皮米级外差干涉仪中低信噪比和采样时间抖动对相位测量的影响

Ultra-precision laser ranging system will be used to measure the inter-satellite displacement for space-borne gravitational waves detection (e.g. Laser Interferometer Space Antenna, LISA) and the next generation of the Earth’s gravity recovery mission (e.g. GRACE follow-on). This tiny displacement results in a phase shift of the beat notes of heterodyne interferometer; hence, the high-precision phase measurement is one of the key techniques in the inter-satellite laser ranging system. In the experiment of testing vacuum magnetic birefringence, we will use heterodyne laser interferometer with an even better sensitivity to measure the frequency (or phase) variation caused by the vacuum birefringence. In LISA mission, the low signal-to-noise ratio (SNR) of the measured signal limits the resolution of phase measurement. To achieve a higher precision of phase measurement, the sampling time jitter is the main noise source. In this project, we will combine the advantages of correlation arithmetic and the digital phase-locked loop to overcome the problem of low SNR. When the received light power is as weak as 100 pico-Watts, the resolution of phasemeter is expected to be about 1E-5rad/√Hz at 0.1Hz. In addition, we will improve the method of pilot tone correction to reduce further the influence of the sampling time jitter, especially when the measured signal doesn’t have a constant frequency, and the resolution of the phase measurement is expected to be 1E-6rad/√Hz at 0.1Hz, which will be improved by one order of magnitude compared to the resolution before correction.

空间引力波探测和下一代地球重力场反演计划都将采用星间激光干涉测距技术来测量卫星间的相对位移。卫星间距的微小变化会引起外差干涉仪拍频信号的相位变化；因此，高精度的相位测量是星间激光测距系统中的关键技术之一。此外，在检验真空双折射效应的实验中，我们将利用外差激光干涉相位测量方法检验真空双折射所引起的微小频率变化，此实验对相位测量的精度提出了更高的要求。LISA计划中，待测信号的信噪比非常低，是干涉仪相位测量的主要噪声来源；如果要求更高精度的相位测量，则采样时间抖动为主要噪声来源。在本项目中，我们将结合相关算法和数字锁相环的优势来减小低信噪比的影响，在接收光功率为100pW的条件下实现相位测量的分辨率达到1E-5rad/√Hz@0.1Hz。另外，在待测信号频率变化的情况下，我们将改进导频修正方法来降低采样时间抖动的影响，将相位测量的分辨率提高一个量级，达到1E-6rad/√Hz@0.1Hz。

星间激光测距系统是未来的空间引力波探测和卫星重力场测量中的重要载荷。外差激光干涉仪的输出信号，需要通过高精度的相位计测量该信号的相位变化。因此，高精度的相位检测技术是星间激光测距系统中的关键技术之一。另外基于锁腔法检验磁场下的真空双折射效应，对相位测量的分辨率提出了更高的需求。该项目主要研究相位计的理论建模、噪声分析、仿真分析，并完成硬件电路的标定测试、噪声本底测试以及环境调制实验。实验表明相位测试系统的主要的噪声来源为采样时间抖动、温度变化导致器件的相移以及输入信号的信噪比。采用导频修正技术和温度控制后，相位测量噪声本底在0.01 Hz处为0.6 μrad/Hz^1/2，相比项目研制之前提高了2个量级(比研究目标好3倍)，可以满足外差锁腔法测量真空双折射效应以及空间引力波探测计划对相位测量精度的需求。