The performance of satellite-to-ground downlink optical communications over Gamma-Gamma distributed atmospheric turbulence are discussed for a direct detection differential phase shift keying system with spatial diversity, which combines the beam coherently before demodulation. Bit-error rate (BER) performances for various numbers of apertures are analyzed and compared for different zenith angle. We also consider the effect of the fiber coupling efficiency and loss ratio of beam coupler to the final BER. The results of numerical simulation shows that the advantage increases with the number of aperture. All the numerical results are verified by Monte-Carlo simulations.
In the field of intersatellite laser communication, there are two high-sensitivity demodulation methods: binary phase-shift keying (BPSK) coherent detection and differential phase-shift keying (DPSK) coherent detection. After taking into account the advantages and disadvantages of BPSK and DPSK, a DPSK heterodyne coherent detection scheme with local oscillation enhancement is proposed. The structure and the principles of this detection system are described, and the theoretical deduction is presented. Moreover, an experimental setup was constructed to test the proposed detection scheme. The offline processing procedure and results are presented. This scheme has potential applications in high-speed intersatellite laser communication.
In the field of satellite communication, space laser communication technology is famous for its high communication rate,
good confidentiality, small size, low power consumption and so on. The design of coherent optical communication
detection device based on modified balanced optical phase-locked loop (OPLL) is presented in the paper. It combined by
local oscillator beam, modulator, voltage controlled oscillator, signal beam, optical filter, 180 degree hybrid, balanced
detector, loop filter and signal receiver. Local oscillator beam and voltage controlled oscillator trace the phase variation of
signal beam simultaneously. That taking the advantage of voltage controlled oscillator which responses sensitively and
tunable local oscillator laser source with large tuning range can trace the phase variation of signal beam rapidly and achieve
phase locking. The demand of the phase deviation is very low, and the system is easy to adjust. When the transmitter
transmits the binary phase shift keying (BPSK) signal, the receiver can demodulate the baseband signal quickly, which has
important significance for the free space coherent laser communication.
The method to realize the integration of laser communication and ranging is proposed in this paper. In the transmitter of two places, the ranging codes with uniqueness, good autocorrelation and cross-correlation properties are embed in the communication data and the encoded with the communication data to realize serial communication. And then the encoded data are modulated and send to each other, which can realize high speed two one-way laser communication. At the receiver, we can get the received ranging code after the demodulation, decoding and clock recovery. The received ranging codes and the local ranging codes do the autocorrelation to get a roughly range, while the phase difference between the local clock and the recovery clock to achieve the precision of the distance.
Laser beam's acquisition, pointing and tracking are crucial technologies of free space optical communication. Fine tracking
system is an important component of APT (acquisition, pointing and tracking) system. Traditional fine tracking system
use CCD or quadrant detector as the position detector of signal light. In order to simplify the system and improve accuracy,
we propose a fine tracking system based on fiber nutation which don’t need a position detector and theoretically prove that
the system is feasible. Meanwhile corresponding fine track system was built, experiment on position detection of the fine
tracking system is done.
The implementation of down-looking Synthetic Aperture Imaging Ladar(SAIL) uses quadratic phase history reconstruction in the travel direction and linear phase modulation reconstruction in the orthogonal direction. And the linear phase modulation in the orthogonal direction is generated by the shift of two cylindrical lenses in the two polarization-orthogonal beams. Therefore, the fast-moving of two cylindrical lenses is necessary for airborne down-looking SAIL to match the aircraft flight speed and to realize the compression of the orthogonal direction, but the quick start and the quick stop of the cylindrical lenses must greatly damage the motor and make the motion trail non-uniform. To reduce the damage and get relatively well trajectory, we make the motor move like a sinusoidal curve to make it more realistic movement, and through a resampling interpolation imaging algorithm, we can transform the nonlinear phase to linear phase, and get good reconstruction results of point target and area target in laboratory. The influences on imaging quality in different sampling positions when the motor make a sinusoidal motion and the necessity of the algorithm are analyzed. At last, we perform a comparison of the results of two cases in resolution.
A structure similar to the direct detection of the DPSK signals using polarization dependent free space Mach-Zehnder interferometer is set up to validate its ability for coherent ranging. M sequence is adopted for its superior performance in code compression. The resultant signal voltage is sampled instead of being zero crossing detected and cross-correlated with the modulation signal sampled at the same rate. The ranging peak appears in the one target setup after proper calibration of the interferometric bias point but doesn’t imply any correct range information. For the two range resolved targets, the output image doesn’t depict two independent peaks. The further study is being conducted.