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A Study of Optical-Fibre Electronically-Scanned White Light Interferometric Sensor Systems

Marshall, R. (1997). A Study of Optical-Fibre Electronically-Scanned White Light Interferometric Sensor Systems. (Unpublished Doctoral thesis, City, University of London)


Starting with a brief review of the development of optical fibre sensors, this thesis comprises work on the study of optical-fibre electronically-scanned white-light interferometers. The theoretical principles of white-light interferometry (WLI) techniques are examined, providing a comparison of these techniques with conventional interferometric methods, highlighting the key advantages of the former. The mechanically- and electronically-scanned WLI methods are also compared and contrasted, highlighting the key advantages of the electronically-scanned method, in that they do not require the use of mechanical phase modulation elements.

An electronically-scanned white-light interferometer (ESWLI) is composed of a sensing interferometer (SI) and a recovery interferometer (RI), operating in tandem. The main part of this work is centred around the design and development of a RI to be employed in an ESWLI. A full theoretical analysis and experimental evaluation of a novel Mach-Zehnder interferometer (MZI) configuration, employed as a RI is presented. The key effects of the SI (which was based upon a modified Fabry-Perot configuration) were also studied, in conjunction with the influence of modal noise in the multimode optical fibre coupler which connects the SI to the RI.

Various methods of enhancing the identification of the central fringe were successfully introduced. These included the method of using dual wavelengths, employing a least-squares curve fitting technique and the use of the centroid method. An investigation of optical dispersion in the dual wavelength configuration was also undertaken.

Examples of the applications of this ESWLI sensing system were also presented. First, the system was employed for the measurement of displacement, which provided an absolute displacement resolution of 40nm. Secondly, the system was studied for the measurement of vibration. Simultaneous measurements of AC and DC stresses were achieved, whereby the system successfully provided information on the direction of the motion associated with the vibration, in addition to its frequency. A photoconductive crystal (PCC) was then used as the detector (rather than a charge- coupled device (CCD) linear array), in a modification of the stress measurement system. Here, the intrinsic non-steady state photo-emf (PEMF) characteristic of the PCC was exploited in conjunction with the MZI to provide higher frequency, smaller amplitude vibration measurements. The system provided mHz sensitivity across a 5kHz range of vibration amplitudes down to the nanometer range.

To conclude this thesis, a cross comparison with previously proposed systems was made with a discussion of the main benefits of this device. A discussion of future work and the further potential of this approach is also presented.

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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