Your questions, comments and suggestions!!

Based on the suggestions of large number of students and laser scientists, this section is being renamed as FAQ. The questions being addressed are of very general nature and are being forwarded to us by readers from different parts of India. This section provides an opportunity for interaction where one can ask their clarification either about the articles on web or any other question related to lasers. The reply will be sent to them after discussing with either the authors of a particular article or the experts in that field. Any comments / suggestions to improve the site welcome.

Some of the questions being addressed are given below:

What does laser stand for?

State Einstein's concept of stimulated emission, which lead to the invention of laser:

Who invented laser?

What is MASER and who invented it?

Why Maser was invented much before the invention of a laser?

What are the basic characteristics of a Laser beam?

What is fluorescence?

What is the importance of fluorescence in the generation of laser?

What is phosphorescence?

What leads to monochromaticity in a laser light?

Is the laser light truly monochromatic? If not what is the typical line width of a laser?

What is coherence?

What is coherence length and coherence time?

What is meant by directionality? What is it due to?

Define beam waist.

Define Rayleigh Range.

Define near field.

Define far field.

For helium neon laser, which has a beam waist of 0.5mm, what will be the value of near field and far field conditions?

How does the intensity in a Gaussian beam vary as it propagates?

Relations between pulse width, energy, pulse repetition rate and power.

Define divergence.

Calculate the value of divergence for a He-Ne laser having a beam diameter of 0.05 cm.

What are the various steps in the generation of laser output?

What is small signal gain?

Why a two level laser is not possible ?

Define radiance?

Estimate the radiance of a one mW He-Ne laser with one mm output diameter and a divergence of one milirad.

Define Irradiance.

What is the relation between intensity and power?

Relations between velocity, frequency, wavelength and time period.

Calculate the frequency of Nd:YAG laser.

Discuss quantum efficiency and operating efficiency..

What happens to the laser light, when it passes through any medium say glass with refractive index as 1.5.

Which of the photons are more energetic i) photons emitted by Nd:YAG laser ii) photons emitted by CO₂ laser.

Why four level laser systems are more efficient as compared to three level lasers?

What are the losses in a laser cavity?

What properties control the active gain in the laser?

State condition for lasing:

What is normal lasing?

What is M²?

What does TEM_pq denote?Show the patterns related to TEM₀₁ and TEM₁₀.

Calculate the number of possible longitudinal modes in a He-Ne laser having a cavity of length of one meter.What is the separation between two modes?

In the above example, if the laser gain profile bandwidth is 500 MHz, how many longitudinal modes are possible?

What are the common techniques of pumping the gain medium?

What are the basic criteria for selecting flash lamps for solid state lasers?

What are the drawbacks of flash lamps?

What are the advantages of diode pumping in solid state lasers?

What is the difference between S- and P- polarization?

Define Slope Efficiency of a Laser?

Define Threshold Pump Power?

Define Wall Plug Efficiency?

Define Luminescence, Fluorescence, and Phosphorescence?

What is a flash lamp?

Why xenon is preferred as the filling gas for the flash lamps?

What is the advantage of using fused silica as the flash lamp envelope?

What is meant by negative resistance in flash lamps?

What is an arc lamp?

What is triggering?

Explain over damped, under damped and critically damped circuits in flash lamp operation.

Explain over voltage, external, parallel and series triggering circuits.

What are the different types of electrode-lamp seals?

What are the basic requirements of an electrical power supply for gas discharge devices?

What are the three basic parts of a flash lamp power supply?

What is a PFN?

What are the factors associated with electrical shock?

Discuss briefly the effect of current in the human body.

What does simmering mean?

What is the star/sun shaped symbol, we see at various places?

Can lasers be dangerous for human beings?

How can lasers prove to be dangerous for eyes?

Why is viewing even the diffused reflection of the pulsed solid-state laser from a roughened surface is usually considered hazardous?

Taking eye as an example, discuss why only particular parts of eye are more susceptible to damage by particular laser radiation and not others.

Which is the most powerful laser in the world?

Why rare earth ions are generally doped as active ions in laser materials?

What are the main lasers being used for medical applications?

Mention the wavelengths of few important Lasers?

What are Eye - safe Lasers?

What are different classes of Lasers?

Briefly describe the working of He-Ne laser.

Briefly discuss the working of carbon dioxide (CO₂) laser

Briefly discuss the working of metal vapour laser.

Briefly describe the working of excimer lasers.

What are the salient points of the argon ion laser?

Describe briefly the unique features of helium- cadmium (He-Cd) laser.

Explain the basic principle of the working of gas dynamic laser (GDL).

Explain the basics related to the working of chemical oxygen-iodine laser (COIL).

Why is that some lasers cannot be Q-switched?

Brief description about He-Ne Laser.

Show energy level diagram of He-Ne Laser.

What type of optics is used in He-Ne Lasers?

What are the characteristics of He-Ne Lasers?

What are the important applications of He-Ne Lasers?

What are the fundamental modes of carbon dioxide?

Show simplified picture of various energy levels of carbon dioxide laser

What is the role of various gases involved in carbon dioxide lasers?

What is the type of optics normally used in CO₂ Lasers?

What are typical properties of sealed off CO₂ lasers?

What are various applications of CO₂ lasers?

Can CO₂ lasers be Q - Switched?

How do you compare CO₂ laser and Nd:YAG laser in terms of spot size, power density and applications?

How do you sense CO₂ radiations?

What is the maximum power of CO₂ laser?

What are typical operating conditions for CO₂ lasers?

What is typical efficiency of CO₂ lasers?

What are the optimal conditions for CO₂ laser operation?

How do you compare DC excited and RF excited CO₂ lasers?

Show simplified energy level diagram of Argon ion lasers.

What are various wavelengths of Argon ion lasers?

What is the comparative strength of various Argon ion laser lines?

Why large currents are required to run Argon ion lasers?

What is so special about Argon ion laser plasma tube?

Why magnetic field is used in Argon ion Lasers?

How a single line in Argon ion laser selected?

What are the requirements for holography and how Argon ion laser is tailored for it?

What is the quality factor M² for Argon ion lasers?

Can Argon ion laser be pulsed?

Name few important properties, like divergence, beam size, life etc, have Argon ion lasers?

What are different applications of Argon ion lasers?

What is the difference between Argon ion and Krypton ion laser?

What are white Lasers?

Brief description of He-Ne Laser.

How is a single line in Argon ion laser selected?

What are the prominent lines of Helium Cadmium Laser?

How do you categorize Helium Cadmium Lasers?

What is the main mechanism of Laser action in Helium Cadmium lasers?

Draw a simplified energy level diagram of Helium Cadmium Laser?

How Cadmium is handled in Helium Cadmium Lasers?

What are the important characteristics of Helium Cadmium Lasers?

What are the applications of Helium cadmium Laser?

What is the wavelength of nitrogen laser?

Why nitrogen lasers cannot be CW

How many types of nitrogen lasers are there?

What is the simplified energy level of nitrogen laser?

How does the lifetime of laser level vary with pressure?

What are the important properties of nitrogen lasers?

What are the important applications of nitrogen Lasers?

What do you mean by Excimer?

Name the most common excimer lasers?

How laser action takes place in excimer lasers?

Explain the energy level diagram of excimer lasers?

What are the important properties of excimer lasers?

What are the major applications of excimer lasers?

What are the wavelengths of Copper Vapour Laser and Gold Vapour Laser?

What are the energy levels in Copper Vapour Laser and how Laser action takes place in Copper Vapour Laser?

How Copper Vapour Lasers are realized?

Why two discharge pulses are required for Copper Vapour Lasers based on Copper halides?

What are typical parameters for Copper Vapour Lasers?

What are applications of Copper Vapour Lasers?

How Gold Vapour Lasers are different from Copper Vapour Lasers?

What are main applications of Gold Vapour Lasers?

How laser action is achieved in dye lasers?

How triplet absorption is minimized?

What are the most common dyes and their tuning wavelength range?

What are the pumping sources for dye lasers?

What are solid-state dye lasers?

How do you compare dye lasers with tunable Ti: sapphire lasers?

What are the important characteristics of dye lasers?

What are the main applications of dye lasers?

How many types of Laser Hazards are there?

What is the effect of various lasers on eye and skin?

What is Maximum permissible exposure (MPE)?

What is the MPE for various lasers?

What is Nominal Hazard Zone?

What are various Laser Classifications?

How do you compare semiconductor lasers with solid-state lasers?

What are the basic components of semiconductor lasers?

What is the difference between homojunctions and heterojunctions in semiconductors?

Draw a band energy diagram of p - n junction under no bias condition?

Draw a band energy diagram of p - n junction under forward bias conditions? How Laser operation occurs in forward bias conditions?

Define the Internal quantum efficiency of laser diodes?

Why indirect band gap materials like silicon cannot be an efficient Laser devices?

What are double-heterostructure (DH) lasers and what are the advantages?

What is the difference between gain guided and index guided laser diodes?

What are Fabry-Perot (FP) diode lasers?

What are surface emitting laser diodes or Vertical Cavity Surface Emitting Laser (VCSEL)?

What is Distributed Feedback Laser (DFB)?

What are quantum well devices in Semiconductor Lasers?

What are the most commonly used materials for Laser Diodes?

What is typical output power vs. drive current characteristics?

What is slope or Quantum efficiency?

What are the typical characteristics of semiconductor lasers?

What is the difference between an LED and a Diode Laser?

What are the main applications of Diode Lasers?

How Free electron lasers differ from other conventional lasers?

What are the main subsystems of FEL?

What is the resonant condition?

How wavelength is changed in FEL?

What are typical velocities of electrons in FEL?

What are the typical parameters of FEL?

How is linewidth related to undulator period?

What are the main applications of FEL?

	What does laser stand for? LASER is the acronym for Light Amplification by Stimulated Emission of Radiation.
	State Einstein's concept of stimulated emission, which lead to the invention of laser: Einstein postulated that, when the population inversion exists between upper and lower levels among atomic systems, it is possible to realize amplified stimulated emission and the stimulated emission has the same frequency and phase as the incident radiation.
	Who invented laser? Though the idea of stimulated emission was given by Albert Einstein way back in 1917, but Theodore Maiman invented the laser as such in 1960. He realized the first Laser using ruby as a lasing medium that was stimulated using high energy flashes of intense light.
	What is MASER and who invented it? MASER stands for Microwave Amplification by the Stimulated Emission of Radiation. Charles H Townes of Columbia University, Alexander Prokhorov and Nikolai G Basov of Moscow University and Joseph Weber of University of Maryland invented it simultaneously in 1951.
	Why Maser was invented much before the invention of a laser? The ratio of probability of spontaneous to stimulated light emission is given by the relation: where ρ is the radiation energy density and is equal to Nhn, N being the number of photons of frequency n per unit volume and k is Boltzmann constant. It is evident that this probability of spontaneous to stimulated light emission depends directly on the frequency of emission or inversely to the wavelength. Thus in the microwave region, stimulated emission is more probable than spontaneous, hence the early production of the maser.
	What are the basic characteristics of a Laser beam? Laser beam has following three basic characteristics: Monochromaticity Coherence Directionality
	What is fluorescence? Wavelength of the emission is longer than the absorption wavelength and the emission stops the moment the excitation ceases.
	What is the importance of fluorescence in the generation of laser? Laser out put can be generated only from those laser materials, which show the property of fluorescence and laser action is possible only at that wavelength, where the fluorescence emission occurs. Scientists examine the energy levels spectroscopically to find fluorescence and evaluate the fluorescence efficiency of the new laser active media. In fluorescence, the emitted wavelengths are always longer than the absorbed wave lengths and emission stops the moment the excitation of the material stops.
	What is phosphorescence? In this process, the emission lasts much after the absorption has ceased to exit.
	What leads to monochromaticity in a laser light? Laser light consists of essentially one wavelength, having its origin in stimulated emission from one set of atomic energy levels. This is possible because laser transition, in principle, involves well-defined energy levels. EM wave of frequency n = (E2 - E1) only can be amplified, n has a certain range which is called line width. This line width is decided by various broadening factors such as Doppler effect of moving atoms and molecules. The generation of laser is such that the laser cavity forms a resonant system and laser oscillation is sustained only at the resonant frequencies of the cavity. This leads to the further narrowing of the laser line width. So laser light is usually very pure in wavelength, we say it has the property of monochromatic.
	Is the laser light truly monochromatic? If not what is the typical line width of a laser? No, laser light is not truly monochromatic. This line width is decided by various broadening factors such as Doppler effect of moving atoms and molecules. Typically, the frequency bandwidth of a commercial He-Ne laser is about 1500MHz (full width at half-maximum, FWHM). In terms of wavelength, it means that at a wavelength of 632.8nm this means a wavelength bandwidth of about 0.01nm. On the other hand, the bandwidth of a typically diode laser with a wavelength of 900nm is about 1nm as compared to LED, which has a bandwidth of approximately 30 - 60 nm.
	What is coherence? Coherence is of two types: temporal and spatial. Temporal coherence is a measure of the correlation between the phases of a light wave at different points along the direction of propagation. Spatial coherence is a measure of the correlation between the phases of a light wave at different points transverse to the direction of propagation. Spatial coherence tells us how uniform the phase of the wave front is.
	What is coherence length and coherence time? Temporal coherence tells us how monochromatic a source is. Suppose the laser emits wavelength λ and λ + Δλ. These waves with slightly different wavelengths would continue to interfere constructively and destructively in space at points dependant on the magnitude of Δλ. Smaller is the value of Δλ, larger will be the distance between points where constructive and destructive interference will take place. This optical path between these two positions is called coherence length lc. This is mathematically represented as: lc =λ2/ Δ λ In terms of time, Δt, the time taken by the waves to cover the above-mentioned two positions, it can be given as: lc = c Δtc : where c is the speed of the light wave.
	What is meant by directionality? What is it due to? As we know that in case of stimulated emission, atoms in an upper energy level are triggered or stimulated in phase by an incoming photon of a specific energy. The incident photon must have an energy corresponding to the energy difference between the upper and lower states. The emitted photons have the same energy as incident photon. These photons are in phase with the triggering photon and also travel in its direction. This leads to directionality in case of a laser. Further, the mirrors placed at opposite ends of a laser cavity enables the beam to travel back and forth in order to gain intensity by the stimulated emission of more photons at the same wavelength, which results in increased amplification due to the longer path length through the medium. The multiple reflections also produce a well-collimated beam, because only photons traveling parallel to the cavity walls will be reflected from both mirrors. If the light is the slightest bit off axis, it will be lost from the beam. The resonant cavity, thus, makes certain that only electromagnetic waves traveling along the optic axis can be sustained, consequent building of the gain.
	Define beam waist. For a Gaussian beam propagating in free space, the spot size w (z) will be at a minimum value w0 at one place along the beam axis. This minimum position is known as the beam waist. The position of the beam waist for a typical laser resonator mode occurs either at a point of focus after it has passed through the lens, or in the region between the two mirrors of an optical resonator. For example, in case of confocal resonator (R1 = R2 = length of cavity, d), it occurs exactly in the middle.
	Define Rayleigh Range. In case of Gaussian beams, it is the distance from the beam waist where the mode area is doubled. In terms of beam radius, it is the distance, from the beam waist where the beam radius is increased by a factor of √2. Mathematically, it can be written as:
	Define near field. The region between the beam waist and the Raleigh range is known as the near field. Or for near field conditions,
	Define far field. The distance beyond near field is known as far field. Divergence is always specified in the far field, which is usually chosen to begin around 10 to 100 times the Raleigh range.
	For helium neon laser, which has a beam waist of 0.5mm, what will be the value of near field and far field conditions? For near field conditions, For far field conditions
	How does the intensity in a Gaussian beam vary as it propagates? The optical intensity is a function of axial and radial distances. Axial distances are the distances along the propagation direction, whereas radial distances are the distances in the transverse plane. On the beam axis, it varies as: Where Io is the maximum value at z=0 i.e. beam waist and drops gradually with increasing z, reaching half its value at z = zR i.e. Rayleigh distance. When z>> zR, The intensity decreases with the distance in accordance with the inverse square law. In the transverse plane, the intensity distribution is of the form: Where Imax is the value of intensity at that value of z, r is the radial distance from the axis and wor is the beam waist radius. Maximum intensity is at a point where z = 0 and r = 0, i.e. at the center of the beam waist.
	Relations between pulse width, energy, pulse repetition rate and power. Peak pulse power (W) = Pulse energy (Joules) / Pulse width (sec) Average Power (W) = : Pulse Energy (Joules) x Pulse Repetition Rate (Hz) Average Power (W) = : Peak pulse power (W) x pulse width (sec) x Pulse Repetition Rate (Hz)
	Define divergence. Far from the beam waist, i.e. z >>zR, the beam radius increases approximately linearly with z, defining a cone with an angle θ. About 86% of the beam power is confined within this cone. The divergence arises because of the diffraction effects associated with the cavity bore. The diffraction-limited divergence is given as: where angle θ is in radians, and both λ and wo are either in meter or centimeter. Higher the diameter of the beam waist, lower is the divergence.
	Calculate the value of divergence for a He-Ne laser having a beam diameter of 0.05 cm.
	What are the various steps in the generation of laser output? Absorption, spontaneous emission, population inversion, stimulated emission, gain build up and finally laser action.
	What is small signal gain? As we know that for laser action, an incident photon must have a higher probability of causing stimulated emission than of being absorbed which is possible only if N2 > N1. The number of photons emitted through stimulated emission process depends upon two factors i.e. N2 and the incident energy. However, after stimulated emission, the atoms return from the excited state to the ground state, reducing the number of N2, thus the capacity of the gain medium for further amplification. The effect is more pronounced, if he incident or pumping energy is large. Small signal gain is defined as the gain in the system under the conditions that the depletion of the excited species are negligible.
	Why a two level laser is not possible ? For laser action, population inversion i.e. N2 > N1 is required. A population inversion cannot be achieved with just two levels because the probability for absorption and for spontaneous emission is exactly the same. The lifetime of a typical excited state is about 10 -8 - 10 -9 seconds, so in practical terms, the electrons revert back to ground level through spontaneous emission of photon almost as fast as we pump them up to the upper level.
	Define radiance? Radiance is usually to the source. It is power divided by the area of the laser beam and the solid angle within which the power is confined. Its units are Watts / m2-steradian. It is also referred as brightness.
	Estimate the radiance of a one mW He-Ne laser with one mm output diameter and a divergence of one milirad. For small angle, the relation between planar angle θ and the solid angle ρ is given as Ω = (π / 4) θ2 One milirad corresponds to Ω = (π / 4) (1 mrad)2 = 0.8 x 10-6 sterad
	Define Irradiance. It is defined as the power per unit area of laser light falling on the target. It is usually referred as power density and expressed as Watt / cm2. Suppose one kilowatt of laser power is falling on a target having a spot size of 10 cm. The irradiance can be estimated as:
	What is the relation between intensity and power? Intensity = Laser Power / Area of the spot. One can see that this is the same relation as that of Irradiance. The term irradiance is usually used when we consider a distant target and the actual power is the one reaching the target. This must take care of factors like attenuation in atmosphere etc. On the other hand, the term intensity is used for targets kept very near to the laser source i.e. typically for material applications point of view. So one can assume the power of the laser as the power falling on the work sample.
	Relations between velocity, frequency, wavelength and time period. Velocity of light:             c = 3 x 10 8 m /sec in vacuum Wavelength: λ (m) Frequency: n (Hz) Time period: T (sec-1) c = n λ n = c / λ λ = c / n n = 1 / T
	Calculate the frequency of Nd:YAG laser. The wavelength of Nd:YAG laser is 1.064 micron or 1.064 x 10-6m. Frequency is given as:
	Discuss quantum efficiency and operating efficiency. Quantum efficiency of a laser is defined as the ratio of the energy of the output photon and the input energy necessary to produce that photon, for a perfect system. The operating efficiency is the ratio of the output energy to the input energy expressed in percentage. Taking Nd:YAG laser as an example, its quantum efficiency is about 80% while its operating efficiency is only few percentage
	What happens to the laser light, when it passes through any medium say glass with refractive index as 1.5. As we know that when light passes through any medium, its velocity changes depending on the refractive index, n, of the medium. In the present case, the velocity in glass would be: The frequency of the laser does not change and this velocity change is reflected in the wavelength change. In the present case, the wavelength in the medium will be reduced in the following manner: For Nd:YAG laser, the wave length in the medium will be reduced from 1.064 micron to 0.71 micron.
	Which of the photons are more energetic i) photons emitted by Nd:YAG laser ii) photons emitted by CO2 laser. The energy of a photon is given as E = h n, where h is Plank's constant and n is the frequency of laser radiation. The wavelength of Nd:YAG and CO2 lasers are 1.064 and 10.6 micron respectively. The corresponding frequencies are The corresponding energies are given as: Or in terms of other familiar units "electron volts (eV)", these energies are: It is clear that photon emitted by Nd:YAG laser is an order more energetic than that emitted by CO2 laser.
	Why four level laser systems are more efficient as compared to three level lasers? Please refer to following figures of three level and four level laser systems; We know that the laser action is initiated only when the population inversion condition is achieved i.e. N2 > N1 where N1 and N2 are the population of the two levels involved in laser action. Further higher is the probability of stimulated emission as compared to absorption, if the difference N2 - N1 is large. In case of three level laser systems, the laser action is initiated only when the excited atoms in level 2 are significantly higher than the number of atoms in ground state at level 1. Since the lifetime of level 3 is of the order of nanoseconds, the excited atoms emit spontaneously and come to a metastable level 2, which has a higher lifetime. In other words, more than half of the atoms should be shifted to level 2 via level 3 to initiate laser action. This requires very strong pumping source. On the other hand, in case of four level lasers, laser action is achieved between levels 3 and 2; both of them are completely empty to start with. So if pumping were able to excite even a fraction of the total ground level atoms, these would shift to metastable level 3 via level 4, which has a short lifetime of the order of nanoseconds. This results in immediate establishment of population inversion condition and thus the laser action. Since the number of atoms to be excited is far smaller in case of four level lasers as compared to three level lasers, the pump power requirements are much smaller too. This leads to higher efficiency in four level lasers.
	What are the losses in a laser cavity? Scattering and absorption due to impurities in the active laser material produce radiation losses The finite dimensions of the laser material, end mirrors and other components produce diffraction losses Imperfect end mirrors produce reflection losses
	What properties control the active gain in the laser? Population inversion Fluorescence efficiency, line width and shape of the spontaneous emission in the wavelength of interest Resonant cavity
	State condition for lasing: Active gain in the laser media should exceed the losses in a complete round trip path of the photons between the end mirrors.
	What is normal lasing? After achieving population inversion, gain build up takes place and laser is generated. Due to laser generation, depletion of population inversion occurs and laser emission ceases. i.e. laser kills it self, unless population inversion and gain are achieved again. In normal lasing, laser output comes in bursts. A typical view of a normal solid-state laser output is shown in the adjoining figure.
	What is M2? It is a measure of beam quality. In simple terms it is the ratio of the divergence of the real beam to that of a theoretical diffraction-limited beam of the same waist size with a Gaussian beam profile.
	What does TEMpq denote? Show the patterns related to TEM01 and TEM10. TEM stands for Transverse Electromagnetic modes. The subscripts p and q represent the number of zero illumination points (between illuminated regions) along x-axis and y-axis respectively.
	Calculate the number of possible longitudinal modes in a He-Ne laser having a cavity of length of one meter.What is the separation between two modes? The number of possible modes can be estimated using the following equation: 2nL=Nλ where L is the length of the cavity, N is number of all possible modes, n is the refractive index of the lasing medium and λ is the wavelength. Assuming refractive index of the medium as one, the number of modes can be calculated as However, all these modes will not be supported. These are limited by the fluorescence curve and only the modes for which the gain of laser of the laser medium G (λ) > 1 would be supported. The separation between two modes can be estimated using simple relation: Δ f = c / 2L, where Δ f is the frequency separation In the above example, this separation is OR one can make use of the following relation and find out the separation in terms of wavelength: This gives:
	In the above example, if the laser gain profile bandwidth is 500 MHz, how many longitudinal modes are possible? As mentioned above that though the number of possible modes exceed three million but the number is restricted by fluorescence curve and only the modes for which the gain of laser of the laser medium G (λ) > 1 would be supported. In the present case, this bandwidth is 500 MHz, and the frequency spacing between two modes is 150 MHz. Thus maximum number of modes can be = (bandwidth / spacing) = 500 / 150 = 3.33. In a practical case only three modes will be excited.
	What are the basic criteria for selecting flash lamps for solid state lasers? Spectral emission characteristics of the flash lamp should match the absorption band of the lasing medium.
	What are the common techniques of pumping the gain medium? Pumping of the gain media is usually performed in one of the following forms: Optical pumping Electrical pumping Chemical pumping So far as solid-state lasers are concerned, it is mainly the optical pumping, which is being used. Optical pumping uses either CW or pulsed light emitted by a powerful lamp or a laser beam. Gas lasers such as He - Ne, carbon dioxide are pumped by electrical means. Chemical lasers such as Hydrogen Fluoride (HF), Deuterium Fluoride (DF), Chemical oxygen Iodine Laser (COIL) are pumped through chemical means.
	What are the drawbacks of flash lamps? The lifetime of lamps is very limited - normally up to a few thousand hours. Flash lamps have a broad emission spectra (see adjoining figure) whereas the absorption spectra of lasing media have more or less discreet absorption peeks. As a result, most of the optical energy being emitted by the flash lamp goes waste. The wall plug efficiency of the laser (electrical to optical efficiency) is low - typically ~ few percent. This results in a higher heat load, making necessary a more powerful cooling system, and the strong thermal lensing and hence a poor beam quality. Electric power supplies for lamp-pumped lasers involve high electrical voltages, which raise additional safety issues. The low pump brightness (compared with that achievable with diode lasers) and the broad emission wavelength range exclude many solid-state gain media.
	What are the advantages of diode pumping in solid state lasers? The main advantages of diode pumping can be summarized as follows: The compactness of the pump source, the power supply and the cooling arrangement makes the whole laser system much smaller and easier to use. A high electrical-to-optical efficiency of the pump source (order of 50%) leads to a high overall power efficiency i.e. wall plug efficiency of the laser. As a consequence, small power supplies are needed, and both the electricity consumption and the cooling demands are drastically reduced, comparing with those for lamp-pumped lasers. The narrow optical bandwidth of diode lasers makes it possible to directly pump certain transitions of laser-active ions without losing power in other spectral regions. It thus also contributes to a high efficiency. Although the beam quality of high power diode lasers is poor, however, end pumping of lasers provide very good overlap of laser mode and pump region, leading to high beam quality and power efficiency. Diode-pumped low-power lasers can be pumped with diffraction-limited laser diodes. This allows the construction of very low power lasers with reasonable power efficiency. The lifetime of laser diodes is long compared with that of discharge lamps: Further it is much easier to replace laser diodes as compared to discharge lamps. Diode pumping makes it possible to use a very wide range of solid-state gain media for different wavelength regions.
	What is the difference between S- and P- polarization? S-polarization (the s stems from the German word Senkrecht meaning perpendicular) and P-polarization (the p means parallel) are the two main ways to describe two types of linearly polarized light. This perpendicular and parallel are with respect to the surface onto which the light is incident or being reflected. So, P-polarization refers to light that is polarized parallel to the plane of incidence and S-polarization refers to light that is polarized perpendicularly to the plane of incidence.
	Define Slope Efficiency of a Laser? Slope efficiency is defined as the slope of the curve obtained by plotting the laser output versus the pump power. It is worth mentioning that for a given pump power, the laser (gain medium) having higher pump absorption efficiency will have higher slope efficiency as well.
	Define Threshold Pump Power? The threshold pump power of an optically pumped laser is the value of the incident pump power for just initiation of Laser action. At this point, the small signal gain equals the losses inside a laser resonator. A low threshold power requires low cavity losses and high pump absorption efficiency. In all practical situations, the pump power used in normal operation is several times higher than the pump threshold power.
	Define Wall Plug Efficiency? The wall-plug efficiency of a laser system is its total electrical-to-optical power efficiency. Though the electrical power consumed should include all the electrical instruments like the chillers and re-circulation systems required for a cooling system, however, the convention is to take into account only the electrical power required for running flash lamps or laser diodes, which are used for pumping the gain medium. It is understandable that diode pumped solid state lasers have much higher wall-plug efficiency as compared to flash lamp pumped lasers.
	Define Luminescence, Fluorescence, and Phosphorescence? Luminescence is a collective term for different phenomena where a substance emits light without being strongly heated, i.e., the emission is not simply thermal radiation. This definition is also reflected by the term "cold light". Fluorescence and Phosphorescence fall under this category. Fluorescence refers to the light emission caused by irradiation with visible or UV light. The light emission occurs typically within nanoseconds to milliseconds after irradiation. It involves the excitation of electrons into states with a higher energy, from where radiative decay is possible. Typically, the emitted wavelengths are longer than the excitation wavelengths. Phosphorescence, on the other hand, refers to a light emission, which can occur over much longer times (sometimes hours) after irradiation. It involves storage of energy in metastable states and its release through relatively slow processes. This release of energy is usually enhanced when thermally activated.
	What is a flash lamp? A flash lamp or flash tube is a gaseous discharge device filled with Nobel gas (usually Xenon or Krypton) that is designed to produce pulsed radiation.
	Why xenon is preferred as the filling gas for the flash lamps? Total radiation output is maximum for xenon compared to other noble gases (except radon, which is radio-active) for the same electrical input.
	What is the advantage of using fused silica as the flash lamp envelope? It has a spectral transmission from 200 to 4000nm, high thermal conductivity and low thermal expansion.
	What is meant by negative resistance in flash lamps? In flash lamps, as the current increases the resistance decreases and this is referred to as negative resistance
	What is an arc lamp? Arc lamps are gas discharge devices designed for continuous radiation. Krypton filled lamps offer high Nd:YAG pumping efficiency because the emission spectra of the lamp is close to the absorption spectrum of the lasing medium.
	What is triggering? Triggering is the initiation of a discharge through a strobe lamp or arc lamp. There are four methods of triggering a flash tube. They are over voltage triggering, series triggering, external triggering and parallel triggering. The most flexible and commonly used design is external triggering.
	Explain over damped, under damped and critically damped circuits in flash lamp operation. Over damped circuit produces current pulse with low peak value. Under damped circuit generates current pulse with oscillatory nature, resulting in current reversing and swinging negative current. Critically damped circuit produces ideal current pulse of high peak value with no current reversal.
	Explain over voltage, external, parallel and series triggering circuits. In the case of over voltage triggering, the bias voltage across the lamp is high enough to produce the break down of the gas in the flash lamp to begin the discharge. But the high voltage trigger switch (like hydrogen thyratron) is an expensive device. In external triggering, a very high voltage of short duration is applied to the trigger wire directly, which is wound outside the envelope of the flash lamp to initiate the discharge. The advantage is that this requires an inexpensive and lightweight transformer In series triggering, the secondary of the trigger transformer is in series with the flash lamp and the energy storage capacitor. It produces a safe, highly reliable and reproducible triggering, resulting in a stable and steady light output. In parallel triggering, the secondary winding of the trigger transformer is connected in parallel to the lamp, with a diode isolating the secondary winding of the transformer from the energy storage capacitor.
	What are the different types of electrode-lamp seals? There are two types of sealing of the electrodes to the quartz tubes, namely, tungsten rod seals and end cap seals. The tungsten rod seal is made with glasses having thermal expansion intermediate between tungsten and the quartz envelope to reduce the stress arising from thermal expansion at material interface. Since this structure (graded seal) is processed at high temperature, flash lamp operation at high current densities with long life is possible. In end cap seals, a circular band of invar, constituting the end cap is incorporated with the quartz tube. It is also called solder seal as lead-indium solder with relatively low melting point, is used to make the seal due to its low manufacturing cost. Since the solder material has a low melting point, it cannot withstand high temperature and high current density, in comparison to rod seals. This limits its operation to moderate current applications.
	What are the basic requirements of an electrical power supply for gas discharge devices? It should provide high voltage sufficient to ionize the gas, with adequate voltage and current for a continuous discharge at a pre-selected current level. i.e. Peak voltage exceeding the ionization potential of the gas and a ballast resistor to limit the current passing through gas. Basic reason for controlling the current is that population inversion is dependent on current and not on voltage.
	What are the three basic parts of a flash lamp power supply? Three basic elements are 1) high voltage DC power supply for charging the energy storage capacitor bank 2) pulse forming net work (PFN) comprising of inductance ( L ), capacitor ( C ) and resistance ( R ) being the resistance at the time of gas discharge for transfer of energy from capacitor to flash lamp.3) trigger circuit to provide a low current & high voltage trigger pulse to initiate ionization of the gas and subsequent gas discharge and flash lamp output.
	What is a PFN? A Pulse Forming Network is comprised of an inductor, capacitor, and power supply that generate an electrical pulse to a flash lamp. The values of the capacitor and the inductor decide the electrical pulse to the lamp.
	What are the factors associated with electrical shock? Damage to human body is basically due to the magnitude of the current and not the voltage. But it may be understood that the voltage and the resistance of the body determines the amount of the current flow. The basic factors are magnitude of the current, the flow route and the duration of the current flow. If it is an a.c. circuit, the frequency is also important as it could affect the function of the heart, for example. Damages can be due to thermal and non-thermal effects. Thermal effect produces burns and non-thermal effect produce electrical breakdown of muscles and nerve cells.
	Discuss briefly the effect of current in the human body. The dry skin of the person has a very high resistance compared to the wet skin and as such the amount of current flow in the latter case will be very high. Further the internal parts of the body have low resistance due to the salty and moist nature of the tissues. The high current thus developed cause damage. The function of the heart is affected by the frequency, unabling it to pump the blood properly.
	What does simmering mean? Simmering is the process of maintaining a steady state of partial ionization in the xenon flash lamp during operation between flashes. Simmering avoids the electrode stress associated with continually discharging across a lamp that is not ionized.
	What is the star/sun shaped symbol, we see at various places? The symbol is a simplified version of the international "sunburst" symbol, which stands for lasers. The standard international symbol is shown on the left in the International "Lasers In Use" warning sign which should be posted in areas where unshielded lasers are in use. You may even find a similar sign or logo inside your CD player or CD-ROM drive as they use low power laser diodes to read the data from the disk.
	Can lasers be dangerous for human beings? Yes. Lasers can be dangerous for the part of the body that is most sensitive to light, the eye and laser beams can also cause skin and clothing burns. This is why at laser shows, high power laser beams are usually well above our heads.
	How can lasers prove to be dangerous for eyes? If a high power laser beam strikes in the eye it can cause a burn on the retina. Just as the magnifying glass can focus the sun and burn a piece of paper, the lens in the human eye focuses the laser beam down to a very small point on the retina which can cause a burn. The focusing effect can concentrate a laser beam up to 100,000 times thus the power density of one watt / cm2 beam entering the eye can be focused to a point with power density increasing to 100,000 watts / cm2. This power density is more than enough to cause a severe retinal burn and loss of vision.
	Why is viewing even the diffused reflection of the pulsed solid-state laser from a roughened surface is usually considered hazardous? Though diffused reflection is not focused on the retina, it forms an image and it is likely that the damage threshold may exceed the limit of accepted value due to the high peak power associated with the solid-state lasers.
	Taking eye as an example, discuss why only particular parts of eye are more susceptible to damage by particular laser radiation and not others. Only those parts of the eye that absorb radiation of a particular spectral region are susceptible to damage. It will not be damaged by radiation of a particular wavelength, if it is not absorbed by it.. For example, lasers in the visible and infrared region are transmitted by the lens and therefore the lens is not damaged. As these radiations reach the retina, it absorbs the same and consequently gets damaged. Similarly, cornea absorbs far infrared and short wavelength lasers and thus gets affected only by lasers emitting these radiations
	Which is the most powerful laser in the world? The Nova facility in the USA, now called the NIF (National Ignition Facility) is certainly the most powerful, among the pulsed lasers. However, in case of CW lasers, the military lasers such as HF and DF lasers, which are in the megawatt range, are most powerful.
	Why rare earth ions are generally doped as active ions in laser materials? In most cases, the rare-earth ions replace other ions of similar size and same valence (charge state) in the host medium; for example, a Nd3+ ion in Nd:YAG (yttrium aluminum garnet) substitutes an yttrium (Y3+) ion. The concentration of laser-active rare-earth dopants in the host medium is in most cases only a small molar percentage. A characteristic property of the trivalent rare-earth ions is that their electronic transitions usually occur within the 4f shell, which is somewhat shielded from the host lattice by the optically passive outer electronic shells. This reduces the influence of the host lattice on the wavelengths, bandwidths and cross sections of the relevant optical transitions.
	What are the main lasers being used for medical applications? LASIK: Laser Assisted in situ Keratomileusis UV 193 nm Excimer laser PRK: Photorefractive Keratectomy UV 193 nm Excimer laser Laser Surgery: Revisualization: For end stage coronary bypass surgery and angioplasty Holmium YAG laser 2.1 micron Hair Removal 700 - 1000 nm Skin resurfacing 3 - 10 micron Vascular and Pigment conditions 532 - 600 nm
	Mention the wavelengths of few important Lasers? Laser Wavelength Excimer - Argon Fluoride (ArF) 0.193 micron Excimer - Xenon Chloride (XeCl) 0.308 micron Excimer - Xenon Fluoride (XeF) 0.351 micron Helium Neon (He-Ne) 0. 6328 micron Chromium: Aluminum oxide (Cr: Al2O3) - (Ruby) 0.6943 micron Neodymium: Yttrium-Aluminum-Garnet (Nd: YAG) 1.064 micron Neodymium: Glass (Nd: Glass) 1.054 micron Erbium: Glass (Er: Glass) 1.54 micron Erbium Yttrium-Aluminum-Garnet (Er:YAG) 2.94 micron Holmium: Yttrium-Aluminum-Garnet (Ho:YAG) 2.1 micron Chromium: chrysoberyl (Cr+3:BeAl2O4) - Alexandrite 0.700 to 0.820 micron Titanium: sapphire (Ti3+:Al2O3) Ti: Sapphire 0.660 to 1.1 micron Carbon dioxide (CO2) 10.6 micron Gallium Arsenide (GaAs) 0. 840 micron Gallium Aluminum Arsenide (GaAlAs) 0.670 - 0.830 micron
	What are Eye - safe Lasers? For a given power levels, Lasers emitting in a wavelength region with relatively low hazards for the human eye are known as eye-safe lasers. Lasers with emission wavelengths longer than 1.4 μm usually fall in this category of "eye-safe", because light in that wavelength range is strongly absorbed in the eye's lens and thus cannot reach the significantly more sensitive retina. Wavelengths between 400 nm and 1400 nm are focused by the curved cornea and lens on to the retina; the optical gain is about 100,000-200,000 times. Viewing a laser beam or Point Source will focus all the light on a very small area of the retina, resulting in a greatly increased power density and an increased chance of damage. Threshold energy density for retinal damage at 1064 nm (Nd:YAG) is 10-6 J/Cm2 and at 694.3 nm (Ruby) it is 10-7 J/Cm2. For laser wavelengths above 1400 nm, damage to the retina occurs at very high energy density, since the transmission of the eye is negligible as shown in the adjoining figure. For example, Er:Glass laser emits radiation at 1540 nm and threshold density of retinal damage is 1 J/Cm2. Such high energy density is not normally encountered at work place and these types of lasers are eye safe.
	What are different classes of Lasers? Class I Lasers These are lasers that are not hazardous for continuous viewing or are designed in such a way that prevent human access to laser radiation. These consist of low power lasers or higher power embedded lasers. (i.e. laser printers) Class 2 Visible Lasers (400 to 700 nm) Lasers emitting visible light of low power normally do not present a hazard because of normal human aversion responses. However may be hazardous, if viewed directly for extended periods of time like many conventional light sources. Class 2A Visible Lasers (400 to 700 nm) Lasers emitting visible light not intended for viewing, and under normal operating conditions would not produce an injury to the eye if viewed directly for less than 1000 seconds. (i.e. bar code scanners) Class 3a Lasers in this category are normally those, which would not cause injury to the eye if viewed momentarily but would present a hazard if viewed using collecting optics like telescope. Class 3b Lasers in this category are those, which present an eye and skin hazard if viewed directly. Class 3b lasers do not produce a hazardous diffuse reflection except when viewed at close proximity. Class 4 Lasers Lasers in this category present an eye hazard from direct, specular and diffuse reflections. In addition such lasers may be fire hazards and produce skin burns.
	Briefly describe the working of He-Ne laser. In He-ne laser, neon atoms are responsible for laser emission. It uses a mixture of helium and neon gases up to a pressure of few torr, with the mixture containing almost ten times helium than neon. Electrical discharge raises the helium and neon atoms to the excited levels. The more abundant helium atoms transfer their excitation energy to neon atoms by collision, thus creating a population inversion condition for neon atoms, resulting in laser emission at 632.8nm.
	Briefly discuss the working of carbon dioxide (CO2) laser. In CO2 laser, carbon dioxide molecule is the active laser medium. It uses a mixture of carbon dioxide, nitrogen and helium, usually in the ratio of 9, 90, 1 percentage respectively. During the electrical discharge, the upper laser levels of carbon dioxide molecules are populated by collision with the abundant excited nitrogen molecules. Helium helps in not only in depopulating lower laser level but also in removing the heat from the system. The population inversion thus generated is responsible for CO2 laser emission at 10.6 micron.
	Briefly discuss the working of metal vapour laser. Metal vapour laser works at high temperatures (above 1000oC) and employs ceramic tubes containing metal pellets of gold or copper, as the case may be, and filled with neon gas. The electrical discharge through neon gas heats the metal pellets generating corresponding metal vapours at low pressure. Metal vapour laser is basically a pulsed laser and as it works at a high repetition rate (few kHz) it appears to be giving continuous output. Copper vapour laser gives output at 511 and 578nm and gold at 628nm.
	Briefly describe the working of excimer lasers. Excimer lasers give pulsed output in the UV region of nanoseconds duration. Excimer lasers employ noble gas molecules, which form compounds that have no stable ground state, but have excited states that are bound temporarily. i.e. they combine for a short period as transient short lived molecule. The diatomic molecule, formed by the union of two atoms, is called an excimer, for example Xenon fluoride (XeF). When a gas mixture of xenon and fluorine is excited in a pulsed discharge device, excited state xenon fluoride (XeF*) is formed. But this metastable excited state exists only for a short period and dissociates as per the reaction XeF* -- Xe + F + hn where the photon hn corresponds to a wavelength of 351nm. It may be noted that as soon as the photon is emitted, XeF* molecule breaks up to form Xe and F. Other important excimer lasers are KrF (krypton fluoride-249nm), CaF2 (calcium fluoride-193nm), ArF (argon fluoride -191nm), XeBr (xenon bromide- 282), and XeCl (xenon chloride-308nm). High-energy electron beams are also used to excite excimer lasers.
	What are the salient points of the argon ion laser? Argon ion lasers require high current density discharge for the generation of ionized argon as the laser medium. In this case, the energy levels responsible for laser action belong to singly ionized argon gas, the lower level being the ground state of argon ion. Since the upper energy level is about 20 ev above the ionized ground state, considerable amount of energy has to be supplied to raise the neutral argon atom to this level. Though there are a number of output wavelengths, most important are at 488 and 514nm.
	Describe briefly the unique features of helium- cadmium (He-Cd) laser. He-Cd laser is a very unique laser in the sense that it is basically an ion laser as the energy levels of ionized gaseous cadmium is used for laser operation. Like metal vapour laser, cadmium metal contained in a reservoir near the anode is heated to give the optimum vapour pressure. The electric discharge generates ionized cadmium gas. Interestingly it has few similarities with He-Ne laser as well. During electric discharge, excited helium atoms produced by collision with electrons, collide with cadmium atoms in the ground state to produce excited levels of cadmium ion. He-Cd laser generates output at 441 and 325 nm.
	Explain the basic principle of the working of gas dynamic laser (GDL). GDL works on the principle that rapid heating and cooling can produce population inversion in molecular systems. When high-pressure hot gas contained in a chamber is rapidly expanded through a bank of convergent -divergent supersonic nozzles to a high mach number, gas is rapidly cooled due to supersonic expansion. Population inversion is created between two molecular energy levels by the differential vibrational relaxation processes. Due to the quickness of expansion, the upper laser level cannot follow the rapid change in temperature and pressure and its population is frozen for a long time downstream of the nozzle. At the same time, the lower level relaxes in a time shorter than expansion time, especially with the addition of a catalyst like water, the lower level population decreases rapidly with in the nozzle and continues to do so till it almost becomes nil downstream of the nozzle. Thus population inversion of a thermally pumped system is produced gasdynamically by rapid expansion through the supersonic nozzle. The laser cavity at the downstream of the nozzle extracts the laser output, perpendicular to the direction of the flow of the gas. In the combustion driven gas dynamic laser (CDGDL) employing CO2 laser, benzene and nitrous oxide are burnt in a combustion chamber with nitrogen added to the mixture. This generates CO2, N2 and water vapour in the required ratio to produce laser output at 10.6μ.
	Explain the basics related to the working of chemical oxygen-iodine laser (COIL). Chemical oxygen iodine laser (COIL) made its debut in 1978. In this laser system, a reaction generated between chlorine and hydrogen peroxide excites oxygen atoms, known as singlet oxygen. These singlet oxygen species transfer their energy to iodine atoms, which are the lasing species. This transfer of energy causes the iodine atoms to become excited, creating a laser with a wavelength of about 1.3 microns. This is the shortest wavelength as compared to other chemical lasers e.g. hydrogen fluoride or deuterium fluoride. This smaller wavelength means that smaller optics can be used to develop a useful system.
	Why is that some lasers cannot be Q-switched? In Q-switching, increase of stored energy and the amplifier gain is limited to only for the period of the lifetime of the upper lasing level. The pumping carried out above this period will result in losing the stored energy by fluorescence. Therefore, more energy can be stored in the active medium if the fluorescent lifetime is greater. In solid-state lasers, the fluorescent lifetime is of the order of few hundred microseconds, which is sufficiently high for Q-switching. But in certain lasers, the upper laser lifetime is too small to build up a large stored energy and as such cannot be Q-switched. Ion lasers fall in to this category
	Brief description about He-Ne Laser. It is a four level atom laser with a mixture of helium and neon. Though it lases at a number of wavelengths, its most popular output is at 633nm (red). Other available outputs are at 543nm (green), 594nm (yellow), 612nm (orange) and 1523nm (infra-red). Though neon is the lasing gas, it is the minor constituent (15% of the total mixture), helium taking the bigger share. Electrical discharge excites the helium atoms to the higher energy states, which are populated by electronic collisions.
	Show energy level diagram of He-Ne Laser.
	What type of optics is used in He-Ne Lasers? To extract a light beam from the resonator, it is necessary that one of the two resonator mirrors, usually called the output coupler, has a reflectivity of 99% so that 1% of the photons incident on it travel out of the resonator to produce an external laser beam. The other mirror, called the high reflector, should be as reflective as possible. The diameter, bandwidth, and polarization of the HeNe laser beam are determined by the properties of the resonator mirrors and other optical components that lie along the axis of the optical resonator.
	What are the characteristics of He-Ne Lasers? The gain of the HeNe laser is inversely proportional to the tube radius; the narrower the discharge tube, the higher the gain. In most HeNe lasers the tube diameter is not larger than a few millimeters. An additional benefit of the small tube diameter is that the emission is restricted to the TEM00 mode; higher order transverse modes cannot oscillate in very narrow tubes. Beam diameters of helium-neon lasers with TEM00 output in the milliwatt range are usually around a millimeter. Divergence of He-Ne laser is of the order of 1 milliradian, which drops when beam diameter increases, because these lasers normally operate near the diffraction limit. Commercial models of He-Ne laser emit continuous beams from a few tenths of a millwatt to 75mW, with most in the 0.5mW to 7mW range.
	What are the important applications of He-Ne Lasers? Interferometers Free-space optical communications Fiber Optic Experimentation. Viewing of holograms. Hologram generation Construction of a basic laser light show Laser surveillance. Laser tachometer. Laser burglar alarm. Laser gyroscope.
	What are the fundamental modes of carbon dioxide? Carbon dioxide molecule is a tri-atomic molecule consisting of two oxygen atoms covalently bonded to a central carbon atom. It has three fundamental modes of vibration, namely, symmetric, bending and asymmetric stretching modes, which are shown in figure below. In the symmetric mode, carbon atom is in the center and the two oxygen atoms oscillate symmetrically along the axis of the molecule in unison, either away from or towards each other. In bending stretch mode, the oscillation of the molecules is in perpendicular direction to the axis. In the asymmetric mode, though the molecules oscillate along the axis, only one of the oxygen atoms comes close to the central carbon atom at a time and as this atom moves away from the center, the other atom comes towards the carbon atom and they alternate the movements.
	Show simplified picture of various energy levels of carbon dioxide laser
	What is the role of various gases involved in carbon dioxide lasers? CO2 laser uses CO2, N2, He and sometimes some hydrogen (H2) and or water vapor mixtures. Nitrogen has only one vibrational mode and its energy level is very near to the CO2 (001) level. Collision between CO2 molecule in the ground state (000) and N2 molecule results in the transfer of energy to CO2 molecule. Consequently, CO2 molecule will be at (001) state. The presence of helium gas helps in accelerating the de-excitation of (010) to (000) level, thus increasing the efficiency of the system. The role of hydrogen or water vapor (2-5 %) is to help (particularly in sealed-tube lasers) to reoxidize carbon monoxide (formed in the discharge) to carbon dioxide.
	What is the type of optics normally used in CO2 Lasers? Total reflecting mirror is a highly polished solid molybdnium or silicon with high reflectivity coatings or gold-coated copper. Germanium is another choice, but this has to be cooled, especially for high output. For high power applications, gold mirrors and zinc selenide windows and lenses are preferred. Recently diamond windows and even lenses are also being used. Diamond windows are extremely expensive, but their high thermal conductivity and hardness make them useful in high-power applications. Output coupler is normally Zinc Selenide (ZnSe) with reflectivity typically around 5-15%.
	What are typical properties of sealed off CO2 lasers? In sealed off low-power lasers and in slow gas flow lasers; the beam quality can be very high. The beam size and divergence angle are between 1 - 7 mm and 2 - 6 mrad respectively. Since the laser is being operated at relatively small pressures, the more dominant form of broadening is Doppler broadening Optimised systems achieve power outputs of up to 60 - 70 Watt per meter of discharged length. Life up to 10000 hrs can be achieved by suitably selecting the gas mixture ratios.
	What are various applications of CO2 lasers? The biggest use of these lasers is for material processing. CO2 lasers are used for cutting materials such as plastic or metal, welding, etching or engraving materials. Relatively high power CO2 lasers are frequently used in industrial applications for cutting and welding, while lower power level lasers are used for engraving. Cutting of plastic materials, wood, die boards, etc which exhibits high absorption at 10.6 μm, require moderate power levels of 20-200 W, whereas cutting and welding metals such as stainless steel, aluminum or copper, require multi-kilowatt powers. Recently, they have been receiving a lot of attention for use in medical procedures. They are useful in surgical procedures because water present in most of the biological tissues absorbs this frequency of light very well. CO2 lasers have been used in surgery to cut skin, stop minor bleeding during surgery, remove or vaporize abnormalities and to perform skin resurfacing etc. Also, it could be used to treat certain skin conditions such as removal of embarrassing or annoying bumps, podules, etc. Because of excellent beam quality, the sealed or no flow CO2 laser is often used in beam-deflected laser marking. The TEA CO2 laser is often used in mask marking. They have been used as a tool to measure distance Because the atmosphere is transparent for CO2 wavelength, these lasers are also being used for military rangefinding and LIDAR applications. Further, the long operation wavelength of CO2 lasers makes them almost eye - safe particularly at lower intensities.
	Can CO2 lasers be Q - Switched? A shorter pulse is possible by using an internal electro-optic modulator, such as a Q-switch (or cavity dumper). It is also very easy to actively Q - switch a CO2 laser by means of a rotating mirror or an electro-optic switch, giving rise to Q-switched peak powers up to gigawatts (GW) of peak power. Previously, Q-switching was limited to military and very high-value applications, because the limited availability of cadmium-telluride (CdTe) modulator crystals. Even then, the modulators had a short lifetime because of poor damage threshold properties of these crystals. However, recent innovations have eliminated these drawbacks. Now, a modulator with advanced growth techniques suitable for CdTe crystals produces devices with very high optical damage threshold. Properties of some resonantly absorbing molecules have also been experimentally investigated by making use of Q-switching techniques. SF6 has been used to passively Q-switch CO2 lasers.
	How do you compare CO2 laser and Nd:YAG laser in terms of spot size, power density and applications? The wavelength of a YAG laser (1.064 microns) is exactly ten times smaller than the CO2 wavelength (10.64 microns) and therefore, has a resulting spot size that is 10 times smaller than a CO2 (in the same set-up). The power density is almost 100 times more in case of YAG laser for the same output power of both the lasers. In materials processing, the shorter wavelength of the Nd:YAG couples better to metal while the longer CO2 wavelength is more suitable for cutting plastics, ceramics and other organic materials.
	How do you sense CO2 radiations? 10.6 μm are totally invisible to the human eye and conventional solid-state sensors cannot work. Therefore, thermal approaches are generally used to measure beam power or determine beam profile. These include low cost CO2 viewing plates, thermo-couple based power meters.
	What is the maximum power of CO2 Laser? 10 MW produced by Russia based on Gas Dynamic principle.
	What are typical operating conditions for CO2 lasers? The typical sealed CO2 tube has an operating voltage of between 3 and 12 kV at 2 to 15 mA DC.
	What is typical efficiency of CO2 lasers? The electrical to optical efficiency of a typical sealed CO2 laser is around 10 - 20 percent as compared to much less than 1 percent for a HeNe laser.
	What are the optimal conditions for CO2 Laser operation? The best efficiency is only achieved when tube diameter and gas pressure are optimal. For example, the optimum gas pressure for a sealed CW CO2 laser using an 8-10 mm inside diameter glass tube is about 14 -15 Torr. Optimum gas pressure varies, as 1/D. Where D is the diameter of the tube.
	How do you compare DC excited and RF excited CO2 Lasers? CO2 lasers can be operated using radio frequency (including microwave) excitation instead of a direct electrical discharge but this results in more complex resonator/electrode configurations, more complex driving electronics and additional safety issues. One of the reasons RF is promoted for high-power fast-flow CO2 lasers are that you don't have internal electrodes that tend to sputter and contaminate the resonator optics. In case of DC excitation, there is voltage drop at the cathode, which results in heat dissipation and doesn't contribute to the discharge. However, a good design of cathode can reduce sputtering contamination a lot. Moreover, the efficiency in DC excited lasers can be achieved greater than 20%, which makes them cheaper as compared to RF excited lasers. For the same output power requirements.
	Show simplified energy level diagram of Argon ion lasers. Population inversion takes place between 4p and 4s level. 4s level has short life time and decays to the ion ground state. Argon ion recaptures and electron and moves to argon atom ground level.
	What are various wavelengths of Argon Ion lasers? Argon ion lasers can generate more than 30 discrete laser lines (wavelengths) ranging from the UV (275.4nm) to near infrared (752 nm) with the majority of the power being developed at the 488nm and 514.5nm lines. However, unlike HeNe lasers, the energy level transitions that contribute to laser action come from ions of argon atoms that have had 1 or 2 electrons stripped from their outer shells. Spectral lines at wavelengths less than 400 nm come from atoms that have had 2 electrons removed. Longer wavelengths come from singly ionized atoms. There are many possible transitions in the UV, visible, and IR portions of the spectrum. With suitable optics coherent light from a single spectral line or many lines may be produced simultaneously.
	What is the comparative strength of various Argon ion laser lines? The comparative strength of some of the important argon ion laser lines are: Wavelength Relative Power 454.6 nm .03 457.9 nm .06 465.8 nm .03 472.7 nm .05 476.5 nm .12 488.0 nm .32 496.5 nm .12 501.7 nm .07 514.5 nm .40 528.7 nm .07
	Why large currents are required to run Argon ion lasers? Argon ion lasers are excited by electric discharge through the gas, after an initial high voltage pulse that ionises the gas. Electrons traveling through the gas collide with atoms and transfer energy through the collision. Since these atoms require large amounts of energy to reach ionisation, many collisions must take place in a short time, which means that high current density is required these types of lasers. Once the gas ions are sufficiently excited, lasing may occur on several different transitions. The ground state of the ion is about 16eV above the neutral atom ground state, so a total of 36eV is required to excite an argon atom from its neutral atom ground state to the upper lasing level. This is a lot of energy considering that electrons can only provide between 2eV and 4eV per collision. Thus, many collisions are required to raise a neutral atom from its ground state to the ion ground state, and then to the upper lasing level. The fact that many collisions are required implies large currents.
	What is so special about Argon ion laser plasma tube? The heart of any argon laser is the plasma tube, and the key component of the plasma tube is the bore. The design of the plasma tube must be such that it can sustain extremely high temperatures without damage while maintaining an excellent vacuum seal. Further, in addition to the heat, the tube material must also be able to withstand the intense UV radiation emitted by ions dropping from the lower laser level to the ground state. Since plasma temperature is in the range of 1500 - 2000o C, there are only few materials that can go into argon plasma tube and survive are: BeO, kovar, tungsten, aluminum nitride, pyrolytic graphite and molybdenum. The material of choice for the bore of an argon ion laser plasma tube is usually BeO since it has a low vapor pressure and can be produced with a high chemical purity. When properly sealed, a plasma tube utilizing a BeO bore will allow the argon gas pressure within the tube to remain at its approximate 1 torr level for many years, thus assuring many hours of reliable laser operation. In addition, BeO is also an excellent thermal conductor. As such, the large amount of heat, generated by the plasma discharge within the bore, is readily conducted to the exterior of the BeO bore where it is then removed by means of forced air cooling (low argon lasers) or flowing water in a water jacket (high power argon lasers). Beryllium oxide is also preferred as it conducts heat 5 times faster then most metals.
	Why magnetic field is used in Argon ion Lasers? In some of the designs, magnetic field is also applied coaxially to the laser tube to further concentrate the current at the center of the laser tube, resulting in higher current density and fewer collisions with the tube walls. Reducing the collisions with the wall of the tube also helps in reducing the tube temperature.
	How a single line in Argon ion laser selected? As Argon ion lasers simultaneously run on several lines unless there is a dispersive element (prism or grating) in the cavity. With an intra-cavity prism, different lines can be selected. Most of these lasers have a hemispherical cavity, with a flat high-reflector mirror and a long-radius output coupler. The mirrors are designed for specific wavelengths. An intra-cavity prism is used for the selection of the various lines, with the prism shaped so that the beams strikes it at or near Brewster's angle on both surfaces. The prism and the high reflector are usually mounted together in a single unit.
	What are the requirements for holography and how Argon ion laser is tailored for it? For applications like holography, one requires a single transverse mode, single line and single frequency i.e. single longitudinal mode argon ion lasers. TEM00 mode operation can be realised by inserting a variable aperture in the resonant cavity. Single line operation is achieved by placing an intracacity prism. However, the number of longitudinal oscillating modes in any laser is approximately equal to the laser line-width divided by the mode spacing. In order to prevent more than one mode to lase and thus to ensure single frequency operation, we need to add an etalon into the cavity.
	What is the quality factor M2 for Argon ion lasers? A quality factor, M2 is defined to describe the deviation of the laser beam from a theoretical Gaussian. For a theoretical Gaussian, M2=1. However, for practical laser systems, the value of M2>1. Helium neon lasers typically have an M2 factor that is less than 1.1. For Argon ion lasers, the M2 factor is typically between 1.1 and 1.3. Collimated TEM00 diode laser beams usually have an M2 ranging from 1.1 to 1.7. For high-energy multimode lasers, the M2 factor can be as high as 3 or 4.
	Can Argon ion laser be pulsed? Pulsed Argon on lasers can be realized in the following manner: Power on Demand power supplies are used for pulsed medical ion laser systems, these power supplies consist of a large capacitor bank charged by a switching supply to enable multi watt lasers to run off common single phase power supplies. An intra-cavity acousto-optic device can mode lock a laser source. The acousto-optic modulator device suitable for a particular wavelength has been used to mode lock the ultraviolet lines at 3511, 3638 and 5145Å from an argon ion laser. Pulses of 0.2 nsec and 0.17 nsec for UV and visible wavelengths have been produced.
	Name few important properties, like divergence, beam size, life etc, have Argon ion lasers? Property Value Strongest Wavelengths 514.5 and 488 nm Power Range Few miliwatts to about 100 W on all the lines Electrical efficiency 0.05 to 0.1 % Small signal gain 0.005 cm-1 Saturation Intensity 16.3 W/cm2 Beam diameter 1 - 2 mm Beam divergence 0.5 mrad Typical operating current 50 A Magnetic Field 600 - 1200 G Operating Life 5000 - 10000 hrs Pressure inside plasma tube 0.1 - 1.0 torr
	What are different applications of Argon ion lasers? Raman Spectroscopy, Microscopy, Flow Cytometry, Forensics to detect latent fingerprints Laser shows for Entertainment Fiber Bragg Grating production Semiconductor Wafer inspection Ophthalmic Surgery Critical cell sorting and classifying for DNA sequencing applications Argon lasers are used for retinal phototherapy particularly for diabetic patients Sources for optical pumping. High power, excellent beam quality, and blue green wavelength, argon lasers being used extensively in high speed printing applications Green line of Argon ion lasers up to one watt has been extensively used for photolithography work. New applications for ion lasers continue to emerge, including producing three-dimensional (3-D) models of parts in a process called stereo lithography and serving as light sources in confocal microscopes.
	What is the difference between Argon ion and Krypton ion laser? Krypton ion laser and argon ion lasers are similar in construction and performance, with the argon system producing higher powers for longer lifetimes. Krypton-ion lasers are almost identical in construction and reliability to argon lasers. Krypton lasers emit at several wavelengths : in the visible range it emits at 406.7 nm, 413.1 nm, 415,4 nm, 468.0 nm, 476.2 nm, 482.5 nm, 520.8 nm, 530.9 nm, 568.2 nm, 647.1 nm, 676.4 nm. The argon laser has its strongest output at 514 nm (green) and 488 nm (blue). The krypton laser is known for its red (647 nm) and yellow (568 nm) output. Krypton ion and argon ion lasers are very similar - they are both rare gas ion lasers, their basic principles of operation are similar, and the same basic hardware configuration and power supplies can usually be used. Differences are primarily in gas fill of the plasma tube and the mirrors/prisms for selecting the output wavelength.
	What are white Lasers? The term 'white light laser' typically refers to one that is capable of producing a set of wavelengths which if mixed in the proper proportion can 'simulate' the effect of a white light source in full color displays and laser shows and also for some spectroscopy applications. However, they generally don't produce a broad spectrum like an incandescent light bulb. Though under some conditions krypton lasers as such can produce wavelengths over the full visible spectrum with lines in the red, yellow, green and blue. However, The most common white light lasers are large frame ion types with a mixture argon and krypton for the gas fill. These lasers use a mix of argon and krypton. Most of them are made for a roughly 60:20:20 ratio of red, green, and blue lines for proper white balance.
	Brief description of He-Ne Laser. It is a four level atom laser with a mixture of helium and neon. Though it lases at a number of wavelengths, its most popular output is at 633nm (red). Other available outputs are at 543nm (green), 594nm (yellow), 612nm (orange) and 1523nm (infra-red). Though neon is the lasing gas, it is the minor constituent (15% of the total mixture), helium taking the bigger share. Electrical discharge excites the helium atoms to the higher energy states, which are populated by electronic collisions.
	How a single line in Argon ion laser selected? As Argon ion lasers simultaneously run on several lines unless there is a dispersive element (prism or grating) in the cavity. With an intra-cavity prism, different lines can be selected. Most of these lasers have a hemispherical cavity, with a flat high-reflector mirror and a long-radius output coupler. The mirrors are designed for specific wavelengths. An intra-cavity prism is used for the selection of the various lines, with the prism shaped so that the beams strikes it at or near Brewster's angle on both surfaces. The prism and the high reflector are usually mounted together in a single unit.
	What are the prominent lines of Helium Cadmium Laser? The transitions in Helium-Cadmium laser are between energy levels of singly ionized Cadmium atoms, and there are about twelve lines. These wavelengths are in the shorter wavelength region, violet and Ultra-Violet (UV). The most prominent wavelengths are 441.6 nm and 325 nm.
	How do you categorize Helium Cadmium Lasers? Helium-Cadmium lasers can be categorized either in Metal vapour Lasers since cadmium is a metal and the lasing action in Helium Cadmium laser occurs between energy levels of cadmium ions OR a Gas Laser since the properties of Helium-Cadmium laser are similar to those of Helium Neon Laser, which is a neutral atom gas laser.
	What is the main mechanism of Laser action in Helium Cadmium lasers? The main mechanism identified for the laser transitions is a process known as Penning ionization, in which highly excited helium atoms transfer their energy to cadmium in a way similar to the operation of He-Ne laser. However, in this case cadmium ions are produced in the process, instead of neutral atoms as in the case of neon, owing to much lower ionization potential of cadmium than of neon. Excitation energy for the helium-cadmium laser is provided by a direct-current discharge passing through the laser tube. Typical discharges are around 700 - 2000 volts, with current densities in the small-diameter bore (2 -mm) of the order of 3 - 5 amperes per square centimeter of cross section. Helium atoms in the laser gas absorb energy from the discharge and then transfer that energy to cadmium ions. The energy levels of cadmium and helium involved in the principal He-Cd lines are shown in the adjoining figure. The most prominent transitions, which can be easily, produced are 441.6-nm (blue transition) and the 325-nm (ultraviolet transition). The most important energy transfer mechanism for the narrow-bore tubes mainly used for blue and ultraviolet lasers is Penning ionization. Penning ionization is a form of chemi-ionization, an ionization process involving reactions between neutral atoms and/or molecules. The process is named after the Dutch physicist Frans Michel Penning, who first reported it in 1927. Chemi-ionization is the formation of an ion through the reaction of a gas phase atom or molecule with an atom or molecule in an excited state and should not be confused with chemical ionization. In Penning ionization, energy from an excited helium atom ionizes a cadmium atom: He* +Cd → He + Cd+ + e-
	Draw a simplified energy level diagram of Helium Cadmium Laser?
	How Cadmium is handled in Helium Cadmium Lasers? Cadmium, a metal, is solid at room temperature and for lasing it needs to be sufficiently heated to have required partial pressure of cadmium vapors in the discharge tube. In case of cadmium, it is not practical to heat the complete chamber to approximately 260oC because of the other components like mirrors, windows, electrodes etc. present in the chamber. Thus the challenge in making HeCd lasers operate continuous-wave (CW) is dealing with cataphoresis. This is the term given to the migration of the positively charged metal ions toward the cathode where they may condense, depleting the supply of vapor and contaminating tube components and optical surfaces. This is achieved by using cataphoresis to control the cadmium vapour distribution. In this process the cadmium metal is heated and vapourized at the anode (which is at positive potential) end of the discharge and is transported towards the cathode end of the discharge by the electric field acting upon the cadmium ions that are produced by the discharge current. The practical problem in Helium-Cadmium laser is to maintain homogeneous distribution of the metal vapor inside the electrical discharge tube. Once the vapours go out of the bore, it may deposit on cold surfaces. Thus it is important that its vapor remains at proper areas. In order to prevent coating of the windows with Cadmium, cold traps are put before the laser windows. The cadmium atoms then condense in a pocket near the cathode region.
	What are the important characteristics of Helium Cadmium Lasers? Laser wavelengths: 441.6, 353.6, 325 nm Small signal gain coeff : 0.2 - 0.3 m-1 Saturation intensity: 0.4 W cm-2 Gas mixture : He:Cd :: 100:1 Gas pressure : 5 - 10 torr Laser gain medium length: 20 - 200 cm Output power : Upto 200 mW Mode : TEMoo or Multimode Life time : Upto 6000 Hrs Starting voltage: 10 kV DC Operating voltage: 700 - 2000 V DC Operating current : Upto 100 mA Beam diameter :0. 3 mm for single mode and 2 - 3 mm for multimode Divergence 1 - 2 mrad M2 : 1.3- 1.5 for single mode and 4 - 5 for multimode. Coherency length: approx. 30cm Overall wall-plug conversion efficiency : 0.003 - 0.02 percent
	What are the applications of Helium cadmium Laser? Lithography Stereo lithography in which the ultraviolet laser is used to make computer-generated models in a plastic material. The blue wavelength is used for printing on photosensitive materials. Flow cytometry Making CD masters Microchip inspection Fluoroscence analysis Diffraction grating fabrication Spectroscopy Nondestructive testing, Laser tumor cancer diagnoses
	What is the wavelength of nitrogen laser? Nitrogen laser is convenient and economical source of short, nanosecond, ultraviolet (337.1 nm) pulses.
	Why nitrogen lasers cannot be CW? For excitation, a fast strong electrical pulse is used where electron collisions cause the preferential population of the upper energy band first. After about 20nSec, the population of molecules at the upper laser level starts decaying to the lower laser level where it will stay because of much longer life time thereby quickly ceasing the laser action after the electrical pulse. Nitrogen lasers are hence self terminating. That is why these lasers cannot operate in CW mode. The pulse length of the low-pressure Nitrogen laser, then, is limited by the lifetime of the upper laser level i.e. 20nS. After this time, population inversion is no longer possible since half of the molecules in the upper energy state have decayed to the lower state.
	How many types of nitrogen lasers are there? Although nitrogen lasers have been operated over a range of partial pressures from a few Torr to more than 1 atmosphere, it is common to divide them into two categories: low pressure and atmospheric pressure. The first approach is a low-pressure design - it is more 'traditional' and requires a vacuum pump. The second approach is a TEA version of the nitrogen laser. TEA lasers (for Transverse Electrical-discharge at Atmospheric pressure) do not require a vacuum system at all as they operate at atmospheric or even greater pressures.
	What is the simplified energy level of nitrogen laser? A fast high-voltage discharge populates the upper laser level, an excited electronic state with 40-ns lifetime, which emits at 337.1 nm when it drops to the lower laser level as shown in the figure. The transition is a vibronic one, in which both electronic and vibrational energy levels change, making it broadband. The lower level has a 10- microsecond decay time, much longer than the upper level, and drops to a metastable state with a lifetime of the order of seconds.
	How does the lifetime of laser level vary with pressure? The lifetime of the upper-lasing level is dependent on the pressure in the laser tube. As pressure rises, the lifetime shortens according to:          t = 36/( 1+12.8*p(bar)) ns                         Or          t = 36/(1+p(torr)/58) ns There is 40 ns upper limit of laser lifetime at low pressures and the lifetime becomes shorter as the pressure increases. For TEA nitrogen laser where the pressure is 760 torr (one atmosphere) the lifetime is about 1 - 3 ns.
	What are the important properties of nitrogen lasers? The important properties can be summarized as follows: Wavelength 337.1 nm Spectral bandwidth 0.1 nm Pulse width (FWHM) < 3.5 ns Pulse energy upto 300 μJ Rep rate upto 100 Hz Beam size 3 × 7 mm Small signal gain: 1-2 cm-1 Saturation intensity; 50 KW/cm2 Beam divergence 4 × 6 mrad Long tube life - 108 shots per fill Spark gap or DC heated thyratron triggered discharge
	What are the important applications of nitrogen Lasers? Pumping source for dye lasers Measurement of air pollution using LIDAR Time-of-Flight Mass Spectrometry DNA Sequencing Laser Ablation Production of fast, dense pulse of photoelectrons for materials testing Biomedical diagnostics Study of fluorescence effects Studies related to Raman scattering Measurement of particle size by light scattering Use in optical coherence tomography (OCT) in the medical sector For device characterization such as gyroscopes and fiber optic sensors.
	What do you mean by Excimer? The name excimer refers to the electronically excited species such as monomers, dimers and other com¬plexes, which exist in the electronically excited state only. Excimers are characterized by short radiative lifetimes of the order of nanoseconds and large cross sections for stimulated emission, which enables an efficient laser operation. The term excimer stands for 'excited dimer' where a dimer refers to a molecule of two identical or similar parts. In the case of excimer lasers as we know which consists of nobel gas halides, both the molecules are different. In this case exciplex should be used for 'excited complex'. Most "excimer" lasers are of the noble gas halide type, for which the term excimer is strictly speaking a misnomer. The correct name for these lasers is exciplex laser. However, we all know these lasers as excimer lasers.
	Name the most common excimer lasers? While a lot of different excimer laser transitions have been used to generate light pulses at various wavelengths between 126nm and about 660nm, the most commonly used excimer lasers are krypton fluoride (KrF, 248 nm), argon fluoride (ArF, 193 nm), xenon chloride (XeCl, 308 nm) and xenon fluoride (XeF, 351 nm).
	How laser action takes place in excimer lasers? Laser action in an excimer molecule occurs because it has a bound excited state, but a repulsive ground state. This is because inert gases such as xenon and krypton do not usually form chemical compound. However, when in an excited state, they can form temporarily-bound molecules with themselves or with halogens such as fluorine and chlorine. This bound state is the upper laser level in the case of excimer laser. The excited compound can give up its excess energy by undergoing spontonaneous or stimulated emission, resulting in a strongly repulsive ground state molecule which very quickly dissociates back into two unbound atoms. Since the excimer molecule returns to the unexcited ground state and separates into atoms, the population inversion condition is achieved the moment excited state is created, since the population of ground level is nil.
	Explain the energy level diagram of excimer lasers? The energy level diagram of excimer laser is shown here. The general principle of the excimer laser transitions is shown in Fig. For example, in case of KrF, the upper laser level is an ionically bound charge transfer state of the rare gas positive ion and the halogen negative ion. For example for Krypton fluoride lasers, it is 2P rare gas positive ion (Kr+) and the 1S halogen negative ion (F-). The upper laser level is populated by a three-body collision involving Kr+, F-, and a third collision partner (called buffer gas, for example Ne or He). While we can see that there is a minimum in the potential energy curve in the upper state it is still rather unstable. The excited laser molecules decay after several nanoseconds via emission of a photon into individual atoms like Kr and F. These form the ground state, which is covalently bonded and consists of separate Kr and F atoms for large inter-nuclear separations.
	What are the important properties of excimer lasers? The bandwidth of excimer lasers is bit large of the order of 0.3 - 0.5 nm. However, for the application of excimer lasers as light sources in submicron line lithography, requires narrower spectral laser bandwidth, higher spectral purity, significantly improved energy stability and higher repetition rate. The cavities of such excimer lasers include highly efficient line-narrowing elements such as high-resolution optical gratings and etalons to achieve bandwidths less than 1pm (0.001 nm). The efficiency of these lasers is relatively quite high (2-4%) as a result of the high quantum efficiency and the high efficiency of the pumping processes. The small signal gain and saturation intensity of excimer lasers is typically in the range of 0.02 - 0. 1 cm-1 and (105 - 10 6 ) W/cm2 respectively. The high gain of the excimer medium requires output-coupling reflectivities of 10-30 % for most efficient energy extraction. Most excimer lasers are used with stable resonators, consisting of a high reflectivity Al or dielectrically coated mirror and a plane CaF2 or MgF2 window as output mirror. The divergence with stable resonator is of the order of (2-4 mrad). However, when lower divergence is required, the lasers may be equipped with unstable resonators that reduce the beam divergence to 200-400 μrad in a beam with 60-70% of the pulse energy obtained with a conventional, stable resonator. Since the pressure of the gas mixture is above atmospheric pressure, Excimer lasers can be operated only in a pulsed regime. Typically for the case of the KrF laser, the gas mixture consists typically of 6% Kr, 0.2% F2 and the remainder is a buffer gas (Ne), reaching a total pressure between 2 and 3 bar. At this high pressure it is impossible to ignite a continuous discharge, since after a short period, a homogeneous discharge will reverse into an arc or spark discharge, which is not suitable for laser generation. Consequently, the excimer laser can only be operated in pulsed high-voltage discharge. The pulse length of excimer laser typically ranges from a few nanoseconds (nS) to about 100nS. This is a relatively short pulse length and leads to high peak power output from excimer lasers. The pulse energies range from few mJ up to 1 J for the powerful units at pulse repetition rates up to about 100 Hz. Excimer lasers can reach a peak power of about 5 MW at UV wavelengths. Typically a 1000 mJ laser with a 20 nS pulse width will yield 5 MW of peak power. The lifetime is defined as the number of pulses, which can be obtained from the laser when operated in the constant energy mode at 50%, rated power at maximum repetition rate, on a single gas fill. A typical value for the number of shots is about 30 - 100 million pulses.
	What are the major applications of excimer lasers? Most of the applications of excimer lasers are in industry and Medical. They account for more than 90 % of the applications whereas rest 10 % is in research. Over the last few decades the excimer laser has obtained the key position among lasers in various sectors of micromachining. Excimer lasers have developed into powerful manufacturing tools mainly because of the reasons that it has short wavelengths and offers excellent quality of machining Major industrial applications of excimer lasers are based on micromaching of different materials as polymers, ceramics and glasses, applied for example in the production of ink jet cartridges by drilling the nozzles and printed circuit board drilling. Theses lasers are also used to drill small precision holes (5 - 10 micron) in various types of plastic and metal packages such as metal containers from the beverage industry, foil packages from the medical device industry or blister packs and plastic ampoules from the pharmaceutical industry. These lasers are excellent for machining repetitive patterns because the use of the mask allows for a series of holes or slits to be processed at the same time. This method is much more efficient than the use of a CO2 or Nd:YAG lasers, which require that each hole or slit be cut individually. For example, an excimer laser can drill 5000 holes in a polymer sheet in approximately 3 seconds, while the same process would require about 50 seconds with a CO2 or Nd:YAG laser Excimer lasers are typically used in machining materials which are hard to machine with other types of lasers, or where very high precision is required. These lasers are also useful for cutting biological tissue where a clean cut is required without thermal damage to the surrounding tissue. Excimer lasers can cut any solid material, from Diamond to the cornea of the eye. The material, the laser wavelength and the average power and / or the repetition rate of the laser determine the rate of most excimer laser machining processes. The largest application of excimer lasers for medical use is in refractive laser surgery. As an ophthalmological tool, excimer laser has been widely used for photoablation process. The precision of excimer laser and, more important, the lack of damage to surrounding tissue, are instrumental for correction of refractive errors or optical problems of the eye, including nearsightedness, farsightedness, and astigmatism. Excimer laser light is typically absorbed in less than a nanometer of tissue. By means of intense excimer pulses, the surface of the human cornea is reshaped to change its refractive power and thus to correct for short or long sightedness. Another medical application where excimer lasers are being used is dermatology for treating a variety of dermatological conditions including psoriasis, vitiligo, atopic dermatitis, alopecia areata and leukoderma. The KrF laser has been of interest in the nuclear fusion energy research in inertial confinement experiments. This laser has high beam uniformity, short wavelength, and the ability to modify the spot size to track an imploding pellet. Lasers with energies as high as 4.5 × 103 joules has been used in the laser confinement experiments. Excimer laser radiation is also being used for changing the structure and properties of materials as oxides, silicon or glass in bulk or thin films, as applied for the production of polycrystalline-silicon thin film transistor (TFT) and active matrix LCD monitors. Synthesis of polysilicon from amorphous silicon can be realized by exposing it to UV light generated by excimer laser. The light is absorbed by the amorphous layer, which melts and crystallizes in polysilicon while cooling. Other important application of excimer lasers is in photolithography for the production of computer chips with critical dimensions below 0.25 μm. Other applications include their use in fabrication of fiber Bragg gratings in telecommunication, high temperature superconducting films, the spectroscopic surface diagnostic, pigment analysis and abla¬tive laser cleaning of stone objects and also varnish removal on paintings, in basic scientific research, as pump sources for tunable dye lasers, mainly to excite laser dyes emitting in the blue-green region of the spectrum.
	What are the wavelengths of Copper Vapour Laser and Gold Vapour Laser? Copper vapour laser : 510.6 nm (green) and 578.2 nm (yellow) Gold vapour laser : 627.8 nm (red)
	What are the energy levels in Copper Vapour Laser and how Laser action takes place in Copper Vapour Laser? Copper vapour laser (CVL) is three-level laser and uses copper vapours as the lasing medium. It produces green laser light at 510.6 nm and yellow laser light at 578.2 nm. It consists of a sealed zirconia tube filled with neon gas at a pressure of 25-40 torr. A solid block of pure copper metal is kept in the middle of the tube. Copper is heated above 10830C (laser operating temperature is about 16500C) to generate copper vapour, which is the active laser medium. High voltage is applied between the two electrodes at the end of the zirconia tube. As a result, the temperature rises inside the tube cavity to about 1400 - 1700 0C, until the Copper evaporates, and the vapour pressure of the Copper is about 0.1 torr. Electrons, accelerated by the high voltage applied to the electrodes, collide with the copper vapour molecules, exciting them into one of the available high laser energy levels. The lasers are self-heated such that most of the energy provided by the discharge current provides heat to bring the plasma tube to the necessary temperature. Excitation occurs by electrons colliding with neutral copper atoms to excite them to the relevant laser-related energy levels. Inelastic collisions of electrons with copper vapour atoms causes excitation of the copper atoms, so the inversion population occur and laser oscillation due to electron transition from upper level (P1/2, P3/2) to the lower meta-stable level (D5/2, D3/2) take place at the 578.2- and 510.6nm, respectively. Two principal outputs having wavelengths of 510.6 nm (green line 2P3/2 - 2D5/2) and 578.2 (yellow line 2P1/2 - 2D3/2) are obtained. The lower laser levels (2D) are metastable leading to self-termination of the laser action. Upper and lower laser levels are shown in the energy level diagram.
	How Copper Vapour Lasers are realized? The Copper Vapour Lasers (CVL) can be realized by two different ways. First one is the development of lasers using copper as such and the second one involves the vapours of copper-bearing compounds, mainly the copper halides, CuCl, CuBr, CuI. In case of copper based CVL lasers, Copper must be heated to 1400 to 17000C in order to achieve a suitable vapour pressure. However, in case of CVL utilizing copper based compounds, it is possible to achieve a sufficient copper concentration for lasing in the 300 to 6000C range depending upon the type of compound. Typically Halide based lasers such as copper bromide (CuBr), copper chloride (CuCl) and copper iodide (CuI) lasers are necessary to be heated to 400, 500, 600 0C, respectively.
	Why two discharge pulses are required for Copper Vapour Lasers based on Copper halides? Typically two energizing pulses in quick succession are required, the first to dissociate vapour molecules, and the second to cause the dissociated ions to lase. The first pulse provides copper atoms by dissociating the halide. The second pulse is delayed until an adequate copper atom concentration has built up. The second pulse is a fast discharge pulse that pumps the copper atoms to the upper laser levels by electron collisions. For better performance, the laser requires fast excitation pulses of rise time of the order of 100ns or less. The time gap between dissociation pulse and excitation pulse should be dependent on how fast the chloride and copper atoms recombine to copper halide.
	What are typical parameters for Copper Vapour Lasers? Typical Parameters of Copper Vapour Lasers: Average Power (W): Upto 200 W Peak Power: 50 - 500 kW Pulse repetition rate: 2 - 40 kHz Green/Yellow ratio 1.5:1 Pulse width (ns) 5 - 50 nS Efficiency (%) Greater than 1 Small signal gain g0 = 0.05 - 0.1 cm-1 Saturation intensity = 9 - 12 W/cm2 Beam Diameter: 5 - 15 mm Divergence: 3 - 5 mrad Tube diameter: 10 - 100 mm Tube length: 50 - 150 cm Warm up time: 45 - 90 min Lifetime: 300 - 800 hours Buffer gas: 25 to 50 Torr of neon
	What are applications of Copper Vapour Lasers? Applications of Copper Vapour Lasers Pumping source for tunable dye lasers and solid-state laser materials such as Ti:sapphire to obtain pico second and femto second ultrashort pulses High-speed flash photography and high-speed imaging with high spatial resolution and temporal resolution Precision material Processing Underwater applications Holography Particle imaging velocimetry Spray Pattern Measurement Flow Visualization Photodynamic therapy and detection of forensic evidence Laser beam can be absorbed by biological tissue components, which may be selectively destroyed. In oncology, the photodynamic therapy based on the effect of simultaneous photochemical reaction between an appropriate sensitize, laser light and oxygen, is used for a selective destructions of pathological tissues Dermatology Copper vapour lasers emitting light at 511 nm (green) ad 578 nm (yellow) have been useful for treating pigmented and vascular lesions, respectively. Nonlinear frequency conversion to the ultraviolet. Harmonic generation can produce 255 and 289 nm from the fundamental copper lines or tunable ultraviolet light from CVL-pumped dye lasers. So one can get pico-second and femtosecond pulses. They are particularly useful for studying Time-spatial resolved spectroscopy. High Resolution spectroscopy Frequency doubled CVL can be used for fabrication of fiber Bragg gratings (FBGs) Copper vapour lasers has an important application in atomic vapour isotope separation (AVLIS) as a pumping source to excite tunable dye lasers
	How Gold Vapour Lasers are different from Copper Vapour Lasers? The principle of operation and structure of the GVL is quite similar to CVL. Even the same laser tube and power supply can be used for both lasers. The only change is that instead of solid copper metal, gold wire is employed to produce gold vapour and it lases at 627.8 nm in the red region. The laser head can withstand a temperature of about 17000C. The discharge circuit, like CVL, makes use of the thyratron. As compared to Copper Vapour Lasers, which can emit more than 200 W of average power, Gold vapour can produce few tens of Watts only. Typical efficiency of GVL (627.8 nm) is 0.2 % as compared to 1.5 %, that of Copper Vapour Laser.
	What are main applications of Gold Vapour Lasers? Gold vapour lasers find their main applications in dermatological and experimental cancer treatment of photodynamic therapy
	How laser action is achieved in dye lasers? Energy band diagram of dye lasers is shown in the adjoining figure. Molecule absorbs light and populates fist excited singlet state S1 with electrons Electrons in upper vibrational levels of S1 undergo vibrational relaxation and the electrons move to the lowest vibrational level of S1. This excitation energy is then rapidly redistributed within the S1 state within a time period of few picoseconds. Thus the molecules very quickly dissipate this very high energy by internal conversion - the electron density moves to the lowest excited state, S1. Internal conversion occurs by the electron density transferring from the vibrational levels of the upper excited state to vibrational levels of a lower excited state, which are overlapping. This is a radiationless transition i.e. it does not emit a photon of energy. The molecule decays from the S1 state to the S0 state and the energy reappears as fluorescence photons. This process may take few nanoseconds. Internal conversion may occur in S0 as well. There is a possibility of transfer of energy from S1 singlet state to T0. This happens in case the lifetime of S1 is more than of the order of 100 nanoseconds. If it happens, then the energy is lost and efficiency of dye lasers is reduced. The simplified energy level diagram of dye lasers is also shown.
	How triplet absorption is minimized? Triplet absorption in excited dye systems is a major factor that limits the proper laser action. That is, the laser pulse terminates before the pump pulse ends. In fact, the laser pulse usually terminates before the intensity of the pump pulse has fallen below the threshold excitation value. Efforts have been made to overcome this situation. For an efficient dye Laser the energy transfer from singlet state S1 to Triplet state T0 should be avoided, as it is the main loss mechanism within the dye molecule. This can be avoided if the dye molecules have low fluorescence lifetime for S1 to S0 transition as compared to time required for transfer of population from S1 singlet state to T0, the triplet state. All successful dye lasers use dyes with typical fluorescence lifetime for the S1-S0 transition of the order of few nanoseconds as compared to 100 nS for S1-T1 transition. The other common method often used is to add a second molecule to the dye solution to act as a triplet-quenching agent. Collisions between quencher and dye molecules are responsible for this de-excitation process. Usually triplet quenchers are laser dye specific: for example, Cycloheptatriene and cyclooctatetraene (COT) are good triplet quencher for rhodamine 6G. Adamantane is also sometimes added to some dyes to prolong their life
	What are the most common dyes and their tuning wavelength range? Some of the Important Dyes and their Wavelength Tuning Range Name of Dye Tuning Range (nm) Rhodamine 6G 573-618 Rhodamine B 600-646 Coumarin 47 436-486 Coumarin 102 454-506 Coumarin 307 478-547 Coumarin 153 517-590 Disodium Fluorescein 535-565 Bromo Fluorescein 530-690 Oxazine 170 672-727 Pyridine 2 710-790 Styryl 9 803-875
	What are the pumping sources for dye lasers? Dye lasers can be operated in both pulsed and CW modes. In the pulsed mode, these are usually pumped either by flash lamps or by other lasers such as pulsed-nitrogen lasers or copper vapour laser or excimer lasers or frequency-doubled Nd:YAG. On the other hand, in the continuous mode, the output of a CW argon ion laser generally is preferred as the pumping source.
	What are solid-state dye lasers? Although liquid dye lasers have been very successful, there has been continuous effort to find new gain media in the solid-state that would simplify the engineering of this class of lasers. The gain media used in these lasers are organic dye doped polymers. Obviously, the solid-state form has many advantages, particularly concerning handling. There are a number of materials, which have been used as solid hosts for laser dyes such as polymers, porous glasses, organically modified silicates or silicate nano-composites, polycom glass (combination of polymer and sol-gel). Further the laser damage threshold also increases significantly. Typically, Rhodamine 6G dye doped in polymer and porous glass composition can withstand pump pulses of 50 ns with energies of 7-10 J/cm2.
	How do you compare dye lasers with tunable Ti: sapphire lasers? Though solid-state lasers based on Ti: sapphire are now being used for a particular wavelength range as these can produce high enough powers without any lifetime restrictions, still dye lasers are dominating the fields of tunable lasers and ultrashort pulse generation for areas such as spectroscopy where large number of wavelengths are required. So as such the future of laser dyes particularly solid-state dye lasers continues to be good.
	What are the important characteristics of dye lasers? Important Properties of Dye Lasers Wavelength Range 400 - 1000 nm Average output Power Few miliwatts to few Watts Slope efficiency Upto 50 % Threshold intensity (1 - 5) W / cm2 Gain (1 - 2.5) cm-1 Saturation Intensity 3.4 x 109W/cm2 Divergence 1 - 2 mrad Beam Diameter 0.4 - 0.6 mm Line width attainable after tuning 0.001 - 0.025 nm
	What are the main applications of dye lasers? Spectroscopy, holography, and biomedical applications. Treatment of port wine stain (PWS) Lithotripsy Isotope separation
	How many types of Laser Hazards are there? Laser Beam Hazards, which include the damage to eye and damage to skin Non - Beam Hazards which include hazards from a number of support systems like high voltage, high current as well as radio frequency power supplies, high pressure arc and flash lamps, heavy duty capacitor banks, gases at high pressure in heavy containers, toxic gases and fumes, carcinogenic and inflammable materials, cryogenic systems etc. The non-beam hazards also include electrical hazards, explosion and fire hazards, and chemical hazards.
	What is the effect of various lasers on eye and skin? Biological Effects of Various Lasers Wavelength Lasers Damage Remarks (200-280 nm) Argon Fluoride, Krypton chloride, Krypton Fluoride Eye Photokeratitis Skin Erythema (Sunburn), Skin Cancer (280-315 nm) Xenon chloride Eye Photokeratitis Skin Erythema (Sunburn), Accelerated Skin Aging, Increased Pigmentation (315-400 nm) Xenon Fluoride, Nitrogen, Helium Chloride Eye Cataract Skin Skin Burn, Pigment Darkening (400-780 nm) Helium Chloride, Helium Neon, Argon, Krypton, Copper Vapour, Frequency doubled Nd:YAG, Gold Vapour, Dye lasers (Visible), Ruby, Ti:Saphire(Visible), Diode laser (Visible) Eye Photochemical and Thermal Retinal Injury, Color and Night Vision Degradation Skin Skin Burn, Photosensitive Reactions (780-1400 nm) Ga As, Nd:YAG/Glass, chemical oxygen iodine laser (COIL) Eye Retinal Burns, Cataract Skin Skin Burn (>1400 nm) HF, Diode Lasers ( IR), He-Ne (IR), Erbium doped YAG/Glass, DF, Carbon Dioxide Eye Corneal Burn Skin Skin Burn
	What is Maximum permissible exposure (MPE)? MPE is defined in ANSI Z-136.1 as "the level of radiation to which a person may be exposed without hazardous effect or adverse biological changes in the eye or skin". The biological effects of laser radiation depend on the wavelength, exposure duration, repetition rate and power / energy levels. The MPE is usually expressed either in terms of radiant exposure in J/cm2 for pulsed lasers or as irradiance in W/cm2 for continuous lasers for a given wavelength and exposure duration. In general, the longer the wavelength, the higher the MPE and for longer exposure times, the MPE is lower.
	What is the MPE for various lasers? PULSED LASERS (MPE for 10 seconds duration - J/cm2) Laser Wavelength Pulse Width Eye Skin Argon Fluoride, Krypton chloride, Krypton Fluoride (200-280 nm) Nanosecond to tens of secs 3 x 10-3 3 x 10-3 Xenon chloride (280-315 nm) Nanosecond to tens of secs 10-2 - 0.1 0.1 Xenon Fluoride, Nitrogen, Helium Chloride (315-400 nm) Nanosecond to tens of secs 0.6 1.0 Helium Chloride, Frequency doubled Nd:YAG, Gold Vapour, Dye lasers (Visible), Ruby, Ti:Saphire(Visible), Diode laser (Visible) (400-780 nm) Nanosecond to microsecond 5 x 10-7 2 x 10-2 Ga As 905 nm Nanosecond to microsecond 1 x 10-6 1.5 x 10-2 Nd:YAG 1064nm millisecond 5 x 10-5 1.0 Nd:YAG 1064nm Nanosecond to microsecond 5 x 10-6 0.1 Erbium doped YAG/Glass 1500nm nanosecond to millisecond 0.1 0.1 CONTINUOUS LASERS. (MPE for 8-hour duration - W/cm2) Laser Wavelength Eye Skin Argon ion 488 / 514 nm 1 x 10-6 0.2 Frequency doubled Nd:YAG 532nm 1 x 10-6 0.2 He-Ne 632.8nm 1.7 x 10-5 0.2 Nd:YAG 1064nm 1.6 x 10-3 0.2 COIL 1.354 micron 4 x 10-2 0.2 HF / DF 2.8 - 4.0 micron 0.1 0.1 CO2 10.6 micron 0.1 0.1
	What is Nominal Hazard Zone? Nominal Hazard Zone (NHZ) is the area where the level of laser radiation is more than MPE and it is necessary to enforce various laser safety control measures to protect the users.
	What are various Laser Classifications? As per the American National Standards Institute, the ANSI Z136 laser safety standards classifies the Lasers in the following categories: Class I lasers are considered safe, based upon current knowledge, under any exposure condition inherent in the design of the product. These Laser systems cannot emit laser radiation levels greater than the Maximum Permissible Exposure and are considered to be incapable of causing eye damage under normal operating or viewing conditions. Maximum power output is of the order of a few microwatts. These low powered devices that use lasers of this category include laser printers, CD players, and survey equipment, and they are not permitted to emit levels of optical radiation above the exposure limits for the eye. Lasers of this class are found in compact disc players. No safety requirements are specified for the use of this class of laser. It may be pointed out that the lasers which are totally enclosed system where access to higher levels of laser radiation is not possible during normal operation also falls in this category. However, whenever the instrument is opened for servicing or repairs, then these lasers are no longer fall in Class 1 and all the necessary precautions applicable to the embedded laser must be followed until the service is complete and the system is again enclosed. Class 1M Lasers are considered incapable of producing hazardous exposure conditions during normal operations unless the beam is viewed through collecting optics like magnifying optical instruments such as telescope. These lasers produce either a large diameter beam or a highly divergent beam. Some of the lasers used for fibre-optic communication systems are Class 1M laser products. These lasers are exempted from any safety measures other than to prevent potentially hazardous optically aided viewing. Class II is a low-power laser that usually emits in the visible portion of the spectrum (0.4 - 0.7 μm). The brightness of the beam normally causes the eyes to blink, well before any permanent damage can occur. These lasers are limited to a radiant power of less than 1 milliwatt, which is below the maximum permissible exposure for momentary exposure of 0.25 second or less. The natural aversion reaction to visible light of this brightness is expected to protect the eyes from damage, but any intentional viewing for extended periods greater than 1000 sec can result in damage. So deliberate staring into the beam should be avoided. Some examples of this class of laser are demonstration lasers for classroom use, laser pointers, laser printers and supermarket scanners. Class II M class is similar to Class II in terms of power and wavelength. Normally these lasers produce either a large diameter beam or a highly divergent beam and as such the total output may be more as compared the output observed in Class II lasers. However, the power densities are safe for accidental viewing because of the diverging nature of laser beams. When viewed with the naked eye, the hazards are the same as for a Class II laser. But these lasers are potentially hazardous if viewed with collecting optics or certain optical aids like magnifying optical instruments e.g. binoculars or a telescope. Lasers used for surveying come under this Class. Class III R lasers are continuous wave visible and infrared lasers with intermediate power levels of 1-5 milliwatts. These lasers have similar applications as encountered for Class II lasers, including laser scanners and pointers. They are considered safe for momentary viewing (less than 0.25 second i.e. blink response), but should not be viewed directly (intrabeam), or with any kind of magnifying optics. These medium-power laser systems though do not pose a fire hazard threat or even eye damage hazard through viewing of diffuse reflections, but nevertheless may be hazardous under direct and reflected beam viewing conditions. Lasers in this class may be used in alignment products. Class III B lasers can be both in the visible and as well as infrared band and are of medium power: continuous wave (5-500 milliwatt), or pulsed (10 joules per square centimeter) These lasers are hazardous to the eye for direct intrabeam viewing and from specular reflections. In general these lasers are safe so far as diffuse reflections are concerned, except towards the high power end. However, longer wavelength and high powers can cause some skin damage.. Specific safety measures are recommended in the standards for control of hazards with this laser class. Examples of applications of this laser type are spectroscopy, confocal microscopy, and entertainment light shows. These lasers are also used in medical applications and research. Class IV lasers emit high power, in excess of 500 mW, the limit for Class IIIB devices, and require stringent controls to eliminate hazards in their use. These lasers can be both continuous as well as pulsed in the visible and infrared ranges. These high-power laser systems are hazardous both to the eye as well as skin. Direct intrabeam viewing, specular and even diffuse reflection can cause severe eye and skin damage. These lasers can also have sufficient energy to ignite materials and thus are fire hazards. Also these lasers can produce hazardous plasma radiation, laser-generated air contaminants, hazardous fumes and byproduct emissions as a result of laser matter interaction. Since most laser eye injuries involve reflections of Class IV laser light, their use requires extreme caution. All reflective surfaces must be kept away from the beam, and appropriate eye protection worn at all times when working with these lasers. In case of pulsed lasers of this class, the power supplies can be fatal and thus all electrical safety precautions must be taken. Class IV lasers can be found in the metal industry, research laboratories, and laser light shows. These lasers are employed for surgery, cutting, drilling, micromachining, and welding.
	How do you compare semiconductor lasers with solid-state lasers? Solid-state and gas lasers work on narrow optical transitions connecting discrete energy levels between which population inversion is achieved by optical or electrical pumping Semiconductor lasers, on the other hand, work on transitions between energy bands in which conduction electrons and valence holes radiative recombination across the band gap that determines the emission wavelength. As compared to other lasers, semiconductor lasers are: Compact and rugged. This ruggedness and small size allow laser diodes to be used in environments and spaces in which other types of lasers cannot operate High efficiency in the range of 30 - 80% Direct excitation with small electric currents, Possibility of direct modulation with applied current Small beam waist However, there are few drawbacks in semiconductor laser diodes as compared to other solid state and gas lasers. These include, their sensitivity to temperature and large beam divergence.
	What are the basic components of semiconductor lasers? Semiconductor lasers also has basic three components A gain medium that amplifies light (p-n junction) An energy source to create population inversion (electrical current through the junction) A resonant cavity for confining the light (cleaving the semiconductor creates a reflective facet which can be used to create a laser cavity). Further to reduce the threshold for laser operation, the reflectivity of output coupler and the total reflector can be realized using dielectric coatings.
	What is the difference between homojunctions and heterojunctions in semiconductors? An interface between two regions of a semiconductor or an interface between two different semiconductor materials is called a junction. Junctions between differently doped regions of the same semiconductor material are called a homojunction, while a junction between two different types of materials is called a heterojunction. A junction between a p-type and an n-type semiconductor is called a p-n junction.
	Draw a band energy diagram of p - n junction under no bias condition? When the contact is made between the 'n' and the 'p' doped material, electrons diffuse from the n region into the p region where they recombine with the abundant holes. Similarly holes diffuse from p region to n region and combine. Electrons leave behind the positively charged donor ions, so some part of n region will be positively charged. Similarly some part of the p side will be negatively charged. Due to the diffusion of both types of carriers away from the junction region, a narrow zone around the junction is totally depleted of mobile charge carriers. This region is called the depletion region. The process happens till dynamic equilibrium takes place: the diffusion of electrons/holes and the drift currents cancel, so in the absence of an external field no net current flows across the junction. In terms of band structure, p-n junction can be represented as shown in the figure: Note that the systemcw of a p-n junction without bias is in equilibrium and hence the Fermi level EFN for n-type and EFP for p-type must be equal implying that there will be band bending. Thus in the absence of a bias, the bottom of the conduction band on the n-side lies lower than that on the p-side. This prevents net diffusion, as the electrons have to overcome a potential barrier qφ.
	Draw a band energy diagram of p - n junction under forward bias conditions? How Laser operation occurs in forward bias conditions? Positive voltage to the p region and negative voltage to the n region is known as forward bias. This allows the current to flow through the junction. On the other hand, the junction is reverse biased if a negative voltage is applied to the p region and positive voltage is applied to n region. Under reverse biased condition, very little current small current flows. Under forward bias conditions, if the external voltage becomes greater than the value of the potential barrier, the current will start flowing through the junction. This is because the negative voltage pushes electrons towards the junction giving them the energy to cross over and combine with the holes, which are being pushed in the opposite direction towards the junction by the positive voltage. Thus forward bias creates extra charge carriers in the junction, lowers the potential barrier, and causes injection of charge carriers, through the junction, to the other side. The laser operation occurs at a p-n junction, that is the boundary region between p-type and n-type materials. When p-n junction diode is forward biased, then there will be injection of electrons into the conduction band along n-side and production of more holes in valence band along p-side of the junction. At the junction, electrons and holes meet and are attracted to each other because of opposite charges. When they meet, they recombine and emit radiation. When a forward-bias voltage is applied to the junction, the barrier height is reduced and some of the electrons in the conduction band will overlap some of the holes in the valence band. It is worth pointing out that pumping the semiconductor raises some electrons to the conduction band where they rapidly distribute themselves into the lowest available energy levels within the conduction band. On the other hand, the electrons in the valence band occupy the lowest energy levels there, pushing the holes to the top of the valence band. Thus under forward bias conditions, there are more number of electrons than the number of holes in the junction region because of higher mobility of electrons as compared to that of holes. In other words a population inversion, the necessary condition for laser operation. In this situation, radiative recombination of the holes and electrons can occur. Electrons fall across the energy gap and recombine with holes. At very low currents, a population inversion does not occur even though recombination radiation is emitted. Under these conditions p-n juction behaves as a light-emitting diode (LED). In comparison, to produce a population inversion, comparatively high current is required within the junction region. This situation is indicated in the adjoining figure where p-n junction is shown under forward bias conditions.
	Define the Internal quantum efficiency of laser diodes? The efficiency for an efficient device can be characterized by the term "internal quantum efficiency" ηINT, defined as and is given as where A is an Einstein coefficient describing radiative recombination and X is a coefficient for nonradiative recombination. Thus it is required that A >> X for efficient photon generation.
	Why indirect band gap materials like silicon cannot be an efficient Laser devices? Indirect-gap semiconductors are inefficient light emitters because in case of indirect band gap materials like silicon, transitions between conduction bands to valence band involve phonon for conservation of momentum. Moreover phonon-assisted photon emission involves three "particles" simultaneously (electron, photon and phonon), its probability is low. Further, where A is an Einstein coefficient describing radiative recombination and X is a coefficient for nonradiative recombination. Thus it is required that A >> X for efficient photon generation. In case of indirect band gap materials, the value of X is very high.
	What are double-heterostructure (DH) lasers and what are the advantages? The double-heterostructure (DH) laser diode, which consists of a thin layer of low bandgap material such as GaAs sandwiched between two high bandgap layers such as AlGaAs, is one of the most commonly studied geometry. The bandgap discontinuity confines the free electrons and holes to the active region, meaning that more electron-hole pairs can contribute to the amplification. Further, the semiconductor with a wider band gap (AlGaAs) will also have a lower refractive index than GaAs. This difference in refractive index is what establishes an optical dielectric wave-guide that ultimately confines photons to the active region. Use of such structures help in confining both the injected electrons and holes and also the emitted photons to a narrow region about the junction. This as such requires less current to establish the required concentration of electrons for population inversion. Typically, the DH laser has a room temperature threshold current density two orders of magnitude smaller then the homojunction device.
	What is the difference between gain guided and index guided laser diodes? This confinement of the laser operation within a stripe region is usually accomplished by either gain guiding or by index guiding. Both these methods confine the light in such a way that the losses due to beam spreading are minimized thereby reducing the current requirements for laser operation. Further, since in the stripe geometry, aperture is limited and the dimensions in the directions parallel and perpendicular to the junction are comparable thus reducing astigmatism. Index-guided lasers employ steps in the index of refraction both parallel and perpendicular to the junction to confine the light. On the other hand, gain guiding structure makes use of composition changes for confinement in the plane of the junction. This is done, by adjusting the charge carrier density in the region, which results in the required refractive index. However, refractive index changes in the direction perpendicular to the junction to confine the light, just as like index-guided devices.
	What are Fabry-Perot (FP) diode lasers? Most of the laser diodes are edge emitters that are also called Fabry-Perot (FP) diode lasers since the cavity is essentially similar to that of a conventional gas or solid-state laser but formed inside the semiconductor laser diode chip itself. The mirrors are either formed by the cleaved edges of the chip or one or both of these are anti-reflection (AR) coated and external mirrors are added for high performance.
	What are surface emitting laser diodes or Vertical Cavity Surface Emitting Laser (VCSEL)? Surface emitting laser diodes (VCSEL: Vertical Cavity Surface Emitting Laser) have also become of interest for special applications. VCSEL have the optical cavity axis along the direction of current flow rather than perpendicular to the current flow as in conventional laser diodes. The active region length is very short compared with the lateral dimensions so that the radiation emerges from the surface of the cavity rather than from its edge. VCSELs emit their beam from their top surface. This approach provides several very significant technical advantages in terms of Beam characteristics and lasing threshold. The beam from a typical VCSEL exits from a circular region 5 to 25 um in diameter. Since this is much larger than for the FP laser diode, the divergence of the resulting beam is much lower. And, because it is also circular, no corrections for asymmetry and astigmatism are required - a simple lens should be able to provide excellent collimation. Lasing threshold drive current is an order less than the edge emitting laser diodes. Further, the packing density of such devices can be an order of magnitude higher than for FP laser diodes. As of now the output power is less and VCSEL technology is in its infancy its potential is just beginning to be exploited.
	What is Distributed Feedback Laser (DFB)? Distributed Feedback Lasers (DFB) is a special category of lasers under edge emitters, which incorporates a distributed grating that acts as a distributed reflector. This results in single mode lasers of high stability, which is the requirement of telecom industry.
	What are quantum well devices in Semiconductor Lasers? Recently quantum well devices for semiconductor technology are being pursued seriously. A quantum well is a very thin layer of semiconductor material between two layers with larger values of band-gap. If the layer is thin enough, 20 nm or less, comparable to the deBroglie wavelength, (λ ≈ h/p), quantum mechanical properties of electrons become important. This changes the energy-level structure of the material. Quantum well devices may incorporate a single quantum well or multiple quantum wells, with a number of alternating thin layers of high-band-gap and low band-gap material. The use of quantum wells in laser devices allows optimizing the properties of the material for the specific application. Quantum well devices offer lower threshold current and higher output power than devices without quantum wells.
	What are the most commonly used materials for Laser Diodes? Most commonly used material for semiconductor lasers are the III-V compounds such as GaAs, AlGaAs, InGaAs and InGaAsP depending upon the desired lasing wavelength. Recently, GaN/AlGaN and InGaN/AlGaN are also being used to achieve emission in the blue and ultraviolet regions.
	What is typical output power vs. drive current characteristics? Figure shows the output power of a semiconductor lasers as a function of current. Above a threshold current, at which the laser diode starts lasing, the laser diode shows almost a linear dependence between optical output power and laser current. Below the threshold the spontaneous emission is predominant and the optical amplification is not sufficient the device behaves like a LED.
	What is slope or Quantum efficiency? Refer to above figure. In the linear region, the slope of the output vs current curve yields the electrical-to-optical power conversion efficiency, also known as slope or quantum efficiency. The values of slope efficiency vary from 30 - 80 %.
	What are the typical characteristics of semiconductor lasers? The output characteristics of these devices are slightly different from those of other type of lasers. Because of their small size these have beam divergence angles of as much as high as 20o - 30o. The high value of divergence of semiconductor lasers is because of diffraction of the light waves when couple out of the laser structure Since the active light-emitting area is rectangle-shaped with different length and breadth, the parallel and vertical divergence are also different. If we focus such a beam, it will be observed that the focus of the vertical and the focus of the parallel divergence are not congruent but are shifted against each other: the effect known as astigmatism. The characteristic curve (output power vs. current) of a semiconductor laser strongly depends on the temperature. Higher the temperature, higher is the threshold current and smaller is the slope of the curve in the laser region. The coherence length of semiconductor laser diodes is low. Typical values for an index guided Fabry-Perot laser, emitting a single spectral line at 825 nm is 7cm, whereas for a gain guided Fabry-Perot laser, the coherence length is 300μm only.
	What is the difference between an LED and a Diode Laser? Difference between Diode Laser can be summarized as follows: Semiconductor Laser LED Generation through stimulated emission Generation by spontaneous emission Monochromatic and coherent light beam Divergent and incoherent light beam Power output kilowatts Power output in miliwatts Require feedback mechanism like optical resonator Does not require feedback mechanism Expensive Cheap Requires temperature and current stability Easy to handle. No such controls are required Generally spectral width less than 5 nm Spectral width upto 100 nm
	What are the main applications of Diode Lasers? Main applications of Diode lasers are in the following areas: Telecommunication Optical storage Solid state Laser pumping Material processing such as Welding, drilling and cutting Medical applications in dermatology, dentistry, ophthalmology, in surgery of tumors, kidney stone Barcode scanning Inspection, measurement and control Laser printers DVD drives Laser pointers
	How Free electron lasers differ from other conventional lasers? Radiation from a Free Electron Laser (FEL) has many common features in common with radiation from a conventional optical laser, such as high power; narrow bandwidth and diffraction limited beam propagation. One of the main differences between the two lasers is the gain medium: In a conventional LASER, the amplification comes from the stimulated emission of electrons bound to atoms, either in a crystal, liquid dye or a gas, whereas the amplification medium of the FEL are "free" (unbound) electrons. The free electrons have been stripped from atoms in an electron gun and are then accelerated to relativistic velocities. Ordinary lasers, however, operate at a fixed frequency. Though efforts have been made to have large number of wavelengths by tuning or having second or third harmonic generation, but still the choice is limited. That limits their usefulness. However, the FELs are ideal for exploring the unknown regions in the spectrum because these are tunable over a broad range of the spectrum. That enables these lasers more useful for material, medical and military applications.
	What are the main subsystems of FEL? The basic FEL system consists of an electron accelerator, an undulator or wiggler in which the electrons emit the syncrotron radiation, and an optical resonator. In FEL, a beam of electrons is accelerated to almost the speed of light. The beam passes through the FEL oscillator, a periodic transverse magnetic field produced by an arrangement of magnets with alternating poles within an optical cavity along the beam path. This array of magnets is called an undulator, or a wiggler, because it forces the electrons in the beam to follow a sinusoidal path. The acceleration of the electrons along this path results in the release of photons (synchrotron radiation). Since the electron motion is in phase with the field of the light already emitted, the fields add together coherently resulting in an exchange of electron energy with the electromagnetic field. This process is induced by the interaction of the electromagnetic radiation with the electrons. Since the radiation is faster than the electrons speeding along their path, the radiation overtakes the electrons flying ahead and interacts with them along the way, accelerating some of them and slowing others down. As a result of energy exchange, the electrons that gain energy begin to move ahead of the average electron, while the electrons that lose energy begin to fall behind the average. In the process, the beam of electrons gradually gets bunched on the scale of the radiation wavelength and this collective motion of bunches radiates powerful coherent synchrotron radiation.
	What is the resonant condition? Over one undulator period, λw, the time difference between the electron bunch and the wave must correspond to the wavelength, λo, of the spontaneously emitted light. Under resonance condition, the wavelength of the emitted radiation, λo, at the resonance depends on the electron energy and the magnitude and periodicity of the undulator and the magnetic field strength according to the relation γ is the relativistic factor and γmc2 is the energy of electrons. K is the undulator parameter, which is proportional to the magnetic field inside the undulator and is given as Where Bw is the undulator magnetic field strength in Tesla and λw is the undulator period length in centimeters.
	How wavelength is changed in FEL? The wavelength of the light emitted can be readily tuned by adjusting the energy of the electron beam or the magnetic field strength of the undulators.
	What are typical velocities of electrons in FEL? One can have an idea about the velocity of electrons from the relation: Where ν is the velocity of electrons, c is the velocity of light and γ is a factor related to energy of electrons as: The electrons having energy of one MeV will have velocity of about 86 % of the velocity of light. Similarly, electrons with energy of 10 Mev and 100 MeV will have velocity of the order of 99.9 % and 99.999% of the velocity of light.
	What are the typical parameters of FEL? Typical values of various parameters are given below: Peak Magnetic field: few kilogauss Wavelength: few Angstroms to 100 mm Number of undulator periods: 100 Undulator period λw: 2 - 10 cm Length of Undulator: 10 meters Electron beam energy: Few MeV to Several GeV Electron beam radius: About 1mm Electron beam pulse: nanoseconds to femtoseconds Efficiency: up to 40 % at longer wavelengths but less at shorter wavelengths Photon beam divergence (FWHM) < μrad Pulse duration (FWHM) ~ 100 fs Excellent beam quality M2 < 1.1
	How is linewidth related to undulator period? The linewidth of the laser radiation is determined by the number Nw of undulator periods; larger the number of periods, narrower is the line width. The linewidth is given as         Δλ / λ = 1 / 2 Nw
	What are the main applications of FEL? Material science for micro machining, metal surface processing, polymer surface processing, electronic material processing, Nanotube synthesis Atmospheric research, Isotope separation Spectroscopic tools for imaging, and to probe dynamical processes in real time on timescales down to tens of femtoseconds. The development of X-ray Free electro lasers will help researchers to take snapshots of chemical bonds being made and broken, and to look at detailed physical processes such as planes of atoms sliding over one another. Medical applications like surgery where the beam needs enough energy to vaporize soft tissue and bone. Some of these applications may be based on the clean cutting of soft tissue. Other uses may include welding tissue to assist in wound healing, repairing nerves, reattaching retinas or monitoring neurological activity. Wavelengths particularly near 6.45 microns have been found optimal for cutting all soft tissues. On the other hand, two wavelengths 7.5 and 7.7 microns have been found to cut through bone particularly cleanly. The progress in Free electron lasers can help in developing dynamic imaging techniques for diagnosing conditions such as progressive degenerative diseases and cancer. Though Army and Air force are concentrating on Solid State lasers and Chemical Lasers respectively, Navy however is pursuing mainly Free Electron Lasers. As per the reported data, Free electron lasers capable of delivering 14 kW have already been developed with immediate goals of developing 100 kW and upgrading later on to a Megawatt level. Magazine depth is an important issue The FEL magazine is much deeper as compared to that of other lasers.

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