Losses and Impulse Response in Parabolic Index Fibers with Square Cross Section

Multimode optical fibers whose cores have parabolic distributions of the refractive index, 1 - 3 (1) are of great practical interest for light transmission over long distances, since their delay distortion is much less serious than t h a t of conventional clad fibers. Since no optical fiber can ever be produced free of random imperfections, it is important to know how statistical irregularities of the fiber affect its performance. Random irregularities of the fiber axis and random changes of the effective width of the fiber cause coupling among its modes. T h e mode losses are functions of the mode number. Absorption losses tend to affect all modes in the same way. However, if we assume t h a t the fiber boundary either consists of an absorptive material or is a rough surface t h a t scatters light, we must expect t h a t higher order modes, whose fields reach strongly into the neighborhood of the fiber boundary, suffer much higher losses t h a n lower order modes t h a t are confined to the vicinity of the fiber axis. Coupling of the low-order modes to the high-loss, high-order modes increases the 195