Laser glasses (or laser-active glasses) are optical glasses (mostly silicate and phosphate glasses) which are doped with some laser-active substance – in most cases, with trivalent rare earth ions such as Nd3+, Yb3+, Er3+, Tm3+, Pr3+ or Ho3+. In principle, glasses can also be doped with laser-active transition metal ions, but that is not common; such ions are mostly used in laser crystals.
Laser glasses (e.g. in the form of cuboids or cylindrical rods with fine-polished and possibly anti-reflection coated end faces) are used as gain media of some solid-state bulk lasers and optical amplifiers. (Such devices are called glass lasers and amplifiers.) The use of such glasses is in principle similar to that of laser crystals and ceramic laser gain media: they are optically pumped with light for which the host glass material is transparent and can then amplify light in some other spectral region(s), typically at longer wavelengths. Laser glasses are also used in fiber lasers and amplifiers.
See the article on optical glasses for fundamental properties of such glasses, not including their laser properties. The following sections explain the main differences between ion-doped laser crystals and glasses.
Note that the term laser glasses is sometimes used with an entirely different meaning: laser safety glasses.
The optical properties of laser glasses are in principle similar to those of laser crystals: there is good transparency in a certain spectral window between the ultraviolet absorption (for photon energies above the band gap energy) and the infrared absorption related to phonon generation.
Glasses are generally optically isotropic, and it is thus not possible to obtain well-defined birefringence. At the same time, there is some thermally induced birefringence, which is usually unwanted. These properties can be a disadvantage when linear polarization of a laser output is needed: enforcing polarization can lead to substantial depolarization loss.
The refractive index of glasses is typically lower than that of the laser crystals, and its temperature dependence is sometimes weaker substantially. The coefficient dn/dT can even be negative.
In contrast to optical crystals, glasses exhibit a certain amount of propagation losses due to their irregular microscopic structure. For the quite limited length of laser glasses, however, such losses usually do not have a substantial detrimental effect; they are usually far smaller than the laser gain.
Optical Transitions for Pumping and Laser Amplification
Rare-earth-doped laser crystals often exhibit sharply defined laser and pump transitions, which typically have a bandwidth of a few nanometers or less. In contrast, laser glasses typically have much broader transitions with bandwidths of the order of tens of nanometers. This difference arises from the fact that in many laser crystals the laser-active ions occupy only a specific site of the crystal lattice, so that all laser-active ions see the same surroundings, whereas glasses offer many different environments to these ions, so that strong inhomogeneous broadening may be expected. Nevertheless, inhomogeneous broadening is not always observed in glasses; for example, Nd3+-doped phosphate glasses often exhibit approximately homogeneous broadening, and their laser performance can be modeled quite accurately with rate equation modeling.
Due to the anisotropy, pump absorption and laser gain in glasses are generally not dependent on polarization or the direction of propagation.
Broadband laser transitions are useful for wavelength tuning of lasers and also for the generation of ultrashort pulses. At the same time, the also broad bandwidth of pump transitions is beneficial because one does not require tight wavelength tolerances for the pump source; one may e.g. use laser diodes without wavelength stabilization.
As the oscillator strengths of laser transitions are often similar for crystals and glasses with the same dopant ions, the broader transition bandwidths of glasses imply lower effective transition cross-sections. (The transitions are “smeared out” over a larger spectral range.) This leads to a higher threshold pump power of a laser, a stronger tendency for spiking behavior, and (for passively mode-locked lasers) a higher tendency for Q-switching instabilities. In an optical amplifier, the gain is broadband, but the peak gain is usually smaller. Also, the saturation intensity and saturation fluence are correspondingly higher, which can be a problem for efficient extraction of stored energy with a single ultrashort pulse in an ultrafast amplifier.
The strong microscopic disorder in glass materials leads to a reduced thermal conductivity. This is detrimental for high-power lasers, mainly because it can lead not only to thermal fracture, but also to strong thermal lensing and thermal depolarization. It is thus more difficult to achieve a high beam quality and linear polarization of generated laser light. To some extent, however, thermal problems can be mitigated by a small or negative dn/dT value.
Production of Laser Glasses
Laser glasses are produced in similar ways as passive optical glasses, just with the addition of some laser-active substance. However, one often adds further substances:
- Some substances (e.g. alumina in silicate glasses) help to better incorporate the laser-active dopant ions, i.e., to avoid clustering of those ions at high doping concentrations.
- Some additional dopants help one to optimize the spectral dependence of the transition cross-sections, for example to obtain a smooth gain profile for ultrashort pulse generation.
- Some dopants help to absorb pump radiation in additional spectral regions and exhibit energy transfer processes to the laser-active ions. Examples of such combinations are Cr/Nd, Sm/Nd, Er/Yb and Tm/Ho.
- In other cases, the depopulation in the lower laser level is the function of a dopant.
The precise chemical composition of a laser glass is often not publicly known; one may e.g. only know that it is a potassium-aluminum-phosphate glass, but not what the relative amounts of various dopants are, and some dopants may not be disclosed by the manufacturer. The composition may not even be completely consistent even between multiple badges of the nominally same glass from the same manufacturer. When switching to a different manufacturer, one may often not be able to obtain a very similar glass.
Apart from the nominal composition, it can be important to avoid contaminations with various unwanted chemical elements. For example, glass melts are often prepared an platinum crucibles, and it may then happen that platinum is oxidized, gets into the glass melt, and is later chemically reduced to metallic platinum again. The metal tends not to be smoothly distributed, but rather to form small platinum inclusions, which can be problematic particularly in high-energy glass amplifiers: they cause parasitic absorption, and the resulting heating can lead to breaking of the glass. Therefore, improved methods, for example based on using an inert gas during the processing, have been developed to obtain virtually platinum-free glasses. This can be taken as an example for the refinement of glass making technology for advanced laser applications.
Laser glasses can be produced in very large dimensions with good optical quality, since no long-range microscopic order needs to be obtained. In contrast, the growth of large laser crystal is rather difficult and time-consuming. However, there are also ceramic laser gain media which can be made in large sizes similar to laser glasses.
While the properties of common laser crystals are often largely known from the scientific literature and often do not widely vary between different manufacturers, the properties of laser glasses with their often proprietary composition are generally not as well known. Therefore, it is particularly important that a manufacturer provides comprehensive specifications concerning many properties:
- basic properties like mass density, doping concentration, Young’s modulus, Poisson’s ratio, fracture toughness
- passive optical properties like refractive index, chromatic dispersion parameters, propagation loss, nonlinear index
- laser-optical (spectroscopic) properties: wavelength-dependent transition cross-sections for absorption and emission bands, upper-state lifetime
- thermal and thermo-optic properties concerning thermal expansion, thermal conductivity, heat capacity, the temperature dependence of the refractive index or the optical path length, elasto-optic coefficient etc.
Comprehensive specifications and the consistency of all properties are important aspects of glass quality.
Use as Optical Fibers
There are also crystalline fibers, which are usually just thin and long rods (and fairly rigid), that have interesting properties but can not offer the versatility of glass fibers.
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See also: optical glasses, glass lasers and amplifiers, laser crystals, laser gain media, rare-earth-doped laser gain media, neodymium-doped laser gain media, ytterbium-doped laser gain media, ceramic laser gain media
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