SBS suppression and acoustic management for high-power narrow-linewidth fiber lasers and amplifiers

Abstract

This paper discusses the transverse acoustic index design of Yb-doped large mode area (LMA) LP01 optical fibers that provide ~10 dB of SBS threshold suppression relative to conventional LMA fibers with homogeneous elastic properties and equivalent optical effective areas Aeff. SBS suppression is achieved with a ramp-like negative acoustic lens structure that refracts the electrostrictive density fluctuations away from the optical mode thereby reducing the acousto-optic interaction that generates the stimulated light scattering. The fundamentals of the SBS process and its mitigation are briefly reviewed. Two figures-of-merit (FsOM) are identified to quantify the SBS suppression capability; the SBS reflectivity RSBS and the SBS threshold power Pth. An initial design of an SBS suppressing Yb-doped double clad fiber is incorporated in the power amplifier stage of a 200 W cw singlefrequency (SF) four stage master-oscillator power-amplifier (MOPA). The MOPA is also exercised as a pulsed amplifier and is utilized to measure the SBS reflectivities and thresholds in passive (i.e. un-pumped) fibers with ~100 ns pulses exhibiting peak powers up to ~250 W. In separate experiments, the SBS suppressing fibers are incorporated into the final stage of the pulsed four stage MOPA and the SBS reflectivities of the active gain fibers are measured. Pulsed SF MOPAs with peak pulsed power outputs ~900 W are demonstrated and confirm the kilowatt SF performance capability of these SBS suppressing fibers.