1814
Research Article
Vol. 37, No. 11 / November 2020 / Journal of the Optical Society of America A
High-speed range and velocity measurement using frequency scanning interferometry with adaptive delay lines [Invited] Christos A. Pallikarakis,*
Jonathan M. Huntley,
AND
Pablo D. Ruiz
Loughborough University, Wolfson School of Mechanical, Electrical and Manufacturing Engineering, Loughborough LE11 3TU, UK *Corresponding author: C.A.Pallikarakis@lboro.ac.uk Received 28 July 2020; revised 7 September 2020; accepted 8 September 2020; posted 10 September 2020 (Doc. ID 403858); published 27 October 2020
Range (i.e., absolute distance), displacement, and velocity of a moving target have been measured with a frequency scanning interferometer that incorporates a 100,000 scan s−1 vertical-cavity surface-emitting laser with 100 nm tuning range. An adaptive delay line in the reference beam, consisting of a chain of switchable exponentially growing optical delays, reduced modulation frequencies to sub-gigahertz levels. Range, displacement, and velocity were determined from the phase of the interference signal; fine alignment and linearization of the scans were achieved from the interferogram of an independent reference interferometer. Sub-nanometer displacement resolution, sub100-nm range resolution, and velocity resolution of 12 µm s−1 have been demonstrated over a depth measurement range of 300 mm. Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. https://doi.org/10.1364/JOSAA.403858
1. INTRODUCTION A variety of pointwise optical techniques have been developed over the years for measurement of distance to a remote target. These include time of flight, phase shifting interferometry, and frequency scanning interferometry (FSI), also known as frequency-modulated continuous-wave (FMCW) lidar [1,2]. FSI is widely regarded as one of the most accurate methods when absolute distance, rather than changes in distance over time, is required. At its simplest, a tunable laser illuminates the target while undergoing a frequency scan at a constant rate. The target range is determined from the modulation frequency of the interference signal, produced when the back-reflected or back-scattered object wave is mixed with a reference wave from the same laser. The drawback of the method in its basic form is that the deduced range is dependent on the tuning rate of the laser, which can drift over time. The use of a separate reference interferometer avoids such errors: target range is determined as a multiple of the reference interferometer length through the ratio of modulation frequencies from the measurement and reference interferometers [3,4]. The inclusion of a gas absorption cell enables the calibration of the reference interferometer length traceable to international standards [5]. As calibration can be performed once per frequency scan, stability requirements of the reference interferometer are relaxed significantly with this approach. 1084-7529/20/111814-12 Journal © 2020 Optical Society of America
Although the accuracy of FSI can be in the range of 1 part in 106 to 1 in 108 , it has long been recognized that the technique’s “Achilles’ heel,” for highly accurate range measurement in practical industrial environments, is motion of the target, or changes in refractive index of the air, that occur during a scan. The error in the calculated range is equal to the intra-scan target displacement (or the effective displacement in the case of refractive index changes), multiplied by an amplification factor (ωc /1ω), where ωc is the angular frequency of the emitted light at the center of the scan, and 1ω is the angular frequency tuning range of the laser [3–7]. Several approaches to solving this problem have been proposed, such as the use of two lasers scanning simultaneously in opposite directions or at different rates [5–7], the generation of a frequency scan rate of opposite sign using four-wave mixing [8], the use of a laser with successive up–down frequency ramps [9], and the addition of a separate laser Doppler velocimeter (LDV) that informs the FSI system of the required error correction due to target motion [10]. When phase or frequency data are acquired from systems with frequency ramps of opposite sign, the average of the two values largely cancels the displacement error, while the difference between them provides a measure of the displacement or velocity, respectively, of the target. The ability to measure both range and velocity is attractive, but the cost and complexity