Experimental optimization of high power Raman fiber lasers at 1495 nm using phosphosilicate fibers

Abstract

We report in this paper on the experimental optimization of high power Raman fiber lasers at the second Stokes order of a phosphosilicate (P-doped) fiber doped with 13 mol% P 2O 5. The lasers were pumped with a 20 W Yb double clad fiber laser at 1070.75 nm and the cavity used two pairs of fiber Bragg gratings (FBGs). One pair is highly reflective at 1248 nm to fully cascade the pump wavelength to the first Stokes order, and the other includes a partial reflective (PR) FBG at 1495 nm to couple the laser out of the cavity. A total of 20 laser configurations were constructed and tested, consisting of four PR FBGs couplers with reflection from 4% to 45% and five fiber lengths from 200 to 1000 m. It is found that the optimal configuration is that with a fiber length of 300 m and a 33% PR FBG. The maximum power obtained from this laser is 6.66 W at 1495 nm with a threshold of ∼1 W and a slope efficiency of 39.18%. The highest slope efficiency achieved with pump power up to 18 W is 55.6% in the shortest fiber laser of 200 m without any PR FBG but a cleaved end. It is observed that the input–output characteristics are almost linear for all the configurations with feedback of PR FBGs ⩾15%, indicating the stimulated scattering to higher Stokes orders is negligible. Laser line width is also observed to become broader with increase in pump power, fiber length and the reflection of the PR FBGs.