Fiber Optics. IX. Waveguide Effects

In recent years techniques for fabricating fibers of diameter comparable to the wavelength have been developed in order to achieve high-resolution fiber optical systems. The performance of such fibers is analyzed by using dielectric waveguide theory, and the results are interpreted from the optical viewpoint of thin film interference and frustrated total reflection. The energy distributions inside (mode pattern) and outside such fibers are computed as a function of the fiber parameters, and the interaction of energy between neighboring fibers is considered. Experimental measurements of the fiber spread function are made for assemblies of close-packed fibers, and its dependence on fiber diameter (d), spacing (t), wavelength (λ), and indices of refraction (n1 and n2) is investigated. It is shown that the “optical diameter” of fibers and the number of modes propagated is critically dependent on the “fiber characteristic term” R given by R=(πd/λ)(n12-n22)12.