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.
This paper proposes a new method for variable rate DPSK in the satellite-to-ground laser communication. Compared to
the general method, this method doesn’t need to change the original communication rate, reduces the complexity of the
system. In theory, SNR can improve above 3 dB when rate into half. We construct a simplified experimental apparatus to
verify the effectiveness of the proposed method. The experimental devices and results are presented. Offline processing
results are very similar to theory, is superior to the traditional means. The further study about doppler effect and clock
synchronization is being conducted.
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.
Camera calibration is one of the indispensable processes to obtain 3D depth information from 2D images in the field of computer vision. Camera self-calibration is more convenient and flexible, especially in the application of large depth of fields, wide fields of view, and scene conversion, as well as other occasions like zooms. In this paper, a self-calibration method based on two vanishing points is proposed, the geometric characteristic of disappear points formed by two groups of orthogonal parallel lines is applied to camera self-calibration. By using the vectors’ orthogonal properties of connection optical centers and the vanishing points, the constraint equations on the camera intrinsic parameters are established. By this method, four internal parameters of the camera can be solved though only four images taken from different viewpoints in a scene. Compared with the two other self-calibration methods with absolute quadric and calibration plate, the method based on two vanishing points does not require calibration objects, camera movement, the information on the size and location of parallel lines, without strict experimental equipment, and having convenient calibration process and simple algorithm. Compared with the experimental results of the method based on calibration plate, self-calibration method by using machine vision software Halcon, the practicability and effectiveness of the proposed method in this paper is verified.