Abstract: Apparatus for sensing or measuring a magnetic field comprises a light source (10) for generating light having at least a component which is polarized, a sensing element (16) arranged to transmit light from the source and comprising material having a substantial Verdet constant which depends on the temperature of the material, means (14-19, 21) for combining light from the source with light from the sensing element to form an interference pattern, means (23) for providing a signal representative of the intensity of the light so combined, and means for deriving first and second signals dependent on the Verdet constant of the material and the temperature thereof, respectively, from the signal representative of light intensity, whereby the intensity of the magnetic field at the element, substantially independent of temperature, can be derived. The invention extends to a method of sensing or measuring magnetic field intensity, and to apparatus and a method for sensing or measuring electric current.

Abstract: Apparatus for producing signal frequency shifts by stimulated Brillouin scattering comprises a pair of optical fibres (112,113) or a single high birefringence fibre which are illuminated by a laser beam to generate two back-scattered signals by stimulated Brillouin scattering and a mixer (117) to combine the two back-scattered signals to produce a beat frequency from the combined signals.