RU2532562C1 - Distributed sensor of acoustic and vibration actions - Google Patents

Abstract

FIELD: measurement equipment.

SUBSTANCE: invention relates to distributed measurements, and namely to distributed sensors of acoustic and vibration actions. In a distributed sensor of acoustic and vibration actions, which includes a sensitive element in the form of a fibre-optic cable and a coherent phase-sensitive optic reflectometer optically connected to it through an optic interface and containing a source of periodic sequence of optic pulses and a receiver of scattered radiation with a photodetector, which are optically connected to the interface, which is intended to convert scattered optic radiation to an electric signal supplied to a processing unit, with that, a source of periodic sequence of optic pulses and the processing unit are electrically connected to a control and synchronisation unit, and the source of periodic sequence of optic pulses and/or the receiver of scattered radiation is of a multichannel type and has the number of channels of at least two and is provided with a possibility of recording reflectograms formed in each of the channels, the receiver of scattered radiation includes a Mach-Zehnder or Michelson nonequiarm interferometer with faraday mirrors; with that, the interferometer has at least two output channels, each of which is connected to the photodetector, and the control and synchronisation unit has the possibility of providing separation and independent processing of signals from each of the output channels of the interferometer.

EFFECT: increasing guaranteed sensitivity and range of action of a distributed sensor of acoustic and vibration actions.

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Description

The invention relates to the field of distributed measurements, namely to distributed sensors of acoustic and vibration effects.

The claimed device can be used to monitor and protect extended objects, such as perimeters and communications, in particular, to monitor the condition of transport pipelines, trunk fiber cables from damage when working near the cable, protect the perimeters of special objects, etc.

A known analyzer of vibroacoustic signals designed to analyze the spectrum of signals and containing optically coupled coherent radiation source, beam splitter, photodetector, amplifier (SU 1589069, 1990). The known device is not intended for monitoring extended objects.

A known diagnostic system designed to track changes in static strains and measure dynamic strains. The system includes a tunable narrow-band light source, a light guide fiber, reflective sensors, such as Bragg gratings located along the length of the fiber, and a signal processing loop. The system can also be used according to the Fabry-Perot scheme (RF patent No. 2141102). The system provides high sensitivity to deformations, but it is very complex and has a low spatial resolution.

A device is known for monitoring the vibro-acoustic characteristics of an extended object, comprising a narrow-band pulsed optical radiation source in the form of a Q-switched fiber laser, a sensitive element in the form of an optical fiber located longitudinally inside or outside an extended object, an optical radiation input unit into the sensitive element, a photodetector, and a processing unit signal with a processor (RF patent No. 2271446). A disadvantage of the known device is the presence of random variations in the carrier frequency of the testing optical pulses injected into the fiber, associated with the pulsed laser mode and the sensitivity of the fiber laser to technical noise. This limits the range, sensitivity and resolution of the device, and also complicates its use in the field.

As the closest analogue (prototype), a device was selected - a distributed sensor of acoustic and vibration effects, containing a sensing element in the form of a fiber-optic cable and optically connected to it through an optical interface, a coherent phase-sensitive optical reflectometer containing a source of a periodic sequence of optical pulses optically connected to the interface and a scattered radiation receiver with a photodetector designed to convert scattered optical and radiation into an electrical signal supplied to the processing unit, the source of the periodic sequence of optical pulses and the processing unit being electrically connected to the control and synchronization unit, while the source of the periodic sequence of optical pulses is made multi-channel, or the scattered radiation receiver is made multi-channel, or the source of a periodic sequence of optical pulses and the scattered radiation receiver is multi-channel (WO 2011162868, A2, 12.29.2011).

A disadvantage of the known device (prototype) is its lack of reliability due to the presence of zones of reduced sensitivity of the distributed sensor of acoustic and vibration effects due to the random distribution of the phase and scattering coefficient of the optical pulse of the coherent phase-sensitive optical reflectometer. This limits the range, sensitivity and resolution of the device.

The present invention is aimed at solving the problem of improving the reliability of the sensor of acoustic and vibration effects.

The technical result of the invention is to increase the guaranteed sensitivity and range of a distributed sensor of acoustic and vibration effects.

The technical result in the implementation of the invention is achieved by the fact that in a distributed sensor of acoustic and vibration effects, containing a sensing element in the form of a fiber optic cable and optically connected to it through an optical interface, a coherent phase-sensitive optical reflectometer containing a source of a periodic sequence of optical pulses optically connected to the interface and scattered radiation receiver with photodetector, designed to convert the scattered radiation optical radiation into an electrical signal supplied to the processing unit, wherein the source of the periodic sequence of optical pulses and the processing unit are electrically connected to the control and synchronization unit, and the source of the periodic sequence of optical pulses and / or the scattered radiation receiver is multi-channel with at least two channels and the possibility of recording reflectograms formed in each of the channels, the scattered radiation receiver contains unequal Mach- a renderer or Michelson with Faraday mirrors, the interferometer has two output channels: in-phase and antiphase, each of which is connected to a photodetector, and the control and synchronization unit is configured to separate and independently process in-phase and antiphase channels, or the interferometer has three output channels : in-phase and with phase shifts of +120 degrees and -120 degrees, each of which is connected to a photodetector, and the control and synchronization unit is configured to be divided I have independent processing of the three output channels, or the interferometer has four output channels with phase shifts of 0 degrees, + 90 degrees, -90 degrees and 180 degrees, each of which is connected to a photodetector, and the control and synchronization unit is designed to provide separation and independent processing four output channels, or a phase modulator is contained in the long arm of the interferometer.

The invention is illustrated by drawings, where:

figure 1 shows a block diagram of a distributed sensor of acoustic and vibration effects;

figure 2 shows a diagram of the implementation of a Michelson interferometer with Faraday mirrors;

figure 3 shows an example of a single trace;

figure 4 shows an example of the imposition of many reflectograms (in the area outside the vibro-acoustic impact, the traces coincide and overlap, and in the region of vibro-acoustic impact, the traces do not coincide and do not overlap);

figure 5 shows an example of the imposition of many differential reflectograms (difference reflectogram is the difference of two successive reflectograms. In the area outside the vibro-acoustic impact, the differential reflectograms are close to zero, and in the region of the vibro-acoustic impact, the differential reflectograms are significantly different from zero); figure 6 shows the dependence of the amplitude of the difference signal (APC) on the coordinates of the point of impact (numerical calculation). Dotted line - conditional level of reduced sensitivity to local effects;

Fig. 7 shows the dependences of APC on the coordinate of the exposure point for two channels differing in the frequency of the optical carrier (duration of test pulses 50 ns).

The device includes:

- source of a periodic sequence of optical pulses

- pulsed laser 1;

- power amplifier 2;

- node 3 input optical radiation into the sensing element and output scattered radiation;

- sensitive element - optical cable 4;

- preamplifier 5;

- photodetector 6;

- block 7 analysis and processing of the electrical signal;

- photo detector 8;

- unequal Mach-Zehnder or Michelson 9 interferometer;

- Faraday mirrors 10;

- phase modulator 11.

The device operates as follows.

The pulsed laser 1 generates a periodic sequence of short pulses (radiation), which, after amplification in the amplifier 2 through the optical input element 3 to the sensor and output of the scattered radiation, is introduced into the sensor - an optical cable 4 located inside or next to the controlled object.

In an optical fiber, the radiation is scattered by the stationary inhomogeneities of the fiber without changing the frequency (Rayleigh scattering).

The scattered radiation through the node 3 of the input of optical radiation into the sensing element and the output of the scattered radiation is supplied to the preamplifier 5 and after amplification it is fed to the photodetector 6, converted into an electrical signal and fed to the analysis and processing unit 7.

With pulsed excitation, the time dependence of the average power of the backscattering signal and, accordingly, the photocurrent of the photodetector 6 (reflectogram) has a form close to the exponent. However, due to the high coherence of the initial radiation, this trace is randomly cut due to the random phase of the interfering scattered radiation. An example of a trace is shown in figure 3.

In the absence of vibroacoustic effects and changes in the carrier frequency of the rectangular test pulse, the reflectograms from different pulses obtained at different instants of time coincide. In the presence of a vibro-acoustic effect on the sensitive element, the reflectograms from different impulses in the impact area turn out to be different. The magnitude of the changes determines the intensity of the impact, and the time delay relative to the testing rectangular pulse uniquely determines the coordinate of the impact.

The nature and coordinate of the impact is determined by the analysis and processing unit 7 from comparing the set of reflectograms by determining the locations of significant changes in reflectograms.

Figure 3 shows a typical trace from a single pulse. Figure 4 shows the result of the superposition of many reflectograms, on which the affected area is clearly visible. Figure 5 shows the result of the imposition of many differential reflectograms with the impact area.

A disadvantage of the known devices is that the magnitude of the change in the amplitude of the reflectogram in a complex way depends not only on the amplitude of the impact, but also on the nature of the trace at the point of influence. The dependence of the amplitude of the difference signal (APC), i.e. the magnitude of the difference of two adjacent traces from the point of application of the external point phase effect, obtained as a result of numerical simulation, is shown in Fig.6.

The proposed device differs significantly from the prototype in that the scattered radiation receiver contains a non-shoulder Mach-Zehnder interferometer or a non-shoulder Michelson interferometer with Faraday mirrors, which allows recording reflectograms and difference signals of several independent channels in one sensor 4.

The use of several independent channels for recording reflectograms and difference signals in one sensor eliminates blind spots, which, in contrast to the prototype, is directed to the invention, that is, to provide the necessary level of sensitivity along the entire fiber.

In development of the independent claim, the interferometer has two outputs: in-phase and antiphase, each of which is connected to the photodetector 8, and the control and synchronization unit provides separation and independent processing of in-phase and antiphase channels.

This feature ensures the elimination of dead zones at the optimal shape and duration of pulses, and therefore, while ensuring maximum resolution and accuracy of localization of external influences and, in addition, at the maximum frequency of recording reflectograms. Another advantage is the increased sensitivity to phase effects with an increase in the difference in the course of the interferometer.

An interferometer can also have three outputs: in-phase and with phase shifts of +120 degrees and -120 degrees, each of which is connected to a photodetector, and the control and synchronization unit provides separation and independent processing of three channels.

This feature also provides the ability to measure the difference in phase incursion between the points of the fiber cable at a distance equal to half the difference in stroke of the unequal interferometer.

The interferometer can also have four outputs with phase shifts of 0 degrees, +90 degrees, -90 degrees and 180 degrees, each of which is connected to a photodetector, and the control and synchronization unit provides separation and independent processing of four channels.

This feature also provides the opportunity to measure the difference in phase incursion between the points of the fiber cable at a distance equal to half the difference in the stroke of the unequal interferometer, and, in addition, increases the sensitivity when using differential reception.

A phase modulator 11 may also be contained in the long arm of the interferometer (see FIG. 2).

This feature also provides the ability to measure the phase difference between the fiber cable points located at a distance equal to half the stroke difference of the unequal arms, and, in addition, increases the sensitivity when using synchronous detection.

The use of the invention allows you to quickly detect violations of the integrity of the perimeter of an extended object or to fix any effects from inside or outside on an extended object. Moreover, the device allows you to determine the coordinates of the location of the defect or the point of impact on the object reliably and with a high degree of accuracy.

Based on the foregoing, we can conclude that the claimed technical result - increasing the range, sensitivity and resolution of the device - is achieved.

Claims (5)

1. A distributed sensor of acoustic and vibration effects, comprising a sensing element in the form of a fiber optic cable and optically connected to it via an optical interface, a coherent phase-sensitive optical reflectometer, containing a source of a periodic sequence of optical pulses and a scattered radiation receiver with a photo detector optically connected to the interface, designed for converting the scattered optical radiation into an electrical signal supplied to the processing unit, moreover, the source of the periodic sequence of optical pulses and the processing unit are electrically connected to the control and synchronization unit, and the source of the periodic sequence of optical pulses and / or the receiver of scattered radiation is made multi-channel with at least two channels and with the possibility of recording reflectograms formed in each channel, differing the fact that the scattered radiation receiver contains a non-shoulder Mach-Zehnder or Michelson interferometer with Faraday mirrors. 2. The distributed acoustic and vibration sensor according to claim 1, characterized in that the interferometer has two output channels: in-phase and antiphase, each of which is connected to a photodetector, and the control and synchronization unit is configured to provide separation and independent processing of in-phase and antiphase channels. 3. The distributed acoustic and vibration sensor according to claim 1, characterized in that the interferometer has three output channels: in-phase and with phase shifts of +120 degrees and -120 degrees, each of which is connected to a photodetector, and the control and synchronization unit the ability to provide separation and independent processing of the three output channels. 4. The distributed acoustic and vibration sensor according to claim 1, characterized in that the interferometer has four output channels with phase shifts of 0 degrees, +90 degrees, -90 degrees and 180 degrees, each of which is connected to a photodetector, and the control unit and synchronization is configured to provide separation and independent processing of the four output channels. 5. The distributed acoustic and vibration sensor according to claim 1, characterized in that a phase modulator is contained in the long arm of the interferometer.

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