Experimental Demonstration of Second-Generation Time-Delay Interferometry With Direct Digital Synthesizer

The direct digital synthesizer (DDS) can be used to generate analog electrical signals with dynamic long-time delays. Based on this capability, this work integrates computer coding with DDS devices to simulate data streams containing noise with dynamic long-time delays in space-borne gravitational wave (GW) detection, and further verifies the critical data processing noise suppression technique, time-delay interferometry (TDI) technique. The TDI technique was proposed to suppress laser frequency noise in space-borne GW detection, which is approximately seven orders of magnitude higher than the GW signals. However, its effectiveness in suppressing laser frequency noise has not been adequately verified through ground-based experiments to date. In this work, we first simulate the data stream for space-borne GW detection using DDS devices, which contains laser frequency noise, GW detector noise floor as well as weak GW signals. The acquired data are then brought into first-generation and second-generation TDI combinations, respectively, to suppress the laser frequency noise. The experimental results demonstrate the variations in the suppression of dynamic long-delay laser frequency noise among different generations of TDI combinations. Notably, with the adoption of the second-generation TDI combination, the laser frequency noise can be suppressed to the level of the detector noise floor, effectively restoring the weak GW signal. During the experiment, the frequency divider is cleverly employed to mitigate the high quantization noise of the DDS device, ensuring that the overall experimental noise level satisfied the requirement of typical space-borne GW detectors. This research offers a ground-based experimental verification system for space-borne GW detection, featuring both long-time delays and low additional noise.