Showing papers on "Fiber optic sensor published in 1987"

Use of highly elliptical core fibers for two-mode fiber devices.

Abstract: The four complex, almost degenerate, second-order eigenmodes of a two-mode fiber having a circular core are reduced to two nondegenerate, linearly polarized second-order eigenmodes with stable-intensity lobe positions in a highly elliptical core fiber. Existing two-mode-fiber devices can be improved by this stabilization of the second-order modes. Practical sensors employing the two spatial modes as the two arms of an interferometer are described. The two arms of an interferometer of this type can have the same group delays, while the difference in phase delays is large.

Structural monitoring system using fiber optics

Abstract: An optical fiber is securely and continuously engaged by a structure such as a pipeline, offshore platform, bridge, building, dam or even a natural object or fluid medium. A light signal is passed into one end of the optical fiber. Any physical movement of the structure, or sectional movements along the optical fiber path, such as deflection, bending, displacement (changes in linear uniformity) or fracture of the structure caused, for example, by strees, strain, pressure, temperature, etc., will necessarily affect the optical fiber. As a consequence, detectable changes will occur in the "electro-optic signature" (for measurements made at the input end of the optical fiber) or in the light signal transmission (for measurements made at the opposite end of the optical fiber). A preferred optic fiber construction comprises a core and cladding contained within a protective buffer coating, wherein the buffer coating includes a particulate material for engaging the cladding in a manner altering reflective or transmission or other "electro-optic signature" characteristics of the fiber when the buffer coating is subjected to radial forces. The particulate material can be uniformly or nonuniformly distributed about the cladding to provide variable sensitivity.

Fiber-optic evanescent-wave methane-gas sensor using optical absorption for the 3.392-μm line of a He–Ne laser

Abstract: Gas sensing using evanescent waves of optical fibers is investigated. Methane gas is detected by means of its strong optical absorption of the 3.392-μm line of a He–Ne laser. A single fiber is used as both a sensor and an optical transmission line. The sensor has a small diameter, ranging from 1.8 to 7 μm, made by heating and expanding a part of a step-index silica fiber. An evanescent wave of 5 to 40% of the total propagating power is generated outside the fiber. When a sensor fiber of 1.8-μm diameter and 10-mm length is used, the minimum detectable concentration of methane is less than the lowest explosive limit of 5%.

Microbend fiber-optic sensor.

Abstract: Intensity modulation induced by microbending in multimode fibers is considered as a transduction mechanism for detecting environmental changes such as pressure, temperature, acceleration, and magnetic and electric fields. A generic microbend sensor has been defined and studied, and its components, such as sensing fiber, light source, optical fiber leads, and detector, have been examined and optimized. Finally, the generic microbend sensor has been tested demonstrating good performance.

Time-division multiplexing of interferometric fiber sensors using passive phase-generated carrier interrogation.

Abstract: The multiplexing and demodulation of interferometric sensors using a time-division/phase-generated carrier approach is described. The scheme uses slightly unbalanced interferometers and laser frequency modulation to effect the interrogation of the sensors, while the time-division multiplexing allows individual sensors to be addressed. Phase sensitivities of ∼20μrad/Hz and cross-talk levels of <−47 dB are demonstrated for a three-sensor network.

Fiber Optic Temperature And Strain Sensors

Abstract: Twin-core, polarimetric and in-fiber Bragg grating temperature and strain sensors for point and distributed measurements are reviewed. New applications to the in-situ monitoring of polymer composites are reported including techniques for measuring stress waves, resolving principal stresses and determining flexural strain variations.

Method for optical pulse transmission through optical fibers which increases the pulse power handling capacity of the fibers

Abstract: A method for transmitting high energy subpicosecond pulses through single-mode optical fiber without stimulating nonlinear effects as are caused by self-phase modulation or Raman generation, which method increases the average power handling capacity of the fiber. The optical pulses, which may be modulated to carry data, as by pulse code modulation, are increased in temporal width before launching into the fiber. The output pulses from the fiber are compressed. Since the nonlinear effects are related to the peak power of the pulses, these effects are avoided while increasing the average power and allowing the use of the available bandwidth of the fiber thereby enabling greater data transmission rates.

Single-mode fiber optics in a long-baseline interferometer

Abstract: We have investigated the potential for using single-mode fiber optics to link two or more telescopes in a large optical to near-IR astronomical interferometer. On an optical bench, we observed the effects of dispersion, temperature, and birefringence on wide-bandwidth interference fringes using up to 30 m of single-mode fiber in each arm of a Twyman-Green interferometer.

High power optical fiber failure detection system

Abstract: A break or leak in an optical fiber transmitting high power laser energy, at average power levels sufficient for material processing, is detected promptly and the laser beam delivery system shut down when the optical fiber begins to fail. Photo detectors monitor the laser power out of the fiber and injection power into the fiber, in particular the light intensities in the fiber input and output couplers. A difference in detector outputs, larger than a set threshold to account for inherent fiber losses, is an indication that a break or leak has occurred.

Laser materials processing with a lensless fiber optic output coupler

Abstract: Laser materials processing apparatus for performing processing with a fiber optic transmitted beam is disclosed. A fiber injecting lens is selected to have a focal length of sufficient length to enable the use, for materials processing, of the diverging beam emitted at an output end of the fiber without the need for a beam focusing lens type output coupler.

The Passive Resonator Fiber Optic Gyro and Comparison to the Interferometer Fiber Gyro

Abstract: Theoretical and experimental work at The Charles Stark Draper Laboratory, Inc. (CSDL) indicates that it is feasible and very attractive to operate the fiber optic gyro in the passive resonant cavity mode. Recent advances in the coherent communications field indicate promise for the ready availability in the near future of narrow line diode lasers, single polarization fibers, and electro-optical waveguide signal processing circuits at the 1.3 and 1.55 micron wavelengths. The purpose of this paper is to describe the operation, characteristics, and performance of a closed loop fiber optic resonator gyro and compare it to the interferometric fiber optic gyro. The limitations and potential of the passive resonator gyro will be discussed.

Fiber optic gradient hydrophone and method of using same

Abstract: The apparatus according to the invention is all-fiber optic device that produces an output indicative of the gradient of an acoustic wave in water The device comprises a neutrally buoyant body having a relatively rigid outer case When it is submerged in water, the motion of the case is the same as that of a water particle in the same vicinity A fiber optic linear accelerometer produces a signal proportional to the component of acceleration

A Nd(3+)-doped cw fiber laser using all-fiber reflectors.

Abstract: We demonstrate a novel all-fiber resonant optical cavity which uses two-fiber reflectors, each formed by a single loop of fiber between the output ports of a fiber directional coupler. The reflectivities of the fiber mirrors are each determined by the coupling ratio and the insertion loss of the fused couplers. When the cavity is formed in this way using a continuous length of Nd(3+)-doped fiber and pumped using a GaAs laser diode, lasing occurs at a wavelength of 1064 nm. Both theoretical and practical descriptions of the device are given.

High-gain rare-earth-doped fibre amplifier at 1.54µm

Abstract: Optical amplifiers are of interest as wideband in-line repeaters for telecommunications and as signal regenerators for a variety of sensor applications. Much current research has concentrated on semiconductor laser amplifiers which are difficult to splice to fiber systems. It is clear that an amplifier consisting of a special optical fiber which is compatible with telecommunication fiber would overcome this problem. This paper describes the first results of an optical fiber amplifier based on an Er3+-doped fiber which has a maximum gain at a wavelength of 1.536 µm. A single-pass gain of 26 dB and a maximum output of 13 dBm at 140 MHz has been demonstrated

Data rate limiter for optical transmission system

Abstract: A single-mode transmission optical fiber may have a bandwidth much greater than that necessary to transmit a desired predetermined maximum data rate The single-mode fiber (11a, 11b) is connected to a band­width limiting device (14,14ʹ) comprising the serial combination of an input optical fiber (16,16ʹ), a mode converter (18,18ʹ,22) and multimode fiber (20,20ʹ) If the multimode fiber is capable of propagating two modes, for example, the mode converter converts the LP01 signal from the transmission fiber to the LP01 and LP11 modes These two modes propagate at different speeds through the multimode fiber Means (12,12ʹ,36) connected to the output end of the multimode fiber (20,20ʹ) detects the two modes If the time delay difference Δτ between the two modes is small with respect to the width of a pulse in the transmitted pulse train, the pulse train will be detected without distortion Transmission at a higher data rate causes Δτ to be large with respect to pulse width Thus, a transmitted pulse causes the generation of a false pulse, whereby operation at the higher data rate is impossible Also disclosed are methods of making suitable mode converters

Water penetration-detecting apparatus and optical fiber cable using same

Abstract: A water penetration-detecting apparatus includes an optical fiber sensor having an optical fiber, and a water-absorbent material disposed along the optical fiber in contiguous relation to the fiber. The water-absorbent material, when absorbing water, expands volumetrically so as to apply a pressure to an outer peripheral surface of the optical fiber to bend the fiber. A detecting means is connected to the optical fiber sensor for detecting a bending of the optical fiber.

Optical fiber sensors using the method of polarization-rotated reflection

Abstract: The method of polarization-rotated reflection is applied to sensors with polarization-maintaining optical fibers. By the use of this scheme, stable measurement can be realized because the fluctuation of the light propagation characteristics in the fiber is canceled automatically without any additional active phase compensation techniques. Several kinds of sensors were designed for the measurement of different physical values. A magnetic field sensor and a temperature sensor were built and the performances were tested to confirm their basic features experimentally.

Acoustooptic modulators for single-mode fibers

Abstract: The design of in-line acoustooptic modulators for single-mode fibers is discussed. The basic configuration consists of a cylindrically symmetric piezoelectric transducer fabricated on the fiber surface, so that the fiber itself acts as a cylindrical acoustic resonator. Depending on the fiber design, the acoustic wave can induce phase, birefringence, or polarization modulation of the light in the fiber. Pairs of the polarization modulators in series can be used to shift the optical frequency. Factors affecting the performance of all of these devices are discussed.

Single-mode fiber components

Abstract: A variety of optical components have been made directly from single-mode fibers. These components can be combined to construct in-line all-fiber systems such as fiber gyroscopes, fiber Raman lasers, local area networks, or coherent lightwave systems. This paper reviews the present state of passive and active single-mode fiber components. Passive components are polarizers, directional couplers, filters, and Faraday rotators. Active components require external control or optical power and include modulators, polarization controllers, frequency shifters, and amplifiers. Polarization is usually important in fiber components and discussions of polarization-maintaining and ordinary single-mode fibers are included.

Fiber optic gyroscope with improved bias stability and repeatability and method

Abstract: A fiber gyroscope provides improved bias stability and repeatability and has a greatly reduced polarizer extinction ratio requirement compared previous devices. This fabrication technique is compatible with both an all fiber gyroscope configuration and an integrated optics or bulk optics device. The present invention comprises a substrate that is preferably formed of fused silica. A pair of optical fibers, or two lengths of one fiber, are mounted to the substrate, and first planar surfaces are formed in the cladding. Optical couplers, polarizers and other components used in forming the rotation sensor are formed on the planar cladding surfaces of the fibers. Polarizers formed on the fibers have a transmission axis aligned with an axis of birefringence of the fiber to remove undesired linear polarization components from optical signals guided by the fibers while permitting a desired linear polarization component to propagate unattenuated. A sensing coil of optical fiber is arranged to guide light between the first and second optical fibers, the first and second optical fibers and the optical coupler cooperating to introduce counterpropagating light waves in the sensing coil.

Method and apparatus of measuring outer diameter and structure of optical fiber

Abstract: A method of and apparatus for examining the structure of an optical fiber such as eccentricity, clad and core diameters, and nonroundness of clad. The structure of the optical fiber can be accurately determined by irradiating the side wall of the optical fiber with a light such as a white light, a monochromatic light or the like in the direction perpendicular to the axis of the optical fiber, detecting an image of the light transmitted through the optical fiber and/or diffraction fringes formed by lights diffracted by the outer edge of the optical fiber to obtain a luminance distribution of the light traversing the optical fiber, and calculating the luminance distribution to thereby obtain accurate structural parameters of the optical fiber.

Intensity Modulated Fiber Optic Sensors Overview

Abstract: Intensity modulated fiber optic sensors have the many distinct advantages associated with fiber optics that makes them suitable for several industrial and military applications. Although, the accuracy of the sensor is far less that for interferometric sensors, the accuracy is more than sufficient for most process control situations. The concepts for intensity modulated fiber optic sensors include: transmissive, reflective, microbending and intrinsic mechanisms. The paper describes the various concepts and applications.

Optical fiber fault locator by the step frequency method.

Abstract: A step frequency method (SFM) is proposed as a new scheme for an optical fiber fault locator. The principle of operation and significant features of the method are described. The feasibility was demonstrated by detecting the discontinuities in a 10-km long multimode fiber using the 830-nm wavelength. The results demonstrate the feasibility of using the SFM in practical fiber optical networks.

Self-referencing multiplexing technique for fiber-optic intensity sensors

Abstract: An all-optical technique for multiplexing and self-referencing a number of intensity modulating fiber-optic sensors is described. The optical transducers are fabricated as integral parts of recirculating optical fiber loops connected in parallel between transmit and receive optical fibers. A portion of an input pulse is tapped off by each sensor loop module. Successive fractions are tapped off on each circulation around the loop and transmitted to a detector. These form pulse trains that characterize each sensor's output. The relative magnitudes of the components of the pulse train are insensitive to lead and connector losses between the sensor modules and the source and detector. Time division multiplexing of the sensor return signals is created by the delays introduced by the fiber leads between the sensor modules. This paper details the response of a single sensor loop versus theoretical performance, and a three sensor system is demonstrated.

Synchronously pumped ring fiber Raman laser

Abstract: An all-fiber ring laser has a single, uninterrupted length of single-mode optical fiber that is formed into a loop by using an optical coupler. Pump signal pulses at a first optical wavelength are introduced into one end of the optical fiber. Each pump pulse propagates through the loop formed in the fiber and then exits the fiber. The pump signal pulses excite the molecules of the optical fiber to cause them to go to a higher, unstable energy level. When the molecules return to a lower energy level, photons are emitted at a second optical frequency that has a wavelength that is shifted from the wavelength of the pump signal to form laser signal pulses. The coupler is a multiplexing coupler that has a first coupling ratio at the wavelength of the pump signal and has a second coupling ratio at the wavelength of the emitted optical signal. The first coupling ratio is preferably close to zero and the second coupling coefficient is greater than 0.5. The laser optical signal recirculates in the loop to stimulate further emission of optical energy at the shifted wavelength in phase with the recirculating optical signal. Each pump pulse is timed to enter the optical fiber loop in synchronism with the recirculating laser signal pulse so as to excite the fiber molecules so as to amplify the recirculating signal pulses. The laser signal has a loop transit time related to the time interval between pump pulses such that a multiple of the loop transit time is substantially equal to a multiple of the time interval.

Distributed fiber-optic temperature sensor using single photon counting detection.

Abstract: Capteur de temperature reparti utilisant la reflectometrie optique dans le domaine temporel. Raman anti Stokes et le comptage de photons

Optical fiber dispersion compensator

Abstract: The various wavelength components propagating in a transmission optical fiber (11) are delayed different times. A conventional dispersion compensation system connects the transmitted signal to wavelength disperser means (15) which spatially separates the different wavelength components of the transmitted signal. Each wavelength component is then propagated by an optical fiber delay line (16-21) the length of which is such that the delay line fibers substantially fully compensate for the differences in propagation times of the wavelength components in the transmission fiber (11). The amount of light collected by the delay line array can be increased and the "dead spots" due to cladding layers between adjacent fiber cores in the array can be eliminated by optimizing fiber shape and by employing staggered, multiple arrays (48-50; 52-­ 55). Also, lens arrays (104) and/or masks can be employed at the input end of the delay line array to reduce or eliminate the cladding effects.

Fiber optic displacement transducer with dichroic target

Abstract: A fiber optic displacement transducer is provided by a fiber optic reflective probe (2), and an axially moveable target (4) having a dichroic optical filter (16) with first and second reflection surfaces (20 and 24) axially spaced by a fixed distance t and reflecting different wavelength light.