With the development of space laser communication, it begins to apply in various scenarios and high-precision time frequency transfer is the important one. In this paper, method of high-precision time-frequency transfer is proposed based on all-digital coherent demodulation. The parallel timing synchronization algorithm in all-digital coherent demodulation is described and its process is implemented in FPGA. The phase time could be acquired in time when operating timing synchronization in BPSK coherent laser communication experiment. Through phase unwrapping, the phase time becomes continuous. Then time transfer precision and frequency stability could be calculated and assessed by standard deviation and Allan variance separately. The phase time precision of below 5ps and frequency stability of below 10-14 /1000s could be realized when the communication rate is 1.25Gbps. Further, the time-frequency precision could be improved after dealing with phase time using linear Kalman filter.
The performance of fiber nutation tracking system based on coherent demodulation is constrained by the nonlinearity of devices in the signal coherent demodulation module. With the influence of this non-linear factors, the reconstruction accuracy of intensity envelope fluctuation signal depends on the input optical power. The influence of optical amplifiers and detectors on the signal intensity calculation in the range of our interest input optical power is analyzed theoretically and verified experimentally.
By rotating the half-wave plate, the variable communication bit rate between 5.12Gbps and 2.56Gbps has been verified at satellite-to-ground optical communication linkage, the communication link used Differential Phase Shift Keying(DPSK) modulation format and the wavelength is 1549.731nm. Without error correcting code and adaptive optics, an average bit error rate of 1.9E-9 was achieved while the link distance exceeded 1500km.
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