Suppression of stimulated Raman scattering in high-power fiber laser systems by lumped spectral filters

Abstract

We present a systematic study on the inhibition of stimulated Raman scattering by lumped spectral filters both in passive optical transport fibers and in fiber amplifiers. This study reveals the parameters that have the strongest influence on the suppression of the Raman scattering (such as the attenuation at the Raman wavelength and the insertion losses at the signal wavelength). These parameters have to be optimized in order to achieve the desired Raman inhibition and/or to minimize the loss in amplifier efficiency. The study is concluded with realistic predictions on the use of spectral filtering elements for Raman scattering inhibition in real-world high power fiber amplifiers. Thus, using for example 10 lumped spectral filters with 20 dB effective Raman attenuation and less than 0.25 dB insertion losses, a maximum Raman threshold increase by a factor of 3 is expected. In this context, long period gratings are proposed as promising filtering elements for Raman inhibition in high power fiber amplifiers. In order to experimentally verify the theoretical predictions and the suitability of long period gratings, a fiber amplifier consisting of 2 m active Ytterbium doped fiber was built. Three long period gratings were consecutively inserted at different positions along the fiber, and the Raman threshold was determined for each situation. It is shown that, with three long period gratings, the Raman threshold (defined as the 20 dB ratio of Raman to signal output power) was increased by about 60%, which offers a good agreement with the theoretical predictions