Polarization properties of a twisted fiber laser.

In this Letter the effects of the twist of fiber laser cavities on the polarization properties of the laser output are investigated experimentally and theoretically. The directions and frequencies of two laser eigenpolarization modes are found to evolve in a much more complicated way as a function of the twist rate, compared with the well-known case of polarization evolution in a twisted passive optical fiber.1 The basic difference between the two cases is a result of the fact that a fiber laser with a linear cavity formed by two mirrors must satisfy an additional condition that the state of polarization (SOP) must come back to the original SOP after a complete round trip inside the cavity.2 The polarization characteristics of fiber lasers under lateral stress, strain, and temperature change have been studied and utilized in recently introduced polarimetric fiber laser sensors.2 – 4 In those studies the fiber laser cavities had linear birefringence, and the external perturbations altered the magnitude of the birefringence, resulting in the change of beat frequency between the two eigenpolarization modes. The change in the polarization-mode beat (PMB) frequency is monitored for the measurement of external perturbations, providing a linear scale factor and a frequency readout without requiring complicated signal processors. The directions of eigenpolarization modes are aligned with the birefringence axes of the fiber. Here we deal with the response of the laser output to another important and frequently encountered external perturbation; i.e., the twist of the cavity fiber. Although the change in the PMB was observed when a fiber laser was twisted,4 to our knowledge the basic mechanism has not been understood. Twisting a fiber induces circular birefringence, and if the fiber has intrinsic linear birefringence, as is usually the case, it results in a general elliptical birefringence.1 The following analysis therefore assumes a combination of linear and circular birefringence. We also found that the existence of short untwisted fiber sections with linear birefringence at the ends of the fiber laser cavity