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 2.5mm and a crosstalk of -35 dB can reach bandwidths within 1.360 -1.700 mm. Using the same crosstalk and a coupling length of 3.7mm, the broadband-weighting-function- weighted mismatched optical couplers can achieve broader bandwidths within 1.230 -1.700 mm. Considering the tolerances of structure parameters, the coupler with a coupling length of 3.4mm which is much smaller than those obtained using the Blackman and Hamming weighting functions (namely, 5.0 and 4.9mm, respectively) satisfies the demands for the C+L-band and a crosstalk level of -35 dB.