Comments about Birefringence Dispersion, with Group and Phase Birefringence in Polarization-Maintaining Fibers

A recent JEOS-RP publication proposed Comments about Dispersion of Light Waves, and we present here complementary comments for birefringence dispersion in polarization-maintaining (PM) fibers, and for its measurement technique based on channeled spectrum analysis. We start by a study of early seminal papers, and we propose additional explanations to get a simpler understanding of the subject. A geometrical construction is described to relate phase birefringence to group birefringence, and it is applied to the measurement of several kinds of PM fibers using stress-induced photo-elasticity, or shape birefringence. These measurements confirm clearly that the difference between group birefringence and phase birefringence is limited to 15-20% in stress-induced PM fibers (bow-tie, panda, or tiger-eye), but that it can get up to a 3-fold factor with an elliptical-core (E-core) fiber. There is also a surprising result with a solid-core micro-structured PM fiber, that is also based on shape birefringence.