The coherent optical communication system modulates the signal to the optical carrier by modulating the amplitude, phase and frequency of the signal in the way of external light modulation (such as DP-QPSK) at the sending end.

Compared with the traditional direct detection system, coherent detection can obtain more signal information through the beat frequency of signal light and local vibration. After the signal arrives at the receiving end, by means of high-speed digital signal processing (DSP) technology, through front-end processing such as equalization, it enters the optical mixer and the optical signal generated by the local oscillator for coherent mixing, so as to realize signal reconstruction and distortion compensation.

Coherent optics is available for both 100G and 400G applications, primarily because it enables service providers to send more data over existing optical fibers, reducing the cost and complexity of network upgrades for bandwidth expansion.

Main advantages 1

Coherent detection is combined with DSP technology

• Remove the barriers to traditional coherent reception

• Various transmission injuries can be compensated in the electrical domain, simplifying the transmission link

• Enables high order modulation formats and polarization multiplexing

Main advantages 2

At the same time, the application of high order modulation scheme makes the spectrum efficiency of coherent optical communication higher than that of traditional system.

The coherent receiver has no special requirement for the fiber channel, so the fiber line can be used in the coherent optical communication. By means of digital signal processing algorithm, the coherent receiver compensates the signal distortion caused by optical fiber dispersion, polarization mode dispersion and carrier phase noise at a small cost.

Main advantages 3

The coherence receiver is about 20dB more sensitive than the ordinary receiver, so the no relay distance in the transmission system will be longer, which reduces the number of amplification on the optical path of distance transmission.

Based on the above reasons, coherent optical communication can reduce the cost of optical fiber erection for long-distance transmission, simplify the design of optical amplification and compensation, and become the main application technology for long-distance transmission network.