Polarization instability in a twisted birefringent optical fiber

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Abstract
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. Direct observation of the fast-axis polarization instability is reported. A full theoretical development of the polarization instability in a twisted, birefringent optical fiber is presented. The theory includes the derivation of and full solutions for the evolution of light in a twisted fiber as well as stability analysis and phase-plane representation of the solutions. The experiment is described in detail; good agreement is 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 10 times are theoretically possible.