A new weighting function we developed called the broadband weighting function is reported. It is numerically shown that excellent results are obtained compared with those obtained using the Blackman and Hamming weighting functions for application to the waveguide structure of mismatched optical couplers that satisty the demand for small length, C+L-band, and low crosstalk level. It is found that a coupler with a coupling length of 2.5 mm and a crosstalk of -35 dB can reach bandwidths within 1.360-1.700 mu m. Using the same crosstalk and a coupling length of 3.7 mm, the broadband-weighting-function-weighted mismatched optical couplers can achieve broader bandwidths within 1.230-1.700 mu m. Considering the tolerances of structure parameters, the coupler with a coupling length of 3.4 mm which is much smaller than those obtained using the Blackman and Hamming weighting functions (namely, 5.0 and 4.9 mm, respectively) satisfies the demands for the C+L-band and a crosstalk level of -35 dB. (C) 2010 The Japan Society of Applied Physics
