Optical Signal Processing Technology In Space Optical Transmission Systems

This thesis revolves two kinds of systems related to space optical transmission: free space quantum key distribution (QKD) system and laser target detection system. Both of them can be summed up in active optical transceiver system. They have common in use of the basic principle, system structure, and devices. Our research is based on these two systems, includes key optical components improvement and innovative applications, system structure optimization design, optical signal processing technology improvement.The free space QKD usually adopts polarization coding, the bit error rate of the system will increase if the polarization state of the photon deviates from the ideal state which is required by the QKD protocol. So, first of all, the origin of the depolarization in the QKD system is analyzed. It indicates that the polarization dependent phase shift difference for the non-polarizing beam splitter (NPBS) is one of the main sources of the depolarization in this system, and it will introduce bit error into the system. We presented an advanced design of receiver in free space QKD system based on BB84protocol. In this system, a novel four-polarization-beam-splitter was used in the receiver. It makes use of reasonable optical path design and optimization to reduce the polarization degradation resulting from the phase shift difference of NPBS. Also it simplifies the receiver fabrication process, and the reflection loss is reduced too. Then we built the transmitter and the receiver for QKD system. The outdoor200m free space QKD experiment was carried out, the bit error rate is0.91%when the average photon number in each optical pulse was about0.1. This result indicates that our design for the QKD system is feasible.Four-polarization-beam-splitter effectively controlled the polarization degradation resulting from the phase shift difference of NPBS. To give full play to the advantages of the device, we put forward and realized the usage of two four-polarization-beam-splitters in the transmitter and receiver of QKD system. They rotate by90degree relative to each other. In this method, the phase shift difference caused by one NPBS was cancelled. As a result, the system bit error rate by the depolarization was decreased. Then we built the transmitter and the receiver for QKD system. The outdoor1.3km free space QKD experiment was carried out, the bit error rate is2.8%when the average photon number in each optical pulse was about0.1. The QKD system is stable and reliable. In order to further verify the advantage of the improved design method, contrast experiment was carried on. One transmitter of the QKD system consists four-polarization-beam-splitter, and another consists separated polarizer and diffraction grating to produce polarized photons and beam combining. The experiment result indicates that our design of four-polarization-beam-splitter simplified the system structure and maintained a good system performance.Laser target detection system is another kind of optical transmission systems we concerned. A frequency-modulated continuous-wave (FMCW) laser target detection system is demonstrated with heterodyne detection. The transmitter utilizes an electro-optic I/Q modulator for the first time to generate carrier-suppressed and frequency-shifted FM modulation. This eliminates the need for an acousto-optic frequency shifter commonly used in heterodyne detection systems. It also allows the use of a much wider modulation bandwidth to improve the range resolution. The distance test in an optical fiber delay line shows that the device has good linear modulation characteristic, the distance detection resolution is up to2,3cm.The capability of complex optical field modulation of the I/Q modulator provides an additional degree of freedom compared with an intensity modulator, which will benefit the target distance and velocity detection. The I/Q modulator is used in the transmitter to realize carrier-suppressed complex optical field modulation in which the positive and the negative optical sidebands can carry independent modulation waveforms. By loading a constant modulation frequency on the lower optical sideband and a wideband linear frequency modulation on the upper sideband, vector velocity and target distance can be measured independently. The wide modulation bandwidth of this system also enabled unprecedented range resolution and the capability of measuring high velocity unambiguously. This system is used to detect the distance and velocity of a spinning disc through free space. Based on the3dB width of the spectra, the resolution of the distance is3.2cm and the resolution of the velocity is0.5m/s, both of which agree with the theoretical value. Then the influence of FM linearity, carrier leakage and image frequency of I/Q modulation for detection results are analyzed.Finally we use this system to realize multi moving targets detection. Remain loading a constant modulation frequency on the lower optical sideband and change the one period frequency modulation signal to varied period frequency modulation in the upper sideband. Then the real distance and velocity of each target could be distinguished. Four different moving targets are used to do the MATLAB simulation.