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An optical fibre based polarisation modulation technique: Development and applications

Chitaree, R. (1994). An optical fibre based polarisation modulation technique: Development and applications. (Unpublished Doctoral thesis, City, University of London)

Abstract

This thesis undertakes an investigation of polarisation modulation techniques arriving from optical transmission in both free-space and optical fibres with the aim of determining relative performances for prospective applications. The system of optimal performance is then applied to make ellipsometric measurements.

Polarisation modulation techniques considered allow control of the state of polarisation through an interferometric method and modulation of the phase difference. Precise state and orientation of polarised light at any instant of time are obtainable. These qualities match the specific requirement of a range of optical instruments, such as polarimeters and ellipsometers.

The evolution and development of various polarisation modulation techniques are discussed in terms of important features such as the operating speed, the number of optical components required and methods of controlling the polarisation. After initial considerations, two configurations were chosen for more detailed experimental study. Their arrangement produces a rotating plane polarised output and are based on interferometric techniques. One configuration is characterised by free-space propagation (Michelson interferometer); whereas, the other uses highly biréfringent (HiBi) fibre as the light propagation medium.

The theoretical and experimental performances of both these configurations are investigated. Full studies focusing on the output quality of the controlled light in terms of the degree of polarisation and ellipticity from the two arrangements were undertaken. These factors are used to determined which configuration offers better performance. Further extensive investigations, particularly on performance-related factors, for the chosen scheme (the optical fibre based system) are carried out to optimise the quality of the rotating plane polarised light emitted. Improvements through a range of the factors were undertaken and are clearly demonstrated.

The achievements resulting from the improvement of the rotating plane polarised light include a reduction in ellipticity (in the range of 10-2), a much smaller effect due to the direction of rotation (a 1-2% asymmetry) and potentially high speed detection (up to 1 kHz). These achievements are considered to be appropriate enough to employ the improved optical fibre polarisation modulation technique to ellipsometric measurements.

The optical fibre modulation scheme is applied to the ellipsometric measurements of a simple system consisting of interfaces of two semi-infinite media. Various materials are investigated and experimental results are shown in terms of the ellipsometric parameters Ѱ and  which then allow an individual sample refractive index to be determined. The accuracy found in measuring the two ellipsometric parameters is within 2% of the parameter mean values. A broader application range is also demonstrated as the optical fibre polarisation modulated ellipsometer is used to characterise ambient-film-substrate samples differing in film thickness. The results gained are then compared with corresponding reference values, obtained from a commercial ellipsometers, in order to demonstrate the degree of sensitivity. The novel implementation of the technique as a thin film based sensor device is demonstrated by detecting the change of a thin film properties on exposure to external stimuli.

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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