Shuhua Yan

The thermal tuning of microresonators is crucial for the accessibility and long-term stability of soliton microcombs. We propose an equivalent resonance probing method that enables operando probing of thermal tuning throughout the soliton generation process. A probe laser, ingeniously Pound–Drever–Hall-locked to a resonance adjacent to the pumped resonance, is used to indirectly monitor the pumped resonance evolution by beating with an atom-referenced pump laser. We characterize the thermal-tuning-induced frequency steps of resonance and the repetition rate with a temperature resolution of 11.3 μK at 1 s averaging time and extract the thermal coupling coefficient of the soliton microcomb to be 2.6 MHz/K. Using these calibrated parameters, we further implement active thermal stabilization that sustains a single soliton for 10 h. Our work provides deep insight into microresonator thermal tuning and offers an efficient pathway for thermal management of soliton microcomb systems.

Soliton microcomb (SMC) arouses wide interest in high-precision distance measurement by virtue of its characteristics in high repetition rate and on-chip capability. However, the high repetition rate brings challenges to frequency locking and signal processing. In this study, we propose a microwave photonic mixing method to simplify the signal processing in SMC-based synthetic wavelength interferometry. In order to achieve this method and obtain the equivalent low-frequency signal, multi-order sidebands with the same frequency interval can be generated and subsequently mixed in the optical domain using an electro-optic modulator (EOM). The microwave photonic mixing method not only decreases the required modulation frequencies for injection locking and frequency-downshifting, but also enables the use of a low-bandwidth photodetector for detection. In the ranging demonstration, the residuals are less than 5 μm compared with a referenced laser interferometer (LI) over a 90 mm distance, and the stability test shows an Allan deviation of 868 nm at an average time of 10 s. It is believed that this microwave photonic mixing synthetic wavelength interferometry for SMC-based distance measurement showcases potential in LIDAR, micro-nano manufacturing, inter-satellite ranging, etc.

Absolute dynamic gravimeters based on cold-atom interferometry provide a promising and efficient approach for gravity mapping. However, there is currently a lack of experimental and quantitative evaluations of the spatial resolution of dynamic atom gravimeters. In this study, we propose a method to test the performance of a shipborne atom gravimeter, wherein the spatial resolution is experimentally evaluated based on acceleration variations induced by the E¨ otv ¨os effect. We developed a shipborne atom gravimeter and conducted a dynamic measurement experiment in a meandering river. The experimental results demonstrate that our shipborne gravimeter achieves a spatial resolution of 525 m. The internal consistency precision of our gravimeter is estimated to be 0.64 mGal, while the external consistency precision is 0.84 mGal. In addition, we demonstrated the adaptability of our gravimeter to diverse survey lines and validated its potential for applications in complex terrains such as meandering rivers and narrow lakes.

The high-repetition-rate dual-microcomb interferometry, characterized by its high precision, rapid measurement speed, and ease of integration, shows significant promise in applications such as precision spectroscopy and high-speed precision ranging. As dual-microcomb interferometry usually requires a specific difference in repetition rates, tuning the repetition rate of the microcomb is crucial for integrating dual-microcomb sources and enhancing the measurement performance, including the precision and the update rate. This work demonstrates a coherent dual-microcomb system driven by a single continuous-wave fiber laser at 1560.49 nm. The system employs a hybrid tuning method combing single-sideband (SSB) modulation for precision pump frequency control (enabling continuous repetition rate tuning across a 4.34 MHz range) with thermal control for coarse tuning. The linear dependence between the repetition rate and pump modulation frequency shows a measured coefficient of 143.58 kHz/GHz. This method enables dual microcombs with MHz-level repetition rate tuning, significantly relaxing the fabrication and pairing requirements for microresonators. The advancement is particularly valuable for dual-comb spectroscopy and ranging applications, including gas detection and satellite formation flying.

High repetition rate dual-microcomb interferometry offers high-precision and fast acquisition, showing broad application prospects in rapid and precise ranging scenarios. In this study, a continuous-wave laser was coupled into two chip-based integrated silicon nitride microresonators to drive two dissipative Kerr soliton optical frequency combs. Their repetition rates, 24.96202 GHz and 24.96121 GHz, were locked to external reference RF sources. Using this dual-microcomb source, we conducted linear displacement experiments within a range of 150 millimeters, with residual errors less than 10 micrometers compared to a reference laser interferometer. In static measurements, the Allan deviation reached 1.6 nanometers at an averaging time of 40 milliseconds. Additionally, with an update rate exceeding 800 kHz, dynamic measurement capabilities were demonstrated by measuring the rapid rotating chopper. To the best of our knowledge, this study is the first to utilize a repetition-frequency-locked coherent dual-microcomb source for ranging experiments, significantly enhancing distance measurement accuracy to the nanometer level compared to free-running systems. The proposed system is of importance for future high-precision LiDAR and remote sensing applications based on dual microcombs, and holds potential in areas such as multi-satellite formation flying and microelectronic device manufacturing.

Optical interferometry has emerged as a cornerstone technology for high-precision length measurement, offering unparalleled accuracy in various scientific and industrial applications. This review provides a comprehensive overview of the latest advancements in optical interferometry, with a focus on grating and laser interferometries. For grating interferometry, systems configurations ranging from single-degree- to multi-degree-of-freedom are introduced. For laser interferometry, different measurement methods are presented and compared according to their respective characteristics, including homodyne, heterodyne, white light interferometry, etc. With the rise of the optical frequency comb, its unique spectral properties have greatly expanded the length measurement capabilities of laser interferometry, achieving an unprecedented leap in both measurement range and accuracy. With regard to discussion on enhancement of measurement precision, special attention is given to periodic nonlinear errors and phase demodulation methods. This review offers insights into current challenges and potential future directions for improving interferometric measurement systems, and also emphasizes the role of innovative technologies in advancing precision metrology technology.

Optical carrier-suppressed single sideband (OCSSSB) signals are known for their easy detection, low noise, and high energy efficiency. Here, we develop a high-performance OCSSSB signal generation system utilizing an integrated TFLN inphase/quadrature (IQ) modulator, along with an automatic bias control (ABC) module. Our device can achieve a side-mode suppression ratio (SMSR) surpassed 20 dB, optimally reaching up to 29.53 dB, which covers a large electrical-optic bandwidth and central wavelength range. For long-term stability, the peak–to– peak fluctuation of SMSR was measured to be 1.79 dB, with an average SMSR value of 23.47 dB for nearly 8 hours. Furthermore, we successfully demonstrate that our device can be implemented in a frequency-modulated continuous-wave (FMCW) ranging detection, allowing the standard deviation no more than ±2.60 mm. Our system offers substantial potential for direct application in various chip-scale integrated fields, including LIDAR and microwave photonics.

The state preparation is a crucial procedure in atom interferometry; however, there is a shortage of detailed experimental studies on determining the optimal method for achieving this. This paper investigates and compares two methods for state preparation: the combined use of microwave and Raman light (M-R) and the combined use of optical pumping, microwave, and Raman light (O-M-W). The experimental results demonstrate that the M-R method improves the efficiency of Raman transitions for atom interference, which is helpful in enhancing the contrast of the interference fringes. The O-M-R method increases the quantity of prepared atoms, thereby enhancing the signal-to-noise ratio of the detected signals. This work helps provide a useful experimental basis and reference for researchers to design a suitable state preparation scheme.

Grating interferometers highly valued for high resolution, straightforward benchmark and insensitivity to environmental disturbances, have become one of the most mainstream methods for ultra-precision positioning. Traditional heterodyne grating interferometers which use a dual-frequency orthogonally polarized laser source suffer nanometer-level periodic nonlinear error due to frequency and polarization mixing, thus becoming incompetent to undertake sub-nanometer measurement. Based on the design of two separated laser beams with a small frequency difference for heterodyne beating, a wavelength-stabilized and quasi-common-path heterodyne grating interferometer is proposed. It incorporates a symmetrical oblique incidence structure where the residual dead path is calibrated and the laser source wavelength is stabilized to the rubidium atomic transition line, diminishing heterodyne phase errors resulted from the optical path difference. This approach not only significantly mitigates periodic nonlinear error to below 0.3 nm, but also enhances measurement resolution, repeatability and interference robustness. The proposed heterodyne grating interferometer is determined for ultra-precision positioning serving for precision machinery manufacturing and scanning beam interference lithography and so forth.

One important parameter in estimating the uncertainty of an atom gravimeter is the equivalent height due to the presence of non-negligible vertical gravity gradient. In this paper, we report a novel method to determine the equivalent height based on the location-dependent Rabi frequency produced by modulating Raman lasers. We theoretically investigate this method and apply it on an atom gravimeter. Experimental results reveal that with this new method, 0.06 μGal uncertainty in gravitational acceleration can be achieved when taking the measurement uncertainty of the equivalent height (0.26247 m ± 0.00021 m) as the only contribution. This uncertainty is negligible compared to the current state-of-the-art gravity measurement uncertainty of 2-5 μGal. The demonstrated method can be widely applicable in most of the existing atom gravimeters for the evaluation of their equivalent height, especially those for high-precision metrology and fundamental physics test.