Lensless fiber-deployed low-coherence interferometer for in-situ measurements in nonideal environments

Abstract

Low-coherence interferometry (LCI) is a well-established optical method used for obtaining geometric measurements, suited for operating in nonideal environments as shown through its use in biomedical science, where it is referred to as optical coherence tomography. However, work on characterizing these technologies' ability to work in-situ within the area of manufacturing is yet to be demonstrated. This research is motivated by the need to develop robust sensors capable of operating in the harsh environment of manufacturing processes in near real-time, providing on-demand process control for the next generation of precise, and highly adaptive schemes of production. The evaluation of a common-path, lensless, spectral-domain, LCI-based sensor for measurements of step heights in air and in the nonideal operating environment of water is demonstrated. Calibration experiments have explored linearity of measurements over a 1-mm investigated axial range with deviations of the order of AE50 nm in air and AE100 nm in water. Step heights of 8, 7, 6, and 5 μm were measured in air and also with the sample and sensing probe submerged in water. Step heights in both media closely align with calibrated specifications given by the manufacturer demonstrating submicrometer accuracy and a precision of AE56 nm in air and AE76 nm in water. © The Authors. Published by SPIE under a Creative Commons Attribution 4.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.

From Light to Displacement: A Design Framework for Optimising Spectral-Domain Low-Coherence Interferometric Sensors for In Situ Measurement

“…It can be seen from Table 1 that a diverse range of applications have already had initial investigation into the use of LCI sensors for monitoring processes. Recent work demonstrating the use of LCI in the domain of a liquid environment has shown that the system is able to provide stable, accurate data during profilometry measurements across various step-heights [12]. An extension of this work demonstrates an embedded LCI sensor for absolute distance measurement of samples operating within a jet of water has also shown significant data stability [13], demonstrating the potential usefulness of the LCI sensor for post-processing verification of parts, utilising a liquid jet for flushing of debris and coolant during measurement.…”

“…As such, further calibration of the system can be undertaken to improve the accuracy of this transfer function. Previous work discovered for one setup that, during measurements in air, this would lead to errors of up to approximately 50 nm and whilst operating in water approximately 100 nm [12]. Hence, depending on the measurement accuracy requirements, further action may be required to compensate for nonlinearities in the system.…”

“…Photon noise also known as shot noise is a statistical variation in the arrival rate of photons on the camera detector governed by Poisson statistics and as such the magnitude is linked to the square root of the signal optical intensity as shown in Equation (12). Dark noise results from a charge accumulation in each pixel due to thermal fluctuations in the silicon which can release electrons.…”

“…The axial resolution in an LCI system is determined by the smallest change in OPD that the system can detect, determined by the spectral resolution of the spectrometer. Previous work utilising the LCI system described in Section 3.2.1 has demonstrated submicrometric accuracy across step heights ranging from 5 µm to 8 µm with a precision of ±56 nm [12]. However, if transparent samples are measured with multiple reflective layers, the smallest discernible distance between layers will be limited by the coherence length of the light source and the axial resolution will be as shown in Equation (17).…”

“…When performing tomographic measurements, the resolution of the instrument is limited by the coherence function due to potential reflectors interfering with one another. It has been shown for measurements of singular reflecting surfaces the accuracy of measurement is not limited by the peak PSF with step-height measurements with submicrometric accuracy [12].…”