A CDMA Method for Multibeacon Codomain Detection by a Quadrant Detector in Free-Space Optical Networking

Networking is an inevitable trend in developing free-space optical communications. The demand for multiple optical terminals to simultaneously establish links with and transmit to a single optical terminal has driven the integration of optical terminals. Phased arrays and rotating parabolic mirrors have been employed for beam control. However, achieving codomain detection of multiple optical spots with a single quadrant detector (QD) remains challenging in integrated detection. This paper proposes a codomain detection method for multiple beacons using a single QD based on pseudorandom code spread-spectrum modulation. This method realizes multiple beacons through code division multiple access (CDMA) by leveraging the orthogonality of pseudorandom sequences. The impact of different spread-spectrum gains on detection sensitivity and accuracy variation law under the interaction of multiple optical spots with different powers is investigated using simulation and experimental verification. The results demonstrate that when the spread spectrum gain is 1023, the detection sensitivity can reach −30 dBm@1 μm. Under conditions of equal-power interaction between two optical spots, the system deviation of position detection is less than ±60 μm, a reduction of 87.8% compared to the ±493 μm disturbance error without using CDMA. Finally, a simultaneous three spots tacking detection system was validated, achieving a detection accuracy of 1 μm, thereby proving the feasibility of CDMA multibeacon detection.