激光天文动力学弱光锁相的研究

On the Study of Weak-Light Phase Locking for Laser Astrodynamical Missions

近年来激光物理与应用的进展促成了激光天文动力学空间任务概念的提出。在激光天文动力学任务概念研究方面 ,必须对由远程航天器上传回的激光进行讯号的测量与处理。激光经过长距离的传输后 ,功率大幅下降 ,因此在空间任务概念的考虑上 ,弱光锁相是计划中关键性的技术。由航天器射来的激光 ,经过望远镜聚光后与本地激光进行锁相 ,由本地激光承载及传达太空激光的相位信息。实验中 ,我们使用 2支半导体激光泵浦非平面环形共振腔钇镏石激光 (Diodelaserpumpednon -planarringcavityNd :YAGlaser) ,分别代表远程的弱光及代表本地的强光 ,建立弱光锁相环路系统 (weak -lightphase -lockedloopsystem)。以中性光强度滤光器 (ND -filter;neutraldensityfilter)减弱光讯号来仿真远程激光传来的弱光。在相位探测部分使用均衡探测法 ,消除激光强度噪声 ,以提高讯噪比。同时配合适当的环路滤波器 ,控制激光频率 ,提高锁相的能力。对 2nW的弱光与 2mW的强光可长时间锁相 ,其均方根相位误差为 57mrad。 2 0 0pW的弱光与 2 0 0 μW的强光锁相时间可达 2h以上 ,其相位误差为 2 0 0mrad。 2 0pW的弱光与 2 0 0 μW的强光锁相时间亦可达 2h以上 ,其相位误差为 1 60mrad。最后 ,我们对 2pW的弱光与 2 0 0 μW的强光锁相 ,

Advances in laser physics and its applications triggered the proposition and development of Laser Astrodynamics. In carrying out the research projects on Laser Space Programs, it is necessary to process the laser signal sent back from the remote spacecraft. After an extremely long distance of travel, the power of this signal is greatly reduced. The weak-light phase-locking technique is the key technique to amplify the signal in these space projects. After collecting the returning laser beam by telescope, it is used to phase-lock a local laser oscillator. The local laser then carries the phase information of the remote spacecraft laser. Experimentally, we used two diode-pumped nonplanar ring cavity Nd:YAG lasers to serve as the remote weak-light laser and the local strong-light laser. We then built an optical phase-locked loop to phase-lock them. The weak-light laser signal was simulated by using ND(neutral density)-filters to decrease light-intensity. In the phase detection, we used balanced detection to eliminate laser intensity noise and improve the S/N ratio. Combined this with an appropriate loop filter, we were able to control the laser frequency and improve the phase-locking ability. We phase-locked a 2 nW weak-light beam and a 2 mW strong-light beam with a 57 mrads (rms) phase error. The locking duration was very long. For a locking of a 200 pW and a 200 μW light beam, the phase error was 200 mrad (rms) and the locking duration was above 2 hours. The phase error of a locking of a 20 pW with a 200 μW light beam was 160 mrad (rms). The locking duration was also more than 2 hours. The last locking we performed was carried out with a 2 pW and a 200 μW light beam: the phase error and the locking duration were 290 mrad (rms) and 1.5 min respectively.