Showing papers on "Fiber optic sensor published in 1995"

Parameter control, characterization, and optimization in the fabrication of optical fiber near-field probes

Abstract: Tip diameter and transmission efficiency of a visible-wavelength near-field optic probe determine both the lateral spatial resolution and experimental utility of the near-field scanning optical microscope. The commonly used tip fabrication technique, laser-heated pulling of fused-silica optical fiber followed by aperture formation through aluminization, is a complex process governed by a large number of parameters. An extensive study of the pulling parameter space has revealed a time-dependent functionality between the various pulling parameters dominated by a photon-based heating mechanism. The photon-based heat source results in a temperature and viscosity dependence that is a complex function of time and fiber diameter. Changing the taper of the optical probe can affect transmission efficiency by an order of magnitude or more.

Optical fiber sensor based on surface plasmon excitation

Abstract: A very sensitive sensor structure for measurement of small changes of the refractive index employing the interaction between the surface plasmon and the guided mode of a single-mode optical fiber is reported. The theoretical treatment of the proposed sensor structure based on planar waveguide approximation is presented. The analysis of the influence of the main parameters of the sensor structure on the sensor performance is carried out with the aim of finding an optimum strategy for designing optical sensors with high sensitivity. The possibility of connecting the sensor to a broadband light source is also discussed.

An overview of fiber-optic sensors

Abstract: Fiber‐optic sensor technology has experienced tremendous growth since its early beginnings in the 1970s with early laboratory demonstrations of fiber‐optic gyros and acoustic sensors and the introduction of the first commercial intensity and spectrally based sensors. These early efforts were followed by a tremendous growth of interest in the 1980s when the number of workers in the field increased from perhaps a few hundred to thousands. The result was the introduction in the 1990s of the first mass produced fiber‐optic sensors that are being used to support navigation and medical applications. The number of fiber‐optic sensors products can be expected to grow tremendously in the years to come as rapid progress continues to be made in the related optoelectronic and communication fields. This paper provides an overview of some of the technologies being used to support fiber‐optic sensor development and how they are being applied.

Recent progress in optical current sensing techniques

Abstract: This paper reviews recent developments in the field of current measurement which employ a wide range of optical and fiber optic techniques. Depending on the current sensing mechanisms involved and the sensing materials used, optical current sensors (OCSs) may be categorized into four main groups: (i) OCSs employing optical fiber as their sensing elements, (ii) OCSs using bulk glass to sense the current, (iii) OCSs using electro‐optic hybrid sensing devices, and (iv) OCSs using magnetic field sensing devices. The operational modes of a variety of OCSs have been grouped and discussed, and several examples given. It can be seen that as a result of an intensive and wide ranging research effort using various approaches, substantial progress in the differing aspects of the optical current sensing techniques considered, such as the sensing material used, the configurations of sensing elements introduced, and the detection schemes adapted, has been achieved during the past years. An overall view of the field shows it to be as an active and exciting research area, highlighting several recently introduced and novel sensing materials and configurations which provide impressive results in this field of instrumentation. Examples of applications for the electric power industry are discussed more extensively.

Optical sensor apparatus for far-field viewing and making optical analytical measurements at remote locations

Abstract: The present invention is an optical sensor apparatus for far-field viewing and imaging as well as for the optical detection and analytical measurement of at least one species of analyte in a remotely-positioned fluid sample. The apparatus employs an imaging fiber comprising a fiber optic array and a Gradient Index lens; and utilizes a remotely-positioned solid substrate having light energy absorbing indicator ligands on an external surface for reactive contact with individual species of analytes when present in a fluid sample. The optical sensor apparatus is able to view an object and provide an image of the object located at a pre-set optical distance remote from the imaging fiber. The apparatus is also able to detect and identify, qualitatively and quantitatively, one or more analytes of interest in a flowing solid, gaseous, or liquid sample at remote locations distanced from the imaging fiber itself.

Optical fiber modulation and demodulation system

Abstract: This invention is directed toward a fiber optic modulation and demodulation system, and more particularly directed toward a telemetry system for relaying signals from sensors in remote, harsh environments. Light is modulated using one or more optical reflective grating and piezoelectric crystal combinations, and demodulated using an interferometer system. The one or more modulators are driven by the responses of one or more sensors thereby modulating one or more carrier wavelengths of a carrier light source. The modulated light signal is transmitted from the sensor or sensors, over an optical fiber, to an interferometer which is used to demodulate the reflected signals and thereby determine the responses of one or more sensors. One embodiment of the invention set forth is that of a telemetry system linking sensors within a borehole to detection and processing equipment at the surface of the earth.

Embedded optical sensor capable of strain and temperature measurement using a single diffraction grating

Abstract: An embedded optical sensor has a plurality of layers (10-20) and an optical fiber (21) with a fiber grating (28), disposed between the layers (14, 16). The layers (10-20) comprise filaments (22) and resin (24) which have different thermal expansion coefficients and the filaments (22) are oriented so as to create unequal transverse residual stresses that act through the geometry of a resin-rich region that surrounds on the grating (28) in the fiber (21). The unequal transverse residual stresses cause birefringence in the grating (28), thereby causing the grating (28) to reflect light (32) having two wavelengths with a predetermined separation, each along a different polarization axis. The wavelength separation and average wavelength between such separation have different sensitivities to temperature and strain, thereby allowing independent temperature and strain measurements using only a single grating. The birefringence is maximized when the filaments (22) of the adjacent layers (10, 12) are oriented at 90 degrees with respect to the longitudinal (Z-axis) of the fiber (21).

Simultaneous strain and temperature sensing with photogenerated in-fiber gratings.

Abstract: A new technique for simultaneous strain and temperature sensing is demonstrated. The approach employs two different types of photogenerated fiber grating, namely, a fiber Bragg grating and a fiber polarization-rocking filter. The method relies on the different dependencies of the fiber refractive index and birefringence on strain and temperature. Both of these measurands can be determined from the effect that they have on the resonant wavelength of each grating. The information is provided in the frequency domain, avoiding the problem of limited unambiguous signal range associated with the use of competing optical fiber interferometers.

Passive temperature-compensating package for optical fiber gratings.

Abstract: We demonstrate a compact, passive temperature-compensating package for fiber gratings. The grating is mounted under tension in a package comprising two materials with different thermal-expansion coefficients. As the temperature rises the strain is progressively released, compensating the temperature dependence of the Bragg wavelength. A fiber grating mounted in a package 50 mm long and 5 mm in diameter exhibited a total variation in Bragg wavelength of 0.07 nm over a 100 °C temperature range, compared with 0.92 nm for an uncompensated grating.

Oscillating fiber optic display and imager

Abstract: A display system includes an image light source for producing a modulated light, an optical fiber having a first end and a second end, the first end of the optical fiber being coupled to the light source, and a deflation device coupled to the second end of the optical fiber, the deflection device deflecting the second end of the optical fiber in a two-dimensional scan pattern for projecting an image onto a viewing surface from the second end of the optical fiber, wherein the projected image is related to the modulated light.

Erbium-doped silica fibers for intrinsic fiber-optic temperature sensors

Abstract: The variation in the green intensity ratio (2H11/2 and 4S3/2 energy levels to the ground state) of Er ions in silica fibers has been studied as a function of temperature. The different processes that are used to determine the population of these levels are investigated, in particular 800-nm excited-state absorption in Er-doped fibers and 980-nm energy transfer, in Yb–Er-codoped fibers. The invariance of the intensity ratio at a fixed temperature with respect to power, wavelength, and doped fiber length has been investigated and shown to permit the realization of a high-dynamic-range (greater than 600 °C), autocalibrated fiber-optic temperature sensor.

In-line fiber etalon (ILFE) fiber-optic strain sensors

Abstract: This paper describes an optical fiber interferometer that uses a short segment of silica hollow-core fiber spliced between two sections of single-mode fiber to form a mechanically robust in-line optical cavity. The hollow-core fiber is specifically manufactured to have an outer diameter that is equal to the outer diameter of the single mode lead fibers, thereby combining the best qualities of existing intrinsic and extrinsic Fabry-Perot sensors. Uniaxial tension and pure bending strength tests are used to show that the new configuration does not diminish the axial strength of bare fiber and reduces the bending strength by 17% at most. Similar tests confirm that the fiber sensor has 1.96% strain to failure. Axisymmetric finite element analysis is used to investigate the reliability of the in-line etalon when it is embedded in a typical thermoset composite, and parametric studies are performed to determine the mechanically optimal cavity length. The fiber optic sensor is tested using low coherence interferometry with pseudo-heterodyne demodulation under strain and temperature fields. The strain response compares well with resistance strain gages, and the temperature tests confirm the low thermal apparent strain of this sensor. >

Bragg intragrating structural sensing

Abstract: Fiber-optic intracore Bragg gratings have been of interest for a variety of sensing applications,1,3 especially for smart structures.4 In most smart structures applications, the strain along a fiber grating sensor is uniform and the strain is measured by the shift of the reflective wavelength of the grating. However, in some situations such as near a fiber debonding zone or near a structural damage point, strong local strain gradients are produced. Under such strain fields, the grating spectrum will not only be shifted but also distorted due to the chirp induced in the grating period by the imposed strain gradient. Based on this spectrum distortion, it is possible to use Bragg gratings as quasi-distributed sensors to sense strain gradients over a small region (sub-centimeter). We have calculated the reflection spectrum of nonuniform Bragg gratings by using the T-matrix method. This has enabled us to determine the effects of strain gradients on the spectrum. Experiments have verified these theoretical results.

Measurement of the Three-Dimensional Permeability of Fiber Preforms Using Embedded Fiber Optic Sensors:

Abstract: An experimental technique is described which can be used to measure simultaneously the three principal permeabilities of fiber reforms made of continuous or short fibers. The technique utilizes embedded optical fibers for detecting the position of the liquid front inside the reform. Expressions were derived for calculating the permeabilities from the three-dimensional measurement of the liquid front. Permeabilities measured by the present fiber optic and by the conventional, pressure drop and flow visualization techniques were compared. Good agreements were found between the results of these techniques.

Application of a fiber Fourier transform spectrometer to the detection of wavelength-encoded signals from Bragg grating sensors

Abstract: We describe an all-fiber Fourier transform spectrometer for decoding the wavelength shifts from a series of Bragg grating sensors. The system described uses an optical fiber Michelson interferometer with one arm wrapped on a piezoelectric driven stretcher capable of inducing a /spl sim/10 cm fiber length change. Passive polarization compensation is utilized which eliminates the possibility of random polarization fading in the interferometer causing apodization of the interferogram. This permits an all-fiber design without the need for speciality optical fiber or external polarization adjustments. A reference Nd:YAG laser is used to stabilize the interferometric scan in a phase-locked loop configuration by way of slow and fast feedback correction elements. A spectral resolution of >

Dual-analyte fiber-optic sensor for the simultaneous and continuous measurement of glucose and oxygen.

Abstract: A fiber-optic sensor for the continuous and simultaneous determination of glucose and oxygen is described. The sensor is comprised of dual-analyte sensing sites in defined positions on the distal end of an imaging fiber (350 microns o.d.). Each sensing site is an individual polymer cone covalently attached to the activated fiber surface using localized photopolymerization. The oxygen sensor consists of a double-layer polymer cone. The inner polymer cone is a hydrophobic gas-permeable copolymer containing an oxygen-sensitive ruthenium dye, and the outer layer is a poly(hydroxyethyl methacrylate) (HEMA) polymer. The glucose sensor is an oxygen sensor with a poly-HEMA outer layer containing immobilized glucose oxidase. The fluorescence images of both sensing sites are captured with a CCD camera, and the measured fluorescence intensities are related to analyte concentrations. Oxygen quenching data for both sensing sites fit a two-site Stern-Volmer quenching model. The sensor has been used to simultaneously monitor independent changes in glucose and oxygen concentrations. Glucose calibration curves were obtained under varying oxygen tensions, and the detection limit is 0.6 mM glucose. The effect of fluctuations in oxygen partial pressure on the glucose response can be used to calibrate the sensor. The sensor response time varies from 9 to 28 s, depending on the different thicknesses of the enzyme layer. The sensor maintains the same sensitivity for 2 days. Multiple glucose sensing sites with different enzymatic activities can be immobilized on the distal end of the fiber, affording control of the linear range.

Fiber devices and sensors based on multimode fiber Bragg gratings

Abstract: New sensing and diagnostic methods are implemented with multimode fiber Bragg gratings (MMFBG) and methods for sensor addressing and multiplexing are described. Real time structural strain and vibration monitoring using optical time domain reflectometry (OTDR) is discussed. The first all-mode, wave optics calculations of the reflectivity of Bragg gratings in highly multimode optical fibers are presented, including the effects of individual mode power changes due to microbending transducer loss upstream from the grating. Two categories of multimode fiber gratings are possible, independent mode and mode coupling, and the fabrication differences between these types of gratings are described. The reflectivity properties of independent mode multimode fiber gratings are examined and compared to single mode fiber gratings. In particular, multimode fiber gratings offer added flexibility in grating design and performance characteristics compared to single mode fiber gratings, because the reflectivity response may be tuned by the spatial periodicity, length, core size, numerical aperture, and mode coupling characteristics of the grating.

Dispersion compensation for optical fiber systems

Abstract: Much of the currently embedded optical fiber was originally designed for light with a wavelength of 1.3 microns. If this fiber is to be used with tomorrow's optically amplified, high-speed, long span-length lightwave system operating at 1.5 microns, the chromatic dispersion in the fiber must be compensated. Dispersion compensation will be required in long-haul l0 Gb/s systems using conventional fiber. Many compensation techniques have been demonstrated and they exhibit a variety of different and often complimentary properties. Transmitter compensation techniques are the most easily implemented but provide a limited amount of compensation. The most commercially advanced technique is negative dispersion fiber. Chirped Bragg gratings are advancing rapidly, but will always be hampered by their narrow bandwidth. The adoption of any particular technique for use in a high-speed network will depend on the constraints imposed by the, as yet, undefined network architecture. >

Fiber optic bending and positioning sensor with selected curved light emission surfaces

Abstract: A curvature or bending and displacement sensor is composed of a fiber optic or light wave guide, for attachment to a member or members being curved or displaced. Light is injected at one end and detected at the other end. Curvature of the fiber results in light loss through an emission surface or surfaces, sometimes in conjunction with a superimposed curvature in a plane other than that of the curvature to be measured, this loss being detected. The loss of light detection is used to produce indication of curvature or displacement. The light emission surfaces extend in various forms, such as a surface strip or band. Particularly, in an example, the emission surfaces extend in a substantially peripheral direction, or in a substantially curved axial direction when in a curved portion of a curved guide. The placement, shape and configuration of the emission surfaces allows adjustment of the linear range of measurement, the overall throughput of light, and the length over which curvature is measured. Two or more light guides can be oriented to given indication of direction of curvature or displacement.

A low-loss downconverting analog fiber-optic link

Abstract: An analog fiber-optic link for concurrent detection and downconversion of microwave signals is reported. Optical amplification is employed in conjunction with electrical power combining of photodetectors to demonstrate link losses of 19.6 and 22.9 dB at RF carrier frequencies of 9 and 16 GHz, respectively. Analytic expressions validating the experimental observations are also developed. The link may be employed to detect phase sensitive or phase-modulated microwave signals and shows excellent potential for application in sensor systems involving remoting of an antenna element. >

Fiber optic power-generation system

Abstract: A fiber optical solar power generating system provides a tower outside a structure to be supplied with solar energy and on which a multiplicity of collectors is provided. An optical fiber trunk carries the collected optical energy to the structure in which a photovoltaic and/or a light/heat transducing stack can be provided and to which light is distributed from the optical fiber trunk so that the transducers need not occupy large areas of the property.

Fiber optic sensor having a multicore optical fiber and an associated sensing method

Abstract: A fiber optic sensor and associated sensing method includes a multicore optical fiber having first and second optical cores adapted to transmit optical signals having first and second predetermined wavelengths, respectively, in a single spatial mode. The first and second optical cores each include a respective Bragg grating adapted to reflect optical signals having the first and second predetermined wavelengths, respectively. Based upon the differences between the respective wavelengths of the optical signals reflected by the respective Bragg gratings and the first and second predetermined wavelengths, a predetermined physical phenomena to which the workpiece is subjected can be determined. For example, the fiber optic sensor can include strain determining means for determining the strain imparted to the workpiece. Alternatively, the fiber optic sensor can include temperature determining means for determining the temperature to which the workpiece is subjected. Further, the fiber optic sensor can include both strain determining means and temperature determining means for simultaneously determining the strain and temperature to which a workpiece is subjected. The fiber optic sensor also includes calibration means for measuring the predetermined physical phenomena, independent of variations in other physical phenomena.

In-vivo biomedical monitoring by fiber-optic systems

Abstract: An overview of fiber optic sensors for in vivo biomedical monitoring is given, with particular attention to the advantages that these sensors are able to offer compared to the performance of traditional devices. >

Instrumentation for interrogating many-element fiber Bragg grating arrays

Abstract: Fiber Bragg grating (FBG) arrays have provided the Smart Structures community with a powerful means for real-time and absolute point measurements of strain throughout extended structures. The mechanical properties of these optical fiber sensors uniquely suit them for unobtrusive and reliable incorporation into composite materials, while wavelength encoding of the measurand enables independent interrogation of many devices distributed along a single fiber. In view of recent progress towards practical and economical FBG array fabrication via the computer-controlled in-line writing process during fiber draw, there is a complementary need for economical instrumentation which makes efficient use of the reflected signals. This is an important issue, especially since the reflectivity of narrow-line, type I gratings produced during fiber draw is typically less than 5%.

Optical fiber distribution frame with fiber testing

Abstract: Disclosed is an optical fiber distribution frame which provides means for testing the patch connections. Optoelectronic circuits are inserted into each module and are optically connected to the incoming optical fiber cable. The circuit includes a light source for launching a test signal into the patch fiber, a detector for monitoring a test signal from the patch fiber, and a microprocessor for controlling the launching and for monitoring the received signals.

A novel technique for optical fiber pH sensing based on methylene blue adsorption

Abstract: A novel fiber optic pH sensor is reported. The sensor is based upon surfacic adsorption of methylene blue dye, producing absorption in the evanescent field surrounding the sensing fiber. The sensor is based upon a 200 /spl mu/m diameter Plastic Clad Silica (PCS) fiber which has its cladding removed in order to expose its core. The linear range of operation is between pH 3 and pH 9 and its response time is considerably shorter than indicator based probes which measure spectral changes of pH sensitive chemicals. Effects of measurand ionic strength are shown to be negligible which is a further important advantage of this sensor over other optical fiber pH sensors. The sensor design is sufficiently flexible to allow it to be configured in probe (reflective) or in-line (transmissive) form. The latter allows the application of time domain reflectometry technique which may be used to achieve multipoint (distributed) sensing. >

System and method for aligning and attaching optical fibers to optical waveguides, and products obtained thereby

Abstract: An input optical fiber (22), an output optical fiber (24, 26) and a waveguide (14) in an integrated optic chip (IOC) (12) intermediate the fibers are coupled together using service and alignment robots (42; 48, 50, 52). The service robot (42) establishes the three-dimensional position of the waveguide. The alignment robot (48, 50, 52) three-dimensionally and angularly aligns the input and output fibers respectively to the input and output legs (16; 18, 20) of the waveguide. An adhesive applying tool (46) coupled with the service robot adheres the input and output fibers respectively to their waveguide input and output legs. Included are specifics for an optical fiber clamp (110) capable of maintaining the orientation of a fiber while it is moved to another location, a vacuum holder (210) for holding and rotating an optical fiber for polarization purposes, goniometer mapping and positioning of an optical fiber with respect to the pivotal axis of the goniometer (324), initial light launching of an optical fiber to a waveguide using the fiber (422) in its cladding mode, planar and angular position alignment of optical fibers to optical waveguides, an alignment procedure for establishing a gap between waveguides, and attaching optical waveguides together.

Apparatus for and methods of splitting fiber optic signals

Abstract: Apparatus and method of providing a fiber optic signal splitter for receiving an incoming fiber optic signal and for splitting the signal into a first plurality of signals, fiber optic splicing apparatus for splicing the first plurality of fiber optic signals to fiber optic splitter apparatus which splits the first plurality of signals into a second plurality of signals greater in number than the first plurality of signals. Combination fiber optic splice tray for splicing an incoming optical fiber to apparatus for splitting optical fiber signals received over the incoming optical fiber.

Temperature distribution measuring apparatus using an optical fiber

Abstract: A temperature distribution measuring apparatus, which inputs pulsed-light into an optical fiber, measures backward Raman-scattered light generated in the optical fiber, and calculates a temperature distribution along the optical fiber in accordance with the ratio of the intensity of anti-Stokes' light to the intensity of Stokes' light of the backward Raman-scattered light. The apparatus comprises an optical filter for extracting the anti-Stokes' light and Stokes' light from the backward Raman-scattered light, an optical attenuator for attenuating the Stokes' light extracted by the optical filter, an optical switch for receiving the anti-Stokes' light extracted by the optical filter and the Stokes' light output from the optical attenuator, and selectively outputting one of the anti-Stokes' flight and Stokes' light, a light-receiving element for detecting the intensity of the light output from the optical switch, an amplifier for amplifying an analog electrical signal produced by the light-receiving element, an amplified analog-to-digital converter for converting the analog electrical signal into a digital signal, and a signal processor for measuring the temperature distribution along the optical fiber by processing the digital signal.

Birefringence dispersion measurement in optical fibers by wavelength scanning

Abstract: We describe a method for measuring modal birefringence in optical fibers. It combines an interferometric technique with wavelength scanning and permits a precise nondestructive measurement of the birefringence along different sections of a long optical fiber. The experimental results for high-birefringence fibers, 10 and 100 m long, are presented. An accuracy of approximately 0.1% is achieved in the spectral range of 600–850 nm.