相干激光通信具有高速率及传输距离长等优势，在星间激光通信中占有重要的地位。相干光通信系统充分利用了光的强度、相位、频率和偏振等信息，不仅能提高频带利用的效率，同时还能够延长光通信中继距离。四相相移键控（QPSK）调制与解调作为一种多进制调制方式，可以大幅度地提升频谱资源的利用率。本文针对相干光激光通信的QPSK调制与解调响应速率的问题，提出了一种改进型的科斯塔斯（Costas）环，实验验证了完整的QPSK调制与解调系统，同时验证了改进型Costas环的性能可行性。

Coherent laser communication has the advantages of high speed and long-distance transmission capabilities, both of which play an important role in inter-satellite communication. A coherent optical communication system makes full use of information on light intensity, phase, frequency, and polarization, which cannot only improve the efficiency of frequency band utilization, but also extend the relay distance across which optical communication is possible. As a type of multi band modulation, quadrature phase shift keying (QPSK) modulation and demodulation can greatly improve the utilization of spectrum resources. Aiming to resolve the problem of the response rate of QPSK modulation and demodulation in coherent optical laser communication, an improved Costas loop is proposed in this study. The results of the experiment verify the entire QPSK modulation and demodulation system and the performance feasibility of the improved Costas loop.