Jonathan A. Greene

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.

Photoinduced refractive-index changes in two-mode, elliptical-core fibers: sensing applications

Photoinduced refractive-index changes in two-mode, elliptical-core optical fibers are shown to affect the differential phase modulation between the LP(01) and the LP(11)(even) modes. This change in beat length is dependent on the amount of strain induced in the fiber while the grating is being formed. We present experimental results that agree with conventional coupled-mode theory and propose the use of such sensors for weighted and distributed applications.

Optical-fiber-based chemical sensors for detection of corrosion precursors and by-products

Optical fiber sensors are a novel and ideal approach for making chemical and physical measurements in a variety of harsh environments. They do not corrode, are resistant to most chemicals, immune to electromagnetic interference, light weight, inherently small and have a flexible geometry. This paper presents recent test results using optical fiber long-period grating (LPG) sensors to monitor corrosion precursors and by-products. With the appropriate coating, the LPG sensor can be designed to identify a variety of environmental target molecules, such as moisture, pH, sulfates, chlorates, nitrates and metal-ions in otherwise inaccessible regions of metallic structures. Detection of these chemicals can be used to determine if the environment within a particular area of an airplane or infrastructure is becoming conducive to corrosion or whether the corrosion process is active. The LPG sensors offer a clear advantage over similar electrochemical sensors since they can be rendered immune to temperature cross-sensitivity, multiplexed along a single fiber, and can be demodulated using a simple, low-cost spectrum analyzer. By coating the LPG sensor with specially designed affinity coatings that selectively absorb target molecules, selective, real-time monitoring of environmental conditions is possible. This sensing platform shows great promise for corrosion by- product detection in pipe networks, civil infrastructure, process control, and petroleum production operations and can be applied as biological sensors for in-vitro detection of pathogens, and chemical sensors for environmental and industrial process monitoring.

Optical fiber corrosion sensors for aging aircraft

Optical fiber corrosion sensors are being developed to address the high service costs associated with current structural maintenance procedures for civilian and military assets. A distributed optical fiber sensor system will help reduce the costs associated with corrosion damage and extend the lifetime of existing assets. Annual national losses in time, labor, materials and systems has been estimated in the billions of dollars. Additional costs arise from system downtime that results from disassembly procedures necessary to locate corrosion damage in remote locations. Furthermore, the potential to damage other system parts during maintenance is increased when disassembly and reassembly occurs. The development of on-line optical fiber sensors capable of detecting corrosion would eliminate a significant portion of the maintenance costs. We present recent test results using optical fiber long-period grating (LPG) corrosion sensors. With the appropriate coating, the sensors can be designed to detect water or metal ions in otherwise inaccessible regions of the aircraft. The LPG sensors can be designed with low temperature cross-sensitivity, multiplexed along a single fiber, and can be demodulated using a simple, low-cost spectrum analyzer.

Multiplexed absolute strain measurements using extrinsic Fabry-Perot interferometers

Extrinsic Fabry-Perot interferometric (EFPI) sensors have previously been demonstrated for relative strain and temperature measurements for smart structure applications. Inherent difficulties in the signal processing of these devices has created the need for absolute measurement capabilities. In this paper, we present an absolute measurement technique based upon white-light interferometric path matching. The system matches a reference gap to the sensing gap of an EFPI. When the difference of these two lengths is within the coherence length of the source, an intensity envelope is created in the system output. Determination of the corresponding path mismatch indicates the size of the sensor gap and hence strain can be determined. This measurement technique is capable of multiplexing an array of EFPI sensors and data will be presented demonstrating four multiplexed devices. Theoretical considerations for system optimization are also presented.

Spatially weighted, grating-based, two-mode, elliptical-core optical fiber vibration sensors

Photoinduced refractive-index changes in two-mode, elliptical-core optical fibers affect the beat length and the sensor sensitivity. Chirped gratings are written by attaching such fibers to cantilever beams positioned in a strained state. We show that fibers with in-line chirped gratings, with the chirp being shaped in the form of a vibration-mode shape, can be used as spatially weighted fiber sensors for vibration analysis. We demonstrate enhanced detection of the first and second modes of vibration of a cantilever beam using this process; vibration mode suppression of the order of 10 dB is obtained.

EFPI manufacturing improvements for enhanced performance and reliability

The recent progress in the performance and reliability of the fiber optic-based extrinsic Fabry-Perot interferometric (EFPI) strain sensor is reported. The developments include refined fabrication techniques and improved quality of constituent elements for enhanced durability and greater operating temperature range, higher strain sensitivity using high-finesse cavities, modified sensor-head for complete strain-field characterization, absolute, real-time and inexpensive measurements employing white light interferometry, and multipoint, distributed sensing using CDMA and path-matching multiplexing techniques. It is shown that these improvements have assisted in overcoming the limitations of the conventional EFPI sensor and made possible the large- scale commercialization of the state-of-the-art EFPI-based strain sensing system.

Grating-based optical-fiber-based corrosion sensors

Optical fiber grating-based sensors are proposed and demonstrated for the detection of corrosion. Two techniques are employed to indirectly monitor corrosion: (1) measuring the corrosion-induced decrease in the residual strain of a metal-coated optical fiber short period grating sensor and (2) monitoring corrosion-induced changes in the dimension of a metal- coated, long-period grating sensor.

Photoinduced Fresnel reflectors for point-wise and distributed sensing applications

Large, photoinduced refractive index changes on the order 10-2 can now be achieved in hydrogen-loaded, GeO2-doped optical fiber. Much of this work has focused on the research and development of Bragg gratings fabricated by exposing the core of fiber to the intensity pattern of two interfering plane waves. Unlike interferometrically formed Bragg gratings, we have exposed optical fibers to a single focused 244 nm laser beam to obtain two Fresnel reflections from each exposure site. Reflectors formed in this way are broadband and have application as markers in OTDR-based distributed sensing systems and as resonant cavities for localized intrinsic Fabry-Perot interferometric strain and temperature sensors.

Applications of "absolute" fiber optic sensors to smart materials and structures

The ability of future materials to autonomously sense and respond to environmental stimuli has been proposed for several years [1, 2, 3]. Some investigators envision Hie large-scale, smart” integrated function structures of Fifty years from now gradually evolving from the discretely instrumented and actuated structures of today and the near future for on-line, nondestructive evaluation.