Fiber-based single-frequency amplifiers, e.g. for gravitational wave detectors, are typically limited by the nonlinear effect of the stimulated Brillouin scattering (SBS). In addition to a decreasing output power level, SBS also initiates a broadband intensity noise in a frequency regime up to several MHz. Several published theories provide models to explain this excess intensity noise. Here, we report new theoretical and experimental studies, which suggest the conversion from phase to intensity noise by an asymmetric Brillouin gain spectrum.
Due to the asymmetric Brillouin gain profile, upper and lower frequency sidebands experience a frequency-dependent gain-factor and a differential non-zero phase shift. Thus, the sidebands do not cancel each other out and the differential non-zero phase shifts provides an additional contribution to the intensity noise. Using a measured asymmetric Brillouin gain spectrum and the 1/f-frequency noise characteristics of the pump source, we are able to reconstruct the shape of the excess intensity noise fairly well.
In addition, further evidence for an asymmetric SBS gain profile has been observed. Due to small back reflections at the fiber end faces corresponding resonances are evident in the intensity noise spectrum. Through the asymmetry of the SBS gain profile and the differential non-zero phase shift, the conditions for the resonator eigenfrequencies are influenced. Therefore, the beat frequency between the upper and lower eigenfrequencies with the carrier are not equal anymore, which becomes noticeable in a splitting of resonances in the intensity noise spectrum.
Sven Hochheim, Jörg Neumann, Dietmar Kracht, and Michael Steinke, "Broadband excess intensity noise due to an asymmetric Brillouin gain spectrum in optical fibers (Conference Presentation)," Proc. SPIE 10897, Fiber Lasers XVI: Technology and Systems, 108971R (Presented at SPIE LASE: February 07, 2019; Published: 13 March 2019); https://doi.org/10.1117/12.2508313.6013182744001.
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