Lenless fiber-deployed low-coherence interferometer for in-situ measurements in non-ideal environments

Low-Coherence Interferometry (LCI) is a well-established optical method used for obtaining geometric measurements, suited to operating in non-ideal environments as shown through its use in biomedical science where it is referred to as Optical Coherence Tomography (OCT). However, work on characterising this 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 non-ideal 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 50 nm in air and 100 nm in water. Step heights of 8 micrometres, 7 micrometres, 6 micrometres and 5 micrometres were measured in air and also with the sample and sensing probe submerged in water. Step heights in both mediums closely align with calibrated specifications given by the manufacturer demonstrating sub-micrometre accuracy and a precision of 56 nm in air and 76 nm in water.