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Design Of Optical Mux/Demux For Wdm Transmission Systems

Filter sidelobe suppression of optical waveguides with spatially tapered coupling strength is examined. Several taper functions (both continuous and quantized) are investigated and compared in order to optimize the filter response, the channel density, the crosstalk, the insertion losses and aspect ratio in prevision of utilizing this component in WDM transmission systems. The Blackman window seems to be the taper function which minimizes the quantization noise and the sidelobe strength. To this last end, the full coupling length must be divided in a suitable number of subintervals compatible with the available fabrication technology having also in mind the filter monolithic integration and its aspect-ratio. Finally a realistic design of tapered co-directional channel waveguide Ti:LiNb03 couplers is covered. The analysis of the filter is carried-out by effective-index method taking in account both in depth and lateral Ti-diffusion in stripe waveguide, the slab investigation being done by numeric integration of Maxwell's equations. The effects of wet and dry treatments are considered and it is found that the wet treatment not only does not eliminate the Li-out-diffusion between channels but also complicates the coupling characteristics because the proton behavior in Ti:LiNb03 crystal is not yet completely stated. As an example we have designed a coupler having a Kaiser taper function centered at λ = 1.02 μm with a bandwidth of 420 Å slightly greater than the uniform counterpart (BW = 400 Å). The channel separation necessary to obtain a crosstalk of -30 dB, however, is only 820 Å vs 3900 Å of uniform coupler.