We report on experimental and theoretical investigations of single frequency high power PCF amplifiers. A model describing the interplay among laser gain, thermals effects, and SBS was developed to study the power limitations of single frequency amplifiers in general, and PCF amplifiers in particular. A distributed noise term was used to initiate the SBS process with the Stokes light spanning multi-frequency channels. The use of thermal and acoustic gradients in conjunction was considered and indicated marginal improvement. In the set of experiments, slope efficiencies as high as 77% were obtained with a maximum output of 427 W. The linewidth was measured and yielded values that were less than 10 KHz. A pump-probe measurement of the Brillouin gain spectrum revealed secondary peaks lying at the highfrequency side. Measurements conducted on a novel PCF, specifically designed to utilize thermal and acoustic gradients in conjunction, showed the existence of two primary gain peaks.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.