All-optical switching and intensity discrimination by polarization instability in periodically twisted fiber filters

Self-induced birefringence effects in a periodically modulated nonlinear medium are investigated. An input beam linearly polarized along the axis of a fiber polarization-rocking filter undergoes a spatial instability as the input power crosses a threshold value that scales linearly with the coupling strength. We describe the potential use of this effect for relatively low-power intensity discrimination, all-optical polarization switching, and small-signal amplification using linearly polarized beams. The wavelength for maximum polarization conversion between the axes can be tuned by varying the input power, and the filter bandwidth narrows with increasing power.