1. Photodetector: Its main function is to use the photoelectric effect to convert light signals into electrical signals. In optical communication systems, the requirements for photodetectors are high sensitivity, fast response, low noise, low cost, and high reliability. The basic principle of photoelectric detection process is light absorption. The commonly used photodetectors in optical communication systems are PIN photodiodes and avalanche diodes (APDs). Comparison of performance between two detectors: Due to the lower price and lower noise of PIN compared to APD, PIN with the same performance is more cost-effective.
2. Optical receiving system: At the receiving end, the function of the receiving antenna is to collect and converge the light field propagating in space onto the surface of the detector.
3. Signal processing
In space optical communication systems, the signal received by the optical receiver is very weak, and in addition, under the interference of high background noise field, it can cause the signal-to-noise ratio S/N at the receiving end to be less than 1. So signal processing is very necessary. The usual measures taken include: firstly, using narrowband filters on the optical channel to process the received optical signal and suppress the interference of background stray light. The basic types of optical filters include absorption filters, interference filters, birefringent filters, and new types of atomic resonance filters The second is to use a preamplifier on the telecommunications channel to convert the weak photocurrent signal generated by the photodetector into a voltage signal, and then further amplify the signal through the main amplifier. Then, methods such as equalization and filtering are used to shape and process the signal, and finally, the information at the starting and sending ends is restored through clock extraction, decision circuits, and decoding circuits.