Showing papers on "Fiber optic sensor published in 1991"

Quadrature phase-shifted, extrinsic Fabry-Perot optical fiber sensors

Abstract: We demonstrate the operation of a quadrature phase-shifted extrinsic Fabry–Perot fiber-optic sensor for the detection of the amplitude and the relative polarity of dynamically varying strain. Two laterally displaced single-mode fibers inserted within a hollow silica tube form the 90° phase-shifted sensing system. A multimode fiber, placed in the tube facing the two fibers, acts as a reflector, thereby creating an air gap that acts as a Fabry–Perot cavity. A theoretical description of the sensor is given, and its operation as a dynamically varying strain sensor is described. Strain sensitivities of 5.54° phase shift/microstrain cm−1 are obtained.

Imaging fiber optic array sensors, apparatus, and methods for concurrently detecting multiple analytes of interest in a fluid sample

Abstract: The present invention provides a unique fiber optic sensor which is able to conduct multiple assays and analysis concurrently using a plurality of different dyes immobilized at individual spatial positions on the surface of the sensor. The present invention also provides apparatus for making precise optical determinations and measurements for multiple analytes of interest concurrently and provides methods of detection for multiple analytes of interest which can be correlated with specific parameters or other ligands for specific applications and purposes.

Single-channel operation in very long nonlinear fibers with optical amplifiers at zero dispersion

Abstract: The author investigates, by means of computer simulations, the performance of a very long, single-channel optical fiber system operating very close to the zero-dispersion wavelength of the fiber. Fiber losses are compensated by optical amplifiers. The optical signal is filtered after each amplifier, is passed through a final optical filter prior to square-law detection, and is finally filtered electrically. It is found that such a system does not work well if the fiber dispersion is strictly constant and if the carrier wavelength of the modulated signal coincides with the zero-dispersion wavelength of the fiber. As a result, the optical signal spectrum spreads to many times its initial width so that power is lost in the optical filters and the signal-to-noise ratio is degraded by the need for admitting a wider band of noise to the receiver. >

Miniature Fabry-Perot interferometers micromachined in silicon for use in optical fiber WDM systems

Abstract: The techniques of silicon micromachining have been used to fabricate a second-generation Fabry-Perot interferometer for use in the near-infrared spectral region. These devices provide a sharp optical transmission peak which can be used as wavelength division demultiplexers in optical fiber communications systems. The wavelength tuning and parallelism control of the mirror elements are achieved electrostatically, by varying the voltage between control electrodes. This second-generation device includes a thin, etch-stopped corrugated diaphragm as the suspension for the movable element. >

Polarization-independent interferometric optical-time-domain reflectometer

Abstract: A polarization-independent interferometric optical-time-domain reflectometer is proposed and demonstrated. The experimental setup is composed of a superluminescent diode and a fiber coupler which combines a polarization-maintaining fiber and a conventional single-mode fiber to achieve a polarization-independent spatial profile of the reflectance in a single-mode fiber or waveguide sample. The polarization independence and the reliability of the measured reflectivity, were confirmed, and the measurement of loss and birefringence in an optical waveguide from its reflectance profile is demonstrated. The influence of dispersion in the fibers on the spatial resolution of the reflectometer is also discussed. >

Method of creating an index grating in an optical fiber and a mode converter using the index grating

Abstract: This invention relates to a method of creating an index grating in an optical fiber comprising disposing a slit mask containing one or more slits over a side of an optical fiber, illuminating the fiber through the slit mask by substantially monochromatic ultraviolet light for a short interval, whereby an index grating line is created and stored in the core of the fiber.

Development of medical pressure and temperature sensors employing optical spectrum modulation

Abstract: Fiber optic Fabry-Perot sensors have been developed whose optical reflectance varies with optical cavity depth (pressure) or with change in a material's refractive index (temperature). These sensors employ a unique combination of features: they are interrogated by an LED; they are designed to operate within a single reflectance cycle; and their returned light is analyzed by a dichroic ratio technique. The sensors use a step index glass fiber and are relatively insensitive to absolute light levels and fiber bending. They have an expanded linear operating range and can be built for low cost disposable applications. Sensor performance meets or exceeds established medical requirements. >

Fiber Optic Sensor for Hydrogen Concentrations near the Explosive Limit

Abstract: This paper discusses a new hydrogen sensor which has been demonstrated that provides a large signal for concentrations of hydrogen in air near the explosive limit This optical fiber sensor is intrinsically safe for use in potentially explosive environments The sensor responds to changes in the optical properties of a thin palladium film exposed to hydrogen

Method and apparatus for the non-invasive measurement of pressure inside pipes using a fiber optic interferometer sensor

Abstract: A method and apparatus for the non-invasive sensing of the pressure within a pipe (or other vessel) is disclosed. An optical source produces a first light beam. This first light beam is split between a first (reference) and a second (measurement) optical fiber. The second optical fiber is associated with the pipe such that circumferential displacements in the pipe, due to changes in internal pressure, result in corresponding displacements in the length of the second optical fiber. Length changes in the optical fibers result in variations in the phase of the light emerging therefrom. The phase difference between the light beams emitted from the first and second optical fibers is then determined and related to changes in the internal pressure of the pipe.

Complete phase-strain model for structurally embedded interferometric optical fiber sensors

Abstract: The relation between a three-dimensional state of strain and the optical phase retardation in a single mode optical fiber is formalized by drawing together classical three-dimensional crystal optics and classical waveguide theory. Neumann's strain optic relations are described in a form usable in optical fiber sensor design. These relations are then combined with weakly guiding fiber theory to develop an integral which relates the optical phase shift in a structurally embedded interferometric optical fiber strain sensor to the induced three-dimensional strain field. This process leads to a previously undisclosed, additional waveguide dispersion term which contributes on the same order to the total strain induced phase retardation as does the term derived by Butter and Hocker (1978). Still, however, waveguide dispersion effects are found to be negligibly small, even in three- dimensional loading. Butter and Hocker's equation and the complete phase-strain model developed herein can give very similar results...

Fiber-Optic Sensors for Concrete Strain-Stress Measurement

Abstract: The paper presents a nondestructive testing method based on fiber optics for measuring strain and stress in concrete structures in situ. When an optical fiber is embedded in concrete, and its refractive index, dimension, and/or shape are changed as a result of the surrounding matrix deformation, the characteristics of the light signal transmitted along the fiber are modified. Optical techniques capable of determining the changes in light characteristics are described. The two most promising techniques for concrete-structures diagnostics are demonstrated in the laboratory using concrete cylinders subjected to unconfined uniaxial compression with optical fibers embedded in a direction parallel and perpendicular to the aplied strain (load). Encouraging experimental results indicate that fiber-optic sensors can be a feasible tool to determine strain/stress distribution in buildings and infrastructures.

High gain multi-mode optical amplifier

Abstract: Indiscriminately exciting the modes of a multi-mode optical fiber amplifier is avoided by an optical fiber amplifier design in which the excitation of pump modes in the core of a multi-mode fiber is controlled by controlling the pump light launching. The pump light is directed substantially along the center axis of a multi-mode fiber within a predetermined launch angle. Rather than exciting all modes, only lower order modes are affected.

Optical detection of partial discharges using fluorescent fiber

Abstract: An optoelectronic system for detecting partial discharges in a sheltered HV device is described. The system consists of a fluorescent plastic optical fiber, with photomultipliers or photodiodes measuring the light intensity at the end of the fiber. The result depends on various factors that might influence the optical detection sensitivity. Information is given on commercially available fluorescent fibers to facilitate the best choice of fiber/photodetector pair for a given application. >

Optical fiber acoustic sensor

Abstract: A new class of hydrophones is currently being developed, which utilize optical beams and interface directly to fiber optic transmission and processing links. Some of these sensors employ an optical fiber itself as the transduction element and possess significant advantages and versatility. The details of these fiber sensors and their advantages are discussed.

Sensing with fiber specklegrams.

Abstract: The fiber specklegram is highly sensitive to the relative modal phases and is of multiplexing capability. Its properties are analyzed and experimentally demonstrated.

Fiber optic pressure and temperature sensors for oil down hole application

Abstract: Detailed studies on fiber optic pressure and temperature sensors for oil down-hole applications are described in this paper. The sensor head is an interferometric based fiber optic senor in which the air-gap will change with the pressure or temperature. For high-speed applications, a novel self-calibrating interferometric/intensity-based (SCIIB) scheme, which realizes compensations for both the light source drift and the fiber loss variation, was used to demodulate the pressure (or temperature) signals. An improved white light system was developed for sensor fabrication. This system is also used as the signal demodulation system providing very high resolution. Experiment results show that the SCIIB system achieves 0.1% accuracy with a 0-8000psi working range for the pressure sensor and a 0-600 degree(s)C working range for the temperature sensor. The resolution of the white light system is about +/- 0.5 nm with a dynamic range up to 10 micrometers. The long -term testing results in the oil site are also presented in this paper.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Miniature Faraday current sensor based on multiple critical angle reflections in a bulk-optic ring.

Abstract: A novel optical element for a bulk glass current sensor that utilizes the Faraday effect is presented with a demonstrated sensitivity of 1 amp-turn and a flat frequency response in the range of 10–104 Hz. The new sensing element overcomes the problems associated with birefringence in optical fiber current sensors and the requirement to make bulk-optic current sensors in complex three-dimensional topologies.

Polarization noise suppression in retracing optical fiber circuits.

Abstract: A novel scheme is proposed in which the presence of a Faraday rotator in a retracing fiber-optic circuit allows the complete, passive, and universal compensation of any birefringence (reciprocal) change occurring in the fiber. The action of the scheme was experimentally tested by inducing thermal and mechanical perturbations on a fiber coil. The results confirm the theoretical expectations and compensating for unwanted birefringence effects.

Optical time domain reflectometer using a tunable optical source

Abstract: In an optical time domain reflectometer (OTDR), an optical signal is coupled to an optical fiber path to be tested, and back-scattered and reflected light from the optical fiber path is detected and used to determine loss-distance characteristics of the path. A control unit varies the wavelength of the optical signal by varying the temperature of a semiconductor laser constituting the optical source, so that the loss-distance characteristics for a plurality of different wavelengths are determined and can be displayed to show any wavelength dependent loss of the optical fiber path, which loss may be due to modal interference associated with closely spaced discontinuities in the path.

Strain sensing using a fiber-optic Bragg grating

Abstract: This paper reports on the development of a passive, fast demodulation system for use with in-fiber Bragg gratings used for strain or temperature measurements. This compact, potentially inexpensive self-referencing system permits absolute strain/temperature measurements over a wide dynamic range with fast temporal response by tracking the wavelength shifts of the narrow-band back-reflected Bragg spectrum. The wavelength, bandwidth and strain sensitivity of a Bragg sensor are discussed, and examples of both static and dynamic strain measurements are shown.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Modified J(1) ... J(4) method for linear readout of dynamic phase changes in a fiber-optic homodyne interferometer.

Abstract: Fiber-optic interferometers have been studied extensively for sensing applications. Recently a technique described as the J1 … J4 technique was reported for the linear measurement of dynamic phase changes in a fiber interferometer that requires no phase bias and for which the measurement is independent of random phase fluctuations. However, the implementation of the J1 … J4 technique is limited because only the magnitude of the J1 … J4 Bessel components can be measured on a spectrum analyzer without information available on the sign of the Bessel function. Here a modified signal-processing technique that overcomes the limitations mentioned above is described.

Fiber-optic apparatus and method for measurement of luminescence and raman scattering

Abstract: A dual fiber forward scattering optrode for Raman spectroscopy with the remote ends of the fibers in opposed, spaced relationship to each other to form a analyte sampling space therebetween and the method of measuring Raman spectra utilizing same. One optical fiber is for sending an exciting signal to the remote sampling space and, at its remote end, has a collimating microlens and an optical filter for filtering out background emissions generated in the fiber. The other optical fiber is for collecting the Raman scattering signal at the remote sampling space and, at its remote end, has a collimating microlens and an optical filter to prevent the exciting signal from the exciting fiber from entering the collection fiber and to thereby prevent the generation of background emissions in the collecting fiber.

Development of a fiber optic current sensor for power systems

Abstract: Numerous techniques have been developed to sense electric current optically, including several using optical fibers as the sensing element. To date, these methods have failed to overcome the fundamental performance problems of the incorporated bulk optic components as well as the instability of the sensing fiber. A description of an all-fiber optic current sensor is presented. A method for producing highly stable sensing coils and techniques for interfacing these coils with novel polarizing fiber are described. The performance of a fully functional prototype device is discussed. Theoretical limits and preliminary characterization data are presented. >

Multi-channel fiber optic rotary joint for single-mode fiber

Abstract: A fiber optic rotary joint device is described which includes a rotor conted to either an input or output fiber optic array and a stator connected to the other of the input or output fiber optic array. A prism is mounted within the rotor for derotating an image of the input array to allow coupling to the output array. A prism rotor and a gear system are provided for rotating the prism at half the speed of the rotor. Optical means are provided for two adjustment tiers (small angular adjustment and fine adjustment) of alignment of the light propagation path for each channel of the array. Mechanical structural features provide maintanance of accurate alignment of optical elements under rotation of the joint. Further mechanical structural features provide resiliency of the gear system to isolate its operation from maintenance of mechanical alignment of optical elements.

Optical fiber electro-optical module

Abstract: OF THE DISCLOSURE An electro-optical module has at least three ports, with at least one port having an active electro-optical device for connection to electrical circuitry and at least one port having an optical port for connection to an optical fiber link. A passive coupler interfaces the various ports. The passive coupler is formed of optical fibers mounted on glass using an adhesive that exhibits refractive index-matching characteristics. The fiber-on-glass coupler is extremely small, allowing for the module to have minimal size. The coupler may be wavelength selective or wavelength insensitive. If a wavelength-selective coupler is utilized, the end surface of an optical fiber is coated with a dichroic filter to provide wavelength sensitivity. An optical fiber adapted to receive light for connection to a detector is selected to be a multi-mode fiber to increase reception of light. The remaining fibers may be single-mode fibers.

Performance of a distributed erbium-doped dispersion-shifted fiber amplifier

Abstract: Continued improvements in the performance of a multi-kilometer, low-erbium-concentration fiber amplifier made by the modified chemical vapor deposition (MCVD) seed fiber method are discussed. Transparency for a -20-dBm signal at 1.55 mu m and fiber length of 11.8 km was achieved with 20 mW of 1.48- mu m copropagating pump power. The wavelength dependence of the gain for this signal level is reported, and the distribution in small signal gain along a 22-km length using an optical domain reflectometer (OTDR) was measured. Simulations are presented to show the effect of erbium concentration, input signal level, and pump power required for transparency of a 50-km span. >

Fiber optic sensors for Navy applications

Abstract: The development of a family of sensors to detect a wide range of energy fields by exploiting their effects on the optical signal in a fiber-optic waveguide is examined. Interferometric, acoustic, magnetic, fiber gyro, and intensity-based sensors are considered, and multiplexing arrays of sensors is discussed. The main thrust of the program has been sensors for antisubmarine warfare. Applications include the all-optical towed array (AOTA), fiber-optic planar arrays, bottom-mounted acoustic sensors, magnetometers for fiber-optic magnetic arrays, and rate-of-rotation sensors for attitude, heading, and navigation systems. These require the high sensitivity and performance of phase-based or interferometric sensors. Other applications that can be satisfied with the simpler, less sensitive intensity-based sensors include damage control sensors being developed for shipboard applications and fly-by-light systems for military aircraft. >

Reflected optical power fiber test system

Abstract: A system for testing an optical fiber used in communications including a light source connectable to provide light to one end of a fiber under test, an optical power detector connectable to receive light from the same end of the fiber under test, a reflector at the other end of the fiber under test, and monitoring means operatively connected to the light source and detector to make a baseline measurement of optical power reflected by the reflector and subsequent measurements of optical power reflected by the reflector and compare the subsequent measurements with the baseline measurement to determine if there has been a change in optical power reflected by the reflector, the baseline measurement and the subsequent measurements involving no more than a single pulse of light from the light source.

System for cancelling phase noise in an interferometric fiber optic sensor arrangement

Abstract: A fiber optic sensor system for cancelling phase noise while it senses a measurand field is disclosed The system includes circuitry for developing a light beam that is frequency modulated at a fundamental frequency First and second unbalanced interferometers, having equal path imbalances, are responsive to the frequency modulated light beam for producing respective first and second optical outputs Detection circuitry is responsive to the fundamental frequency, the second harmonic of the fundamental frequency, and the first and second optical outputs for producing a plurality of sine and cosine signals containing phase difference and phase noise components A signal processor is responsive to the plurality of sine and cosine signals for cancelling the phase noise components and only producing output sine and cosine components of the difference in the phase shift between the first and second unbalanced interferometers The system can further include a demodulator for demodulating the sine and cosine components to produce an output signal proportional to the phase shift difference between the first and second unbalanced interferometers

Evaluation of the output error in an optical passive ring-resonator gyro with a 90 degrees polarization-axis rotation in the polarization-maintaining fiber resonator

Abstract: A previously developed method is used to evaluate the error in the optical passive ring-resonator gyro. This scheme is shown to have sufficient accuracy to meet high-grade aircraft-navigation requirements with enough tolerance for such resonator parameters as splice misalignment, finesse, and temperature. >