Abstract: A vibrating element transducer is provided in which a vibrating element and a substantially non-vibrating reference element are positioned side by side and illuminated via an optical fibre. The surfaces of the vibrating element and the reference element are formed at slightly different distances from the end of the optical fibre. The differing distances are arranged such that interference between the light reflected from the vibrating element and from the reference reflector can be used to determine the frequency of vibration of the vibrating element.

Abstract: An optical displacement sensor comprises first and second optical fibers which transmit light to and receive light reflected from first and second fixed reflectors. A third optical fiber transmits light to and receives light reflected from a moving reflector on the end of a core which moves in response to the displacement to be measured. The intensities of the light reflected to the fibers allows the displacement to be determined with immunity from variations such as in transmissivity and refractive index of a medium between the fibers and the reflectors.

Abstract: A vibrating sensor comprises an elongated element arranged to vibrate at a frequency dependent upon an external stress applied to the sensor, and an optical fiber for coupling light into an end of the element to excite it into vibration. A light-absorbing layer is provided on a longitudinal surface of the element such that the element will change its configuration, e.g. by bending, as the layer absorbs light and heats that side of the element, this alteration in configuration causing a reduction of radiation which is absorbed so that the element restores. The element may define a Fabry-Perot interferometer between its opposite ends.

Abstract: An optical displacement sensor suitable for use in drowned environments where the refractive index of a medium in the optical path may vary, comprises a light source emitting first to fourth wavelengths. The first and second wavelengths are within the infrared region of the spectrum, whereas the third and fourth wavelengths are in the visible region of the spectrum. A zone plate focuses the first and second wavelengths to respective positions substantially at the extremes of movement of a reflective element whose position is to be measured. The zone plate also focuses the third and fourth wavelengths to respective positions. A cold mirror is interposed between the zone plate and the reflective element and is arranged to transmit the first and second wavelengths and to reflect the third and fourth wavelengths. The ratio of the intensities of the light reflected from the cold mirror gives information about the refractive index of the medium in the optical path.