Phosphate Glass

definition:
Glass materials mainly containing phosphorus pentoxide (P2O5).

Phosphate glass is a glass material mainly composed of phosphorus pentoxide (P2O5), and usually also contains other chemical components. It can serve as a laser gain medium and can also be used in bulk lasers and optical fibers. Its main advantage lies in its high compatibility with rare earth ions (see rare earth doped gain media), such as erbium (Er3+), ytterbium (Yb3+), and neodymium (Nd3+) ions.
Phosphate glass can be doped with high concentrations of laser active rare earth ions without causing adverse effects such as clustering, which can lead to quenching effects and affect laser performance. The doping concentration in phosphate doped erbium fiber is much higher than that in quartz fiber, and it is possible to increase it by several percentage points. This can achieve short fiber lasers and amplifiers, which is not the only advantage of this glass, but also includes:

• A short fiber laser resonant cavity can achieve a large free spectral range, making it easier to achieve single frequency operation.  
• Distributed feedback lasers themselves are very short, while highly doped phosphate fibers can achieve more effective pump absorption.  
• Ultra short pulse fiber lasers are less susceptible to nonlinear effects if shorter fibers are used.  

Some other properties of phosphate glass include:

1. The spectral range of high optical transmittance ranges from 0.4 μ m to 2 μ m, slightly narrower than quartz glass.  
2. The transition cross-section and upper energy state lifetime of rare earth ions in phosphate glass are very suitable. For example, fiber amplifiers suitable for obtaining high gain bandwidth have a wide and smooth spectral shape due to the transition cross-section.  
3. The transition temperature of phosphate glass is relatively low, about 365 ° C. Therefore, the port of phosphate glass is more prone to melting when heated during high-power operation. Therefore, when using phosphate glass to achieve high-power fiber lasers and amplifiers, attention should be paid to not melting the glass during pump injection.  
4. Phosphate glass has a much lower fiber damage threshold and much lower thermal conductivity than quartz glass.  
5. Unlike many gain media, its photothermal coefficient dn/dT is less than 0. This indicates that the direct heat has a negative impact on the thermal lens effect (defocusing), while the contribution of additional stress and expansion effects is greater than zero (focusing); The overall result is that the thermal lens effect is very weak.  
6. The nonlinear coefficient of phosphate glass is very low, three times smaller than that of quartz glass.  

The mixture of phosphate and fluoride glass is called fluorophosphate glass. Similarly, there are also phosphosilicate glass and aluminum phosphate glass. These glasses are also commonly used as laser gain media.  

The device obtained by combining phosphate fiber and quartz fiber may have problems because the two different materials have different transition temperatures, making fusion very difficult (although it is also possible).