Introduction

A laser is composed of an active laser medium, or gain medium, and a resonant optical cavity. The Laser gain medium transfers external energy into the laser beam. It is a material with controlled purity, size, concentration, and shape, which amplifies the beam by the process of stimulated emission. The laser gain medium, in general, is pumped, by an external energy source including a flash lamp, another laser source, electric gas discharge, exothermic chemical reactions etc. . The pump energy is absorbed by the laser medium, exciting some of its particles into high-energy state where these can interact with light both by absorbing photons or by emitting photons. Under certain conditions, as mentioned in earlier sections, the amount of stimulated emission due to light that passes exceeds the amount of absorption resulting in amplification. Thus the basic components of a laser are:

• Lasing material e.g. crystal, glass, gas, semiconductor, dye, etc.
• Pump source that adds energy to the lasing material, e.g. flash lamp, electrical current to cause electron collisions, radiation from a laser, chemical reactions etc.
• Optical cavity, which consists of reflectors, acts as the feedback mechanism for light amplification.

In this section, we would like to discuss various types of lasers like, solid state lasers, semiconductor lasers, dye lasers, excimer lasers, gas lasers, gas dynamic lasers, chemical lasers, X-Ray lasers, Free Electron lasers etc. Our intention is to provide salient features of various systems, without going into intricate details. The reader is advised to go through the various references for details at the end of the section.


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.

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:

• Optical pumping
• Electrical pumping
• Chemical pumping

So far as solid-state lasers are concerned, it is mainly the optical pumping, which is being used. Optical pumping uses either cw or pulsed light emitted by a powerful lamp or a laser beam. 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:

• The price per watt of generated pump power is much lower for lamps, compared with laser diodes used for diode pumping.
• Very high pump powers (particularly peak powers) can be generated.
• Lamps are fairly robust, e.g. quite immune to voltage or current spikes.
• However, device lifetime, power efficiency, cooling and thermal lensing are not really important issues e.g. when a flash lamp is operated with low pulse repetition rate and low average power, as required e.g. in engraving and marking systems.

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.

The main disadvantages associated with Flash lamp pumping includes:

• The lifetime of lamps is very limited - normally up to a few thousand hours.
• Flash lamps have a broad emission spectra (see adjoining figure) whereas the absorption spectra of lasing media have more or less discreet absorption peeks. . As a result, most of the optical energy being emitted by the flash lamp goes waste.
  
  

  
  

• The wall plug efficiency of the laser (electrical to optical efficiency) is low - typically ( few percent. This results in a higher heat load, making necessary a more powerful cooling system, and the strong thermal lensing and hence a poor beam quality.
• Electric power supplies for lamp-pumped lasers involve high electrical voltages, which raise additional safety issues.
• The low pump brightness (compared with that achievable with diode lasers) and the broad emission wavelength range exclude many solid-state gain media.

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.

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.

• Low-power lasers (up to roughly 200 mW) can be pumped with small edge-emitting laser diodes. These exhibit a diffraction limited beam quality and make it quite easy to achieve the same for the solid-state laser.
• Broad area diodes typically generate several watts and are suitable for pumping solid-state lasers with output powers up to a few watts. Their beam quality is quite asymmetric, but normally still sufficient for achieving a diffraction-limited laser output without using complicated optics.
• High power diode bars emit tens of watts (or even >100 W), allowing for higher output powers, particularly when several bars are combined. Their beam quality is strongly asymmetric and quite poor; as a result their radiance is much lower than that of lower-power diodes. However, beam shapers are often used to improve the beam quality and to make the beam symmetric.
• For the highest powers, diode stacks are often used. These have a still worse beam quality and lower brightness, but can provide multiple kilowatts. We can stack the bars into a two dimensional structure, it is reported that 1 cm long bars are stacked to form an emitting area. The average power is about 100W/cm², peak power 1kW/cm².

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 compactness of the pump source, the power supply and the cooling arrangement makes the whole laser system much smaller and easier to use.
• A high electrical-to-optical efficiency of the pump source (order of 50%) leads to a high overall power efficiency i.e. wall plug efficiency of the laser. As a consequence, small power supplies are needed, and both the electricity consumption and the cooling demands are drastically reduced, comparing with those for lamp-pumped lasers.
• The narrow optical bandwidth of diode lasers makes it possible to directly pump certain transitions of laser-active ions without losing power in other spectral regions. It thus also contributes to a high efficiency.
• Although the beam quality of high power diode lasers is poor, however, end pumping of lasers provide very good overlap of laser mode and pump region, leading to high beam quality and power efficiency.
• Diode-pumped low-power lasers can be pumped with diffraction-limited laser diodes. This allows the construction of very low power lasers with reasonable power efficiency.
• The lifetime of laser diodes is long compared with that of discharge lamps: Further it is much easier to replace laser diodes as compared to discharge lamps.
• Diode pumping makes it possible to use a very wide range of solid-state gain media for different wavelength regions.

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.

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.

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 ⁴F_3/2 to ⁴I_11/2 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, ⁴F_3/2, 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 ⁴I_11/2 quickly reduces to zero as excited species jump to the ground state ⁴I_9/2 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.

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:

• Nd:YAG = Nd:Y₃Al₅O₁₂ (yttrium aluminum garnet,) : the classical choice for 1064 nm, but also usable at 946 nm and 1320 nm (and a few other lines); isotropic; still very common particularly for high power lasers and Q - switched lasers.

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.

• Nd:Cr:GSGG laser: Nd³⁺ is the active ion in this case also, the host being gadolinium scandium gallium garnet (GSGG), sensitized with Cr³⁺. GSGG is a material with higher fracture limit and sensitization with triply ionized chromium (Cr³⁺) gives it a far better efficiency, compared to Nd:YAG, because Cr³⁺ absorbs the unutilized part of the emission spectra of the flash lamp and emits in the band corresponding to the absorption band of Nd³⁺ ion. Its lasing wavelength is 1064 nm.
• Nd: YVO₄ (yttrium vanadate,) for 1064 nm, 914 nm and 1342 nm: very high pump and laser cross sections and larger gain bandwidth, compared with Nd:YAG, thus particularly attractive for low - threshold lasers; also good properties for high power operation with good beam quality (low dn/dT); birefringent
• Nd:YLF = Nd:YLiF₄ (yttrium lithium fluoride) for 1047 nm and 1053 nm: birefringent, long upper state life time, weak thermal lensing: useful for high power Q switched lasers.
• Nd:GdVO₄ (gadolinium vanadate) for 1064 nm and 1341 nm: similar to Nd:YVO₄, but having a larger gain bandwidth.
• Nd:GGG (gadolinium gallium garnet): often used for high power heat capacity lasers
• Nd:YAP ( yttrium aluminum phosphate) : high thermal conductivity, birefringent
• Nd:glass : Neodymium atoms are also used as the active elements in Nd:glass lasers. The doping level is usually 1% or less. The absorption spectrum and energy-level diagrams of Nd:glass are similar to those of Nd:YAG, but the glass absorption peaks are much broader and less distinct. The reason for this is that glass is not a crystalline structure as is YAG. Glass is a supercooled fluid and has a random amorphous structure. Neodymium ions in a YAG crystal all have the same spacing from neighboring atoms and very similar environments. In glass the atomic distances and distribution are random, and each ion has a different environment. This causes the energy levels of different ions to shift differently, broadening all the absorption and emission lines considerably. This also results in a somewhat longer lifetime for the upper lasing level. This means that Nd:glass has a higher efficiency than Nd:YAG in the pulsed mode and a broader output linewidth.

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.

Ytterbium-doped laser crystals and glasses have a number of interesting properties, which differ from those e.g. of Nd: doped host materials.

• They have a very simple electronic level structure, with only one excited state manifold (²F_5/2) within reach from the ground state manifold (²F_7/2) with near-infrared photons.
• Pumping and amplification involve transitions between different sublevels of the ground state and excited state manifolds.
• The quantum defect is always rather small, making them suitable for high power lasers.
• The gain bandwidth of the laser transitions is typically quite large, compared to Nd : doped crystals thus making them suitable for applications involving wide wavelength tuning, generation of ultra short pulses in mode - locked lasers.
• The upper state life times are relatively long; typically of the order of 1-2 milliseconds, which is beneficial for Q - switching.

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.

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.

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⁺)****(Check original) 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****(Check original)
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.

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.

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:

Special properties of the Ti:sapphire gain medium can be summarized as follows:

• Sapphire μmonocrystalline Al₂O₃) has an excellent thermal conductivity, alleviating thermal effects even for high laser powers and intensities.
• The Ti³⁺ ion has a very large gain bandwidth μmuch larger than that of rare earth doped gain media), allowing the generation of very short pulses as well as wide wavelength tenability.
• The maximum gain and laser efficiency is obtained around 800 nm. The possible tuning range is ˜650 nm to 1100 nm, although different mirror sets are normally required for covering this huge range, and exchanging mirror sets is a somewhat tedious task. However, the number of required using ultra broadband mirrors could reduce mirror sets.
• There is also a wide range of possible pump wavelengths, which however are located in the green spectral region, where powerful laser diodes are not available. In most cases, several watts of pump power are used, sometimes even up to 20 W. Originally, Ti:sapphire lasers were in most cases pumped with 514-nm argon ion lasers, which are powerful, but very inefficient, expensive to operate, and bulky. Other kinds of green lasers, which are now being widely used, are frequency doubled solid-state lasers based on neodymium doped gain media such as Nd: YAG, Nd: YLF.
• The upper state lifetime is rather short (3.2 - 3.6 μs), and the saturation power is very high. This means that the pump intensity needs to be rather high, so that a strongly focused pump beam and thus a pump source with high beam quality is required.
• Despite the huge emission bandwidth, Ti:sapphire has relatively high laser cross sections, which reduces the tendency of Ti:sapphire lasers for Q - switching instabilities.

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.

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.

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:

• Since no tedious growth of single crystals is required, ceramic lasers can be significantly less expensive than conventional lasers.
• The ceramic materials can be custom-fabricated with spatially tailored doping concentrations and index profiles.
• The biggest advantage of ceramic laser to the single crystal laser is the scaling to the large aperture size. Samples of the size of 10 by 10 by 2 centimeters have already been fabricated and are being used in heat capacity solid-state laser applications. Demonstration of a large aperture sample of 1m x 1m in the ceramic forming process has already been reported for its application for the meter-size ceramic lasers for laser fusion driver.
• The slabs of these materials can be obtained regularly, on time, and without unexpected additional costs. Ceramic materials can be made any size and shape. The time required to produce the slabs from start to finish is much shorter than the time to grow crystal boules-days instead of weeks. In addition, multiple samples can be fired in one furnace at the same time.
• Ceramic slabs are also tougher than single-seed crystal slabs and much less apt to undergo a catastrophic fracture. When a crystal slab fractures, the fracture can "run," extending some distance from the original crack and often branching or making a random turn into the center of the crystal to relieve stress. Because cracks are impeded by grain boundaries, ceramic fractures don't run as easily or randomly.
• Ceramics also measure lower residual stress, which is stress that resides in a material after it has been manufactured. Significant residual stress distorts the laser beam and can make the material more susceptible to cracking.
• Further, Ceramics can accommodate higher concentrations of dopants (rare-earth ions such as neodymium), which could permit pumping at wavelengths that might otherwise be impractical.
• Dopant concentrations are highly homogeneous in ceramics and can be controlled precisely. In crystals, dopants tend to segregate toward the bottom of the growing boule.
• Ceramics also offer the possibility of novel composite structures. For example, a single slab could have an "active" layer of YAG doped with neodymium ions and another layer composed of YAG doped with chromium ions. Such a design is called a passive Q-switch, which turns on the laser after saturation. Another possible approach is to embed different powders with the same host before sintering the slab to create a gradation of neodymium ions or incorporate the passive Q - switch.

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.

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.

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.


References:

W. Koechner, "Solid-state laser engineering", Springer, 6th edition, 2006, ISBN: 0-387-29094-X
http://www.rp-photonics.com/
http://cord.org/cm/leot/course03_mod03/mod03_03.htm
http://www.mrl.columbia.edu/
http://www.repairfaq.org/sam/lasers
http://www.colorado.edu/physics/
http://www.photonics.com/printerFriendly.aspx?contentID=87148&Publication=2
http://en.wikipedia.org/w/index.php?title=Diode_pumped_solid_state_laser&redirect=no
https://www.llnl.gov/str/April06/Soules.html
http://www.laserclub.org/crystal-laser.htm#
http://www.patentstorm.us/patents/5526372-fulltext.html

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