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Studies of multimode fibre linked white interferometric sensor systems

Wang, Q. (1997). Studies of multimode fibre linked white interferometric sensor systems. (Unpublished Doctoral thesis, City, University of London)


This thesis undertakes a detailed analysis of both the phase and intensity noise in a multimode fibre linked white light interferometric system. Several signal processing schemes have been introduced to ease the central fringe identification and to increase the accuracy of the central position measurement.

A detail investigation of modal noise induced by the multimode fibre link in the WLI system has been carried out. The relationship between modal noise and the parameters of the illuminating source and the linking optical fibre, including the coherence length of the source, the length of the linking fibre, the core diameter of the fibre and type of the fibre have been experimentally studied. A supporting theory has been developed and the results from the experiment are in good agreement with those from the theoretical analysis.

Two signal processing schemes have been developed to increase the relative intensity of the central fringe of the output in a dual wavelength system. One uses an electrical circuit to square the output of the two wavelength system directly, and the other uses two detectors to detect the different wavelengths components of the output of the system. Then, these two output are multiplied and squared using an analogue electrical circuit. With the use of these signal processing schemes, the central fringe becomes more dominant. The comparison of the two signal processing schemes has also been given.

A system consisting of two Michelson interferometers linked with a multimode fibre and illuminated with a dual wavelength source is investigated. The output signal to noise ratio of the system is measured by using a curve fitting method when different linking fibres are used. The result show that the output signal to noise ratio of the system increases as the core diameter of the linking fibre increases when the linking fibre is deliberately shaken. The effect of the modal noise on the central position measurement is also investigated.

A signal processing scheme based on curve fitting algorithm using a cosine function is introduced for this system. The theoretical resolution of the system with the use of the signal processing scheme is given when both intensity noise and phase noise is present. The displacement measurement repeatability of the system is experimentally measured over a displacement range of 80(im. The results show that the measured repeatability (standard deviations) of the system with the use of the signal processing scheme is better than 5nm, which is very close to the value of the theoretical resolution of, 1.9nm.

Publication Type: Thesis (Doctoral)
Subjects: T Technology > TA Engineering (General). Civil engineering (General)
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
[thumbnail of Wang thesis 1997 PDF-A.pdf]
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