Nonlinear Polarization Effects in Birefringent Optical Fiber

In many experiments involving nonlinear optics in fibers, the effect of the light polarization on the system is completely neglected. The fiber is often assumed to be either completely isotropic or to be highly polarization preserving. In either of these two cases, it is valid to assume that the light polarization does not change in the fiber and has little effect on the nonlinear process under investigation. The motivation for this work was to identify and research several areas in which the light polarization and the fiber birefringence play a key role. The fast axis polarization instability arises in a weakly birefringent fiber as a result of competition between the natural fiber birefringence and the nonlinear ellipse rotation. In this work, the first direct observation of the fast axis polarization instability has been reported. The effect has been analyzed theoretically, and excellent agreement was obtained between theory and experiment. As a result of the instability, very small variations in either the input power or the input polarization to the fiber result in large changes in the output polarization. A crossed polarizer at the fiber end converts the polarization variation into intensity information. Thus, the modulation depth of an input pulse has been increased from 15% to 100%. Modulation gains of as much as ten are theoretically possible. A powerful and increasingly common method to mode -lock a variety of lasers is additive pulse mode-locking (APM). In this technique, a nonlinear medium is placed in an external cavity which is coupled to the main laser cavity. As a result of interference at the common mirror between the pulses returning from each cavity, significant pulse shortening may be obtained. Optical fiber is often used as the nonlinear medium, and polarization effects and fiber birefringence are generally neglected. In this work, a computer model has been developed in order to simulate the effect of using a birefringent fiber. The simulations indicate that although the birefringence often has little qualitative effect on the system, in certain cases the birefringence may be exploited for enhanced mode-locking.