A theoretical and experimental analysis of SBS suppression through modification of amplifier seed
Theoretical and experimental investigations of stimulated Brillouin scattering (SBS) are conducted in Yb-doped fiber amplifiers when the amplifier is simultaneously seeded with multiple distinct frequencies or with a phase modulated signal. To this end, detailed models of the SBS process are developed consisting of both a steady-state approach described mathematically by a coupled set of ordinary differential equations and also transient effects described by a coupled set of partial differential equations. For the multi-frequency seeded case, the equations are solved in the steady-state limit and include the effects of four-wave mixing (FWM), intrinsic and external thermal gradients, and laser gain. In one configuration of the multi-seeded case, the signals are separated at twice the acoustic frequency of the fiber medium in order to create nonlinear Brillouin gain coupling between the seeds and Stokes signals, which suppresses the SBS process in the highest frequency seed. The concept is theoretically investigated for the two and three seeded cases. It is shown that for this scheme, FWM becomes quite significant making this concept unlikely in a practical application requiring single-frequency output. Alternatively, a novel concept is developed to suppress SBS in fiber amplifiers that relies on laser gain competition among multiple seeds to create both a favorable thermal gradient and a reduced effective length for the SBS process. In one configuration, the amplifier is simultaneously seeded with a broadband (Δ ) and single-frequency Δ seed. In this case, several experiments are performed to validate the theoretical predictions with experiments leading to a 203 W polarization maintaining (PM), co-pumped monolithic fiber amplifier demonstration. To the best of our knowledge, this output power is the highest reported in the literature to date for such an amplifier. A time-dependent model of the SBS process initiated from random thermal noise is also developed to study SBS suppression under phase modulated pump conditions. The SBS suppression is characterized for several phase modulation schemes. It is found that the SBS suppression for a white-noise phase modulation (WNS) which broadens the pump spectrum, depends significantly on the length of fiber and only in the long fiber limit follows the often quoted threshold enhancement formula of where , and describe the SBS threshold of the single-frequency case, the effective linewidth of the pump, and the spontaneous Brillouin linewidth respectively. In addition, the SBS threshold is characterized as a function of modulation amplitude and frequency for a single-sinusoidal phase modulation scheme.
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