16.1.1 Elliptical Core Optical Fiber

Most polarization-maintaining (PM) fibers work on the basic principle of creating two decoupled paths by introducing anisotropic stress generated by the different thermal expansions of the glass materials across the optical core region. In comparison, the elliptical core single-mode fiber employs the distinct property of geometry or form birefringence, rather than stress, to achieve its polarization-preserving characteristics. By separating the propagation constants of the two fundamental modes, the elliptical core fiber is able to reduce intermodal coupling. As a result, the polarization is maintained over a significant distance to be used in interferometric sensors.

In 1961 the first solution of an elliptical dielectric rod was published by Lyubimov et al. [1]. Later the next year, Yeh [2] outlined the extensive analysis of the elliptical core waveguide. When fiber optics emerged as the likely candidate for long-distance transmission lines, Dyott and Stern [3] analyzed the slightly elliptical core optical fiber as an annoying limitation in telecommunication links based on the group delay difference between the two orthogonal fundamental modes, which was followed by a study by Schlosser [4] of delay distortion due to elliptical deformation. Ramaswamy et al. [5] and Dyott et al. [6] soon realized that when the ellipticity and the core-to-cladding index difference are made sufficiently large, intermodal coupling is reduced, polarization is maintained, and elliptical core fiber applications eventually emerged generally concentrated in interferometric sensors. This larger...