source：techbriefs
Laser crystals can be considered the “engines” of solid-state lasers. They are used for gain media, for frequency conversion, and to manage laser characteristics and performance. Like the engine of a car, if laser crystals are clean and working properly, they allow the larger system to operate at a higher level. In the case of a laser system, operating at a high level means creating a stable beam and reaching high optical powers. Some advantages of laser crystals over other solid-state gain media are that they typically offer less absorption, a narrower emission bandwidth, higher transition cross-sections, and higher thermal conductivity. Laser crystals are critical for enabling a wide variety of applications including laser materials processing, laser surgery, sensing, defense applications like rangefinding, and more.
Because laser crystals are sensitive optical components and are often used with high-power lasers, depositing the correct coatings onto them without introducing any defects is essential. While the complex geometries and high laser-damage threshold (LDT) requirements make the fabrication of laser crystals challenging, keeping several key considerations in mind helps ensure that the crystal and its coating behave as intended.
Laser crystals are optical crystals typically doped with transition metals or rare earth ions. There are many different crystal types and shapes and each crystal has its own unique set of attributes that must be considered. Some common crystal shapes include rods, cubes, and zigzag slabs used to reduce thermal lensing and stress-induced birefringence.
Raw boules, or synthetically grown ingots, of crystals are cut, ground, and polished to the tightly-toleranced specifications needed for the application. The parallelism and perpendicularity of the different faces of the crystal must be tightly controlled since the alignment of the crystal inside a laser cavity is crucial to proper functioning. Protecting previously polished surfaces while polishing the other surfaces is critical to maintaining surface quality. Polishing is carefully monitored to minimize subsurface damage, which could lead to light loss and even complete failure if high-power laser light scatters off of defects or is absorbed.
In-process metrology ensures that requirements for surface figure, parallelism, perpendicularity, dimensional specifications, and surface quality have been met. Careful cleaning of all polished surfaces prior to the deposition of the coatings is also important for preventing the introduction of any contamination such as slurry or blocking substances. Ultrasonic cleaning removes any leftover polishing compounds before coating. This is especially helpful for cleaning ground surfaces, as they are harder to clean by hand than polished surfaces. Finally, a manual inspection using a high-magnification microscope verifies cleanliness and quality, determining if an additional manual cleaning step is required.
COATING
Most laser crystals have two surfaces that need to be polished and coated, but depending on the crystal geometry, up to six different polished and coated surfaces may be needed. Coating multiple surfaces increases the complexity of the coating process. The specific order in which coatings are applied must be considered to preserve the surface quality of the remaining crystal faces and not damage any coatings that have already been applied. The tooling and blocking techniques used during coating are also critical to protecting already-coated surfaces and preventing unwanted overspray onto other surfaces. Tooling is designed to allow for the expansion of different materials during coating without getting damaged. In certain cases, polishing and coating steps are alternated. This is common when the surfaces adjacent to each other are both coated all the way to their edges.
Thin film coatings are deposited to improve transmissive and reflection properties. The specific coatings used are entirely dependent on the end application’s wavelength, power levels, environmental requirements (temperature, humidity, vacuum, radiation, salt spray, etc.), laser design, and other factors. The coatings are applied as single-band and multi-band wavelengths according to the customer’s specifications. Chamber geometry and evaporation techniques are important parameters that must be met in order to have perfect uniformity between all the parts. Multiband coatings are very carefully designed for repeatability with very discrete layer thickness control to get low-loss, non-absorbing films. Sometimes a whole crystal slab is coated, diced into smaller pieces, and then coated again to cover the newly-created surfaces.
Electron-beam (e-beam) coatings are slightly porous, and their behavior can slightly shift based on absorbing moisture or temperature increases, which drive out absorbed moisture. Figure 4 shows an example of how a change in temperature can impact spectral performance. Historical data and testing at the end-application’s operating conditions inform how the crystal will behave in the field. Other coating techniques such as ion-assisted deposition (IAD) and ion beam sputtering (IBS) can minimize shifting or eliminate it altogether by compressing the films to limit moisture intrusion. However, these techniques may introduce stress to the crystal and lower its laser damage threshold (LDT), so all requirements must be prioritized against each other.
For extremely difficult specifications such as narrow-band or multiband coatings, the placement of each individual crystal in the coating chamber is important to maintain repeatability (Figure 5). Parts are specifically arranged in the chamber to ensure uniformity among all parts. Any thickness errors are evaluated to determine if they will affect the crystal’s final performance.
CONFIRMING LASER CRYSTAL QUALITY
A wide range of in-process and post-process metrology including spectrophotometers, interferometers, high-power microscopy, dimensional gauging, photothermal absorption, and laser damage testing is used to verify key specifications. This is essential for optical suppliers to be confident that all customer requirements are actually met.
As many laser applications continue to move to higher powers, maintaining tight dimensional tolerances, high laser damage thresholds, and precise spectral performance becomes increasingly important for laser crystals. Speak to your optical component supplier when sourcing laser crystals to ensure that they have factored in the above considerations into their quotes and designs. Aligning on these requirements early on will reduce the likelihood of design iterations and make it more likely that your crystals will behave as needed in your final system.