Abstract: A fiber optic sensor includes two reflective elements in series. The first reflective element is formed as a partially mirrored surface on the end of a single-mode optical fiber lead which is bonded into a ferrule. A sleeve is used to join the ferrule to a second ferrule which is turn is bonded to a length of single-mode optical fiber. The second reflective element is a partially mirrored surface on the cleaved end of the second fiber. The second fiber may be affixed to or embedded in a structure to be monitored and changes its optical path length in response to a condition of the structure. Light introduced into the sensor is reflected from the first or second reflective element and thus follows two optical paths. The path length difference between the two optical paths is twice the optical path length of the second fiber.

Abstract: An interoferometric fiber optic sensing system uses three optical fibers. A sensing optical fiber is applied to a structure to be monitored to detect displacement or the like by changing its optical path length. A reference optical fiber has a fixed optical path length. An adjustable length optical fiber is controllably adjusted in its optical path length. The three optical fibers form optical paths whose light outputs are caused to interfere. The adjustable length optical fiber is adjusted until an interference fringe appears. The quantity to be detected is derived from the maximum of the interference fringe. Several sensing optical fibers can be multiplexed; by staggering their optical path lengths, their interference fringes can be separated sufficiently to resolve them.

Abstract: An interoferometric fiber optic sensing system uses three optical fibers. A sensing optical fiber is applied to a structure to be monitored to detect displacement or the like by changing its optical path length. A reference optical fiber has a fixed optical path length. An adjustable length optical fiber is controllably adjusted in its optical path length. The three optical fibers form optical paths whose light outputs are caused to interfere. The adjustable length optical fiber is adjusted until an interference fringe appears. The quantity to be detected is derived from the maximum of the interference fringe. Several sensing optical fibers can be multiplexed; by staggering their optical path lengths, their interference fringes can be separated sufficiently to resolve them.