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Abstract

This thesis is concerned with the application of external optical feedback in a laser diode to interferometric optical sensing using the effects of ’self-mixing interference’ and examines some of the practical issues for the development of such sensors.

A theory is described which accounts for the behaviour of a laser diode with weak external optical feedback and shows how a phase-dependent interferometric signal can be obtained from such a system.

Experimental techniques are described which have been developed to overcome some of the inherent difficulties in carrying out practical work with optical feedback in laser diodes and enable accurate and systematic interpretation of experimental results. These include the estimation and accurate measurement of feedback strength, the accurate alignment of components and the measurement of wavelength changes due to feedback.

The principles of operation of a simple self-mixing interference-based wide- range displacement sensor have been demonstrated. The choice of components for such a system has been considered in detail and interferometric fringes have been produced for target distances of over one metre.

The problem for interferometric sensing systems of the wavelength instability with respect to temperature variations of laser diodes has been addressed with particular reference to the effects of optical feedback. A method of using weak optical feedback to reduce the wavelength tuning coefficient of a single longitudinal mode has been proposed and demonstrated experimentally.

Experimental results are presented in which the frequency of the intensity modulation due to feedback is twice the value expected for the length of the external cavity. This is shown to be due to misalignment of the external reflector causing light to make a double pass of the external cavity. It is shown that with the external reflector misaligned the system is mechanically more stable and the resulting modulation frequency doubling offers a potential doubling in the resolution of a self-mixing interference based sensing system.

Publication Type:	Thesis (Doctoral)
Subjects:	Q Science > QC Physics
Departments:	School of Science & Technology Doctoral Theses School of Science & Technology > Engineering

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