Why erbium-doped glass?
In the last several decades, significant advancements have been made in phosphate-based laser technology in terms of improved output power, shorter pulse duration, reduced system size, and new operating wavelengths. Er:Glass lasers commonly emit at the eye-safe wavelengths of 1540nm, 1550nm, or 1570nm, which is highly-beneficial in rangefinding and other situations where people may be exposed to the beams. These wavelengths benefit from high transmission through atmosphere. 1540nm also experiences minimal absorption by melanin, making Er: glass lasers optimal for aesthetic laser applications on patients with darker complexions.
Figure 1. Energy states of erbium. Er:Glass lasers are typically pumped with a 800nm or 980nm laser and emit at 1540nm or 1570nm.
Phosphate glass reaches high transmissions and can be doped with rare-earth atoms such as erbium and ytterbium so that it can reach population inversion and lase when exposed to a pump wavelength of 800nm or 980nm (Figure 1). Er: glass could also be pumped by photons at 1480nm, but this is undesirable because efficiency could be lowered by pumping and stimulated emission occurring in the same wavelength and energy band. Phosphate glasses also benefit from chemical stability and high laser-induced damage thresholds (LIDTs), making Er:Glass and other doped phosphate glasses ideal candidates for NIR laser gain media.
Phosphate glasses have a higher solubility of rare earth ions than silicate glasses, which feature a more rigid matrix structure. However, they feature a narrower bandwidth than silicate glasses and are slightly hygroscopic, meaning that they absorb more moisture from air. Therefore, they are limited to applications in their bandwidth and systems where they will be sufficiently protected from moisture by coatings or other optics.
Many of the applications discussed earlier, particularly laser rangefinding for defense applications, often require small Er: glass components with extremely tight dimensional tolerances. These finely-polished slabs of laser glass can then be dropped into assemblies with little to no alignment required. They can get down to the size of a SIM card and often do not feature bevels because they are so small (Figure 2). This makes edge chipping more likely. Achieving tight parallelism and surface quality specifications on these small components can be incredibly challenging. The clear aperture, or portion of the optical surface that must meet all specifications, is often nearly 100%, leaving little to no room for error around the edges of optical surfaces.
Figure 2. Er:Glass slabs used for laser rangefinding and other NIR laser applications are often the size of a common SIM card or smaller.
So why go through all of this trouble? Previous solutions often involved larger subassemblies of multiple crystal components attached to a Nd: YAG bar. These additional components could include Brewster plates, saturable absorbers for passive Q-switching, or frequency conversion crystals. Frequency conversion crystals are important in rangefinder or other open-air applications because the emission wavelength of neodymium is much more hazardous than erbium and must be shifted to a longer wavelength before it can be safely transmitted over long distances.
Rangefinder applications often have a shock and vibration requirement, which makes bonding multiple components together while meeting all specifications difficult. Moving from these old designs to a single, polished piece of Er: glass accomplishing the same tasks with various coatings reduced system size and cost. YAG crystals are often used at Brewster’s angle, but the same effect can be accomplished by using coatings. Since Er:Glass slabs need to be coated anyway, it is beneficial to add in this type of coating to pack in as much functionality as possible and save cost elsewhere.
Because phosphate glasses are slightly hygroscopic, if uncoated Er: glass is left outside for several days it can degrade. Surface quality must be controlled prior to coating to prevent moisture from moving into the glass. Coatings deposited on the polished surfaces of the final glass slab help protect them from this degradation.
Common specifications for small, high-precision Er: glass slabs are <5 arcmin perpendicularity for the edges, <10 arcsec of perpendicularity for the ends, and a surface quality better than 10-5 scratch dig. These demanding specifications require a clean environment, highly controlled processes, and minimized touch time.
Laser glass normally just has two polished surfaces on the ends while the rest of the surfaces are ground, but some of the sides of these Er: glass slabs are also polished and highly toleranced to simplify alignment. The choice of which sides to polish and coat first, which sides to polish before or after dicing, and when to use single-side or double-side polishing all determine cost and yield. The difference in yield between an uninformed process and a process optimized by an experienced manufacturer can easily be as large as a factor of three.
In order to reduce touch time and improve yields, it is optimal to have all manufacturing and coating performed at a single location. Every time the partially finished part is shipped between different locations the likelihood of contamination and damage greatly increase, along with additional queue time.
MULTIPLE HIGH-LIDT COATINGS
One challenge with manufacturing small Er:Glass slabs for rangefinding and other precision NIR applications is that multiple coatings are often deposited on different facets of the component. This is difficult because of the required fixturing and protection of pristine uncoated surfaces before coating. It is also a challenge for manufacturers to avoid overspray or blow-by on the back side of the slab, which has to be protected during coating. The ends feature anti-reflective (AR) coatings with high lase-induced damage thresholds (LIDTs). The edges also feature high LIDT AR coatings to let in the pump beam. The pump power is always higher than that of the emission. Some four-sided slabs even have additional coatings for built-in high-reflectivity cavity mirrors, wavelength discrimination, and pump light rejection.
——From one article written by Cory Boone and Mike Middleton.