欧洲、日本和美国正在进行的研究和开发工作已经产生了基于相干传输技术的各种实验通信系统。这些系统有希望超过直接探测系统的作用距离和带宽性能。但是,为了改善接收机性能,必须克服几个障碍,特别是压窄半导体激光线宽问题。相干光传输与目前在发射端采用直接强度调制,而在接收端采用直接强度探测的光纤系统相比有两个主要优点。第一,由于光混频增益使接收灵敏度改善了10～20分贝,这就导致传输距离增大。第二,可能使电滤 Ongoing research and development efforts in Europe, Japan and the United States have resulted in a variety of experimental communication systems based on coherent transmission technology. These systems promise to exceed the reach and bandwidth performance of direct-probe systems. However, in order to improve receiver performance, several obstacles must be overcome, in particular, to narrow the semiconductor laser linewidth. Coherent optical transmission has two major advantages over direct fiber optic systems that use direct intensity modulation at the transmitting end and optical fiber systems that use direct intensity detection at the receiving end. First, the receive sensitivity is improved by 10 to 20 dB due to the optical mixing gain, which leads to an increase in the transmission distance. Second, it is possible to electrofilter