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?

	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

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