Abstract

Directional couplers are key elements in the design of many important optical devices such as power combiners, power splitters, modulators, switches, polarizers and filters. Inherently, the directional coupler is a frequency dependent device and the wavelength dependency of its coupling ratios finds application in the construction of wavelength-division multi/demultiplexer for exploiting the broad bandwidth available from an optical-fiber network. The material dispersion, waveguide dispersion, and the mode confinement of a waveguide mode change with the system operating frequency. This also changes the coupling coefficients and hence the coupling length for a pair of coupled waveguides. However, the frequency sensitivity of the power transfer efficiency is small, when the guides are identical, since only the effect of coupling length variation is utilized, as the two guides are always phased matched. On the other hand, if the guides are nonidentical, but phase matched only at the design frequency, then their power transfer efficiency will be very sensitive with the operating wavelength and it can be used as a realistic narrow band-width filter. So far most of the published simulation work is on the use of planar structures [1,2], and these are not suitable for practical applications. However, Burke et al. [3] have presented results for a nonsynchronous optical filter incorporating rib waveguides and using the spectral index method for the simulations. In the work presented in this paper, an accurate characterization of optical filters is carried out by using nonidentical coupled waveguides with two-dimensional confínement but using the more rigorous finite element method and the least squares boundary residual method.

© 1996 Optical Society of America