LEAKY-TO-BOUND MODE TRANSITION IN A DEPRESSED-CLADDING PLANAR OPTICALWAVE-GUIDE

An analysis is given for the way in which leaky modes of an open plana r symmetric waveguide with depressed cladding migrate as the frequency increases into bound propagating modes. For simple step profiles it i s well known that leaky modes start from complex waves, evolve through a nonphysical transition as frequency increases, and then finally bec ome bound modes at cutoff and higher frequencies. We consider the case of depressed cladding (or W) profiles and show that the nonphysical t ransition attenuates with increasing depth and width of the depression ; i.e., the deeper and wider the depression, the narrower the frequenc y band over which the transition period exists. One indicated conseque nce is that leaky modes for the W profile have greater influence on th e continuous (i.e., radiation-mode) spectrum than do leaky modes of or dinary profiles. Some previously known effects of depressed cladding, such as nonzero cutoff of the fundamental mode, are derived in a new w ay from the underlying model, and a new phenomenon concerning behavior of the propagation constant at the onset of nonzero cutoff is identif ied.