In space laser communication, the wide divergence angle of beacon light leads to substantial spatial losses, compounded by background light and detector noise; this results in compromised precision in the detection of the beacon light position. To solve this problem, a high-precision detection technique and communication composite technology employing a four-quadrant detector (QD) with beacon spread-spectrum modulation are proposed. Pseudo-random sequences (PRNs) are employed to spread the beacon communication spectrum, with the spread-spectrum signal utilized to modulate the intensity of the transmitted beacon light at the transmitter end. At the receiver, QD photocurrent signals are cross-correlated with an identical PRN that is used for modulation. The strong auto-correlation properties of PRNs, which are uncorrelated with noise, enhance the output signal-to-noise ratio (SNR), enabling precise position detection and beacon communication under high-SNR conditions. Theoretical analysis is used to explore the effects of spreading gain on the sensitivity of system detection and the precision of position detection. The experimental results demonstrate that the beacon spread-spectrum modulation scheme effectively detects the position of the light spot. At a received optical power of −37 dBm and spreading sequence PRN depths of 1023, 127, and 31, the root-mean-square error (RMSE) values are 0.983 μm, 2.876 μm, and 7.275 μm, respectively. This corresponds to improvements of 14.96 dB, 10.29, dB, and 6.26 dB compared to direct detection precision (30.811 μm). Additionally, under an identical signal bandwidth, the sensitivity improves by 14.6 dB, 10.1 dB, and 6.4 dB, respectively. The proposed beacon spread-spectrum scheme mitigates the limitations of hardware reception sensitivity and position-detection precision, demonstrating its potential application in high-precision detection in long-distance interstellar laser communication.

Free space optical communication is developing towards laser communication networking. A novel method of quadrant detector (QD) multi-spot position detection based on orthogonal frequency division multiplexing (OFDM) is proposed for multi-spot laser communication systems. The mathematical model is constructed, and the Monte Carlo method is used to validate it. The position calculation of three beams incident on QD is simulated. The influence of key parameters on the accuracy of position detection is analyzed when the spots are at the same position and a different position. The results show that when the SNR of the system is 31.74 dB, the radius of the Gauss spot is 2 mm, the number of FFT (fast Fourier transform) points is 1024, and the center of the multi-spot is at the same position of the detector target; the accuracy of signal position detection calculated by the equation is 1.433 μm, and the simulation results are 1.351 μm, 1.354 μm, and 1.389 μm, respectively. When the center position of the multi-spot is at different positions of the detector target, the detection accuracy calculated by the formula is 1.438 μm, 1.433 μm, and 1.434 μm, respectively, and the simulation results are 1.419 μm, 1.387 μm, and 1.346 μm, respectively. The experimental results verify the effectiveness of the proposed multi-target simultaneous detection method. This article proposes a new multi-spot position detection method which can not only achieve one-to-multiple node laser communication but also improve the accuracy of point position detection.

In gravitational wave measurement and inter-satellite laser communication systems, the relative rotation and motion between the transmitter and receiver terminals introduces small angular deviations over a link distance of thousands of kilometers, which need to be measured with high accuracy and sensitivity. The heterodyne coherent angle measurement has a higher measurement accuracy and detection sensitivity compared with the traditional direct detection technique, which performs angle measurement through the phase of a beat frequency signal. In this paper, we propose a four-quadrant heterodyne coherent angle measurement technique with μrad accuracy and nW-level detection sensitivity. A mathematical model of a differential wavefront sensing (DWS) angle solution was formulated, and a Monte Carlo simulation system was built for performance testing. An experimental system was devised to assess the accuracy and sensitivity of the heterodyne coherent measurement method and to compare the performance with that of the direct detection method. The experimental results showed that for azimuth and pitch axes, the accuracy of the heterodyne coherent angle measurement was 2.54 μrad and 2.85 μrad under the same signal power of −16 dBm, which had a 5-fold improvement compared with direct detection. The sensitivity of the heterodyne coherent detection was −50 dBm at the 20 μrad accuracy threshold, which was a 1000-fold improvement compared with direct detection. This research is of great significance for the phase measurement and tracking system in the field of gravitational wave detection and has a guiding role in system design work in the field of inter-satellite laser communication.

In inter-satellite laser communication systems, angular deviations between the boresights of terminals exert an impact on system performance, thereby imposing higher requirements on performance parameters such as the angle measurement accuracy and range of the system. Conventional coherent angle measurement systems feature a fixed baseline, which results in insufficient dynamic adjustment capability for accuracy and range. To address the aforementioned issues, this paper proposes a variable-baseline coherent angle measurement method based on nutation scanning. By altering the baseline length through a nutation scanning strategy, this method achieves dynamic adjustment of angle measurement accuracy and range, enhancing the adaptability of the angle measurement system in different scenarios. For long-distance communication, a small nutation scanning angle (short baseline) is used to adapt to significant satellite relative position changes via a large angle measurement range, while a large nutation scanning angle (long baseline) is adopted for short-distance communication to ensure stable tracking with high angle measurement accuracy. Firstly, a mathematical model of the system is established, and the relationships between the nutation scanning angle, angle measurement accuracy, and angle measurement range are derived. The Monte Carlo method is employed to construct a variable-baseline angle measurement simulation system for data verification, and a tabletop experimental system is built for performance analysis. Experimental results show that when the nutation scanning angle γ increases from 10 μrad to 100 μrad, the baseline length increases from 0.05 μm to 0.5 μm, the angle measurement accuracy improves from 56.9 μrad to 6.35 μrad, and the angle measurement range narrows from 880 μrad to 560 μrad. These results are consistent with the trend of theoretical derivation and correspond to the simulation results. This study verifies the feasibility of dynamically regulating angle measurement accuracy and range via the nutation scanning angle and provides a reference for the design of space laser communication systems.