We present an absolute distance measurement system using a phase-stable dual-comb system with 56.09 MHz repetition rate and 2 kHz repetition rate difference. A relative phase stability of 0.1 rad in 0.5 ms between two combs is achieved using a mutual locking scheme. The dual-comb ranging system combines the time-of-flight (TOF) method, synthetic-wavelength interferometry (SWI), and carrier wave interferometry (CWI). Each method provides a particular ambiguity range and resolution, and they can be applied simultaneously and linked to enhance the precision and measurement rate of the ranging system. The experimental results demonstrate that a precision of 1.2 μm is obtained without time averaging, and the precision can be improved to 3 nm with only 10 ms averaging time using the SWI method described in this study. The precision reaches a sub-nanometer when the averaging time exceeds 0.1 s. A system with high accuracy and short averaging time would enhance fast measurement performance in various industrial applications. The ambiguity range is about 2.67 m in our system, we test the performance of the system with 1.5 mm range at 1.5 m distance.

Long-term mutual coherence is a key factor that affects the signal-to-noise ratio and resolution of a dual-comb interferometer. To realize a phase-stable dual-comb interferometer configuration, tightly phase-locked loop systems or digital error correction methods with external optical reference are commonly used. This paper presents a self-referencing digital error correction method based on the short-term spectral characteristics of interferograms to reduce the cost and complexity of the phase-stable dual-comb interferometer configuration. In our experiment, fully mutual coherence of a dual-comb interferometer is reconstructed and 1 Hz theoretical linewidth in 1 s acquisition time is achieved by digitally compensating for time jitter, center frequency jitter, and carrier-envelope-phase jitter, offering an effective technique for advanced dual-comb applications.

A phase-stable dual-comb interferometer measures materials’ broadband optical response functions, including amplitude, frequency, and phase, making it a powerful tool for optical metrology. Normally, the phase-stable dual-comb interferometer is realized via tight phase-locking methods. This paper presents a post-correction algorithm that can compensate for carrier wave phase noise and interferogram timing jitter. The compensating signal is a beat between two combs using a free-running continuous wave laser as an optical intermediary. In our experiment, sub-hertz relative linewidth, ~1 ns relative timing jitter, and 0.2 rad precision in the carrier phase is demonstrated.

Dual-comb system parameters have significant impacts on the ranging accuracy. We present a theoretical model and a numerical simulation method for the parameter optimization of a dual-comb ranging system. With this method we investigate the impacts of repetition rate difference, repetition rate, and carrier-envelope-offset frequency on the ranging accuracy. Firstly, the simulation results suggest a series of discrete zones of repetition rate difference in an optimal range, which are consistent with the experimental results. Secondly, the simulation results of the repetition rate indicate that a higher repetition rate is very favorable to improve the ranging accuracy. Finally, the simulation results suggest a series of discrete optimal ranges of the carrier-envelope-offset frequency for the dual-comb system. The simulated results were verified by our experiments.

We investigate a novel two-channel grating encoder that can perform simultaneous measurements of six-degree-of-freedom (DOF) motions of two adjacent sub-components of synthetic-aperture optics such as pulse-compression gratings(PCGs) and telescope-primary mirrors. The grating encoder consists of a reading head and two separate gratings, which are attached to the back of the sub-components, respectively. The reading head is constructed such that there two identical optical probes can share the same optical components. The two probes are guided to hit each of the two gratings and can detect six-DOF motions simultaneously and independently. For each probe, the incident beam propagates through both a three-axes grating interferometry module and a three-axes diffraction integrated autocollimator-module, which detects translational and rotational movement, respectively. By combining the two modules it is possible to perform six-DOF measurement for a single point. The common-path configuration of the two probes enable identical responses to environmental variation, which ensures high accuracy.