Solid-state lasers are based on gain media such as glass or crystals. Both glass and lasers are used by optically pumping them with light, so they can amplify light in other spectral regions, usually at longer wavelengths.
Usually, laser glasses (typically phosphate and silicate glasses) are doped with trivalent rare earth metal ions like Nd3+, Er3-, Yb3+, etc. They are available in the form of finely polished cylindrical or cuboid rods. The end-faces may also be coated with anti-reflection coating.
Laser crystals are usually doped with laser-active transition metal ions, though they may also be doped with rare-earth ions. Commonly used host crystal materials are oxides like YAG (yttrium aluminum garnet), vanadates (gadolinium, yttrium, or lutetium vanadate), tungstate crystals, fluorides, borates, and apatites. Different types of doped laser crystals are suitable for specific applications.
Host crystals must be able to withstand extreme conditions imposed by laser radiation inside a laser cavity. Differences in hardness across hosts will affect how easily they may be cut and polished to maintain high quality. The most commonly-used garnet crystals doped with erbium or neodymium are highly versatile and have a variety of applications.
All crystals and glasses must be highly transparent so absorption and scattering rates are low. Comparing glass lasers doped with Nd gives versus Nd-doped crystals paints a picture of how the two types of materials behave.
Doped laser crystal properties
The uniqueness of the composition and various shapes of laser crystals strongly influence the output laser beam in doped crystal lasers. Laser crystals have a narrower absorption and emission bandwidth than laser glasses. This is because the laser-active ions typically occupy only a specific site on the crystal lattice and there is order.
Crystals also have higher thermal conductivity. This makes it easier to cool crystal lasers efficiently with fluid coolants and helps them achieve high output with high repetition rates. High power outputs are possible with doped crystal lasers.
One of the most common and widely-studied high-power laser is the Nd:YAG solid-state laser. It’s typically doped with around 1% (atm.) of neodymium. It’s widely used for rangefinding, material processing, laser target designation, surgery, pumping other lasers, and research.
But Nd:YAG lasers, like most crystal lasers, have very narrow absorption bands. As a result, even though repetition rates of 400 pps and higher can be achieved with a high output with a xenon-krypton flash lamp, efficiency lies at around 1%. Nd:YAG lasers are usually pulsed down to fractions of a nanosecond.
A crystal laser also has higher transition cross-sections as compared to glass. This leads to a higher chance of induced transition due to radiation. In other words, a crystal laser has a higher probability of absorption or stimulated emission.
Doped laser glasses
Probably the biggest advantage of rare-earth-doped glasses is they allow for a larger gain bandwidth than laser crystals. Laser-active ions in glasses can exist in various environments and lead to inhomogeneous broadening of transitions. Laser glasses allow solid-state bulk lasers and amplifiers to extract a higher maximum gain for a given frequency. Broader wavelength tuning ranges are possible as a result.
Transition-metal doped laser gain media also tend to offer larger tuning ranges than rare-earth-doped media. This is because electrons in the former have a stronger interaction with the host lattice. Output powers in glass lasers can reach hundreds or thousands of milliwatts.
In ultrashort pulse lasers, the passive mode-locking technique can generate shorter pulses in doped laser glasses than in crystals. Short laser pulses have great benefits in medical applications, material processing, and research.
The shorter, intensive light beams help to achieve extremely precise cut edges, minimizing temperature load in the immediate surroundings. It increases the precision of surgical procedures and reduces complications, such as in eye surgeries. Tattoos are removed with minimal damage to the tissue around them. In areas like multiphoton microscopy, ultrafast spectroscopy, and optical coherence tomography, ultrashort pulse lasers allow the precise observation, recording, and influence of ultrafast processes.
Glasses are therefore more commonly used in the form of bulk pieces or rare-earth-doped optical fibers based on silica hosts. These fibers have high optical confinement, which allows lasers to operate even on difficult transitions where gain efficiency is low.
Special fiber glasses, such as those made of fluoride glass, have especially low phonon energies. This means transmissions in the mid-infrared region are good and metastable state lifetimes are longer.
Lasers relying on upconversion processes often use glass gain media.
Disadvantage: The disadvantage of glass over crystal is it has a much lower thermal conductivity. As a result, the waste heat is retained in the material for longer. This leads to a greater rise in temperature and a slower transition from lower lasing level to the ground state in Nd:glass. Lasing is therefore quickly stopped as the temperature rises. As a result, Nd: glass lasers can only operate in the pulsed mode.
Such glasses can’t be used in high-power lasers because they could suffer thermal fracture, thermal lensing, and thermal depolarization. It’s harder to achieve a high beam quality and linear laser light in laser glasses, though it’s possible to an extent by a small dn/dt value.
Inferior thermal properties also lead to higher threshold pump power and a stronger tendency of these lasers for instabilities of passively mode-locked lasers where the pulse energy is unstable.
To Sum Up
Here’s a quick look at how doped laser glasses compare with doped laser crystals:
Lower average cross sections of ions in glass due to inhomogeneous broadening
Higher transition cross-sections
Broader transitions with bandwidths ~ tens of nanometers; allows large wavelength tuning & ultrashort pulses
Smaller absorption & emission bandwidth, of a few nanometers or less
Lower thermal conductivity due to high microscopic disorder = lower output powers
Higher thermal conductivity, higher power outputs possible
Laser-induced birefringence in some fused silica glasses
Larger glasses of high optical quality are easy to produce
Growing large crystals is slow and difficult
Doping concentrations of glasses and crystals must be carefully optimized for efficiency. In the short length, high doping densities can lead to better pump absorption. But at the same time, there may be loss of energy due to quenching.