CN112697181B - A frequency modulation-based phase-sensitive optical time-domain reflectometry device and method - Google Patents

Abstract

The present invention provides a phase-sensitive optical time domain reflection device and method based on frequency modulation. The multi-frequency optical pulse sequence is obtained by dividing the laser into two paths and synchronously modulating the output by an intensity modulator. The spectrum of each optical pulse is determined by a fixed Single frequency and non-overlapping linear chirps; this sequence can generate back Rayleigh scattering signals with separate frequency bands in the sensing fiber, and the digital bandpass filters of different frequency bands are used to obtain the signal of each multi-frequency optical pulse. Rayleigh scattering pattern: Correlative processing is performed on the successively obtained Rayleigh scattering patterns in turn, and finally the disturbance position and the disturbance size are obtained. The present invention provides a phase-sensitive optical time-domain reflection device and method based on frequency modulation, which solves the problem of signal aliasing caused by inputting multiple pulses in a repeating cycle in a traditional device, shortens the detection interval, and improves the measurement accuracy. The speed is greatly improved, which improves the system's ability to measure broadband large vibrations.

Description

A frequency modulation-based phase-sensitive optical time-domain reflectometry device and method

technical field

The present invention relates to the technical field of optical fiber sensing, and more particularly, to a phase-sensitive optical time domain reflection device and method based on frequency modulation.

Background technique

-OTDR是一种能够实现振动测量的分布式光纤传感技术。当激光光源输出光经过调制器调制成脉冲光注入传感光纤后，通过探测器探测后向瑞利散射光，根据散射光接收的时延来得到相应光纤位置处的信息。由于

-OTDR使用的是窄线宽光源，因此探测器接收到的是脉冲宽度内后向瑞利散射光相互干涉叠加后的结果。当外界扰动作用于光纤时，对应位置处的折射率将发生变化，从而导致该位置处光相位发生了改变，最终表现为该位置处散射光强度的剧烈起伏。Phase Sensitive Optical Time Domain Reflectometer

-OTDR is a distributed optical fiber sensing technology that enables vibration measurement. When the output light of the laser light source is modulated by the modulator into pulsed light and injected into the sensing fiber, the backscattered light is detected by the detector, and the information at the corresponding fiber position is obtained according to the time delay of receiving the scattered light. because

-OTDR uses a narrow linewidth light source, so the detector receives the result of mutual interference and superposition of back Rayleigh scattered light within the pulse width. When the external disturbance acts on the optical fiber, the refractive index at the corresponding position will change, resulting in the change of the light phase at the position, and finally manifested as the violent fluctuation of the scattered light intensity at the position.

In order to ensure that the scattered light measured before and after the two measurements does not overlap in the time domain, the system must wait for the detector to receive the scattered light at the end of the fiber before performing the next measurement, which makes the measurement speed limited by the sensing distance. , that is, the system's ability to measure broadband vibration is limited.

-OTDR(CP-

OTDR)系统(WO2017093588A1)，其能够通过啁啾脉冲的时频关系对外界扰动引起的瑞利散射谱的频移进行补偿，从而实现对扰动的定量测量。一方面，该方法的测量速度仍然受到传感距离的限制，使其无法测量频率较高的外界扰动；另一方面，由于CP-

OTDR单次测量的频移量仅为啁啾脉冲扫频范围的3％-5％，因此其应变测量范围同样受到测量速度的影响。In 2016, a Spanish research group proposed a method based on chirped pulses and direct detection.

-OTDR(CP-

OTDR) system (WO2017093588A1), which can compensate the frequency shift of the Rayleigh scattering spectrum caused by external disturbance through the time-frequency relationship of the chirped pulse, thereby realizing the quantitative measurement of the disturbance. On the one hand, the measurement speed of this method is still limited by the sensing distance, making it impossible to measure external disturbances with higher frequencies; on the other hand, due to the CP-

The frequency shift of a single measurement of OTDR is only 3%-5% of the frequency sweep range of the chirped pulse, so its strain measurement range is also affected by the measurement speed.

-OTDR系统还无法满足频分复用条件。Frequency division multiplexing is a common method used to improve the measurement speed based on optical time domain reflectometry. For example, the optical frequency-division multiplexing phase-sensitive optical time-domain reflectometer (CN201210124995.3) proposed by Zhou Jun, Cai Haiwen and others of Shanghai Jiaotong University in 2012 can detect high-frequency vibration, but it has to increase the pulse width to Avoid spectral aliasing during measurement, which loses positioning accuracy. In addition, in 2016, the distributed optical fiber sensing system and its vibration detection and positioning method (CN201610719172.3) proposed by He Zuyuan, Liu Qingwen and others of Shanghai Jiaotong University realized multi-channel frequency division multiplexing by using swept frequency pulses under the coherent structure. It overcomes the problems of low response bandwidth and positioning accuracy, but its system complexity is also greatly improved. At present, due to the phenomenon of mutual interference within a single pulse and between multiple pulses, the existing chirped pulses and direct detection based on

- The OTDR system cannot yet meet the frequency division multiplexing conditions.

SUMMARY OF THE INVENTION

-OTDR系统测量速度存受限于传感距离技术缺陷，提供了一种基于频率调制的相位敏感光时域反射装置及方法来实现对高频扰动的测量。The present invention is to overcome the existing chirped pulse-based and direct detection type

- The measurement speed of the OTDR system is limited by the technical defects of the sensing distance. A phase-sensitive optical time domain reflectometry based on frequency modulation and a method are provided to realize the measurement of high frequency disturbance.

For solving the above-mentioned technical problems, the technical scheme of the present invention is as follows:

A phase-sensitive optical time domain reflection device based on frequency modulation, comprising a laser light source, a frequency modulation device, an optical amplification and filtering module, a sensing module, a signal acquisition and demodulation device; the signal acquisition and demodulation device includes a photoelectric detection device device, acquisition card and demodulation device; of which:

The laser light emitted by the laser light source is modulated by the frequency modulation device to obtain a high extinction ratio multi-frequency pulse signal;

After the optical amplification and filtering module amplifies the optical power of the high extinction ratio multi-frequency pulse signal and filters out the noise generated by the amplification, the optical signal is input into the signal acquisition and demodulation device through the sensing module;

The signal acquisition and demodulation device converts the optical signal into an electrical signal through the photodetector and outputs it to the acquisition card;

其中，K为扫频速率，υ₀为中心频率，Δt为互相关峰偏移量。The demodulation device performs decoding processing on the data of the acquisition card, and separates the Rayleigh scattering pattern data with non-overlapping frequency bands from the frequency domain by using N digital bandpass filters with different frequency bands that do not overlap each other. Obtain the Rayleigh scattering patterns of N multi-frequency pulses, and perform cross-correlation between the measured Rayleigh scattering patterns and the Rayleigh scattering reference pattern according to a window of a certain length, and the Rayleigh scattering patterns at the vibration positions will be shifted. , so the offset position of the correlation peak is the vibration region, and the magnitude of the strain is determined by the offset of the cross-correlation peak, that is,

Among them, K is the frequency sweep rate, υ ₀ is the center frequency, and Δt is the offset of the cross-correlation peak.

In the above scheme, the device adopts frequency modulation to improve the spectral utilization rate. Although multiple multi-frequency pulses are input to the sensing fiber at a certain interval, they will cause them to alias each other in the time domain. However, due to the different frequency bands of the multi-frequency pulses, it can be The digital band-pass filter of the frequency band separates the measurement results of each multi-frequency pulse from the frequency domain, thereby obtaining multiple scattering curves within one measurement time. By correlating multiple scattering curves, the disturbance information at the corresponding position can be obtained. The corresponding speed of the device to external disturbance is effectively improved, and the measurement range of vibration frequency and vibration magnitude are greatly improved.

In the above solution, the device adopts frequency modulation to improve the measurement speed, injects N pulses within the same tour time _TR , and according to Nyquist's law, the perturbation frequency that the device can respond to simultaneously increases by N times. The improvement of the measurement speed means that the disturbance variation between two measurements is smaller, which reduces the relative frequency shift and improves the measurement accuracy, and also reduces the requirement for the sweep frequency range.

In the above solution, the device only needs to change the driving circuit and data processing mode by using frequency modulation, and the optical path adopts a direct detection structure, which is simpler in structure and easy to implement.

Wherein, the frequency modulation device includes a signal generator, a first frequency modulator, a second frequency modulator, a pulse modulator, a first coupler and a second coupler; wherein:

The splitting ratio of the first coupler is 50:50, which is used to split the laser into two beams and input them to the first frequency modulator and the second frequency modulator respectively;

One channel of the signal generator repeatedly outputs N linear frequency modulation pulses of different frequency bands for driving the first frequency modulator, and the repeated output interval is T; the other channel generates synchronously output N fixed frequency sinusoidal pulse signals for use In order to drive the second frequency modulator, the repeated output interval is T; at the same time, the output signal of the synchronization port of one of the channels is used to drive the pulse modulator; wherein, the product of N and T needs to be equal to the optical pulse in the sensing fiber. Tour time _TR in ;

The second coupler combines the output optical signals of the first frequency modulator and the second frequency modulator into a multi-frequency optical signal;

The pulse modulator modulates the multi-frequency optical signal into a high extinction ratio multi-frequency pulse signal.

Wherein, the expression of the tour time _TR is:

Among them, L is the sensing length, n is the refractive index, and c is the speed of light in vacuum.

In the above solution, the laser light source is a narrow linewidth laser with a linewidth ranging from 100KHz to 10MHz, which is used to suppress the interference between scattered lights generated by different multi-frequency pulses and avoid aliasing of the measurement spectrum.

In the above scheme, the characteristics of the multi-frequency pulse frequency domain are: the spectrum of each multi-frequency pulse signal consists of two parts, one part is a linear frequency sweep; the other part is a fixed single frequency; the minimum interval between the two parts is greater than the linear sweep frequency range. , the linear part sweeps the same frequency range but the frequency bands do not overlap. The time-domain characteristics of multi-frequency pulses are: the time interval and pulse width of the multi-frequency pulses are the same before and after. The first frequency modulator and the second frequency modulator are optical intensity modulators, which provide a bias voltage to make them work in a suitable working area, and are driven by the multi-frequency pulse signal generated by the signal generator, thereby outputting multi-frequency optical pulses .

In the above solution, the pulse modulator is a semiconductor optical amplifier for outputting pulsed light with a high extinction ratio.

Wherein, the optical amplification and filtering module includes an optical amplifier, an optical filter and an adjustable attenuator; wherein:

The optical amplifier amplifies the optical power of the high extinction ratio multi-frequency pulse signal;

the optical filter filters out the noise brought by the optical amplifier;

The adjustable attenuator adjusts the filtered optical power and inputs the signal into the sensing module.

In the above scheme, the optical amplifier is an erbium-doped fiber amplifier, which is used to amplify the pulsed optical power; the optical filter is a band-pass filter, which is used to filter out the noise brought by the optical amplifier; the adjustable attenuation The device is used to adjust the optical power to prevent nonlinear effects and protect the circuit.

Wherein, the sensing module includes a circulator and a sensing fiber; the input end of the sensing fiber is connected to the output end of the adjustable attenuator; the input end of the circulator is connected to the sensing fiber; The output end of the device is connected to the photodetector; wherein:

The adjustable attenuator inputs a signal into the sensing fiber, and the circulator is connected to the photodetector.

In the above solution, the sensing fiber is a single-mode fiber with a length of 10Km.

In the above scheme, the laser light emitted by the laser light source is divided into two channels through the synchronous modulation output of the intensity modulator to obtain a multi-frequency optical pulse sequence. Each optical pulse spectrum is composed of a fixed single frequency and a linear chirp with non-overlapping frequency bands. It can generate backward Rayleigh scattering signals with frequency bands separated from each other, and the Rayleigh scattering pattern of each multi-frequency optical pulse can be obtained by using digital bandpass filters of different frequency bands; the Rayleigh scattering patterns obtained successively are processed in turn. , and finally obtain the disturbance position and its disturbance size. The method effectively improves the response speed of the device to external disturbances, the vibration magnitude and the vibration frequency measurement range will be greatly improved, and it is suitable for broadband large vibration detection.

A phase-sensitive optical time-domain reflectometry method based on frequency modulation, comprising the following steps:

S1: modulate the laser to obtain a high extinction ratio multi-frequency pulse signal;

S2: Amplify the optical power of the multi-frequency pulse signal with high extinction ratio, filter the noise and transmit it;

其中，K为扫频速率，υ₀为中心频率，Δt为互相关峰偏移量，完成对光信号的解调。S3: Receive the transmitted optical signal and demodulate the optical signal; separate the Rayleigh scattering pattern data with non-overlapping frequency bands from the frequency domain by using N digital bandpass filters with different and non-overlapping frequency bands, respectively. Obtain the Rayleigh scattering patterns of N multi-frequency pulses, and perform cross-correlation between the measured Rayleigh scattering patterns and the Rayleigh scattering reference pattern according to a window of a certain length, and the Rayleigh scattering patterns at the vibration positions will be shifted. , so the offset position of the correlation peak is the vibration region, and the magnitude of the strain is determined by the offset of the cross-correlation peak, that is,

Among them, K is the frequency sweep rate, υ ₀ is the center frequency, Δt is the offset of the cross-correlation peak, and the demodulation of the optical signal is completed.

Wherein, the step S1 specifically includes the following steps:

S11: Divide the incident laser into two beams through the coupler, and input them into the first frequency modulator and the second frequency modulator respectively;

S12: The signal generator generates two kinds of driving signals to drive the first frequency modulator and the second frequency modulator to modulate the two laser beams respectively;

Among them, one channel of the signal generator repeatedly outputs N linear frequency modulation pulses of different frequency bands to drive the first frequency modulator, and the repeated output interval is T; the other channel generates synchronously output N fixed-frequency sinusoidal pulse signals to drive the second frequency modulator , the repeated output interval is T; the product of N and T needs to be equal to the travel time _TR of the optical pulse in the sensing fiber;

S13: Combine the output optical signals of the first frequency modulator and the second frequency modulator into a multi-frequency optical signal through a coupler;

S14: Drive the pulse modulator with the output signal of the synchronization port of one channel of the signal generator, and the pulse modulator modulates the multi-frequency optical signal into a high extinction ratio multi-frequency pulse signal.

The expression of the tour time _TR is specifically:

Among them, L is the sensing length, n is the refractive index, and c is the speed of light in vacuum.

Among them, the signal generator generates two kinds of driving signals specifically:

Among them, V ₀ is the driving signal amplitude; i is the ith multi-frequency pulse; K is the sweep rate; τ _p is the multi-frequency pulse width; ΔF is the multi-frequency pulse bandwidth; f ₀ is the multi-frequency pulse starting frequency; Δf is the The minimum interval between the linear sweep part of the first multi-frequency pulse and the fixed single frequency; rect( ) is a rectangular function;

The DC bias voltage makes the first frequency modulator and the second frequency modulator work at a suitable operating point, and the signal generator drives the first frequency modulator and the second frequency modulator through a multi-frequency pulse signal, and outputs synchronously. to the pulse modulator; the multi-frequency optical pulse E(t) output after laser modulation is:

E(t)= _Echirp (t)+ _Esingle (t)

Among them, E ₀ is the output signal amplitude; i is the ith multi-frequency pulse; K is the sweep rate; τ _p is the multi-frequency pulse width; ΔF is the multi-frequency pulse bandwidth; m is the modulation depth; Δf is the first multi-frequency pulse The minimum interval between the linear frequency sweep part of the pulse and the fixed single frequency, rect(·) is a rectangular function.

Wherein, the step S3 specifically includes the following steps:

S31: During the transmission process of the high extinction ratio multi-frequency pulse signal, the generated back Rayleigh scattering signal is converted into an electrical signal by the photodetector and output to the acquisition card;

S32: Fourier transform the backscattered signals I(t) of the N multi-frequency pulses obtained by the data acquisition card to obtain I(f); wherein, I(f)=I ₁ (f)+I ₂ (f ); I ₁ (f) consists of the linear part of the spectrum of each multi-frequency pulse and the internal interference of Rayleigh scattered light of a fixed single frequency; I ₂ (f) consists of the linear part of the spectrum of each multi-frequency pulse It is composed of mutual interference with fixed single-frequency Rayleigh scattered light;

S33: Since the frequency bands in I ₂ (f) are different and non-overlapping, I ₁ (f) and I ₂ (f) frequency bands are different and non-overlapping, by using N digital bandpass filters with different frequency bands and no overlapping, respectively The data in I ₂ (f) are respectively taken out and subjected to inverse Fourier transform to restore N scattering patterns obtained by N multi-frequency pulses;

S34: Perform the cross-correlation operation on the N scattering patterns respectively, the offset position of the correlation peak is the vibration area, and the magnitude of the strain variable is determined by the offset of the cross-correlation peak, namely:

Among them, K is the frequency sweep rate, υ ₀ is the center frequency, and Δt is the offset of the cross-correlation peak. Therefore, within the same tour time _TR , the measurement speed is increased by N times, and the response bandwidth is also increased by N times.

Compared with the prior art, the beneficial effects of the technical solution of the present invention are:

The present invention provides a phase-sensitive optical time domain reflection device and method based on frequency modulation. The multi-frequency optical pulse sequence is obtained by dividing the laser into two paths and synchronously modulating the output by an intensity modulator. The spectrum of each optical pulse is determined by a fixed The single frequency and the non-overlapping linear chirps are composed of linear chirps; this sequence can generate back Rayleigh scattering signals with separate frequency bands in the sensing fiber. Rayleigh scattering pattern: Correlative processing is performed on successively acquired Rayleigh scattering patterns, and finally the disturbance position and its disturbance size are obtained. This method effectively improves the response speed of the device to external disturbances, and its vibration frequency measurement range and vibration magnitude measurement range will be greatly improved.

Description of drawings

1 is a schematic structural diagram of a phase-sensitive optical time-domain reflectometry device based on frequency modulation;

Fig. 2 is the output timing diagram of the frequency modulation device;

Fig. 3 is a schematic diagram of the interval before and after the multi-pulse injection into the optical fiber;

Fig. 4 is the time domain schematic diagram of multi-frequency pulse;

5 is a schematic diagram of the frequency domain of the backward Rayleigh scattering light generated by different multi-frequency pulses;

Wherein: 1, laser light source; 2, frequency modulation device; 201, signal generator; 202, first frequency modulator; 203, second frequency modulator; 204, pulse modulator; 205, first coupler; 206, second coupler; 3. optical amplifier and filter module; 301, optical amplifier; 302, optical filter; 303, adjustable attenuator; 4, sensing module; 401, circulator; 402, sensing fiber; 5, Signal acquisition and demodulation device; 501, photoelectric detector; 502, acquisition card; 503, demodulation device.

Detailed ways

The accompanying drawings are for illustrative purposes only, and should not be construed as limitations on this patent;

In order to better illustrate this embodiment, some parts of the drawings are omitted, enlarged or reduced, which do not represent the size of the actual product;

It will be understood by those skilled in the art that some well-known structures and their descriptions may be omitted from the drawings.

The technical solutions of the present invention will be further described below with reference to the accompanying drawings and embodiments.

Example 1

As shown in FIG. 1 , the present invention provides a phase-sensitive optical time domain reflection device based on frequency modulation, including a laser light source 1, a frequency modulation device 2, an optical amplification and filtering module 3, a sensing module 4, signal acquisition and demodulation Device 5; the signal acquisition and demodulation device 5 includes a photodetector 501, a capture card 502 and a demodulation device 503; wherein:

The laser light emitted by the laser light source 1 is modulated by the frequency modulation device 2 to obtain a high extinction ratio multi-frequency pulse signal;

After the optical amplification and filtering module 3 amplifies the optical power of the high extinction ratio multi-frequency pulse signal and filters out the noise generated by the amplification, the optical signal is input into the signal acquisition and demodulation device 5 through the sensing module 4 ;

The signal acquisition and demodulation device 5 converts the optical signal into an electrical signal through the photodetector 501 and outputs it to the acquisition card 502;

其中，K为扫频速率，υ₀为中心频率，Δt为互相关峰偏移量。The demodulation device 503 decodes the data of the acquisition card 502, and separates the Rayleigh scattering pattern data with non-overlapping frequency bands from the frequency domain by using N digital bandpass filters with different frequency bands that do not overlap with each other. , respectively obtain the Rayleigh scattering patterns of N multi-frequency pulses, and perform cross-correlation between the measured Rayleigh scattering pattern and the Rayleigh scattering reference pattern according to a window of a certain length, and the Rayleigh scattering pattern at the vibration position will occur. Therefore, the offset position of the correlation peak is the vibration area, and the magnitude of the strain is determined by the offset of the cross-correlation peak, namely

Among them, K is the frequency sweep rate, υ ₀ is the center frequency, and Δt is the offset of the cross-correlation peak.

In the specific implementation process, the laser light source 1 is a narrow linewidth laser, the selected linewidth is 1MHz, the corresponding coherence time is 500ns, and the coherence length is 100m. The purpose is to suppress the mutual interference between the scattered light generated by the multi-frequency light pulses before and after.

In the specific implementation process, the photodetector 501 is a photodetector, and the bandwidth must be greater than 10GHz; the sampling rate of the acquisition card 502 is 15GSa/s; the demodulation device 503 is a signal processing system, which digitally filters the data of the acquisition card 502 and performs related demodulation.

More specifically, the frequency modulation device 2 includes a signal generator 201, a first frequency modulator 202, a second frequency modulator 203, a pulse modulator 204, a first coupler 205 and a second coupler 206; wherein:

The splitting ratio of the first coupler 205 is 50:50, which is used to split the laser into two beams and input them to the first frequency modulator 202 and the second frequency modulator 203 respectively;

One channel of the signal generator 201 repeatedly outputs a 100ns chirp for driving the first frequency modulator 202; the other channel generates a synchronous output 100ns sine pulse signal for driving the second frequency modulator 203 At the same time, the output signal of the synchronization port of one of the channels is used to drive the pulse modulator 204 to perform pulse synchronization output; the output timing is shown in FIG. 2 .

The second coupler 206 combines the output optical signals of the first frequency modulator 202 and the second frequency modulator 203 into a multi-frequency optical signal;

The pulse modulator 204 modulates the multi-frequency optical signal into a high extinction ratio multi-frequency pulse signal.

More specifically, as shown in FIG. 3 , the expression of the tour time _TR is specifically:

Among them, L is the sensing length, n is the refractive index, and c is the speed of light in vacuum.

The characteristics of the multi-frequency pulse spectrum are: the spectrum of each multi-frequency pulse signal consists of two parts, one part is a linear frequency sweep; the other part is a fixed single frequency; the interval between the two parts is greater than the linear frequency sweep range, and the linear part has the same frequency sweep range (F=1GHz), the minimum interval of the linear part between different pulses is ΔF=0.1GHz, the minimum interval between the single frequency and the linear part in each pulse is 1.1GHz+(N-1)(F+ΔF), N is the multi-frequency number of pulses. The characteristics of the multi-frequency pulse time domain are: the repeated output interval is 20us, and a total of N=5 pulses with the same width (100ns) are input.

The time-domain and frequency-domain characteristics of the multi-frequency pulse are shown in Figure 4.

More specifically, the optical amplification and filtering module 3 includes an optical amplifier 301, an optical filter 302 and an adjustable attenuator 303; wherein:

The optical amplifier 301 amplifies the optical power of the high extinction ratio multi-frequency pulse signal;

The optical filter 302 filters out the noise brought by the optical amplifier 301;

The adjustable attenuator 303 adjusts the filtered optical power, and inputs the signal into the sensing module 4 .

In the specific implementation process, the optical amplifier 301 is an erbium-doped fiber amplifier, which is used to amplify the pulsed optical power; the optical filter 302 is a band-pass filter, which is used to filter out the noise brought by the optical amplifier 301; The adjustable attenuator 303 is used to adjust the optical power to prevent nonlinear effects and protect the circuit.

More specifically, the sensing module 4 includes a circulator 401 and a sensing fiber 402; the input end of the sensing fiber 402 is connected to the output end of the adjustable attenuator 303; the input end of the circulator 401 is connected to the The sensing fiber 402 is connected; the output end of the circulator 401 is connected to the photodetector 501; wherein:

The adjustable attenuator 303 inputs a signal into the sensing fiber 402 , and the circulator 401 is connected to the photodetector 501 .

In a specific implementation process, the sensing fiber 402 is a single-mode fiber with a length of 10Km.

In the specific implementation process, the device adopts frequency modulation to improve the spectral utilization rate. Although multiple multi-frequency pulses are input into the sensing fiber 402 at a certain interval, they will cause them to alias each other in the time domain. However, due to the different frequency bands of the multi-frequency pulses, The measurement results of each multi-frequency pulse can be separated from the frequency domain according to the digital band-pass filters of different frequency bands, so that multiple scattering curves can be obtained within one measurement time. By correlating multiple scattering curves, the disturbance information at the corresponding position can be obtained. The corresponding speed of the device to external disturbance is effectively improved, and the vibration size and vibration frequency measurement range are greatly improved.

In the specific implementation process, the device adopts frequency modulation to improve the measurement speed, and injects N pulses within the same tour time _TR . According to Nyquist's law, the perturbation frequency that the device can respond to simultaneously increases by N times. The improvement of the measurement speed means that the disturbance variation between two measurements is smaller, which reduces the relative frequency shift and improves the measurement accuracy, and also reduces the requirement for the sweep frequency range.

In the specific implementation process, the device only needs to change the driving circuit and data processing mode by using frequency modulation, and the optical path adopts a direct detection structure, which is simpler in structure and easy to implement.

Example 2

More specifically, on the basis of Embodiment 1, a frequency modulation-based phase-sensitive optical time-domain reflectometry method is provided, comprising the following steps:

S1: modulate the laser to obtain a high extinction ratio multi-frequency pulse signal;

S2: Amplify the optical power of the multi-frequency pulse signal with high extinction ratio, filter the noise and transmit it;

其中，K为扫频速率，υ₀为中心频率，Δt为互相关峰偏移量，完成对光信号的解调。S3: Receive the transmitted optical signal and demodulate the optical signal; separate the Rayleigh scattering pattern data with non-overlapping frequency bands from the frequency domain by using N digital bandpass filters with different and non-overlapping frequency bands, respectively. Obtain the Rayleigh scattering patterns of N multi-frequency pulses, and perform cross-correlation between the measured Rayleigh scattering patterns and the Rayleigh scattering reference pattern according to a window of a certain length, and the Rayleigh scattering patterns at the vibration positions will be shifted. , so the offset position of the correlation peak is the vibration region, and the magnitude of the strain Δε is determined by the offset of the cross-correlation peak, that is,

Among them, K is the frequency sweep rate, υ ₀ is the center frequency, Δt is the offset of the cross-correlation peak, and the demodulation of the optical signal is completed.

More specifically, the step S1 specifically includes the following steps:

S11: Divide the incident laser into two beams through the coupler, and input them into the first frequency modulator and the second frequency modulator respectively;

S12: The signal generator generates two kinds of driving signals to drive the first frequency modulator and the second frequency modulator to modulate the two laser beams respectively;

Among them, one channel of the signal generator repeatedly outputs N linear frequency modulation pulses of different frequency bands to drive the first frequency modulator, and the repeated output interval is T; the other channel generates synchronously output N fixed-frequency sinusoidal pulse signals to drive the second frequency modulator , the repeated output interval is T; the product of N and T needs to be equal to the travel time _TR of the optical pulse in the sensing fiber;

S13: Combine the output optical signals of the first frequency modulator and the second frequency modulator into a multi-frequency optical signal through a coupler;

S14: Drive the pulse modulator with the output signal of the synchronization port of one channel of the signal generator, and the pulse modulator modulates the multi-frequency optical signal into a high extinction ratio multi-frequency pulse signal.

More specifically, the tour time _TR expression is specifically:

Among them, L is the sensing length, n is the refractive index, and c is the speed of light in vacuum.

More specifically, the signal generator generates two driving signals as follows:

Among them, V ₀ is the driving signal amplitude, i is the ith multi-frequency pulse; K is the frequency sweep rate; τ _p is the multi-frequency pulse width; ΔF is the multi-frequency pulse bandwidth; f ₀ is the multi-frequency pulse starting frequency; Δf is the The minimum interval between the linear sweep part of the first multi-frequency pulse and the fixed single frequency, rect( ) is a rectangular function;

The DC bias voltage makes the first frequency modulator and the second frequency modulator work at a suitable operating point, and the signal generator drives the first frequency modulator and the second frequency modulator through a multi-frequency pulse signal, and outputs synchronously. to the pulse modulator; the multi-frequency optical pulse E(t) output after laser modulation is:

E(t)= _Echirp (t)+ _Esingle (t)

Among them, E ₀ is the output signal amplitude; i is the ith multi-frequency pulse; K is the sweep rate; τ _p is the multi-frequency pulse width; ΔF is the multi-frequency pulse bandwidth; m is the modulation depth; Δf is the first multi-frequency pulse The minimum interval between the linear frequency sweep part of the pulse and the fixed single frequency; rect(·) is a rectangular function.

More specifically, the step S3 specifically includes the following steps:

S31: During the transmission process of the high extinction ratio multi-frequency pulse signal, the generated back Rayleigh scattering signal is converted into an electrical signal by the photodetector and output to the acquisition card;

S32: Fourier transform the backscattered signals I(t) of the N multi-frequency pulses obtained by the data acquisition card to obtain I(f); wherein, I(f)=I ₁ (f)+I ₂ (f ); I ₁ (f) consists of the linear part of the spectrum of each multi-frequency pulse and the internal interference of Rayleigh scattered light of a fixed single frequency; I ₂ (f) consists of the linear part of the spectrum of each multi-frequency pulse It is composed of mutual interference with the Rayleigh scattered light of a fixed single frequency; as shown in Figure 5.

S33: Since the frequency bands in I ₂ (f) are different and non-overlapping, I ₁ (f) and I ₂ (f) frequency bands are different and non-overlapping, by using N digital bandpass filters with different frequency bands and no overlapping, respectively The data in I ₂ (f) are respectively taken out and subjected to inverse Fourier transform to restore N scattering patterns obtained by N multi-frequency pulses;

S34: Perform the cross-correlation operation on the N scattering patterns respectively, the offset position of the correlation peak is the vibration area, and the magnitude of the strain variable is determined by the offset of the cross-correlation peak, namely:

Among them, K is the frequency sweep rate, υ ₀ is the center frequency, and Δt is the offset of the cross-correlation peak. Therefore, within the same tour time _TR , the measurement speed is increased by N times, and the response bandwidth is also increased by N times.

In the specific implementation process, the vibration frequency response bandwidth of this embodiment depends on the repetition frequency f of the multi-frequency optical pulse, and its response bandwidth is 0.5f obtained from the Nyquist theorem. When the sensing length is 10Km, the response bandwidth of a typical system is 5KHz, and the response bandwidth of this embodiment can reach 25KHz, which is increased by 5 times.

The vibration size measurement range of this embodiment is affected by the repetition frequency f of the multi-frequency optical pulse. The frequency variation caused by the two measurements cannot be greater than 5%, otherwise the correlation of the scattering patterns obtained from the previous and previous measurements will decrease, resulting in demodulation errors. When the repetition frequency f increases, the frequency change caused by the two measurements is relatively reduced, so that the measurement range of the vibration size is relatively improved.

It can be seen from the specific example that the present invention proposes a phase-sensitive optical time domain reflection device and method based on frequency modulation, which solves the problem of signal aliasing caused by inputting multiple pulses in the repetition period in the traditional device, and shortens the The detection interval time is shortened, and the measurement speed is greatly improved, which makes the system improve the broadband large vibration measurement capability.

Obviously, the above-mentioned embodiments of the present invention are only examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. For those of ordinary skill in the art, changes or modifications in other different forms can also be made on the basis of the above description. There is no need and cannot be exhaustive of all implementations here. Any modifications, equivalent replacements and improvements made within the spirit and principle of the present invention shall be included within the protection scope of the claims of the present invention.

Claims (5)

1. A phase-sensitive optical time domain reflection device based on frequency modulation, characterized in that it comprises a laser light source (1), a frequency modulation device (2), an optical amplification and filtering module (3), a sensing module (4), A signal collection and demodulation device (5); the signal collection and demodulation device (5) comprises a photodetector (501), a collection card (502) and a demodulation device (503); wherein: The laser light source (1) emits laser light and is modulated by the frequency modulation device (2) to obtain a high extinction ratio multi-frequency pulse signal; After the optical amplification and filtering module (3) amplifies the optical power of the high extinction ratio multi-frequency pulse signal and filters out the noise generated by the amplification, the optical signal is input to the signal acquisition and solution through the sensing module (4). in the adjusting device (5); The signal acquisition and demodulation device (5) converts an optical signal into an electrical signal through the photodetector (501) and outputs it to the acquisition card (502); The demodulation device (503) decodes the data of the acquisition card (502), and converts the Rayleigh scattering pattern data with non-overlapping frequency bands from N digital bandpass filters with different and non-overlapping frequency bands from the data of the acquisition card (502). Separate in the frequency domain, respectively obtain the Rayleigh scattering patterns of N multi-frequency pulses, and perform cross-correlation between the measured Rayleigh scattering pattern and the Rayleigh scattering reference pattern according to a window of a certain length. The scattering pattern will be offset, so the offset position of its correlation peak is the vibration area, and the magnitude of its strain is determined by the offset of the cross-correlation peak, that is,

Among them, K is the frequency sweep rate, υ ₀ is the center frequency, and Δt is the offset of the cross-correlation peak; The frequency modulation device (2) comprises a signal generator (201), a first frequency modulator (202), a second frequency modulator (203), a pulse modulator (204), a first coupler (205) and a Two couplers (206); wherein: The light splitting ratio of the first coupler (205) is 50:50, and is used to split the laser into two beams and input them to the first frequency modulator (202) and the second frequency modulator (203) respectively; One channel of the signal generator (201) repeatedly outputs N chirps of different frequency bands for driving the first frequency modulator (202), and the repeated output interval is T; the other channel generates N fixed synchronous outputs A sinusoidal pulse signal of a frequency is used to drive the second frequency modulator (203), and the repeated output interval is T; at the same time, the synchronous port output signal of one of the channels is used to drive the pulse modulator (204); wherein, N The product of T and T needs to be equal to the travel time _TR of the optical pulse in the sensing fiber; The second coupler (206) combines the output optical signals of the first frequency modulator (202) and the second frequency modulator (203) into a multi-frequency optical signal; The pulse modulator (204) modulates the multi-frequency optical signal into a high extinction ratio multi-frequency pulse signal; The optical amplification and filtering module (3) includes an optical amplifier (301), an optical filter (302) and an adjustable attenuator (303); wherein: The optical amplifier (301) amplifies the optical power of the high extinction ratio multi-frequency pulse signal; The optical filter (302) filters out the noise brought by the optical amplifier (301); The adjustable attenuator (303) adjusts the filtered optical power, and inputs the signal into the sensing module (4); The sensing module (4) includes a circulator (401) and a sensing fiber (402); the input end of the sensing fiber (402) is connected to the output end of the adjustable attenuator (303); the circulator (401) The input end is connected to the sensing fiber (402); the output end of the circulator (401) is connected to the photodetector (501); wherein: The adjustable attenuator (303) inputs a signal into the sensing fiber (402), and the circulator (401) is connected to the photodetector (501). 2. a kind of phase-sensitive optical time domain reflectometry device based on frequency modulation according to claim 1, is characterized in that, described tour time _TR expression is:

Among them, L is the sensing length, n is the refractive index, and c is the speed of light in vacuum. 3. a phase-sensitive optical time domain reflection method based on frequency modulation, is characterized in that, comprises the following steps: S1: modulate the laser to obtain a high extinction ratio multi-frequency pulse signal; S2: Amplify the optical power of the multi-frequency pulse signal with high extinction ratio, filter the noise and transmit it; S3: Receive the transmitted optical signal and demodulate the optical signal; separate the Rayleigh scattering pattern data with non-overlapping frequency bands from the frequency domain by using N digital bandpass filters with different and non-overlapping frequency bands, respectively. Obtain the Rayleigh scattering patterns of N multi-frequency pulses, and perform cross-correlation between the measured Rayleigh scattering patterns and the Rayleigh scattering reference pattern according to a window of a certain length, and the Rayleigh scattering patterns at the vibration positions will be shifted. , so the offset position of the correlation peak is the vibration region, and the magnitude of the strain Δε is determined by the offset of the cross-correlation peak, that is,

Among them, K is the frequency sweep rate, υ ₀ is the center frequency, Δt is the offset of the cross-correlation peak, and the demodulation of the optical signal is completed; Wherein, the step S1 specifically includes the following steps: S11: Divide the incident laser into two beams through the coupler, and input them into the first frequency modulator and the second frequency modulator respectively; S12: The signal generator generates two kinds of driving signals to drive the first frequency modulator and the second frequency modulator to modulate the two laser beams respectively; Among them, one channel of the signal generator repeatedly outputs N linear frequency modulation pulses of different frequency bands to drive the first frequency modulator, and the repeated output interval is T; the other channel generates synchronously output N fixed-frequency sinusoidal pulse signals to drive the second frequency modulator , the repeated output interval is T; the product of N and T needs to be equal to the travel time _TR of the optical pulse in the sensing fiber; S13: Combine the output optical signals of the first frequency modulator and the second frequency modulator into a multi-frequency optical signal through a coupler; S14: Drive the pulse modulator with the output signal of the synchronization port of one channel of the signal generator, and the pulse modulator modulates the multi-frequency optical signal into a high extinction ratio multi-frequency pulse signal; The step S3 specifically includes the following steps: S31: During the transmission process of the high extinction ratio multi-frequency pulse signal, the generated back Rayleigh scattering signal is converted into an electrical signal by the photodetector and output to the acquisition card; S32: Fourier transform the backscattered signals I(t) of the N multi-frequency pulses obtained by the data acquisition card to obtain I(f); wherein, I(f)=I ₁ (f)+I ₂ (f ); I ₁ (f) consists of the linear part of the spectrum of each multi-frequency pulse and the internal interference of Rayleigh scattered light of a fixed single frequency; I ₂ (f) consists of the linear part of the spectrum of each multi-frequency pulse It is composed of mutual interference with fixed single-frequency Rayleigh scattered light; S33: Since the frequency bands in I ₂ (f) are different and non-overlapping, I ₁ (f) and I ₂ (f) frequency bands are different and non-overlapping, by using N digital bandpass filters with different frequency bands and no overlapping, respectively The data in I ₂ (f) are respectively taken out and subjected to inverse Fourier transform to restore N scattering patterns obtained by N multi-frequency pulses; S34: Perform the cross-correlation operation on the N scattering patterns respectively. The offset position of the correlation peak is the vibration area, and the magnitude of the strain variable is determined by the offset of the cross-correlation peak, namely:

Among them, K is the frequency sweep rate, υ ₀ is the center frequency, and Δt is the offset of the cross-correlation peak. 4. a kind of phase-sensitive optical time domain reflection method based on frequency modulation according to claim 3, is characterized in that, described tour time _TR expression is specifically:

Among them, L is the sensing length, n is the refractive index, and c is the speed of light in vacuum. 5. a kind of phase-sensitive optical time domain reflection method based on frequency modulation according to claim 4, is characterized in that, the signal generator produces two kinds of driving signals specifically:

Among them, V ₀ is the driving signal amplitude, i is the ith multi-frequency pulse; K is the frequency sweep rate; τ _p is the multi-frequency pulse width; ΔF is the multi-frequency pulse bandwidth; f ₀ is the multi-frequency pulse starting frequency; Δf is the The minimum interval between the linear sweep part of the first multi-frequency pulse and the fixed single frequency, rect( ) is a rectangular function; The DC bias voltage enables the first frequency modulator and the second frequency modulator to work at appropriate operating points, and the signal generator drives the first frequency modulator and the second frequency modulator through a multi-frequency pulse signal, and outputs synchronously. to the pulse modulator; the multi-frequency optical pulse E(t) output after laser modulation is: E(t)= _Echirp (t)+ _Esingle (t)

Among them, E ₀ is the output signal amplitude; i is the ith multi-frequency pulse; K is the sweep rate; τ _p is the multi-frequency pulse width; ΔF is the multi-frequency pulse bandwidth; m is the modulation depth; Δf is the first multi-frequency pulse The minimum interval between the linear frequency sweep part of the pulse and the fixed single frequency; rect(·) is a rectangular function.

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