In a space-integrated ground information network, high-speed laser communication is a key technology for high-speed intersatellite data transmission. Owing to its high sensitivity and strong anti-interference capabilities, coherent laser communication has become an ideal choice for long-distance inter-satellite communication. However, traditional analog coherent demodulation technology requires a complex optical analog lock-in loop (OPLL) control system and places high demands on the performance of the local oscillator laser, which increases the complexity and cost of the system. To address these issues, a new carrier recovery method is explored in this study and combined with digital coherent demodulation technology to reduce the system complexity.

A carrier recovery method for a coherent receiver based on semiconductor laser injection locking technology is investigated, and digital demodulation technology is used to realize signal reception. Manchester-coded binary phase-shift keying (BPSK) modulation technology is used at the transmitter to effectively retain the carrier component in the transmitted optical signal. The carrier component in the modulated light is filtered and amplified via semiconductor laser injection locking technology at the receiver, which is frequency-shifted by 160 MHz using an acousto-optic modulator before entering the 90° optical hybrid. The frequency shift reduces the effect of low-frequency beat noise on the system. A balanced photodetector with alternating current coupling is used to detect the mixed signal, and an oscilloscope is employed to collect the signal. The collected signal is processed offline to obtain a demodulated encoded signal.

Experiments are conducted on carrier recovery based on injection-locking technology at the receiver end. The impacts of different modulation depths, signal rates, and injection ratios on the performance of the local oscillator light after injection locking are analyzed. A decrease in the modulation rate and increases in the modulation depth and injection power can increase the proportion of the signal components contained in the recovered carrier spectrum, thereby reducing the quality of carrier recovery. In practical applications, selecting an appropriate modulation depth and injection power is beneficial for improving the carrier recovery quality. The demodulation of BPSK coherent communication signals at 5 Gbit/s is achieved, and the impact of various parameters is compared using the error vector magnitude (EVM). For signals with the baud rate of 5 GHz, the baseband signal is successfully demodulated at modulation depths of 0.33, 0.42, and 0.49. In these cases, increasing the modulation depth results in a gradual decrease in the EVM, indicating that a higher modulation depth is beneficial for reducing the EVM. However, for signals with the baud rate of 4 GHz, the EVM at the modulation depth of 0.42 is lower than that at 0.49. This is because in previous experiments, for signals with the modulation depth of 0.49 at 4 GHz, the depression near the carrier component is reduced, and the local oscillator light mixed with the signal components can not successfully demodulate the baseband signal. Therefore, the EVM can be used to measure the combined quality of the local oscillator light recovered by injection locking and the signal light. For signals at 3 GHz, owing to the small depression near the carrier component, the signal can not be demodulated even at the modulation depth of 0.33, and the change in the EVM is not significant at this time.

The proposed coherent communication system based on the semiconductor laser injection-locking carrier recovery technology combines the advantages of high-bandwidth optical locking and digital coherent tuning. A homodyne carrier signal can be recovered from the signal light using an optical method and effective mixed signals can be obtained using a distributed feedback (DFB) laser with a linewidth in the MHz range. Complex electronic feedback loops are not required, significantly reducing the system complexity. As a result, the demodulation of BPSK coherent communication signals at 5 Gbit/s without BER is achieved. This research can provide a simple and effective technical method for the carrier recovery of local oscillator light and is expected to be applied to high-precision coherent demodulation scenarios, including space coherent laser communication, lidar, and optical computing.