Pico-watt and femto-watt weak-light phase locking

Advances in laser physics and its applications have triggered the proposition and development of Laser Astrodynamics. In carrying out research projects on Laser Space Programs, it is necessary to process the laser signal sent back from remote spacecraft. After traveling an extremely long distance, the power of this signal is greatly reduced. Weak-fight phase-locking is the key technique used for signal amplification in these space projects. After the returning laser beam is collected 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. We used diode-pumped non-planar 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 using ND (neutral density)-filters to decrease the fight intensity. In the phase detection, we used balanced detection to eliminate laser intensity noise and improve the SIN ratio. Combining 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-fight beam with a 57 mrads (rms) phase error. The locking duration was very long. Locking of a 200 pW and a 200 muW light beam, with phase error of 200 mrad (rms) and duration of over 2 hours was achieved. The phase error for locking a 200 muW to a 20 pW light beam was 160 mrad (rms). The locking duration was also longer than 2 hours. The last locking performed was carried out with a 2 pW and a 200 muW light beam. The phase error and the locking duration were 290 mrad (rms) and 1.5 min respectively.