USA As Represented by Secretary of the Navy

US Government As Represented by the Secretary of the Army

Lloyd C Bobb

Howard D Krumboltz

Jon P Davis

Barbara J White

Gerald D Ferguson

Arthur Samouris

Kimball Kramer

Michael Rankin

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.+1), wherein R=the ratio of the amount of strain in the second portion which will cause the optical waveguide to fail to the predetermined amount of strain in the structure, .alpha.=the ratio of the length of the first portion to the length of the second portion, and .beta.=the ratio of the cross-sectional area of the first portion to the cross-sectional area of the second portion.

Optical fiber strain-to-failure sensor

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. The intensity of light transmitted to the detector will vary inversely with the distance apart the fiber ends are; therefore, this distance can be measured and correlated with strain to a high degree of accuracy.

Optical fiber strain sensor for measuring maximum strain

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. When the maximum strain experienced is to be measured, light is launched into the free end of the fiber and reflected back through the fiber by the reflective surface. Its intensity is then detected and measured. The intensity of light transmitted to the detector will vary inversely with the distance between the fiber end and the reflective surface; therefore, this distance can be measured and correlated with strain to a high degree of accuracy.

Single-ended optical fiber strain sensor for measuring maximum strain

A method of measuring the position of a liquid surface within a vessel is scribed. An optical fiber is provided which has first and second ends and has an energy-absorbing element of a predetermined length disposed upon a portion of the cladding thereof such that there is a thermal interface between the energy-absorbing element and the cladding. The optical fiber is then positioned in the vessel so that the energy-absorbing element disposed thereon extends a known distance into the vessel along a known depth gradient of the vessel and so that the energy-absorbing element will intersect the liquid surface over the anticipated range of positions thereof. Single-frequency, coherent light is then transmitted through the core of the optical fiber by launching it into the first end of the optical fiber. A pulse of energy is applied across the entire length of the energy-absorbing element to heat it, and then the transmitted light is received from the second end of the optical fiber. The change in phase of the transmitted light resulting from applying the pulse of energy is then measured, and the position of the liquid surface is then determined in accordance with the change in phase.

Method of measuring liquid level with a thermal interface detection

Apparatus is provided to detect electromagnetic radiation, in which a radion-absorbing element is disposed on a short section of an optical waveguide to provide a thermal interface therebetween. Radiation is absorbed by the element, which thereby heats the waveguide, causing it to change its optical pathlength in proportion to the radiation absorbed. Interferometer apparatus is connected to measure this change in optical pathlength as a change in the interference condition. This device is highly sensitive and can be operated at room temperature.

Electromagnetic radiation detector utilizing an electromagnetic

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.

Thermal phase modulator and method of modulation of light beams by

A physical property of a fluid or of any optical fiber is measured using an ptical fiber interferometer. A conductive material is disposed upon the surface of a region of a light tranmitting optical fiber and the region having the conductive material is disposed in the fluid. Light energy is applied to one end of the fiber and transmitted light is received at the other end of the fiber. Electrical energy is applied to the conductive material disposed upon the surface of the fiber to heat the region of the fiber and cause a change in the optical path length of the light transmitted through the fiber. The physical property of the fluid or optical fiber is determined in accordance with the change in the optical path length or phase of the received light caused by applying the electrical energy to the conductive material. Thermal conductivity is measured using a series of short energy pulses and determining the average phase change. The flow rate of a fluid is measured by measuring the phase change and applied electrical energy upon heating the fiber to an equilibrium temperature. The conductive material is gold and it encircles the fiber. The gold may be disposed on the jacket of the fiber or the jacket may be removed before disposing the gold.

Optical fiber sensor for measuring physical properties of fluids

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.

Tapered optical fiber sensor

A physical property of a liquid or of any optical fiber is measured using optical fiber interferometer. A conductive material is disposed upon the surface of a region of a light transmitting optical fiber and the region having the conductive material is disposed in the liquid. Light energy is applied to one end of the fiber and transmitted light is received at the other end of the fiber. Electrical energy is applied to the conductive material disposed upon the surface of the fiber to heat the region of the fiber and cause a change in the optical path length of the light transmitted through the fiber. The physical property of the liquid or optical fiber is determined in accordance with the change in the optical path length of the received light caused by applying the electrical energy to the conductive material. A series of short energy pulses is provided and the average phase change is determined. The conductive material is gold and it encircles the fiber. The gold may be disposed on the jacket of the fiber or the jacket may be removed before disposing the gold.

Optical fiber sensor for measuring physical properties of liquids

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.

Optical fiber refractometer launching light at a non-zero launch angle

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.

Optical fiber refractometer

Magnetometers disposed as magnetostrictive segments on optical fiber elems are incorporated in multiple order gradiometers to reduce the size, weight and cost thereof. In the preferred embodiments, such reductions are greatly enhanced by consolidating a plurality of magnetometers on individual optical fiber elements, which also serves to decrease the number of devices associated with the magnetometers in the multiple order gradiometers.

Fiber optic magnetometers for multiple order gradiometers

The phase of light passing through an optical waveguide is modulated by aying electric current to heat the waveguide and thereby alter the length and refractive index thereof. An application for the thermal phase modulation concept is provided in a Mach-Zehnder interferometer.

Thermal modulation of light beams

A sensor system is disclosed for measuring small physical perturbations in he environment using an optical fiber interferometer in the Fabry-Perot configuration operating at maximum sensitivity. A single frequency laser source is focused on one end of a single mode optical fiber with highly polished, highly reflective flat ends. An element responsive to the ambient magnetic or electric field alters the fiber's optical path length, thereby affecting the intensity of light transmitted through the fiber. A detection and feedback system detects the transmitted light and readjusts the optical path length to one which corresponds to maximum sensitivity.

Optical fiber interferometer

A magnetic field gradiometer includes a geometric arrangement of fiber op sensors through which light from a single frequency laser is transmitted. Having a pair of single-mode optical fibers, each of which are coupled to a magnetostrictive element, each sensor is formed in the manner of a Mach-Zehnder interferometer which is oriented to detect the spatial variation in a particular orthogonal component of the magnetic field by phase comparison of the light transmitted through its respective pair of fibers.

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Warminster, PA, United States of America

Lloyd C Bobb