Kent A. Murphy

Quadrature phase-shifted, extrinsic Fabry-Perot optical fiber sensors

We demonstrate the operation of a quadrature phase-shifted extrinsic Fabry-Perot fiber-optic sensor for the detection of the amplitude and the relative polarity of dynamically varying strain. Two laterally displaced single-mode fibers inserted within a hollow silica tube form the 90 degrees phase-shifted sensing system. A multimode fiber, placed in the tube facing the two fibers, acts as a reflector, thereby creating an air gap that acts as a Fabry-Perot cavity. A theoretical description of the sensor is given, and its operation as a dynamically varying strain sensor is described. Strain sensitivities of 5.54 degrees phase shift/microstrain cm(-1) are obtained.

Elliptical-core two mode optical-fiber sensor implementation methods

Experimental methods for the practical implementation of few-mode elliptical-core sensors are described. Techniques for desensitizing the lead-in and lead-out fibers are discussed, and results of a vibration sensor embedded in a graphite-epoxy composite are presented. A scheme using a single-mode elliptical-core fiber as the lead-in fiber and an offset circular-core single-mode fiber as the lead-out fiber is successfully implemented. Detection techniques for few-mode fiber sensors are reviewed, and a novel fringe-counting method to unambiguously detect the amplitude and direction of dynamic strain is reported. >

Temperature-insensitive and strain-insensitive long-period grating sensors for smart structures

We demonstrate that temperature- and strain-insensitive long-period gratings can be fabricated in conventional optical fibers. The former is employed to measure strain with resolution of 20 µ? under thermal fluctuations in the surroundings, while the latter is used to detect temperature variations as small as 0.8°C in the presence of axial strain.

WHITE-LIGHT SCANNING FIBER MICHELSON INTERFEROMETER FOR ABSOLUTE POSITION-DISTANCE MEASUREMENT

A white-light fiber interferometer working in the spatial domain, using two fiber ends in a hollow tube as the sensing head and an electric magnetic actuator-mirror reflector as the path-compensation-measurement element, is presented. Analysis and preliminary experiments have demonstrated a repeatability of 0.5 microm (2sigma) for position-distance measurement, and the measurement uncertainty was estimated to be 1.5 microm (2sigma) over a distance range of 150 microm. Suggestions for further improving the measurement accuracy and response speed are also given.

Optical fibre based absolute extrinsic Fabry - Pérot interferometric sensing system

The design and principle of operation of a novel fibre optic sensing system is presented. The absolute extrinsic Fabry - Perot interferometric (AEFPI) system is shown to possess high sensitivity, wide dynamic range and a real-time output signal. Applications of the AEFPI system to high-performance civil structures are suggested. Multiplexing of many such sensors is achieved by employing the path matching configuration.

Extrinsic Fabry-Perot sensor for strain and crack opening displacement measurements from -200 to 900 degrees C

The authors present the design of a fiber optic extrinsic Fabry-Perot interferometric sensor element and its operation in several applications. The sensor has been demonstrated in a coal-fired combustor for the measurement of both thermally induced strains and crack opening displacements in ceramic materials. As a strain sensor, the device is demonstrated at temperatures ranging from -200 to 900 degrees C. It is shown that a Fabry-Perot strain gage with a one centimetre gage length can be operated in differential mode with 0.01 microstrain resolution in real-time, and in an absolute mode with 0.5 microstrain resolution with a three-second scan time.

Sapphire-fiber-based intrinsic Fabry-Perot interferometer

A sapphire optical fiber intrinsic Fabry–Perot interferometric sensor is demonstrated. A length of multimode sapphire fiber that functions as a Fabry–Perot cavity is spliced to a silica single-mode fiber. The interferometric signals of this sensor are produced by the interference between the reflection from the silica–sapphire fiber splice and the reflection from the free end face of the sapphire fiber. This sensor has been demonstrated for temperature measurement. A resolution of 0.2°C has been obtained over a measurement range of 310°C to 976°C.

Fiber-optic liquid-level sensor

Abstract A novel intensity-based liquid-level sensor that operates up to 300°C is described. Liquid-level measurements for different liquids with refractive indices ranging from 1.33 to 1.635 have been obtained with extinction ratios between 20 and 39 dB. The hysteresis for the measurement of lubricating oil is 0.175 mm.

Fabry–Perot fiber-optic sensors in full-scale fatigue testing on an F-15 aircraft

We report results from fiber-optic-sensor field tests on an F-15 aircraft mounted within a full-scale test frame for the purpose of fatigue testing. Strain sensitivities of the order of 0.01 μm/m have been obtained.

Optical fiber sensing technique for impact detection and location in composites and metal specimens

We present a novel technique for effective detection and location of impacts in metals and graphite/epoxy composite laminates. This scheme employs the highly sensitive extrinsic Fabry-Perot interferometric (EFPI) optical fiber-based sensing system for recording the differential arrival times of impact-generated acoustic signals using a set of four sensors whose location is predetermined. The sensors are surface-mounted on an aluminum sample and completely embedded in the composite specimen. A mathematical model is coded into a computer program to enable real-time, online determination of impact locations. The precise location of the impact can be deduced typically with a 0.5 mm resolution and an accuracy better than 5 mm. An improvement in the sensitivity of this system is proposed by using high-finesse Fabry-Perot cavities which modify the output transfer function curve of the sensor.