Solid State Lasers
Study of characterstics of Solid State Lasers
Solid-state laser consists of a host and an active ion doped in the solid host material. The Active ion must have sharp fluorescent line, broad absorption bands and high quantum efficiency for the wavelength of interest. The host material must be strong, and fracture resistant, with high thermal conductivity and high optical quality. Glasses and crystalline materials have shown to have these characteristics, when doped with rare earth ions. Silicate glasses, phosphate glasses, crystalline material like, garnets, aluminates, metal oxides, fluorides, molybdates, tungstates, etc, are very good hosts. Important active ions are rare earth ions like, neodymium, erbium, holmium and transition metals like, chromium, titanium, nickel, etc. Some of the important solid state lasers are, Ruby, Nd:YAG, Nd:Glass, Nd:Cr:GSGG, Er:Glass, Alexandrite, Titanium: sapphire, etc.

Basic parts of a flash pumped solid-state laser are given in the adjoining figure. All the solid-state laser materials used as the active medium have their absorption bands in the visible region. Consequently, optical pumping with flash lamps having their emission spectra in the visible region is used as the excitation mechanism. Flash lamp pumped Solid state lasers are, in general, very inefficient, as only a very small region of the emission spectra is used in the absorption process, absorption band of the active ion being very narrow and rest being unutilized. Pumping using Diode lasers with precisely matching output with the absorption band of the active medium have improved the efficiency of solid state lasers considerably, some times almost touching 100%. But since the output power of the diode laser being rather low, solid-state laser output is also low. To overcome this drawback, stacks of diodes are employed to increase their total output, thus generating very high power laser giving as good as the flash lamp pumped laser systems. The real advantage of a diode pumped solid-state laser that it is very compact, light weight and small in size, with long life.
Basic parts of Solid State Laser System
There are a large number of solid-state lasers and we will discuss only some of the very important solid-state lasers and their salient features.
Pumping of Solid State Lasers
Pumping of the gain media is usually performed in one of the following forms: 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. Optical pumping can be realized by light from powerful incoherent sources. The incoherent light is absorbed by the active medium so that the atoms are pumped to the upper laser level. This method is especially suited for solid state or liquid lasers whose absorption bands are wide enough to absorb sufficient energy from the wide band incident incoherent light sources.

Optical pumping is a resonant process; the incident photon energy hn must be equal to the energy differences between the excited states and normal states. We can express optical pumping as hn + A → A*, where A is the atom at normal state, A* is the corresponding atom at excited state. So if there are lasers whose light wavelengths are within the absorption bands of the active medium, we can use these laser lights for pumping. Since the bandwidth of laser light is very narrow, the pumping efficiency can be very high. Laser pumping is not limited to solid-state lasers, it can also be used for liquid and gas lasers. In fact diode laser pumping has become the dominant means of optical pumping for reasons discussed below.

The first ever laser, the ruby laser reported by Maiman, was pumped with a discharge lamp viz. flash lamp. Though not very efficient, still there are few advantages; for example: Discharge lamps used for laser pumping can be grouped in two categories: arc lamps and flash lamps. Arc lamps are usually optimized for continuous operation, whereas flash lamps find their applications in pulsed lamps.

In most of the cases, laser rod and lamp are placed within an elliptical pump chamber with reflective walls, so that a larger percentage of the generated pump light can be absorbed in the laser rod (as shown in the figure). Cooled water or an ethylene glycol mixture is circulated to remove the excess heat. In addition to rod geometries, slab lasers can also be pumped through flash lamps. Here, an array of lamps pumps a slab through its large face, possibly from both sides. The pump light may be injected through a layer of cooling fluid.
Typical Flash Lamp - Laser Rod configuration
The main disadvantages associated with Flash lamp pumping includes: The second technique under optical pumping is through diode lasers .The lasers based on this type of pumping are known as Diode Pumped Solid State Lasers (DPSSL) or sometimes the all-solid state lasers.

Because optical pumping is a resonant process, the wavelengths of the pumping diode lasers must be within the absorption bandwidth of the active medium to be pumped, the nearer to the absorption peak wavelength the better. The adjoining figure show the absorption spectral of Nd:YAG laser which has a peak absorption value at 810 nm. GaAs / AlGaAs quantum well (QW) diode lasers operating at about 800 nm can be used to pump this laser. Likewise, Nd:Glass has a absorption peak at 802 nm and thus can also be pumped by the same laser.
Absorption spectra of Nd:YAG crystal
However, for Yb:YAG laser and Yb:glass laser, the best absorption wavelengths are 960 and 980 nm respectively, we can pump them using InGaSa/GaAs strained quantum well (QW) lasers in the 950-980 nm range.

In case of diode laser pumping, the absorption efficiency is about 0.90~0.98, whereas for flash lamp pumping, it is about 0.17. Further the energy quantum efficiency for diode laser pumping is about 1.4 times as large as flash lamp pumping, with typical value of 0.82 and 0.59 respectively. So the overall pumping efficiency of diode laser is about 7~8 times that of lamp pumping.

The advantage is quite clear. For normal pumping processes, because of the low efficiency of pumping and the required high pumping power to maintain proper power output, a large fraction of the pumping power is wasted as harmful heat. This heat has to be properly removed, i.e., the laser has to be properly cooled to maintain proper working conditions. While for diode pumped lasers, much of the absorbed power is used for final population inversion, the ratio of thermal generation from the absorbed radiation power for diode laser pumping is much less than that for lamp pumping. Thus the power required for diode pumping is far less than the lamp pumping; the absolute value of thermal burden of diode laser pumping is also strikingly small compared with lamp pumping. This makes it possible for more compact laser designs.

We can divide diode laser pumping into four types according to the degree of integration of the diode lasers: single stripe, diode array, diode bar and diode stack. Normally the pumping power increases with the integration degree. There are basically two types of pump geometry, longitudinal pumping (pump beam enters the laser medium along the resonator axis) and transverse pumping (pump beam incident on the active medium from transverse directions to the resonator axis). For longitudinal pumping, the beam needs to be concentrated to a small and circular spot. These two types of pumping viz. edge pumping and side pumping are shown in the following figures.
The main advantages of diode pumping can be summarized as follows
The main disadvantage of diode pumping (as compared to lamp pumping) is the significantly higher cost per watt of pump power.
Ruby laser
The World's first solid-state laser, invented by Maiman in 1960, now has only a historical importance. The laser host is Aluminium oxide (Al2O3) with triply ionized chromium (Cr3+) as the active ion.

This first material used was synthetic ruby. Ruby is crystalline alumina (Al2O3) in which a small fraction of the Al3+ ions have been replaced by chromium ions, Cr3+. It is the chromium ions that give rise to the characteristic pink or red color of ruby and it is in these ions that a population inversion is set up in a ruby laser.

The two broad absorption regions centered on 400 nm and 550 nm are both used for optical pumping of the ruby. Thus most of the useful pump light for a ruby rod lies in the blue-green portion of the visible spectrum. In a ruby laser, a rod of ruby is irradiated with the intense flash of light from xenon-filled flashtubes. Light in the green and blue regions of the spectrum is absorbed by chromium ions, raising the energy of electrons of the ions from the ground state level to the broad F bands of levels. Electrons in the F bands rapidly undergo non-radiative transitions to the two metastable E levels. A non-radiative transition does not result in the emission of light; the energy released in the transition is dissipated as heat in the ruby crystal. The metastable levels are unusual in that they have a relatively long lifetime of about 4 milliseconds (4 x 10-3 s), the major decay process being a transition from the lower level to the ground state. This long lifetime allows a high proportion μmore than a half) of the chromium ions to build up in the metastable levels so that a population inversion is set up between these levels and the ground state level. This population inversion is the condition required for stimulated emission to overcome absorption and so give rise to the amplification of light. In an assembly of chromium ions in which a population inversion has been set up, some will decay spontaneously to the ground state level emitting red light of wavelength 694.3 nm in the process. This light can then interact with other chromium ions that are in the metastable levels causing them to emit light of the same wavelength by stimulated emission. As each stimulating photon leads to the emission of two photons, the intensity of the light emitted will build up quickly through this cascading process.
Energy level diagram of chromium ions in Ruby
The ruby laser is often referred to as an example of a three-level system. More than three energy levels are actually involved but they can be put into three categories. These are; the lower level form which pumping takes place, the F levels into which the chromium ions are pumped, and the metastable levels from which stimulated emission occurs. It is a three level laser and as such threshold for laser action is nearly 300 to 400 times when compared with Nd:YAG laser (four level laser) of similar dimensions. Working of this laser has already been discussed earlier. Some important properties of Ruby are listed below:
Important Properties of Ruby
Property Value
Density 3.98 g/cc
Melting Point 2040°C
Young's Modulus 345 Gpa
Compressive Strength 2.0 Gpa
Hardness 9 Mhos, 2000 Knoop
Thermal Expansion 5.8 x 10-6 / °C {20° to 50°C} ; 7.7 x 10-6 / °C {20° to 200°C }
Thermal Conductivity 46.02 W / μm∙K) { 0°C } ; 25.10 W / μm∙K) {100°C} ; 12.55 W / μm∙K) {400°C}
Refractive index at 700 nm 1.7638 Ordinary Ray ; 1.7556 Extraordinary Ray
Birefringence 0.008
Refractive Index vs. Chromium Concentration 3 x 10-3 (Δn / % Cr2O3)
Crystallographic orientation, optical (c - axis) to rod axis 60° within 5°
Fluorescent Lifetime at 0.05% Cr2O3 3 ms at 300 K
Fluorescent Linewidth 5.0 Å at 300K
Output Wavelength 6.94.3 nm
Major Pump Bands 404 nm and 554 nm


Concept of Maiman's Ruby laser is shown below.
Concept and components of first Ruby laser Maiman
Other types of laser operate on a four level system and, in general, the mechanism of amplification differs for different lasing materials. However, in all cases, it is necessary to set up a population inversion so that stimulated emission occurs more often than absorption.
Neodymium Class of Lasers
Neodymium, with chemical symbol as Nd, is a chemical element belonging to the group of rare earth metals. In laser technology, it is widely used in the form of the trivalent ion Nd3+ as the laser-active dopant of gain media based on various host materials, including both crystals and laser glasses.

The strongest laser transition is that from 4F3/2 to 4I11/2 for 1064 nm, but other transitions are available with longer or shorter wavelengths. In order to achieve lasing on those, lasing at the 1064-nm line needs to be suppressed by inserting an appropriate wavelength filter. Neodymium atoms in the ground state absorb photons and are raised in energy to one of the pump bands. The states in these bands have lifetimes on the order of 10-8 seconds, and the atoms quickly drop to the upper lasing level by radiation less transition. The upper lasing level, 4F3/2, has a fluorescent lifetime of about 0.3 msec. A population inversion develops and lasing occurs, with the atoms dropping to the lower lasing level. his level is very close to the ground state, and excited atoms rapidly return to the ground state by another radiation less transition.

The population in level 4I11/2 quickly reduces to zero as excited species jump to the ground state 4I9/2 via multi-phonon emission.

Since the lifetime of the lower states is much smaller than that of the upper states, there is normally negligible population in all these levels, so that neodymium-doped gain media exhibit pure four - level behaviour.
Prominent lines of Nd in YAG
The greatest consideration in the design of a solid-state laser is spectral matching of the pump source to the absorption spectrum of the laser rod. Xenon flashlamps provide the most efficient operation of ruby lasers. Krypton arc lamps and flashlamps are best with neodymium lasers. The krypton flashlamp produces most of its output light in the infrared region of the absorption bands of Nd:YAG and Nd:glass. Thus, it is the best spectral match for these laser materials. Krypton flashlamps are however, not widely used because of their cost. They are far more expensive than xenon lamps, and the xenon lamps also have sufficient output in the desired spectral region, thus making their lower efficiency acceptable.

The most common neodymium-doped gain media are: The most studied of all the solid state lasers, Nd:YAG was lased in 1964. In Nd:YAG laser, YAG is the host and triply ionized neodymium (Nd3+) is the active ion responsible for the laser output at 1064 nm wavelength. It is a four level laser with high fluorescence efficiency. An Nd:YAG rod of 75mm length and 6mm diameter lases at a very low threshold of less than a Joule with a matching pulse forming network and a xenon-krypton gas mixture flash lamp. It has a high thermal conductivity and can be cooled with fluid coolant efficiently and produce high output at repetition rate of 400 pulses per second (pps) or better. But its efficiency is around 1% due to its very narrow absorption bands; consequently most of the visible output of the flash lamp is unutilized. Typical neodymium doping concentrations are of the order of 1% (atm.). High doping concentrations can be advantageous e.g. because they reduce the pump absorption length, but too high concentrations lead to quenching of the upper state lifetime via up conversion processes. The YAG absorption lines form sharp spikes within closely packed bands. The two important pumping bands in Nd:YAG lasers are in the regions of 730-760 nm and 790-820 nm. Since both of these bands are in the near infrared, these wavelengths are the most desirable for optical pumping of YAG lasers.

Water cooling of the rod combined with the high thermal conductivity of YAG provides a cooling effect sufficient that small-diameter Nd:YAG laser rods may be operated in the CW mode. YAG is the only widely used solid-state laser material capable of CW operation, although other CW solid-state lasers are under development. As glass has a much lower thermal conductivity than YAG implying that the waste heat is retained in the lasing material longer, resulting in a greater temperature rise. For this reason the transition from the lower lasing level to the ground state in Nd:glass occurs much more slowly as the laser temperature rises during operation. This quickly quenches lasing and requires that Nd:glass lasers operate in the pulsed mode only.

Though this laser also is a four level laser with Nd3+ as the active laser ion, it cannot generate output at high repetition rate due to its very low thermal conductivity, but it can produce much higher energy output as compared to Nd:YAG laser. As both silicate glasses and phosphate glasses are used as hosts; depending on the hosts lasing is at 1061 nm or 1054 nm respectively. Nd:glass lasers typically can be pulsed only once every few seconds, but the larger rods can deliver pulse energies of several hundred joules for relatively small systems and kilojoules for larger ones. They are the most efficient solid-state lasers, and the least expensive. This makes glass popular where high-energy pulses are required. Finally, these neodymium-doped glasses (mostly silicate and phosphate glasses) can be used for laser applications. However, silicate glasses are often more attractive for neodymium-doped optical fibers, which are suitable for fiber lasers and amplifiers.

In all these media (except for glasses), the neodymium dopant ions replace other ions (often yttrium) of the host medium, which have about the same size.
Some Important properties of Nd:YAG crystals are given below
Property Value
Chemical formula Nd3+:Y3Al5O12
Crystal structure Cubic
Density 4.56 g/cm3
Moh hardness 8 to 8.5
Young's modulus 280 GPa
Tensile strength 200 MPa
Melting point 1970 °C
Thermal conductivity 10 to 14 W / μm K)
Thermal expansion coefficient 7 to 8·10-6/K
Birefringence None (only thermally induced)
Refractive index at 1064 nm 1.82
Temperature dependence of refractive index 7 - 10 x 10-6/K
Nd density for 1% atm. doping 1.36∙1020 cm-3
Fluorescence lifetime 230 μs
Absorption cross section at 808 nm 7.7 ∙10-20 cm2
Emission cross section at 1064 nm 28∙10-20 cm2
Gain bandwidth 0.6 nm
Ytterbium doped class of solid-state lasers
Ytterbium is a chemical element belonging to the group of rare earth metals having chemical symbol as Yb. Presently, it has acquired a prominent role in the form of the trivalent ion Yb3+, which is used as a laser-active dopant in a variety of host materials, including both crystals and glasses. It is often being used for high power lasers and for wavelength- tunable solid-state lasers. Energy levels of Yb3+ ions in Yb:YAG, and the usual pump and laser transitions are shown in the adjoining figure.
Energy levels of Yb3+ ions in Yb:YAG
Ytterbium-doped laser crystals and glasses have a number of interesting properties, which differ from those e.g. of Nd: doped host materials. In addition to neodymium and ytterbium, there are other dopants have also been attempted in YAG crystals and laser glasses. For example, Erbium doped laser materials can emit at various wavelengths e.g. Er: Glass laser emits radiation at 1540 nm 1.54 μm, suitable for eye-safe laser applications; Er:YAG emitting at 2.94 μm and is used in dentistry and for skin resurfacing: Er:YAG can also emit at 1645 nm and 1617 nm, as well as at 550 nm and 561 nm; Er:YLF emits at 1730nm and Er:GSGG emits at 2.8 μm.

Active elements from Erbium doped Yttrium Scandium Gallium Garnet crystals (Er:Y3Sc2Ga3012 or Er:YSGG) single crystals are designed for diode pumped solid-state lasers radiating in the 3 μm range. Er:YSGG crystals show the potential of their application alongside with the widely used Er:YAG, Er:GGG and Er:YLF crystals. Flash lamp pumped solid-state lasers based on Cr,Nd and Cr,Er doped Yttrium Scandium Gallium Garnet crystals (Cr,Nd:Y3Sc2Ga3012 or Cr,Nd:YSGG and Cr,Er:Y3Sc2Ga3012 or Cr,Er:YSGG) have a higher efficiency than those based on Nd:YAG and Er:YAG. Active elements prepared from YSGG crystals are optimum for medium power pulse lasers with the repetition rates up to several tens of cycles.
Comparative generation characteristics
Crystal type Er:YSGG Er:YAG
Er concerntation, at. % 38 33
Pumping wavelength, nm 966 964
Stimulated radiation wavelength, μm 2.797; 2.823 2.830
Generation threshold, mW 72 418
Max. Power output at pumping power 720 mW, 966 nm 201 51
Slope efficiency, % 31.1 16.9

The advantages of YSGG crystals compared with YAG crystals, however, are lost when large size elements are used because of the inferior thermal characteristics of YSGG crystals.
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.
Transmission spectra of human eye
Obviously, the quality "eye-safe" depends not only on the emission wavelength, but also on the power level and the optical intensity, which can reach the eye. With sufficient power, as e.g. reached with a fiber amplifier or with a Q- switched laser, the eye can of course still be damaged, e.g. by overheating the eye's lens. 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 1J/cm2. Such high energy density is not normally encountered at work place and these types of lasers are eye safe.

Solid-state lasers have been designed to operate at various wavelengths, but the band of wavelengths from about 1.4 mm to 1.6 mm is of great interest because of eye safety reasons. Maximum permissible exposure levels for eye in this band are several orders of magnitudes greater than invisible and one micron band [ANSI, Z136.1-1993]. Er: Glass class of lasers is gaining much attention due to its radiation wavelength of 1540 nm which is not only safe for eye, but can also be used for rage finder applications as there is an atmospheric window for this wavelength.

On the other hand, eye-safe lasers in the range of 2 - 3 μm wavelength are being used in the fields of coherent Doppler velocimetry, gas detection, space applications and medical operations since water exhibits a strong absorption spectrum in this wavelength region. Er: YAG laser, which emits at 2.94 μm, also falls in this category. Unlike in Nd:YAG lasers, the frequency of Er:YAG lasers is strongly absorbed by water due to atomic resonances. This restricts its use in range finder applications and many other laser applications e.g. surgery, which have water present. Because of this limitation Er:YAG lasers are far less common than relatives such as Nd:YAG and Er: glass.

In addition to the Er: glass, which is the main workhorse in the area of eye-safe lasers, some of the other flash pumped eye safe solid state lasers are Thulium doped YAG, Tm3+:YAG ( around 2 μm ), Holmium doped YAG, Ho:YAG ( 2.1 μm ), Chromium doped YAG , Cr4+:YAG ( 1.35 - 1.55 μm ) , Er:YLF (1730nm), Er:YAG (2940nm), Er:Cr:GSGG ( 2.8μm) and Ho:YLF (2060nm).

Since wavelength around 1.5 micron is of interest, apart from direct generation of this eye safe laser radiation, it has also been generated by shifting the 1064nm output of Nd:YAG to 1540nm by Stimulated Raman Scattering (SRS) technique. Other commonly used technique is based on optical parametric oscillators (OPOs), which involves KTP and periodically poled KTP (PPKTP) crystals.

The energy level diagram of Erbium in glass matrix is shown in the adjoining figures along with the typical absorption and emission cross-sections for the most prominent 4I13/2 → 4I15/2 transition in Er+3spectra.
Energy level diagram of Erbium in glass matrix
Absorption and emission cross-sections for 4I13/2 → 4I15/2 transition in Er+3spectra
Table lists the important properties of these erbium-doped glasses. Recently Ytterbium and chromium have also been co-doped with erbium for 1.54-micron applications. Erbium laser glass with Yb ion doping has been found suitable for microlaser system using moderate power diode pumping laser system (DPSS). Using these glasses, the cw 1540 nm has also been obtained with good beam quality and stable output. Chromium co-doping with erbium, on the other hand, in laser glasses are especially suitable for the high power xenon lamp-pumping laser.
Properties of Er: doped Laser Glasses
Property Value
Center lasing wavelength 1.535 μm
Stimulated emission cross section 8.0 x 10-21 cm2
Fluorescence life time 7.9 msec
Refractive index 1.533 at 0.6 μm: 1.521 at 1.535 μm
Temp coeff. Of refractive index, dn/dt -10 x 10 -7 /°C between 20 - 40°C
Transformation temp 450°C
Softening temp 485°C
Thermal coeff of expansion 8.2 x 10-6 /°C between 20 - 40°C
Density 2.90 g / cc

Tunable Lasers
Normally, stimulated emission in solid-state laser is in the form of photons. But it is possible to couple stimulated emission of photons with the phonons, the vibrational quanta of lattice, where the fixed total energy of laser transition can be partitioned between photons and phonons in a continuous way. This has resulted in a new class of solid-state laser called vibronic laser, where a phonon is emitted or absorbed with each electronic transition. Historically, the first tunable vibronic laser, Nickel doped in Magnesium Fluoride (Ni:MgF2) was lased in 1963 at Bell Labs. This followed by a series of vibronic lasers, using nickel, vanadium, cobalt etc. as the dopant material and MnF2, MgO, MgF2, ZnF2 etc. as the host crystal. All these flash pumped lasers worked at cryogenic temperature. Optically pumped Ho:BaY2F6 was the first tunable vibronic laser to operate at room temperature. It can be seen from literature that, chromium (Cr3+) plays a very important role, as dopant, in many of the tunable solid-state lasers. In tunable lasers, output is tunable from visible to infra-red. Some of the important tunable lasers are Alexandrite (BeAl2O4), Emerald (Be3Al2Si6O18) and Titanium:sapphire (Ti:Al2O3) lasers.
Alexandrite Laser
Alexandrite laser was invented in 1974. The laser material is Cr3+ doped chrysoberyl (Cr+3:BeAl2O4). It is tunable from 700 to 820nm, is mechanically strong, chemically stable, has high average power capability, high thermal coefficient, performs better at higher temperature, can be Q-switched and can also be made to lase in the CW mode. As a 3-level system its function is very much akin to that of the ruby laser and lases at a fixed wavelength of 680nm, has high threshold for laser action with low efficiency. As a 4-level laser its function is that of a vibronic laser: that is, phonons, as well as photons, are emitted during lasing. The wavelength tuning is accomplished by controlling the branching of energy between phonons and photons during lasing. Alexandrite lasers have been tuned across most of the spectrum between 701 and 860 nm. The central part of the tuning range is from 700 - 820 nm. Using non-linear wavelength conversion processes such as harmonic generation and Raman shifting, light has been generated at wavelengths from the deep IR (20 μm) to the VUV. In addition to its broad absorption bands throughout the visible spectrum, alexandrite exhibits narrow R line absorption features at wavelengths near 680 nm. These properties together with its long fluorescence lifetime make it an excellent material for both flashlamp and diode pumping. Alexandrite's thermo-mechanical properties make it an excellent performer in high power laser applications.
Material Properties of Alexandrite (Cr+3: BeAl2O4)

Property Value
Operating wavelength 700 - 820 nm
Crystal structure Rhombic
Lattice parameters a = 5.47 Å : b = 9.39 Å : c = 4.42 Å
Hardness 8.5 mohs
Density 3.79 g/cm3
Refractive index 1.74 - 1.75
Axial characteristic Biaxial
Thermal conductivity 0.23 W/cm °K
Stimulated emission cross-section at 300°K 3.0 x 10-19 cm2
Lifetime 260 x 10-6 sec
Absorption loss at 750 nm 0.001 - 0.003 cm-1
Cr dopant concentration 0.03 - 0.50 at. %


Simplified energy level diagram of alexandrite, as a 4-level laser, shown here.
Simplified energy level diagram of Alexandrite Laser
It may be noticed that the upper laser level μmeta-stable level in the figure) is above the energy storage level and consequently the upper lasing level gets more populated from the transitions from the storage level with the rise in temperature of the system. The resulting transitions to ground level are vibronic in nature. i.e. photon emission is accompanied by lattice phonon creation giving rise to 4-level operation.
Emerald laser
Room temperature operation of alexandrite laser induced the search for other materials with similar properties, which resulted in the development of emerald laser in 1980. Chromium doped in beryllium aluminium silicate (Cr3+ in Be3Al2Si6O18) is the common name for emerald. It is a vibronic 4-level laser. The gain and emission cross-section of emerald is almost twice that of alexandrite. It has lower lasing threshold compared to alexandrite and ruby has many similarities with alexandrite like working better at higher temperature and excitation by flash lamps. Further, it is tunable from 730nm to 840nm and can be Q-switched and mode-locked. With its wide spectral bandwidth, it is capable of generating ultra-short pulses. Emerald, like alexandrite operates in a vibronic four level, phonon terminated mode and exhibits gain over a 695-835 nm wavelength range. Its broad fluorescence bandwidth, together with a high gain cross section and 65 μs room temperature fluorescence lifetime, make emerald an excellent laser material for high power, Q-switched, or mode-locked operation. Highly efficient quasi-cw (continuous-wave) laser operation has been achieved in emerald over the 720-842 nm tuning range.
Titanium:sapphire laser (Ti3+:Al2O3)
Titanium:sapphire laser is an important member of the family of vibronic lasers. In this case trivalent titanium is doped in the sapphire host material. Presently, it is the most widely used crystal for wavelengths tunable lasers. It combines the excellent thermal, physical and optical properties of Sapphire with the broadest tunable range of any known material. It can be lased over the entire band from 660 to 1100 nm. Frequency doubling provides tunability over the blue-green region of the visible spectrum. Ti:Sapphire crystals are active media for highly efficient tunable solid-state lasers. They demonstrate good operation in the pulsed-periodic, quasi-CW and CW modes of operation. Ti:Sapphire is a 4-level, Vibronic laser with fluorescence lifetime of 3.2 - 3.6 μm. The peak of the absorption band is 490 - 500 nm which makes it an excellent material for pumping with a variety of sources operating in the green-argon ion, copper vapour, frequency-doubled Nd:YAG or Nd: YLF, and dye lasers are routinely used. Excitation by flash lamp is very difficult due to its short fluorescence lifetime at room temperature. Nevertheless, flash lamp pumping was carried out in 1984 employing a coaxial flash lamp, operating with a pulse width of 5 μs. These flash lamps were specially designed to allow short fluorescence lifetime. These factors and broad tunability make it an excellent replacement for several common dye lasing materials. Peter Moulton was the first scientist, who demonstrated this laser in 1982.

Titanium-doped sapphire (Ti3+:sapphire) is also a widely used transition metal doped gain medium for femtosecond solid state lasers. Immediately after its demonstration, Ti:sapphire lasers quickly replaced most of the dye lasers, which had previously dominated the fields for ultrashort pulse generation and widely wavelength tunable lasers. These ultra short pulses from Ti:sapphire lasers can be generated using passive mode locking, where a pulse duration around 100 fs is easily achieved. However, using advanced precision dispersion compensation techniques, pulses of the order of 5 - 10 fs have also been obtained. Ti:sapphire lasers are also very convenient for pumping test setups of new solid state lasers such as based on neodymium or ytterbium doped gain media, since they can easily be tuned to the required pump wavelength and allow to work with very high pump brightness due to their excellent beam quality and high output power of typically several watts.
Properties of Er: doped Laser Glasses

Property Value
Crystal structure Hexagonal
Lattice parameters a = 4.748 Å; c = 12.957 Å
Axial characteristic Uniaxial
Tuning range 660 - 1100 nm
Pumping range 450 - 532 nm
Ti dopant concentration 0.02 - 0.35 at. %
Refractive index 1.76
Birefringence 0.0082
Density 3.98 g/cm3
Hardness 9 Mohs
Thermal conductivity at 25°C 0.33 - 0.35 W / cm °K
Specific heat at 18°C 761 J / kg °K
Thermal expansion coefficient (20 - 100°C) (4.78 - 5.31) x 10-6 / °K
Absorption coefficient at 510 nm 0.5 - 2.5 cm-1


The adjoining figure shows the energy diagram of the absorption and emission bands of the 3d1 Ti3+ ion. In the diagram, the 2T2 level is the ground state, while the 2E level is the excited state. The closely spaced vibrational sublevels broaden the electronic energy levels. The Ti:sapphire laser is called a vibronic laser because of the close blending of the electronic and vibrational frequencies.
Energy diagram of Ti:Sapphire Laser
The absorption band of Ti3+ is in the blue green spectral region, whereas the emission spectrum is slightly red shifted as shown in the figure given here:
Absorption and emission spectra of Ti:Sapphire Laser
Special properties of the Ti:sapphire gain medium can be summarized as follows

If the requirements in terms of pulse duration and output power are less stringent, Ti: sapphire lasers may be replaced with Cr:LiSAF (LiSrAlF6 ) or Cr:LiCAF (LiCaAlF6 ) lasers, which can be pumped at longer (red) wavelengths, where laser diodes are available. These Cr: doped materials are promising new solid-state laser material with a reasonably good tuning range. In the case of LiCAF, the peak lasing wavelength is at 780 nm with a tuning range from 720 to 840 nm. Whereas LiSAF has an even wider tuning range, covering 780-1010 nm with peak lasing wavelength is at 825 nm.

Nonlinear frequency conversion can be used to further extend the range of emission wavelengths of a Ti: sapphire laser system. The simplest possibility is frequency doubling to access the blue, ultraviolet and green spectral region. Another approach is to pump an optical parametric oscillator (OPO), offering a wide tuning range in the near or mid infrared spectral region.
OPO Based frequency conversion setup
The output wavelengths of the OPOs are usually tuned by changing the pump Ti: sapphire wavelength. This technique of tuning the OPO wavelength is mechanically simpler than the more common technique of angle-tuning the OPO crystal (which requires a physical rotation of the OPO crystal). In addition, it avoids the redirection of the output beam due to crystal rotation. Another advantage of pump-wavelength tuning is that it is possible to achieve rapid tuning with no moving parts by using an electronically tunable Ti: sapphire laser. The Ti: sapphire laser can be tuned using a conventional multi-plate birefringent filter. A typical OPO based frequency conversion set up for obtaining wavelengths in the range of 1.5 and 2.5 μ m for LIDAR applications is given in the adjoining figure.

A diode-pumped Nd:YLF or Nd: YAG laser is frequency doubled using to pump Ti: sapphire laser. Tuning of the Ti: sapphire laser can be accomplished by the computer-controlled, stepper-motor rotation of a birefringent filter or electro-optical or acousto-optical elements. This tunable radiation is subsequently used to pump one of two optical parametric oscillators to produce tunable mid-IR radiation. Frequency doubling can be accomplished using non-linear crystals like KTP (Potassium titanyl phosphate: KTiOPO4) or LBO (Lithium Triborate: LiB3O5), whereas OPO materials like RTA (Rubidium titanyl arsenate: RbTiOAsO4 ), CTA (cesium titanyl arsenate : CsTiOAsO4 ), RTP ( Rubidium titanyl phosphate :RbTiOP04), can be finally used for obtaining the wavelengths in the range of 2-5 micron.

It is worth mentioning that materials like Potassium Titanyl Arsenate (KTiOAsO4 or KTA) is an excellent optical non-linear crystal developed recently for non-linear optical and electro-optical device applications. The non-linear optical and electro optical coefficients are higher in these materials as compared to KTP and they have the added benefit of significantly reduced absorption in the 2.0 - 5.0 μm region. The large non-linear coefficients are combined with broad angular and temperature bandwidths. Additional advantages of the Arsenates are low dielectric constants, low loss tangent and ionic conductivities orders of magnitude less than KTP. Single crystals of these Arsenates are chemically and thermally stable, non-hygroscopic and are highly resistant to high intensity laser radiation. Crystals of KTA are important for second harmonic generation (SHG), sum and difference frequency generation (SFG)/(DFG), optical parametric oscillation (OPO), electrooptical Q-switching and modulation and as substrates for optical waveguides. OPO devices based on these crystals are reliable, solid state sources of tunable laser radiation exhibiting energy conversion efficiencies above 50%. KTA has a very high damage threshold. No optical damage has been observed at the levels of 10 - 20 GW/cm2 with the picoseconds dye laser pulses.
Wavelength selection
In tunable lasers, wavelength selection is an essential requirement. Some of the wavelengths tuning techniques for selecting a specified wavelength are the use of prism, grating, intra-cavity etalon, birefringent filter etc. The most commonly used technique is the birefringent filter, which was demonstrated in 1973. It consists of a single thin birefringent material located inside the laser cavity at the Brewster angle, with the birefringent axis lying in the plane of the crystal. If the wavelength of interest corresponds to an integral number of full wave retardation, laser functions as if the filter is absent and the specific wavelength is emitted. The laser polarization is modified for any other wavelength and suffers heavy losses at the Brewster surfaces. The losses for the unwanted wave lengths can be increased by increasing the number of crystal plates, which are similarly aligned. By rotating the birefringent crystal in its own plane, the wavelength tunability is achieved.
Birefringent Filter
The adjoining figure depicts the birefringent tuning element employed in most of the tunable lasers. The birefringent element is usually made of crystal quartz or calcite and is mounted at Brewster's angle. Light traveling through this element is resolved into two components, one polarized along the fast axis and one polarized along the slow axis. These two components travel at different speeds and, thus, become more out-of-phase as they travel through the element. The thickness of this element is adjusted such that it results in a retardation of one full wavelength for the wavelength of interest for the slow ray. When this element is used where wavelength band is present and passes through it, only one of the wavelengths will actually be retarded by exactly one wavelength. Other wavelengths will be retarded slightly more or less. The wavelength that is retarded by exactly one full wavelength will emerge with its polarization unchanged. All other wavelengths will have an elliptical polarization with a horizontal component. These horizontal components will be reflected from Brewster's-angle surfaces in the system, producing losses for all wavelengths except the one passed unchanged by the filter.

Additional filter elements can be added to achieve narrower bandwidths. The second element is twice the thickness of the first, and the third element is four times the thickness of the first. Each additional element further reduces the output line width.

The birefringent filter is tuned by rotation about an axis perpendicular to its optical surfaces. If the filter is positioned so that its "slow" axis is horizontal, the slow component of the light experiences the greatest retardation. The angle between the slow axis and the light transmission direction changes with the rotation of the filter. It becomes minimum when the slow axis lies in a vertical plane. Reduction of this angle also reduces the retardation effect. This allows the slow ray to travel faster as the slow axis becomes more vertical. Under these conditions, a different wavelength will experience exactly one full wave retardation at different angular orientations of the filter.
Ceramic Lasers
Solid-state laser technologies made a significant progress last decade. Major part of this progress is attributed to the laser diode (LD) pumping. In the late nineties, the new solid-state laser material, ceramic YAG, achieved prominence because of its tremendous application potential. Two Japanese groups developed ceramic laser by different techniques. Dr. Ikesue demonstrated the first laser oscillation in 1995. But his method, hot press method, is good for microchip lasers only and has limited scalability. Dr. Yanagitani, Konoshima Chemical Company, published the patent on pure chemical method for ceramic YAG laser component. Nanometer size precursor and nano-YAG-crystal grow to micro-crystals with grain size of 10 micron through the solid phase crystal growth. The technique has become a major milestone for the major applications of solid-state lasers including National Ignition Facility (NIF) Programs and Solid-State Heat Capacity Laser (SSHCL) programme for Directed Energy Weapon systems.

The fast growing interest in the development of these ceramic lasers has led to intensive research in this area. Over the past few years, polycrystalline ceramics have emerged as a viable alternative to the single-crystal hosts, which are based on a rare-earth dopant in a crystalline host material. A ceramic laser is a real revolution in solid-state lasers. It has a nature of crystalline laser like large and homogeneously broadened emission cross-section, thermal conductivity, and mechanical constant. But the fabrication process is really glass-like-fabricated.

Lasers based on these ceramic materials have several advantages: The ceramic laser materials are being produced by forming a nanopowder of ingredients into the desired shape followed by sintering in vacuum to form an aggregate of micro crystals that exhibit optical and thermal qualities almost identical to those of a single seed crystal. Livermore researchers are experimenting with several methods to make transparent ceramics. Like Japanese scientists, they begin with a solution of yttrium, neodymium, and aluminum salts and add a solution of ammonium hydrogen carbonate. The precipitate is then filtered, washed, and dried. At this point, the co-precipitated amorphous carbonate is made up of agglomerates of particles measuring about 10 nanometers in diameter. The particles are heated to about 1,100°C to decompose the carbonates and obtain particles of neodymium-doped yttrium-aluminum-garnet (Nd:YAG) measuring about 100 nanometers in size. Highly agglomerated, the particles are treated ultrasonically, and then the large particles are removed to obtain a uniform small size. In a process called slip casting, a suspension of the fine powder is poured into a plaster of paris mold and allowed to settle. Excess water is poured off, and the mold is set aside to absorb most of the remaining water and dry. The result is a porous structure called a preform structure, which is removed from the mold. The preform still contains many pores and is only about 40 to 45 percent dense. The preform structure is then fired in a vacuum at high temperature for many hours. This sintering process involves surface atom diffusion, resulting in the particles fusing together and decreasing the total surface energy. Some of the pores are squeezed out, and the structure shrinks but still retains its overall shape. Additionally, many physical and thermal properties undergo dramatic improvements during sintering.

Because the sintering process still leaves a few trapped pores, the ceramic parts are subjected to a 1- to 2-hour treatment in a hot isostatic press. The press drives out the last pores by heating the sample to high temperatures under enormous pressure of the order of several hundred megapascals. Provided that no impurities exist, the remaining trapped pores collapse, and the finished part achieves the greater than 99.99 percent theoretical density required for nearly perfect transparency.

Recently, Japanese scientists have developed techniques to produce ceramic parts that rival the transparency of traditional crystals (grown from a single seed) and exceed a single crystal's fracture resistance and robustness of manufacturability.

In addition to the National Ignition Facility (NIF) Programs Directorate and Solid-State Heat Capacity Laser (SSHCL), Livermore researchers have also been looking at other possible applications of these remarkable materials for use in other Livermore lasers. Potential applications include scalable components and advanced drivers for laser-driven fusion power plants.

In its current configuration, the SSHCL has four transparent ceramic insulators, called amplifier slabs, measuring 10 by 10 by 2 centimeters that are pumped by 16 arrays of battery-powered laser diode bars.

With the transparent ceramic slabs in place, the SSHCL can generate 25,000 watts of light for up to 10 seconds at 10-percent duty cycle. The SSHCL is pulsed, turning on and off 200 times per second to generate a beam that can penetrate a 2.5-centimeter-thick piece of steel in 2 to 7 seconds depending on the beam size at the target. The system recently achieved 67,000 watts of average power with five ceramic slabs for short fire durations. The laser, which is powered by batteries, is being pursued as part of the U.S. Army's program to develop directed-energy technologies to defend against missiles, mortar shells, and artillery. Unlike chemical lasers designed for the same purpose, an SSHCL is small enough to be installed on a transport vehicle or helicopter. An SSHCL can also be used to clear land mines. Its pulses can dig through several centimeters of dirt to expose and neutralize a mine.

Yamamoto and colleagues are designing a megawatt-class, solid-state ceramic laser that builds on the success of the ceramics in the SSHCL. The new design features 16 ceramic laser slabs measuring 20 by 20 by 4 centimeters.

Tests show that the transparent ceramics exceed specifications. The amount of scattered light, for example, is similar to that measured from single crystals of Nd:GGG or Nd:YAG. The ceramic slab contains tens of thousands of boundaries between microcrystallites, or "grain boundaries," in the path of the laser light. However, the laser light passing through doesn't "see" the many grain boundaries that measure less than 1 nanometer wide. "The performance of transparent ceramic slabs in the SSHCL is astounding, easily meeting or surpassing the performance of the crystal Nd:GGG slabs."

Recently, scientists at Nanyang Technological University in Singapore, at the University of Electro-Communications in Tokyo and at Konoshima Chemical Co. Ltd. in Japan reported what they believe is the highest efficiency reported from a diode-pumped ceramic Yb:Y2O3 laser. Pumping the laser with 976-nm radiation, of which 2.8 W was absorbed in the ceramic material, they observed 1.74 W of output at 1078 nm with a slope efficiency of 82.4 %. Their Yb: Y2O3 sample had 8 percent atomic doping, producing three absorption peaks and three emission peaks. Two of the emission peaks viz. at 950 and 1031.2 nm overlaid absorption peaks and were poor candidates for laser action. To maximize the quantum efficiency, the scientists pumped their ceramic laser at 976 nm and obtained laser action at 1074 nm. With an end-pumped, 3 × 3 × 2-mm-long Yb:Y2O3 ceramic sample inside a 5-cm-long, nearly hemispheric resonator, they obtained the 82.4 percent slope efficiency.
Heat Capacity Lasers
High average power output of the order of even few tens of kilowatts is not obtainable from solid-state lasers due to their poor efficiency and thermal constraint. Solid-state lasers have very low efficiency and consequently the unutilized input energy heats up the laser rod.

There are three possible modes of laser operation. The first one is single shot operation like the one being used in laser range finders or at higher energy level in the NIF, which uses laser glass as the lasing media. In this case, the thermal effects in the solid-state medium are not very important and the cooling is provided through ambient atmosphere after the laser shot. Since it is a single shot or the time between the shots is much larger, the laser media gets cooled by the time next shot is fired. Net result is that there is no thermal gradient in the lasing media at the time of firing a shot.

The second mode of operation is steady state operation, which is typically the case for most of the solid-state lasers based on Nd: YAG. The laser medium is continuously cooled while it is also being pumped. Solid-state lasers have very low efficiency and consequently the unutilized input energy heats up the laser rod. As the cooling fluid flows over the surface of the laser medium, either rod or a disc, a temperature gradient is developed between the center of the medium and the surface. This is due to the fact that the cooling of the surface is faster than the of the central region of the medium, since the cooling of the same takes place depending on the thermal conductivity of the material, which is very poor. As the material cooling rate is rather low compared to that of surface, temperature gradient is produced in the material. If the thermal gradient increases beyond a certain value, laser action becomes more and more inefficient and may even cease. Further, thermal birefringence thereby produced also considerably reduces the beam quality of the laser. Figure shows the temperature gradient in a laser medium under steady state operation. This temperature profile induces a tensile stress in the medium. Higher the power levels, more is the waste heat deposited in the material and thus higher are the thermo-mechanical stresses (of tensile nature). Since there is a limit to which a material can be subjected to stresses before the fracture limit, it sets the limit how much power we can extract in lasers in steady state conditions.
Temperature profile in lasing media before start of lasing operation in any mode
 
Temperature profile in lasing media when laser is running in a steady state operation
The limitation of steady state operation can be overcome by operating the laser in a novel mode i.e. heat capacity mode, which is intermediate between the above two modes viz. single shot and a steady state. In this case, single shots are rapidly fired on a time scales, which are short as compared to thermal diffusion times through the laser medium. Under these conditions, the build up of thermal gradients is avoided and the device basically has the thermo-optic properties of a single shot device. The waste heat generated during lasing is stored in the active medium, whose temperature rises from the initially achieved starting value to a temperature where laser operation ceases. At this point the medium is again cooled to the initial temperature so that new lasing sequence can begin. Lasing times of many seconds typically up to 10 sec can be achieved generating up to megawatts of levels of burst power during this time. This burst operation makes the heat capacity laser concept more suitable for applications, which require large amount of energy, but for a short period of time. In the HCL concept, there is inversion of the temperature profile through out the medium, as compared to the normal steady state lasing approach. In fact, total energy that can be extracted depends on the heat capacity of the active medium and the temperature difference over which it is operated. To generate higher output, one has to choose a material with higher heat capacity and avail a technique to increase the temperature difference, like cooling the system to liquid nitrogen temperature.

Figure shows the temperature gradient in a laser medium under heat capacity mode operation. This temperature profile induces a compressive stress in the medium. Higher the power levels, more is the waste heat deposited in the material and thus higher are the thermo-mechanical stresses (of compressive nature). Since for laser materials, compressive fracture strength is about 5 - 6 times higher than that of the tensile strength, the laser can be pumped much harder thereby yielding higher outputs.
Temperature profile in lasing media just after vigorous cooling and before initiation of new burst
 
Temperature profile in lasing media immediately after laser burst is over in heat capacity mode
After every burst, the laser material is vigoursly cooled for making it ready for the next run. In the beginning of the new run, the temperature profile in the material will be such that the surfaces will be cooler as compared to the center of the material. As soon as we start firing new burst, the surface temperature starts rising faster as compared to the center of the laser material, thus first making the profile even and then inverted later on. On the other hand, in steady state operation, the surface temperature is always less than the center of the laser medium to start with, and this difference continuously increases till a steady state is reached. The temperature difference decides the maximum output power, one can extract from the material. In terms of thermal gradient, the thermal gradient goes on increasing with time in case of steady state operation, whereas in case of heat capacity mode, these gradients rather decreases first for some time and then increases later on. Time within which the gradients develop for the laser to cease really decides the duration of the burst.

The heat capacity concept was demonstrated in 2001 at Lawrence Livermore National Laboratory by constructing a 10 kW average power that operates in this mode. Initial demonstration used laser glass as a lasing medium and pumped was carried out using flash lamps. Although the prototype uses Nd: glass for its laser amplifier disks, the upgraded versions use Nd:GGG. Compared with Nd: glass, Nd: GGG boasts a higher mechanical strength and higher thermal conductivity, which, in combination, allows to rapidly cool the disks between runs and reduce the turnaround time between laser firings. To pump these Nd: GGG amplifier disks, the SSHCL uses arrays of laser diodes instead of flash lamps because diode arrays are more compact and efficient than flash lamps and, more importantly, diode radiation generates less heat in the Nd: GGG laser crystals. The Nd: GGG is also twice as efficient in converting pump energy to output beam energy. However, there was a big challenge to grow the crystals large enough to manufacture the nine 13-square-centimeter slabs needed for the upgraded 100-kilowatt laser. Northrop /Grumman Poly-Scientific, the commercial partner responsible for growing the crystals, have attempted to produce high-optical-quality Nd: GGG crystals up to 15 centimeters in diameter.

In 2001 itself, Professor Ueda of Univ. of Electro-Communications, Tokyo Japan demonstrated the potential of ceramic lasers. Ceramic rods of Nd: YAG of 100 mm length and 3 mm diameter pumped by diode lasers were reported to yield powers of the order of 2 kW with a potential to deliver up to 10 kW. High efficiency operation was demonstrated in the end-pumping scheme and the optical-optical efficiency was measured to be about 60% in 1% and 2% doping. This was almost the best data for a single crystal and was a clear evidence to show the high quality of ceramic YAG material. Since ceramic laser media has many advantages over their crystal counterpart (see section on ceramic lasers), scientist working at Lawrence Livermore National Laboratory immediately thought of using ceramic media for their heat capacity laser programme.

During the last several years, Lawrence Livermore National Laboratory has been developing high-power solid-state lasers for tactical battlefield applications. These lasers are based on a compact, flexible, single-aperture, mobile architecture that can be readily scaled to engagement-level powers (~ 100 kW). Looking at the potential of ceramic media for heat capacity lasers, the lasing medium is a series of diode-pumped solid-state ceramic slabs, producing a beam at a wavelength of approximately 1 micron. During lasing operations, the waste heat is stored in the slabs. In a field device, the slabs would be rapidly interchanged with cool slabs, after several accumulated seconds of lasing.

It has been reported in 2005 that the laboratory laser has four ceramic YAG slabs pumped by diodes at a pulse repetition rate of 200 Hz. The aperture size is 10x10 cm2. With this laser, routine operation has been achieved at a time-averaged power of about 25 kW (125 J, 200 Hz) for several seconds. Since the laser has a pulsed format, this is the power averaged over an interval longer than several pulses. The pulse length is about 0.5 ms, giving a duty factor of 10%. The time-averaged power is the same as the equivalent CW power. With the transparent ceramic slabs in place, the SSHCL can generate 25,000 watts of light for up to 10 seconds at 10-percent duty cycle. This pulsed SSHCL can generate a beam that can penetrate a 2.5-centimeter thick piece of steel in 2 to 7 seconds depending on the beam size at the target. The system recently achieved 67,000 watts of average power with five ceramic slabs for short fire durations. The laser, which is powered by batteries, was conceived as part of the U.S. army's program to develop directed-energy technologies to defend against missiles, mortar shells, and artillery. SSHCL is small enough to be installed on a transport vehicle or helicopter. This SSHCL can also be used to clear land mines. Its pulses can dig through several centimeters of dirt to expose and neutralize a mine.

Based on the success of ceramic slabs, efforts are on to upgrade the system up to 100 kW (500J, 200 Hz) by designing a megawatt-class, solid-state heat capacity ceramic laser based on 16 ceramic laser slabs measuring 20 by 20 by 4 centimeters.
Dye Lasers
Dye Lasers use an organic dye as the gain medium. The wide gain spectrum of available dyes allows these lasers to have high degree of tunability with high resolution and high power. The dye laser initially developed by Schmidt and Schafer and Sorokin and Lankard in 1967 was based on flash lamp pumped. Since the dyes used in tunable dye lasers are fluorescent, another light source is always required to pump the dye in order to achieve the population inversion. The pump beam used to excite the large dye molecules and produce the population inversion is a strong light source either a flash lamp or another laser focused on the dye stream. Typical absorption and emission spectra of a common laser dye molecule, which are large organic molecules, is shown in the adjoining figure. One can see that both emission band and the absorption bands are quite broad but the emission band is lower in frequency as compared to absorption band. The dye will absorb only those wavelengths of light, which are shorter than those, which it emits, since some input energy will always be absorbed in the form of vibrations or heat. The characteristics of the light used in the excitation determine the characteristics of the laser. If a pulsed source like flash lamp is used to pump the dye laser, the beam will also be pulsed, on the other hand, if a continuous-wave laser like argon laser pumps the laser, the dye laser's beam will also be continuous.

The energy absorbed by the dye creates a population inversion, moving the electrons into an excited state. Typically, the dye molecule de-excites spontaneously into a metastable state having relatively longer lifetime. It stays there till stimulated emission occurs from the de-excitation of other molecules in the dye. Since the dyes commonly employed are large and have many lower-energy states available for the excited electron to decay into, a large range of de-excitation energies and thus, a large range of output wavelengths are available to the dye. Typically, in the case of Rhodamine 6G, this spectrum of usable wavelengths is quite large, about 130 nm. The dyes most commonly used in dye lasers are Rhodamine 6G combined and Coumarin which are dissolved in a liquid and pumped through the optical cavity in order to prevent any one part of the dye from becoming exhausted during the excitation process. Now- a- days solid dye cells are also being employed in order to increase convenience and make the system portable. In spite of the large gain bandwidth of organic dyes, no one dye can cover the entire visible spectrum; typical tunable range for various dyes is given the following table:

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

The most important attribute of the dye laser is its tunability, which gives the user access to essentially any wavelength in the visible and near-visible spectrum. The spectral range of ion-laser-pumped CW dye lasers is essentially complete coverage from 400 to 1000 nm. It is even possible to extend their CW tuning range by using nonlinear optical methods to generate wavelengths further into the ultraviolet and infrared region. Energy band diagram of dye lasers is shown in the figure. Energy band diagram of Dye Lasers
Simplified picture of singlet states is shown in the figure below.
Simple Energy band diagram of Dye Lasers
Typically the dye molecules are large organic molecules and have many internal degrees of freedom both vibration and rotation resulting in broad overlapping of energy levels. For laser oscillation the dye the intense pump laser using either flash lamps or other lasers such as Argon or Krypton excites molecule's absorption band. The process is as follows: 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.

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. Most 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

Since most organic dyes have a large range of wavelengths over which amplification can occur (called the gain bandwidth), lasers built around them can be composed of light waves spanning a range of wavelengths in the spectrum. This makes possible the ability to select the wavelength of the laser light through the adjustment of a prism or grating. This tunability feature allows certain specific applications to be performed at minimal cost as compared to having large number of different monochromatic lasers.

Some of the salient features of dye lasers are listed below:
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

Applications
Semiconductor Lasers
Semiconductor lasers or diode lasers or laser diodes as they are generally referred to, were invented almost half a century ago by Robert N Hall and Marshall Nathan in 1962. Diode lasers of the sixties required threshold current densities of 1000 A/cm2 at 77 K temperatures and two orders of magnitude greater, or 100,000 A/cm2 at about 300 K. Moreover these lasers were pulsed. The main challenge was to operate these lasers at room temperatures continuously with low threshold current densities. The first laser diode to achieve continuous wave operation was a double heterostructure operation demonstrated in 1970 simultaneously by Zhores Alferov at Iaffe Physico-Technical Institute, St. Petersburg Russia and Morton panish and Izuo Hayashi at Bell Labs. Continuous developments have resulted in laser diodes with shorter and shorter wavelengths, increasing output power and an improved beam quality. Today reliable laser diodes stacks with powers in the range of kilowatts are available in the market for applications like Diode pumped solid-state lasers. In addition, compared to other types of lasers, laser diodes use very little power. Most laser diodes can operate with voltage as low as 2 V with power requirements determined by their current setting. Electrical to optical efficiencies in excess of 50% are typical in the case of laser diodes. In this way, Laser diodes have thus grown to a key component in modern photonics technology. Advances in crystal growth technologies, such as metallorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE), the development of double heterostructure lasers and subsequently quantum well lasers, materials passivation technologies, thermal management technologies; all have further contributed to one of the most enabling technological industries today, that of high-power semiconductor lasers.

As compared to other lasers, semiconductor lasers are:
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 and lower spectral purity.

The optical gain in a semiconductor lasers are achieved through the recombination of injected holes and electrons resulting in emission of photons in a forward-biased semiconductor p-n junction. This represents the direct conversion of electricity to light, which is a very efficient process, and practical diode laser devices can achieve more than 50% percent electrical-to-optical power conversion rate, that is almost an order of magnitude larger than most other lasers. Over the past few years, efforts are on for a gradual replacement of other laser types by diode laser based-solutions, as the considerable challenges to engineering with diode lasers are being sorted out. At the same time the compactness and the low power consumption of diode lasers have enabled important new applications such as storing information in compact discs and DVDs, and the practical high-speed, broadband transmission of information over optical fibers.

Semiconductor lasers also have basic three components Under normal conditions, absorption of photon results in the generation of an electron hole pair: the condition, which is used for detection of light. The recombination of an electron hole pair results in spontaneous emission of photon: the principle of operation of light emitting diodes (LED). Electron hole combination can be stimulated by a photon thus inducing emission of identical photons: the principle of operation of semiconductor 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.

In a semiconductor laser, the transitions are associated with the electron states in the conduction band and valence band. Since the upper and lower energy states are continuous and hence the semiconductor has a broad gain spectrum implying that the output is not sharp. Thus coherence and mono chromaticity of these lasers are poor.

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.

Before we go into the basics of semiconductor lasers, we will briefly outline some of the fundamental points related to semiconductor physics. However, for details reader can consult any textbook on semiconductor physics.

Semiconductor materials are crystalline or amorphous solids whose electrical conductivity is somewhere between that of an insulator and a conductor. Examples of Semiconductors materials are silicon, germanium and gallium arsenide (GaAs), They are neither good conductors nor good insulators: that is why the name semi-conductors. They have a small number of free electrons because the atoms are closely grouped together in a crystalline pattern called known as crystal lattice. However, their ability to conduct electricity can be greatly enhanced by adding certain impurities to this crystalline structure thereby, producing more free electrons than holes or vice versa.

Semiconductors contain two types of mobile charge carriers, holes and electrons.

The holes are positively charged while the electrons are negatively charged.

A semiconductor may be doped with donor impurities such as antimony in silicon so that it contains mobile charges, which are primarily electrons. Semiconductor material with electrons as majority carriers is known as n-type semiconductor

Similarly, a semiconductor may also be doped with acceptor impurities such as boron in silicon, so that it contains mobile charges, which are mainly holes. Semiconductor material with holes as majority carriers is known as p-type semiconductor

Electrical and optical properties of semiconductors are determined by the energy distribution of electrons in these materials.

The energy of electrons in solids, just like the energy of electrons in atoms, is limited to certain discrete values. In crystalline solids, these energy levels are grouped into bands, known as allowed energy bands.

In semiconductors, the last completely filled band is called the valence band. This first empty band above the valence band is called the conduction band. Energy Gap between the Valence band and Conduction band is known as Band Energy Gap The energy gap between these two bands is called forbidden gap or band-gap, Eg.

Insulators have large band-gap energies. The material is an insulator if the energy band-gap is larger than about 3.5 eV. For example, band-gap energy of diamond, which is a good insulator, is Eg = 6eV. The band-gap energy of semiconductors is typically between 0.2 eV and 3.5 eV. Materials with Eg < 0.2eV are generally considered as metals.

The semiconductor contains no electrons at a temperature of absolute zero, T = 0K and its conduction band is empty, and thus behaves like an insulator. As the temperature increases, some electrons are thermally excited into the first empty band, i.e. the conduction band.

Fermi level indicates the occupation conditions of electrons or holes in the semiconductor; it is the energy level to which carriers occupy. Fermi level (EFP) for p-type is near the valence band and EFN for the n-type is near the conduction band.

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.

Once 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. Simplified p-n junction diagram under no bias 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 system 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.

However, this equilibrium can be disrupted, by applying an external electric field usually known as biasing the p-n junction.

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.

The width of depletion layer increases with an increase of a reverse voltage and decreases with an increase in the application of a forward voltage

The p-n junction is the basis of optoelectronics devices, such as the light emitting diode (LED), laser diodes,

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. Simplified p-n junction diagram under no bias 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 junction 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.

One can see that EFN is not equal to EFP in case of biased p-n junction. Fermi levels in such situations are known as Quasi Fermi Level.

Under forward biased condition, the recombination process has to be such that the carriers recombine radiatively. For this the probability of band-to-band recombination, which is the most desirable process for generating high-energy photons, is to be exploited.

In an efficient semiconductor laser or even LED, most of the carriers that recombine must result in production of photons, in other words, the recombination should be a result of band-to-band radiative transition. All other recombination processes in which electron energy is lost in producing phonons, as is the case in indirect band gap materials, or in recombination with ionized impurities are undesirable. The efficiency for an efficient device can be characterized by the term "internal quantum efficiency" ηINT, defined as
ηINT   =   Number of band to band radiative recombinations

Number of carriers crossing junction
and is given as
Internal quantum efficiency
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. This explains why indirect band gap materials like silicon are not efficient semiconductor laser materials.

The energy of the photon resulting from this recombination is equal to that associated with the energy band gap. In light-emitting diodes (LED) this light energy is transmitted out through the sides of the junction region. In semiconductor lasers the junction forms the active medium, and the reflective ends of the laser material provide feedback. By imposing the appropriate feedback conditions required by all lasers, stimulated emission dominates and laser action can occur. The structure of the laser diode creates an optical cavity in which the light photons have multiple reflections. Ensuring the light is properly reflected is very important for the operation of the device. Usually, the cleaved ends of the laser diode, with no further coating, form the mirrors for output coupling. The typical reflectivity at the interface between gallium arsenide and air is approximately 36%. However, mirror of higher reflectivity is desired at one end to ensure that the laser output comes from one end. Further to reduce the threshold for laser operation, the reflectivity of output coupler and the total reflector can be realized using dielectric coatings. More sophisticated coatings can then be applied to these facets to tailor their reflectivity. Alternatively, Bragg gratings can be inserted at the ends of the cavity to provide reliable single-frequency operation capable of high-speed modulation.

The thickness of the junction region is small, typically around one micron. Thus, light traveling in the plane of the junction is amplified more than light perpendicular to it and the laser emission is parallel to the plane of the junction.

Though effective photon generation is essential for Semiconductor lasers, however, it is equally important to ensure that these photons come out of the active region of the device; otherwise the device will not be efficient. The generated photons may fail to escape from the active region because of the reabsorption of these photons in bulk material between the active layer and the surface. Thus for a bright, efficient, photon emitter we need to ensure that as many carriers as possible recombine soon after crossing the junction, and do not escape to travel into the bulk material far from the junction. In an ideal device electrons and holes should be "trapped" in the region where recombination is desired. Though initial work on these devices has been on homojunctions (e.g. p-type and n-type GaAs), but these devices somehow suffer from poor electron - hole confinement, poor optical confinement, and inefficient injection of carriers thus requiring very high degree of doping. This implies that the threshold current densities are very high and the device has to work as pulsed and also at low temperature typically 77 K. This trapping can be affected by the use of heterostructures semiconductor structures that use heterojunctions. A heterojunction is a junction between layers of different properties (e.g.: different band gap energies) but having almost the same lattice structure. The Double Heterojunction laser (DH-laser) uses four different layers, for example an n-GaAs -layer followed by a N-GaAlAs-layer, then a p-GaAs-layer and eventually a P-GaAlAs one where N and P indicate larger band gaps. The aluminium containing layers have a lower refractive index. Hence this structure with the p-layer as the active region in between them provides good wave guidance. These structures confine the injected electrons and holes to a narrow region about the junction. This requires less current to establish the required concentration of electrons for population inversion. Further these structures also help in photon confinement. A dielectric waveguide around the optical gain region helps to increase the photon concentration and elevate the probability of stimulated emission. This reduces the number of electrons lost traveling off the cavity axis.

For example heterostructure laser diode can be fabricated using two different band gap materials namely GaAs and AlGaAs. 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.

As mentioned earlier, that the homojunctions are no longer being used because of poor confinement of both carriers as well as emitted photons thus paving the way for heterojunctions for the present day practical devices which dominate most applications. These devices have basically a stripe geometry, in which the gain region is confined to a narrow stripe region. 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.

Gain-guided lasers are comparatively easier to fabricate as compared to index-guided lasers, but they have weaker confinement, which leads to somewhat poorer beam quality and stability. Further Gain-guided devices also have somewhat larger astigmatism than index-guided devices. These factors restrict the use of gain-guided devices for some applications where beam quality is important, but for most of the applications gain-guided diode lasers are suitable.

Another rapidly upcoming area of semiconductor laser technology is the development of high-power linear arrays. These devices are high-radiance diode sources suitable for applications like pumping solid-state lasers. The array is fabricated as a bar with a number of stripe laser sources. These devices are capable of emission of kilowatts of optical power. Such lasers are usually available as bar structures and may be stacked to form two-dimensional arrays.

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.

However, recently, 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. The reflectors at the ends of the cavity are dielectric mirrors made from alternating high and low refractive index quarter-wave thick multilayer.VCSELs emit their beam from their top surface. The cavity is formed of a hundred or more layers consisting of mirrors and active laser semiconductor all formed epitaxially on a bulk substrate. 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.

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.

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.

Conventional semiconductor lasers can emit wavelengths upto few microns depending upon the band gap of the material. However, in order to generate longer wavelengths, new class of semiconductor lasers, known as quantum cascade lasers can be considered. In conventional semiconductor lasers, the lasers action is due to interband transitions through the recombination of electron hole pairs across the band gap, On the other hand, in Quantum Cascade lasers laser emission is achieved through the use of intersubband transitions in a repeated stack of semiconductor multiple quantum heterostructures. Wavelength range from 2.75-250 μm has been generated using these structures.

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.

The important characteristics of semiconductor lasers are listed below:

Diode output power vs drive current Adjoining 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. The current through the junction must exceed a minimum threshold value. It means that it must provide enough holes and electrons so that the radiation generated by their recombination exceeds the losses. Losses may arise from several reasons such as spreading of light out of the active region, transmission of light through the mirrors, and absorption of light by free carriers in the junction.

This steeply rising light output curve can be extrapolated backward to the zero light output intercept, which defines the threshold current. Further, 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%.
Quantum efficiency
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. Inside the laser, the light waves are limited to the active zone. Since the active light-emitting area is rectangle-shaped with different length and breadth, the parallel and vertical divergence are also different. This results in the appearance of an elliptical spot at some distance from the emitting area. The ratio of vertical to parallel divergence, measured in the far field, is called the ratio of axes. 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. In order to make a diverging beam parallel, a positive (convex) lens can be used to produce a collimated or focused beam. However, without further correction, the beam profile will be elliptical and the focal distances in both the axes will not be the same due to the astigmatism. Usually a pair of wedge-shaped prisms can be used to circularize the elliptical spot shape. By adjusting the relative orientations of the two prisms, it is fairly easy to effectively correct for this beam characteristic. The astigmatism may, however, be corrected, by the use of cylindrical lenses to form a circular profile. Still the beam divergence is substantially larger than what one is accustomed to with lasers. However there are some like Vertical cavity surface emitting lasers (VCSEL), which have square or round emitting areas and, therefore, can produce relatively symmetrical beam.

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 threshold current density for laser operation increases rapidly with increasing temperature. Typically, at the cryogenic temperature of 77 K, the threshold current in a gallium arsenide laser is about one tenth that of the room temperature value. This means that cooling to cryogenic temperatures changes the operating characteristics of the laser. The shift in the threshold current is due to the temperature dependent nature of the carrier concentration in the active layer, whereas the decrease in slope is due to an increasing probability for non-emitting recombination processes. Further, increase in temperature also affects the spectral distribution. With the increase of temperature, the crystal expands and thus increasing the resonator length. Further the refractive index increases whereas the bang gap decreases with increase in temperature. Net result is the output spectral lines drift to longer wavelengths. Typically, the wavelength shift for 808 nm diodes is generally around 3 nm per 10oC. Thus, it is necessary to stabilize the laser temperature. Most of the semiconductor diodes are operated at 77 - 200 K.

Another effect of temperature is on the life of semiconductor lasers. When the temperature is reduced by about 10 degrees, the lifetime is almost increased twice. This is why these lasers are mounted onto a heat sink to avoid an overheating by power dissipation. Life time up to 100.000 hours have been reported

Since charge carriers electrons and holes are injected into the device from the n- and p- side, these lasers are also sometimes called injection lasers.

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.

Typical gas lasers, with a diffraction-limited beam emerging from an aperture around 1.5 millimeters in diameter usually have a circular beam with divergence of a few tenths of a degree. Thus semiconductor diode lasers typically have much lower radiance than other types of lasers. Also, their radiation cannot be focused so well as the light from better-collimated lasers.

Typical gain and Saturation intensity of semiconductor lasers is 103 cm-1 and 2.5 x 109 W/cm2 respectively.

Laser diodes offer many advantages, including small size, lightweight, low power consumption, and high efficiency. They have become widely used as light sources for a wide variety of applications, including compact disk players, printers, magneto-optic data storage, and optical-fiber telecommunications.
Difference between Diode Laser 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

Most Common semiconductor materials and their wavelengths

Material Wavelength
GaAs/AlGaAs 720 - 850 nm
GaAs/InGaAs 900 - 1100 nm
GaAs/AlGaInP 635, 650, 670 nm
GaN/InGaN 380, 405, 450, 470 nm
InP/InGaAsP 1000 - 1650 nm

Applications
References


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Updated: 16 January, 2014
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