Showing papers on "Fiber optic sensor published in 1990"

Fiber Optic Bragg Grating Sensors

Abstract: This paper describes a new method that we have developed to generate Bragg grating filters in germania-doped communication fibers and discusses their applications as sensors. The gratings are formed by exposing a short section of the core, through the side of the fiber, to an interference pattern of intersecting coherent beams of UV light. A permanent periodic index modulation is produced by the interference fringes. It forms a phase grating which acts as a band rejection filter, passing wavelengths that are not in resonance with the grating and strongly reflecting wavelengths which satisfy the Bragg condition. The grating wavelength is sensitive to changes temperature and strain. Measurements of the shift in Bragg wavelength with these quantities are reported and compared with computed estimates of the sensitivity. A large number of independent gratings can be easily written along a length of fiber to make a distributed sensor system. We have also demonstrated that Bragg gratings can wavelength tune laser diodes, which could be used for detecting the wavelength shift of the Bragg sensors.

Three-parameter optical fiber sensor and system

Abstract: A single sensor is provided as part of a fiberoptic probe to measure up to three parameters, namely pressure (or force or displacement), temperature, and heat flow or fluid velocity. A solid elastomeric optical element is formed at the end of optical fiber transmission medium, and adjacent light reflective and temperature dependent materials are formed on the resulting convex surface of the optical element. The amount of light reflected is proportional to the force or pressure against the element. The temperature dependent material is preferably a luminescent material. Over the luminescent material is formed a layer of material that is absorptive of infrared radiation, thereby allowing a determination of characteristics of heat or fluid flow by measuring the rate at which heat is carried away from the infrared heated layer. The sensor can be formed at the end of a single optical fiber, thereby having extensive applications where a very small sensor is required. One such application is a medical or clinical one, where the sensor is mounted in a catheter for providing pressure, flow and temperature of the blood in a blood vessel.

Elliptical-core two mode optical-fiber sensor implementation methods

Abstract: Experimental methods for the practical implementation of few-mode elliptical-core sensors are described. Techniques for desensitizing the lead-in and lead-out fibers are discussed, and results of a vibration sensor embedded in a graphite-epoxy composite are presented. A scheme using a single-mode elliptical-core fiber as the lead-in fiber and an offset circular-core single-mode fiber as the lead-out fiber is successfully implemented. Detection techniques for few-mode fiber sensors are reviewed, and a novel fringe-counting method to unambiguously detect the amplitude and direction of dynamic strain is reported. >

Embedded fiber-optic Fabry-Perot ultrasound sensor

Abstract: A fiber-optic ultrasound sensor is presented. The sensor consists of a continuous length of single-mode optical fiber with a built-in Fabry-Perot interferometer. The acoustic pressure produces changes in the index of refraction along the interferometer cavity through the strain-optic effect, thus modulating the reflected power of the light propagating in the fiber. The dielectric internal mirrors that form the interferometer are fabricated by joining a fiber coating with a TiO/sub 2/ film at one end to an uncoated fiber by electric arc fusion splicing. Experimental results have been obtained for sensors embedded in plastic and graphite composite materials, using ultrasound waves in the range from 100 kHz to 5 MHz. Values for the optical phase shift amplitude as large as 0.5 rad were obtained at an acoustic frequency of 200 kHz for a 1.1-cm-long interferometer embedded in plastic. >

Distributed amplification for lightwave transmission system

Abstract: Bidirectional lightwave transmission is restored and uniform amplification of lightwave signals over long spans of optical fiber is achieved by employing distributed amplification over the spans. Distributed amplification is achieved with an amplifying optical fiber which includes a long length of optical fiber having a dilute rare-earth dopant concentration substantially in the fiber core region, and a corresponding pump signal generator at at least one end of the doped fiber having the appropriate wavelength and power to cause amplification of optical signals by both Raman effects and stimulated emission from the rare-earth dopants. Dilute concentrations are understood as the range of concentrations substantially satisfying the condition that the gain from the rare-earth dopant, when near saturation, is substantially equal to the fiber loss.

A distributed fiber optic sensor based on cladding fluorescence

Abstract: The fiber for the sensor is formed by cladding fused silica during drawing with polydimethyl siloxane into which an organic fluorescent dye, 9, 10-diphenylanthracene, has been dissolved. Upon side illumination at a wavelength within the excitation range of the dye, the cladding fluoresces; some of this fluorescence is coupled into guided modes in the fiber core through the evanescent fields of these modes. In the presence of oxygen, fluorescent emission by the dye is diminished. For the sensor described, the rubbery liquidlike nature of the polydimethyl siloxane cladding allows rapid diffusion of gases, and the intensity of the guided fluorescence is observed to drop by 30% in less than 5 s when the ambient atmosphere changes from pure nitrogen to pure oxygen. The advantages of this sensing technique, and some of the possibilities for new sensors based on this principle, are discussed. >

Fiber optic gyroscope

Abstract: In a fiber optic gyroscope comprising a light source, a fiber optic coupler, an optical integrated circuit having a function of a polarizer and a branching optical waveguide, and a fiber optic coil, and detecting an angular velocity applied to the fiber optic coil about the axis thereof, a polarization maintaining optical fiber having its length L is connected to an input/output end of the optical integrated circuit nearer the light source in an optical system of optical fiber from the light source to the optical integrated circuit, the polarization maintaining optical fiber of its length L resulting in a difference in group delay time between the orthogonal two polarization modes and the difference at least exceeding a coherence length of light from the light source. An optical system of optical fiber from the light source to the optical integrated circuit except for the polarization maintaining optical fiber of its length L is formed by a single mode optical fiber, and the fiber optic coupler is formed by a single mode optical fiber.

Coupling of optical devices to optical fibers by means of microlenses

Abstract: This invention is concerned with coupling optical energy efficiently by means of a microlens at an end of an optical fiber, the microlens being capable of improved coupling efficiency exceeding 55 percent (-2.5dB), with coupling efficiencies of about 90 percent or more (less than 0.45dB and as low as 0.22dB loss) for uncoated lenses and of 95 percent or more for antireflection coated lenses being obtainable. An optimal microlens shape is substantially a hyperboloid of revolution having a relatively short focal length, f, e.g., for a mode radius of the optical device ω₀=1µm and a mode radius of the optical fiber ω₁=5µm, f≃12µm. The microlens having the substantially hyperboloid of revolution shape may be produced by laser micromachining technique. The optical fiber with the novel microlens at its end may be used in optical communication packages comprising the fiber and an optical device. The latter may be selected from semiconductor lasers and amplifiers, fiber amplifiers, pump sources for fiber amplifiers, and may include light receiving devices such as photodetectors.

Fiber Optic and Laser Sensors VII

Abstract: This book contains articles on fiber optic and laser sensors Included are these topics: Fiber optic sensor development at universities, Fiber optic sensing techniques, Magnetics, and Acoustics and pressure sensors

Remote measurement of physical variables with fiber optic systems

Abstract: The invention relates to methods and devices for measuring physical parameters by converting a fraction of the intensity of the interrogating light into a positive optical signal with wavelengths and/or light propagation modes different from those of the interrogating light, and having at least one characteristic which is a known function of the physical parameter being measured. The invention is adapted to the measurement of distributed forces and/or temperatures along a continuous length of optical fiber, and to the non-invasive coupling of information into an optical fiber from the side at any point or a multiplicity of points. The optical fiber is so designed that information signals coupled into it simultaneously at different points are separable at one end of the fiber and measurable without interference from each other.

Analysis of input-polarization-induced phase noise in interferometric fiber-optic sensors and its reduction using polarization scrambling

Abstract: The dependence of the phase shift of an interferometric fiber sensor on the input state of polarization is analyzed, and it is shown that fluctuations in the input polarization to a fiber interferometer can lead to the generation of excess phase noise. The relationship between this effect and the variation in visibility with input polarization is described and theoretically confirmed. The use of depolarized source light to eliminate input-polarization-induced excess phase noise is theoretically and experimentally demonstrated. >

Interferometric stain measurement by arbitrarily configured surface-mounted, optical fibers

Abstract: The phase change of light traveling in a single-mode, arbitrarily configured optical fiber when the fiber is subjected to an arbitrary strain field is described by an integral equation. Assumptions concerning strain transfer from the strained body to a surface mounted optical-fiber sensor are experimentally verified. The general mathematical description is applied to two interferometric optical-fiber strain sensor geometries and is verified by experiment. The lack of precise data about the mechanical and optical material properties of the fiber is circumvented through calibration. Extensions of this model to both embedded fibers and nonlinear fiber response are discussed. >

Biomedical fiber optic probe with frequency domain signal processing

Abstract: A fiber optic probe incorporating a luminescent composition is used to monitor conditions within a living subject. Response light from the fiber optic is detected and a frequency domain representation of the response light is derived. Characteristics of the frequency domain representation are used to derive values for luminescence lifetimes or similar decay parameters and these values in turn are translated into values of the conditions to be sensed.

Surface-Enhanced Raman Scattering Fiber-Optic Sensor

Abstract: A fiber-optic system was developed for exciting and collecting surface-enhanced Raman scattering (SERS) signals generated from a sensing plate tip having silver-coated microparticles deposited on a glass support. Various fiber parameters, such as fiber type, fiber-substrate geometry, and other experimental parameters, were investigated to obtain the optimum conditions for the SERS fiber-optic device. In addition, analytical figures of merit relevant to the performance of the SERS fiber-optic sensor, such as SERS spectral characteristics, reproducibility, linear dynamic range, and limit of detection, were also investigated.

Multifunction, distributed optical fiber sensor for composite cure and response monitoring

Abstract: Bragg gratings have been exposed into the core of optical fibers at specific locations to implement distributed, discrete gages for measurement of strain and temperature. Demonstrations using these devices to monitor composite specimen curing and bending are discussed. The sensor signals are also used in a control loop to drive an actuator for active damping of the test sample.

Optical fiber sensor with localized sensing regions

Abstract: A coupling mechanism for a non-identical dual core optical fiber. The fiber is sensitized along selected portions of its length by coating the fiber in a pattern of bands of specified spacing with material that is sensitive to the presence of a predetermined parameter or environmental field. By virtue of inequality of propagation properties and mismatch of optical phase of the fields of the two cores, evanescent wave coupling is largely or wholly suppressed except in those portions of length coated in the specified spatially periodic manner. Optical power launched into one of the two cores remains propagating within that core alone over an extended length of the fiber, except for that period of time when the predetermined parameter or environmental field is sensed. The specific spatial periodicity of the pattern of coating bands on the outside of the fiber acts in concert with the nonidentical propagation parameters of the two cores to effect intercore coupling of optical energy. This coupling, representing sensitivity to any environmental field which causes strain in the coating, is effective only on those portions of the dual core fiber length which bear a specific coating pattern.

Coherence tuned fiber optic sensing system, with self-initialization, based on a multimode laser diode

Abstract: In this paper we study the potential of using multimode laser diodes as an alternative to low coherence length sources normally used white light fiber optic interferometric sensors. A simple theoretical model is introduced to demonstrate the autocorrelation function of such sources. An experimental setup of two interferometers in tandem was used to study the coherence properties of the multimode laser diode Mitsubishi ML-4406 and then to demonstrate the possibility of using it in coherence tuned multiplexing systems. The main advantage of using such sources is to launch more optical power into the monomode fiber and hence to improve the system resolution.

Fiber optic microbending sensor arrays including microbend sensors sensitive over different bands of wavelengths of light

Abstract: A fiber optic sensing system detects changes in a material based upon physical deformation of a fiber bonded to the structure in which stress, strain or failure is to be detected. Deformation of the fiber affects propagation of light therethrough, permitting detection based upon detected changes in light throughput.

Ultrafast, low power, and highly stable all-optical switching in an all polarization maintaining fiber Sagnac interferometer

Abstract: An ultrafast, low-power, and highly stable all-optical switch in a nonlinear Sagnac interferometer is reported. To achieve low-power, highly stable, and walkoff free switching, use is made of a small-core dispersion-shifted polarization-maintaining fiber loop (200 m in length) which has a small group delay difference between the wavelengths of the input signal and the control pulse. To achieve complete polarization stability, a wavelength-sensitive polarization-maintaining fiber coupler is employed. Highly stable and walkoff free all-optical switching is demonstrated at 5 Gb/s. >

Handbook of Fiber Optics: Theory and Applications

Abstract: Introduction. The Optical Fibers: The Optical Fibers. Wave Propagation in Lightguides. Auxiliary Components for Optical Fiber Systems. Optical Fiber Measurements. Semiconductor Light Sources and Detectors: Semiconductor Light Sources: Light-Emitting Diodes and Injection Lasers. Photodetectors. Optical Fiber Communication Systems: Optical Fiber Transmitter and Receiver. Fiber Optic Communication Networks. Optical Fiber Sensors, Passive Applications, and Integrated Devices: Physical Phenomena for Optical Fiber Sensors. Optical Fiber Sensors. Miscellaneous Passive Applications of Optical Fibers. Integrated Optical Fiber Devices. Index

Fault location technique for in-service branched optical fiber networks

Abstract: A fault location technique using an optical time-domain reflectometer (OTDR) for branched optical fiber networks is described. The required dynamic range of the OTDR for locating a fiber failure occurring after N-way optical power splitting has been studied. In addition, it has been demonstrated that by using an OTDR operating at 1.65 mu m, it is possible to locate one fiber failure among 16-branched fibers without disrupting the 1.31- mu m and 1.55- mu m services. >

Fiber-optic interferometric sensor for the detection of acoustic emission within composite materials.

Abstract: An optical-fiber Michelson interferometric acoustic emission sensor is described. The sensor uses ordinary singlemode fiber and is embedded in the composite material under test. Signals are demodulated through the active homodyne. This system provides a novel approach for material nondestructive evaluation.

Monitoring system for fiber optic cables utilizing an OTDR for detection of signal loss and automatic location of faults in the cable

Abstract: A monitoring system continuously monitors fibre optic cables for faults and then enables automatic location of the fault in the cable. In the system an optical monitoring signal is generated and connected as an output signal to a first end of the fiber optic cable for transmission to the second end. The signal is then returned through the fiber optic cable to provide a return signal at the first end of the cable. A comparator at the first end compares the output signal and the return signal and provides corresponding difference data. An optical time domain reflectometer detects the location of a signal loss along the fiber optic cable and a data processing system determines the location of the signal loss from the corresponding difference data. A single optical time domain reflectometer can be used for monitoring a plurality of fiber optic cables.

Measurement of sodium hydroxide concentration with a renewable reagent-based fiber-optic sensor

Abstract: A fiber-optic caustic sensor is described in which a weakly buffered acid-base indicator reagent (bromothymol blue) is continuously delivered to the sensor tip which consists of a length of Nafion hollow fiber membrane, an analyte exchange region, and an optical cell. In this study, caustic concentrations of 1.5-23.0% (w/w) (0.38-7.2 M) were measured, with a relative standard deviation of 1.6%. Sensor performance was studied at several reagent compositions, at various reagent delivery rates, in the presence of brine, and with respect to probe-to-probe reproducibility and long-term use

Tunable diode laser frequency modulation spectroscopy through an optical fiber: High‐sensitivity detection of water vapor

Abstract: Frequency modulation spectroscopy through a single‐mode optical fiber has been demonstrated for monitoring H2O vapor at 7665 cm−1. Using a 2.5 mW distributed feedback laser and 100 m of optical fiber, a minimum detectable absorption of 5×10−7 was achieved. To obtain this sensitivity limit, very large spurious signals and technical noise were suppressed using a dual‐channel detection scheme.

Fiber optic Fabry-Perot strain gauge

Abstract: A local all-fiber Fabry-Perot interferometric strain sensor is reported. The device makes use of a simple semireflective fusion splice technique and is operated in reflection mode. Five sensors of gauge lengths of approximately 10 mm were fabricated and adhered to the surface of a cantilever beam. The phase-strain sensitivity, Delta phi / Delta L, was measured to be 2.24+or-0.07*10/sup 7/ m/sup -1/ ( lambda =632.8 nm) and found to be symmetric with respect to tension and compression. >

Development of a polarimetric optical fiber sensor for electronic measurement of high pressure

Abstract: The theoretical understanding of the principle of pressure-induced polarization coupling is discussed, and the improved construction, operation, and temperature desensitization of a high-pressure (up to 100 MPa) fiber-optic sensor in two configurations is described. The sensor exploits the effect of polarization coupling between two orthogonally polarized eigenmodes of a highly birefringent, polarization-preserving optical fiber which serves as the sensing element. An idea of temperature desensitization of the sensor output signal is demonstrated. The requirements for an electronic measurement system based on the sensor are discussed, including indentification of the parametric and functional specifications and constraints of such a system. >

Passive fiber optic sensor with omnidirectional acoustic sensor and accelerometer

Abstract: A mismatched path length fiber optic interferometer is optically coupled to an optical fiber and configured to form an omnidirectional acoustic sensor. A second mismatched path length fiber optic interferometer is optically coupled to the optical fiber and configured as a first gradient sensor. A second fiber optic gradient sensor is also optically coupled to the optical fiber. A detector optically coupled to the omnidirectional acoustic sensor and to the gradient sensors converts optical signals output therefrom to electrical signals indicative of the magnitude and direction of changes in an acoustic field. The omnidirectional acoustic sensor may include a length of optical fiber wrapped around the housing while the gradient sensors are mounted inside the housing. The housing perferably has a volume that is adjustable for controlling the buoyancy thereof. Each gradient sensor preferably comprises a pair of mandrels formed to enclose chambers. Optical fiber coils are formed on the mandrels. Both the chambers are filled with a fluid and placed in fluid communication through a tube that defines a sensing axis between the mandrels such that acceleration of the housing along the first sensing axis causes a fluid pressure differential on the first and second optical fiber coils.