Showing papers on "Fiber optic sensor published in 1985"

Polarization-state control schemes for heterodyne or homodyne optical fiber communications

Abstract: In heterodyne or homodyne optical fiber communications, the fluctuation of the polarization state in the fiber may deteriorate the receiver sensitivity. This paper reviews the principles and features of six polarization-state control schemes so far proposed, including two new attempts, one using Faraday rotators and the other using rotatable fiber cranks (RFC's), both of which have been proposed very recently. Experiments simulating actual optical heterodyne receivers are also described.

Pulse compression in optical fiber filters

Abstract: A method is described for compressing optical pulses at any wavelength in an optical fiber. It uses the negative dispersion property of permanent phase gratings created within the fiber. No external grating pair is required.

Distributed, spatially resolving optical fiber strain gauge.

Abstract: A distributed, spatially resolving optical fiber strain gauge in which the core of the optical fiber is written with periodic grating patterns effective for transmitting and reflecting light injected into the core. Spectral shifts in the transmitted and reflected light indicate the intensity of strain or temperature variations at positions of the grating corresponding to the associated wavelengths of injected light.

Fluid flow sensing apparatus for in vivo and industrial applications employing novel differential optical fiber pressure sensors

Abstract: An optical fiber fluid flow device is provided for in vivo determination of blood flow in arteries. The device includes a fiber optical fluid pressure measuring device having at least first and second optical fiber sensors which optical fiber sensors are positioned in the blood passage and in a restricted flow area in the blood passage and the two fiber optical pressure fluid sensors are connected to an interferometer associated with an opto-electronic demodulator which has an output signal representing the differential pressure between the two sensed area. The device also has utility in industrial applications.

Wavelength-dependent transmission of monomode optical fiber tapers.

Abstract: The transmission of light as a function of wavelength through biconical tapers formed in sections of monomode and low-mode number optical fibers is considered. It is found that in the monomode propagation regime the transmission varies sinusoidally with wavelength with a 50% peak loss and a 50-nm period, for tapers typical of those used in the manufacture of fused-biconical directional couplers. The observed dependence of transmission on wavelength can be understood using a coupled-mode formalism and leads to new insights into the operation of monomode fused-biconical directional couplers.

Multimode fiber-lens optical coupler

Abstract: A novel expanded beam coupling arrangement for use in association with single mode fibers is disclosed. An appropriate length of multimode fiber is fused to the endface of an input single mode fiber, where the length of the multimode fiber is chosen to provide the desired lensing conditions of the input beam. The multimode fiber is thus used as a lens, but provides many advantages over prior art optical connectors which use conventional quarter-pitch GRIN lenses epoxied to the fiber endfaces. In particular, the misalignment associated with the epoxied arrangement is reduced since the multimode fiber-lens connector of the present invention may be chosen to comprise the same outer diameter as the single mode fiber. Additionally, the use of a section of optical fiber as a lens allows for a fused connection to be used instead of an epoxied connection, which results in a more stable and rugged interface between the fiber and the lens.

Apparatus and method for performing laser material processing through a fiber optic

Abstract: Pulse laser energy in the near infrared and visible spectrum is passed through a single fiber optic at power levels required for material and metal processing. A neodymium-YAG laser used in pulsed mode is coupled to the end of a quartz fiber optic which transmits peak powers in the kilowatt range. In order to transmit higher amounts of average power, a prepared fiber end allows beam coupling through core-air and core-cladding zones. The beam at the output of the fiber optic is focused to achieve power densities capable of cutting, drilling, and welding of metals etc. The main advantage is greater flexibility of laser beam manipulation.

Enzyme-based fiber optic sensor.

Abstract: Application a un systeme modele utilisant la phosphatase alcaline comme enzyme immobilisee, le p-nitrophenylphosphate comme substrat et le p-nitrophenoxyde comme produit detecte

Determination of single-mode fiber coupler design parameters from loss measurements

Abstract: A method is presented for determining the minimum spacing between the fiber core centers in a single-mode fiber coupler which consists of two substrate blocks each provided with a fiber fixed in a curved groove. Immersion liquids with known refractive indices are placed on the top surface of one coupler block and the introduced power loss of the light guided within the fiber is measured. By fitting these results to calculated curves the minimum spacing may be determined with an error of less than 0.2 μm.

Remote displacement measurement using a passive interferometer with a fiber-optic link.

Abstract: Remote displacement measurement is demonstrated using a Fabry-Perot cavity with a multimode optical fiber link. The sensing cavity modulates, as a function of its length, the spectrum of a light-emitting diode (LED). The light returns via the fiber and is analyzed by a tunable reference cavity. A closed-loop control causes the reference cavity to track the sensing cavity length within 2 x 10 to the -12th m. Displacement range is 2 x 10 to the -6th m. The reference cavity length is measured interferometrically, using a laser, to obtain the sensing cavity length. Advantages of this sensing technique include compatibility with multimode fiber-optic components, high immunity to optical losses, and large dynamic range.

Review of long wavelength single-mode optical fiber reflectometry techniques

Abstract: This paper reviews the worldwide activity on long-wavelength ( >1.1-\mu m) single-mode optical fiber reflectometry and compares and contrasts the many different approaches being taken to equipment design.

Fiber-optic sensor with two different wavelengths of light traveling together through the sensor head

Abstract: A fiber-optic system for measuring a desired environmental parameter, such as temperature, is disclosed. The system includes a fiber-optic sensing head that alters light of a prescribed frequency as a function of the environmental parameter to be measured. Light not of the prescribed frequency is not so altered. In a preferred embodiment of the fiber-optic sensing head, a semiconductor material is positioned between two substantially parallel optical fibers at a distal end of the fibers. One fiber serves as in input channel and the other fiber serves as an output channel. Both fibers approach the semiconductor material from the same direction. Light channeled to the semiconductor material is reflected at the distal end of the input fiber so as to pass through the semiconductor material, and then is reflected again at the distal end of the output fiber so as to pass back out the output fiber. The semiconductor material absorbs the light of the prescribed frequency as a function of temperature. By monitoring the intensity of the light of the first frequency at the output channel, a measurement of the temperature to which the fiber-optic sensing head is subjected can be determined. To remove the influence of input light intensities, fiber attenuation, and the like, from the temperature measurement, lights of two frequencies, only one of which is the prescribed frequency, are selectively directed to an optical detector through the fiber-optic sensing head and through a separate fiber-optic reference channel. The four measurements thus obtained at the optical detector--prescribed frequency through sensing head, prescribed frequency through reference channel, non-prescribed frequency through sensing head, and non-prescribed frequency through reference channel--are appropriately processed in order to generate a signal that has all variations or influences removed therefrom except the temperature variations of the sensing head.

Fiber Optic Sensors: An Overview

Abstract: Fiber optic sensor technology is explained in terms of theory and applications. Various components common to all fiber optic sensors are compared. Two classes of sensing devices are emphasized: amplitude-modu-lated sensors and phase-modulated sensors. Specific examples of amplitude-modulated devices-one a pressure sensor using an optical reflection technique and another an accelerometer using a microbend technique-are described. Four types of fiber optic interferometers used in phase-modulated sensors are discussed. The effect of the optical fiber jacket on the sensitivity of phase-modulated sensors is considered. A miniature pressure sensor and a highly sensitive fiber optic accelerometer, both employing phase modula-tion, are described.

Super radiant light source

Abstract: 57 A super-radiant light source having an output with a low temporal coherence well suited, for example, to optical inertial sensor applications, includes a single mode optical waveguide such as an optical fiber having its core doped with an active laser material, such as neodymium. Pump light is coupled into the optical fiber at an intensity sufficient to produce a significant amplification of the spontaneous emission. The reflectivity at the exit end of the fiber is low enough so that even with a high gain for the light in one traverse through the fiber, the fiber is operated well below threshold for laser oscillation. The resulting emission is of high intensity, but has a low temporal coherence. In one form, the pump light is coupled into one end of the fiber and a dichroic filter is interposed in the light path between the pump light and the optical fiber to pass pump light into the core and reflect the spontaneous emission of the active material back into the core and toward the output.

Broadband spectrometer with fiber optic reformattor

Abstract: The invention provides a system having a broadband spectrometer with a fiber optic reformattor for field use in detecting and identifying gas clouds within a field of view. The system includes a grating type spectrometer, a fiber optic reformattor, a focal plane detector array and electronics for background signal subtraction techniques for generating spectral signature data which is analyzed for gas detection and outputting a decision making signal for recognition.

New optical gyroscope based on the ring passive phase conjugator

Abstract: A new optical rotation sensor is described. It is a ring passive phase conjugator in which the ring may consist of a multimode fiber. A nonreciprocal phase shift in the ‘‘passive’’ like fiber ring activates a grating movement and subsequent frequency detuning of the beams in a photorefractive four‐wave mixer. This device has the advantages of natural reciprocal behavior of phase conjugate beams (essential for rotation sensing) and has several adjustable controlling parameters. It reveals a new class of interferometry in which changes in the ring’s optical phases, the beam’s intensities and losses, and the mixing crystal’s efficiency and electric field modulate a frequency detuning of the oscillating beams.

Gated fiber optic rotation sensor with extended dynamic range

Abstract: A fiber optic rotation sensor, employing the Sagnac effect comprising all fiber optic components positioned along a continuous, uninterrupted strand of fiber optic material. The rotation sensor includes a detection system utilizing a modulator or modulators for phase modulating at first and second harmonic frequencies light waves which counter-propagate through a loop formed in the fiber optic strand. Each modulator is operated at a specific frequency to eliminate amplitude modulation in the detected optical output signal. A phase sensitive detector generates a feedback error signal proportional to the magnitude of the first harmonic in the output optical signal. The feedback error signal controls a modulator which controls the amplitude of the second harmonic driving signal for the second harmonic phase modulator such that the first harmonic component in the output signal from the rotation sensor is cancelled or held within a small range of amplitudes.

Multi-channel fiber optic sensor system

Abstract: A time division multiplexing, fiber optic data gathering system, including two or more sensors formed on an optical fiber, and a phase sensitive detector. Each sensor includes two reflectors separated by a section of the fiber having length L. Each reflector may be activated so that when an interrogating light signal propagates in a first direction past the activated reflector, a portion of the interrogating light signal will be reflected back into a direction opposite the first direction. Each reflector may also be deactivated so that the interrogating signal may propagate unhindered past the deactivated reflector. Variations in the optical path length between the reflectors of a sensor, due to changes in an external parameter of interest, will cause phase modulations that are extracted in the phase sensitive detector by homodyne or heterodyne techniques. In a preferred embodiment, the activated reflectors produce a specific type of acoustic field resulting from superposition of two acoustic waves, which acoustic field will interact with the interrogating light signal so as to reflect a portion of the interrogating light signal by 180°.

Single-mode fiber optic saturable absorber

Abstract: A fiber optic saturable absorber for processing optical signals is characterized by an optical fiber (20) from which a portion (40) of the cladding (36) is removed to form a facing surface (28). A light-absorbing substance (30) having non- linear light-absorbing characteristics is applied to the facing surface (28) such that a portion of the optical signal energy is transferred from the fiber (20) to the substance (30) where it is absorbed. The device selectively attenuates the optical signal and noise, and can be used to reduce pulse waveform distortion caused by pulse broadening and by amplification of system noise.

Optical sensing systems

Abstract: A multiplex optical sensing system comprises an optical fibre 5 arranged to be subjected at the longesth length to fibre deforming forces during operation of the system and means 1, 2, 3 for producing a plurality of pulse trains of coherent light in which the frequency of frequencies of F1 + ΔF1, F2 + ΔF2, F3 + ΔF3 of the pulses in respective pulse trains lie within a different frequency channel for transmis­ sion along said optical fibre 5. The optical fibre 5 is provided along its length with a number of equally spaced discon­ tinuities 6 to 14 which effectively divide the fibre 5 into a plurality of discrete fibre elements so that a small proportion of each light being pulse being transmitted along the fibre 5 will be reflected back along the fibre from each of the discontinuities 7 to 14 whereby, within any one of the pulse trains, each reflected light pulse after the first is superim­ posed upon either the previously reflected pulse from the preceding discontinuity or a reference light signal of the same frequency or a frequency with a constant difference frequency to the said transmitted light signal to produce an electrical signal in square law photo-detection means 15, the difference between respective electrical signals correspond­ ing to successive fibre elements being dependent upon the length of the fibre elements so that changes in optical path length of these elements produced by the incidence of deforming forces will result in changes in the electrical signals which will be detected.

Temperature independent fiber optic sensor

Abstract: Disclosed is an improved fiber optic sensor of the type in which a fiber optic waveguide component of the sensor is configured to be responsive to an external parameter such that curvature of the fiber optic waveguide is altered in response to forces induced by changes in the external parameter being sensed. The alteration of the curvature of the fiber optic waveguide causes variations in the intensity of light passing therethrough, these variations being indicative of the state of the external parameter. The improvement comprises coating material covering the exterior portion of the fiber optic waveguide, the coating material having an expansion coefficient and thickness such that distortion of the fiber optic waveguide caused by thermally induced stresses between the coating material and the glass fiber is substantially eliminated. Also disclosed is a support member for supporting the curved fiber optic waveguide, the support member and fiber optic waveguide being configured and arranged to minimize the effects of thermal stress tending to separate the waveguide from the support member.

Analysis of a fiber-optic passive loop-resonator gyroscope: Dependence on resonator parameters and light-source coherence

Abstract: Analysis has been made of a fiber-optic passive loop resonator, the heart of the gyroscope The performance characteristics are heavily dependent on the resonator parameters, such as coupling intensity loss of the input light, coupling coefficient between two fiber lines in an evanescent-field directional coupler, and losses of the light traveling in the fiber loop, as well as on the temporal coherence of the input light A two input and two output system is considered for a passive loop-resonator gyroscope Differential output operation of the two output signals is described

All-optical towed and conformal arrays

Abstract: A method and apparatus for remotely activating and analyzing fiber optic ed and conformal sensor arrays using fiber optic transmission and reception with multiplexed input and output signals using optical wavelength division, electronic frequency division, or both.

Optical contact evanescent wave fiber optic coupler

Abstract: A single mode, evanescent wave, fiber optic coupler. The coupler comprises a pair of substrates, each substrate having an optically worked fiber embedded therein and wherein the substrates are secured together such that the optical fibers are in optical contact and are in effective fusion with each other.

Self-aligning optical fiber directional coupler and fiber-ring optical rotation sensor using same

Abstract: A continuously drawn optical fiber comprising a core and cladding having different refractive indices and forming a single-mode guiding region, and the outer surface of the fiber having a cross-section forming a pair of orthogonal exterior flat surfaces so that the location of the guiding region can be ascertained from the exterior geometry of the fiber, the guiding region being offset from the center of gravity of the transverse cross-section of the fiber and located sufficiently close to at least one of the flat surfaces to allow coupling to a guided wave through that surface by exposure or expansion of the field of the guiding region.

Monomode fiber-optic strain gauge with simultaneous phase- and polarization-state detection

Abstract: A fiber-optic sensor for the measurement of strain is described, in which the measurand-induced changes in both phase and polarization state are simultaneously recovered. A simple optical arrangement based on the fiber Fabry–Perot interferometer is used to realize a high-resolution instrument with enhanced measurement range.

Fiber laser sensor

Abstract: A fiber laser sensor is comprised of two fiber lasers which are each optically coupled to a third fiber so that energy is exchanged between each of the laser fibers and the third fiber. The coupling is accomplished by arranging the laser fibers parallel to and on either side of a third fiber and properly spacing the three fibers to produce optical coupling along the length of the fibers. The central fiber ends are non-reflective so that light generated by one of the lasers and coupled to the central fiber is lost except for a small fraction of the energy transferred to the other laser. Accordingly, the structure operates with the least loss when the two lasers oscillate at a common frequency but 180° out-of-phase. The resonant cavities of the two fiber lasers are made non-identical by making the path length one cavity slightly longer than the other to produce Vernier tuning of the laser resonant frequency orders relative to each other with only one common resonant frequency. The device can be used as a sensor because the common laser output frequency is sensitive to changes in the optical path length of either or both of the laser cavities.

Fiber optic seam tracking apparatus

Abstract: An arc welding head (13) suitable for robotic welding has a fiber optic bundle (12) and remotely located laser light source (24) which is imaged onto an end of the fiber optic bundle (12) and optically linked to substantially a single fiber of the fiber optic bundle (12). The light source (24) is then repetitively scanned across the coherent arrangement of fibers within the bundle (12), such that, a substantial portion of the transmitted light will be sequentially delivered to each individual fiber within a row of fibers and subsequently delivered to a workpiece (11). This arrangement provides a high signal to noise ratio making the laser light reflected from the workpiece (11) easily distinguishable from the optical "noise" generated by the welding arc flash.

Force measuring apparatus and method using stress-induced birefringence in a single-mode optical fiber

Abstract: The force measuring apparatus includes a single-mode optical fiber having a defined anisotropy, whereby coherent light suppled to one end of the optical fiber is guided as two waves having linear orthogonal polarizations, respectively. The optical fiber is arranged between a pair of parallel planar pressure plates, the orientation of the optical fiber being such that the aforementioned linear polarizations are parallel with and normal to the remote surfaces of the pressure plates, respectively. Upon the application of a force to be measured in the compressive direction normal to one remote surface of a plate, a phase shift is produced between the waves which is a function of the magnitude of the load to be measured, as determined by interferometer means arranged at the other end of the optical fiber. The optical fiber may be of a helical configuration having turns that are contained between successive pairs of pressure plates, respectively, arranged in a stack, thereby achieving a multiplication of the measuring effect with increased accuracy of the corresponding measurement.