Birefringence and polarization mode dispersion in a coil of a single-mode fiber

Modal birefringence introduced by lateral stress resulting from bending is evaluated experimentally as a function of bending curvature in wide region of bending radius from 2.2 to 30 mm by a technique utilizing birefringent-matching stimulated four-photon mixing at pump wavelengths of 1.06 and 1.34μm. Wavelength dependence of polarization mode dispersion is also measured in the 0.8–1.6-μm wavelength region by an improved spatial technique based on optical heterodyne detection. The results obtained experimentally on these quantities are compared with those calculated theoretically. It is found that the birefringence evaluated experimentally agrees well with that calculated theoretically, even for a bending radius as small as 2 mm. As for polarization mode dispersion, the theoretical evaluation of the normalized frequency dependence of the modal dispersion agrees well with that obtained experimentally with respect to curve tendency against the V value and the magnitude of the dispersion far from the cutoff V value. However, it is observed that the modal dispersion drastically decreases with the V value in the region of 1.6 < V < 1.8 in the experiment, whereas the theory predicts that the dispersion becomes constant over the V-value region of V > 1.2. This discrepancy is considered to be due to difference between the actual stress distribution resulting from bending and the calculated one obtained by using the slab approximation to evaluate bending-induced birefringence.