Interferometric signals in space-borne Gravitational Wave Detectors are measured by digital phasemeters. The phasemeter processes signals generated by multiple interferometers, with its primary function being micro-radian level phase measurements. The Science Interferometer is responsible for inter-spacecraft measurements, including relative ranging, absolute ranging, laser communication, and clock noise transfer. Since the scientific interferometer incorporates multiple functions and various signals are simultaneously coupled into the heterodyne signal, establishing a suitable evaluation environment is a crucial foundation for achieving micro-radian level phase measurement during ground testing and verification. This paper evaluates the phase measurement noise of the science interferometer by simulating the heterodyne signal and establishing a test environment. The experimental results show that when the simulated heterodyne signal contains the main beat-note, upper and lower sideband beat-notes, and PRN modulation simultaneously, the phase measurement noise of the main beat-note, upper and lower sideband beat-notes all reach 2π μrad/Hz^1/2@(0.1 mHz–1 Hz), meeting the requirements of the space gravitational wave detection mission. An experimental verification platform and performance reference benchmark have been established for subsequent research on the impact of specific noise on phase measurement performance and noise suppression methods.

Space gravitational wave detection uses a three-satellite formation scheme, with the distance between satellites reaching hundreds of thousands or millions of kilometers. According to the principle of laser heterodyne interferometry, the distance change between the inter-satellite inertial references caused by the gravitational wave event is converted into the phase change of the heterodyne interference signal. The payload for measuring the phase change information is the phasemeter. The mission requires that the phasemeter’s ranging accuracy is 1 picometer, and the corresponding phase measurement accuracy is required to reach 2π μrad/Hz^1/2 @(0.1 mHz–1 Hz). Due to the inter-satellite Doppler effect, the dynamic range of the interference signal frequency reaches 5 MHz to 25 MHz. Due to the sampling jitter noise of the interference signal, it is necessary to suppress the noise through a single pilot tone. This paper introduces the development of the phasemeter, which uses a single pilot tone to suppress sampling jitter noise. The test results show that when the dynamic range of the interference signal frequency is 5 MHz to 25 MHz, the phasemeter meets the mission indicator requirement of 2π μrad/Hz^1/2 @(0.1 mHz–1 Hz).

As the primary electronic payload of laser interferometry system for space gravitational wave detection, the core function of the phasemeter is ultra-high precision phase measurement. According to the principle of laser heterodyne interferometry and the requirement of 1 pm ranging accuracy of the phasemeter, the phase measurement noise should reach 2π μrad/Hz^1/2@(0.1 mHz–1 Hz). The heterodyne interference signal first passes through the quadrant photoelectric detector (QPD) to achieve photoelectric conversion, then passes through the analog-to-digital converter (ADC) to achieve analog and digital conversion, and finally passes through the digital phase-locked loop (DPLL) for phase locking. The sampling timing jitter of the heterodyne interference signal caused by the ADC is the main noise affecting the phase measurement performance and must be suppressed. This paper proposes a sampling timing jitter noise suppression system (STJNSS), which can set system parameters for high-frequency signals used for inter-satellite clock noise transmission, the system clock of the phasemeter, and the pilot frequency for suppressing ADC sampling timing jitter noise, meeting the needs of the current major space gravitational wave detection plans. The experimental results after the integration of SJNSS and the phase meter show that the phase measurement noise of the heterodyne interferometer signal reaches 2π μrad/Hz^1/2@(0.1 mHz–1 Hz), which meets the requirements of space gravitational wave missions.

As one of the key payloads for space-borne gravitational wave detection (SGWD), the phasemeter is primarily responsible for conducting phase measurements of heterodyne signals. A phase extraction precision at the micro-radian level constitutes a crucial performance metric for intersatellite heterodyne interferometry. In this work, an ultra-stable hexagonal optical bench was developed using hydroxide-catalysis bonding technology. Different beat-notes were generated in accordance with the requirements of four experimental stages, which were applied to simulate the main beat-note of the inter-satellite scientific interferometer, thereby verifying the phase measurement performance of the phasemeter for beat-notes. Experimental results demonstrate that the phase extraction precision meets the index requirement of $2\pi \mu \mathrm{rad}/\sqrt{\mathrm{Hz}}$ for SGWD missions. Based on the test environment of the ultra-stable hexagonal optical bench, the feasibility of the phasemeter’s core phase measurement function was verified, laying a solid foundation for subsequent research on its auxiliary functions and extended tests.

A ranging communication system is a key technology for achieving precise ranging and scientific data exchange in space gravitational wave detection, with the aim of realizing the symmetry of interferometer arms. This system is integrated into the phase measurement payload, the ’phasemeter’. Achieving high-ranging accuracy and low-bit error rate communication while mitigating the impact of phase noise has become a focus of current research. This paper starts with the coding methods for ranging communication and analyzes phase modulation noise based on Binary Phase Shift Keying (BPSK). The study found that the main lobe phase noise caused by BPSK modulation is approximately 158 $\mathrm{\mu }\mathrm{r}\mathrm{a}\mathrm{d}/\sqrt{\mathrm{H}\mathrm{z}}$, which is two orders of magnitude higher than the phase-tracking criteria for gravitational wave detection. To address this, this paper proposes a Bit-Balanced Code (BBC) sequence design and optimization method aimed at eliminating main lobe noise. The experimental results show that the optimized BBC sequence improves the metrics of even autocorrelation, odd autocorrelation, maximum spectral amplitude, and even cross-correlation by 7.17, 2.83, 1.22, and 7.16, respectively, compared to the original sequence. Furthermore, experiments have demonstrated that the BBC sequence is insensitive to random data and can achieve dynamic bit balancing to eliminate the DC component. The proposed BBC sequence design method can serve as a reference for technologies related to space gravitational wave detection.

In space gravitational wave detection, the phase information of interfering signals is read out by a phasemeter, typically output sampling at a MHz frequency. To transmit the phase information between space and ground, it must be downsampled; however, spectral aliasing during downsampling will affect the performance of the phasemeter.This paper presents a four-module cascaded downsampling filter (FCDF) with detailed module parameter design. On-board experiments conducted in a phasemeter environment demonstrate that the FCDF achieves a passband attenuation of less than $8.68\times {10}^{-6}$ dB and a stopband attenuation exceeding 160 dB, enabling downsampling from 80 MHz to 3.4 Hz. Additionally, the FCDF offers improved low-frequency noise suppression, which can enhance phasemeter performance.