In practical applications, the antenna in a coherent-detection system receives the signal beam, couples it into an optical fiber, and then mixes the frequency with that of the local-oscillator beam. This process is affected by the performance of the detector itself, as well as the influence of external factors on the detection performance of the system. Consequently, the optical power of the signal reaching the surface of the detector is smaller than that of the received signal. Therefore, it is necessary to study the factors affecting the detection sensitivity of a coherent-detection system and their influence on the system performance.