Abstract: An accurate measurement method and apparatus using an optical fiber are disclosed. A total change in optical length in an optical core in the optical fiber is determined that reflects an accumulation of all of the changes in optical length for multiple segment lengths of the optical core up to a point on the optical fiber. The total change in optical length in the optical core is provided for calculation of an average strain over a length of the optical core based on the detected total change in optical length.

Abstract: This application relates to methods and apparatus for fiber optic sensing which can provide information about the environment in which the fiber optic is deployed. In particular the application relates to fiber optic based sensing of the mechanical impedance of the environment. The method comprises using an interrogator (201) to interrogate an optical fiber (104) which is coupled to a first element (202; 802) which is responsive to electromagnetic fields. In use a varying electric current (I), which may be an alternating current, is applied so as to induce a varying force (F) on said first element. The optical radiation backscattered from within the optical fiber is analyzed to determine a measurement signal indicative of a variation in the backscattered radiation corresponding with said electric current applied. The first element may be a first conductor (202) and the varying current may be supplied to the first conductor, or to a second conductor (701).

Abstract: An image data set acquired by an optical coherence tomography (OCT) system is corrected for effects due to motion of the sample. A first set of A-scans is acquired within a time short enough to avoid any significant motion of the sample. A second more extensive set of A-scans is acquired over an overlapping region on the sample. Significant sample motion may occur during acquisition of the second set. A-scans from the first set are matched with A-scans from the second set, based on similarity between the longitudinal optical scattering profiles they contain. Such matched pairs of A-scans are likely to correspond to the same region in the sample. Comparison of the OCT scanner coordinates that produced each A-scan in a matching pair, in conjunction with any shift in the longitudinal scattering profiles between the pair of A-scans, reveals the displacement of the sample between acquisition of the first and second A-scans in the pair.

Abstract: A serial weak FBG interrogator is disclosed. The serial weak FBG interrogator may include a CW tunable laser or pulsed laser utilized as a laser source and an EDFA. The serial weak FBG interrogator may also be an interrogation of a single sensor system by utilization of a DFB laser which utilizes a single sensor, which may be an interferometer sensor, an extrinsic Fabry-Perot interferometer or a wavelength-modulated sensor. The serial weak FBG interrogator may also include a computer system or CSPU.

Abstract: A method for estimating a parameter includes: generating an optical signal, the optical signal modulated via a modulation signal; transmitting the modulated optical signal from a light source into an optical fiber, the optical fiber including a plurality of sensing locations disposed along the optical fiber and configured to reflect light; receiving a reflected signal including light reflected from the plurality of sensing locations; and combining, in parallel, each of a plurality of reference signals with the reflected signal to estimate a value of the parameter.

Abstract: A wavelength-swept light source apparatus comprises a light source that emits a wavelength-swept light that varies in a predetermined cycle, a mode hop detector that detects a mode hop of the wavelength-swept light emitted from the light source; and a control unit that controls at least one of a parameter that defines a specified period having a predetermined fixed or variable time length provided in the predetermined cycle and a control parameter of the light source, thereby to set an occurrence timing of the mode hop detected by the mode hop detector outside of the specified period.

Abstract: An optical semiconductor element includes a ring modulator, and a light absorbing material provided at a position apart from a path for a modulated light which is guided by the ring modulator, the light absorbing material absorbing a light leaked out of a ring waveguide of the ring modulator, and increasing a temperature of the ring waveguide.

Abstract: An optical semiconductor element includes a ring modulator, and a light absorbing material provided at a position apart from a path for a modulated light which is guided by the ring modulator, the light absorbing material absorbing a light leaked out of a ring waveguide of the ring modulator, and increasing a temperature of the ring waveguide.

Abstract: An optical fiber sensing spring of the present invention includes a longer cylindrical tension coil spring, a shorter cylindrical compression coil spring and a longer cylindrical tension coil spring that are connected in series to form a spring having the same inner and outer diameters. A single-mode optical fiber manufactured with a long section of fiber grating is placed in the manufactured spring. This section of the fiber grating is tensed to two ends of the cylindrical compression coil spring having a predetermined gauge length, and the force applied is released. A pre-tensing method for providing a maximum tolerable compression strain and manufacturing technology for this sensing element are then achieved. Thus, the self-tensed optical fiber sensing spring structure satisfies a standard 0.9 mm outer diameter, is fully spring jacketed by the full spring jacketing and protection layer, and forms a linear stress-strain relationship in an elasticity stretched elastic region.

Abstract: An optical fiber strain sensor system and method are provided that use pixels of a three-dimension (3-D) pixel sensor to sense the respective light beams passing out of the ends of a reference fiber and a measurement fiber and for converting the respective light beams into respective electrical signals. Because 3-D camera pixels have photodiodes that are directly connected by switches to integrators within the same die, the need to use separate TIAs and phase detection circuitry in each receive channel is eliminated, which reduces system complexity and overall cost. In addition, omitting the separate TIAs and phase detection circuitry for each channel eliminates the phase uncertainty that can occur when using those components, and thus improves measurement precision.

Abstract: An optical device includes an optical reflector based on a coupled-loopback optical waveguide. In particular, an input port, an output port and an optical loop in arms of the optical reflector are optically coupled to a directional coupler. The directional coupler evanescently couples an optical signal between the arms. For example, the directional coupler may include: a multimode interference coupler and/or a Mach-Zehnder Interferometer (MZI). Moreover, destructive interference during the evanescent coupling determines the reflection and transmission power coefficients of the optical reflector.

Abstract: In an integrated optical circuit, light from a light source is polarized and coupled to a first and second strip waveguide. A waveguide coupling element couples the two optical signals from the two strip waveguides to different polarization modes of an optical fiber line. The optical fiber line is connected to a measuring head, which reflects the optical signal and in which a phase difference between the two optical partial signals is modulated in a magnetic field. In the waveguide coupling element, the reflected signal is split into two optical partial signals having the same polarization and the phase difference between the two partial signals is determined. A phase modulator device provides for closed-loop operation. Compared to fiber-optical concepts, the number of splices is reduced.

Abstract: A broadband fiber optic sensor array is formed along a length of single mode optical fiber, with the individual sensing elements formed by introducing local perturbations (e.g., changes in diameter) along the length of the optical fiber. The sensor array requires only a single light source input and a single (conventional) optical spectrum analyzer output and is capable of providing individual measurements (such as local temperature or pressure) for each sensing element disposed along the length of fiber. The individual transmission spectra of the sensing elements forming the array are smooth and strongly overlap, and a method has been developed for determining the characteristics of the individual elements from the variations in the total (combined) transmission spectrum.

Abstract: A magnetic field direction detector for detecting whether a magnetic flux has a component of field from a first side or a second side of a detection axis; the magnetic field direction detector comprising: a first magneto-resistive sensor; and a perturbation generator; wherein the perturbation generator causes an external magnetic field to be perturbed so as to cause the apparent direction of flux to change by an angle ?1 at the first magneto-resistive sensor.

Abstract: Embodiments of the present disclosure are directed towards techniques and configurations for MEMS sensing device configured to determine inertial change applied to the device. In one instance, the device may comprise a laser arrangement configured to generate a light beam, and a waveguide configured to split the light beam into two portions. The waveguide may include two arms through which the respective portions of the light beam may respectively pass, and disposed substantially parallel with each other and joined together around their respective ends to recombine the portions into a light beam. One of the arms may be deformable. A deformation of the arm may result in a change of an optical path length of a portion of the light beam traveling through the arm, causing a detectable change in light intensity of the recombined light beam outputted by the waveguide. Other embodiments may be described and/or claimed.

Abstract: A fiber optic sensor comprising a first light source, a second light source, a first optical coupler operatively connected with the first light source, and a second optical coupler operatively connected with the second light source. The first optical coupler directs first and second optical waves along a first optical path into first and second ends of an optical fiber such that the first and second optical waves interfere to form a first combined optical wave. The second optical coupler directs the third and fourth optical waves along a second optical path into the first and second ends of the optical fiber such that the third and fourth optical waves interfere to form a second combined optical wave. The second optical path is longer than the first optical path by a predetermined distance. Detectors receive the first and second combined optical waves and output information with respect thereto.

Abstract: The system includes a first optical device and a second optical device. The second optical device has a second facet. The first optical device has a waveguide on a base. The waveguide terminates at a waveguide facet. The waveguide facet is positioned so as to exchange light signals with the second facet. The first optical device also has one or more actuators that are each configured to move the waveguide facet relative to the second facet.

Abstract: A twisted, multicore fiber communicates light input to each core to an output. The twisting mitigates relative time delays of the input light traveling through each of the cores in the multicore fiber to the output caused by bending of that multicore fiber. An example application is in an optical network that includes an optical input terminal and an optical sensor connected by a twisted multicore connecting fiber. One example of twisted multicore optical fiber is helically-wrapped, multicore fiber.

Abstract: There are provided a method and a system for characterizing the CMRR of an ICR under test, which employ highly coherent light from two continuous-wave (CW) single-frequency lasers whose respective optical frequencies mutually differ by an offset defining an “Intermediate Frequency” (fIF) in the rf electrical baseband. The method involves the coherent mixing of light from these two lasers in the ICR under test. A “tone” in the rf electrical baseband at frequency fIF is generated by the beating of light from the two single-frequency lasers as they interfere on the photodetectors of the ICR. The resulting tone at frequency fIF in the output electrical signals of the ICR is then detected and analyzed to characterize the CMRR of the ICR.

Abstract: In certain embodiments, a system for detecting an agent includes a resonator device configured to receive an agent. The resonator device is also configured to transmit light received from a light source, the transmitted light having an altered peak wavelength due to the presence of the received agent. The system further includes a filter device configured to filter the transmitted light having the altered peak wavelength such that the transmitted light having the altered peak wavelength does not reach one or more detectors of a detector array configured to receive transmitted light not filtered by the filter device. The system further includes a processing system operable to determine that the one or more detectors of the detector array are not generating a signal, the absence of the signal being generated by the one or more detectors of the detector array indicating the presence of the agent.

Abstract: An optical sensor (10) comprises an optical cavity defined by a dielectric body and responsive to one or more physical environmental conditions, and a waveguide (70) having a terminal end spaced apart from the optical cavity such that light is optically coupled from the terminal end of the waveguide (70) to the optical cavity. The waveguide (70) is arranged such that, in use, it is maintained at a first temperature that would not damage the optical coupling to the optical cavity when the dielectric body is maintained at a second temperature sufficient to damage the optical coupling to the optical cavity.

Abstract: An apparatus and method for reducing the visibility of speckle in coherent light comprises propagating the coherent light through a multimode optical waveguide 106 to reduce a coherence length of the coherent light by intermodal dispersion. A vibratable membrane mirror 104 is arranged to reflect the coherent light either before or after passing through the multimode optical fiber further to reduce the coherence length by distributing wave packets across different propagating modes and to erase the modal structure by averaging the distribution of energy across the modes leading to a more uniform illumination. The invention may be used for speckle removal in a laser scanning projector.

Abstract: Disclosed herein is a packing container including: a packing container body including a leading-out section which contains an optical probe having a first end section for incoming of a laser beam and a second end section for outgoing of the incoming laser beam, which leads out the first end section of the optical probe thus contained to the exterior and which is sealed, and a window section by which the laser beam going out from the second end section of the contained optical probe is led out to the exterior; and a light-transmitting member which closes the window section and permits the laser beam to pass therethrough.

Abstract: A distance measuring apparatus and method that enable high-precision and high-speed measurement by canceling variations of a delay circuit in the apparatus are provided. Pulsed light is branched into first and second reference light, and transmitted measurement light, and the difference in detection time among the first reference light along a first path with no optical variations, the second reference light along a second path with an optical delay, and received measurement light from an object to be measured is measured. The received measurement light and the first reference light are temporally separate, distance is calculated from the difference in detection time between the received measurement light and the first reference light.

Abstract: A fiber optic current or magnetic field sensor uses a sensing fiber in a coil for measuring a current or a magnetic field and has a retarder for converting between linearly polarized light and elliptically polarized light. The retardation of the retarder, its temperature dependence as well as its azimuth angle in respect to the plane of the fiber coil are optimized in dependence of the birefringence in the sensing fiber in order to minimize the influence of temperature variations and manufacturing tolerances on the overall scale factor of the sensor.

Abstract: An optical signal generator includes a single-mode laser; a reflecting mirror to define another cavity different from a cavity of the single-mode laser, and reflect a part of output light from the single-mode laser to return the part of the output light to the single-mode laser; an intensity modulator provided between the single-mode laser and the reflecting mirror; and a phase adjuster, provided between the single-mode laser and the reflecting mirror, to adjust a frequency difference between a signal on state and a signal off state generated in accordance with intensity modulation by the intensity modulator.

Abstract: An agent sensing system may comprise an emitter optical resonator, a functionalized optical resonator, and a reference optical resonator. The emitter optical resonator may be configured to emit light at one or more system peak wavelengths. The functionalized optical resonator may be optically coupled to the emitter optical resonator and configured to propagate the emitted light in the absence of a particular agent, and filter the emitted light in the presence of the particular agent. The reference optical resonator may be optically coupled to at least one of the emitter optical resonator and the functionalized optical resonator such that an intensity of light propagated by the reference optical resonator is based at least on whether light emitted by the emitter optical resonator is filtered or propagated by the functionalized optical resonator.

Abstract: Low-cost interface property measuring device and method enabling high-precision and simple measurement of an interface property.

Abstract: An optical detection system for monitoring a pipeline. The optical detection system includes a host node in the vicinity of or remote from, a pipeline to be monitored. The optical detection system includes (a) an optical source for generating optical signals, and (b) an optical receiver.

Abstract: A sensor probe for detecting an environmental state within a bodily lumen includes an elongated body having an optical guide with an outcoupling surface, which is part of an end surface of the elongated body, facing a reflective surface of an overhanging part. The sensor probe further includes a photodetector which detects a property of light which is composed of light in the optical guide that is reflected from the outcoupling surface of the optical guide and of light in the optical guide that is reflected from the overhanging part. The sensor probe also includes a hydrogel material having a property that changes when getting into contact with an environmental material and/or when detecting an environmental change. The hydrogel material is provided between the end surface of the elongated body and the overhanging part.

Abstract: The present invention relates to a system for sensing of a disturbance on an optical link. Data traffic from an optical source with a short coherence length is transmitted along the link to a receiver station on one or more of a plurality of time-division-multiplexed channels. One of the channels is used to transmit encoded phase information relating to the phase characteristics of the optical source output. At the receiver station, the actual phase characteristics of the arriving light from the optical source is compared with the encoded phase information. Since a physical disturbance of the link is likely to alter the actual phase characteristics of the arriving light but not the encoded phase information, it is possible to determine if a physical disturbance has occurred. The system can conveniently be used to monitor an optical link carrying communications traffic.

Abstract: An improved optic system for measuring and/or controlling displacement, force, pressure, position, or chemistry is disclosed. This apparatus allows for more accurate, robust, and economical communication between the transducer (or control input element) and the reader device (or control output), allows the use of a single optic fiber and/or or a gap for the communication link, and produces substantial insensitivity to attenuation due to mechanical, chemical, thermal, and radiation effects acting on the optic fiber or open space in which the signal propagates. It is also significantly immune to interference from electromagnetic radiation, since the link can be easily produced as a non-conductor which will not propagate unwanted electrical energy or lightning, and is intrinsically safe from igniting fires or explosions. It also facilitates use on rotating machinery and remote location of the transducer by the ability to transmit the signal across a large gap or air space.

Abstract: The present invention relates to a method of and apparatus for evaluating the position of a disturbance, in particular using a waveguide having a plurality of overlap regions. A position sensor is provided including: an optical waveguide; a transmission stage for launching a sensing signal into the waveguide; a receiving stage arranged to receive a returned sensing signals which returned sensing signal is a time distributed signal derived from backscattered components of the sensing signal, the waveguide being arranged along a path having a plurality of overlap regions such that a disturbance in an overlap region causes a first disturbance feature and a second disturbance feature in the returned sensing signal; and, monitoring means for monitoring the returned signal, such that a respective time of return can be associated with the first and second disturbance feature. Both return features can then be used to evaluate the position of the disturbance so as to enhance the resolution of the position sensor.

Abstract: A sensor for measuring force, the sensor including: a light source; and a mixing medium in optical communication with the light source and exposed to the force; wherein four wave mixing of light interacting with the mixing medium provides a signal that indicates the force.

Abstract: A system including methods and apparatuses for responding to an entity's superposition of states. The system is also able to respond to a first entity's state of entanglement, as well as respond to either the superposition of states or entanglement of any other entity that is entangled with the first entity. The system is capable of effecting its responses to superpositions of states or entanglements in modes that can preserve or demolish the superposition of states or entanglements of the entities it interacts with. The system is able to selectively switch between these modes of responses, and can also effect its responses in a delayed-choice manner. The system also encompasses applications of the methods and apparatuses for purposes including quantum communication, quantum computation, and quantum cryptography.

Abstract: An intensity modulated broadband optical signal source is arranged to produce optical signals in a plurality of wavelength bands. A demultiplexer/multiplexer pair receives optical signals from the optical signal source and produces a separate optical signal for each wavelength band. An optical coupler is arranged to receive the separate optical signals. Optical fiber loops connected to the optical coupler guides both clockwise and counter clockwise waves for each of the separate optical signals. The optical signals propagate through a sensor in each loop and then combine in the optical coupler to form a plurality of interference signals in each wavelength band. A multiplexer is optically coupled to the optical coupler to receive the interference signals therefrom, and a detector array having a selected detector corresponding to the each of the plurality of wavelength bands is arranged to provide an electrical signal that indicates the interference signals for each wavelength band.

Abstract: The invention relates to studies of internal structures of objects with the aid of optical means. According to the invention an optical system (15) of the delivering device for low coherence optical radiation, in a particular embodiment, an optical fiber probe (8), includes at least two lens components (19), (20), which have a positive focal power and are positioned substantionally confocally. This ensures a constant propagation time for the low coherence optical radiation propagating from a given point of the transverse scanning surface (28) or (39) to a corresponding conjugate point of the image plane (22). That provides elimination of the transverse scanning related aberration of the optical path length for low coherence optical radiation directed towards the object (11) both for a flat transverse scanning surface (28) and for a transverse scanning surface (39) having a curvature.

Abstract: A multi-channel low coherence interferometer having sensing and reference arms, at least one of which has variable delay. The sensing arm includes an optical switch for connecting to two or more probe arms. The distal ends of the probe arms collect source light backscattered from a sample. The backscattered light collected by the distal end of a probe arm is combined with reference light and a low coherence interferometric signal is produced by a sweep of a variable delay of the device or a sweep of a variable frequency laser light source. The interference signal produced by the interaction of reference and sensing light at a detector measures backscattered light, which may be used to characterize the sample. The low coherence interferometric signals can provide information about the morphology, physical nature, composition, and properties of the sample.

Abstract: A system and method for superheterodyne detection in accordance with the invention. The system comprises a first conversion unit for performing a first heterodyne operation on an optical input signal to generate an electrical IF signal. A second conversion unit is electrically or optically coupled to the first conversion unit. The second conversion unit performs a second heterodyne operation to generate an electrical output signal suitable for signal processing.

Abstract: A fiber optic acoustic sensor that detects pressure waves from all incident angles features multiple mandrels orthogonally disposed to provide omni-directional sensing capability. The topology of the mandrels prevents frequency response drop-off characteristic of cylindrical sensors at wavelengths smaller than ½ the length of the acoustic cylinder. The larger operating bandwidth and omni-directional sensing capability makes this fiber optic acoustic sensor a suitable choice for a wide range of applications.

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 (43, 44) 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 method and device are presented for use in determining a rate of rotation of an object. The device comprises a light guide comprising an arrangement of a plurality of coupled optical resonators arranged along a curvilinear optical path. This allows for determining a change in at least one of the light phase and frequency affected by the light propagation through the curvilinear path during the device rotation, said change being indicative of the rotation rate of the light guide.

Abstract: A high-voltage component, having a first end and a second end, whereby the first end is on a high-voltage potential with respect to the second end. An insulating part, is arranged between the first end and the second end, and an optical fiber is integrated in the high-voltage component and extends from the first end to the second end. A capillary extends from the first end to the second end and is arranged within the insulating part. The inside diameter of the capillary exceeds the outside diameter of the fiber, and the fiber is arranged within the capillary. The capillary includes a protective medium to achieve a dielectric strength in the capillary, which dielectric strength is suitable for the operating conditions.

Abstract: An image data set acquired by an optical coherence tomography (OCT) system is corrected for effects due to motion of the sample. A first set of A-scans is acquired within a time short enough to avoid any significant motion of the sample. A second more extensive set of A-scans is acquired over an overlapping region on the sample. Significant sample motion may occur during acquisition of the second set. A-scans from the first set are matched with A-scans from the second set, based on similarity between the longitudinal optical scattering profiles they contain. Such matched pairs of A-scans are likely to correspond to the same region in the sample. Comparison of the OCT scanner coordinates that produced each A-scan in a matching pair, in conjunction with any shift in the longitudinal scattering profiles between the pair of A-scans, reveals the displacement of the sample between acquisition of the first and second A-scans in the pair.

Abstract: A direct detection method and apparatus for a fiber optic acoustic sensor array systems using an in-line Michelson sensor TDM array and an interferometric section having two acousto-optic modulators that produce optical pulses that are frequency shifted with respect to each other. Direct detection is accomplished according to the equation: I(t)=A+B cos [?1??2+2?(f1?f2)t], with the phase shift difference ?1??2 between two paths containing the acoustic phase information and the frequency f1?f2 being the difference between the RF frequencies for the two acousto-optic modulators.

Abstract: An apparatus for measuring coherent crosstalk (CXT) light of the invention generates in a light source section, measurement light which has been modulated to a sawtooth wave shape and applies this to an object of measurement; sends transmission light and CXT light emerging from the object of measurement to a light receiving section via a variable optical attenuator; applies an electrical signal photoelectric-converted in an optical receiver, to a frequency filter, to thereby extract a beat component corresponding to a frequency difference between the transmission light and CXT light; controls the modulation period of the measurement light so that the power of the beat component becomes a local maximum; varies an optical attenuation amount of a variable optical attenuator, while keeping constant the optimized modulation period; and measures with high accuracy the amount of CXT light generated in the object of measurement, based on a rate of variation in the power of the beat component caused at that time.

Abstract: An optical current sensor having a reflection interferometer (1, 10) has, in its fiber-optic lead (2), a polarization-maintaining first fiber branch (20) for two forward-traveling orthogonally polarized waves and a polarization-maintaining second fiber branch (20?) for two backward-traveling orthogonally polarized waves. In this case, the two fiber branches (20, 20?) are connected to one another via a coupler (8) on the sensor side. The first fiber branch (20) is connected to a light source (4) and the second fiber branch (20?) is connected to the detector (5). A means for phase shifting (7) is funcionally connected to at least one of the fiber branches (20, 20?). It is thus possible to achieve a quasi-static control of the phase shift of the waves, so that less stringent requirements can be made of the means for phase shifting than of the phase modulators that are usually used in current sensors of this type.

Abstract: A logical element including an optical junction coupled to at least two optical inlets and to at least one optical outlet. Incoming light beams of coherent monochromatic light beams and the same uniform frequency are applied to the optical inlets, and their super-positioning is provided as an outgoing light beam(s) via the optical outlet to another logical element or to a light intensity gauge. The light intensity gauge measures light intensity in specific zone(s) of an interference pattern created by the outgoing light beam, dependent on phase shift difference between the components of the incoming light beams, and the measured intensity is correlated with intensity ranges predetermined to conjugate to logical integer values, such as Boolean or other integer numeric values. A multiplicity of logical elements can be installed to provide an optical processor. Parallel use of the same logical element is provided by the simultaneous application of sets of light beams.

Abstract: A device capable of efficiently detecting a single-photon signal preserves a photon characteristic such as polarization or angular momentum. The device can include a beam splitter that splits an input photon state into modes that are distinguished by states of a characteristic of signal photons in the input photon state, a non-destructive measurement system capable of measuring a total number of photons in the modes without identifying a photon number for any individual one of the modes; and a beam combiner positioned to combine the modes after output from the non-destructive detection system.

Abstract: A sensor array employs a parameter to induce a time-varying phase angle ? on an optical signal that comprises a phase generated carrier. The phase angle ? is calculated through employment of only four samples, where all the four samples are based on the optical signal.