Michael F. Gunther

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.

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.

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.

Extrinsic Fabry-Perot Optical Fiber Sensor

Phase-modulated fiber optic sensors have been shown to possess high sensitivities for the measurement of strain, temperature, vibration, pressure and other parameters.1 Fabry-Perot (FP) sensors that are based on multiple beam interference eliminate the need for a reference arm and do not require sophisticated stabilization techniques as in the case of Mach-Zehnder and Michelson interferometers.2 Several techniques to create intrinsic optical fiber Fabry-Perot interferometers have been described in the past.3-5 In a recent paper, we described an optical fiber extrinsic FP interferometer and used it as a sensor of microdisplacements and thermally- induced strain.6 The fiber interferometer was classified as extrinsic because the FP cavity was an air-gap between two fiber ends and the sensor output was immune to perturbations in the input/output fiber.

Fiber optic impact detection and location system embedded in a composite material

We present an impact detection and location system which uses fiber optic extrinsic Fizeau interferontric sensors embedded in a graphite/epoxy composite laminate. The acoustic signals generated by the impact events are detected by four fiber optic sensors. The fiber sensor and the embedding process are described. Also developed are a mathematical method and computer program that allow calculations of unambiguous impact location from the sensor data. The impact location can be determined with a 0.5 millimeter resolution and an accuracy typically less than five millimeters.

Embeddable distributed moisture and pH sensors for nondestructive inspection of aircraft lap joints

Two distributed fiber optic sensors for use in the prevention and monitoring of corrosion in aircraft are described. These sensors, based on optical fibers that are intrinsically sensitive to either water or changes in pH, will alert maintenance personnel to the presence of water in lap joints and other inaccessible critical areas. Furthermore, the sensors can also locate precisely where the moisture infiltration has occurred. In a typical application, a sensor fiber would be embedded in a lap joint along the bottom panel of an aircrafts body, or on a wing, where water is likely to collect. Changes in the optical transmission through the fiber can be monitored either periodically or continuously to determine the extent of water penetration.

Optical fiber sensors for measurement of strain and acoustic waves

A surface acoustic wave (SAW) sensor based on an extrinsic Fabry-Perot interferometer is described. A single-mode fiber, used as the input/output fiber, and a multimode fiber, used solely as a reflector, form an air-gap that acts as a low-finesse Fabry-Perot cavity. The Fresnel reference reflection from the glass/air interface at the front of the air-gap interferes with the sensing reflection from the air/glass interface at the far end of the air-gap. Strains in the silica tube housing the two fibers change the air-gap length, thereby altering the phase difference between the reference and sensing reflections. A theoretical analysis of the interaction between the strain induced by elastic stress wave fields and the fiber sensor is presented. A dual optical wavelength stabilization technique is used to minimize signal drifts. Signal to noise ratios on the order of 39 dB are obtained with a sensitivity of 4 degree(s)/microsecond(s) train cm-1 for strain measurements. The sensor was also attached to a steel cantilever beam and submerged in liquid nitrogen. The sensor was used to measure strain at liquid nitrogen temperatures with typical errors of less than 10%.

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 mounted within a full-scale test frame for the purpose of fatigue testing at the Structures Test Facility, Wright Patterson Air Force Base, Ohio. Static and dynamic loading data obtained using multiple extrinsic Fabry- Perot fiber optic sensors is presented. The output fringes from two quadrature phase shifted Fabry-Perot sensors were linearized using computerized software. The results compare well with data obtained from conventional strain gauges located adjacent to the fiber optic sensors. Strain sensitivities on the order of 0.01 (mu) /m were observed.