Detection of Magnetic Fields Using Fibre Optic Interferometric Sensors.

The principle aim of the work described in this thesis is to determine a suitable optical detection system for d.c. and low frequency magnetic fields which are likely to be encountered in practical magnetometer applications. To construct a sensitive magneotmeter one arm of an optical fibre Mach-Zehnder interferometer has been magnetically sensitised using a magnetostrictive material. Since the signal frequency range of interest was in the region of 0.01 to 10Hz, clearly the signal was in the same frequency band as the environmental noise associated with ambient temperature and pressure variations. Initially, a technique was developed to measure the magnetic field from the shift of the total fringe pattern generated by a modified Mach-Zehnder interferometer and a minimum detectable magnetic field of 10e-7 tesla.m. was obtained. This minimum detectable magnetic field has been improved by a number of modifications. A technique has been developed which utilises an a.c. bias field to put the magnetic signal on a carrier so that it can be measured at a frequency where the amplitude of the interferometer 1/f noise is much reduced. To maintain maximum interferometric sensitivity to this signal active homodyne demodulation techniques have been developed to maintain the interferometer at quadrature by compensating for the environmental noise. A minimum detectable magnetic field of 5x10e-10 tesla.m. has been achieved with this system. As an alternative to the Mach-Zehnder interferometer a Fabry-Perot interferometer, which utilises multiple-beam interference, has been considered. This type of interferometer consists of a single fibre with high reflectivity coatings on its ends. Such an interferometer has been used as a sensor and as an external cavity in laser frequency stabilisation scheme