Showing papers on "Fiber optic sensor published in 2002"

Sensitivity characteristics of long-period fiber gratings

Abstract: We present a detailed investigation into the sensitivity of long-period fiber gratings (LPFGs) as a function of temperature, strain, and surrounding refractive index, with particular attention to the higher order cladding modes and the possibilities for ultrasensitive sensors. From a general theoretical analysis, we identify a general sensitivity factor which offers new physical insight into LPFG behavior and represents a useful design aid in conjunction with a set of measurand-specific sensitivity factors. Our analysis reveals the existence of turning points in the mode dispersion characteristics at which ultrasensitive operation may be obtained. In an extensive set of coordinated experiments, we verify the theoretical predictions with close agreement and provide demonstrations of the device behavior close to the turning points. Alternative sensor schemes for temperature, strain, and refractive index based, respectively, on measurement of the dual resonance characteristic of the modes and on the transmission characteristics close to the turning points, utilizing higher order modes of the LPFG, are presented. For two variables at least, we record the highest LPFG sensitivities yet reported.

Fiber Optic Sensors

Abstract: Fiber optic sensors have become indispensable tools for biomedical study because of their unique features such as (1) high sensitivity, (2) small footprint and endoscopic compatibility, (3) multiple agent distributive sensing capability, and (4) immunity from electromagnetic interference. In this chapter, we will provide a brief overview on the basic principle of fiber optic sensors, types of fiber optic sensors, and their applications to biomedical sensing. As a result of the page limitation, important work in this area may not be included in this chapter. Readers may find those contents in the listed reference material. Keywords: fiber optic sensors; light; sensing mechanism; biomedical sensing

Temperature and vibration insensitive fiber-optic current sensor

Abstract: A robust interferometric fiber-optic current sensor with inherent temperature compensation of the Faraday effect is presented. Sensor configurations based on Sagnac and polarization-rotated reflection interferometers are considered. The sensing fiber is residing and thermally annealed in a coiled capillary of fused silica. The capillary is embedded in silicone within a ring-shaped housing. It is theoretically and experimentally shown that the temperature dependence of the birefringent fiber-optic phase retarders of the interferometers can be employed to balance the temperature dependence of the Faraday effect (0.7/spl times/10/sup -4///spl deg/C). Insensitivity of the sensor to temperature within 0.2% is demonstrated between -35/spl deg/C and 85/spl deg/C. The influence of the phase retarders on the linearity of the sensor is also addressed. Furthermore, the sensitivity to vibration of the two configurations at frequencies up to 500 Hz and accelerations up to 10 g is compared. High immunity of the reflective sensor to mechanical perturbations is verified.

Relative humidity sensor with optical fiber Bragg gratings

Abstract: A novel concept for an intrinsic relative humidity (RH) sensor that uses polyimide-recoated fiber Bragg gratings is presented. Tests in a controlled environment indicate that the sensor has a linear, reversible, and accurate response behavior at 10-90% RH and at 13-60 degrees C. The RH and temperature sensitivities were measured as a function of coating thickness, and the thermal and hygroscopic expansion coefficients of the polyimide coating were determined.

Distributed fiber Brillouin strain sensing with 1-cm spatial resolution by correlation-based continuous-wave technique

Abstract: This letter describes improvement in the spatial resolution of distributed fiber Brillouin strain sensing by the correlation-based continuous-wave technique, which we proposed previously. The spatial resolution of 1 cm has been achieved for a partly stretched fiber. This result shows the technique is applicable to smart structures or materials as a nervous system to monitor strain distribution.

Solid-state light source

Abstract: A solid-state light source includes a semiconductor light source for emitting light and an optical system having a fiber optic element. The fiber optic element has an input for receiving emitted light from the semiconductor light source. The fiber optic element also has an output for emitting light received from the solid-state light source. The semiconductor light source and the fiber optic element in aggregate form an illumination path.

Fiber optic interferometric vital sign monitor for use in magnetic resonance imaging, confined care facilities and in-hospital

Abstract: A fiber optic monitor that utilizes optical phase interferometry to monitor a patient's vital signs such as respiration, cardiac activity, blood pressure and body's physical movement The monitor, which is non-invasive, comprises an optical fiber interferometer that includes an optical fiber proximately situated to the patient so that time varying acousto-mechanical signals from the patient are coupled into the optical fiber Responsive thereto, the interferometer generates a time-varying optical intensity resulting from the interference of optical signals, which are detected at a photo-detector A signal processor coupled to the optical detector provides one or more processed output signals indicative of the vital functions The monitor system has broad applicability, from routine monitoring of infants at home to detection of apnea, arrhythmia, blood pressure and trauma The system can be implemented in embodiments ranging from a low cost in-home monitor for infants to a high end product for in hospital use The monitor can be integrated with other sensors such as an EKG, a video or still camera, an oxygen sensor, a carbon dioxide sensor, temperature sensor or a microphone to get additional required information depending on the application When integrated and combined with EKG information, the monitor provides ballisto-mechanical information of the heart for early diagnosis of cardiac conditions or prediction of events or for correcting corrupted EKG signals due to time varying magnetic and electric fields In some embodiments of the monitor, the system can be made portable so that the patient can walk around while still being continuously monitored for vital signs Another suitable design measures blood pressure continuously and non-invasively by containing the fiber optic sensor in a cuff that wraps around an arterial wall of the patient The fiber optic monitor may be designed for use in a variety of settings including an operating room, a recovery room, an intensive care unit, a magnetic resonance imaging laboratory, a computerized tomography scan laboratory and an elderly care facility

Delamination detection in CFRP laminates with embedded small-diameter fiber Bragg grating sensors

Abstract: Newly developed small-diameter fiber Bragg grating (FBG) sensors were applied for the detection of the delamination in carbon fiber reinforced plastic (CFRP) cross-ply laminates Since the diameter of the FBG sensor was very small, the sensor was easily embedded into the 0° ply along the reinforcing carbon fibers The reflection spectra from the FBG sensor were measured at various delamination lengths through four-point bending test The form of the spectrum changed sensitively, as the delamination length increased Moreover, the spectrum was calculated theoretically for confirmation of the measured spectrum The calculated result reproduced the change in the measured spectrum very well Then, we proposed the intensity ratio in the spectrum as an effective indicator for the prediction of the delamination length This indicator has a potential to be applied to other laminate configurations

Wavelength-agile fiber laser using group-velocity dispersion of pulsed super-continua and application to broadband absorption spectroscopy

Abstract: A swept-wavelength source is created by connecting four elements in series: a femtosecond fiber laser at 1.56 μm, a non-linear fiber, a dispersive fiber and a tunable spectral bandpass filter. The 1.56-μm pulses are converted to super-continuum (1.1–2.2 μm) pulses by the non-linear fiber, and these broadband pulses are stretched and arranged into wavelength scans by the dispersive fiber. The tunable bandpass filter is used to select a portion of the super-continuum as a scan-wavelength output. A variety of scan characteristics are possible using this approach. As an example, an output with an effective linewidth of approximately 1 cm-1 is scanned from 1350–1550 nm every 20 ns. Compared to previous scanning benchmarks of approximately 1 nm/μs, such broad, rapid scans offer new capabilities: a gas sensing application is demonstrated by monitoring absorption bands of H2O, CO2, C2H2 and C2H6O at a pressure of 10 bar.

Wavelength exchange in a highly nonlinear dispersion-shifted fiber: theory and experiments

Abstract: With a suitable arrangement of two pumps and two signals with respect to the zero-dispersion wavelength of a fiber, simultaneous wavelength exchange between two signals can be realized by four-wave mixing in the fiber. We have demonstrated near-complete wavelength exchange between two signals at 1573.4 and 1579.9 nm with two 0.25-W pumps in a 1-km-long highly nonlinear dispersion-shifted fiber. We also have evaluated the bit-error-rate performance of wavelength exchange with a 10-Gb/s signal, and obtained a power penalty of less than 1 dB for the exchanged signal.

Fiber-loop ring-down spectroscopy

Abstract: Pulsed, visible and near-infrared laser light is coupled into an optical fiber, which is wound into a loop using a fiber splice connector. The light pulses traveling through the fiber-loop are detected using a photomultiplier detector. It is found that once the light is coupled into the fiber it experiences very little loss and the light pulses do a large number of round trips before their intensity is below the detection threshold. Measurements of the loss-per-pass and of the ring-down time allow for characterization of the different loss mechanisms of the light pulses in the fiber and splice connector. This method resembles “cavity ring-down absorption spectroscopy” and is well suited to characterize low-loss processes in fiber optic transmission independent from power fluctuations of the light source. It is demonstrated that by measuring the ring-down times one can accurately determine the absolute transmission of an optical fiber and of the fiber connector. In addition it is demonstrated that the tech...

Real-time cure monitoring of smart composite materials using extrinsic Fabry-Perot interferometer and fiber Bragg grating sensors

Abstract: Real-time cure monitoring of composite materials is very important to improve the performance of advanced composite materials. It is very difficult to monitor the cure process online using conventional methods. Fiber optic sensors in smart composite materials provide a unique opportunity to monitor the cure process of composite materials in real time by using embedded sensors. In this paper, extrinsic Fabry-Perot interferometer (EFPI) and fiber Bragg grating (FBG) sensors are embedded in carbon/epoxy composite laminates and used to monitor the cure process simultaneously. Furthermore, measurements of residual strains of composite laminates during the cure have been performed. The results show that both EFPI and FBG sensors can be used to monitor the strain development of composite laminates with and without damage during cure. An excellent correlation between the EFPI and FBG sensors is presented.

Fiber optic Bragg grating sensor based on hydrogels for measuring salinity

Abstract: We report a new type of optical salinity sensor with a fiber Bragg grating (FBG) coated with hydrogels. The sensing mechanism in this device is based on mechanical stress that is induced in the chemically sensitive water swellable polymers (hydrogels) coating when the water escapes from it. The stress in the hydrogels coating stretches and shifts the Bragg wavelength of the FBG. Varying the composition of the gel, the sensor can be used to response to different trigger stimulus. Here the sensors for measuring salinity are demonstrated.

Dependence of temperature and strain coefficients on fiber grating type and its application to simultaneous temperature and strain measurement

Abstract: We report an investigation of the dependence of the temperature and strain coefficients on the grating type for fiber Bragg gratings that are UV inscribed in B/Ge-codoped fiber with and without hydrogenation. The results reveal that all types of grating exhibit similar strain sensitivities but markedly different temperature sensitivities, greater for gratings inscribed in hydrogen-free rather than hydrogenated fiber and substantially less in type IA gratings than all others. The sensitivity characteristics of these gratings have been used to implement a new type of dual-grating sensor for simultaneous measurement of temperature and strain that has properties superior to those of previously reported structures.

Cavity-ring-down principle for fiber-optic resonators: experimental realization of bending loss and evanescent-field sensing.

Abstract: A novel measurement principle for fiber-optic sensing is presented. Use of a cavity-ring-down scheme enables measurements of minute optical losses in high-finesse fiber-optic cavities. The loss may be induced by evanescent-field absorption, fiber bending, fiber degradation, Bragg gratings, or any other effect that might change the fiber transmission or cavity reflector properties. The principle is proved to be rather insensitive to ambient perturbations such as temperature changes. A high-sensitivity measurement of loss due to bending is presented as a proof-of-principle. With a cavity finesse of 627 a sensitivity for induced loss of 108 ppm (4.68 × 10-4 dB) is achieved. Preliminary measurements of evanescent-field absorption are also discussed.

Distributed sound speed measurements for multiphase flow measurement

Abstract: A multiphase flow meter distributed system is disclosed that is capable of measuring phase flow rates of a multiphase fluid. The distributed system includes at least one flow meter disposed along the pipe, an additional sensor disposed along the pipe spatially removed from the flow meter, and a multiphase flow model that receives flow related parameters from the flow meter and the additional sensor to calculate the phase flow rates. The flow meter provides parameters such as pressure, temperature, fluid sound speed and/or velocity of the fluid, and the additional sensor provides a parameter indicative of pressure and or temperature of the fluid. Depending on production needs and the reservoir dimensions, the distributed system may utilize a plurality of flow meters disposed at several locations along the pipe and may further include a plurality of additional sensors as well. The distributed system preferably uses fiber optic sensors with bragg gratings. This enables the system to have a high tolerance for long term exposure to harsh temperature environments and also provides the advantage of multiplexing the flow meters and/or sensors together.

Tunable microfluidic optical fiber

Abstract: We describe a class of active, tunable optical fiber that incorporates multiple microfluidic plugs into interior fiber microchannels. The propagation characteristics of certain optical modes of these fiber waveguides can be usefully manipulated by controlling the positions and optical properties of these plugs, using actuators and pumps formed on the fiber surface. These hybrid microfluidic/silica structures offer versatile tuning capabilities in a format that preserves the advantages of conventional, passive optical fiber. As an example, we report an all-fiber, narrowband filter, whose transmission wavelength and attenuation are independently adjustable via microfluidic tuning.

Effect of thermal residual stress on the reflection spectrum from fiber Bragg grating sensors embedded in CFRP laminates

Abstract: When FBG sensors are embedded in CFRP laminates, the reflection spectrum from the FBG sensors splits into two peaks because of the non-axisymmetric thermal residual stress. This deformation of the spectrum will lead to misreading in strain measurements or crack detection in the laminates. In the present research, three types of FBG sensors: uncoated normal, polyimide-coated normal, and polyimide-coated small-diameter FBG sensors, were embedded in CFRP cross-ply laminates, and reflection spectra from the sensors were measured during the fabrication process of the laminates. Through the comparison of results obtained for the three FBG sensors, it was found that the effect of thermal residual stress on the reflection spectrum could be decreased when the optical fiber was coated with polyimide and its diameter was small in the present laminate configuration and embedment position. Furthermore, these changes of the spectra during the curing process could be simulated by theoretical calculation considering the birefringence effect.

Characteristics of short-period blazed fiber Bragg gratings for use as macro-bending sensors

Abstract: The characteristics of short-period blazed fiber Bragg gratings for use as macro-bending sensors are discussed. This sensor is able to detect macro bending with the transmitted power variation of the first side mode in the blazed fiber Bragg grating. Since an incident ray experiences different variations of tilt angles with respect to bending direction, the blazed fiber Bragg grating has different coupling efficiencies of the first side mode, which can be reduced considerably in the case of twisted blazed fiber Bragg gratings.

Continuous-wave fiber optical parametric oscillator

Abstract: We report continuous-wave operation of singly resonant fiber optical parametric oscillators. In a cavity formed by 100 m of highly nonlinear fiber and two fiber Bragg gratings, the pump power threshold was 240 mW; the output wavelength could be tuned over 80 nm by tuning of the pump. We also obtained an internal conversion efficiency of 30%, compared with the maximum theoretical value of 50%, by use of a 1-km-long cavity.

Simultaneous measurement of strain and temperature using a fiber Bragg grating and a thermochromic material

Abstract: A new scheme for simultaneous measurement of temperature and strain using fiber Bragg grating (FBG) and a thermochromic material is presented. It consists of a dual system with two different sensing elements included in the same fiber: a fiber Bragg grating to measure the strain and a thermochromic effect based optical fiber sensor to measure the temperature variations and to compensate the cross sensitivity of the fiber Bragg grating with temperature. Experimental results of the optical dual system are shown and discussed.

L-band multiwavelength fiber laser using an elliptical fiber

Abstract: In this letter, we propose and demonstrate a simple and novel technique to get multiwavelength operation in the L-band by using an elliptical core erbium-doped fiber. The principle of operation is based on the anisotropic gain effect in a polarization-maintaining fiber. Stable multiwavelength operation is achieved at room temperatures. The lasing lines may be controlled by a polarization controller.

Cure monitoring of composite laminates using fiber optic sensors

Abstract: In this paper, we present the simultaneous measurement of the strain and temperature during cures of various composite laminates using fiber Bragg grating/extrinsic Fabry-Perot interferometric (FBG/EFPI) hybrid sensors. The characteristic matrix of the hybrid sensor is derived analytically. For the fabrication of the three types of graphite/epoxy composite laminate, two FBG/EFPI hybrid sensors were embedded in each composite laminate in two mutually perpendicular directions. We performed the real-time measurement of fabrication strains and temperatures at two points within the composite laminates during the curing process in an autoclave. Through these experiments, FBG/EFPI sensors are proven to be a good choice for efficient smart monitoring of composite structures.

Fiber grating environmental sensing system

Abstract: A fiber grating environmental sensing system is described that has the ability to measure strain, vibration, humidity and water content. Fiber gratings are used to measure axial strain to accomplish these goals. An approach is described that uses matched fiber grating sensors and filters to allow for thermal compensation greatly reducing one of the key performance issues facing these systems. The system may be deployed in such applications as roadways and paved surfaces, bridges, buildings and aircraft and spacecraft. Means are described to implement transducers optimized for specific environmental measurements as well as means to demodulate the fiber grating sensors.

Intrinsic fiber Fabry–Perot temperature sensor with fiber Bragg grating mirrors

Abstract: A fiber Fabry-Perot interferometer (FFPI) sensor is formed with broadband (~3 nm, 3-dB bandwidth) fiber Bragg grating (FBG) mirrors. Repetitively modulating a distributed-feedback laser produces chirping that modulates the reflectance of the FFPI. Because the reflectance of the FBG mirrors varies with optical frequency, the fringes in the sensor reflectance modulation are distinguishable, making it possible to extend the sensor dynamic range versus that of a FFPI sensor with conventional wavelength-dependent mirrors. An ambient temperature is determined in the range from 25 to 170 degrees C with a resolution of 0.005 degrees C.