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

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.