Suppression of stimulated Raman scattering employing long period gratings in double-clad fiber amplifiers.

We report on the suppression of stimulated Raman scattering (SRS) in a double-clad fiber amplifier using long-period gratings (LPGs). The LPGs, fabricated with a CO(2) laser, achieve SRS suppression by coupling the Stokes wavelength from the active core into the cladding. With only three LPGs inserted into a fiber pulse amplifier, the extractable Raman-free output power was nearly doubled. A numerical simulation of the setup shows good agreement with the experimental results.