Currently, the construction of low-orbit satellite constellation is booming in the world. To make full use of carrier resources and improve launch efficiency, the extra-vehicular launch envelope size of spaceborne laser communication payload need to be smaller and flatter. Tracking mirror is the decisive factor of launch size because its rotating mechanism of external load and itself cannot be packed in to the cabin. . Therefore, we propose the design architecture of the tracking mirror with eccentric rotation axis and independent temperature control. the scanning space of the short side during the movement is reduced by tilting the rotation axis of the pendulum mirror to one side,. To decrease the height of the support axis for the pendulum mirror and minimize the overall launch envelope, the mirror is positioned horizontally during the launch stage. The design of the eccentric pendulum mirror utilizes rigid body isolation and radiation temperature control. It considers various factors including material properties, adhesive layers, and film layers. Additionally, it implements multi-point flexible installation and lens radiation temperature control techniques, while fine-tuning parameters through finite element analysis. The design achieved a 67.5mm reduction in the size of the transmitting envelope, accounting for 43% of the total height of the original mechanism. In the temperature range of 23~33℃, the RMS value of the pendulum mirror type changes to (1/166λ)/℃, which shows a linear trend. Lab test results show that the surface RMS value changes to (1/161λ)/℃ in the range of 21~32℃, which aligns well with the simulation value. The pendulum mirror has passed the mechanical and thermal vacuum environment test and achieved the design requirements, ensuring the smooth delivery of loads and on-orbit operation. Additionally, it introduces an innovative design optimization method for space eccentric tracking pendulum mirrors.
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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