Coherent modulation and heterodyne detection are mainly used in coherent optical communication. The so-called coherent modulation is to use the signal to be transmitted to change the frequency, phase and amplitude of the optical carrier (instead of just changing the intensity of the light as intensity detection does), which requires that the optical signal has a certain frequency and phase (rather than a certain frequency and phase as natural light does not), that is, it should be coherent light. A laser is a coherent light. The so-called heterodyne detection is to use a beam of laser generated by local oscillation and the input signal light to mix in the optical mixer, so as to obtain the IF signal with the same regularity as the frequency, phase and amplitude of the signal light.
At the transmitter, the signal is modulated to the optical carrier by external modulation. When the signal optical transmission reaches the receiving end, it is first coherently coupled with a local oscillator signal, and then detected by the balanced receiver. Coherent optical communication can be divided into heterodyne detection and zero difference detection according to the unequal or equal frequency of local oscillator and signal. The former optical signal is the IF signal after photoelectric conversion. It needs to be demodulated twice before it can be converted into baseband signal. The latter optical signal is directly converted into baseband signal after photoelectric conversion without secondary demodulation, but it requires the local optical frequency and signal optical frequency to match strictly, and requires the phase locking of the local oscillator light and signal light.
High sensitivity and long relay distance
One of the main advantages of coherent optical communication is that coherent detection can improve the sensitivity of the receiver. Under the same conditions, the sensitivity of coherent receiver is increased about 20dB compared with the common receiver, which can reach the high performance close to the limit of shot noise, so the non relay transmission distance of optical signal is also increased.
Large communication capacity
Another main advantage of coherent optical communication is that it can improve the selectivity of the receiver. In direct detection, in order to suppress the interference of noise, a narrow-band filter is usually placed in front of the detector. In the coherent heterodyne detection, the mixing light of signal light and local oscillator light is detected, so only the noise in the IF frequency band can enter the system, and other noises are filtered by the microwave IF amplifier with narrow bandwidth. It can be seen that heterodyne detection has good filtering performance, which will play an important role in the application of inter satellite optical communication.
Signal modulation mode
In the traditional optical communication system, only intensity modulation can be used to modulate the light. In coherent optical communication, in addition to amplitude modulation of light, a variety of modulation formats such as PSK, DPSK, QAM can be used, which is conducive to flexible engineering applications. Although this increases the complexity of the system, compared with the traditional optical receiver, which only responds to the variation of optical power, coherent detection can detect all information carried by the amplitude, frequency, phase and polarization state of light, so coherent detection is a holographic detection technology, which is not available in traditional optical communication technology.
Connet develops single frequency fiber laser and polarization maintaining fiber amplifeir to meet the needs of coherent optical communication. CoSF-D and CoSF-R low noise ultra narrow linewidth single frequency fiber lasers are ideal light sources for coherent optical communication. These single frequency fiber lasers have extremely low phase noise and frequency noise, ultra narrow linewidth, excellent frequency stability and polarization stability, and are suitable for coherent communication systems with zero difference detection or heterodyne detection. In addition, Connet can provide external modulation laser transmitter based on LiNbO3 optical modulator technology, and realize signal modulation methods widely used such as PSK and DPSK. Cascaded low noise fiber amplifier is used to improve the output power and meet the needs of long-distance laser coherent communication.