Observation of Polarization Instability & Modulational Gain in Optical Fiber

Abstract

When birefringence induced via the optical Kerr effect in a weakly birefringent fiber is comparable to the natural fiber birefringence, polarization instability and asymmetry between the fast and slow fiber axes arise. A signature of the instability is that small changes in input intensity may result in large changes in intensity transmitted through a crossed polarizer at the fiber exit, leading to the possibility of substantial amplitude modulation (AM) gain. We use linearly polarized light with 15% amplitude modulation from a Q-switched Nd: YAG laser to make the first direct observations of both AM gain and the asymmetry. A polarizer which nulls the fiber output at low power serves as the analyzer. A fully modulated output beam may be obtained for an input near the fast axis, leading to an AM gain of 6. Theoretically, AM gain may be as high as 10 for a less modulated input beam. Light aligned near the fast axis also undergoes significant pulse shaping. At low peak power, the transmitted pulse is flat-topped. With increasing peak power, the central portion is transmitted more strongly than the wings. Substantial pulse narrowing is observed at high peak power. For light aligned near the slow axis neither significant pulse shaping nor AM gain is observed. The experimental results show excellent qualitative agreement with the theory.