Polarization modulation-quadrature phase shift keying (PM-QPSK) modulation technology and coherent detection technology are used to realize the long distance transmission for 40/100 Gb/s optical module. Coherent detection in conjunction with advanced PM-QPSK modulation formats is a main research field 40/100 Gb/s-long haul transmission since such scheme can achieve higher spectrum efficiency than conventional direct-detection systems. The re-emergence of coherent detection is attributed to the advanced development of high-speed analog-to-digital converters (ADC). High-speed ADC samples mixing frequency of baseband signal, and realizes coherent reception by digital signal processing (DSP) chip working in a digital balanced way. Digital signal processing chip can realize timing recovery, signal recovery, polarization and polarization mode dispersion (PMD) tracking, and the dispersion compensation.