CN117335871A - Three-acousto-optic coherent optical time domain reflection system for low-frequency noise suppression - Google Patents
Three-acousto-optic coherent optical time domain reflection system for low-frequency noise suppression Download PDFInfo
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Abstract
The invention belongs to the field of low-frequency signal detection of a coherent light time domain reflection system, and particularly relates to a three-acousto-optic coherent light time domain reflection system for low-frequency noise suppression. According to the invention, three pulses with different frequencies are generated by constructing the three acousto-optic modulation modules, the intermediate frequency band signals are used as main signals, two adjacent frequency bands are used as reference signals, and the phase difference reference compensation mode is used to effectively inhibit low-frequency noise, so that the signal-to-noise ratio of the low-frequency signals is improved. The invention effectively suppresses the low-frequency noise of the coherent optical time domain reflection system caused by the frequency drift of the laser, better restores the time domain signal of the low-frequency noise, improves the signal to noise ratio of the low-frequency signal, and provides a new thought and solution for realizing the low-frequency noise suppression of the coherent optical time domain reflection system, thereby forcefully pushing the application and development of the coherent optical time domain reflection system in the field of the low-frequency signal.
Description
Technical Field
The invention belongs to the field of low-frequency signal detection of a coherent light time domain reflection system, and particularly relates to a three-acousto-optic coherent light time domain reflection system for low-frequency noise suppression.
Background
A distributed acoustic wave sensing system (DAS) is a sensing system that utilizes optical fibers as sensing elements and signal transmission media to obtain dynamic strain information. The DAS system can realize remote, distributed and multipoint real-time quantitative detection of dynamic strain and has the advantages of simple structure, long sensing distance, high detection precision, strong environmental adaptability and the like. Therefore, the distributed acoustic wave sensor system is widely applied to various national defense and industrial fields such as perimeter safety, resource exploration, oil and gas pipeline detection, communication line detection, high-speed rail, ship, airport monitoring and the like.
Phase-sensitive optical time domain reflectometry (Φ -OTDR) has attracted considerable interest from researchers due to its advantages of distributed sensing in the sensing field, cost effectiveness, wide dynamic range, good spatial resolution and high accuracy, especially in geophysical applications, becoming an ideal choice for industrial applications and replacing traditional geophones, which may require better performance of Φ -OTDR in the low frequency region. However, in the phase demodulation of Φ -OTDR, when the frequency of the disturbance signal is high, the laser frequency drift will cause a phase accumulation on the time axis, which will introduce disturbance information to the demodulated signal. When the disturbance frequency is low and near the laser frequency drift, the disturbance signal cannot be distinguished from the disturbance caused by the frequency drift. In addition, the influence of various noises such as single-mode fiber low Rayleigh scattering intensity, coherent fading, pulse modulation noise and the like is limited, and the sensing performance of the system is reduced, so that the application of phi-OTDR in ultra-low frequency detection is limited. In order to improve the low frequency detection performance of Φ -OTDR, many researchers have studied laser frequency drift, low frequency noise suppression, and the like.
Ideally, the rayleigh back-scattering returned from the sensing fiber exhibits a stable scattering curve in the time domain when there is no disturbance on the fiber under test. However, since the optical phase at a certain location is related to the frequency of the light wave, the frequency variation of the laser source changes the phase difference between the two points, resulting in a variation of the speckle pattern of the backscattered signal. It has been demonstrated in 2015 that commercial lasers always have slow frequency drift, which can cause significant noise in the low frequency domain, limiting the ability of Φ -OTDR to measure low frequency events.
Disclosure of Invention
Aiming at the problems or the defects, the invention provides a three-acousto-optic coherent optical time domain reflection system for low-frequency noise suppression, which is used for generating detection signals with three frequencies by utilizing the three-acousto-optic coherent optical time domain reflection system and realizing low-frequency noise suppression by selecting a main signal and a reference signal to perform phase difference compensation in order to solve the problem that the low-frequency application of the existing phi-OTDR is limited due to the fact that the frequency drift of a laser causes significant noise in a low frequency domain during measurement.
A three-acousto-optic coherent optical time domain reflection system for low-frequency noise suppression comprises a narrow linewidth laser, optical fiber couplers 1, 2, 3 and 4, an acousto-optic modulator 1, 2 and 3, an erbium-doped optical fiber amplifier, a band-pass filter, a circulator, a balance detector, an acquisition card, an arbitrary waveform generator and an upper computer.
The narrow linewidth laser generates signal light, the signal light is divided into two paths through the 1*2 optical fiber coupler 1, wherein at least 80% of the signal light is input into the three-acousto-optic modulation module, and the rest part is used as intrinsic light to be input into the 1*2 coupler 4.
The optical fiber couplers 2 and 3 and the acousto-optic modulators 1, 2 and 3 form a tri-acousto-optic modulation module; the signal light with the ratio not lower than 80% is input into the 1*3 optical fiber coupler 2, split by equal proportion and respectively injected into the acoustic-optic modulator 1, the acoustic-optic modulator 2 and the acoustic-optic modulator 3, and 3 acoustic-optic modulator modulation pulses are controlled by an arbitrary waveform generator to be pulse light with three different frequencies; finally, the pulse light after being polymerized by the 1*3 coupler 3 is transmitted to the erbium-doped fiber amplifier.
The erbium-doped fiber amplifier amplifies the polymerized pulse light, then filters noise of other light wavelengths except the light source wavelength through a band-pass filter, the filtered signal is injected into the tested optical fiber through a circulator, and Rayleigh scattered light returned by the tested optical fiber is transmitted to a 1*2 coupler 4 polymerized in equal proportion through the circulator to beat frequency with the intrinsic light, and then is received by the balance detector. The received signals are converted into digital signals through the acquisition card and transmitted to the upper computer for signal processing, and the acquisition card is triggered synchronously by pulse signals given by the arbitrary waveform generator and the acousto-optic modulator.
Further, the arbitrary waveform generator controls the three acousto-optic modulators to generate detection light pulses with time delay pulses, and the time delay meets the following requirements:
τ d >τ+PW1
τ d >τ+PW2
τ d >τ+PW3
τ is the propagation time of pulse signals in a disturbance area (namely measuring distance) in the optical fiber, PW1, PW2 and PW3 are 3 pulses modulated by an arbitrary waveform generator, the pulses are sequentially triggered, and the time delay between two adjacent pulses is τ d . At any time, ensure that when one frequency sweeps through the range, the other two frequencies are near but outside the range, τ d The larger the value, the further two frequencies are from the distance. When the results of the three pulses are mapped to the same position axis, the vibration position is found first, the middle position signal of the time sequence is used as a main signal, and the signals at the two sides are used as reference signals for the frequency drift compensation of the laser. Thus, the intermediate signal contains the vibration signal and the frequency drift signal, while the two-sided signal contains only laser frequency drift information that is highly similar to the intermediate signal. Therefore, the phase noise caused by the frequency drift of the laser can be restrained by using the phase difference reference signal, and the signal-to-noise ratio of the low-frequency signal of the coherent optical time domain reflection system is improved.
Further, the probing frequency of the probing light pulses generated by the acousto-optic modulator must be properly selected to have minimal phase correlation. In order to eliminate the phase correlation between every two frequencies and with independent probing, the different frequencies must satisfy:
△f＝f 3 -f 2 ＝f 2 -f 1 ≥v g /4L
where Δf is the frequency difference of the two acousto-optic modulators modulation, f 1 、f 2 、f 3 Is the corresponding modulation frequency of the acousto-optic modulator 1, 2, 3,v g is the group velocity in the fiber and L is the distance length of the disturbance zone, i.e. the physical distance of the measurement gap.
In summary, the present invention provides a three acousto-optic coherent optical time domain reflection system for low frequency noise suppression, which generates three pulses with different frequencies through a three acousto-optic modulation module, uses an intermediate frequency band signal as a main signal, uses two adjacent frequency bands as reference signals, and uses a phase difference reference compensation mode to effectively suppress low frequency noise, thereby improving the signal-to-noise ratio of the low frequency signal. The invention can further promote the practical engineering application of the coherent optical time domain reflection system in the field of low-frequency signal detection, and provides a new thought and solving method for realizing the low-frequency noise suppression of the coherent optical time domain reflection system.
Drawings
FIG. 1 is a block diagram of the structure of the present invention;
FIG. 2 is a schematic diagram of the pulse modulation of the present invention;
FIG. 3 is a graph of modulation spectrum of a three-acousto-optic system according to an embodiment;
FIG. 4 is a diagram of a three-acousto-optic system signal demodulation phase space-time waterfall diagram according to an embodiment;
fig. 5 is a diagram showing a comparison of (a) time domain and (b) frequency domain of a 1Hz signal phase demodulated by the three-acousto-optic system according to the embodiment.
Reference numerals: 1-narrow linewidth laser, 2, 3, 7, 12-optical fiber coupler, 4, 5, 6-acousto-optic modulator, 8-erbium-doped optical fiber amplifier, 9-band-pass filter, 10-circulator, 11-measured optical fiber, 13-balance detector, 14-data acquisition card, 15-upper computer, 16-arbitrary waveform generator.
Detailed Description
The present invention will be described in detail and fully with reference to the following examples and the accompanying drawings.
The embodiment is realized by the following technical scheme, and a three-acousto-optic coherent optical time domain reflection system shown in fig. 1 is firstly built. 1*2 the optical fiber coupler 1 divides the light emitted by a narrow linewidth laser with the light source wavelength of 1550.12nm into two parts, one part is taken as intrinsic light, and not less than 80% of signal light is equally divided into an acousto-optic modulator 1, an acousto-optic modulator 2 and an acousto-optic modulator 3 through one 1*3 optical fiber coupler 2 to be modulated into detection pulses. The three acousto-optic modulators need to have different pulse frequencies, the modulated detection pulse is injected into an erbium-doped fiber amplifier for amplification through a coupler 3 of 1*3, the amplified signal is injected into a circulator through a band-pass filter, the circulator is connected into a tested optical fiber, the backward Rayleigh scattered light and the intrinsic light of the tested optical fiber are received by a 200MHz balanced photoelectric detector after being beaten by a 1*2 coupler 4, and then the signals are fed into a 250 MSa/s and 16-bit acquisition card (DAQ), and finally demodulation processing is carried out by an upper computer.
In the embodiment, the distance L of the disturbance area is 4m, and the selected acousto-optic modulator frequency is 60MHz, 80MHz and 100MHz, so that the frequency selection requirement is met. The pulse modulation principle diagram of the three acousto-optic scheme is shown in figure 2, wherein the pulse repetition frequency of the acousto-optic modulator is 1kHz, the pulse width is 100ns, and the time delay tau is delayed d Setting 500ns, the parameters set by the acousto-optic modulator are controlled by an arbitrary waveform generator.
With the optical path configuration shown in fig. 1, a probe light pulse with a 500ns delay is generated by three acousto-optic modulators, ensuring that when one frequency sweeps through the gauge, the other frequency is near but outside the gauge. In this embodiment, PW2 is used as the main signal of the sweep frequency, and PW1 and PW3 are used as reference signals for laser frequency drift compensation. When the results of the three pulses are mapped on the same position axis, the vibration position is found first, the pre-trigger pulse PW1 will move a little to the right, the pulse PW2 is located in the disturbance area, and the post-trigger pulse PW3 will move a little to the left. Thus, the PW2 signal contains a vibration signal and a frequency drift signal, while PW1, PW3 contain signals that contain only laser frequency drift information that is highly similar to PW 1. Thus, the following formula is utilized:
the vibration signal of the pulse PW2 can be obtained. Wherein,for pulse PW2Phase signal after line low frequency noise suppression, < >>Phase signals comprising vibrations and frequency drift for pulse PW2,/->Is the phase signal of the pulses PW1, PW 3.
First, spectrum analysis is performed on the collected data stream signals, and the spectrum analysis result is shown in fig. 3. It can be seen that the spectrum of the signal acquired after the system modulation is concentrated at 60MHz, 80MHz and 100MHz, which is consistent with the frequency of the acousto-optic modulator we are using.
The signals of each frequency band are demodulated, and the demodulated signal phase space-time waterfall diagram is shown in fig. 4. It can be seen that the signals passing through the acousto-optic modulators with different modulation frequencies are demodulated and then are presented in different areas on the waterfall diagram, the disturbance positions are adjacent but not overlapped, the demodulation signals of fig. 4 (b) are used as main signals, the demodulation signals of fig. 4 (a) and (c) are used as reference signals, the signals of fig. 4 (a) and (c) only contain frequency drift signals which are similar to those of the signals of (b), and the compensated demodulation phase signals can be obtained through phase difference.
The comparison of the 1Hz signal phase time domain and frequency domain before and after compensation is shown in fig. 5. Fig. 5 (a) is a phase-time domain comparison of the demodulation 1Hz signal for different system schemes, and fig. 5 (b) is a phase-frequency domain comparison of the demodulation 1Hz signal for different system schemes. It can be seen that the three-acousto-optic scheme has an obvious inhibition effect on the frequency drift of the original signal after being compensated, and has an obvious improvement on the signal-to-noise ratio, and compared with the original system, the three-acousto-optic scheme has a 5dB improvement.
According to the embodiment, three pulses with different frequencies are generated by constructing the three acousto-optic modulation modules, the intermediate frequency band signals are used as main signals, two adjacent frequency bands are used as reference signals, and the phase difference reference compensation mode is used for effectively suppressing low-frequency noise, so that the signal-to-noise ratio of the low-frequency signals is improved. The invention effectively suppresses the low-frequency noise of the coherent optical time domain reflection system caused by the frequency drift of the laser, better restores the time domain signal of the low-frequency noise, improves the signal to noise ratio of the low-frequency signal, and provides a new thought and solution for realizing the low-frequency noise suppression of the coherent optical time domain reflection system, thereby forcefully pushing the application and development of the coherent optical time domain reflection system in the field of the low-frequency signal.
Claims (4)
1. A three acousto-optic coherent optical time domain reflection system for low-frequency noise suppression is characterized in that: the device comprises a narrow linewidth laser, optical fiber couplers 1, 2, 3 and 4, an acousto-optic modulator 1, 2 and 3, an erbium-doped optical fiber amplifier, a band-pass filter, a circulator, a balance detector, an acquisition card, an arbitrary waveform generator and an upper computer;
the narrow linewidth laser generates signal light, the signal light is divided into two paths through a 1*2 optical fiber coupler 1, wherein at least 80% of the signal light is input into a three-acousto-optic modulation module, and the rest part is used as intrinsic light to be input into a 1*2 coupler 4;
the optical fiber couplers 2 and 3 and the acousto-optic modulators 1, 2 and 3 form a tri-acousto-optic modulation module; the signal light with the ratio not lower than 80% is input into the 1*3 optical fiber coupler 2, split by equal proportion and respectively injected into the acoustic-optic modulator 1, the acoustic-optic modulator 2 and the acoustic-optic modulator 3, and 3 acoustic-optic modulator modulation pulses are controlled by an arbitrary waveform generator to be pulse light with three different frequencies; finally, the pulse light after being polymerized by the 1*3 coupler 3 is transmitted to an erbium-doped optical fiber amplifier;
the erbium-doped fiber amplifier amplifies the polymerized pulse light, then filters noise of other light wavelengths except the light source wavelength through a band-pass filter, the filtered signal is injected into a tested optical fiber through a circulator, and Rayleigh scattered light returned by the tested optical fiber is transmitted to a 1*2 coupler 4 polymerized in equal proportion through the circulator to beat frequency with the intrinsic light, and then is received by a balance detector; the received signals are converted into digital signals through the acquisition card and transmitted to the upper computer for signal processing, and the acquisition card is triggered synchronously by pulse signals given by the arbitrary waveform generator and the acousto-optic modulator.
2. The three acousto-optic coherent optical time domain reflectometry system for low frequency noise suppression of claim 1, wherein: the arbitrary waveform generator controls the three acousto-optic modulators to generate detection light pulses with time delay pulses, and the time delay meets the following requirements:
τ d >τ+PW1
τ d >τ+PW2
τ d >τ+PW3
τ is the propagation time of pulse signals in a disturbance area in the optical fiber, PW1, PW2 and PW3 are 3 pulses modulated by an arbitrary waveform generator, the pulses are triggered in sequence, and the time delay between two adjacent pulses is τ d ；
When the results of the three pulses are mapped to the same position shaft, firstly finding out the vibration position, taking the middle position signals of the time sequence as main signals and the signals at the two sides as reference signals for frequency drift compensation of the laser; thus, the intermediate signal contains a vibration signal and a frequency drift signal, while the two-side signal contains only laser frequency drift information which is highly similar to the intermediate signal, and phase noise caused by laser frequency drift can be suppressed by using the phase difference reference signal.
3. The three acousto-optic coherent optical time domain reflectometry system for low frequency noise suppression of claim 2, wherein: the detection frequency of the detection light pulse generated by the acousto-optic modulator is as follows:
△f＝f 3 -f 2 ＝f 2 -f 1 ≥v g /4L
where Δf is the frequency difference of the two acousto-optic modulators modulation, f 1 、f 2 、f 3 Is the corresponding modulation frequency, v, of the acousto-optic modulator 1, 2, 3 g Is the group velocity in the fiber and L is the distance length of the disturbance zone.
4. The three acousto-optic coherent optical time domain reflectometry system for low frequency noise suppression of claim 2, wherein:
the PW2 is used as a main signal of frequency sweep, PW1 and PW3 are used as reference signals of laser frequency drift compensation, and the following formula is utilized:
the vibration signal of the pulse PW2 can be obtained; wherein,for the phase signal of pulse PW2 after low-frequency noise suppression, < >>Phase signals comprising vibrations and frequency drift for pulse PW2,/->Is the phase signal of the pulses PW1, PW 3.
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