Liheng Shi

Ultrafast, high-precision roll-angle metrology is crucial for dynamic motion control yet remains challenging because roll is not directly accessible to conventional geometric optical methods. Here, we present a roll-angle measurement method based on a polarization-modulated optical frequency comb (PMOFC). By modulating the offset frequency, a sequence of femtosecond pulses with periodically varying polarization was generated. After passing through a birefringent target and a fixed-angle analyzer, the roll angle is retrieved from the corresponding intensity sequence. Experimental results demonstrate an absolute non-ambiguous range of 0°–90°, and a single-shot measurement time of ∼71 ns. At the worst-case angular position, the Allan deviation decreases below 20 ^{′ ′} after 3.93 μs of averaging. These results establish a practical route to high-rate absolute roll-angle measurement.

A dual-comb absolute-ranging payload delivered to China’s Tiangong space station aboard Tianzhou-9 has demonstrated sustained on-orbit interferogram acquisition in an extravehicular environment. The system reports 13 µm precision at a 1 kHz update rate and stable operation over months, advancing traceable space metrology for formation flying and deployable observatories.

High-precision six-degree-of-freedom (6-DOF, pitch, yaw, roll, axial displacement, and two lateral displacements) measurement is essential in advanced manufacturing and precision assembly, particularly in aerospace, semiconductors, and precision engineering. However, achieving high-precision absolute 6-DOF measurement directly on the target remains challenging. This study proposes a novel method for absolute 6-DOF measurement directly on non-cooperative targets, employing low-coherence light and temporal phase-shifting. This method extracts position and orientation information from the coherence envelope shape and the geometric texture of the target background without relying on cooperative targets. The proposed method achieves favorable results in verifying the resolution, linearity, and repeatability indicators of 6-DOF. Additionally, a simulation of the 6-DOF tracking of a satellite antenna central support plate during the manufacturing process demonstrates good agreement between the measured and reference values. This study presents a novel approach for high-precision absolute and direct 6-DOF measurements on non-cooperative targets, offering significant guidance for the processing and assembly of aerospace components in advanced manufacturing.

Dual-comb ranging (DCR) has emerged as a representative method for absolute distance measurement; however, its precision remains susceptible to intensity noise in practical implementations. This study establishes a comprehensive theoretical model for intensity noise in a linear optical sampling DCR system. The model is composed of two parts: one focuses on the influence of the system's hardware parameters on the signal-to-noise ratio (SNR) of dual-comb interferograms, and the other investigates the impact of key algorithm parameters on ranging precision. We design corresponding experiments to validate the model. A scheme is proposed to mitigate photodiode detector saturation based on the model, and the ranging precision is improved to 0.12 µm at a 2 ms refresh rate. Finally, the study proposes evaluation criteria for assessing DCR systems under intensity noise, enabling comparison of noise levels across different systems. This work provides theoretical insights for designing DCR systems and improving ranging precision.

The laser scanning and ranging system (LSRS) is a pivotal technology for conducting high-precision multi-dimensional measurements, with diverse applications in remote sensing, satellite formation flying, and surface profilometry. However, the capability to achieve accurate measurement of the current system is limited by the beam-pointing mechanism. Specifically, the rotation angle of the beam-pointing mechanism is too small to achieve zero-position self-calibration, and this mechanism can introduce errors that exceed the precision of distance measurement. Herein, we propose a self-referenced LSRS that can directly determine the measurement origin and avoid motion-induced optical path length errors into the interferometric arms. This system is achieved by placing the reference arm of the measurement system after the beam-pointing mechanism in the optical path. Using a dual-comb ranging system with a precision of ∼0.8 μm, we validated the performance of our design through scanning experiments with hollow retroreflectors and surface profile reconstruction of a gauge-block assembly. The experimental results demonstrate that our design achieves accurate measurements without pre-calibration or post-compensation, thereby enhancing its applicability to next-generation space missions.

Heterodyne interferometry is a powerful tool for achieving high precision and fast measurement. We developed an angle measurement system based on heterodyne interferometry by combining discrete equal-spacing longitudinal modes of optical frequency comb with an acousto-optic modulator. Using a self-designed grating-corner-cube sensor, this method can achieve a two-dimensional angle measurement with sub-arcsecond accuracy and megahertz (MHz) update rate. We experimentally demonstrate a precision of 0.073 arcsec under a 3 MHz update rate, and comparison residuals are kept within 0.063 arcsec over 300 arcsec when compared to a piezo stage. In the dynamic measurement of a 40 Hz frequency, the continuous sinusoidal motion of 0.05 arcsec can be clearly distinguished and reconstructed.

Coherence scanning interferometer (CSI) enables 3D imaging with nanoscale precision. However, the efficiency of such a system is limited because of the restriction imposed by the acquisition system. Herein, we propose a phase compensation method that reduces the interferometric fringe period of femtosecond-laser-based CSI, resulting in larger sampling intervals. We realize this method by synchronizing the heterodyne frequency with the repetition frequency of the femtosecond laser. The experimental results show that our method can keep the root-mean-square axial error down to 2 nm at a high scanning speed of 6.44 µm per frame, which enables fast nanoscale profilometry over a wide area.

The dual-comb technique is a powerful tool in industrial inspection and scientific research and is capable of realizing ultrahigh-resolution and fast broadband spectral measurements. We propose an absolute angular-position measurement method based on dual-comb spectroscopy. With a simple layout, the absolute angular position can be naturally determined through the traceable and wide-amplitude spectra of the autocollimation diffracted beams of the target grating. We experimentally demonstrate that a precision of 0.12 arcsec in the dynamic range of approximately 6660 arcsec, along with a 1 kHz repetition rate difference, is achieved. Compared with a commercial autocollimator, over 1000 arcsec, the comparison residuals are kept within ±0.3 arcsec.

Real-time measurement of the thickness and group refractive index is crucial for semiconductor devices. In this paper, we proposed a fast synchronous method for measuring the thickness and group refractive index distribution of solid plates based on line-field dispersive interferometry. The proposed method measured the line-field distribution in an illuminated region through a single step. A low-cost spectrometer calibration method using an eight-channel dense wavelength division multiplexer was developed for verification. The line-field distribution of a three-step silicon wafer was successfully measured within 3.3 ms. The combined uncertainties for the geometrical thickness and group refractive index were −4, respectively.