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We propose a novel scheme that combines a polarization-beam splitter (PBS) and a 90° hybrid into a single device to demodulate the dual-polarization quadrature phase-shift keying (DP-QPSK) signal by introducing a photonic crystal structure into a 4 × 4 multimode interference coupler. Since no separated PBS is utilized, the demodulator has much smaller chip size compared with a conventional DP-QPSK demodulator which contains two PBSs and two 90° hybrids. Simulation with 2-D finite-difference time-domain method indicates that the demodulator can operate very well for demodulating the DP-QPSK signal. The numerical simulation demonstrates a common-mode rejection ratio larger than 20 dB and phase errors smaller than ±5° of the device over the C-band wavelength range.