Optical gain spectra of InGaAsP MQW for photonic crystal waveguide (PCWG) were simulated by the $k\cdot p$ method with considering the variation of group velocity and the natural broadening synthetically. The dependence of the first mode's gain maximum on the width of PCWG was discussed. To improve the mode characteristics and the gain performances in the 2-D PCWG with relative large width, we proposed a new structure by combining a microcavity inside the 2-D PCWG. Mode characteristics in proposed structure were analyzed and the transmission performance was simulated by the FDTD method. The simulation results show that improved longitudinal mode characteristics can be obtained even in the W3 PCWG with relative wide waveguide width because of the additional frequency selecting mechanism provided by the microcavity.
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