Abstract: Photons can propagate concurrently in two different directions along optical paths in a generalized Mach Zehnder interferometer (GMZI). A counterpropagating GMZI can include a first set of input ports and a second set of input ports, a first set of output ports and a second set of output ports, and optical components interconnected to form a GMZI that can selectably establish a first optical path between one of the first set of input ports and one of the first set of output ports and a second optical path between one of the second set of input ports and one of the second set of output ports. The first optical path and the second optical path can include an overlapping portion though which photons on the first and second optical paths propagate in opposing directions.

Abstract: An optical fiber ring interferometer is provided, which is based on a common light path for two or more light beam pairs preferably originated from two or more light sources of a substantially different spectrum or from a single light source split spectrum and whereas each light beam of a specific pair is propagating in relative opposite directions, wherein at least one pair of light beams is utilized to detect acousto-mechanical events and to provide information regarding location and other characteristics of detected environmental disturbance.

Abstract: Technologies for quantum sensing are disclosed. In the illustrative embodiment, a sensor system may be operated by coupling an electromagnetic wave from a measurement line to a first resonator. The sensor system includes a second resonator that is non-reciprocally coupled to the first resonator. In the absence or a perturbation, there is no reciprocal coupling between the first resonator and the second resonator, but a perturbation may cause reciprocal coupling between the first and second resonator. With appropriate selection of the non-reciprocal coupling, the signal at the output of the measurement line may allow for fast determination of whether the perturbation is present.

Abstract: There is provided apparatuses to detect occurrence and location of damages on optical fiber links in advance by converting an optical span in optical network to an interferometry based sensing media. The interferometry based sensing media may enable detection of mechanical perturbation or mechanical vibration occurred on optical fiber links across optical network. The system employed with the interferometry based sensing media can detect occurrence of mechanical perturbation or mechanical vibration as well as discover the location of such event occurred using standard interferometry based sensing techniques.

Abstract: The inventive circular chiral fiber polarizer is operable to convert linearly polarized light to circularly polarized light, may be advantageously fabricated in an “in-fiber” manner and to comprise desirable extinction ratio characteristics, and may also serve as an interface between a sequentially positioned polarization maintaining (PM) fiber, and a single mode (SM) fiber.

Abstract: A photonic device such as a polarization controller includes a coupler, such as a 2×2 MMI coupler, or a series of such couplers. Couplers may be interspersed with other components such as phase shifters. Photodetectors such as photodiodes are coupled to input and output lines of the coupler or couplers, for example via taps. In various embodiments, all of the couplers include photodetectors monitoring light power for at least two of their inputs and outputs, and at least one of the couplers includes photodetectors monitoring at least three of its inputs and outputs. The arrangement of photodetectors can provide sufficient information for feedback control of the photonic device and/or determining the state of polarization of light within the photonic device. Signals from some photodetectors can be used to estimate light power at locations in the photonic device lacking photodetectors.

Abstract: A measuring device measures an electrical current and contains a light source for generating a polarized primary light signal for feeding into a Faraday sensor unit, and a detector for detecting a secondary light signal provided by the Faraday sensor unit and polarization-altered in relation to the primary light signal. An optical-electrical compensation element, by which the polarization alteration of the secondary light signal can be compensated via an opposite polarization alteration, and a measurement signal, according to the opposite polarization alteration, for the electrical current can be deduced. A method for measuring an electrical current by use of the measuring device is further disclosed.

Abstract: An illustrative downhole sensing system includes a phase-squeezer assembly, an interferometer with a downhole sensor on the sensing path, and a receiver. The phase squeezer assembly provides a phase-squeezed laser beam, preferably with a squeeze parameter greater than 2. Certain embodiments include a pulse generator that gates the phase-squeezed laser beam to form a sequence of phase-squeezed laser pulses, and may further include a compensator that converts the sequence of pulses into a sequence of double pulses with a slight frequency shift between the pulses in each pair. The interferometer conveys a reference portion of the phase-squeezed laser beam along a reference path and a sensing portion of the phase-squeezed laser beam along a sensing path. A downhole sensor along the sensing path provides the sensing portion of the phase-squeezed laser beam with a measurement-parameter dependent phase shift relative to the reference portion of the phase-squeezed laser beam, which is measured by the receiver.

Abstract: A hybrid external cavity multi-wavelength laser using a QD RSOA and a silicon photonics chip is demonstrated. Four lasing modes at 2 nm spacing and less than 3 dB power non-uniformity were observed, with over 20 mW of total output power. Each lasing peak can be successfully modulated at 10 Gb/s. At 10?9 BER, the receiver power penalty is less than 2.6 dB compared to a conventional commercial laser. An expected application is the provision of a comb laser source for WDM transmission in optical interconnection systems.

Abstract: An optical fiber sensor system and a method for determining a location of a disturbance having a signal processor with a plurality of activation cells adapted to react to components of a back-scattered signal and label the disturbance.

Abstract: An image processing method includes the steps of providing at least one image of at least one object or part of the at least one object, and providing a coordinate system in relation to the image, providing at least one degree of freedom in the coordinate system or at least one sensor data in the coordinate system, and computing image data of the at least one image or at least one part of the at least one image constrained or aligned by the at least one degree of freedom or the at least one sensor data.

Abstract: An interferometric system includes a polarization separation element (10), a first polarization conversion element (11), a Mach-Zehnder interferometer (2) including a first (4) and second (5) arms connected to one another by a first (6) and second (7) ends in order for a first and second beams (20, 21) having the same polarization to pass through the interferometer in a reciprocal manner in opposite directions of propagation, respectively, so as to form a first and second interferometric beam (22, 23), a second polarization conversion element (11) for obtaining an interferometric beam (24), the polarization of which is converted, a polarization-combining element (10), and a detection element (8) suitable for detecting an output beam (25).

Abstract: An embodiment relates to a Fourier transform spectrometer comprising: a coherent light source; an interferometer adapted to separate the coherent light source into two or more parts in order to generate through frequency or phase-induced effects, interferences between the two or more parts; detection means adapted to detect the interferences, wherein the coherent light source comprises a frequency comb generator having a frequency repetition rate, and the detection means are adapted to detect the beating of pairs of frequencies of the frequency comb separated by the frequency repetition rate or a multiple of the frequency repetition rate.

Abstract: In one embodiment, a Sagnac interferometer-type fiber-optic sensor includes a synchronous detection circuit to carry out synchronous detection of detected light signal with a phase modulation angular frequency of a phase modulator. A signal processing circuit calculates and outputs the magnitude of current to be measured using the signal detected in the synchronous detection circuit. A phase modulator driving circuit controls the driving of the phase modulator. The phase modulator driving circuit controls a phase modulation depth of the phase modulator so that the amplitude of the second-order harmonics and the fourth-order harmonics obtained by carrying out the synchronous detection of the detected light signal with the phase modulation angular frequency becomes the same.

Abstract: A bidirectional guided optical router includes an evanescent-field optical coupler having three input ports, three output ports, a first central waveguide, a second lateral waveguide and a third lateral waveguide and an evanescent-field-based optical coupling zone in which the first, second and third waveguides are disposed so as to allow evanescent-field-based coupling between the first central waveguide and either one of the lateral waveguides. The 3×3 optical coupler has a length L of between 3154×Leq and 2×Leq such that an optical beam coupled on the first input port having a power p and propagating on the first waveguide in the forward direction is distributed according to the following distribution: a first secondary beam having a power 90% of p/2 on the second output port, another secondary beam having a same power ?90% of p/2 on the third output port.

Abstract: An apparatus comprising an encoded pressure signal propagating in a fluid flowing in a conduit. An optical fiber measurement element has a reflector on one end and is disposed around at least a portion of the conduit. A light source injects a second optical signal and a third optical signal propagating in first and second optical fibers, respectively. A delay section is disposed in the second optical fiber. The second optical signal and the third optical signal are directed into the optical fiber measurement element and are reflected back from the reflective end such that at least a portion of the reflected second and third optical signals propagate through the second and first optical fibers respectively to an optical detector. The optical detector senses an interference between the reflected optical signals and outputs a first signal related thereto.

Abstract: In one embodiment, a Sagnac interferometer-type fiber-optic sensor includes a synchronous detection circuit to carry out synchronous detection of detected light signal with a phase modulation angular frequency of a phase modulator. A signal processing circuit calculates and outputs the magnitude of current to be measured using the signal detected in the synchronous detection circuit. A phase modulator driving circuit controls the driving of the phase modulator. The phase modulator driving circuit controls a phase modulation depth of the phase modulator so that the amplitude of the second-order harmonics and the fourth-order harmonics obtained by carrying out the synchronous detection of the detected light signal with the phase modulation angular frequency becomes the same.

Abstract: The instant disclosure describes an optical device for the interferometric analysis of the surface condition of an object, including: a light source; an optical fiber capable of receiving the incident light wave and transmitting said wave to the object; a detector capable of detecting a combination between a light wave reflected by the optical fiber and a light wave returned by the object; and in which the optical fiber has a free end in the shape of a cone, with a vertex angle of between 15 and 25 degrees, the tip of the cone having dimensions of less than 50×50 nm, and the tip of the cone being placed, while in use, at a distance of between 5 and 50 ?m from the surface of the object.

Abstract: An optical ingredient-measuring apparatus is provided for measuring a concentration of an optically rotative substance. The apparatus includes a sensor main body which detects a phase difference between linear polarized light beams that propagate through an optical fiber loop in opposite directions, a circular polarized input component interposed in the middle of the optical fiber loop having first and second converters that convert the linear polarized light propagating through the optical fiber loop into left-hand and right-hand circular polarized light, and a concentration detector, installed in the sensor main body, which calculates the concentration of the substance in the sample based on the detected phase difference.

Abstract: An apparatus for detecting data in a fluid pressure signal in a conduit comprises an optical fiber loop comprises a measurement section and a delay section wherein the measurement section is disposed substantially circumferentially around at least a portion of the conduit, and wherein the measurement section changes length in response to the fluid pressure signal in the conduit. A light source injects a first optical signal in a first direction into the measurement section and a second optical signal in a second direction opposite the first direction into the delay section. An optical detector senses an interference phase shift between the first optical signal and the second optical signal and outputs a first signal related thereto.

Abstract: An optical system including: a photon source; first directing elements configured to direct photons to follow a first path through the optical system; second directing elements configured to direct photons to follow a second path through the optical system, wherein the second path is the reverse of the first path, photons travelling through the first path having a different polarization to those travelling through the second path; and a mechanism varying the relative phase shift between photons following the first path and photons following the second path.

Abstract: A biaxial optical gyroscope is provided, which realizes mode separation using the waveguide devices with different material, and the biaxial optical gyroscope includes: a surface Plasmon Polariton Y-type mode splitter (1), a Y waveguide integrated optical device (2), a Y waveguide integrated optical chip (3), a first polarization-maintaining fiber coil (41), a second polarization-maintaining fiber coil (42), directional couplers (51, 52) and detectors (61, 62), wherein the two output ends of the surface Plasmon Polariton Y-type mode splitter (1) are respectively connected to the Y waveguide integrated optical device (2) and the Y waveguide integrated optical chip (3), and the output ends of the Y waveguide integrated optical device (2) and the Y waveguide integrated optical chip (3) are respectively connected to the first polarization-maintaining fiber coil (41) and the second polarization-maintaining fiber coil (42).

Abstract: A sensing coil for a Sagnac interferometer current sensor is disclosed, the sensing coil (14) being composed of an optical fiber (for example a spun polarizing Hi-Bi fiber) that is arranged in use to transmit a single elliptical polarization state and the sensing coil comprising at least two interconnected loops (15 and 16 or 26 and 27 or 28 and 29). At least one of the loops is arranged in use to enclose a current conductor (11, 12 or 25) and the loops are interconnected such that light propagating in a first direction (23 or 30) in the first loop will propagate in a second, opposite, direction (24 or 31) in the other or, if more than one, in each other loop, whereby the sensing coil provides minimal sensitivity to rotational movement. Also disclosed are a sensing unit (FIG. 5) that incorporates the sensing coil and a current sensor (FIG. 1) that incorporates the sensing unit.

Abstract: A disturbance, such as vibration from human activity, is located along a fiberoptic waveguide configuration (301-304) with two interferometers (801, 802) of the same or different types, such as Mach-Zehnder, Sagnac, and Michelson interferometers. Carrier signals from a source (101) are split at the interferometer inputs (201, 202) and re-combined at the outputs (701, 702) after propagating through the detection zone (401), where phase variations are induced by the disturbance (501). Phase responsive receivers (901, 902) detect phase relationships (1001, 1002) between the carrier signals over time. A processor (1101) combines the phase relationships into composite signals according to equations that differ for different interferometer configurations, with a time lag between or a ratio of the composite signals representing the location of the disturbance.

Abstract: A method in which a ribbon fiber, containing a plurality of waveguides each having a first end and a second end, is wound into a coil and the second end of a first waveguide is coupled together with the first end of a second waveguide such that light in said first waveguide will transfer to said second waveguide.

Abstract: An optical sensor includes at least a portion of an optical waveguide having a hollow core. The optical waveguide substantially confines a first optical signal and a second optical signal within the hollow core as the first optical signal and the second optical signal counterpropagate through the optical waveguide. Interference between the first optical signal and the second optical signal is responsive to perturbation of the at least a portion of the optical waveguide.

Abstract: Disclosed is a fiber optic interferometer including: a wideband optical source having a decoherence time ?DC; a coil including N turns of a fiber optic with length L; an optical element separating the incident beam into first and second beams coupled to first and second ends of the fiber respectively, so the first beam travels through the fiber optic in a first direction and the second beam travels through the fiber optic in a counter propagating direction; and a detector detecting the intensity of the output beam. The fiber optic is a high polarization mode dispersion type, and the length L of the fiber optic coil is more than twice the fiber correlation length, so the fiber operates in a coupled PMD mode, and the propagation differential group delay between two orthogonal polarization states, accumulated over the length of the fiber, is greater than the decoherence time of the source.

Abstract: An optical sensor includes at least a portion of an optical waveguide having a hollow core generally surrounded by a cladding. The cladding substantially confines a first optical signal and a second optical signal within the hollow core as the first optical signal and the second optical signal counterpropagate through the optical waveguide. Interference between the first optical signal and the second optical signal is responsive to perturbation of the at least a portion of the optical waveguide.

Abstract: An optical fiber vibration sensor includes a light source, an optical receiver, an optical dividing/coupling portion, a signal processing unit, and a fiber loop portion made of an optical fiber. A part of an optical fiber composing the fiber loop portion is installed inside a housing of a main body of the optical fiber vibration sensor as an optical fiber for delay and another part of the optical fiber composing the fiber loop portion is installed outside the housing as a vibration detecting optical fiber.

Abstract: The invention relates to a spectrograph (11) comprising a waveguide (10) provided with accesses (10; 10b, 12), a means for injecting two guided contra-propagative waves by each accesses in such a way that a spatial interference is formed in the waveguide, means (19, 20, 14, 16) for detecting the energy of the evanescent wave of the guided field produced by the interference of said contra-propagative waves.

Abstract: In one embodiment, a Sagnac interferometer-type fiber-optic sensor includes a synchronous detection circuit to carry out synchronous detection of detected light signal with a phase modulation angular frequency of a phase modulator. A signal processing circuit calculates and outputs the magnitude of current to be measured using the signal detected in the synchronous detection circuit. A phase modulator driving circuit controls the driving of the phase modulator. The phase modulator driving circuit controls a phase modulation depth of the phase modulator so that the amplitude of the second-order harmonics and the fourth-order harmonics obtained by carrying out the synchronous detection of the detected light signal with the phase modulation angular frequency becomes the same.

Abstract: An apparatus and method for sensing hazardous materials utilizes first and second optical fibers and a recirculator coupled to a substrate. The first optical fiber is coupled to the substrate and has first and second opposing ends and a first substance embedded therein. The first substance is reactive to a first hazardous material type. The second optical fiber is coupled to the substrate and has first and second opposing ends and a second substance embedded therein. The second substance is reactive to a second hazardous material type. The at least one recirculator is coupled to the substrate and configured such that when light propagates from one of the ends of at least one of the first and second optical fibers, at least some of the light is directed by the at least one recirculator into the opposing end of the respective optical fiber.

Abstract: An apparatus and method for using a counter-propagating signal method for locating events is disclosed. The apparatus and method uses a Mach Zehnder interferometer through which counter-propagating signals can be launched. If the sensing zone of the Mach Zehnder interferometer is disturbed, modified counter-propagating signals are produced and the time difference between receipt of those signals is used to determine the location of the event. A microcontroller receives feedback signals which adjusts polarization controllers so that the polarization states of the counter-propagating signals can be controlled to match the amplitude and/or phase of the output signals. Detectors are provided for detecting the modified signals.

Abstract: An apparatus for detecting data in a fluid pressure signal in a conduit comprises an optical fiber loop comprises a measurement section and a delay section wherein the measurement section is disposed substantially circumferentially around at least a portion of the conduit, and wherein the measurement section changes length in response to the fluid pressure signal in the conduit. A light source injects a first optical signal in a first direction into the measurement section and a second optical signal in a second direction opposite the first direction into the delay section. An optical detector senses an interference phase shift between the first optical signal and the second optical signal and outputs a first signal related thereto.

Abstract: A fiber optic sensor system employs at least one light source that operates to generate light having one or more desired wavelengths. A first optical fiber based sensor transparent to a desired light wavelength operates to sense a magnetic field emitted from a predetermined electrical conductor or a current flowing through the electrical conductor. A temperature sensor that may be another optical fiber based sensor operates to sense an operating temperature associated with the first optical fiber based sensor in response to the light generated by the light source. Signal-processing electronics adjust the sensed current to substantially compensate for temperature induced errors associated with the sensed current in response to the measured operational temperature of the fiber optic sensor.

Abstract: To measure a sum of electrical currents in different conductors, light is led in an optical sensing fiber around all the conductors. The sensing fiber can be wound around the conductors in a single loop or in several individual loops. Interspersed polarization maintaining fibers and retarders can be used to transfer the light between individual loops for preventing undesired signals from stray magnetic fields. The method has high accuracy is particularly suited for measuring large currents that sum up to zero.

Abstract: To achieve an apparatus capable of measuring a light absorption coefficient f a sample with high sensitivity. A ring down spectroscope uses a wavelength-variable femtosecond soliton pulse light source 1. Pulse light is input to a loop optical fiber 6 through a first light waveguide 4 and a wavelength selective switch 5. Ring down pulse light is input to a homodyne detector through the wavelength selective switch 5. On the other hand, pulse light propagating in the first light waveguide 4 is split and input to light waveguides constituting a second light waveguide 20 through an optical directional coupler 8 and a first optical switching element 12. The pulse light propagating in the second light waveguide 20 is input to the homodyne detector as reference light and used for synchronous detection. The plural light waveguides constituting the second light waveguide 20 differ in optical length in accordance with the length of the optical fiber 6, and can slightly change the optical length.

Abstract: A fiber-optic sensor, a method of configuring a fiber-optic sensor, and a method of using a fiber-optic sensor are provided. The fiber-optic sensor includes an optical fiber coil having a length and a laser source optically coupled to the coil. The laser source has a coherence length. Light from the source is transmitted to the coil as a first signal propagating along the coil in a first direction and a second signal propagating along the coil in a second direction opposite to the first direction. The optical paths of the first signal and the second signal are substantially reciprocal with one another and the first signal and the second signal are combined together after propagating through the coil to generate a third signal. The coherence length is greater than 1 meter or is in a range between 200 microns and 10 centimeters.

Abstract: A method is provided for measurement of dispersion or other optical and mechanical properties within a waveguide by inducing four-photon mixing at different locations within the waveguide by timing a pump signal to counter-collide with and abruptly amplify or attenuate one or both of a probe pulse and a signal pulse at each location. The measurement of the components of the resulting mixing signal created by each collision is used to calculate dispersion defined by the location at which the collision occurred. By combining the measurements from all of the locations, a spatial map of dispersion or other optical or mechanical properties within the waveguide can be generated.

Abstract: Embodiments of the present invention provide a sensor probe for fiber-based electric current sensors. The sensor probe includes a conductor of spiral shape and a relatively straight optical fiber being placed at a through-hole formed by the spiral shape conductor. An electric current conveyed by the spiral shape conductor causes polarization direction of a light traveling along the optical fiber to rotate. By detecting the amount of rotation that the light experiences at the ends of the optical fiber, the amount of current carried by the spiral shape conductor is determined. Method of making the sensor probe is also disclosed.

Abstract: Systems and methods are disclosed for a modified Sagnac interferometer having a plurality of gratings that can be reflective or transmissive. The gratings allow measurement of wavelength spectra in counter-circulating beams of the interferometer. In one embodiment, diffraction geometries at each pair of neighboring gratings are configured so that diffractive and angular contributions reinforce each other at the second of the pair of gratings. In one embodiment, diffraction geometries at the gratings are configured so that the exiting beams of the interferometer satisfy the crossing condition wherein the exiting beams are on the opposite sides of a reference beam axis for a design wavelength input beam. Also disclosed are techniques for restoring the reinforcement and/or crossing conditions when these conditions are not otherwise met.

Abstract: Passively biased fiber-optic Sagnac interferometric sensor architecture, for gyroscope and current sensor in particular, is disclosed. One embodiment uses a 3×3 coupler entirely made of circular polarization maintaining fiber that serves as a beam splitter and meanwhile a passive bias. An alternative is to use a 3×3 hybrid coupler consisting of two linear polarization maintaining fibers and one conventional single-mode fiber, with the former two connected in a common interferometric sensor circuitry, and with the latter one cut short at both ends to form matched terminations. Still another alternative is an integral unit of Faraday rotator, whose central part is a fiber-optic magneto-optic 45° rotator, with a “zero to fast” fiber-optic quarter wave plate attached to one side, and a “fast to zero” fiber-optic quarter wave plate to the other. Advantages of passive bias are simplicity in construction, no need of manual adjustment and operational stability.

Abstract: A disturbance, such as vibration from human activity, is located along a fiberoptic waveguide configuration (301-304) with two interferometers (801, 802) of the same or different types, such as Mach-Zehnder, Sagnac, and Michelson interferometers. Carrier signals from a source (101) are split at the interferometer inputs (201, 202) and re-combined at the outputs (701, 702) after propagating through the detection zone (401), where phase variations are induced by the disturbance (501). Phase responsive receivers (901, 902) detect phase relationships (1001, 1002) between the carrier signals over time. A processor (1101) combines the phase relationships into composite signals according to equations that differ for different interferometer configurations, with a time lag between or a ratio of the composite signals representing the location of the disturbance.

Abstract: An electro-optical high-voltage sensor includes a waveguiding sensing fiber of an electro-optical material. The electrical field of the voltage to be measured is substantially parallel to the longitudinal axis of the sensing fiber. The sensing fiber carries two orthogonally polarized light waves, with the applied field affecting the birefringence between the waves. Using an electro-optical waveguiding fiber in this configuration allows the voltage between two widely spaced points to be accurately measured.

Abstract: A RF-synchronization system includes a laser that creates pulse trains for synchronization. A modulation means transfers the timing information of the pulse train into an amplitude modulation of an optical or electronic system. A synchronization module changes the driving frequency of the modulation means until it reaches a phase-locked state with the pulse train.

Abstract: An apparatus and method for using a counter-propagating signal method for locating events is disclosed. The apparatus and method uses a Mach Zehnder interferometer through which counter-propagating signals can be launched. If the sensing zone of the Mach Zehnder interferometer is disturbed, modified counter-propagating signals are produced and the time difference between receipt of those signals is used to determine the location of the event. A microcontroller receives feedback signals which adjusts polarisation controllers so that the polarisation states of the counter-propagating signals can be controlled to match the amplitude and/or phase of the output signals. Detectors are provided for detecting the modified signals.

Abstract: A perimeter security system is disclosed which includes a first cable (40) and a second cable (60) buried beneath the ground in a zig-zag pattern. The first cable (40) has a first fibre (44) and a further fibre (42). Second cable (60) has a second fibre (62). The first and second fibres (44) and (62) are connected by a coupler (52) at one end so that light can be launched into the first and second fibres (44) and (62) to propagate in one direction. The further fibre (42) is connected to a coupler (70) which also connects to the other end of the first and second fibres (44) and (62) so light can be launched into the fibres from the other end and travel in the opposite direction.

Abstract: An apparatus and method for using a counter-propagating signal method for locating events is disclosed. The apparatus and method uses a Mach Zehnder interferometer through which counter-propagating signals can be launched. If the sensing zone of the Mach Zehnder interferometer is disturbed, modified counter-propagating signals are produced and the time difference between receipt of those signals is used to determine the location of the event. A microcontroller receives feedback signals which adjusts polarisation controllers so that the polarisation states of the counter-propagating signals can be controlled to match the amplitude and/or phase of the output signals. Detectors are provided for detecting the modified signals.

Abstract: An apparatus and method for using a counter-propagating signal method for locating events is disclosed. The apparatus and method uses a Mach Zehnder interferometer through which counter-propagating signals can be launched. If the sensing zone of the Mach Zehnder interferometer is disturbed, modified counter-propagating signals are produced, and the time difference between receipt of those signals is used to determine the location of the event. Polarisation controllers receive feedback signals so that the polarisation states of the counter-propagating signals can be controlled to match the amplitude and/or phase of the signals. Detectors are provided for detecting the modified signals.

Abstract: To perform distributed sensing with an optical fiber using Brillouin scattering, a light pulse is transmitted into the optical fiber, where the transmitted light pulse has a first frequency. Backscattered light and optical local oscillator light are combined, where the backscattered light is received from the optical fiber in response to the transmitted light pulse, and where the optical local oscillator light has a second frequency. A frequency offset is caused to be present between the first frequency of the transmitted light pulse and the second frequency of the optical local oscillator light, where the frequency offset is at least 1 GHz less than a Brillouin frequency shift of the backscattered light. Spectra representing Stokes and anti-Stokes components of the backscattered light are acquired, where the Stokes and anti-Stokes components are separated by a frequency span that is based on the frequency offset.