Objective In the satellite coherent laser communication system, the signal modulation format is mainly phase- shift keying (PSK), which is not compatible with the coherent reception of on- off keying (OOK). In view of the problem that the satellite coherent optical communication receiver is incompatible with various modulation formats, a coherent communication receiver compatible with OOK and binary PSK ( BPSK) is built experimentally. At the communication rate of 1 Gbit/ s, when the modulation format is OOK, and the signal optical power is - 54. 6 dBm, the bit error rate (BER) is 10(-3), and there is a distance of 3. 3 dB from the shot noise limit; when the modulation format is BPSK, and the signal optical power is - 57. 95 dBm, the BER is 10-3, and there is a distance of 4. 2 dB from the shot noise limit. The multisystem compatible coherent receiver shares a common structure with many coherent receiver hardware at present and has high receiving sensitivity. It verifies the feasibility of the multi- system compatible technology of satellite coherent laser communication and is of great significance. Methods As the current mainstream satellite communication modulation formats, BPSK and OOK will exist for a long time. Therefore, this paper experimentally builds a coherent receiver compatible with OOK and BPSK. Firstly, the optimal local optical power of the balanced detector used in this experiment is measured by the experimental setup shown in Fig. 4. When the optical modulation format of the signal is OOK, the in- phase ( I) and quadrature (Q) signals are firstly complexed, then the modulus of the complex signal is calculated, and finally the threshold judgment method is used for verification. When the optical modulation format of the signal is BPSK, the real-time carrier phase difference is calculated by the complex digitization and IQ arctangent method, and the baseband signal can be obtained after the carrier recovery of the BPSK signal. The demodulation of the overall signal relies on offline processing, and Fig. 8 shows the specific flow of offline processing. Results and Discussions After the experiment in Fig. 4, we finally set 12. 8 mW as the optimal optical power of the local oscillator in this experiment. In this paper, when the optical frequency of the local oscillator and the signal is small, the high- speed oscilloscope is used to demodulate the OOK signal. The recovered baseband signal and eye diagram are shown in Fig. 7. When the optical power of the signal close to the shot noise limit is measured, the BER is calculated through offline processing. When the signal optical modulation format is BPSK, the baseband signal will be recovered, and the BER will be calculated after the I and Q signals are directly collected for offline processing. In this experiment, the communication rates of OOK and BPSK are both 1 Gbit/ s. With the multi-system compatible coherent detection technology applied, the receiving sensitivity of the OOK signal is 3. 3 dB away from the shot noise limit. The main reasons are as follows. The responsivity of the detector used is 0. 85 A/ W, and the ideal responsivity is 1. 25 A/W when the quantum efficiency is 1 in the 1550 nm band, which will cause a loss of 1. 67 dB. The output of the optical 90 degrees bridge goes into the detector fiber, and there is a connection flange, which will cause a loss of 0. 3 dB. The remaining loss of 1. 33 dB may be caused by the following reasons: energy lost due to the transmittance of the detector window, imperfect heterodyne efficiency, and ADC quantization loss. With the help of the multi-system compatible coherent detection technology, the receiving sensitivity of the BPSK signal is 4. 2 dB away from the shot noise limit, which is 0. 9 dB higher than the distance between the receiving sensitivity of the OOK signal and the shot noise limit. The loss may be caused by an inaccurate phase- locking error from the inaccurate phase difference calculation at a low signal- to-noise ratio due to an extremely low signal optical power. Conclusions In this paper, a multi-system compatible coherent detection device for satellite laser communication is built experimentally, and the corresponding demodulation algorithm and offline processing method are given. When the communication rate is 1 Gbit/ s, and the BER is 10-3, the receiving sensitivity of the multi-system compatible coherent detection device is only 3. 3 dB away from the shot noise limit for the OOK signal, and that for the BPSK signal is only 4. 2 dB away from the shot noise limit, which realizes high-sensitivity and multi- system coherent reception. It is worth mentioning that an advantage of coherent OOK is that in the PSK coherent communication system, when the carrier recovery algorithm or the phase-locked loop cannot achieve carrier synchronization for some reason, the coherent reception of OOK can be used as an important alternative method, with only a slight loss of sensitivity. In addition, multi-system compatibility is not limited to OOK and BPSK. For quadrature PSK (QPSK) modulated signals, we only need to change the square operation of eliminating the communication term to the fourth power operation. After carrier synchronization, both I and Q signals become baseband data. Multi-system compatible coherent reception can greatly improve the flexibility and interactivity of satellite laser communication networks in the future, so it is of great significance.
