Showing papers on "Fiber optic sensor published in 1994"

Characteristics of erbium-doped superfluorescent fiber sources for interferometric sensor applications

Abstract: The characteristics of 1.55 /spl mu/m Er-doped superfluorescent fiber sources (SFS's), intended for fiber-optic gyroscope (FOG) applications, are explored theoretically and experimentally. With proper selection of the source configuration, fiber length, pump wavelength, pump power, and fiber composition, we show that it is possible to meet the stringent requirements of the FOG, including a high output power, broad emission bandwidth, and excellent spectral thermal stability. Variations of the mean wavelength, spectral width, and output power of the SFS with fiber length, pump power, pump wavelength, and temperature are modeled for representative sources pumped near 980 nm or 1.48 /spl mu/m, and are shown to be in good agreement with experimental results. The effects of a multimoded pump, erbium ion pair, and optical feedback are also assessed. This study indicates that the Er-doped SFS is an excellent candidate for the FOG and for other applications requiring spatial coherence and low temporal coherence. >

Fiber optic sensor and methods and apparatus relating thereto

Abstract: There is disclosed fiber optic sensor which detects a sample in contact with the sensor by surface plasmon resonance (SPR) measurements, as well as methods and apparatus relating thereto. The fiber optic SPR sensor of this invention employs a limited range of incident angles and uses incident light having multiple wavelengths. In preferred embodiments, both an in-line transmission-based fiber optic SPR sensor and a terminated reflection-based fiber optic SPR sensor are disclosed. The fiber optic SPR sensor includes a surface plasmon supporting metal layer in contact with an exposed portion of the optical fiber core, and may optionally contain one or more additional layers deposited on the surface plasmon supporting metal layer. In further embodiments, methods are disclosed for detecting a sample by contacting the sample with the fiber optic SPR sensor of this invention, as well as sensing apparatus which contain the fiber optic SPR sensor in combination with a source of electromagnetic radiation of multiple wavelengths and a detection device.

Reciprocal reflection interferometer for a fiber-optic Faraday current sensor

Abstract: A reciprocal fiber-optic reflection interferometer for remote measurement of electrical current through the Faraday effect is described. The effects of polarization cross coupling because of nonideal elements are eliminated with a low-coherence source. Nonreciprocal birefringence phase modulation is employed for detection of the Faraday phase shift. The theoretical predictions are confirmed by measurements with a piece of straight fiber as the sensing element in a 100-turn solenoid. Currents from 0 to 40 A have been measured with a linear response and a noise limit of ∼0.015A/Hz.

Combined distributed temperature and strain sensor based on Brillouin loss in an optical fiber

Abstract: We present a novel distributed sensor that utilizes the temperature and strain dependence of the frequency at which the Brillouin loss is maximized in the interaction between a cw laser and a pulsed laser. With a 22-km sensing length, a strain resolution of 20 µ? and a temperature resolution of 2°C have been achieved with a spatial resolution of 5 m.

Graded-index polymer optical fiber for high-speed data communication

Abstract: We successfully obtained a high-bandwidth (1 GHz km) and low-loss (90 dB/km at 0.572 μm of wavelength) graded-index polymer optical fiber by using the interfacial-gel polymerization technique, in which we used an unreactive component to obtain the quadratic refractive-index distribution. This high-bandwidth graded-index polymer optical fiber makes it possible to transmit a high-speed optical signal in a short-range network, which is not possible when we use the step-index type of polymer optical fiber commercially available.

Diagnostic system for fiber grating sensors

Abstract: An optical sensor diagnostic system includes a tunable narrow wavelength-band source (9) which provides a variable wavelength light (44) into an optical fiber (32, 52) Reflective sensors (54, 58), such as Bragg gratings, are disposed along the fiber (52) in the path of the variable light (44) The sensors (54, 58) transmit light (56, 60) having a minimum transmission wavelength which varies due to a perturbation, such as strain, imposed thereon A tuner control circuit (42) drives the tunable light source (9) to cause the source light (44) to scan across a predetermined wavelength range to illuminate each sensor at its minimum transmission wavelength The power of the transmitted light is converted to an electrical signal by a detector (64) and monitored by a signal processor (68) which detects drops in transmitted power level and provides output signals on lines (71) indicative of the perturbation for each sensor The system may be configured in open loop mode to measure static strains, or closed loop mode to track static strains and measure dynamic strains Also, the system may be used in a Fabry-Perot configuration to provide a very sensitive strain detection system Further, the system may be configured in reflection or transmission mode

Active multipoint fiber laser sensor

Abstract: A remote active multipoint fiber laser sensor includes a plurality of fiber lasers (12, 14, 16), each having a pair of Bragg gratings (18, 20), embedded in a fiber (10) and excited by a common pump light (30). The lasers (12, 14, 16) lase at different longitudinal modes (lasing wavelengths) and emit light (32, 34, 36), at their respective wavelengths (μ1, μ2, μn). The lasing wavelength of each laser shifts due to perturbations, such as strain or temperature, applied thereto. The output light (32, 34, 36) is fed to a spectrum analyzer (50) where the wavelength shift is analyzed. A signal processor (54) reads the wavelength shift and provides a signal on lines (56) indicative of the perturbation at each of the lasers/sensors (12-16). Alternatively, a single laser may be used as a single sensor. Alternatively, birefringent fiber may be used as the fiber cavities (21) and the two polarizations are beat together to form a lower difference or ''beat'' frequency, thereby allowing lower frequency detection devices to be used.

Fiber-optic dual-technique sensor for simultaneous measurement of strain and temperature

Abstract: Polarimetric and two-mode differential interferometric schemes incorporated in an elliptical-core fiber are able to resolve strain and temperature simultaneously with resolutions of 10 /spl mu/m/m and 5/spl deg/C, respectively. A technique, based on the evaluation of the condition number of a matrix, is shown to be useful in evaluating comparative merits of multiparameter sensing schemes. The determinant of the beat length matrix is expressed in terms of mode propagation constants, and a method for designing specialized fibers suitable for simultaneous measurement of strain and temperature is proposed. Experimental results for four fibers are presented and cross-sensitivity issues are discussed. >

Low cost optical fiber RF signal distribution system

Abstract: A fiber optic RF signal distribution system has a plurality of antenna stations, each station including an RF antenna. A central RF signal distribution hub receives and transmits signals external to the system. A pair of optical fibers connects each antenna station directly to the distribution hub with the connections being in a star configuration. A connectorized laser module permits direct coupling of a distributed feedback semiconductor laser to the end of each optical fiber without an intervening connector. The module accepts an APC connector with the angled end of its optical fiber in an aperture plane in a connector housing. A distributed feedback laser is mounted in the connector housing with a lens for illuminating the aperture plane and hence, the end of the fiber at an acute angle to the axis of the fiber. Furthermore, the illumination is defocused at the aperture plane for loose optical coupling to the fiber and minimized reflection to the DFB laser.

Electro-Optics Handbook

Abstract: All-inclusive opto electronics guideA valuable "must-have" tool for electronic and optical engineers, this Handbook is the only single-volume, tell-it-all guide to the use of optical devices and light in electronics systems. Developed by a towering figure in the field, this manual familiarizes you with UV, VUV and X-Ray lasers; visible, solid-state, semiconductor and infrared gas lasers; FEL and ultrashort laser pulses; visible and infrared optical materials; infrared and imaging detectors; optical fibers and fiber optic sensors; holography; laser spectroscopy and photochemistry; high resolution lithography for optoelectronics; and much more. In this up-to-the-minute edition you'll find new chapters on optical communications, electro-optic devices, and high intensity optical fields, in addition to extensively updated material throughout, and abundant charts, diagrams and data tables.

Comblike dispersion-profiled fiber for soliton pulse train generation.

Abstract: A novel optical fiber [comblike dispersion-profiled fiber (CDPF)] that consists of a chain of alternating segments of standard telecommunication fiber and dispersion-shifted fiber is proposed for the generation of a soliton pulse train based on nonlinear transformation of an optical beat signal. A totally integrated all-optical fiber source of a 59.1-GHz train of 2.2-ps solitons is demonstrated with a CDPF. For a beat signal generator we use a dual-frequency erbium fiber laser incorporating fiber grating reflectors that provides 16-kHz linewidths and a low phase noise of optical beating (<5 × 10−5). Significant suppression of stimulated Brillouin scattering, which is essential for this technique, is achieved in the CDPF.

Fiber laser source/analyzer for Bragg grating sensor array interrogation

Abstract: This paper reports on the application of a calibrated, narrow-linewidth, single-frequency, continuously wavelength-tunable erbium fiber laser to the interrogation of a multipoint Bragg grating temperature sensor. The fiber laser was wavelength-tuned, through an array of three fiber Bragg grating sensors, to determine the temperature of each individual grating. The temperatures of the three gratings were measured as a function of grating Bragg wavelength. The minimum wavelength resolution, due to electro-mechanical repeatability, of the fiber laser source/analyzer was determined to be approximately 2.3 picometers. This corresponds to a frequency resolution of approximately 300 MHz. >

Control of the dynamic range and sensitivity of a surface plasmon resonance based fiber optic sensor

Abstract: The refractive index sensitivity and dynamic range of a recently developed fiber optic based surface plasmon resonance sensor have been investigated. The sensor response was studied using a set of aqueous refractive index standards. The sensitivity to refractive index was found to be between 5.0×10−4 and 5.0×10−5 index of refraction units. The dynamic range of an unmodified sensor was between 1.25 and 1.40 index of refraction units. The dynamic range was tuned for ranges between 1.00 and 1.40 index of refraction units with the addition of thin high refractive index overlayer films. This range is significant since it includes both gas and aqueous chemical sample refractive indices. The upper limit of the refractive index dynamic range was extended to 1.70 by use of a sapphire fiber core material.

Laser beam mode selection by computer generated holograms

Abstract: Preface Introduction Modes of Coherent Beams Modal Beams in Scalar Approximation Excitation of Modes in Optical Fibers and Laser Cavities Complex Eikonal Method Amplitude-and-Phase Relations for Free-Space Propagation of Mode Beams Gaussian and Bessel Modes Modans as the Computer Generated Holograms Physical Observability of Modes and the Concept of Modans Multifocal and Multibeam Holographic Optical Elements Modans on Crossed Diffraction Gratings Generating Functions Method for the Synthesis of Modans Digital Holography Methods for Generation of Modans Coherent Modes Selection Optical Stages for Mode Selection with the Aid of Modans Light Beam Formation with Required Composition of Modes The Analysis of Modal Composition in Coherent Light Beams Modal Spectral Transforms Modan's Performances Perturbations of Modal Functions Modal Composition in Presence of Perturbations Perturbation Matrix Evaluation in the Case of Sampling the Modal Functions Energy Efficiency of Modans Accuracy of Analysis or Formation of Coherent Beams Accuracy of Modal Spectral Transforms Modan's Development and Experimental Investigation Computer Generation of Modans by Diffractive Optics Technology Measurements of Efficiency and Operating Performances of Single-Beam Modans Correlative Investigations of Two Beam Modans Measurements of Modal Power Distribution in Graded-Index Fibers Mode-Selective Excitation of the Fiber with the Aid of Modans Tolerances for Launching Conditions in Modan-Aided Excitation of the Fiber Fiberoptic Sensors and Communications with Mode Selection Fiberoptic Communication Line with Modal Multiplexing Mode-Selective Fiber Optic Sensors Modan Perturbation Effects in Sensor Operation Fiber Optic Sensor of Pressure with Reflective Computer Generated Optical Elements Experimental Investigation of Microshift Sensor Employing Modans Appendices: The Proof of the Properties of Orthogonal Modes Sinusoidal Modes in a Real Form Analytical Equations for Gaussian Modes Excitation in Optical Fibers and Cavities Random-Fields Expansions with Decorrelated Coefficients References. Index.

Programmable fiber optic delay line

Abstract: This paper describes a novel programmable delay line that generates up to 50 ns of true time-delay in discrete 10 ns intervals. The delay line consists of an externally modulated wavelength tunable fiber laser and a six-element wavelength multiplexed fiber Bragg grating array, with the grating spacing set to yield the desired delay. >

Experimental demonstration of compression of dispersed optical pulses by reflection from self-chirped optical fiber Bragg gratings.

Abstract: Dispersion compensation is demonstrated experimentally by pulse compression with the use of chirped optical fiber Bragg gratings. The gratings chirp is self-induced by the Gaussian intensity profile of the 240-nm wavelength beam used for holographic sidewriting of the grating. Chirped pulses generated by a 1.55-microm gain-switched distributed-feedback laser with an initial pulse duration of 21 ps and a spectral width of 0.7 nm are compressed to 13 ps, in good agreement with theory.

Integrated optical Young interferometer.

Abstract: The integrated optical configuration of a Young interferometer is proposed for refractometry and chemical sensing. We coupled light into an integrated optical Y branch by fixing a laser diode directly at the input of the optical device. We solved the problem of ambiguity in the interference order by operating the laser diode at currents below threshold, resulting in visibility modulation of the interference fringes caused by the low coherence length of the emitted light. A very compact device results that measures the refractive index of liquids or gases. An electronic scanning technique by means of a CCD array provides a fast readout without the need for moving parts.

Fiber optic evanescent field absorption sensor: effect of launching condition and the geometry of the sensing region

Abstract: Fiber optic evanescent wave absorption sensors based on four different designs are described. The launching condition and the geometry of the sensing region are found to be the main parameters that can influence the sensitivity of the sensor. High sensitivity is achieved by launching the selected rays into the fiber having tapered sensing regions. The experimental results obtained are qualitatively explained using a simple model based on meridional rays.

Fiber optic sensor for in vivo measurement of nitric oxide

Abstract: A fiber optic sensor for measurement of in vivo nitric oxide concentrations in a subject. The sensor contains a nitric oxide-sensing compound in a polymer matrix attached to an optical fiber. The sensor may be placed in any blood vessel, including one within the heart of a subject for continuous measurement of nitric oxide concentrations in blood.

Evanescent-absorption coefficient for diffuse source illumination: uniform- and tapered-fiber sensors

Abstract: A comparative study of evanescent-wave fiber-optic absorption sensors based on uniform and tapered fibers has been carried out. The expressions for an effective evanescent-absorption coefficient have been derived for diffused or Lambertian source illumination. It has been shown that the sensitivity of sensors depends on the numerical aperture of the fiber, the taper ratio, and the refractive index of the absorbing fluid. The higher the sensitivity the smaller the range of functional refractive indices of the fluid. In the case of taper, which fiber (with a low or high numerical aperture) has maximum sensitivity depends on the refractive index of the fluid.

Optical waveguide including singlemode waveguide channels coupled to a multimode fiber

Abstract: An intrusion-alarmed optical fiber communication system, where light from two or more sources are launched with a planar channel waveguide launcher into two or more modes of a multimode graded-index transmission fiber, is disclosed. Input fibers containing the source light waves and the output fibers are in direct contact with the waveguide channels. Waveguide channels redistribute the light from input channels to output channels by rerouting, crossing, merging, or splitting channels. Waveguide launcher precisely launches data light into the fundamental mode and intrusion monitor light into high order modes of a multimode graded-index fiber. Fiber intrusion attenuates light in high order modes, but much less of the data in the fundamental mode, thereby forming a basis for the intrusion-alarmed system. Waveguide launcher permits light from a plurality of sources to be launched selectively into several types of fibers: singlemode fibers, multimode fibers, multicore fibers, multimode fibers having high refractive-index ring profile within the fiber core boundary. Waveguide channel configuration permit other versatile functions to be performed; these include optical time domain reflectometry, channel feedback stabilization of the diodes, launching light from more than one light source into the fundamental mode of a multimode graded-index fiber, and other functions. Means for launching and propagating the fundamental mode in multimode graded-index fiber in order to increase bandwidth capacity of said fiber are disclosed. Optical-electronic intrusion-alarmed systems with synchronous phase sensitive detection of intrusion are described, including reference recovery means. In order to decrease false alarm rates, information on transmitter light source variations are transmitted to the receiver via digital bit stream. A precise fiber optic sensor system based on the waveguide launcher is disclosed.

Raman spectroscopy using a fiber optic probe with subwavelength aperture

Abstract: Raman spectroscopy with subwavelength spatial resolution of a diamond sample was recorded using a tapered fiber optical probe in conjunction with a conventional Raman spectrometer. The experiment demonstrates the potential of suboptical wavelength resolution analytical spectroscopy. The tapered fiber optical probe with an aperture of around 100 nm, served as the means for delivering pump radiation while simultaneously collecting the Stokes radiation from the diamond specimen. Comparing the magnitude of the Raman scattering measured with the submicron single mode fiber probe to similar signals obtained with a nontapered probe made of the same type of fiber, illustrates the potential increase in effective optical aperture resulting from the close approach of the fiber to the surface.

Gain controllable optical amplifier and applications thereof

Abstract: An optical fiber which amplifies an input signal by a pump source, a compensation-signal source which injects a compensation-signal propagating contra-directionally with respect to the input signal Into the optical fiber, a gain detector which measures a spontaneous emission light of the pump source at an input side of the optical fiber and a compensation-signal source controller which controls output of the compensation-signal source based on output of the gain detector are provided. In addition, a coupler and a wavelength selective reflector which reflects a light of the same wavelength as the compensation-signal selectively, are provided at the input side of the optical fiber.

Interferometry with Rayleigh backscattering in a single-mode optical fiber.

Abstract: A novel distributed sensing technique based on reflectometry is proposed. The technique uses the interference of the light waves backscattered within the resolution range of a reflectometer to detect the optical path changes in a single-mode fiber. Experimental results are presented, and practical realization of an intrusion detector that employs this technique is briefly discussed.

A micromachined pressure sensor with fiber-optic interferometric readout

Abstract: A high sensitivity, batch fabricated, fiber-optic pressure sensor has been fabricated using silicon micromachining technology. The transducer consists of a fiber positioning v-groove, a 45° stationary mirror and a silicon membrane, micromachined in silicon by anisotropic etching in KOH solution, and a single mode optical fiber. The membrane and optical fiber end form a Fabry-Perot cavity whose length varies with pressure. The generated optical interference fringes are used to detect and measure the change in membrane deflection. Pressure range of operation is dictated by the thickness, size and material of the membrane. The sensor described here was designed for low pressure range (0–25 mm Hg) applications. Temperature sensitivity and stability problems which are commonly encountered with currently available piezoresistive and capacitive pressure sensors are significantly reduced by the inherent differential nature of interferometric measurement and the use of all silicon construction. The fabrication, packaging and testing of the sensor are described in this paper. The performance of the sensor was evaluated and found to compare favorably with theoretical predictions.

Fiberoptic pressure sensor having drift correcting means for in situ calibration

Abstract: An improved intensity-encoded fiber optic sensor incorporating novel drift correction means is disclosed. Disclosed is an improved technique for long-term temporal drift cancellation in a fiber optic pressure sensor by periodically applying pressure to the sensor tip (160) in order to ascertain the measured voltage at which the sensor diaphragm (176) contacts other elements of the sensor (182). This measured voltage is subtracted from an initial calibration voltage, and the result is applied to the measured signal as a constant correction term.

Optical fiber module with surface emitting laser

Abstract: An optical fiber module comprises a light-emitting chip based on a semiconductor substrate, a vertical cavity surface emitting laser formed on a primary plane of the light-emitting chip, a guiding hole opened on a secondary plane of the light-emitting chip, and an optical fiber stuck to the secondary plane of the light-emitting chip. The position of the guiding hole is aligned to that of the surface emitting laser, and the guiding hole is close to the surface emitting laser. By inserting a tip of the optical fiber into the guiding hole, the optical fiber is coupled optically to the surface emitting laser only with mechanical positioning. The light-emitting chip is mounted on an electronic circuit board with the primary plane attached to it. Since the optical fiber is taken out from a secondary plane of the light-emitting chip, the assembly of the optical fiber module is simply done by coupling an optical fiber to the light-emitting chip after the light-emitting chip is attached to the electronic circuit board bonded on a package.

Fiber optic strain sensor and read-out system

Abstract: An optical sensor is described comprising a first optical fiber and a first reflective member. The first optical fiber has a first predetermined length of graded-index multimode optical fiber attached to its end. The first reflective member and the first optical fiber are arranged such that a gap is formed between the unattached end of the graded-index multimode optical fiber and the first reflective member. Also described is a rugged sensor design incorporating the first optical fiber and first reflective sensor as well as a method for providing the predetermined length of graded-index multimode fiber at the end of the first optical fiber. Finally, a readout system for an optical sensor is also described which comprises two optical paths differing in length by an amount equal to an optical path distance in the optical sensor. The readout system also comprises an optical source having a wavelength differing from that of the optical sensor, a modulator and a controller. The readout system provides a linear output signal that is independent of temperature and vibration effects associated with the read-out system.

Suppression of stimulated scattering in optical time domain reflectometry

Abstract: Optical time domain reflectometry methods and apparatus are proposed in which the back-scattered optical radiation used to produce output signals is restricted to that resulting from Rayleigh scattering of light launched into a fiber 2 at a first wavelength and that in an anti-Stokes spectral band resulting from Raman or Brillouin scattering of optical radiation at the first wavelength. A first set of output signals produced in dependence upon the anti-Stokes back-scatter may be normalized to the geometric mean of a second set of output signals, produced in dependence upon the Rayleigh back-scatter at the first wavelength, and a third set of output signals, produced in dependence upon Rayleigh back-scatter resulting from light launched into the fiber at the anti-Stokes wavelength. Growth in the intensity of optical radiation in a first Stokes (Raman or Brillouin) spectral band may be inhibited, so as to suppress stimulated scattering, for example by making the fiber 2 high loss at the Stokes wavelength and/or launching into the fiber 2, simultaneously with the pulse at the first wavelength, an additional pulse at a third wavelength equal to that of a Stokes spectral line resulting from inelastic scattering of optical radiation at the first Stokes wavelength. The first wavelength may be chosen so as to minimize the transmission loss, in view of the average losses at the first and detected wavelengths or the increase with wavelength of the launch power which can be used before stimulated scattering has a specified effect.

Real-time analog and digital demodulator for interferometric fiber optic sensors

Abstract: A real-time mixed signal (analog and digital) demodulator for interferometric fiber optic sensors operating at high signal levels was designed, implemented, and tested. The inputs to this fiber optic sensor demodulating instrument are a phase modulated optical signal and an electrical carrier reference signal. This high performance demodulator has an intrinsic 84 dB linear dynamic range that is switch selectable over a 193 dB range, a demodulated signal output amplitude-frequency product of 500,000 rad (DOT) Hz (-3 dB), and an absolute accuracy to better than 0.4%. The group delay from the sensor input is 4.8 microsecond(s) to the demodulated analog output, and 2.3 microsecond(s) to the digital demodulated output. One of the unique simplifying design attributes of this arctangent type demodulator eliminates the need for jump detector/counter circuitry. The details of this design, the tests performed, and the test results are examined.