Showing papers on "Fiber optic sensor published in 1989"

Electric current sensors employing spun highly birefringent optical fibers

Abstract: The theory of highly elliptically birefringent fibers fabricated by spinning a linearly birefringent fiber during the draw is described. These fibers are particularly interesting for application as Faraday-effect fiber current monitors, since, in contrast to conventional fibers, they can be wound in small multiturn coils while retaining their sensitivity. The fiber and its application in three optical schemes are modeled using Jone calculus and are also experimentally investigated. A simple optical configuration is proposed, combining the elliptically birefringent fiber and a broad-spectrum light source. An accurate, compact, and robust current monitor is obtained. The sensor is characterized by a measurement repeatability of +or-0.5%, a temperature drift of 0.05%/ degrees C and a sensitivity of 1 mA RMS/Hz/sup 1/2/. Further, the performance of this sensor with optimized fiber length for a given measurement bandwidth is predicted. >

Apparatus and method for reducing phase errors in an interferometer

Abstract: A fiber optic rotation sensor comprises a fiber optic interferometer loop formed from a highly birefringent optical fiber, and a short coherence length source for introducing light into the inteferometer loop to provide a pair of waves which counter-propogate therethrough. A detector is included to detect the phase difference between the waves after they have traversed the loop to provide an indication of the loop rotation rate, in accordance with the Sagnac effect. Phase errors are reduced by selecting the coherence length of the source and the birefringence of the fiber, so that the loop is comprised of plural fiber coherence lengths. The term "fiber coherence length" should be distinguished from source coherence length. Fiber coherence length is the length of fiber required for the optical path length difference between the two polarization modes of a single mode fiber to equal one coherence length of the light source. In addition, phase errors are reduced by providing a birefringent waveguide between the source and the loop such that light propagates from the source to the loop in an optical path having a path length difference which is at least equal to a coherence length of the source. Phase errors are further reduced by positioning the detector to intercept the optical output signal from the loop such that light wave components in orthogonal modes are spatially averaged.

Statistical-mode sensor for fiber optic vibration sensing uses.

Abstract: A method of sensing vibration using the detection of changes in the spatial distribution of energy in the output of a multimode optical fiber has been demonstrated. Two implementations of the sensor have been built and tested. The first implementation involved simple optical processing of the output fiber speckle pattern using spatial filtering. The second implementation involved projecting the pattern on a CCD array and digitally processing observed changes in the intensity distribution. A mathematical model has been developed which has shown good agreement with observed sensor behavior. The sensor technique has been used to detect induced structural vibration in laboratory test specimens. Simple field testing has also demonstrated the ability of the technique to detect personnel and vehicles passing over a buried and electrically undetectable sensing cable. The sensing technique is compatible with off-the-shelf components and fiber cable and even allows for simultaneous telecommunication and sensing using the same optical fiber cable. Near term application of this technology could provide significant benefits for vibration sensing, intrusion detection, and acoustic sensing.

Theoretical noise performance of coherence-multiplexed interferometric sensors

Abstract: If a number of fiber-optic interferometric sensors are arranged so that their outputs are returned to the user via a common optical bus, then some method of distinguishing the returns from different sensors must be used to recover individual signals. One such method involves using light with a short coherence length, so that returns from different sensors will be mutually incoherent. The interferometric signal associated with each sensor can then be recovered via appropriate optical processing. The author considers sensors multiplexed using this technique and calculates their noise performance. It is found that for systems with only a few sensors, the minimum detectable phase is limited by the noise associated with incoherent interference; this can be minimized by using light with as short a coherence length as is practical. The maximum number of sensors that can be multiplexed is limited by optical power loss. A ladder topology is tentatively found to give the best performance. >

Fiber optic sensors, apparatus, and detection methods using fluid erodible controlled release polymers for delivery of reagent formulations

Abstract: An improved fiber optic sensor, sensing apparatus, and methods for making optical determinations are provided. The fiber optic sensor employs a fiber optic strand to convey light energy and at least one polymer matrix comprising a fluid erodible, continuous release polymer and a releasable reagent formulation able to react with a molecule or analyte of interest. The optic sensors and sensor construction has been demonstrated to be accurate, precise, and of long duration.

Multiple-point temperature sensor using optic fibers

Abstract: A multiple-point temperature sensor, with optic fibers, uses the effect of variation in the birefringence in a birefringent optic fiber as a function of temperature. This sensor has a plane polarized, wide spectrum source coupled to a monomode optic fiber for preservation of polarization, along one of the neutral axes, with weak coupling points distributed along this fiber. A polarizer at 45° to the neutral axes of the fiber is placed at the output of this sensing fiber. The output radiation is analyzed by spectroscopy, for example in a Michelson interferometer, capable of being swept, associated with a detector. The interferometer shifts needed to detect the extreme values of the transmission function are measurements of temperature deviations.

Optical-fiber Fabry–Perot embedded sensor

Abstract: A reflectively monitored optical-fiber Fabry-Perot interferometer was embedded in a graphite-epoxy composite material. Its performance as a temperature sensor was demonstrated from 20 to 200 degrees C. The change in relative phase shift with temperature, Deltao/oDeltaT, was measured to be 8.0 x 10(-6)/ degrees C for this embedded sensor. This value is 4% lower than for one employing a similar fiber in an air ambient. A thermal expansion coefficient for the composite material in the direction of the fiber axis is estimated from these data to be 2.1 x 10(-7)/ degrees C.

Fiber optic sensors

Abstract: Serveral improvements in fiber optic sensing systems are disclosed. One improvement incorporates a dielectric filter applied directly to the sensing end of an optical fiber detector which has the characteristic that it reflects back a reference light beam of one wavelength while passing a sensing signal of a different wavelength which is modulated in some predetermined fashion beyond the filter. Both light signals are reflected back through the fiber and are, accordingly, attenuated in the identical or nearly identical fashion in response to various noise sources. The ratio of the intensity of the two signals is proportional to the sensed parameter and renders the system essentially self-compensating. The dielectric filter is preferably directly coated onto the exit end of the fiber through vapor deposition techniques. Time division multiplexing is used to sequentially fire plural light sources and a single photodetector is used for measuring the intensity of the reflected back signals of the various wavelengths. Another improvement encompasses the detection unit in which the input and reflected signals are separated using an off-axis parabolic mirror which provides excellent separation efficiency. According to another principal feature of this invention, a concave surface is formed at the fiber end which increases the numerical aperture of the fiber which provides greater sensitivity to the deflection of a pressure sensitive diaphragm spaced from the confronting the fiber end.

High-sensitivity fiber-optic accelerometer.

Abstract: We describe a novel accelerometer in which the sensing element is a weighted diaphragm. The displacement of the diaphragm produced by acceleration is measured using a miniature hemispherical air-spaced Fabry-Perot interferometer, of which one mirror is mounted on the diaphragm. The interferometer is illuminated by a diode laser and addressed through a monomode optical fiber.

Linear readout of dynamic phase change in a fiber-optic homodyne interferometer.

Abstract: A new technique that provides linear measurement of dynamic phase change in a no-feedback, no-phase-bias fiberoptic interferometer is described. The phase measurement is unaffected by random changes in phase, source intensity, and fringe visibility. A minimum detectable phase shift of 0.1 rad has been measured for the configuration reported.

Fiber-optic sensor for humidity.

Abstract: A novel fiber-optic humidity sensor is described. It is based on reversible sorption of water from the ambient atmosphere in a porous thin-film interferometer that sits on the tip of a fiber. The sorbed water changes the refractive index of the thin films and thus the reflectivity of the interferometer; the resulting modulation of the reflected intensity is detected. The sensor is insensitive to electromagnetic interference and aggressive chemicals and is extremely small (micrometers).

Temperature compensated fiber-optic pressure sensor

Abstract: A pressure sensor comprises a chamber formed from two members micromachined in silicon or a similar substance. The members define a chamber with at least one pressure sensitive membrane and an optic fiber extending through the chamber parallel to the membrane. The membrane may have an optical grating formed thereon which may be coated with a surface plasmon supporting substance. Light is injected into the fiber with a wavelength that couples with the grating on the membrane, either in a Bragg relationship or in coupling to a surface plasmon. The coupling, and thus the light lost from the fiber, varies with separation between the membrane and fiber and thus with the pressure outside the chamber. The members also include a thicker wall which is not pressure sensitive but which couples with an identifiably distinct portion of the light in the optic fiber to provide a temperature compensated reference. The pressure sensor can be made very small and rugged for combustion chamber pressure sensing in an engine and for other uses where very small pressure sensors are required.

Applications of fiber-optic sensors

Abstract: The authors discuss the basic principles and applications of optical-fiber sensor technology, outlining the various sensing mechanisms which can be utilized to influence the propagation of light in an optical fiber and the various sensor types-intensity, phase-modulated (interferometric), intrinsic, and extrinsic. Sensors for the measurement of temperature, pressure, liquid level, chemical parameters, acoustic effect, magnetic field, and rotation (gyroscope) are briefly described. >

Characteristics of laser diodes for interferometric use.

Abstract: Low-coherence-length light from laser diode sources has applications in extending the useful range of interferometric fiber optic sensors. The characteristics of two commercial low-coherence laser diodes were investigated and compared with theoretical models to determine the operational characteristics of the devices. Reasonable trends in the comparison were seen.

Laser catheter system

Abstract: A fiber optic device for remote delivery of intense ultraviolet optical signals comprising an excimer laser source and an optical fiber. The output of the pulsed, transverse discharge, high pressure laser source is coupled to an articulating reflection mechanism which directs the pulsed laser output to an input end of the optical fiber. The optical fiber serves to transmit the light from the laser source to a remote location or target. The excimer laser further comprises a segmented first elongated laser electrode and a coextensive second laser electrode which is substantially solid. The discharge is stabilized by inductors connected to each segment of the first electrode and further by preionization electrodes located adjacent and coextensive with the second electrode. The preionization electrodes comprise a central conductor surrounded by a dielectric sleeve. The laser further comprises a closed gas system for the lasing medium gas. The gas system comprises a circulating blower, a heat exchanger and a cryogenic trap. The optical fiber comprises a central core having a substantially constant index of refraction and a cladding having an index of refraction which varies as the radius of the cladding. Thus, the invention advantageously permits high intensity ultraviolet light to be generated over extended periods of time utilizing a sealed laser and furthermore delivers the high intensity ultraviolet light to a remote location through an optical fiber with minimal damage to the fiber. The system is particularly well suited for application as a percutaneously introduced laser angioplasty system for removal of vascular obstructions.

Miniature fiber optic pressure sensor

Abstract: A pressure sensor comprises a chamber formed from two members micromachined in silicon or a similar substance. The members define a chamber with at least one pressure sensitive membrane and an optic fiber extending through the chamber parallel to the membrane. The membrane has an optical grating formed thereon which may be coated with a surface plasmon supporting substance. Light is injected into the fiber with a wavelength that couples with the grating on the membrane, either in a Bragg relationship or in coupling to a surface plasmon. The coupling, and thus the light lost from the fiber, varies with separation between the membrane and fiber and thus with the pressure outside the chamber. The members may also include a thicker wall which is not pressure sensitive but which couples with an identifiably distinct portion of the light in the optic fiber to provide a temperature compensated reference. The pressure sensor can be made very small and rugged for combustion chamber pressure sensing in an engine and for other uses where very small pressure sensors are required.

Composite integrity monitoring

Abstract: An optical frequency domain distributed strain sensor for determining the strain distribution along an optical fiber includes an optical source that provides a polarization controlled optical interrogation signal having a frequency that varies in a recurring linear manner. The interrogation signal is injected into a sensor fiber embedded within a composite structure that places the fiber under strain. A portion of the interrogation signal is backscattered from the sensing fiber as a consequence of the strain experienced by the fiber and is mixed with a reference signal to produce beat frequency signals. The frequency of the beat signals is directly related the to position of backscatter in the sensing fiber while the amplitude of each beat frequency signal is directly related to the integrated strain-induced birefringence up to the backscatter point. An in-line fiber polarizer and an associated controllable polarizer control the polarization state of the interrogation signal in the sensor fiber to provide zero point sensitivity compensation and controllable testing for ambiguous strain points.

Optical fiber switch employing a Sagnac interferometer

Abstract: Switching in less than 1 ns of an optical signal at 632 nm is demonstrated in a 1 m optical fiber Sagnac interferometer. Full signal modulation was obtained with a pump power of 24 W at a wavelength of 532 nm.

Porous plastic optical fiber sensor for ammonia measurement.

Abstract: A porous plastic optical fiber has been developed for use in chemical gas sensing. This porous plastic waveguide, which was made with copolymer materials, has an interconnective porous structure as well as uniformity of pore size. These sensors are based on in-line optical absorption within the porous plastic fiber core and have much greater sensitivities than sensors based on evanescent coupling to a surrounding medium. Furthermore, this fiber simultaneously exhibits very high gas permeability and liquid impermeability. This combination makes the fiber particularly suitable for gas concentration measurements in aqueous samples. An ammonia gas sensor has been tested to demonstrate the effectiveness of this porous plastic waveguide.

Planar fiber-optic interferometric acoustic sensor

Abstract: A sensing portion for an interferometric acousto-optic sensor and a sensor using that sensing fiber are disclosed. The sensor fiber has an outer coating of a material having a low bulk modulus. This coating greatly enhances the sensitivity of the sensor fiber. In a preferred embodiment, the material of low bulk modulus embeds the sensor fiber and any included reference fiber. Most preferably, the material having a low bulk modulus is a polymer such as polyurethane. The sensing portions made according to the present invention will usually be incorporated into sensors used for underwater exploration.

Fiber optic vibration sensor for remote monitoring in high power electric machines.

Abstract: A novel fiber optic vibration sensor is presented, based on the motion detection of an elastic cantilever by means of the optical triangulation technique. The sensor was designed to work at a fixed frequency of 100 Hz for applications in high power electrical plants where insulation and EM immunity requirements make fiber optic sensing the most suitable choice. The working linear range reaches a peak amplitude of 0.25 mm; resolution is 0.1 microm; accuracy is better than +/-3%; and sensitivity is 10 mV/microm. Design details and experimental results are reported.

Fiber polarimetric stress sensors.

Abstract: Simple polarimetric force sensors based on high birefringence fibers are investigated theoretically and experimentally. The sensor consists of a linearly polarized light source, a birefringent fiber, an analyzer or polarizing prism and two photo detectors. At the input, one of the polarization modes is excited. At the output, power in the orthogonal polarization mode is detected. In the theoretical investigation, the sensor is modeled as a concatenation of three birefringent fiber sections with the short middle section as the stress sensing element. The high birefringent fibers are treated as anisotropic media and the effects of stress on the fiber birefringence and the rotation of birefringence axes are studied. In particular, the presence of protective jackets is taken into account in the theoretical consideration. The sensor response is expressed as a function of the applied force, the interaction length, and the direction of force relative to the birefringence axes of the fiber. Experimental results based on high birefringent fibers agree very well with the theoretical predicted behavior.

Observation of ultrafast nonlinear polarization switching induced by polarization instability in a birefringent fiber rocking filter

Abstract: Ultrafast all-optical switching between two orthogonal linear polarizations in a periodically twisted fiber filter is discussed. The dependence of the switching characteristics on the input power and operating wavelength is considered. >

Erbium-doped fiber amplifier coupling device

Abstract: This invention is directed toward reducing substantially the loss incurred when two optical fibers of different mode diameters are coupled together. In this invention, the coupling of one fiber to that of another where the fibers are of different mode diameters is not with a tapered fusion, core-to-core splice, but with an evanescent field fiber coupler. Briefly, to achieve amplification of a weak optical signal, an amplifying fiber is inserted into the system at one or a plurality of locations. More specifically, at a desired location, the transmission fiber is severed to provide two sections and the severed sections are transversely coupled to the amplifying fiber. The weak signal from the first section of the transmission fiber is coupled into the amplifying fiber via a first evanescent field type of fiber coupler; and the amplified signal is coupled from the amplifying fiber to the second section of the transmission fiber via a second evanescent field type of fiber coupler. The first evanescent field type of fiber coupler is designed to couple substantially all of the signal energy from the first section of the transmission fiber to the amplifying fiber and to couple substantially none of the laser diode pump signal or energy from the amplifying fiber to the transmission fiber. The second evanescent field type of fiber coupler is designed to couple substantially all of the signal energy in the amplifying fiber to the transmitting fiber and substantially no laser diode pump energy is transferred from the amplifying fiber to the transmission fiber. Either or both cut ends of the transmission fiber can be coupled to monitor devices to detect and determine the value of the signal being coupled into and/or out of the amplifying fiber to control, for example, the gain of the amplifying fiber.

Optical fiber distribution system for an optical fiber sensor

Abstract: A device for distributing signals within a sensor system used for monitoring the presence of a specific analyte in a composition by means of an analyte-sensitive indicator molecule is disclosed. The sensor system includes a signal-generating component, a single-fiber sensor tip that houses the indicator molecule and a signal-measuring component. In one embodiment, the signal-generating component generates a first optical signal of a wavelength that is sensitive to the indicator molecule, and a second optical signal of a wavelength that is predictably altered by the indicator molecule. The distribution system includes lengths of optical fiber, a dividing connector, a mixing connector, a transmitting connector and a tip connector. The dividing connector connects at least three intermediate fibers to the signal-generating component to receive intermediate signals. The mixing connector connects a mixing fiber to the intermediate fibers to receive the intermediate signals and blend them into a single mixed signal. The transmitting connector connects a transmitting fiber to the mixing fiber to thereby receive a portion of the mixed signal, and connects a comparison fiber to the mixing fiber to transmit a portion of the mixed signal to the signal-measuring component. The tip connector connects the transmitting fiber to the sensor tip fiber to transmit a portion of the mixed signal to the sensor tip, and connects an indicator fiber to the sensor tip to transmit a portion of the resulting indicator signal returned from the sensor tip to the signal-measuring component.

Overview Of Mach-Zehnder Sensor Technology And Applications

Abstract: The development of the technology for generic fiber optic interferometric sensors is outlined. Attention will be paid to recent developments of remote interrogation/demodulation and to the avoidance of polarization fading. The second part of the article describes some of the applications which have received most attention.

Wavelength division multiplexing using a tunable acousto-optic filter

Abstract: A fiber-optic communication system in which two or more lasers of different wavelengths are modulated with respective data signals. The laser outputs are coupled to a common optical fiber. A receiving station receives all the signals on the optical fiber. A tunable acousto-optical filter in the receiving station is electrically tuned to the laser frequency carrying the desired data signal and passes only the optical signal with that frequency to an optical detector.

Fiber-optic sensing of voltages by line integration of the electric field

Abstract: A fiber-optic sensor that is sensitive to the line integral of the electric field is presented. The sensor detects electric-potential differences by adding the scalar products of the local electric fields along the integration path times the path-segment vectors. This is achieved by exploiting symmetry properties of the converse piezoelectric effect. The sensor principle is experimentally demonstrated. Important performance parameters including integration accuracy, dynamic range, linearity, and bandwidth are examined.