Re-entry bodies are subject to extreme conditions, among them the rigorous shock, vibration, and loading characteristics
that can often induce noise or loss of measurement. Restrictions by the Department of Energy on spark sources within a
sealed body require the exclusive use of fiber optics for sensing. A joint effort between Los Alamos National Laboratory
and Lambda Instruments has developed and evaluated a white light interferometric fiber sensor to address these concerns
while measuring displacements between high explosive components in potential flight applications. The sensor offers
advantages with electro-magnetic immunity, non-contact sensing elements, and high sensitivity to movement. Gap
values are calculated from the extrema of the sinusoidal wavelength pattern created by the Fabry-Perot cavity between
the lens and explosive surface, collected by an optical spectrum analyzer and interpreted by an external computer. This
paper focuses on the interferometric concept and experimental data received from the unit in real-time during centrifuge
tests. Results from single and multimode versions are presented and reported in their effectiveness for 0-2 millimeter
measurements.
Recent progress on the development of optical fiber sensors for both static and dynamic characterization of new ceramic-matrix composite aerospace structural materials is presented. Optical fiber sensors are employed over electrical strain gage equivalents due primarily to their lower mass, all dielectric construction, and ability to survive much higher temperatures than traditional wire-based strain gage instrumentation. In addition, the fiber sensor design ensures that it is shielded from surface shear strains that typically lead to fatigue-induced failure of wire-filament gages exposed to dynamic loading.
We present recent progress on the commercial development of optical fiber strain gages for both static and dynamic applications up to 1650°C. The low mass and all dielectric nature of optical fiber sensors resists debonding in high vibration environments and completely eliminates noise induced by electromagnetic fields. In addition, the fiber sensor design ensures that it is shielded from surface shear strains that typically lead to debonding and fracture of wire filament gages exposed to long-term cyclical loading.
Ytterbium (Tb) doped double-cladding fiber (DCF) lasers and amplifiers are being developed for a number of industrial and military applications. There are several key factors for maximizing the output power of these devices. Lambda Istruments is concentrating on two areas: component development and optical fiber development. The component development effort has focused on grating devices and pump couplers. Stable, highly reflective short-period fiber Bragg gratings are produced in DCF rapidly, have low insertion loss and can be customized for many different laser/amplifier applications. Long-period gratings are also being developed for possible novel use in fiber laser and amplifier applications. A proprietary fiber coupler under development is currently capable of an 85% coupling efficiency. The second focus for Lambda is the development of polarization maintaining Yb-doped DCF. Recent efforts have shifted towards making large mode area versions of these fibers to reduce nonlinear effects at high powers.
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 sensor have numerous advantages over conventional sensing technologies. One exciting capability of optical fiber sensor is their ability to operate in high temperature environments. While most conventional strain, pressure, etc. sensors do not operate reliably over 300 degrees C, fused silica based optical fiber sensor can survive up to 900 degrees C. High temperature materials such as sapphire and silicon carbide can be used to construct sensors that can survive up to 2000 degrees C. A suite of high temperature strain, pressure, and temperature sensor are being developed using the Extrinsic Fabry-Perot Interferometer technology for NDE characterization of advanced composite materials. These sensors have been demonstrated in several applications. High temperature strain and temperature sensors were used to conduct fatigue testing of composite compounds at 600 degrees C. High temperature pressure senors are being developed as microphones for high temperature acoustic testing. The result from these specific high temperature applications are presented along with future applications and directions for these sensors.
Optical fiber sensors have numerous advantages over conventional sensing technologies. One such advantage is that optical fiber sensors can operate in high temperature environments. While most conventional electrical-based sensors do not operate reliably over 300 degrees C, fused silica based optical fiber sensors can survive up to 900 degrees C, and sapphire based optical fiber sensors can survive up to 2000 degrees C. Using both fused silica and sapphire technologies, we present result for high temperature strain, pressure, and temperature sensors using Extrinsic Fabry-Perot INterferometric-based and Bragg grating sensors. High temperature strain and temperature sensors were used to conduct fatigue testing of composite coupons at 600 degrees C. The results from these specific high temperature applications are presented along with future applications and directions for these sensors.
We report test results using optical fiber sensor to measure dynamic strain and temperature on ceramic-matrix composite (CMC) specimens at temperatures up to 600 degrees C. For strain sensing we are employing extrinsic Fabry-Perot interferometric strain gages fabricated with gold-coated optical fibers and attached to the CMC specimens using high- temperature ceramic adhesive. For temperature measurements, specially fabricated Bragg and long-period grating sensors are being employed.
Microelectromechanical systems or MEMS are miniature devices that have several advantages over conventional sensing and actuating technology. MEMS devices benefit form well developed integrated circuit production methods which ensure high volume, high yield processes that create low-cost sensors and actuators. OPtical fiber interconnected MEMS will provide new functionality in MEMS devices such as multiplexed operation for distributed sensing applications. This paper presents approaches in optical fiber to MEMS interfacing and some preliminary results.
Optical fiber sensors, because of their small size, low weight, extremely high information carrying capability, immunity to electromagnetic interference, and large operational temperature range, provide numerous advantages over conventional electrically based sensors. Fiber-based sensors have found numerous applications in industry for process control, and more recently for monitoring the health of advanced civil structures. This paper presents preliminary results from optical fiber sensor designs for monitoring acceleration and magnetic field.
Optical fiber sensing techniques are ideal for applications where high-temperature, electromagnetic interference, or vibration cause traditional electrical sensors to become unreliable. Gold-coated, silica-based optical fibers can withstand temperatures up to 900 degrees C and sapphire fibers can be employed for temperatures as high as 2000 degrees C. We present dynamic strain and temperature measurements of ceramic matrix composite specimens using extrinsic Fabry-Perot interferometric fiber optic strain and temperature sensors. The extremely low-mass and rugged construction of the sensors will allow them to survive high- cycle, high-temperature fatigue testing.
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.
Significant weight and space savings have been realized in avionic equipment and structures by using composite materials. Optical fiber provides improved communication between equipment and components on board the aircraft. The marriage of these two technologies by embedding optical fiber in the composites achieves improved signal transmission and reduced weight. The goal of this work is to provide reliable, low-profile, optical interconnects with composite-embedded optical fiber for communication with opto-electronic circuit cards and modules. The design and results of a prototype embedded multimode optical beamsplitter utilizing in-fiber 45 degree(s) dielectric coatings are presented.
We present recent progress in the development of optical fiber sensors for early detection of corrosion on aging metallic aircraft. Optical fiber sensing techniques being investigated include fiber optic Bragg grating strain sensor to monitor the mass reduction of metal `witness' capillary tubes and extrinsic Fabry-Perot interferometric strain gages to monitor pillowing in lap joints.
In the past, optical fiber switches have typically been constructed from plastics or ceramics. However, the inability of these materials to operate effectively at high temperatures has greatly restricted the utilization of these devices. Recently, fiber optic switches have been manufactured from two thermally stable materials: carbon- carbon and BS50, a high temperature ceramic. The integration of these dimensionally stable materials into the fabrication of the optical switch will allow the switch to be utilized in an increased number of applications including optics, aerospace, mechanical, medical, and electronics. Preliminary testing included examining these new optical switches for structural damage due to the manufacturing process and testing the switches to demonstrate that the fibers could be realigned after processing. The tests concluded that no structural damage was induced, and the critical fiber realignment was achieved.
Optical fiber gratings have recently emerged as attractive sensors for non-destructive evaluation of materials and structures. We present photoinduced Bragg and long-period gratings (LPGs) for monitoring corrosion in aging infrastructure. These two sensors are described based upon fabrication techniques, sensing mechanisms, sensitivities, and cross sensitivities. It will be demonstrated that while Bragg gratings need to be prestrained to detect corrosion of metals, the modulation of the evanescent field of the cladding modes in-long-period grating can be employed for corrosion monitoring. It will also be shown that sensitivities of LPG-based corrosion sensors to ambient temperature fluctuations can be reduced significantly.
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.
A novel fiber optic differential pressure transducer and multiplexing system is described for real-time pressure measurements of airflow on an actuator- and SMA-controlled airfoil. The design of the pressure transducer is based upon extrinsic Fabry-Perot interferometric technology and incorporates a micromachined silicon diaphragm as the pressure sensitive element. The pressure transducer has a full scale operating range of -10 to 10 psig and a resolution of greater than 0.01 psi. Data is presented demonstrating the reproducible performance of the fiber optic sensor after repeated cycling and also at various temperatures. Finally, various multiplexing techniques and results are described.
The continuing development of the extrinsic Fabry-Perot interferometric sensor (EFPI) has led to a number of improvements to the original design. Manufacturing improvements have enabled the sensor to be employed in many diverse applications. This paper describes newly developed techniques used to manufacture the EFPI sensors and presents their use in advanced aerospace applications.
We present modifications in the design and operation of the conventional optical fiber extrinsic Fabry-Perot interferometric (EFPI) sensor to obtain real-time, self-calibrated, on-line and absolute strain measurements. The absolute EFPI (AEFPI) system utilizes the concept of white light interferometry to interrogate a fiber Fabry-Perot cavity and demodulation of the output signal may be carried out using a number of simple techniques like path matching or optical spectrum analyzer detection. The limitations of the conventional EFPI strain sensors are listed and it is shown that the modified AEFPI system overcomes most of these drawbacks by virtue of the fact that the information is wavelength-encoded. The AEFPI system is used to determine the strain during loading tendons that are commonly utilized in pre-stressed concrete. Other major applications include strain measurements in high-performance aerospace materials and structures under extreme mechanical vibrations and temperature variations. Preliminary experimental results are presented and applications to smart structures are proposed.
We propose the use of modal interferometers to detect changes in the transmitted signal in high-finesse extrinsic Fabry-Perot interferometric cavities for real-time, absolute strain and temperature measurements. The short length of the cavity ensures a large free spectral range of the resulting output Airy pattern and by tracking the wavelength shift of one peak, the applied perturbation may be completely characterized. The same principle is also proposed to detect the signal reflected from fiber Bragg gratings for strain and temperature sensing. The relative merits and demerits of this demodulation scheme are discussed and preliminary experimental results are presented.
We discuss two of the latest demonstrations of the extrinsic Fabry-Perot interferometer (EFPI) sensor, namely a novel silica fiber-based hydrogen sensor and a high-temperature, sapphire fiber-based displacement sensor for surface strain measurements at temperatures up to 2000 degrees C. To modify the EFPI sensor for the detection of hydrogen, the sensor is sputter coated with a 2 micrometers layer of palladium. Based on the signal-to-noise ratio of the sensor, a minimum detectable hydrogen concentration of 35 ppm has been obtained. For displacement measurements at temperatures above the melting point of silica, we discuss results using a sapphire fiber EFPI sensing head demodulated using white light scanning Michelson interferometer.
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. As the only fiber-optic sensors subcontractor to Northrop Corporation on the Navy/Air Force-sponsored Smart Metallic Structures (SMS) program, Fiber & Sensor Technologies (F&S) is developing the optical fiber fatigue gage instrumentation for a multiplexed, in situ structural health monitoring system for aging aircraft. In March, 1995, F&S successfully demonstrated the system on a full-size F/A-18 wing-box spar fully instrumented with 12 of F&S' patented EFPI optical fiber strain gages. F&S is now in process of up-scaling the signal processing system in addition to the optics and intends to demonstrate a second generation multipoint sensor system capable of simultaneously monitoring strains at up to 60 different sites throughout the aircraft later in 1995 or early 1996.
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.
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.
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.
A modified design of the extrinsic Fabry-Perot interferometric (EFPI) optical fiber sensor, for complete characterization of multi-component strain fields, is proposed. The novel EFPI includes a sensor head with two input fibers such that the respective reflections from the output multimode fibers end face are in quadrature. Any strain field possessing a component along the line joining the axes of the two fibers causes the initial phase difference to be modulated. The measured changes in phase difference is employed to determine this transverse component of the strain field. Absolute measurements are possible using the AEFPI sensing system. Applications of the modified sensor-head to smart materials and structures are discussed.
Optical fiber Bragg gratings, first demonstrated by Hill in 1978, will see increasing deployment as sensors in smart structure applications due to their minimal insertion loss and small profile. Bragg grating sensors provide narrowband reflections from a periodic index discontinuity. Strain and temperature changes occurring on the grating induce a shift in the reflected wavelength. Currently, the most basic Bragg grating demodulation system consists of monitoring the reflected wavelength with an optical spectrum analyzer, but use of these devices is limited due to high costs and poor frequency response. In this paper, we propose using dual-mode optical fiber as an intensity based interrogation system. The modal birefringence of the fiber is wavelength dependent and therefore, the phase relationship between two modes changes as a function of the Bragg grating reflected signal. We describe the optical architecture for this demodulation system and provide experimental data for temperature and vibration response. It is predicted that with an optical source operating at 300 microwatts, less than 1 microstrain resolution is achievable.
In the design of aerospace materials, developmental testing is conducted to characterize the behavior of the material under severe environmental conditions of high stress, temperature and vibration. Such materials are designed to withstand these environmental extremities without undergoing mechanical failure. Of importance is the performance of the materials at elevated temperatures, particularly the yield strength, which must be known so that the component can be designed to operate below such limits. Currently, instrumentation used in high-temperature tests must rely on long throw instrumentation, such as a knife-edged extensometer, to probe the surface of the specimen. This often results in reduced accuracy and limits the flexibility available for material evaluation. In this paper, we present the development of a fiber optic strain gage which was used for creep measurement of a stainless steel specimen (Type 304) heated to a temperature of 1400 degree(s)F (760 degree(s)C) during tensile loading tests. Surface attached fiber optic strain gages designed for a .3% dynamic range were completely immersed in a 1400 degree(s)F (760 degree(s)C) furnace throughout the duration of the test. The yield strength of the steel specimen evaluated with optical strain sensors agreed well with the manufacturing specifications. In addition, strain measured with the optical fiber strain sensor showed a difference of less than 0.01% compared to a ceramic knife-edged extensometer.
To perform a high temperature strain measurements, the sapphire extrinsic Fabry-Perot interferometer (EFPI) sensor head was attached to a silicon carbide rod and placed in a RF induction heater mounted on an MTS load frame. The high-temperature ceramic adhesive is designed to withstand temperatures up to 4000° F. As a basis for comparison, a gold-coated silica EFPI sensor was attached along side the sapphire EFPI.
Initially, strain data was recorded at room temperature as the silicon carbide rod was slowly loaded to 4500 lbs. When the temperature was stabilized at a maximum of 2000° F, the SiC rod was again loaded to determine the effects of temperature on strain. Although the sapphire sensor was capable of measuring strain at higher temperatures, die induction heater, being poorly insulated, was not able to further heat the test material.
The purpose of the second experiment was to determine the survivability and repeatability of tiie sapphire EFPI due to high-cycle fatigue loading. The long-term survivability of the sensor, in both static and fatigue loading, has been demonstrated. Although the load frame was shutoff after 200,000 cycles, one would expect to be able to fatigue load tiie sensor for millions of cycles since the components of the EFPI sensing head are free to move inside the alignment tube. This is consistent widi data obtained for similar silica opdcal EFPI sensors diat were fadgue loaded to over 10 million cycles as compared to resistive strain gages that typically can be cycled only to 60,000 at 2000°F.
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.
Multilayer ceramic actuators (MCAs) are of interest for u in actuated materials because of their small
hysterisis, small size, and durability, and the relatively large range of displacements made possible by
their multilayer construction. The processing of the MCA elements requires involved detail and time, in
part to ensure the removal of all organic components contained in the polymer binder and the noble
metal electrode layers.1
Piezoelectric/electrostrictive materials are a unique class of nonconducting, anisotropic materials which change in dimension due to the application of an electric field and thereby may be used as mechanical actuators. The most widely used actuation materials for acoustic transduction applications are piezoceramics, such as lead zirconate titanate (PZT) and lead magnesium niobate (PMN). Disadvantages of these materials include relatively high creep and hysteresis, the tendency of the ferroelectric dielectric material to retain electric potential after the alternating electric field to which it is subjected reverses polarity, thus causing electrostatic action to lag the applied voltage. The need to study the geometrical, material, and time dependent nonlinear behavior, as well as the interaction effects between sensors and actuators, is increasingly apparent, although a unified approach for modeling the local and global response of a nonlinear active material system has not been accomplished. In this paper we discuss the use of optical fiber-based short gage length Fabry-Perot sensors to experimentally verify an analytical model and allow determination of the nonlinear behavior of actuator elements without affecting their material properties.
This paper describes initial results of a fiber optic-based sensor during on-site testing performed by FEORC and Fiber and Sensor Technologies at Ingersol-Rand. Advantages of the fiber optic sensor are a demonstrated enhanced survivability, higher sensitivity, smaller size, electromagnetic interference immunity, and reduced risk of explosion. The conventional wire strain gages typically survive only a few minutes attached to the drill steel and drive chain, while the fiber sensors described here have survived over 400 hours and are currently still functioning properly. The tests described include the demonstration of strain energy measurements on the drive chain and drill steel, and displacement measurements of the piston within the drifter. All of the sensors tested can be used as both a laboratory evaluation and testing tools, as well as being an integral part of a proposed control and health monitoring system.
This paper describes a fiber optic system for detection of acoustic emission location in which extrinsic Fabry-Perot interferometric fiber sensors are incorporated in two different configurations: (1) attached to a thin aluminum panel, and (2) embedded in a graphite/epoxy composite laminate. The system computes the coordinate of the acoustic emission source using sensor recorded differential arrival times of the acoustic signals generated by the source. The impact location can be determined with a 0.5 millimeter resolution and an accuracy typically less than five millimeters.
We discuss fatigue test results using extrinsic Fabry-Perot interferometric (EFPI) strain gages on a F-15 fighter within a full-scale test frame at the Structures Test Facility, Wright Patterson Air Force Base, Ohio. A linear array of EFPI sensors were surface-mounted to the trailing edge of the F-15 wing to monitor strain concentration along the inboard flap hinge. Two types of EFPI strain gages and strain gage support systems were employed; linearized output from the standard, differential EFPI sensor system determined dynamic strain on the flap hinge during the application of load, whereas, absolute EFPI strain sensors were implemented to measure residual strain or cracks in the aluminum hinge.
A technique that uses embedded or attached high temperature optical fiber sensors is demonstrated for the intelligent processing of multilayer ceramic actuator elements. Presented are the results of the fiber optic strain sensor used to monitor internal displacements associated with the burnout of organic binders from a green actuator sample. Also presented is the method of operation of the low-finesse Fabry-Perot interferometric sensor, and post processing results obtained using the same type of sensor for tracking actuator performance and hysterisis.
A high-temperature sapphire strain gage based on the in-line extrinsic fiber- optic Fizeau interferometer was used to measure strain imparted by a 4000 lb compressive load applied at a temperature of 1100 degree(s)C. Experimental strain sensitivities on the order of 1 (mu) (epsilon) were obtained.
This paper reports the use of embedded extrinsic Fabry-Perot interferometric (EFPI) optical fiber sensors for the evaluation of composite materials containing arrays of piezoelectric actuator elements. The EFPI sensors are used in both differential and absolute measurement configurations. The extended 2D array of many small rod actuators is electrically driven through a pair of conductive electrode plates placed on top of and below the laminate. By applying an electrical potential difference between the plates, the actuator elements may be made to elongate axially. Two-dimensional spatial control of the resulting actuation function may be achieved by the interconnection of multiple conducting electrode addressing elements across the laminate.
Techniques are being investigated to spatially weight the strain sensitivity of a two-mode fiber strain sensor by mechanically altering the shape of the fiber along its length. The goal of this process is to obtain fibers that will be single-mode (V < 2.4) over a well-defined sensing length and dual-mode (2.4 < V < 3.8) over the remaining section of fiber. The single- mode section will be relatively insensitive to applied strain, while the dual-mode section will detect a distributed strain field associated with structural vibration mode shapes. The integrated strain will be over the two-mode section of the fiber, which will change with the changing wavelength of the interrogation beam.
This paper describes two optical fiber waveguide modification methods which allow the synthesis of effective modal filters and sensors for the analysis and active feedback control of vibrating structures. Both methods allow the sensitivity of the fiber to be varied along its length when used as a long gauge length integrating sensor. The performance of these sensors is similar to that of shaped piezoelectric film elements which have been demonstrated for several years for similar applications as modal filters, except the fiber sensors allow low- profile geometries in both dimensions orthogonal to the fiber axis. The first method uses modal birefringence between the LP01 and LP11 modes in two-mode fibers modified by tapering the waveguide during or subsequent to fabrication, with the sensitivity of a differential section of fiber along its length determined by the local core radius. By adjusting the taper geometry as a function of position along the fiber, individual vibration modes in a structure may be measured, with information concerning other orthogonal modes suppressed. The second method uses spatially weighted photorefractive index gratings in similar fibers to achieve modal selectivity. For both cases, we describe new methods for generating tapers and two-mode gratings, and present theoretical data and experimental results for such sensors embedded in composite laminates.
The effects of such parameters as exposure time, exposure power, and elevated temperature exposure on the growth and performance of photoinduced two-mode gratings are presented. We also present the results of an investigation into the possibility of utilizing the two polarization axes of an elliptical-core two-mode fiber to monitor two vibration modes simultaneously with one optical fiber. Two separate gratings are written onto a separate axis of a single two-mode fiber. Each polarization will be used to monitor a different vibration mode of a thin cantilever beam. The effects of the formation of a second grating on the performance of the first grating are investigated, and results are presented. These results will determine the feasibility of using the two polarization axes to monitor two vibrational modes of a structure.
Photoinduced refractive-index gratings are shown to affect the differential phase modulation between the LP and LP11 modes copropagating within the core of a germanium-doped (Ge-doped) two-mode elliptical-core (e-core) fiber. The beat length of fibers containing such gratings is shown to vary as a function of strain applied to the fiber leading to a new type of weighted optical fiber sensor whose sensitivity varies as a function of length along the structure to which it is attached or embedded. This novel type of spatially-weighted distributed fiber sensor is shown experimentally to function as an efficient vibration mode filter by enhancing either the first or second mode of a thin cantilever beam. These results support future applications of spatially-weighted/distributed optical fiber sensors for the evaluation and control of vibrating structures. I.
Two-mode, elliptical-core optical fibers are demonstrated in weighted, distributed and selective vibration-mode-filtering applications. We show how appropriate placement of optical fibers on a vibrating structure can lead to vibration mode filtering. Selective vibration-mode suppression on the order of 10 dB has been obtained using tapered two-mode, circular-core fibers with tapering functions that match the second derivatives of the modes of vibration to be enhanced. We also demonstrate the use of chirped, two-mode gratings in fibers as spatial modal sensors that are equivalents of shaped piezoelectric sensors.
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