Methods for Managing Stimulated Brillouin Scattering in Narrow Linewidth Fiber Raman Amplifiers

Abstract

Optical fiber amplifiers and lasers are becoming increasingly important for applications ranging from telecommunications, industrial manufacturing, remote sensing, and medicine because of their robustness and advantages in beam quality, energy efficiency, size, and weight over traditional solid state laser designs. But while technological advances in rare-earth doped active fibers have allowed for power scaling of such devices comparable to other solid state lasers, their spectral coverage has remained primarily limited to three discrete wavelength bands centered near 1.06, 1.55, and 2.0 µm where Yb3+, Er3+, and Tm3+ ions, respectively, exhibit optical gain. To meet increasing demands outside of the three commercially available spectral windows, alternatives have been proposed including fibers doped with additional rare-earth active ions and devices based on optical nonlinear frequency conversion. Fiber Raman amplifiers (FRAs) are one such technology that have recently received considerable attention. Unlike doped active fibers that rely on specific atomic transitions, FRAs provide gain through stimulated Raman scattering effects originating from wavelength independent resonant molecular vibrations of an optically transparent material. Consequently, FRAs can operate at wavelengths across much of the near and shortwave infrared spectrum where silicate-glass-based optical fibers have their lowest attenuation. Despite their advantage in increased spectral coverage, power scaling of FRAs remains particularly challenging due to the relatively low gain of silicate-glass fibers and the onset of parasitic nonlinearities including stimulated Brillouin scattering (SBS), especially when narrow linewidth spectral purity is required. Therefore, it is the aim of this dissertation to discuss FRA technology as it pertains to achieving high-power laser devices. Fundamental physics and material considerations are examined for increasing Raman gain within fibers and amplifier architectures are discussed including constraints in laser output power due to component limitations. The physics of SBS within optical fibers is thoroughly reviewed leading into a detailed discussion of several techniques that can be used for managing the formation of SBS in FRA devices. These include Raman gain fibers employing selective transverse doping profiles for tailoring the fiber acoustic waveguide properties, the use of cascaded fibers within the amplifier gain stage, and methods for suppressing SBS by introducing variations in waveguide parameters along the longitudinal direction of the gain fiber.