Abstract: A sensor and method are disclosed for determining if a region, defined by two end points, in a structure has exceeded a predetermined amount of strain. The sensor has an optical waveguide which has two ends for receiving and emitting light and which is fixable at two locations thereon to respective ones of the end points to define a sensing region therebetween. The sensing region has a first portion with a first length and a first cross-sectional area, and a second portion with a second length and a second cross-sectional area which is smaller than the first cross-sectional area. The lengths and cross-sectional areas are sized so that the optical waveguide has a strain failure point equal to the predetermined amount of strain. The lengths and cross-sectional areas are approximately sized according to the formula R=.alpha.+1/ (.alpha./.beta.

Abstract: An optical fiber strain sensor is provided which remembers the maximum stn a structure has experienced in a given timeframe. A reflective surface is fixed at one location on the structure, and a multi-mode optical fiber with a flat end is fixed at another location on the structure and is positioned so that its flat end is in contact with the reflective surface. A microbore capillary tube attached to the reflective surface encloses the fiber at its flat end and frictionally retains it in contact with the reflective surface. Increasing strain in the structure pulls the fiber end away from the reflective surface in proportion to the amount of strain; however, when strain is decreasing, the fiber buckles rather than moving closer to the reflective surface. Therefore, the maximum strain is reflected as a maximum distance between the fiber end and the reflective surface.

Abstract: An optical fiber strain sensor is provided which remembers the maximum stn a structure has experienced in a given timeframe. A pair of multi-mode optical fibers with flat ends meet end-to-end within a microbore capillary tube with which the fibers have frictional contact. The fibers are fastened to the structure at two points a known distance apart on either side of the capillary tube. Increasing strain in the structure pulls the fiber ends apart in proportion to the amount of strain; however, when strain is decreasing, the fibers buckle rather than move closer together. Therefore, the maximum strain is reflected as a maximum distance between the fiber ends. When the maximum strain experienced is to be measured, the free end of one fiber is connected to a light source and the free end of the other fiber is connected to a detector.

Abstract: Apparatus is provided for modulating the phase of a coherent light beam pagating through the core of a single-mode waveguide. A light-absorbing element is connected to a portion of the cladding of the waveguide such that there is a thermal interface therebetween. A means is provided for launching light into the portion of the cladding of the waveguide having the light-absorbing element connected thereto. The light-absorbing element absorbs the light and produces heat in proportion to the light's energy. The heat thus produced is transferred to the waveguide, changing its optical path length and varying the phase of the light propagating therethrough. The intensity of the light launched into the cladding is variable and is controlled by a power supply. The modulator may be used in an interferometer in which a signal is directed to the power supply to control the light intensity for various purposes.

Abstract: A sensor is disclosed in which light is launched into the core of a singlode optical fiber having a tapered-down or narrow waist region between 2 tapers along an internal portion of the length thereof. The fiber is positioned so that a physical effect to be measured bends the fiber in the narrow waist region. The intensity of the light exiting from the core of the fiber is measured and the displacement is determined in accordance with the measured intensity. The narrow waist region of the fiber has a V.sub.co parameter of less than 1. A bellows which changes dimension with a change in pressure may be coupled to the fiber to bend it in the waist region in proportion to pressure changes. A magnetostrictive element which changes dimension with changing magnetic field may be coupled to the narrow waist region so that a change in dimension causes a displacement which bends the fiber.

Abstract: The index of refraction of a liquid is measured using an optical fiber rectometer having a light transmitting optical fiber by immersing a portion of the optical fiber in the liquid and launching light into one end of the optical fiber at a selected non-zero launch angle with respect to the fiber axis. Light transmitted through the optical fiber is detected at the other end of the optical fiber and a determination is made of the index of refraction of the liquid in accordance with the detected light and the selected non-zero launch angle. By varying the launching angles of the light the range of the refractometer is increased. The light transmitting optical fiber is provided with a region having at least one tapered portion for further increasing the range of the refractometer. The tapered portion of the optical fiber is disposed between a refractive end of the optical fiber and the light source for providing single-ended operation.

Abstract: The index of refraction of a liquid is measured using an optical fiber refractometer having a light transmitting optical fiber by immersing a portion of the optical fiber in the liquid and launching light into one end of the optical fiber at a selected non-zero launch angle with respect to the fiber axis. Light transmitted through the optical fiber is detected at the other end of the optical fiber and a determination is made of the index of refraction of the liquid in accordance with the detected light and the selected non-zero launch angle. By varying the launching angles of the light the range of the refractometer is increased. The light transmitting optical fiber is provided with a region having at least one tapered portion for further increasing the range of the refractometer. The tapered portion of the optical fiber is disposed between a refractive end of the optical fiber and the light source for providing single-ended operation.