Measurements of beat length in short low-birefringence fibers

Summary form only given. We show a simple technique for birefringence measurements based on a fiber optic loop mirror (FOLM). We demonstrate birefringence measurements in fiber spans as short as 0.07 of a beat length. To measure fiber birefringence, we twisted the fiber and observed the dependence of the FOLM transmission on the fiber twist. The fine-adjustment of birefringence of one of the ports was needed in measurements. The adjustment mechanism produces low-birefringence by applying pressure over a fiber section of several millimeters and rotates the principal axes of this section.