Optimization of Chirp and Tilt of Fiber Bragg gratings for Raman Emission Suppression

In recent years, chirped and tilted fiber gratings (CTFBG) have become promising all-fiber components for the suppression of Stimilated Raman Scattering (SRS) in high power fiber lasers (HPFLs). A CTFBG acts as a smooth wideband filter with an engineered central wavelength. It can be used to couple SRS light from fiber core mode to fiber cladding modes [1] . However, a CTFBG may fail to achieve SRS suppression because of the residual core-to-core Bragg reflection that typically attains −20 dB [2] , [3] . Moreover, the chirp direction with respect to light flow has an impact on the reflected spectrum of a CTFBG, and thus one direction must be advantageous over the other. Here, we report the impact of the tilt angle and chirp direction of a CTFBGs inserted at the output of a HPFL. Several CTFBGs are compared, with tilt angles spanning 4 ◦ − 6 ◦ and for both chirp directions. We conclude that CTFBGs optimal for SRS suppression should be oriented with SRS entering from the long wavelength chirp side, as well as their tilt angle should be close to 5 ◦ .