Wavelength dependence of the principal states of polarization in short segments of doped fiber

Due to various imperfections, such as core ellipticity, as well as internal and external stresses, single mode fibers are birefringent. In general, the magnitude and orientation of the birefringence varies randomly along the fiber. This leads to the well known phenomena of polarization mode dispersion in fiber-optic communication and to special polarization effects such as polarization tuning in fiber lasers. Er-doped fibers tend to have larger birefringence and differential group delay than un-doped fibers. The larger values of the birefringence and its wavelength dependence can be used in the design of fiber laser. We show here measurements of the differential group delay and of the principal states of polarization, and their wavelength dependence, in short Er-doped fibers. In addition, we present the polarization characteristics for a round-trip propagation in a doped fiber. This last measurement can be used to analytically describe the tuning characteristics of a fiber laser with intra-cavity polarizer.