Investigations on the Extreme Frequency Shift of Phosphosilicate Random Fiber Laser

Random fiber lasers (RFLs), which employ random distributed feedback (RDFB) provided by the intrinsic Rayleigh scattering (RS) along the passive fiber, have gained wide attention in the last decade. A major advantage of Raman gain based RFL is wavelength flexibility, and the so-called cascaded Raman conversion can extend the operating wavelength considerably. However, frequently used silica fibers often need multiple Raman shifts to reach the final desired wavelength due to its peak frequency shift of ~13.2 THz. A good alternative is the phosphosilicate fiber which has a gain peak at ~39.8 THz. Nevertheless, the simultaneous existence of silica-related and phosphorus-related gain peaks in the phosphosilicate fiber intensifies the gain competition, further limiting the power scaling and the enhancement of spectral purity, even making it difficult to reach the desired wavelength. In this article, we experimentally explored the influences of the pump wavelength and bandwidth on the lasing performance of a phosphosilicate RFL, and measured the extreme frequency shift of the phosphorus-related Raman gain peak. Besides, by the aid of a modified power balance model, we qualitatively simulated the impacts of pump spectrum and boundary condition on the laser output. The simulation agrees well with the experimental results, and may also explain the parasitic lasing in the prior reports of phosphosilicate RFLs. As a result, the extreme frequency shift of phosphosilicate RFL is measured to be ~1 THz. And through optimizing the pump wavelength and pump bandwidth, 99.24% high spectral purity random lasing at 1237 nm with 33.1 W output power is obtained. This work may help to reveal the gain competition in phosphosilicate RFL and guide the future laser design.