An analysis is given for the way in which leaky modes of an open planar symmetric waveguide with depressed cladding migrate as the frequency increases into bound propagating modes. For simple step profiles it is well known that leaky modes start from complex waves, evolve through a nonphysical transition as frequency increases, and then finally become bound modes at cutoff and higher frequencies. We consider the case of depressed cladding (or W) profiles and show that the nonphysical transition attenuates with increasing depth and width of the depression; i.e., the deeper and wider the depression, the narrower the frequency band over which the transition period exists. One indicated consequence is that leaky modes for the W profile have greater influence on the continuous (i.e., radiation-mode) spectrum than do leaky modes of ordinary profiles. Some previously known effects of depressed cladding, such as nonzero cutoff of the fundamental mode, are derived in a new way from the underlying model, and a new phenomenon concerning behavior of the propagation constant at the onset of nonzero cutoff is identified.
© 1993 Optical Society of AmericaFull Article | PDF Article
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