Glass Lasers and Amplifiers
Glass lasers are those solid-state lasers where the gain medium is a laser glass, as opposed to a laser crystal or a ceramic laser gain medium. Similarly, optical amplifiers can be built with such laser gain media.
In most cases, a laser glass is doped with trivalent rare earth ions such as Nd3+, Yb3+, Er3+, Tm3+, Pr3+ or Ho3+. Most common are lasers and amplifiers based on Nd3+-, Yb3+- and Er3+-doped silicate or phosphate glasses.
Usually, glass lasers are meant to be bulk lasers, based on a solid glass piece in the form of a cuboid or a rod. However, fiber lasers are also almost all based on some glass, which is drawn into a fiber and provides a waveguide.
Large Glass Lasers and Amplifiers
One of the main attractions of laser glasses (in contrast to laser crystals) is that they can be fabricated in very large sizes at a reasonable cost. This is essentially because no long-range microscopic order needs to be achieved – in contrast to single crystals, where it is difficult or at least very time-consuming to grow large pieces.
A large glass gain medium allows one to store large amounts of excitation energy. That energy is typically extracted either by Q switching in case of a laser or by single-pass or multi-pass amplification in case of an optical amplifier.
Large Gain Bandwidth
Reduced Emission Cross Sections
The peak emission cross section and gain of a laser glass are substantially lower than those values of a laser crystal with a similar doping concentration; this is essentially because the optical transitions are “smeared out” over a wider spectral region.
In case of a Q-switched laser, that can be even advantageous: the lower gain makes it easier to suppress premature lasing, i.e., the Q switch does not need to introduce very high losses before pulse build-up. In case of passive Q switching, one obtains a higher pulse energy for a given mode radius, essentially because of the higher saturation fluence of the gain medium. However, a high-gain laser crystal is preferable for achieving the shortest possible pulse duration.
Even for an optical amplifier, the reduced magnitude of emission cross sections can be beneficial, because the gain could otherwise become unnecessarily high, and gain saturation would become detrimental. However, the high saturation fluence can make it impossible to extract all the stored energy with a single ultrashort pulse, because a too high peak intensity may cause laser-induced damage. Chirped pulse amplification can then be a solution which is also reduces strong nonlinear effects.
On the other hand, the reduced transition cross sections are often detrimental, e.g. when an amplifier with high gain is needed, or a Q-switched laser for very short pulses.
A definite disadvantage of glasses is the relatively low thermal conductivity. Therefore, a glass laser or amplifier cannot be operated with a high pulse repetition rate and correspondingly high average power; it would otherwise overheat for excessive thermal loading, and this could lead to damage (e.g. breaking of the glass) or to strong thermal induced beam distortions.
Broad Pumping Transitions
Not only the laser transitions, but also the pump transitions are spectrally quite broad in glasses. This is advantageous because more broadband pump radiation can be used. One may thus use laser diodes without wavelength stabilization; in some cases, even lamp pumping (e.g. with xenon lamps) is used. For rather low pulse repetition rates and moderate average output powers, the latter can be a quite economical solution despite the low wall-plug efficiency.
Application Example: Laser-induced Nuclear Fusion
An interesting application of large glass laser amplifiers is inertial confinement fusion (e.g. at the National Ignition Facility (NIF) in Livermore, California), where an extremely high pulse energy (in the megajoule region) in combination with a pulse duration in the femtosecond domain is required. Here, glass amplifiers are basically the only available option.
The NIF glass amplifiers are based on over 3000 neodymium-doped phosphate glass slabs, each with a weight of 42 kg . They are intensely pumped with flash lamps, but only with a couple of shots per day. The extraction of the high energy with high beam quality requires very sophisticated engineering.
Small Glass Lasers
Other glass lasers are based on a small cuboid or rod-shaped piece of laser glass with dimensions of only a few millimeters, and are usually diode-pumped. Typically, one uses a glass rather than a laser crystal because of the broader gain bandwidth, which allows one to obtain relatively short pulses in the picosecond or even femtosecond domain, or in other cases wavelength tuning over a wider spectral range.
Due to the non-ideal thermal properties of the laser glass, the output power (average power in case of pulsed operation) is usually limited to well below 1 W at least in cases where diffraction-limited beam quality is needed, although there are exceptions . In that respect, a fiber laser could easily perform better, but a bulk glass laser allows one to realize a much shorter laser resonator, which is required or at least desirable in various cases. For example, one can realize mode-locked lasers with very high pulse repetition rates , one can more easily achieve single-frequency operation .
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|||Information on glass amplifiers at the National Ignition Facility, https://lasers.llnl.gov/about/how-nif-works/beamline/amplifiers|