RU2695058C1 - Multichannel fiber-optic device for recording vibration effects with one receiving registration module - Google Patents

Abstract

FIELD: optics.

SUBSTANCE: invention relates to fiber-optic sensor systems. Multichannel fiber-optic device for detecting vibration effects includes: series-connected highly stable narrow-band radiation source; optical signal amplifier (booster); driver-controlled acoustooptic modulator for generating probing pulses; optical circulator; a receiving recording module located after the optical circulator and consisting of an optical amplifier of a weak back-scattered signal from the measurement channels, a narrow-band optical filter, an optical signal receiver, which receives signals from all N-channels, an analogue-to-digital converter, after which a computing device is arranged with possibility of final processing of detected signals and display of information. Also comprises optical switch 1xN between said optical circulator and polled N-channels, used together with determination of the current working channel of the polled N-channels by binding to the length of the i-th channel or to the distance to the first connector after the i-th optical coil from possible several optical coils of different length, installed in channels after optical switch 1xN under condition of equal length between two and/or several channels for establishment of different optical length in channels. Difference in the optical length of the channels must not be less than the resolution threshold of the device.

EFFECT: providing a plurality of measuring channels of a phase-sensitive reflectometer with one receiving channel without a complex timing circuit for pulses arriving in measurement channels and pulses arriving at the receiver, as well as reducing the number of expensive components.

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Description

Technical field

The invention relates to fiber-optic sensor systems based on phase-sensitive reflectometry, used in monitoring systems of extensive branched objects, and can be used to monitor infrastructure communication lines, objects with many controlled perimeters, in oil well logging systems based on the method of phase-sensitive reflectometry.

The level of technology

The method of phase-sensitive reflectometry makes it possible to detect a time-random backscatter signal (trace). This backscattered signal does not change under the condition of frequency stability and in the absence of thermal or mechanical effects on the sensitive optical fiber. Electronic processing of the recorded signal today is predominantly developed to isolate mechanical effects, the frequency range of which lies in the range from 10 Hz to 10 kHz, which is in good agreement with the range of acoustic signals of various nature that cause vibration of the sensor fiber. This vibration occurs when acoustic waves propagate from a source of vibration, for example, a walking person, a moving train, or operating equipment. Each type of impact generates an acoustic wave at a certain frequency, generating characteristic oscillations in certain parts of a random back-scattered signal, which can then be recognized using mathematical processing.

The basic device and coherent reflectometry method has been described in US Pat. No. 5,149,847 (IPC G01H 9/00; G01L 1/24; G01L 11/02; G08B 13/12; G08B 13/186; (IPC1-7): G08B 13/10 ; G08B 13/18, published 1993-03-16). The method of coherent reflectometry includes the following basic operations: - placement of a sensitive fiber-optic cable along the monitoring object; - supply of coherent optical radiation pulses of a certain length to the line; - reception of backscatter signals and signal extraction, indicating the fact of external influence on disturbances in the indicated backscatter signals.

The basic method corresponds to the basic schemes of devices for the implementation of the method, as well as the set of derived schemes of coherent reflectometry devices.

The increase in channels in phase-sensitive reflectometry systems is carried out by introducing optical splitters or optical switches into the circuit. When using optical splitters, the radiation is divided between all channels, a small part of the total radiation goes into one channel (in the case of the same percentage division of radiation by optical radiation, no more than Umax / N dB goes to each measuring channel, taking into account the losses in the optical splitter, where Umax - input power to the optical splitter, N - the number of channels). In this case, the receiver must also have N channels. When using optical switches, only the loss of the switch itself is introduced, however, the radiation enters the measuring channels variablely. In this case, the receiving part can contain both one (1) common for all channels receiver and N receivers, its own for each channel. However, in the case of a single receiver, it is necessary to have a tight binding on the time of the polled channel and data coming from it, which leads to the need to use cumbersome and complex feedback schemes or reduce the sampling time.

Chinese patent CN104198030A (publ. 10.12.2014) presents an analogue of the proposed technical solution - a circuit using an optical separator 1xN, N channels, an optical switch 1xN for collecting radiation from these channels. In this device, channel separation is carried out to achieve a higher discretization due to a specific sequence of sending pulses to each measuring channel.

The main disadvantage of this scheme is that many measuring channels are physically located in one cable and serve to increase the signal resolution from the same monitored section. The separation of the individual fibers in this invention is done with the aim of increasing the discretization with low characteristics of the component base of the ADC device.

The device according to the patent of the Russian Federation RU 2650853 C1 (IPC G01D 5/00 (2006.01), priority of the invention: February 17, 2017, state registration date: April 17, 2018) was selected as a prototype. In this device, a fiber-optic distributed vibroacoustic sensor based on a phase-sensitive reflectometer contains a narrow-band radiation source, a fiber-optic amplifier that amplifies the radiation source, an acousto-optical modulator operating in a pulsed mode and introducing a frequency shift into the optical radiation, a fiber-optic splitter into M-channels in the case of M> 1, and each channel consists of an optical fiber, a circulator and a fiber-optic erbium amplifier in the receiving part of the channel, an amplifier of a narrow-band optical filter and then a photoreceiver module with an output to a channel of a multi-channel ADC with a number of inputs not less than the number of channels involved, thus, the outputs of all channels are connected to their inputs of a multi-channel ADC. At the output of the ADC, the digital processor for generating control pulses, control, data processing and transmission, the frequency-pulse driver board and the AOM driver are sequentially installed; all the fibers of the M-channels are laid along each other side by side.

The main disadvantage of the prototype is the need to use separate receiving parts for each measuring channel, namely: optical circulator, fiber optic erbium amplifier in the receiving part of the channel, amplifier, narrow-band optical filter, photoreceiver module, and a multi-channel ADC with the number of inputs not less than the number of channels; and the fact that all the fibers of the M-channels are laid along each other side by side. This leads to an increase in losses in each measuring channel, an increase in the cost of the final product, as well as the impossibility of laying the measuring channels in different directions, that is, the controlled area for each of the channels is the same.

DISCLOSURE OF INVENTION

The object of the invention is to provide a plurality of measuring channels of a phase-sensitive reflectometer with a single receiving channel without a complex time synchronization circuit of pulses entering the measuring channels and pulses arriving at the receiver, as well as reducing the number of expensive components.

This problem is solved by the proposed multi-channel fiber-optic device for recording vibration effects with one receiving registration module, which includes: highly stable narrow-band radiation source; an acousto-optic modulator with a driver for it to form pulses entering the measuring channels; fiber-optic circulator, which transmits radiation from the source to the measuring channels, as well as backscattered from the measuring channels to the receiving part; optical switch 1xN, where N is the maximum number of polled channels; optical coils at the beginning of the i-th measuring line (used for binding to the length of each channel, if the lengths of two and / or more measuring channels are pairwise identical) or optical coils located at the beginning of each measuring line (used for binding to the distance to the first connector after coils); receiving module consisting of a weak signal amplifier, narrowband filter, signal receiver, ADC; as well as a computing device.

The technical result is achieved due to the fact that the fiber-optic device for recording vibration effects with an increased number of polled channels consists of a highly stable narrow-band radiation source 1, from which the radiation enters the optical signal amplifier (booster) 2, after which it is modulated by an acousto-optic modulator 3 controlled by a driver to the acousto-optic modulator 4, so that subsequent radiation pulses through the circulator 5 (its connector 5.2 in Fig. 1) fall into the optical reklyuchatel 1xN 6 and reaching the end of the i-th channel, time to disperse and to return back through the 1xN switch 6 when it is set to channel i. Next, a weak back-scattered signal from the measuring channels through the circulator 5 after connector 5.3 enters the receiving module and, amplifying on the optical amplifier 7, passes through the narrow-band optical filter 8 to the receiver 9, after which the ADC 10 is digitized for subsequent transfer to the computing device 11, where it is produced final processing of the registered signals, the definition of the working channel and the display of information.

Unlike the prototype, the proposed device additionally contains an optical switch 1xN between connector 5.2 of the optical circulator 5 and the polled N-channels, used in conjunction with determining the current working channel from the polled N-channels by binding to the length of the i-th channel or to the distance to the first connector after the i-th optical coil of several possible optical coils of different lengths, installed in the channels after the 1xN optical switch with equal length between two and / or several channels to establish different optical lengths in the channels. In this case, the difference in the optical length of the channels must be at least the device resolution threshold.

The possibility of carrying out correct measurements without complicated time synchronization of pulses and pulses arriving at the measuring channels arriving at the receiver is accomplished by binding to the conditions: a) the length of the i-th channel, or b) the distance to the first connector after the i-th coil.

When implementing the device by condition a): during the operation of the device, the length of the i-th channel remains unchanged, to determine which channel the signal comes from, possibly by reference to this length.

Under the condition of equal length between two and / or several channels, after the 1xN optical switch, the installation of optical fiber coils of different lengths is provided to establish different lengths in the measuring channels. In this case, the difference in the optical length of the channels should be no less than the resolution of the device. If the channel lengths are different, then the optical coils are not installed.

When implementing the device according to condition b): the optical coils located after the optical switch 1xN have different lengths of optical fiber, and the difference in the lengths in the coils is chosen so that this length is not less than the resolution of the phase-sensitive OTDR.

List of figures

FIG. 1 shows the scheme of the proposed device.

FIG. Figure 2 shows the working recorded signal from channel 1 with coils located at the beginning of the line.

FIG. Figure 3 shows the working recorded signal from channel 2 with coils located at the beginning of the line.

FIG. 4 shows the working recorded signal from channel 1 at various lengths of lines.

FIG. Figure 5 shows the working recorded signal from channel 2 at various lengths of lines.

The implementation of the invention

FIG. 1 shows the scheme of the proposed device. A fiber-optic device for detecting vibration effects with an increased number of polled channels contains: narrow-band radiation source 1, optical signal amplifier (booster) 2, acousto-optic modulator 3, driver of an acousto-optic modulator 4, optical circulator 5, optical switch 1xN 6, N optical fiber coils, one on each of the measuring channels (case a) or M coils of optical fiber (0≤M≤N), at the beginning of the measuring channels provided that the lengths of these channels are equal (case b), optical sky amplifier 7, a narrow-band optical filter 8, the radiation detector 9, the ADC 10, the computing device 11.

Continuous radiation from narrow-band radiation source 1 is amplified by an optical signal amplifier (booster) 2. Next, the continuous amplified radiation hits an acousto-optic modulator 3, controlled by an acousto-optic modulator driver 4, where pulses of duration t and repetition period T are generated from continuous radiation. Pulse duration t is directly related with the energy parameters of the system and, as a result, with the maximum range of the system, as well as with the spatial resolution of the PR system by:

where c is the speed of light; n is the effective refractive index of the fiber.

The repetition period depends on the length of the monitored section L and must comply with the condition that the next impulse will not be formed until backscattered radiation from the end of the line from the previous impulse comes, thus:

The generated pulses of the optical signal through the circulator 5 fall on the optical switch 1xN 6, which with a given sequence switches the measuring channels of the system.

Case a). At the output of each channel of the optical switch 1xN 6 is a coil of optical fiber. Each coil belongs to a specific channel and has a certain length that differs from the others by a minimum of the length of the spatial resolution of the system (or the maximum possible spatial resolution of the system if the system operates in different modes). So, for example, if the spatial resolution of the PR system is ± 5 m (pulse duration τ = 100 ns), then the difference in the coil lengths must be at least Δl = 10 m, i.e. coil 1 has a length l ₁ = 10 m, coil 2 long l ₂ = 20 m, etc., coil N long l _N = l ₁ + (N-1) ⋅Δl. Each coil is connected to a monitored line through an optical patch cord to obtain an accurately detectable peak from the end of a specific coil. Obtaining the values of the length of the peak from the first connector after the coil determines the channel on which the system operates at a particular point in time.

Case b). Coils are put only in the case of equal length channels. Binding occurs along the length of the i-th channel.

Thanks to this scheme, it is possible to carry out correct measurements without complicated time synchronization of the pulses entering the measuring channels and the pulses arriving at the receiver.

The switching period of the optical switch 1xN 6 must be no less than the time required for the impulse to pass the monitored measuring channel and back, and set so that from each channel during one switching cycle, at least one back-scattered signal comes to the radiation receiver 9.

The switching period of the 1xN 6 optical switch can be set constant with the possibility of “cutting off” the extreme (first and / or last) reflectograms provided that at least one uncut backscattered signal is received from each channel.

After returning back the scattered signal from each channel re-passes through the optical circulator 5 and is sent to the receiving module, passing successively the optical amplifier 7 and the narrow-band optical filter 8, enters the radiation receiver 9 and the signal is digitized by the ADC 10. Next, it is transmitted to the computing device 11.

An example of the signal registered by the receiver in the proposed device is shown in FIG. 2, 3, 4, 5. The switching between two (N = 2) channels was chosen as an example. The graphs are visible variants of the trace recorded by the system:

1) The working recorded signal from channel 1 with coils located at the beginning of the line is shown in FIG. 2;

2) The working recorded signal from channel 2 with coils located at the beginning of the line is shown in FIG. 3;

3) The working recorded signal from channel 1 with different line lengths is shown in FIG. four;

4) The working recorded signal from channel 2 with different line lengths is shown in FIG. five;

The working channel is determined by the position of the peak from the coil connector or the output connector, which is individual and permanent for each channel.

In the case of determining the position of the peak from the coil connector, the first visible peak located at the same place on all channels is the peak from the input connector, after which for each channel the peak from the i-ro channel coil connector will be located individually; definition of the working channel. The difference in the length of the coils in this case must be greater than the error in measuring the distance of the system. The distance to the peak from the coil connector of the i-th channel determines the working channel.

In the case of determining the channel by its length (or output connector), the binding is performed by a back-reflected peak at the end of the i-th channel. In this case, the difference in channel lengths should be greater than the error in measuring the distance of the system. The distance to the peak from the output of the i-th channel determines the working channel.

Claims (1)

A multichannel fiber-optic device for detecting vibration effects, which includes: a highly stable narrow-band radiation source 1 connected in series; optical signal amplifier (booster) 2; driver-controlled 4 acousto-optic modulator 3 for the formation of probe pulses; optical circulator 5; receiving registration module, located after the optical circulator and consisting of an optical amplifier 7 of a weak signal backscattered from the measuring channels; narrowband optical filter 8; optical signal receiver 9, which receives signals from all N-channels; analog-to-digital converter 10; after which the computing device 11 is located with the possibility of final processing of the registered signals and information display, characterized in that it additionally contains an optical switch 1xN between the specified optical circulator 5 and the polled N-channels, used in conjunction with determining the current working channel from the polled N-channels due to binding to the length of the i-th channel or to the distance to the first connector after the i-th optical coil of several possible optical coils of different lengths, is set infused into the channels after 1xN optical switch provided between two equal length and / or several channels for establishing different optical length in the channels, the difference of the optical length of the channels should be less than the threshold authorization device.

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