drawings fig1 shows a planar waveguide 10 comprising an inner dielectric member 11 and two outer dielectric layers 12 and 13 which are in contact with the major surfaces of member 11 in order to provide wave guidance primarily within the inner dielectric member 11 or core region of this waveguide the refractive index of the outer layers is less than that of member 11 notwithstanding the fact that the width w of the core is many times greater than its thickness t such a waveguide is capable of propagating optical wave energy polarized along directions parallel to both transverse dimensions of the core region in the absence of any extraneous coupling mechanism a beat length l can be defined within which energy is completely exchanged between the two orthogonally polarized waves i e the energy reappears in the same polarization after completely being transferred to the other polarization for a single mode fiber this length l is given as where u03b4 u03b2 is the difference in the propagation constants of the two orthogonally polarized waves it is apparent that by increasing u03b4 u03b2 the beat length can be reduced since mechanical perturbations having spatial periods that are comparable to the beat length cause unwanted coupling from one polarization to another the beat period is advantageously made smaller than the perturbation periods typically introduced by the fabrication process or.