Optical Vernier sampling using a dual-comb-swept laser to solve distance aliasing

Abstract

Optical interferometry using comb-swept lasers has the advantage of efficiently reducing the acquisition bandwidth for high-speed and long-range detection. However, in general, the use of a comb-swept laser involves a critical limitation in that the absolute distance cannot be measured, and, thus, multiple layers cannot be distinguished when measuring each position. This is because of the distance ambiguity induced by optical aliasing, in which there is periodic repetition of the frequency of an interferometric signal owing to discrete spectral sweeping, which does not occur in conventional optical interferometry that uses a continuous swept laser. In this paper, we introduce an optical Vernier sampling method using a dual-comb-swept laser to measure the absolute distances in a multi-layer target. For this, we designed a new type of dual-comb-swept laser to include two different free spectral ranges (FSRs) in separated wavelength bands to provide a stable lasing condition. Using a principle similar to that of a Vernier caliper for length measurement, the two different FSRs can be used to recover a higher frequency of an optical interferometric signal to measure longer distances from different layers in a target. Using the dual-comb-swept laser in optical interferometry, we solved the optical aliasing issue and measured the absolute distances of three layers separated over 83 mm using a point-scanning imaging setup and the simultaneous absolute distance of the top surfaces separated over 45 mm using a full-field imaging setup at 14 and 8 times lower acquisition bandwidth than a conventional continuous swept laser that is based on optical interferometry.

High-Precision Silicon-Integrated Frequency-Modulated Continuous Wave LiDAR Calibrated Using a Microresonator

“…Among them, the LiDAR has the advantages of high measurement accuracy, high speed, compatibility with noncooperative target measurement, and so forth. It has been widely used in large absolute distance measurement − and has become a hotspot of current research. , …”

Programmable Optical Vernier‐Sampled Coherent Ranging

“…The maximum detection distance without ambiguity in the OVS method ( d max ) can be determined as half of the least common multiple (LCM) of d limit1 and d limit2 . [ 20 ] To maximize d max , d limit1 , and d limit2 must be related to the relative prime. The last term of Equation () shows that d limit can be changed by ( N step – 1) under fixed n and B .…”

“…The dual‐comb‐swept laser comprises two different etalon comb filters and a fiber Fabry–Perot tunable filter (FFP–TF) and sequentially emits two different free spectral ranges (FSRs) of the comb output. [ 20 ] In the OVS method using the dual‐comb‐swept laser, every target distance generates a subsampled frequency pair owing to the two different sequential FSRs of the comb spectra output. Thus, the actual interference frequency can be recovered by comparing pairs of subsampled frequencies.…”

“…The maximum detection range is determined by a combination of the FSRs of the two etalon comb filters. [ 20 ]…”

“…For example, a subsampled coherent ranging (e.g., circular-ranging optical coherence tomography (OCT)) utilizes a stepped-frequency-swept laser (stepped-FSL) (also called a comb-swept laser) to achieve an optical subsampling effect. [15][16][17][18][19][20][21] This approach has been demonstrated to produce compressive OCT images in a depth range of ≈3 cm. [15] However, a subsampled interference signal cannot directly represent the actual target distance because of a distance-aliasing effect.…”