Due to its bandwidth advantage, space laser communication has become an effective means to solve the bottleneck of microwave communication, build a space-based broadband network, and realize the real-time transmission of massive amount of earth observation data. The space laser communication terminal has characteristics of small size, lightweight, low power consumption, etc., which are suitable for satellite payload and meet the increasing communication needs of aerospace activities. In future, each communication satellite will carry multiple laser communication terminals that can serve multiple targets simultaneously. Therefore, laser communication terminals are being developed in the direction of miniaturization and integration. Traditional laser communication terminals use external modulation methods to achieve intensity or phase modulation of optical signals. Optical transmitters comprise multiple independent components, such as lasers, modulators, and bias controllers, and the system’s structure is complex. The phase modulation of the optical signal is realized using the direct modulation of the chirp-managed laser (CML), without an external modulator, bias controller, etc., with small size, low power consumption, low equipment complexity, and low cost. In addition, it can adapt to the continuous, high-speed, and integrated development of optical communication networks.

In this study, the chirp effect of the CML is used for phase modulation to generate a return-to-zero differential phase shift keying (RZ-DPSK) signal. RZ-DPSK has several advantages, such as high sensitivity, good reliability, simple receiver, and its receiving sensitivity is 3 dB higher than that of on-off keying (OOK) modulation method. It has received extensive attention in the engineering field. Using the chirp effect of the laser, the phase shift of the optical field is achieved by controlling the magnitude of the injected current, and the driving signal is simply pre-encoded using MATLAB to generate a three-level signal, thereby accurately controlling the phase change of the carrier signal. The error rate performance of RZ-DPSK estimated using this modulation method was tested and compared with that of the traditional external modulation method. The performance difference between the two methods was analyzed.

This study first uses the binary sequence "1110100" to verify the system principle. The schematic of the transmitter and receiver experimental schemes are shown in Figures 3 and 7, and the signal rate is 2.5 Gb/s. The output wavelength of the CML laser is 1552.544 nm, and the output optical power is 9.14 dBm. The receiving end includes erbium-doped fiber amplifier (EDFA), optical filter, optical delay interferometer, and balanced detector to receive and demodulate RZ-DPSK optical signal and restore the baseband electrical signal. To further reduce the spontaneous radiation noise caused by the amplification process, an optical filter with a bandwidth of 0.05 nm is placed after the EDFA. The signal waveform after demodulation is shown in Figure 8. 2⁷-1 pseudo-random binary sequence (PRBS) is used for bit error rate test. The pseudo-random signal is demodulated by the delay interferometer, and the output signal eye diagram of the balanced detector is shown in Figure 9. As a comparative experiment, the receiving end based on LiNbO₃ external modulation and the CML system use the same receiving device. The schematic of the two systems is shown in Figure 10. The bit error rate curves of RZ-DPSK system based on CML transmitter and LiNbO₃ transmitter are shown in the Figure 11. When the system error rate is 10^-9, the receiving sensitivity of CML and LiNbO₃ transmitters is -36.98 and -45.72 dBm, respectively. Compared with the LiNbO₃ transmitter, the sensitivity of the CML system is reduced by 8.74 dB. When the error rate of the forward error correction limit is 10^-3, the sensitivity of the CML transmitter is -48.1 dBm, which is only 1.8 dB less than the -49.9 dBm of the LiNbO₃ transmitter. The error characteristics of the two are the same, and thus, an error-free transmission can be realized. The CML transmitter has a simple structure, small size, and low power consumption, and the performance of the receiver is equivalent to that of external modulation when the limit error rate of the forward error correction is 10^-3, which shows a significant development prospect.

This study introduces the principle of signal coding and modulation of CML laser and realizes the direct modulation of 2.5 Gb/s RZ-DPSK signal based on the CML laser without differential coding and external modulator. The performance index of the modulation signal is analyzed. At the same time, the bit error rate performance of the transmission system based on the CML laser and the system based on the LiNbO₃ transmitter are compared. The results show that the sensitivity of the transmitter based on the CML is -48.1 dBm when the limit bit error rate of forward error correction is 10^-3. Compared with the sensitivity of LiNbO₃-based transmitter system (-49.9 dBm), the difference of the sensitivity of CML-based transmitter system is only 1.8 dB and the error characteristics are basically the same. Further, the CML-based transmitter system has a good transmission performance. In terms of hardware, the CML-based transmitter system has a simpler structure, low power consumption, small size, and lightweight, which can better adapt to the continuous high-speed and integrated development of space optical communication networks.