Two-dimensional angle measurement with sub-arcsecond precision and MHz update rate using heterodyne interferometry with optical frequency comb.

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.

Femtosecond-laser-based full-field three-dimensional imaging with phase compensation.

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.

Multi-pulse sampling dual-comb ranging method.

A multi-pulse sampling dual-comb ranging (MS-DCR) method is proposed in this paper. Four sampling pulses and two signal pulses separated in the time domain are generated in a repetition period by fiber delay. Through multi-pulse linear optical sampling, eight cross-correlation interferograms (IGMs) are generated in an updating period. The proposed method realizes the multiplication of IGMs so that additional ranging results can be obtained. The experimental results demonstrate that we suppress any random noise by averaging the ranging results and improve the precision of the time-of-flight (TOF) method and carrier-wave interferometric (CWI) method simultaneously. The precision of TOF is improved from 3.85 µm to 1.39 µm without time averaging and that of CWI is improved from 25 nm to 11 nm. The TOF result can link to the interferometric phase with 15 ms averaging, and a precision of 0.48 nm is reached with 0.5 s averaging. The proposed technique overcomes the limitations of linear optical sampling in conventional dual-comb interferometers and achieves faster and higher precision distance measurements without decreasing the unambiguity range.