Transmission of optical radiation by sharp bends in fiber-optical waveguides

A theoretical and experimental investigation is reported of the behavior of optical radiation near single sharp bends in quartz multimode fiber-optical waveguides. The theory uses conformal mapping of complex-variable functions to transform a curved waveguide into a straight one. Allowance is made for spatial distortions of the electromagnetic fields at the bend. The experimental method uses the experimental observation that almost all (~80%) of the optical radiation which has escaped from the core into the cladding in the bend area, is returned to the core when the waveguide becomes straight again. The technique involves measuring the dependence of the optical radiation intensity at the exit from the bent waveguide on the variable refractive index of the external medium in which the bend is immersed. By using this technique, it is possible to determine experimentally the interval of the propagation constants of the cladding modes excited at the bend, the intensity of the residual radiation which does not leave the waveguide core for the given bend radius, and the critical bend radius for which all the core radiation escapes into the cladding in the bend area. The theory satisfactorily explains the results.