Mismatched optical couplers with variable widths of waveguide tapered by Hamming function are numerically investigated in the demand of short-length, broadband, and low crosstalk. We used global search algorithm and beam propagation method to seek optimal structure parameters of coupling waveguide. The coupler length is 3.6 mm within the C L-band (1.53-1.61 mu m) for variable widths of waveguide at crosstalk level of -35 dB. Comparison with constant width of waveguide, the constant width of waveguide has a coupler length of 4.4 mm and can only achieve -20 dB of crosstalk within the C-band (1.53-1.565 mu m). Obviously, the waveguide with variable widths has the advantage over constant width for the demand of short-length, bandwidth, and low-crosstalk. (C) 2009 Elsevier GmbH. All rights reserved.
