CN111609918A - Optical fiber distributed vibration sensing system based on envelope detection circuit - Google Patents
Optical fiber distributed vibration sensing system based on envelope detection circuit Download PDFInfo
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Abstract
The invention provides an optical fiber distributed vibration sensing system based on an envelope detection circuit, which modulates divided first path direct current laser into pulse light and sends the pulse light to a sensing optical fiber, so that the sensing optical fiber carries out vibration detection based on the pulse light to generate backward transmitted Rayleigh scattering light, then couples second path direct current laser and the Rayleigh scattering light to generate two paths of scattering light interference signals with phase difference of 180 degrees, carries out coherent detection on the two paths of scattering light interference signals, filters out common mode components and obtains beat frequency electric signals. After passing through an envelope detection circuit consisting of full-wave rectification and low-pass filtering, the beat frequency electric signal is shifted from a carrier to a baseband in a frequency domain, so that the requirement of a system on the sampling rate of an acquisition card is reduced; the envelope of the beat frequency electric signal is obtained in the time domain, the digital demodulation in the coherent detection is avoided, and the burden of a processing device is reduced. The invention combines the full-wave rectification circuit with the coherent detection, and improves the sensing distance while ensuring the high signal-to-noise ratio of the coherent detection.
Description
Technical Field
The invention belongs to the field of optical fiber sensing, and particularly relates to an optical fiber distributed vibration sensing system based on an envelope detection circuit.
Background
For the optical fiber distributed sensor, the whole optical fiber serves as a continuous sensing medium, so that a single distributed optical fiber sensor is equivalent to connecting a large number of point sensors together, thereby greatly reducing the cost. The optical fiber distributed vibration sensing can perform distributed detection on the vibration condition of each position on the whole optical fiber link to obtain information such as the position, frequency and size of a vibration signal.
Distributed sensing is typically achieved by techniques such as Optical Time Domain Reflectometry (OTDR) or Optical Frequency Domain Reflectometry (OFDR). The time domain measurement can be carried out in an induction range of tens of kilometers, and the spatial resolution of the time domain measurement can reach a meter or sub-meter level; while the spatial resolution of the frequency domain measurement can reach millimeter or sub-millimeter level, the sensing range is usually limited to several kilometers. In the practical application of long distance such as pipeline detection, boundary security protection and the like, the spatial resolution of meter or sub-meter level can meet the requirement, so that higher requirements are provided for the sensing distance and the signal-to-noise ratio of time domain measurement at present. In the application of the traditional distributed optical fiber sensing system, the detection mode of the scattered light intensity is generally adopted, and the detection mode can be realized through an optical detection structure of direct detection and coherent detection. For the direct detection mode, the structure is simple, the cost is low, but the sensing distance is limited due to the inherent attenuation of the optical fiber; coherent detection has homodyne and heterodyne structures, which can improve signal strength and enhance signal-to-noise ratio, but generally requires digital demodulation to acquire useful signals, increasing the burden of a processing device (i.e., a computer), and the system is difficult to meet the real-time requirement in practical application.
Disclosure of Invention
The invention provides an optical fiber distributed vibration sensing system based on an envelope detection circuit, which aims to solve the problems that the transmission distance is short, the signal-to-noise ratio is low and a processing device needs to carry out a large amount of digital demodulation when the vibration measurement is carried out by an optical fiber based on a time domain measurement technology at present.
According to a first aspect of the embodiments of the present invention, an envelope detection circuit-based optical fiber distributed vibration sensing system is provided, including a laser, a first coupler, a waveform modulator, a circulator, a sensing optical fiber, a second coupler, a balanced detector, an envelope detection circuit, an acquisition card, and a processing device, where an output end of the laser is connected to an input end of the first coupler, a first output end of the first coupler is connected to a first end of the circulator through the waveform modulator, a second end of the circulator is connected to the sensing optical fiber, and a third end of the circulator is connected to a first input end of the second coupler; the second output end of the first coupler is connected with the second input end of the second coupler; the first output end and the second output end of the second coupler are correspondingly connected with the first input end and the second input end of the balance detector, and the output end of the balance detector is connected with the processing device sequentially through the envelope detection circuit and the acquisition card;
the first coupler divides the direct current laser generated by the laser into two paths, the first path of direct current laser is transmitted to the waveform modulator, and the second path of direct current laser is transmitted to the second coupler as reference light;
the waveform modulator modulates the intensity of the first path of direct current laser to form pulse light with a set pulse width, and transmits the pulse light to the circulator, so that the circulator transmits the pulse light to the sensing optical fiber through a second end of the circulator;
after receiving the pulsed light, the sensing optical fiber generates backward-transmitted rayleigh scattered light and transmits the rayleigh scattered light to the circulator, so that the circulator transmits the rayleigh scattered light to the second coupler through a third end of the circulator;
the second coupler couples the reference light and the Rayleigh scattered light to generate two paths of scattered light interference signals with phase difference of 180 degrees;
the balance detector carries out beat frequency on the two paths of scattered light interference signals, filters out common mode components, obtains beat frequency electric signals and sends the beat frequency electric signals to the envelope detection circuit;
the envelope detection circuit performs full-wave rectification on the beat frequency electric signal, performs low-pass filtering on the full-wave rectified beat frequency electric signal, and moves the beat frequency electric signal from a carrier to a baseband so as to obtain a scattering signal;
the acquisition card acquires the scattering signal and sends the scattering signal to the processor;
and the processing device positions the vibration point on the sensing optical fiber according to the scattering signal and obtains the relevant vibration information at the vibration point.
In an optional implementation manner, the processing device obtains the mean square deviation of the intensities of the multiple groups of scattering signals for each point on the sensing optical fiber, compares the mean square deviation of each point, and locates the vibration point on the sensing optical fiber.
In another optional implementation manner, for each vibration point on the sensing optical fiber, the processing device forms a matrix with voltage signals corresponding to multiple sets of scattering signals at the vibration point, and performs fourier analysis on the matrix to obtain the vibration frequency and intensity at the vibration point.
In another alternative implementation, the waveform modulator includes a waveform generator and an acousto-optic modulator, an output terminal of the first coupler is connected to a first input terminal of the acousto-optic modulator, an output terminal of the waveform generator is connected to a second input terminal of the acousto-optic modulator, and an output terminal of the acousto-optic modulator is connected to a first terminal of the circulator;
the waveform generator sends the generated sound wave with the set pulse width to the acousto-optic modulator;
the acousto-optic modulator performs acousto-optic modulation on the first path of direct current laser by using the sound wave with the set pulse width to generate pulse light with the set pulse width.
In another optional implementation manner, the waveform modulator is connected to the first end of the circulator sequentially through a first optical amplifier and a first optical filter, and the first optical amplifier performs optical amplification processing on pulsed light with a set pulse width formed by the waveform modulator; the first optical filter filters the pulse light after the light amplification treatment so as to filter noise introduced by the amplification treatment;
the third end of the circulator is connected with the second input end of the balance detector sequentially through a second optical amplifier and a second optical filter, and the second optical amplifier performs optical amplification processing on Rayleigh scattered light output by the third end of the circulator; and the second optical filter filters the Rayleigh scattered light subjected to the light amplification treatment so as to filter noise introduced by the amplification treatment.
In another optional implementation manner, the envelope detection circuit includes a full-wave rectification circuit and a low-pass filter, wherein an input end of the full-wave rectification circuit is connected to an output end of the balanced detector, an output end of the full-wave rectification circuit is connected to an input end of the low-pass filter, and an output end of the low-pass filter is connected to the acquisition card;
the full-wave rectification circuit is used for performing full-wave rectification on the beat frequency electric signal;
the low-pass filter is used for performing low-pass filtering on the full-wave rectified beat frequency electric signal and moving the beat frequency electric signal from a carrier to a baseband.
In another optional implementation manner, the full-wave rectification circuit comprises first to fifth resistors, a first diode, a second diode, a first operational amplifier and a second operational amplifier, wherein a first end of the first resistor is used as an input end of the full-wave rectification circuit and is connected with an output end of the balanced detector; the first end of the first resistor is connected with the output end of the second operational amplifier through a third resistor and a fifth resistor in sequence, the second end of the first resistor is connected with the anode of the second diode through the second resistor and is connected with the cathode of the first diode and the inverting input end of the first operational amplifier, the non-inverting input end of the first operational amplifier is grounded, the output end of the first operational amplifier is connected with the cathode of the second diode, and the anode of the first diode is connected with the cathode of the second diode;
the positive pole of the second diode is sequentially connected with the middle node of the third resistor and the fifth resistor and the inverting input end of the second operational amplifier through the fourth resistor, the non-inverting input end of the second operational amplifier is grounded, and the output end of the second operational amplifier is used as the output end of the full-wave rectifying circuit and is connected with the input end of the low-pass filter.
In another alternative implementation manner, the low-pass filter includes a sixth resistor, a capacitor and a third operational amplifier, wherein a first end of the sixth resistor is used as an input end of the low-pass filter and is connected with an output end of the full-wave rectification circuit;
the second end of the seventh resistor is connected with the non-inverting input end of the third operational amplifier and is grounded through the capacitor, the inverting input end of the third operational amplifier is connected with the output end of the third operational amplifier, and the output end of the third operational amplifier is used as the output end of the low-pass filter and is connected with the acquisition card.
In another optional implementation manner, when the pulse width of the pulsed light is fixed, the spatial resolution of vibration detection is fixed, the effective spectrum width in the beat frequency electrical signal is fixed, the low-pass filter is used to perform low-pass filtering processing on the beat frequency electrical signal, and when the beat frequency electrical signal is moved from a carrier to a baseband, the frequency spectrum cost in the beat frequency electrical signal within the effective spectrum width range is not filtered, and meanwhile, other high-frequency costs in the beat frequency electrical signal are filtered.
In another alternative implementation, the repetition period T of the pulsed light is greater than the total time length of the pulsed light transmitted back and forth in the sensing fiber.
The invention has the beneficial effects that:
1. compared with direct detection, the invention has wider detection range by combining coherent detection and envelope detection, ensures the sensing distance and can save the digit of the acquisition card. Because light is transmitted in the optical fiber with fixed transmission loss, the traditional direct detection mode detects a light intensity signal; coherent detection detects the amplitude signal of light, and the light intensity is proportional to the square of the amplitude of the light, which corresponds to the square of the attenuation of the light. The combination of coherent detection and envelope detection keeps the characteristic of low attenuation loss of coherent detection, thereby having wider detection range than direct detection and ensuring the sensing distance. Compared with direct detection, the detection mode has smaller attenuation under the condition of detecting the same distance, thereby saving the digit of the acquisition card.
2. The demodulation mode provided by the invention ensures the advantages of high common mode rejection ratio and high signal-to-noise ratio of coherent detection, and has higher signal-to-noise ratio compared with direct detection.Regardless of the direct detection or coherent detection method, the noise mainly contains the following components: shot noise, thermal noise, relative intensity noise, and P exists between the signal-to-noise ratio of coherent detection and the signal-to-noise ratio of direct detection when the signal increase due to coherent detection is greater than the noise increase due to shot and RINL/PSIn a proportional relationship of (1), wherein PLIs a reference optical power, PSTo detect optical power. The proposed combination of coherent detection and envelope detection preserves this high signal-to-noise ratio.
3. The demodulation mode provided by the invention avoids digital demodulation in coherent detection, thereby reducing the burden of a computer. In the conventional coherent detection process, if the envelope of the scattering signal is to be obtained, the envelope of the scattering signal is usually obtained by means of digital demodulation, and the process increases the burden of a computer, and the requirement on real-time performance in practical application is difficult to meet. The demodulation method adopts full-wave rectification and low-pass filtering, and the circuit is used for obtaining the envelope of the scattering signal, so that the burden of a computer is reduced, and the requirement of the system on real-time performance is improved.
4. The demodulation mode provided by the invention moves the signal from the high-frequency carrier to the baseband, and reduces the requirement of the system on the sampling rate of the acquisition card, thereby reducing the cost. For coherent detection, the scattered signal is loaded onto a certain carrier. The proposed demodulation mode moves signals from a high-frequency carrier to a baseband in a full-wave rectification and low-pass filtering mode, reduces the requirement of a system on the acquisition rate of an acquisition card due to a sampling theorem, and further reduces the cost.
5. The demodulation mode provided by the invention can filter high-frequency noise in the detection signal while ensuring the spatial resolution of the system, and can improve the signal-to-noise ratio of the detection system. The filtering function is designed in the demodulation mode of coherent detection and envelope detection, and the cut-off frequency of the low-pass filter can be changed by adjusting the value of the resistor and the capacitor, so that the cut-off frequency is slightly larger than the bandwidth of a detection signal, thereby ensuring the spatial resolution of a system, filtering high-frequency noise and improving the signal-to-noise ratio.
Drawings
FIG. 1 is a schematic structural diagram of an embodiment of a fiber-optic distributed vibration sensing system based on an envelope detection circuit according to the present invention;
FIG. 2 is a circuit diagram of the envelope detection circuit of the present invention;
FIG. 3 is a frequency-intensity spectrum of a beat frequency electrical signal output by a balanced detector;
FIG. 4 is a frequency-intensity spectrum of a beat frequency electrical signal processed by an envelope detection circuit;
FIG. 5 is a schematic structural diagram of another embodiment of the optical fiber distributed vibration sensing system based on the envelope detection circuit according to the present invention;
FIG. 6 is a schematic comparison of curve analysis using direct detection and detection according to the present invention;
FIG. 7 is a comparison of spectral analysis using direct detection and detection by the present invention.
Detailed Description
In order to make the technical solutions in the embodiments of the present invention better understood and make the above objects, features and advantages of the embodiments of the present invention more comprehensible, the technical solutions in the embodiments of the present invention are described in further detail below with reference to the accompanying drawings.
In the description of the present invention, unless otherwise specified and limited, it is to be noted that the term "connected" is to be interpreted broadly, and may be, for example, a mechanical connection or an electrical connection, or a communication between two elements, or may be a direct connection or an indirect connection through an intermediate medium, and a specific meaning of the term may be understood by those skilled in the art according to specific situations.
Referring to fig. 1, a schematic structural diagram of an embodiment of the optical fiber distributed vibration sensing system based on an envelope detection circuit according to the present invention is shown. The optical fiber distributed vibration sensing system based on the envelope detection circuit can comprise a laser, a first coupler, a waveform modulator, a circulator, a sensing optical fiber, a second coupler, a balance detector, an envelope detection circuit, an acquisition card and a processing device, wherein the output end of the laser is connected with the input end of the first coupler, the first output end of the first coupler is connected with the first end of the circulator through the waveform modulator, the second end of the circulator is connected with the sensing optical fiber, and the third end of the circulator is connected with the first input end of the second coupler; the second output end of the first coupler is connected with the second input end of the second coupler; the first output end and the second output end of the second coupler are correspondingly connected with the first input end and the second input end of the balanced detector, and the output end of the balanced detector is connected with the processing device (such as a computer and the like) through the envelope detection circuit and the acquisition card in sequence.
The first coupler divides the direct current laser generated by the laser into two paths, the first path of direct current laser is transmitted to the waveform modulator, and the second path of direct current laser is transmitted to the second coupler as reference light; the waveform modulator modulates the intensity of the first path of direct current laser to form pulse light with a set pulse width, and transmits the pulse light to the circulator, so that the circulator transmits the pulse light to the sensing optical fiber through a second end of the circulator; after receiving the pulsed light, the sensing optical fiber generates backward-transmitted rayleigh scattered light and transmits the rayleigh scattered light to the circulator, so that the circulator transmits the rayleigh scattered light to the second coupler through a third end of the circulator; the second coupler couples the reference light and the Rayleigh scattering light to generate two paths of scattering light interference signals with phase difference of 180 degrees, and transmits the two paths of scattering light interference signals to the balanced detector; the balance detector carries out beat frequency on the two paths of scattered light interference signals, filters out common mode components, obtains beat frequency electric signals and sends the beat frequency electric signals to the envelope detection circuit; the envelope detection circuit performs full-wave rectification on the beat frequency electric signal, performs low-pass filtering on the full-wave rectified beat frequency electric signal, and moves the beat frequency electric signal from a carrier to a baseband so as to obtain a scattering signal; the acquisition card acquires the scattering signal and sends the acquired scattering signal to the processor; and the processing device positions the vibration point on the sensing optical fiber according to the scattering signal and obtains the relevant vibration information at the vibration point.
In this embodiment, the envelope detection circuit may include a full-wave rectification circuit and a low-pass filter, wherein an input end of the full-wave rectification circuit is connected to an output end of the balanced detector, an output end of the full-wave rectification circuit is connected to an input end of the low-pass filter, and an output end of the low-pass filter is connected to the acquisition card; the full-wave rectification circuit is used for performing full-wave rectification on the beat frequency electric signal; the low-pass filter is used for performing low-pass filtering on the full-wave rectified beat frequency electric signal and moving the beat frequency electric signal from a carrier to a baseband.
As shown in fig. 2, the full-wave rectification circuit may include first to fifth resistors R1 to R5, a first diode VD1, a second diode VD2, a first operational amplifier a1, and a second operational amplifier a2, wherein a first end of the first resistor R1 is used as an input end of the full-wave rectification circuit and is connected to an output end of the balanced detector; a first end of the first resistor R1 is connected with an output end of the second operational amplifier a2 sequentially through a third resistor R3 and a fifth resistor R5, a second end of the first resistor R1 is connected with an anode of the second diode VD2 through a second resistor R2 and is connected with a cathode of the first diode VD1 and an inverting input end of the first operational amplifier a1, a non-inverting input end of the first operational amplifier a1 is grounded, an output end of the first resistor R1 is connected with a cathode of the second diode VD2, and an anode of the first diode VD1 is connected with a cathode of the second diode VD 2; the positive electrode of the second diode VD2 is sequentially connected to the intermediate node between the third resistor R3 and the fifth resistor R5 and the inverting input terminal of the second operational amplifier a2 through the fourth resistor R4, the non-inverting input terminal of the second operational amplifier a2 is grounded, and the output terminal thereof is connected to the input terminal of the low-pass filter as the output terminal of the full-wave rectifier circuit. The low pass filter may include a sixth resistor R6, a capacitor C, and a third operational amplifier A3, wherein a first end of the sixth resistor R6 is connected to an output end of the full wave rectifying circuit as an input end of the low pass filter; the second end of the sixth resistor R6 is connected to the non-inverting input terminal of the third operational amplifier A3 and grounded through the capacitor C, the inverting input terminal of the third operational amplifier A3 is connected to the output terminal thereof, and the output terminal of the third operational amplifier A3 is used as the output terminal of the low pass filter and connected to the acquisition card.
Since the spectral components of the rayleigh scattering signal are directly related to the pulse width of the pulsed light, as shown in fig. 3 and 4, the curves from top to bottom in fig. 3 are used to respectively represent the frequency-intensity spectrum of the beat frequency electrical signal output by the balanced detector for the pulsed light with gradually increasing pulse width. As can be seen from fig. 3, as the pulse width of the pulsed light gradually increases, the spectrum width of the beat frequency electrical signal output by the balanced detector gradually decreases. Similarly, the curves from top to bottom in fig. 4 are used to respectively show the frequency-intensity spectrum of the beat frequency electric signal processed by the envelope detection circuit for the pulsed light with gradually increasing pulse width. As can be seen from fig. 4, after the beat frequency electric signal is processed by the envelope detection circuit, the beat frequency electric signal is shifted from the carrier to the baseband. Since the width of the pulsed light determines the spatial resolution of the system, when the spatial resolution of the system is required to be constant, the width of the pulsed light is constant, and accordingly the effective spectrum component of the rayleigh scattering signal is also constant. Therefore, the invention introduces an envelope detection circuit to carry out low-pass filtering on the beat frequency electric signal output by the balance detector and move the beat frequency electric signal from the carrier to the baseband, thereby on one hand, unnecessary frequency spectrum components can be filtered and the signal-to-noise ratio is improved, on the other hand, the beat frequency electric signal is moved from the carrier to the baseband, the requirement of the system on the sampling rate of the acquisition card is reduced, and the sampling cost is reduced. When the pulse width of the pulse light is fixed, the spatial resolution of vibration detection is fixed, the effective spectrum width in the beat frequency electric signal is fixed, the low-pass filter is used for performing low-pass filtering processing on the beat frequency electric signal, and when the beat frequency electric signal is moved from a carrier to a baseband, the low-pass filter is used for ensuring that the spectrum cost in the beat frequency electric signal within the effective spectrum width range is not filtered, and simultaneously other high-frequency cost in the beat frequency electric signal needs to be filtered, so that the selection of the size of a sixth resistor R6 and a capacitor C in the low-pass filter is particularly important, different sixth resistors R6 and capacitors C are selected according to different spatial resolution requirements, and the cut-off frequency of the low-pass filter is fixed
The low-pass filter adopts an active filter circuit, so that the filter parameter can not change along with the change of the load. In order to facilitate vibration detection, the repetition period T of the pulse light is larger than the total time length of the pulse light transmitted back and forth in the sensing optical fiber.
According to the embodiment, the direct current laser is divided into two paths by the first coupler, one path of direct current laser and Rayleigh scattered light reversely transmitted back by the sensing optical fiber are subjected to coherent detection, full-wave rectification is performed on beat frequency electric signals obtained after the coherent detection, the coherent detection and the full-wave rectification are combined, the high signal-to-noise ratio of the coherent detection is ensured, the detection range can be greatly improved, and the sensing distance is ensured; according to the invention, a first path of direct current laser is modulated into pulse light with a set pulse width and sent to a sensing optical fiber, so that the sensing optical fiber performs vibration detection based on the pulse light to generate backward transmitted Rayleigh scattering light, then a second path of direct current laser and the Rayleigh scattering light are coupled to generate two paths of scattering light interference signals with a phase difference of 180 degrees, then a balance detector is used for performing coherent detection, the two paths of scattering light interference signals are subjected to beat frequency, common mode components are filtered, and then a full-wave detection circuit is used for performing full-wave rectification and low-pass filtering on the beat frequency electric signals, so that not only can the signal-to-noise ratio be improved, but also almost all unnecessary signals introduced when the sensing optical fiber performs vibration detection based on the pulse light can be filtered, and therefore, digital demodulation in coherent detection; the envelope detection circuit moves the beat frequency electric signal from a carrier to a baseband, so that the requirement of a system on the sampling rate of a collecting card is reduced, and the sampling cost is reduced; in conclusion, the invention carries out full-wave rectification and low-pass filtering on the beat frequency electric signal, and moves the beat frequency electric signal from a carrier to a baseband in a frequency domain, thereby reducing the requirement of a system on the sampling rate of a collecting card; the envelope of the beat frequency electric signal is obtained in the time domain, so that the digital demodulation in the coherent detection is avoided, and the burden of a processing device is reduced. In addition, the whole system of the invention can realize the positioning of the vibration point and the related vibration information at the vibration point.
The transmitted light intensity of the sensing optical fiber is almost unchanged under the condition of no external disturbance aiming at the same position on the sensing optical fiber, but once vibration occurs, the external vibration can cause the change of the axial length and the refractive index of the sensing optical fiber, so that the phase of Rayleigh scattered light is influenced, and the signal intensity reflected on the intensity is changed along with time. By utilizing the principle, the processing device obtains the intensity of a plurality of groups of scattering signals for each point on the sensing optical fiber to calculate the mean square error, compares the mean square error of each point and positions the vibration point on the sensing optical fiber. When a certain position on the sensing optical fiber vibrates, the amplitude of the change of the signal intensity at the position along with the time is possibly small and is difficult to detect, so that the invention does not judge whether the signal intensity at the position changes along with the time aiming at each point on the sensing optical fiber when positioning the vibration point, but judges whether the signal intensity at the point changes along with the time aiming at each point on the sensing optical fiber by taking the intensity of a plurality of groups of scattering signals to calculate the mean square error, thereby enlarging the vibration intensity difference of the vibration point, and when the mean square error of a certain point is obviously larger than that of other points, the point is taken as a positioning point. In addition, the processing device can combine a plurality of groups of voltage signals corresponding to the vibration points of the scattering signals into a matrix aiming at each vibration point on the sensing optical fiber, and perform Fourier analysis on the matrix to obtain vibration frequency and intensity information of the vibration point, so that the detection accuracy of the vibration information is high.
Referring to fig. 5, it is a schematic structural diagram of another embodiment of the optical fiber distributed vibration sensing system based on the envelope detection circuit according to the present invention. FIG. 5 differs from the embodiment of FIG. 1 in that the waveform modulator may include a waveform generator and an acousto-optic modulator, the output of the first coupler being connected to a first input of the acousto-optic modulator, the output of the waveform generator being connected to a second input of the acousto-optic modulator, the output of the acousto-optic modulator being connected to a first end of the circulator; the waveform generator sends the generated sound wave with the set pulse width to the acousto-optic modulator; the acousto-optic modulator performs acousto-optic modulation on the first path of direct current laser by using the sound wave with the set pulse width to generate pulse light with the set pulse width. The waveform modulator is connected to the first end of the circulator in this order through a first optical amplifier and a first optical filter, and the first optical amplifier optically amplifies pulse light having a set pulse width formed by the waveform modulator; the first optical filter filters the pulse light after the light amplification treatment so as to filter noise introduced by the amplification treatment; the third end of the circulator is connected with the second input end of the balance detector sequentially through a second optical amplifier and a second optical filter, and the second optical amplifier performs optical amplification processing on Rayleigh scattered light output by the third end of the circulator; and the second optical filter filters the Rayleigh scattered light subjected to the light amplification treatment so as to filter noise introduced by the amplification treatment.
In one example, a narrow linewidth laser with a center wavelength of 1550.12nm and a linewidth of 200Hz emits continuous laser light, which is divided into two paths by a 90:10 coupler, wherein the path with low power is used as reference light, the path with high power is modulated by a 200MHz acousto-optic modulator, and an arbitrary waveform generator is used for driving the acousto-optic modulator to modulate direct current laser light into pulsed light with a pulse width of 50ns and a repetition period of 520 mus. The pulse light is amplified by the optical amplifier and then filtered by the optical filter to filter noise introduced by amplification, and then enters from the first port of the circulator and enters into the sensing optical fiber of 52km (the sensing optical fiber is a common single-mode optical fiber) through the second port of the circulator. Applying a vibration signal of 200Hz to the position 51.24km of the sensing optical fiber, enabling backward scattering light to enter through a second port of the circulator, emitting from a third port, amplifying through an optical amplifier, filtering through an optical filter, and coupling with reference light through a 1:1 optical coupler so as to generate a beat frequency signal with the center frequency of 200MHz, wherein two output ends of the coupler are respectively connected with two input ends of a double-balanced detector, and the balance detector converts the optical signal into an electric signal. The output of the balance detector is connected to an envelope detection circuit for demodulation, and then is collected by a collection card, the sampling rate of the collection card is 2Gsa/s, and the sampling time is longer than the period of the vibration signal. The acquisition card transmits the acquired signals to the computer, and the computer stores and calculates the signals. Fig. 6(a) and (c) are graphs corresponding to the case of direct detection, and fig. 6(b) and (d) are graphs corresponding to the case of detection using the present invention. From the comparative analysis of fig. 6, it can be seen that the scatter signal can be detected both with the direct detection and with the detection of the present invention, but a stronger scatter signal can be obtained with the detection of the present invention. In addition, the upper curve in fig. 7 is a spectrum curve of a scattering signal obtained when the present invention is used for detection, and the lower curve is a spectrum curve of a scattering signal obtained when direct detection is used, and as can be seen from the comparison in fig. 7, the frequency of the scattering signal obtained in both the two ways is 200Hz, but the signal-to-noise ratio is obviously higher by about 8dB when the present invention is used for detection than when direct detection is used.
Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. This application is intended to cover any variations, uses, or adaptations of the invention following, in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice within the art to which the invention pertains. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.
It will be understood that the invention is not limited to the precise arrangements described above and shown in the drawings and that various modifications and changes may be made without departing from the scope thereof. The scope of the invention is to be controlled solely by the appended claims.
Claims (10)
1. An optical fiber distributed vibration sensing system based on an envelope detection circuit is characterized by comprising a laser, a first coupler, a waveform modulator, a circulator, a sensing optical fiber, a second coupler, a balance detector, an envelope detection circuit, an acquisition card and a processing device, wherein the output end of the laser is connected with the input end of the first coupler, the first output end of the first coupler is connected with the first end of the circulator through the waveform modulator, the second end of the circulator is connected with the sensing optical fiber, and the third end of the circulator is connected with the first input end of the second coupler; the second output end of the first coupler is connected with the second input end of the second coupler; the first output end and the second output end of the second coupler are correspondingly connected with the first input end and the second input end of the balance detector, and the output end of the balance detector is connected with the processing device sequentially through the envelope detection circuit and the acquisition card;
the first coupler divides the direct current laser generated by the laser into two paths, the first path of direct current laser is transmitted to the waveform modulator, and the second path of direct current laser is transmitted to the second coupler as reference light;
the waveform modulator modulates the intensity of the first path of direct current laser to form pulse light with a set pulse width, and transmits the pulse light to the circulator, so that the circulator transmits the pulse light to the sensing optical fiber through a second end of the circulator;
after receiving the pulsed light, the sensing optical fiber generates backward-transmitted rayleigh scattered light and transmits the rayleigh scattered light to the circulator, so that the circulator transmits the rayleigh scattered light to the second coupler through a third end of the circulator;
the second coupler couples the reference light and the Rayleigh scattered light to generate two paths of scattered light interference signals with phase difference of 180 degrees;
the balance detector carries out beat frequency on the two paths of scattered light interference signals, filters out common mode components, obtains beat frequency electric signals and sends the beat frequency electric signals to the envelope detection circuit;
the envelope detection circuit performs full-wave rectification on the beat frequency electric signal, performs low-pass filtering on the full-wave rectified beat frequency electric signal, and moves the beat frequency electric signal from a carrier to a baseband so as to obtain a scattering signal;
the acquisition card acquires the scattering signal and sends the scattering signal to the processor;
and the processing device positions the vibration point on the sensing optical fiber according to the scattering signal and obtains the relevant vibration information at the vibration point.
2. The envelope detection circuit-based optical fiber distributed vibration sensing system of claim 1, wherein said processing means finds the mean-square-difference of the intensities of the multiple sets of scattered signals for each point on the sensing optical fiber, compares the mean-square-difference of each point, and locates the vibration point on the sensing optical fiber.
3. The envelope detection circuit-based optical fiber distributed vibration sensing system according to claim 1 or 2, wherein the processing device forms a matrix from voltage signals corresponding to a plurality of groups of scattered signals at each vibration point on the sensing optical fiber, and performs fourier analysis on the matrix to obtain the vibration frequency and intensity at the vibration point.
4. The envelope detection circuit-based fiber optic distributed vibration sensing system of claim 1 wherein said waveform modulator comprises a waveform generator and an acousto-optic modulator, an output of said first coupler being connected to a first input of said acousto-optic modulator, an output of said waveform generator being connected to a second input of said acousto-optic modulator, an output of said acousto-optic modulator being connected to a first end of said circulator;
the waveform generator sends the generated sound wave with the set pulse width to the acousto-optic modulator;
the acousto-optic modulator performs acousto-optic modulation on the first path of direct current laser by using the sound wave with the set pulse width to generate pulse light with the set pulse width.
5. The envelope detection circuit-based optical fiber distributed vibration sensing system according to claim 1 or 4, wherein the waveform modulator is connected to the first end of the circulator through a first optical amplifier and a first optical filter in sequence, and the first optical amplifier optically amplifies the pulse light with a set pulse width formed by the waveform modulator; the first optical filter filters the pulse light after the light amplification treatment so as to filter noise introduced by the amplification treatment;
the third end of the circulator is connected with the second input end of the balance detector sequentially through a second optical amplifier and a second optical filter, and the second optical amplifier performs optical amplification processing on Rayleigh scattered light output by the third end of the circulator; and the second optical filter filters the Rayleigh scattered light subjected to the light amplification treatment so as to filter noise introduced by the amplification treatment.
6. The envelope detection circuit-based fiber optic distributed vibration sensing system of claim 1, wherein said envelope detection circuit comprises a full-wave rectifying circuit and a low-pass filter, wherein an input of said full-wave rectifying circuit is connected to an output of said balanced detector, an output of said full-wave rectifying circuit is connected to an input of said low-pass filter, and an output of said low-pass filter is connected to said acquisition card;
the full-wave rectification circuit is used for performing full-wave rectification on the beat frequency electric signal;
the low-pass filter is used for performing low-pass filtering on the full-wave rectified beat frequency electric signal and moving the beat frequency electric signal from a carrier to a baseband.
7. The envelope detection circuit-based fiber distributed vibration sensing system of claim 6, wherein said full-wave rectification circuit comprises first to fifth resistors, a first diode, a second diode, a first operational amplifier and a second operational amplifier, wherein a first end of the first resistor is connected to an output terminal of the balanced detector as an input terminal of the full-wave rectification circuit;
the first end of the first resistor is connected with the output end of the second operational amplifier through a third resistor and a fifth resistor in sequence, the second end of the first resistor is connected with the anode of the second diode through the second resistor and is connected with the cathode of the first diode and the inverting input end of the first operational amplifier, the non-inverting input end of the first operational amplifier is grounded, the output end of the first operational amplifier is connected with the cathode of the second diode, and the anode of the first diode is connected with the cathode of the second diode;
the positive pole of the second diode is sequentially connected with the middle node of the third resistor and the fifth resistor and the inverting input end of the second operational amplifier through the fourth resistor, the non-inverting input end of the second operational amplifier is grounded, and the output end of the second operational amplifier is used as the output end of the full-wave rectifying circuit and is connected with the input end of the low-pass filter.
8. The envelope detection circuit-based fiber distributed vibration sensing system according to claim 6 or 7, wherein said low pass filter comprises a sixth resistor, a capacitor and a third operational amplifier, wherein a first end of the sixth resistor is connected to an output end of the full wave rectification circuit as an input end of the low pass filter;
the second end of the sixth resistor is connected with the non-inverting input end of the third operational amplifier and is grounded through the capacitor, the inverting input end of the third operational amplifier is connected with the output end of the third operational amplifier, and the output end of the third operational amplifier is used as the output end of the low-pass filter and is connected with the acquisition card.
9. The envelope detection circuit-based optical fiber distributed vibration sensing system according to claim 6, wherein when the pulse width of the pulse light is constant, the spatial resolution of vibration detection is constant, the effective spectrum width in the beat frequency electrical signal is constant, and when the beat frequency electrical signal is subjected to low-pass filtering processing by using the low-pass filter and is shifted from a carrier to a baseband, the spectrum cost in the beat frequency electrical signal within the effective spectrum width range is ensured not to be filtered, and other high-frequency costs in the beat frequency electrical signal are filtered.
10. The envelope detection circuit-based fiber optic distributed vibration sensing system of claim 1, wherein the repetition period T of the pulsed light is greater than the total length of time that the pulsed light travels back and forth in the sensing fiber.
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