Fabry-Perot Fiber Optic Sensors Research Articles

The mechanical performance of pultruded composite rods with embedded fiber-optic sensors

Fiber-optic strain sensors are successfully embedded in glass- and carbon-fiber-reinforced polymer (GFRP and CFRP) tendons during pultrusion. The study of the performance of the embedded Fabry-Perot fiber-optic sensors under conditions of static and dynamic loading when exposed to both low and high temperature extremes, is presented. The experiments entailed subjecting the GFRP and CFRP tendons to sinusoidal and trapezoidal load waveforms of about 11 kN magnitude inside a temperature chamber. The temperature in the chamber was varied from −40 to 60°C in increments of 20°C. The strain output from the embedded sensors was compared to that from externally mounted extensometers as well as to theoretical strain values. It was determined that the performance of the Fabry-Perot sensors was not affected by ambient temperatures falling within the range of −40 to +60°C and the sensor readings conformed very well with the corresponding extensometer and theoretical readings.

Phase-demodulation error of a fiber-optic Fabry–Perot sensor with complex reflection coefficients

The influence of reflector losses attracts little discussion in standard treatments of the Fabry-Perot interferometer yet may be an important factor contributing to errors in phase-stepped demodulation of fiber optic Fabry-Perot (FFP) sensors. We describe a general transfer function for FFP sensors with complex reflection coefficients and estimate systematic phase errors that arise when the asymmetry of the reflected fringe system is neglected, as is common in the literature. The measured asymmetric response of higher-finesse metal-dielectric FFP constructions corroborates a model that predicts systematic phase errors of 0.06 rad in three-step demodulation of a low-finesse FFP sensor (R = 0.05) with internal reflector losses of 25%.

Performance of embedded short-gage-length optical fiber sensors in a fatigue-loaded reinforced concrete specimen

This paper reports the performance of short-gage-length optical fiber sensors embedded in a reinforced concrete specimen. Embedded extrinsic Fabry-Perot optical fiber sensors, attached to steel reinforcement rods, were used to monitor local displacements and strain in a concrete cross-beam specimen. The strain data obtained from the fiber sensors were compared with data obtained from collocated foil strain gages. Absolute extrinsic Fabry-Perot interferometric sensors, capable of measuring absolute displacement and strain, were embedded in a steel-reinforced concrete specimen. Loading experiments performed on this specimen also yielded absolute strain information. The specimen was cyclically loaded for more than 100000 cycles in this geometry and the sensors still provided quantitative strain information regarding this specimen.

A Model of Embedded Fiber Optic Fabry-Perot Temperature and Strain Sensors

A model was developed for embedded intrinsic and extrinsic Fabry-Perot fiber optic sensors. This model relates the strains and the temperature changes in the material surrounding the sensor to the reflected light intensity. The model consists of three parts. Submodel I relates the temperature and the strains in the material to the temperature and the strains inside the sensor. Submodel 2 relates the temperature and the strains inside the sensor to the change in sensor length and to the changes in the optical properties of the sensor. Submodel 3 relates the changes in sensor length and optical properties to the changes in the intensity of the reflected light. On the basis of the model, a computer code SENSOR1 was written which can be used to calculate the reflected light intensity for specified strains and temperature changes inside the material. The model and the code were verified by measuring changes in the light in tensity of an intrinsic Fabry-Perot sensor embedded in a graphite-epoxy composite plate subjected to loads varying with time. The changes in light intensities were measured and were also calculated by the code. The measured and calculated light intensities were found to be in good agreement.

Measurements of Strain and Temperature with Embedded Intrinsic Fabry-Perot Optical Fiber Sensors

The use of embedded intrinsic Fabry-Perot sensors in measuring strains and temperature changes was explored. Tests were performed with intrinsic Fabry-Perot sensors embedded in plates made of Fiberite T300/976 graphite-epoxy and of Fiberite APC2-PEEK graphite thermoplastic unidirectional tapes. The plates were also in strumented with thermocouples and strain gauges. The plates were subjected either to three point bending at room temperature, or to a uniform temperature change without an applied external stress. The temperatures and strains measured by the Fabry-Perot sensors agreed well with the thermocouple and strain gauge data, showing that with a single sensor and a single, constant wavelength light source, either the axial strain component or the temperature change can be measured.

Optical fiber Fabry-Perot sensors for smart structures

Fiber Fabry-Perot interferometers (FFPIs) utilizing internal mirrors have been developed to sense temperature, strain, acoustic waves and other physical perturbations in structural materials, and have been successfully embedded in composites and in metals. The construction, performance and application of the FFPI sensors to smart structures are described.

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.

Embedded Fabry-Perot fiber optic strain sensors in the macromodel composites

We introduce the use of fiber optic strain sensors embedded within a macromodel composite employed to study and validate micromechanical theories. Fabry-Perot (FP) fiber optic strain sensors (FOSSs) embedded within a macromodel composite are shown to be an accurate and precise means of making local internal strain measurements in and around damage events. The measurements made by the sensors compare closely to those obtained from resistance strain gauge data verified through presently accepted micromechanics. The optical strain sensors effectively measure both the signature of fiber fracture and the resulting strain concentration due to the damage event. The sensors add a new dimension to the validation and development of micromechanics. The approach assists in the formulation of new concepts for interpretation and prediction of actuator/sensor response in smart materials.

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.

Development of medical pressure and temperature sensors employing optical spectrum modulation

Fiber optic Fabry-Perot sensors have been developed whose optical reflectance varies with optical cavity depth (pressure) or with change in a material's refractive index (temperature). These sensors employ a unique combination of features: they are interrogated by an LED; they are designed to operate within a single reflectance cycle; and their returned light is analyzed by a dichroic ratio technique. The sensors use a step index glass fiber and are relatively insensitive to absolute light levels and fiber bending. They have an expanded linear operating range and can be built for low cost disposable applications. Sensor performance meets or exceeds established medical requirements.

Gross talk in a fiber-optic Fabry–Perot sensor array with ring reflectors

We demonstrate a fiber-optic interferometric sensor array based on the in-line Fabry-Perot configuration, which uses fiber ring reflectors to effect the low-reflectivity elements required between each sensor section. The intrinsic optical cross talk that occurs between the sensor sections owing to multiple reflections is analyzed and compared with experimental results obtained with a four-sensor system.