We propose a structural optimization method based on a real-coded micro-genetic algorithm to realize a weakly guided 2 × 2 multimode interference (MMI) coupler with low imbalance and excess loss over a wavelength range from 1520 to 1580 nm. The proposed method was applied to silica-based 2×2 MMI couplers with a relative refractive index difference of 5.5%. The optimized result showed an imbalance of less than 8.4×10−3 dB, an excess loss of less than 0.14 dB, and a normalized output power of more than 48% over the operation wavelength range. The proposed method achieved an optimized 2×2 MMI coupler after 250 times of propagation analysis per wavelength, which is less than 6.7% of those by the conventional methods for 4×4 and 1×4 MMI couplers, and was proven to be more effective than the conventional methods. To consider realistic optical devices, 2×2 MMI couplers whose values of structural parameters are close to the optimized values within the accuracy of typical fabrication tolerance are also analyzed. The results are comparable to those of the optimized 2×2 MMI coupler.
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