《Talking about high-speed coherent optical communication》

Coherent Optical Communication in English, is a technology in the field of optical fiber communication, mainly the sending end uses coherent modulation technology, and the receiving end uses heterodyne detection technology for information transmission; The coherence of light means that two light waves have the same wavelength, the same vibration direction and a constant phase difference during transmission.

Compared with traditional incoherent optical communication, coherent optical communication has the technical advantages of longer transmission distance, larger transmission capacity and higher receiving sensitivity, so it has been widely concerned by all walks of life.

In the eighties of the last century, countries such as the United Kingdom, the United States and Japan carried out a lot of research on coherent technology, AT&T and Bell Laboratories, in 1989 and 1990, between two stations 35 kilometers apart in Pennsylvania, successively carried out 1.7G signal non-relay coherent transmission experiments with 1.3μm and 1.55μm wavelengths. In the 1990s, it was found that the DWDM+EDFA technology combination could solve the problem of expansion and long-distance transmission of optical communication more simply and effectively, so the development of coherent transmission technology was temporarily cold.

Around 2008, with the development of the mobile Internet, network data traffic increased explosively, and the backbone network (> 1000 km) and metropolitan area network (100~1000 km) faced by the pressure of upgrading and expansion increased sharply, and at this time DWDM+EDFA technology has tended to the extreme, and the development potential is getting smaller and smaller. On the other hand, with the maturity of DSP and optical device technology, coherent optical communication based on these technologies is suitable for relaying DWDM+EDFA technology to break the technical bottleneck of long-distance and large-bandwidth optical fiber communication, so coherent optical communication ushered in the second development opportunity.

The outbreak of data traffic has also promoted the large-scale construction of data centers, and then the development of cloud computing has changed the IT infrastructure of data centers, making distributed data centers mainstream. Distributed data centers need to communicate with each other to share data, balance loads, provide backups, and scale capacity when needed. Distributed data centers urgently need the support of coherent transmission technology due to their large bandwidth and long-distance optical interconnection requirements (80~120km).

In summary, the upgrade and expansion of backbone and metro networks, as well as the optical interconnection of data centers, urgently need the support of coherent optical transmission technology.

【Advantages and applications】
The main advantages of coherent optical communication include:

1) High sensitivity and long relay distance 2) Good selectivity and large communication capacity 3)It has a variety of modulation methods

High-speed coherent optical communication belongs to the world's most advanced optical transmission technology, currently only in the hands of a few large enterprises, mainly foreign II-VI, NeoPhotonics, Ciena and other companies. China's related industries started a little late, and have formed industrial clusters centered on Suzhou, Chengdu, Qingdao and Wuhan.

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

【Why develop high-speed coherent optical communication】
With the development of the mobile Internet and the realization of the national strategy of counting from east to west, the demand for optical fiber transceiver modules by cloud operators and telecom operators continues to grow rapidly, and higher requirements are constantly put forward for the transmission bandwidth and distance of optical modules. Traditional optical modules cannot meet the transmission requirements of large bandwidth (over 100G) and long distance (more than 80 km) at the same time, and coherent optical modules make up for this gap.

Talking about high-speed coherent optical communication

Coherent optical transmission technology will greatly change the current network structure and ultra-long-distance transmission capability, greatly reducing the complexity of system equipment. The development of coherent optical transmission products will drive the development of related industries such as integrated optical transmitter ICT, integrated optical receiver ICR, and high-speed flexible PCB board in the upstream of the industrial chain, and provide local supplier options for downstream optical communication equipment vendors and Internet enterprises (data center optical interconnection).