Analysis of SBS Gain Shaping and Threshold Increase by Arbitrary Strain Distributions

Fibers with a longitudinal strain distribution can be used to shape the stimulated Brillouin scattering (SBS) gain spectrum or to increase the SBS threshold power. Theoretical analysis and experimental results on the SBS gain spectrum of coiled fibers with a continuous and arbitrary distribution of longitudinal strain are presented. The concept of a critical gain factor for determining the SBS threshold power with a simple formula is reviewed for fibers with different parameters and spectral shapes. A fiber coiling machine for high coiling forces is described for realizing permanent strain distributions with a maximum strain of more than 3%. SBS spectra were broadened to a spectral width of 1.7 GHz. Measurements verify a significant SBS gain suppression by a factor of 40, which is in excellent agreement with the theoretical analysis. Finally, synthetic triangular and broadened Lorentzian SBS spectral shapes with a spectral width of 550 MHz are demonstrated. The results can be used to suppress SBS in high-power fiber lasers and amplifiers or to tailor almost any arbitrary SBS spectral shapes, which could be useful for slow-light or active optical filter applications.