Experimental scheme and noise analysis of weak-light phase locked loop for large-scale intersatellite laser interferometer

Abstract

In the current space gravitational wave (GW) detection, satellites are separated by millions of kilometers. As a result, watts of laser from one satellite is attenuated to the picowatt level at the other end due to the Gaussian beam divergence and the finite aperture of the telescope. Establishing an effective interferometry with such weak-light is a major challenge. The key is to enhance the weak-light while preserving its phase information, which carries the actual GW signal. This can be accomplished by employing an optical phase-locked loop (PLL) to lock the phase of a local oscillator (LO) laser to the weak-light and then sending the power-amplified LO back to the interferometer on the other satellite. Although shot-noise-limited performance of the picowatt level weak-light PLL has been achieved for high frequencies, it remains elusive for frequencies below 0.1 Hz. Here, we propose a three-step experimental scheme to identify the main noise sources of the weak-light PLL, which turn out to be the low-frequency phase measurement noise, the weak-light shot noise, and the laser phase noise. In this paper, the first step experiment result shows that the out-loop phase noise can be suppressed to a level less than 6 × 10−6 rad/√Hz from 6 mHz to 1 Hz by first using the special pilot-tone technique in the PLL to directly reduce the sampling time jitter noise in the digital phasemeter. The out-loop phase noise is mainly limited by the signal amplitude variation and differential time jitter noise of the reference clock.

Weak-Light Phase-Locking Time Delay Interferometry with Optical Frequency Combs

“…Generally speaking, the WLPL noises are related to the phase measurement noise, the photodetector noise, the weak signal noise, the laser phase noise, etc. [ 18 , 19 ]. Now, the performance in the higher frequency band can already reach the limit of the shot noise, but in the lower frequency band there is still the residual noise possibly due to the thermal effects or the electrical noises [ 20 , 21 , 22 ].…”

Weak-light phase locking aided by frequency division phase meter for intersatellite laser interferometry

“…Samuel等 [13] 根据锁 相环模型优化锁相环带宽, 最小化由散粒噪声和激 光相位噪声引起的误差相位标准差, 实验设计的数 字锁相环可以实现最低接收功率30 fW的弱光锁 相, 周跳率小于0.01 s -1 . Liang等 [14] 从理论上分 析了电子噪声、激光振幅噪声和散粒噪声对信噪比 的影响, 通过优化电路参数和光功率, 提高了信噪 比, 实现了13 pW光功率的低噪声锁相. 并设计实 验验证弱光锁相中主要的噪声来源 [15] .…”