Abstract: An interferometric sensor includes an optical fiber polarimetric sensing element (5) for detecting a required measurand and is configured so as to derive interference output signals, which are simultaneously monitored by photodetectors (9, 11), from both conventional, for example, Michelson, and polarimetric interferometer arrangements. The sensor therefore offers the high resolution of a conventional interferometer in conjunction with the increased dynamic range of a polarimetric device.

Abstract: A temperature sensor comprises a single-mode all-fiber Michelson interferometer. The interferometer includes a bi-directional coupler and signal and reference fibers having mirrored ends. These two fibers are of unequal lengths to provide an optical path length in-balance and the signal fiber may have a metal temperature sensing probe at its mirrored end. The injection current of a laser light source for the interferometer is modulated with a ramping signal so that a linearly moving interference pattern is produced at the output by mixing of the reflected beams propagated in the signal and reference fibers.

Abstract: A flowmeter for measuring the velocity of fluid flow by monitoring the vortex shedding frequency of the fluid flow comprises a single mode optical fibre sensing element (1) for detecting vortex shedding and utilizes interferometric techniques for producing an electrical output signal corresponding to the vortex shedding frequency. The sensing element (1) comprises at least part of the signal arm (2) of an interferometer (3) which also includes means (12) for deriving a reference signal from the illuminating light source (4). The flow induced oscillation of the sensing element (1) caused by vortex shedding produces modulations of the interferometer output which is monitored by a photodetector (14) which, in turn, produces a modulated electrical output signal which can be processed by a signal processing system (16-19) to identify the vortex shedding frequency and, hence, produce a measurement of the flow velocity.

Abstract: Optical pressure sensing apparatus comprises an optical fibre interferometer arranged to sense the movement of a pressure responsive element and produce an interference signal at its output which can be monitored and processed to measure changes in the pressure applied to the pressure responsive element. The interferometer comprises a signal arm coupled to the pressure responsive element and a fixed reference arm. A light source supplies light to the input of the interferometer via an optical fibre and monitoring and processing electronics connected to the output determine the optical phase shift between the light beams propagated in the signal and reference arms and produce a pressure reading.

Abstract: A polarmetric fiber sensor comprises a lead fiber (1) and a shorter length of sensing fiber (2). Both fibers comprise higly birefringent optical fiber having orthogonal eigen modes and are spliced together with their eigen modes mutually rotated by 45.degree.. The sensing fiber (2) has a reflective distal end (6). Linearly polarized light from a laser source (3) is launched into one mode of the lead fiber (1) and light reflected from the distal end of the sensing fiber is recovered from the other mode of the lead fiber via a beam splitter (7) and photodiode (8). The resulting electrical signal produced by the photodiode (8) corresponds to the modal birefringence of the sensing fiber (2) and changes in this birefringence are monitored by processing means (9) to detect changes in a physical parameter sensed by the sensing fiber.