Study on picometer-level laser interferometer readout system in TianQin project

As a central payload of TianQin space-borne gravitational wave detector, inter satellite laser interferometer allows to measure distance variance with extremely high precision. The readout system, or phasemeter, is often based on all digital phase locked loop in an FPGA. However, sampling jitter is one of the main inevitable noise sources during digitization. Here we use the pilot tone correction to suppress sampling jitter noise. To obtain accurate correction coefficient rather than preset values by normal pilot tone (NPT) correction, a least squares method (LSM) is presented. The experimental results show that the noise floor after pilot tone correction reaches 5 µrad/Hz1/2, which meets the TianQin project displacement noise requirement of 1 pm/Hz1/2. Besides, the correlation between the signal corrected before and after by LSM method is less than half of that by NPT method.