With the continuous development of communication technology, the space laser communication system and standards are also constantly evolving and improving. The modulation technology types for space laser communication are varied. The traditional space laser communication generally adopts the intensity modulation/direct detection (IM/DD) mode, which is easy to implement and economical but has low detection sensitivity and is prone to interference. In contrast, laser communication systems with phase modulation/coherent detection have good wavelength selectivity, allowing for high-speed data transmission while maintaining high reception sensitivity. Most laser communication devices only have a single modulation communication method, which results in a single application scenario and poor interconnectivity among various engineering projects, and can no longer satisfy the requirements of satellite internet. Because different communication systems have different advantages and disadvantages, they are suitable for different application scenarios. If multiple modulation formats can be compatible with a single system and can be switched freely, then this system is suitable for various practical conditions, significantly improving the reliability, flexibility, and efficiency of space optical communication systems. Moreover, communication costs can be reduced by selecting the appropriate communication system. Therefore, many researchers have studied laser communication systems that are compatible and switchable with multiple modulation formats.

The multi-compatible system developed in this study is based on a double parallel Mach-Zehnder modulator (DPMZM). Multiple emission formats can be achieved by changing the bias voltage on the DPMZM electrodes to operate at different operating points. First, we introduce the bias control method, which combines power and pilot detection methods. It uses a stepwise control method, which is mainly divided into two steps: coarse scanning and fine tracking. In the coarse scanning process, the power detection method is mainly used with a large step value; in the fine tracking process, the pilot signal method is mainly used with a small step value. Second, a numerical simulation of the pilot signal method is conducted to demonstrate the feasibility of the bias control method. Subsequently, a multi-compatible optical emission system is designed using field programmable gate array (FPGA) to interact with multiple modules and achieve multiformat optical emission and coherent demodulation functions. Furthermore, we build a test block diagram for this system and test its transmission signal quality and receiver performance, and conduct thermal vacuum experiments. Finally, we conduct a simulated space environment experiment to verify the adaptability of this system to micro-vibrations in satellite laser communication environments.

The simulation results confirm that the feedback signals of each branch in the pilot method of the bias control algorithm employed in this study exhibit a contrast exceeding 20 dB under ±0.05 V_π offset (Figs. 5 and 6), thereby satisfying the system’s design requirements. During testing of emission performance in the multicompatible system, the quadrature phase shift keying (QPSK) transmission signal at a 5 Gbit/s rate is repeatedly tested at room temperature, yielding test results that consistently surpass 9%, with the lowest error vector magnitude (EVM) reaching 5.03% [Fig. 11(a)]. The binary phase shift keying (BPSK) transmission signal at a 2.5 Gbit/s rate exhibits the lowest EVM of 3.43% [Fig. 11(b)], whereas the on-off keying (OOK) transmission signal at the same rate has an EVM of 3.76% [Fig. 11(c)]. To assess the overall communication performance of the system, the transmitting system is integrated with the receiving demodulation system. Under an optical power of -50 dBm, the bit error rate (BER) measured by the system is 3.9×10^-3 (Fig. 12). Additionally, the stability is verified in a thermal vacuum environment test. At an optical power of -47.5 dBm, the BER test results for 9 h of continuous operation exhibit an overall stability of 1×10^-4 (Fig. 13). Furthermore, in the simulated space environment test, the system is loaded with NASDA spectrum, resulting in a BER higher than 8×10^-6 (Fig. 15). These findings demonstrate the terminal’s adaptability to micro-vibrations in satellite laser communication environments.

In this study, we developed a multi-compatible system, which is based on a closed-loop bias control algorithm and is compatible with OOK/BPSK/QPSK. When transmitting QPSK signals with a communication rate of 5 Gbit/s, the receiver’s communication sensitivity reaches -50 dBm, the decoding BER is less than 1×10^-7, and the transmission EVM is lower than 9%. It can provide high-speed and high-quality optical signal sources for space laser communication. Through experimental verification, the communication system has good vacuum and temperature adaptability as well as good adaptability to microvibrations in the space environment. It can effectively address the unification of different constellations and has important research significance.