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White light interferometric sensor systems

Wang, D. N. (1995). White light interferometric sensor systems. (Unpublished Doctoral thesis, City, University of London)

Abstract

Optical fibre sensor technology has been developed extensively over the past decade. It provides a means of measurement which is flexible, accurate, free of electromagnetic interference and non-destructive and is suitable for a chemically hazardous environment.

Among various kinds of optical fibre sensors explored so far, the interferometric type has received much attention as it exhibits the highest level of sensitivity. However, conventional interferometric optical fibre sensors have only limited unambiguous range. To overcome this difficulty, white light interferometry has been developed, which provides the possibility of performing an absolute and unambiguous measurement of physical parameters.

This thesis represents a study of the light sources, scanning techniques and applications of white light interferometric sensor systems.

As a technique of using non-monochromatic light sources, white light interferometry is based on coherence theory which is introduced in Chapter 2. The main low coherence light sources and scanning techniques currently used are also discussed.

Multimode laser diodes have become more and more popular in white light interferometry because of their large output power and good coupling efficiency into optical fibres, but they have relatively long coherence lengths, which may cause some difficulties in identifying the central fringe in the output fringe pattern as such a fringe provides a reliable reference position thus posessing crucial importance in performing high precision measurement.

It can be seen from Chapter 3 and Chapter 4 that, by the use of two wavelength or multiwavelength combination source techniques, especially when the wavelengths are optimized, the central fringe identification becomes relatively straightforward. The methods of selecting the optimum wavelength combinations are also demonstrated, with the help of computer simulations.

The price paid for the adoption of two wavelength or multiwavelength combination sources is an increased system complexity and alignment difficulty. As demonstrated in Chapter 5, an Ar-ion laser pumped Sm3+-doped fibre provides an efficient and flexible means to generate a "pseudo" two wavelength and multiwavelength combination source without any additional difficulties in alignment. The fibre fluorescent sources may become highly competitive if a laser diode pumped system can be realized.

A conventional remote sensing white light interferometric system consists of two interferometers, of which one has to be scanned to recover the signals. An alternative optical scanning technique has been developed in Chapter 6, which shows distinct advantages over existing techniques by offering the possibility of eliminating one of the interferometers and thus simplifing the whole sensor system.

A simple optical sensor for eye length measurement is presented in Chapter 7, to explore the applications of white light interferometry. The principles of the system and the preliminary measurement results carried out on a simulated eye are presented.

Finally, the whole study is summarized and future research work is suggested.

Publication Type: Thesis (Doctoral)
Subjects: T Technology > TK Electrical engineering. Electronics Nuclear engineering
Departments: School of Science & Technology > Engineering > Electrical & Electronic Engineering
School of Science & Technology > School of Science & Technology Doctoral Theses
Doctoral Theses
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