A mismatched optical coupler with waveguide weighted by the Blackman function is numerically investigated in the demand of short length, C+L-band, and low crosstalk. Utilizing the full factorial design, the structure parameters of coupling waveguide are obtained by beam propagation method. In the condition of crosstalk of -35 dB, the mismatched optical coupler with proper selected waveguide structure parameters is found to have a coupling length of 3.60 mm in the transmission wavelength ranges of C+L-band (1.53-1.61 mu m). Obviously, the selection and design of waveguide structure are very important to satisfy the qualities of a mismatched optical coupler for the demand of short length, broad bandwidth, and low crosstalk. (C) 2008 Elsevier B.V. All rights reserved.