# Phase Alignment of an Array Optical Telescope System Using Balanced Detection
###### Abstract  
Differential phase shift keying (DPSK) modulation and multi-aperture receiving are effective means for suppressing flickering, deviation, and fragmentation of the light spot by atmospheric turbulence. What is challenging in coherent beam combination of such an array receiver system is to detect and compensate for phase deviation of sub-apertures. In this paper, a method of phase alignment of an array optical telescope system using balanced detection was proposed and demonstrated. The improved Mach Zehnder Interferometer (MZI) can demodulate the digital signal and recover the phase difference at the same time. It also brings a 3 dB gain to the receiver and improves the detection sensitivity of the system. Adequate simulations with OptiSystem and MATLAB were carried out to show that the power value remains near the ideal state of 2.75 mW, and the bit error rate is less than \(10^{-9}\) after phase compensation, which indicates the effectiveness and accuracy of the proposed method. Furthermore, taking the communication interruption difference of ninety degrees as an example, the system bit error rate was reduced from 1 to \(10^{-35}\), and communication was established again. phase alignment; balanced detector; an array optical telescope system; differential phase shift keying; the improved Mach Zehnder Interferometer +
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laser communication relay demonstration (LCRD) and verification test finally proved to be a success in the USA. It adopted DPSK and pulse position modulation (PPM)-compatible communication systems for the first time and achieved long-distance satellite-to-ground laser communication with a communication rate of 2.88 Gps and a communication distance of 45,000 km [6; 7; 8]. It goes a step further in NASA's Integrated LCRD Low-Earth Orbit User Modem and Amplifier Terminal (ILLUMA-T) project [8]. Furthermore, the High-speed Communication with Advanced Laser Instrument(HICALI) project has been initiated by the National Institute of Information and Communications Technology (NICT), aiming for a 10 Gbps-class laser link using DPSK [9]. For the receiving system, large aperture telescopes are restricted by high cost and complex structures, such as the Shack-Hartman sensor, variant mirror, fast mirror, and other devices [10; 11]. Reducing the receiving aperture of the telescope is the simplest way to overcome atmospheric turbulence. The United States completed the lunar laser communication demonstration system (LLCD) test in 2013 [12]. The test verified the equivalent relationship between multi-aperture and large aperture [13]. Compared with a large aperture, the multi-aperture receiving system has the advantages of lower cost, improved diffraction limit performance, smaller gravitational effect, scalability, and so on [14; 15]. In this work, DPSK modulation and multi-aperture receiving technology were adopted to improve the performance of the communication system. However, the tilt of optical axes brings uncertain phase deviation. Studying the phase deviation of sub-apertures is a breakthrough direction to ensure the quality of high-resolution imaging and space optical communication. The novelty of this paper is the innovated Mach Zehnder Interferometer (MZI) and the establishment of the autodyne detection mode, which replaced one optical signal and one zero position light with two optical signals. It brought a 3 dB gain to the demodulation input so that the sensitivity of the system was improved, and the transmission performance was optimized. Further, differently from existing methods, which implemented DPSK demodulation and optical signal phase difference detection separately using different technologies [16; 17; 18], this study combined the two. Specifically, we finished the identification by using the voltage signal from the differential output port of the balance detector after we had connected the demodulator and the balance detector.