Novel Embedded Metrology Instruments for the Light Controlled Factory

Abstract

This thesis aims to document the work done over the previous thirty-six months as part of the Light Controlled Factory (LCF) research project. The first part of the thesis aims to familiarise the reader with a) the problem that we are trying to solve, b) some standard metrology terms and c) state-of-the-art coordinate measuring machines and techniques such as laser tracker and photogrammetry. The environment is the main limiting factor when trying to provide accurate measurements in large scale manufacturing, such as aircraft production. In the following chapters the solution is referred to as AMS, Absolute Multilateration Between Spheres. The idea involves interferometry in order to measure the distance between the centres of two spheres whilst the optical path is shielded from environmental disturbances.

Two computational AMS models, the ray tracing and Gaussian beam models, were developed in order to test and confirm the validity of the solution. Their capabilities were investigated and compared. The data gathered suggest that using feedback inherent to the system it is possible to achieve sub-micron accuracy when aligning the interferometer. The experimental work involved the development of a Mach Zhender and the AMS interferometers. The interference patterns were analysed and the data gathered from the quadrature detection were interpreted. It was therefore possible to define the direction of the movement of the interferometric pattern and whether or not it had moved to the next fringe (or by a whole fringe).

Even though the diode laser used was surprisingly stable (30MHz) without any external systems involved, a feedback loop was developed in order to stabilise its frequency. For this a series of equipment was required i.e. frequency generator, lock-in amplifier, PID controller etc. A number of recordings of the interferometric signals, I&Q, for short (a few seconds) and long (12-hours) term time scales was taken when the laser was unlocked, frequency stabilised whilst using only the P term of the PID controller and frequency stabilised whilst using the PI terms. At the same time the beat frequency and Allan deviation of the laser was calculated. A combination of the results led to the conclusion that when using the feedback controller loop, the laser was locked to better than 0.4 MHz at 104 seconds.

At the same time, a hollow core optical fibre was designed and fabricated in order to develop an all-fibre acetylene gas cell for a collaboration project with the National Physical Laboratory. Their system already contains an acetylene reference cell (50 Torr) for monitoring frequency and frequency stability of the laser in a Frequency Scanning Interferometry (FSI) system. However, using an optical bre gas cell implies that the absorption peaks can have a much smaller pressure shift as the pressure inside the bre can be reduced whilst increasing the fibres length. A comparison of pressure shifts for the acetylene spectrum of the three di erent cells along with the Gaussian lineshape ts confirm that the centers of the acetylene peaks can be determined with higher accuracy when using the bre gas cell and the device can be implemented in an FSI system. Converting this device into an all-fibre design created issues regarding back-reflections. To minimise back-re ections a series of experiments was performed that involved tapering a single mode bre and inserting it into the hollow core bre, and coupling light in and out of a hollow core bre using angled polished patch cables. The back-re ections were minimised to-42dB and less than-66.3 dB respectively. The second option was therefore more promising and a technique was developed for implementing angle polished fibres in an all-fibre gas cell device. This involved coupling light in and out of a hollow core breusing angle cleaved pigtails whilst relying on a tight fit inside a capillary. The preliminary results were positive though the technique requires further improvement.

Date of Award	24 Mar 2021
Original language	English
Awarding Institution	University of Bath
Supervisor	William Wadsworth (Supervisor) & Patrick Keogh (Supervisor)