The Working Principle and Main Advantages of Coherent Optical Communication

Coherent optical communication transmission technology is currently the main technical direction to achieve 400G and 100G transmission rates in data centers and network infrastructure. Utilize more dimensions, polarizations, amplitudes, phases and frequencies of light waves to carry more modulation information, thereby expanding the transmission capacity.

Coherent modulation and heterodyne detection techniques are mainly used in coherent optical communication.

Coherent modulation is to use the signal to be transmitted to change the frequency, phase and amplitude of the optical carrier (rather than just changing the intensity of the light like intensity detection), and requires the optical signal to be coherent light, such as laser light.

Heterodyne detection is to use a laser beam generated by local oscillation and the input signal light to mix in an optical mixer to obtain an intermediate frequency signal whose frequency, phase and amplitude change according to the same rules as the signal light.

Ⅰ. The working principle of coherent optical communication

At the sending end, the external modulation method is used to modulate the signal onto the optical carrier for transmission. When the signal optical transmission reaches the receiving end, it is first coherently coupled with the local oscillator optical signal, and then detected by the balanced receiver. In heterodyne detection, the optical signal is converted into an intermediate frequency signal after photoelectric conversion, which requires secondary demodulation to be converted into a baseband signal.

Ⅱ. The main advantages of coherent optical communication

1. High sensitivity and long relay distance

Coherent detection in coherent optical communication can improve receiver sensitivity. Under the same conditions, the sensitivity of the coherent receiver is about 20dB higher than that of the ordinary receiver, which can achieve high performance close to the shot noise limit, thus increasing the transmission distance of the optical signal without relay.

2. Good selectivity and large communication capacity

Coherent optical communication can improve receiver selectivity. In direct detection, the receiving band is relatively large. In order to suppress the interference of noise, a narrow-band filter is usually placed in front of the detector, but its frequency band is still very wide.

In the coherent heterodyne detection of coherent optical communication, the mixed light of the signal light and the local oscillator light is detected, so only the noise in the intermediate frequency band can enter the system, while other noises are detected by the microwave intermediate frequency with narrow bandwidth. Amplifier filtering, heterodyne detection has good filtering performance.

At the same time, due to the excellent wavelength selectivity of coherent detection, the coherent receiver can greatly reduce the frequency interval of the frequency division multiplexing system, that is, dense wavelength division multiplexing (DWDM), which replaces the large frequency interval of traditional optical multiplexing technology. Division multiplexing realizes the potential advantage of higher transmission rates.

3. With a variety of modulation methods

In coherent optical communication, in addition to amplitude modulation of light, various modulation formats such as PSK, DPSK, and QAM can also be used, which is conducive to flexible engineering applications.

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