Development course of coherent optical communication

In the field of optical communication, wider bandwidth, longer transmission distance and higher receiving sensitivity have always been the goals pursued by researchers. Although the application of wavelength division multiplexing (WDM) and erbium-doped fiber amplifier (EDFA) greatly improves the bandwidth and transmission distance of optical communication system, with the explosive growth of information generated by the application of communication technologies such as video conferencing and the popularization of the Internet, higher transmission performance requirements are put forward for the physical layer as the basis of the whole communication system. The optical communication system adopts intensity modulation/direct detection (IM/DD), that is, the transmitter modulates the intensity of the optical carrier and the receiver detects the envelope of the optical carrier. Although this structure has the advantages of simplicity and easy integration, its single channel bandwidth is limited because it can only adopt ASK modulation format. Therefore, this traditional optical communication technology will inevitably be replaced by more advanced technology. However, with the bursting of the communication bubble today, the application of new optical communication technology will inevitably bring the demand for new communication equipment. Faced with the high price of optical devices, the high cost of upgrading large-scale communication equipment is unacceptable to operators, so the research and development of new optical fiber communication technology is also facing great risks for equipment manufacturers. How to improve the performance of optical communication system on the basis of existing equipment has become a realistic problem. In this context, coherent optical communication technology, which was highly anticipated more than 20 years ago, has been put on the table again.

The theory and experiment of coherent optical communication began in 1980s. Because coherent optical communication system is recognized as having the advantages of high sensitivity, many countries have done a lot of research work on coherent optical transmission technology. After ten years of research, coherent optical communication has entered the practical stage. Britain, the United States, Japan and other countries have conducted a series of coherent optical communication experiments. At&T Company. In 1989 and 1990, T and Bell Company conducted the experiments of 1.7Gbit/s FSK outfield coherent transmission with wavelengths of 1.3μm and 1.55μm, respectively. 1990, NTT company conducted a 2.5Gbit/s CPFSK coherent transmission experiment between Oita and five stations in Seto Inland Sea, with a total length of 43 1 km. Until the end of 1980s, the development of EDFA and WDM technology slowed down the development of coherent optical communication technology. In the meantime, the sensitivity and information capacity of each channel are no longer concerned. However, after 20 years of development and wide application of direct detection WDM system, new symptoms begin to appear, which indicates that the application of coherent light transmission technology will be paid attention to again. In digital communication, expanding the capacity of C-band amplifier, overcoming the deterioration of fiber dispersion effect and increasing the capacity and range of free space transmission have become important considerations. In analog communication, sensitivity and dynamic range become the key parameters of the system, which can be greatly improved by related optical communication technologies. In digital transmission system, DPSK and DQPSK have been widely used, which indicates that phase-sensitive coding and transmission technology will become a trend. Detection sensitivity and spectral efficiency are the keys to this trend. Other factors affecting the selection of detection scheme include the security and reliability of the physical layer and the adaptability of the network, both of which can benefit from amplitude, frequency and polarization coding using coherent light technology. Compared with incoherent transmission, coherent analog transmission also has great advantages, especially in the dynamic range. Although analog communication is not as widely used as digital communication, analog transmission plays an important role in many special environmental applications.

At the same time, in this short twenty years, optical devices have made great progress, the output power, linewidth, stability and noise of lasers, the bandwidth, power capacity and mode rejection ratio of photodetectors have been greatly improved, and the performance of microwave electronic devices has also been greatly improved. These advances make it possible to commercialize coherent optical communication systems.