CN112697181B - Phase-sensitive optical time domain reflection device and method based on frequency modulation - Google Patents
Phase-sensitive optical time domain reflection device and method based on frequency modulation Download PDFInfo
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Abstract
The invention provides a phase sensitive optical time domain reflecting device and method based on frequency modulation, wherein laser is divided into two paths, and a multifrequency optical pulse sequence is obtained through synchronous modulation output of an intensity modulator, wherein each optical pulse frequency spectrum consists of a fixed single frequency and linear chirp without overlapping frequency bands; the sequence can generate backward Rayleigh scattering signals with mutually separated frequency bands in the sensing optical fiber, and a Rayleigh scattering pattern of each multi-frequency optical pulse is obtained by using digital band-pass filters with different frequency bands; and carrying out correlation processing on the successively acquired Rayleigh scattering patterns in sequence to finally obtain the disturbance position and the disturbance size. The phase sensitive optical time domain reflection device and method based on frequency modulation provided by the invention solve the problem of signal aliasing caused by a plurality of pulses injected in a repetition period in the traditional device, shorten the detection interval time, greatly improve the measurement speed and improve the broadband large-vibration measurement capability of the system.
Description
Technical Field
The invention relates to the technical field of optical fiber sensing, in particular to a phase-sensitive optical time domain reflection device and method based on frequency modulation.
Background
Phase-sensitive optical time domain reflectometer
OTDR is a distributed fiber sensing technique that enables vibration measurements. After the output light of the laser light source is modulated into pulse light by the modulator and injected into the sensing optical fiberAnd detecting backward Rayleigh scattered light by a detector, and obtaining information at the corresponding optical fiber position according to the receiving time delay of the scattered light. Due to the fact that
OTDR uses a narrow linewidth light source, so the detector receives the results of the superposition of the interference of the backscattered rayleigh light within the pulse width. When external disturbance acts on the optical fiber, the refractive index at the corresponding position changes, so that the optical phase at the position changes, and finally the optical phase appears as a sharp fluctuation of the scattered light intensity at the position.
In order to ensure that the scattered light measured in two times does not alias in the time domain, the system must wait for the detector to receive the scattered light at the end of the optical fiber before performing the next measurement, which makes the measurement speed limited by the sensing distance, i.e. the system has limited measurement capability for broadband vibration.
In 2016, the Spanish subject group proposed a system based on chirping and direct detection
-OTDR(CP-
An OTDR) system (WO2017093588a1) which can compensate the frequency shift of the rayleigh scattering spectrum caused by the external disturbance through the time-frequency relationship of the chirped pulses, thereby realizing the quantitative measurement of the disturbance. On one hand, the measuring speed of the method is still limited by the sensing distance, so that the method cannot measure external disturbance with high frequency; on the other hand, due to CP-
The frequency shift amount of the OTDR single measurement is only 3% -5% of the chirp pulse frequency sweep range, so the strain measurement range is also influenced by the measurement speed.
Frequency division multiplexing is a common method for increasing the measurement speed based on the optical time domain reflectometry technique. For example, Zhoujun, Chua's foreign language at Shanghai university of traffic in 2012An optical frequency division multiplexing phase-sensitive optical time domain reflectometer (CN201210124995.3) is proposed, which enables detection of dither, but which has to increase the pulse width to avoid spectral aliasing during measurement, thereby losing positioning accuracy. In addition, in 2016, the distributed optical fiber sensing system and the vibration detection and positioning method (CN201610719172.3) thereof proposed by the source of he shanghai traffic university and the liuqing, etc., adopt sweep frequency pulses to realize multiplexing frequency division multiplexing under a coherent structure, and although the problems of low response bandwidth and low positioning accuracy are overcome, the system complexity is also greatly increased. At present, because of the phenomenon of mutual interference between a single pulse and a plurality of pulses, the existing chirp pulse and direct detection type are based on
The OTDR system has not yet satisfied the frequency division multiplexing condition.
Disclosure of Invention
The invention aims to overcome the defects of the prior chirp pulse-based direct detection type
The measuring speed of the OTDR system is limited by the technical defect of sensing distance, and a phase sensitive optical time domain reflection device and a method based on frequency modulation are provided for realizing the measurement of high-frequency disturbance.
In order to solve the technical problems, the technical scheme of the invention is as follows:
a phase sensitive optical time domain reflection device based on frequency modulation comprises a laser light source, a frequency modulation device, an optical amplification and filtering module, a sensing module and a signal acquisition and demodulation device; the signal acquisition and demodulation device comprises a photoelectric detector, an acquisition card and a demodulation device; wherein:
the laser light source emits laser light, and the laser light is modulated by the frequency modulation device to obtain a multi-frequency pulse signal with a high extinction ratio;
the optical amplification and filtering module is used for amplifying the optical power of the high extinction ratio multi-frequency pulse signal and filtering noise generated by amplification, and then the optical signal is input into the signal acquisition and demodulation device through the sensing module;
the signal acquisition and demodulation device converts optical signals into electric signals through the photoelectric detector and outputs the electric signals to the acquisition card;
the demodulation device decodes the data of the acquisition card, separates the Rayleigh scattering pattern data with non-overlapping frequency bands from a frequency domain by using N digital band-pass filters with different and non-overlapping frequency bands to respectively obtain Rayleigh scattering patterns of N multi-frequency pulses, and performs cross-correlation operation on the measured Rayleigh scattering patterns and a Rayleigh scattering reference pattern according to a window with a certain length, wherein the Rayleigh scattering patterns at the vibration position are shifted, so that the related peak is shifted, namely a vibration area, and the size of the dependent variable is determined by the shift of the cross-correlation peak, namely the shift of the related peak is determined by the shift of the cross-correlation peak, namely the vibration area
Where K is the sweep rate, upsilon 0 At the center frequency, Δ t is the cross-correlation peak offset.
In the above scheme, the device adopts frequency modulation to improve the spectrum utilization rate, although a plurality of multi-frequency pulses input into the sensing optical fiber at certain intervals can cause mutual aliasing in the time domain, the frequency bands of the multi-frequency pulses are different, and the measurement results of the multi-frequency pulses can be separated from the frequency domain according to digital band-pass filters of different frequency bands, so that a plurality of scattering curves can be obtained within one measurement time. By carrying out correlation operation on a plurality of 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 frequency measurement range and the vibration size measurement range are greatly improved.
In the scheme, the device adopts frequency modulation to improve the measurement speed and the same itinerant time T R N pulses are injected in the device, and the disturbance frequency which the device can respond to is increased by N times according to the Nyquist law. The improvement of the measurement speed means that the disturbance variable quantity between two measurements is smaller, so that the relative frequency shift quantity is reduced, the measurement precision is higher, and the requirement on the sweep frequency range is also reduced.
In the above scheme, the device adopts frequency modulation and only needs to change on a driving circuit and a data processing mode, and the light path adopts a direct detection structure, so that the structure is simpler and easy to realize.
The frequency modulation device comprises 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, and the first coupler is used for splitting laser into two beams which are respectively input into the first frequency modulator and the second frequency modulator;
one channel of the signal generator repeatedly outputs N linear frequency modulation pulses with different frequency bands for driving the first frequency modulator, and the repeated output interval is T; the other channel generates sinusoidal pulse signals synchronously outputting N fixed frequencies for driving the second frequency modulator, and the repeated output interval is T; simultaneously using a synchronous port output signal of one of the channels for driving the pulse modulator; wherein, the product of N and T is equal to the travel time T of the optical pulse in the sensing optical fiber R ；
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 multi-frequency pulse signal with a high extinction ratio.
Wherein the patrol time T R The expression is as follows:
where L is the sensing length, n is the refractive index, and c is the speed of light in vacuum.
In the above scheme, the laser light source is a narrow linewidth laser, the linewidth range of the laser is 100 KHz-10 MHz, and the laser is used for inhibiting interference between scattered light generated by different multi-frequency pulses and avoiding aliasing of measurement frequency spectrum.
In the above scheme, the frequency domain of the multi-frequency pulse is characterized in that: the frequency spectrum of each multi-frequency pulse signal consists of two parts, wherein one part is linear frequency sweep; the other part is a fixed single frequency; the minimum interval of the two parts is larger than the linear frequency sweep range, and the frequency sweep ranges of the linear part are the same but the frequency bands are not overlapped. The multi-frequency pulse time domain is characterized in that: the time interval of the front multifrequency pulse and the time interval of the rear multifrequency pulse are the same, and the pulse width is the same. The first frequency modulator and the second frequency modulator are light intensity modulators, provide bias voltage to enable the light intensity modulators to work in a proper working area, and are driven by a multi-frequency pulse signal generated by the signal generator, so that multi-frequency light pulses are output.
In the above scheme, the pulse modulator is a semiconductor optical amplifier, and is configured to output pulsed light with a high extinction ratio.
The optical amplifying and filtering module comprises 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 noise brought by the optical amplifier;
the adjustable attenuator adjusts the filtered optical power and inputs signals into the sensing module.
In the above scheme, the optical amplifier is an erbium-doped fiber amplifier for amplifying pulse optical power; the optical filter is a band-pass filter and is used for filtering noise brought by the optical amplifier; the adjustable attenuator is used for adjusting the optical power to prevent the nonlinear effect from being caused and protecting the circuit.
The sensing module comprises a circulator and a sensing optical fiber; the input end of the sensing optical fiber is connected with the output end of the adjustable attenuator; the input end of the circulator is connected with the sensing optical fiber; the output end of the circulator is connected with the photoelectric detector; wherein:
the adjustable attenuator inputs signals into the sensing optical fiber and is connected into the photoelectric detector by the circulator.
In the above scheme, the sensing fiber is a single mode fiber, and the length of the sensing fiber is 10 Km.
In the scheme, two paths of laser emitted by a laser source are synchronously modulated and output through an intensity modulator to obtain a multi-frequency optical pulse sequence, each optical pulse frequency spectrum consists of a fixed single frequency and non-overlapping frequency band linear chirp, backward Rayleigh scattering signals with mutually separated frequency bands can be generated in a sensing optical fiber, and Rayleigh scattering patterns of each multi-frequency optical pulse can be obtained by using digital band-pass filters with different frequency bands; and carrying out correlation processing on the successively acquired Rayleigh scattering patterns in sequence to finally obtain the disturbance position and the disturbance size. The method effectively improves the response speed of the device to external disturbance, greatly improves the vibration magnitude and the vibration frequency measurement range, and is suitable for wide-frequency large vibration detection.
A phase sensitive optical time domain reflection method based on frequency modulation comprises the following steps:
s1: modulating laser to obtain a high extinction ratio multi-frequency pulse signal;
s2: amplifying the optical power of the high extinction ratio multi-frequency pulse signal, filtering noise and transmitting;
s3: receiving the transmitted optical signal and demodulating the optical signal; separating Rayleigh scattering pattern data with frequency bands which are not overlapped from a frequency domain by using N digital band-pass filters with different and non-overlapped frequency bands, respectively obtaining Rayleigh scattering patterns of N multi-frequency pulses, and performing cross-correlation operation on the measured Rayleigh scattering patterns and Rayleigh scattering reference patterns according to a window with a certain length, wherein the Rayleigh scattering patterns at the vibration position are shifted, so that the related peak is shifted to form a vibration area, and the dependent variable is determined by the offset of the cross-correlation peak, namely
Where K is the sweep rate, upsilon 0 And the central frequency is, and delta t is the offset of the cross correlation peak, so that the demodulation of the optical signal is completed.
Wherein the step S1 specifically includes the following steps:
s11: dividing the incident laser into two beams by a coupler, and respectively inputting the two beams into a first frequency modulator and a second frequency modulator;
s12: two driving signals are generated by a signal generator to drive a first frequency modulator and a second frequency modulator to modulate two beams of laser respectively;
the signal generator repeatedly outputs N linear frequency modulation pulses with different frequency bands to drive the first frequency modulator through one channel, and the repeated output interval is T; the other channel generates sinusoidal pulse signals synchronously outputting N fixed frequencies to drive a second frequency modulator, and the repeated output interval is T; the product of N and T is equal to the travel time T of the optical pulse in the sensing optical fiber R ；
S13: combining output optical signals of the first frequency modulator and the second frequency modulator into a multi-frequency optical signal through a coupler;
s14: a synchronous port output signal of one channel of the signal generator is used for driving the pulse modulator, and the pulse modulator modulates the multifrequency optical signal into a multifrequency pulse signal with a high extinction ratio.
Wherein the patrol time T R The expression is specifically as follows:
where L is the sensing length, n is the refractive index, and c is the speed of light in vacuum.
The signal generator generates two driving signals, specifically:
wherein, V 0 Is the drive signal amplitude; i is the ith multifrequency pulse; k is the sweep frequency rate; tau. p Is a multi-frequency pulse width; Δ F is the multi-frequency pulse bandwidth; f. of 0 For multi-frequency pulsesAn initial frequency; delta f is the minimum interval between the 1 st multifrequency pulse linear frequency sweeping part and the fixed single frequency; rect (-) is a rectangular function;
the first frequency modulator and the second frequency modulator work at proper working points through direct-current bias voltage, and the signal generator drives the first frequency modulator and the second frequency modulator through multi-frequency pulse signals and synchronously outputs the multi-frequency pulse signals to the pulse modulators; the multifrequency light pulse E (t) output after laser modulation is as follows:
E(t)＝E chirp (t)+E single (t)
wherein, E 0 Is the output signal amplitude; i is the ith multifrequency pulse; k is the sweep frequency rate; tau. p Is a multi-frequency pulse width; Δ F is the multi-frequency pulse bandwidth; m is the modulation depth; Δ f is the minimum interval between the 1 st multifrequency pulse linear sweep portion and the fixed single frequency, and rect (-) is a rectangular function.
Wherein, the step S3 specifically includes the following steps:
s31: in the transmission process of the high extinction ratio multi-frequency pulse signal, a generated backward Rayleigh scattering signal is converted into an electric signal through a photoelectric detector and is output to an acquisition card;
s32: fourier transformation is carried out on backscattering signals I (t) of N multi-frequency pulses obtained by a data acquisition card to obtain I (f); wherein I (f) ═ I 1 (f)+I 2 (f)；I 1 (f) The method is characterized by comprising the following steps of (1) forming by internal interference of Rayleigh scattered light of a linear part of a frequency spectrum of each multifrequency pulse and internal interference of fixed single-frequency Rayleigh scattered light; i is 2 (f) The interference of the linear part of the frequency spectrum of each multifrequency pulse and the Rayleigh scattered light of a fixed single frequency is formed;
s33: due to I 2 (f) Different and non-overlapping frequency bands in (1) 1 (f) AndI 2 (f) frequency bands are different and have no overlap, and I is respectively filtered by N digital band-pass filters with different frequency bands and no overlap 2 (f) The data in the step (1) are respectively taken out and subjected to inverse Fourier transform, so that N scattering patterns obtained by N multi-frequency pulses are restored;
s34: performing cross-correlation operation on the N scattering patterns respectively, wherein the relative peak of each scattering pattern is in a vibration area, and the magnitude of the dependent variable is determined by the offset of the cross-correlation peak, namely:
where K is the sweep rate, upsilon 0 At the center frequency, Δ t is the cross-correlation peak offset. Thus at the same cruising time T R In addition, the measurement speed is increased by N times, and the response bandwidth is also increased by N times.
Compared with the prior art, the technical scheme of the invention has the beneficial effects that:
the invention provides a phase sensitive optical time domain reflection device and method based on frequency modulation.A laser is divided into two paths, and a multi-frequency optical pulse sequence is obtained by synchronous modulation output of an intensity modulator, wherein each optical pulse frequency spectrum consists of a fixed single frequency and linear chirp without overlapping frequency bands; the sequence can generate backward Rayleigh scattering signals with mutually separated frequency bands in the sensing optical fiber, and a Rayleigh scattering pattern of each multi-frequency light pulse is obtained by using digital band-pass filters with different frequency bands; and carrying out correlation processing on the successively acquired Rayleigh scattering patterns in sequence to finally obtain the disturbance position and the disturbance size. The method effectively improves the response speed of the device to external disturbance, greatly improves the vibration frequency measurement range and the vibration size measurement range,
drawings
FIG. 1 is a schematic structural diagram of a phase sensitive optical time domain reflectometer based on frequency modulation;
FIG. 2 is a timing diagram of the output of the frequency modulation device;
FIG. 3 is a schematic diagram of the spacing between the front and rear of a multi-pulse injection fiber;
FIG. 4 is a schematic time domain diagram of a multifrequency pulse;
FIG. 5 is a frequency domain diagram of the backward Rayleigh scattered light generated by different multifrequency pulses;
wherein: 1. a laser light source; 2. a frequency modulation device; 201. a signal generator; 202. a first frequency modulator; 203. a second frequency modulator; 204. a pulse modulator; 205. a first coupler; 206. a second coupler; 3. a light amplifying and filtering module; 301. an optical amplifier; 302. an optical filter; 303. an adjustable attenuator; 4. a sensing module; 401. a circulator; 402. a sensing optical fiber; 5. a signal acquisition and demodulation device; 501. a photodetector; 502. collecting a card; 503. and a demodulation device.
Detailed Description
The drawings are for illustrative purposes only and are not to be construed as limiting the patent;
for the purpose of better illustrating the present embodiments, certain elements of the drawings may be omitted, enlarged or reduced, and do not represent the size of an actual product;
it will be understood by those skilled in the art that certain well-known structures in the drawings and descriptions thereof may be omitted.
The technical solution of the present invention is further described with reference to the drawings and the embodiments.
Example 1
As shown in fig. 1, the invention provides a phase-sensitive optical time domain reflection device based on frequency modulation, which includes a laser light source 1, a frequency modulation device 2, an optical amplification and filtering module 3, a sensing module 4, and a signal acquisition and demodulation device 5; the signal acquisition and demodulation device 5 comprises a photoelectric detector 501, an acquisition card 502 and a demodulation device 503; wherein:
the laser light source 1 emits laser light, and the laser light is modulated by the frequency modulation device 2 to obtain a multi-frequency pulse signal with a high extinction ratio;
the optical amplification and filtering module 3 amplifies the optical power of the high extinction ratio multi-frequency pulse signal and filters noise generated by amplification, and then the optical signal is input into the signal acquisition and demodulation device 5 through the sensing module 4;
the signal collecting and demodulating device 5 converts the optical signal into an electrical signal through the photoelectric detector 501 and outputs the electrical signal to the collecting card 502;
the demodulation device 503 decodes the data of the acquisition card 502, separates the rayleigh scattering pattern data with non-overlapping frequency bands from the frequency domain by using N digital band-pass filters with different and non-overlapping frequency bands, respectively obtains N rayleigh scattering patterns of multi-frequency pulses, and performs cross-correlation operation on the measured rayleigh scattering pattern and a rayleigh scattering reference pattern according to a window with a certain length, wherein the rayleigh scattering pattern at the vibration position is shifted, so that the related peak is shifted to a vibration region, and the magnitude of the dependent variable is determined by the shift of the cross-correlation peak, i.e. the rayleigh scattering pattern at the vibration position is shifted to a vibration region
Where K is the sweep rate, upsilon 0 At the center frequency, Δ t is the cross-correlation peak offset.
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 100 m. The purpose is to suppress the mutual interference between scattered light generated by the front and back multifrequency light pulses.
In the specific implementation process, the photodetector 501 is a photodetector, and the bandwidth must be greater than 10 GHz; the sampling rate of the acquisition card 502 is 15 GSa/s; the demodulation device 503 is a signal processing system, and performs digital filtering and related demodulation on the data of the acquisition card 502.
More specifically, the frequency modulation apparatus 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, and the first coupler is used for splitting laser into two beams which are respectively input into the first frequency modulator 202 and the second frequency modulator 203;
one channel of the signal generator 201 repeatedly outputs 100ns of chirp for driving the first frequency modulator 202; the other channel generates a sinusoidal pulse signal of synchronous output 100ns for driving the second frequency modulator 203; simultaneously, the synchronous port output signal of one channel is used for driving the pulse modulator 204 to perform pulse synchronous output; the output timing thereof 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 multifrequency optical signal into a multifrequency pulse signal with a high extinction ratio.
More specifically, as shown in fig. 3, the cruising time T R The expression is specifically as follows:
where L is the sensing length, n is the refractive index, and c is the speed of light in vacuum.
The multi-frequency pulse spectrum is characterized in that: the frequency spectrum of each multi-frequency pulse signal consists of two parts, one part is linear frequency sweep; the other part is a fixed single frequency; the interval between the two parts is larger than the linear sweep frequency range, the linear part sweep frequency range is the same (F is 1GHz), the minimum interval of the linear part between different pulses is delta F is 0.1GHz, the minimum interval of the single frequency and the linear part in each pulse is 1.1GHz + (N-1) (F + delta F), and N is the number of the multi-frequency pulses. The multi-frequency pulse time domain is characterized in that: the repeated output interval is 20us, and the total input N is 5 pulse widths which are the same (100 ns).
The time domain and frequency domain characteristics of the multifrequency pulse are shown in fig. 4.
More specifically, the optical amplifying 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 multifrequency pulse signal;
the optical filter 302 filters noise caused by the optical amplifier 301;
the adjustable attenuator 303 adjusts the filtered optical power and inputs the signal into the sensing module 4.
In a specific implementation process, the optical amplifier 301 is an erbium-doped fiber amplifier, and is configured to amplify pulsed optical power; the optical filter 302 is a band-pass filter and is used for filtering noise brought by the optical amplifier 301; the adjustable attenuator 303 is used to adjust the optical power to prevent non-linear 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 optical fiber 402 is connected with the output end of the adjustable attenuator 303; the input end of the circulator 401 is connected with the sensing optical fiber 402; the output end of the circulator 401 is connected with the photoelectric detector 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, the sensing fiber 402 is a single-mode fiber with a length of 10 Km.
In the specific implementation process, the device adopts frequency modulation to improve the spectrum utilization rate, although a plurality of multi-frequency pulses are input into the sensing fiber 402 at certain intervals to cause mutual aliasing in the time domain, the measurement results of the multi-frequency pulses can be separated from the frequency domain according to digital band-pass filters in different frequency bands due to different frequency bands of the multi-frequency pulses, and therefore a plurality of scattering curves can be obtained within one measurement time. Through carrying out correlation operation on a plurality of 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 magnitude and the vibration frequency measurement range are greatly improved.
In the specific implementation process, the device adopts frequency modulation to improve the measurement speed, and the measurement speed is increased within the same itinerant time T R N pulses are injected in, and the disturbance frequency which can be responded by the device is simultaneously increased by N times according to the Nyquist law. The improvement of the measurement speed means that the disturbance variable quantity between two measurements is smaller, so that the relative frequency shift quantity is reduced, the measurement precision is higher, and the requirement on the sweep frequency range is also reduced.
In the specific implementation process, the device adopts frequency modulation and only needs to change a driving circuit and a data processing mode, and a light path adopts a direct detection structure, so that the structure is simpler and easy to realize.
Example 2
More specifically, on the basis of embodiment 1, a phase-sensitive optical time domain reflection method based on frequency modulation is provided, which includes the following steps:
s1: modulating laser to obtain a high extinction ratio multi-frequency pulse signal;
s2: amplifying the optical power of the high extinction ratio multi-frequency pulse signal, filtering noise and transmitting;
s3: receiving the transmitted optical signal and demodulating the optical signal; separating Rayleigh scattering pattern data with frequency bands which are not overlapped from a frequency domain by using N digital band-pass filters with different and non-overlapped frequency bands, respectively obtaining Rayleigh scattering patterns of N multi-frequency pulses, and performing cross-correlation operation on the measured Rayleigh scattering patterns and Rayleigh scattering reference patterns according to a window with a certain length, wherein the Rayleigh scattering patterns at the vibration position are shifted, so that the related peak is shifted to form a vibration area, and the magnitude delta epsilon of the strain is determined by the offset of the cross-correlation peak, namely
Wherein K is sweep frequency rate, upsilon 0 And the central frequency is obtained, and the delta t is the offset of the cross correlation peak, so that the demodulation of the optical signal is completed.
More specifically, the step S1 specifically includes the following steps:
s11: dividing the incident laser into two beams by a coupler, and respectively inputting the two beams into a first frequency modulator and a second frequency modulator;
s12: two driving signals are generated by a signal generator to drive a first frequency modulator and a second frequency modulator to modulate two beams of laser respectively;
wherein, the signal generator repeatedly outputs N different frequency-band chirp pulses to drive the first oneA frequency modulator for repeatedly outputting a signal with an interval of T; the other channel generates sinusoidal pulse signals synchronously outputting N fixed frequencies to drive a second frequency modulator, and the repeated output interval is T; the product of N and T is equal to the travel time T of the optical pulse in the sensing optical fiber R ；
S13: combining output optical signals of the first frequency modulator and the second frequency modulator into a multi-frequency optical signal through a coupler;
s14: the pulse modulator is driven by the output signal of the synchronous port of one channel of the signal generator, and the multi-frequency optical signal is modulated into a multi-frequency pulse signal with a high extinction ratio by the pulse modulator.
More specifically, the cruising time T R The expression is specifically as follows:
where 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:
wherein, V 0 I is the ith multifrequency pulse for the amplitude of the drive signal; k is the sweep frequency rate; tau. p Is a multi-frequency pulse width; Δ F is the multifrequency pulse bandwidth; f. of 0 Is the start frequency of the multifrequency pulse; delta f is the minimum interval between the 1 st multifrequency pulse linear frequency sweeping part and the fixed single frequency, and rect (-) is a rectangular function;
the first frequency modulator and the second frequency modulator work at proper working points through direct-current bias voltage, and the signal generator drives the first frequency modulator and the second frequency modulator through multi-frequency pulse signals and synchronously outputs the multi-frequency pulse signals to the pulse modulators; the multifrequency light pulse E (t) output after laser modulation is as follows:
E(t)＝E chirp (t)+E single (t)
wherein E is 0 Is the output signal amplitude; i is the ith multifrequency pulse; k is the sweep frequency rate; tau is p Is a multi-frequency pulse width; Δ F is the multi-frequency pulse bandwidth; m is the modulation depth; delta f is the minimum interval between the 1 st multifrequency pulse linear frequency sweeping part and the fixed single frequency; rect (-) is a rectangular function.
More specifically, the step S3 specifically includes the following steps:
s31: in the transmission process of the high extinction ratio multi-frequency pulse signal, a generated backward Rayleigh scattering signal is converted into an electric signal through a photoelectric detector and is output to an acquisition card;
s32: fourier transform is carried out on backscattering signals I (t) of N multi-frequency pulses obtained by a data acquisition card to obtain I (f); wherein I (f) ═ I 1 (f)+I 2 (f)；I 1 (f) The system is composed of internal interference of Rayleigh scattering light of a linear part of a frequency spectrum of each multi-frequency pulse and internal interference of fixed single-frequency Rayleigh scattering light; I.C. A 2 (f) The interference of the linear part of the frequency spectrum of each multifrequency pulse and the Rayleigh scattered light of a fixed single frequency is formed; as shown in fig. 5.
S33: due to I 2 (f) Different and non-overlapping frequency bands in (1) 1 (f) And I 2 (f) Frequency bands are different and have no overlap, and I is respectively filtered by N digital band-pass filters with different frequency bands and no overlap 2 (f) The data in the step (2) are respectively taken out and subjected to inverse Fourier transform, so that N scattering patterns obtained by N multi-frequency pulses are restored;
s34: the N scattering patterns are subjected to cross-correlation operation respectively, the relative peak of the N scattering patterns is shifted to form a vibration area, and the magnitude of the dependent variable of the N scattering patterns is determined by the offset of the cross-correlation peak, namely:
wherein K is sweep frequency rate, upsilon 0 At the center frequency, Δ t is the cross-correlation peak offset. Thus at the same cruising time T R In addition, the measurement speed is increased by N times, and the response bandwidth is also increased by N times.
In a specific implementation process, the response bandwidth of the vibration frequency of the embodiment depends on the repetition frequency f of the multi-frequency light pulse, and the response bandwidth is 0.5f obtained by the nyquist theorem. When the sensing length is 10Km, the response bandwidth of a typical system is 5KHz, and the response bandwidth of the embodiment can reach 25KHz, which is improved by 5 times.
The vibration magnitude measurement range of the present embodiment is affected by the repetition frequency f of the multi-frequency light pulse. The frequency variation caused by two measurements cannot be larger than 5%, otherwise the correlation of scattering patterns obtained by the previous and subsequent measurements will be reduced to cause demodulation errors. When the repetition frequency f is increased, the frequency variation caused by two measurements is relatively reduced, so that the measurement range of the vibration magnitude is relatively improved.
According to the specific example, the invention provides the phase sensitive optical time domain reflection device and method based on frequency modulation, which solve the problem of signal aliasing caused by driving a plurality of pulses in a repetition period in the traditional device, shorten the detection interval time, greatly improve the measurement speed and improve the broadband large-vibration measurement capability of the system.
It should be understood that the above-described embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. Other variations and modifications will be apparent to persons skilled in the art in light of the above description. This need not be, nor should it be exhaustive of all embodiments. Any modification, equivalent replacement, and improvement made within the spirit and principle of the present invention should be included in 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 is characterized by comprising a laser light source (1), a frequency modulation device (2), an optical amplification and filtering module (3), a sensing module (4) and a signal acquisition and demodulation device (5); the signal acquisition and demodulation device (5) comprises a photoelectric detector (501), an acquisition card (502) and a demodulation device (503); wherein:
the laser light source (1) emits laser, and the laser is modulated by the frequency modulation device (2) to obtain a high extinction ratio multi-frequency pulse signal;
the optical amplification and filtering module (3) amplifies the optical power of the high extinction ratio multi-frequency pulse signal, filters noise generated by amplification, and inputs the optical signal into the signal acquisition and demodulation device (5) through the sensing module (4);
the signal acquisition and demodulation device (5) converts an optical signal into an electric signal through the photoelectric detector (501) and outputs the electric signal to the acquisition card (502);
the demodulation device (503) decodes the data of the acquisition card (502), separates the rayleigh scattering pattern data with non-overlapping frequency bands from the frequency domain by using N digital band-pass filters with different and non-overlapping frequency bands, respectively obtains the rayleigh scattering patterns of N multi-frequency pulses, and performs cross-correlation operation on the measured rayleigh scattering pattern and a rayleigh scattering reference pattern according to a window with a certain length, wherein the rayleigh scattering pattern at the vibration position is shifted, so that the related peak is shifted to form a vibration region, and the magnitude of the dependent variable is determined by the shift of the cross-correlation peak, namely the shift of the cross-correlation peak
Where K is the sweep rate, upsilon 0 Is the center frequency, Δ t is the cross-correlation peak offset;
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 second coupler (206); wherein:
the splitting ratio of the first coupler (205) is 50:50, and the first coupler is used for splitting laser into two beams which are respectively input into the first frequency modulator (202) and the second frequency modulator (203);
one channel of the signal generator (201) repeatedly outputs N chirp pulses of different frequency bands for driving the first frequency modulator (202), and the repeated output interval is T; the other channel generates synchronous output N sinusoidal pulse signals with fixed frequency for driving the second frequency modulator (203), and the repeated output interval is T; simultaneously using the sync port output signal of one of the channels for driving the pulse modulator (204); wherein, the product of N and T is equal to the travel time T of the optical pulse in the sensing optical fiber R ；
The second coupler (206) combines the output optical signals of the first frequency modulator (202) and the second frequency modulator (203) into a multifrequency optical signal;
the pulse modulator (204) modulates the multifrequency optical signal into a multifrequency pulse signal with a high extinction ratio;
the optical amplification and filtering module (3) comprises 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 noise brought by the optical amplifier (301);
the adjustable attenuator (303) adjusts the filtered optical power and inputs a signal into the sensing module (4);
the sensing module (4) comprises a circulator (401) and a sensing optical fiber (402); the input end of the sensing optical fiber (402) is connected with the output end of the adjustable attenuator (303); the input end of the circulator (401) is connected with the sensing optical fiber (402); the output end of the circulator (401) is connected with the photoelectric detector (501); wherein:
the adjustable attenuator (303) inputs signals into the sensing optical fiber (402) and is connected into the photoelectric detector (501) by the circulator (401).
2. The device according to claim 1, wherein the device is a phase-sensitive optical time domain reflectometer based on frequency modulationCharacterized in that said itinerant time T R The expression is as follows:
where 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 by comprising the following steps:
s1: modulating laser to obtain a high extinction ratio multi-frequency pulse signal;
s2: amplifying the optical power of the high extinction ratio multi-frequency pulse signal, filtering noise and transmitting;
s3: receiving the transmitted optical signal and demodulating the optical signal; separating Rayleigh scattering pattern data with frequency bands which are not overlapped from a frequency domain by using N digital band-pass filters with different and non-overlapped frequency bands to respectively obtain Rayleigh scattering patterns of N multifrequency pulses, and performing cross-correlation operation on the measured Rayleigh scattering patterns and Rayleigh scattering reference patterns according to a window with a certain length, wherein the Rayleigh scattering patterns at the vibration position shift, so that the related peak shifts, namely a vibration area, and the magnitude delta epsilon of the strain is determined by the shift of the cross-correlation peak, namely
Where K is the sweep rate, upsilon 0 The central frequency is delta t, and the cross correlation peak offset is delta t, so that the demodulation of the optical signal is completed;
wherein the step S1 specifically includes the following steps:
s11: dividing the incident laser into two beams by a coupler, and respectively inputting the two beams into a first frequency modulator and a second frequency modulator;
s12: two driving signals are generated by a signal generator to drive a first frequency modulator and a second frequency modulator to modulate two beams of laser respectively;
the signal generator repeatedly outputs N linear frequency modulation pulses with different frequency bands to drive the first frequency modulator through one channel, and the repeated output interval is T; the other channel generates sinusoidal pulse signals synchronously outputting N fixed frequencies to drive a second frequency modulator, and the repeated output interval is T; the product of N and T is equal to the travel time T of the optical pulse in the sensing optical fiber R ；
S13: combining output optical signals of the first frequency modulator and the second frequency modulator into a multi-frequency optical signal through a coupler;
s14: a synchronous port output signal of one channel of the signal generator is used for driving a pulse modulator, and the pulse modulator is used for modulating the multifrequency optical signal into a multifrequency pulse signal with a high extinction ratio;
the step S3 specifically includes the following steps:
s31: in the transmission process of the high extinction ratio multi-frequency pulse signal, a generated backward Rayleigh scattering signal is converted into an electric signal through a photoelectric detector and is output to an acquisition card;
s32: fourier transform is carried out on backscattering signals I (t) of N multi-frequency pulses obtained by a data acquisition card to obtain I (f); wherein I (f) is 1 (f)+I 2 (f)；I 1 (f) The method is characterized by comprising the following steps of (1) forming by internal interference of Rayleigh scattered light of a linear part of a frequency spectrum of each multifrequency pulse and internal interference of fixed single-frequency Rayleigh scattered light; I.C. A 2 (f) The interference of the linear part of the frequency spectrum of each multi-frequency pulse and the Rayleigh scattering light of a fixed single frequency is formed;
s33: due to I 2 (f) The middle frequency bands are different and have no overlap, I 1 (f) And I 2 (f) Frequency bands are different and have no overlap, and I is respectively filtered by N digital band-pass filters with different frequency bands and no overlap 2 (f) The data in the step (1) are respectively taken out and subjected to inverse Fourier transform, so that N scattering patterns obtained by N multi-frequency pulses are restored;
s34: the N scattering patterns are subjected to cross-correlation operation respectively, the relative peak of the N scattering patterns is shifted to form a vibration area, and the magnitude of the dependent variable of the N scattering patterns is determined by the offset of the cross-correlation peak, namely:
where K is the sweep rate, upsilon 0 At the center frequency, Δ t is the cross-correlation peak offset.
4. The phase-sensitive optical time domain reflectometry method based on frequency modulation as in claim 3, wherein the tour time T is R The expression is specifically as follows:
where L is the sensing length, n is the refractive index, and c is the speed of light in vacuum.
5. The phase-sensitive optical time domain reflectometry method based on frequency modulation as in claim 4, wherein the signal generator generates two driving signals specifically:
wherein, V 0 I is the ith multi-frequency pulse for the amplitude of the driving signal; k is the sweep frequency rate; tau. p Is a multi-frequency pulse width; Δ F is the multifrequency pulse bandwidth; f. of 0 Is the start frequency of the multifrequency pulse; delta f is the minimum interval between the 1 st multifrequency pulse linear frequency sweeping part and the fixed single frequency, rect (-) is a rectangular function;
the first frequency modulator and the second frequency modulator work at proper working points through direct-current bias voltage, and the signal generator drives the first frequency modulator and the second frequency modulator through multi-frequency pulse signals and synchronously outputs the multi-frequency pulse signals to the pulse modulators; the multifrequency light pulse E (t) output after laser modulation is as follows:
E(t)＝E chirp (t)+E single (t)
wherein E is 0 Is the output signal amplitude; i is the ith multifrequency pulse; k is the sweep frequency rate; tau. p Is a multi-frequency pulse width; Δ F is the multifrequency pulse bandwidth; m is the modulation depth; delta f is the minimum interval between the 1 st multi-frequency pulse linear frequency sweeping part and the fixed single frequency; rect (-) is a rectangular function.
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