Dual-pulse long-distance distributed vibration detection device and method based on OFDR principle

Abstract

The application discloses a kind of based on the principle of OFDR's double-pulse long-distance distributed vibration detection device, including narrow linewidth laser light source and to-be-measured interferometer, and the application also includes auxiliary interferometer and phase noise demodulation and elimination mechanism, narrow linewidth laser light source includes NLL laser and first fiber coupler, the first output channel and the second output channel of first fiber coupler are connected with to-be-measured interferometer and auxiliary interferometer respectively, and to-be-measured interferometer and auxiliary interferometer are connected with phase noise demodulation and elimination mechanism.The application uses the above structure based on the principle of OFDR's double-pulse long-distance distributed vibration detection device, utilizes two homologous pulses to carry out Rayleigh scattering detection, mixes with auxiliary interferometer, realizes the elimination of phase noise, breaks through the limit of coherence length to range, realizes long range, frequency response high vibration signal detection, and only needs to mix in optical path twice, demodulation algorithm is simple, and phase correction speed is fast.

Description

TECHNICAL FIELD

[0001] The present application relates to a vibration measurement technology, and in particular to a double-pulse long-distance distributed vibration detection device and detection method based on the principle of OFDR. BACKGROUND

[0002] Compared with the traditional sensing system, the distributed optical fiber sensing system has the advantages of anti-electromagnetic interference, corrosion resistance, high measurement accuracy, and multi-point detection, and is widely used in vibration detection within the range of optical fiber lines.

[0003] At present, the main development directions of distributed optical fiber sensing technology include: phase-sensitive optical time domain reflectometry (Phase-sensitive Optical Time Domain Reflectometry, OTDR), chirped-pulse optical time domain reflectometry (Chirped-pulse Optical Time Domain Reflectometry, CP-OTDR) and time-gated optical frequency domain reflectometry (Time-gated Digital Optical Time Domain Reflectometry, TGD-OFDR).

[0004] Among them, The basic principle of OTDR technology is to inject a highly coherent light pulse into the device to be measured, and then detect the backscattered Rayleigh scattering signal generated in the device to be measured by an optical detector. This basic principle was first proposed by Taylor in 1993, but later in order to improve the signal-to-noise ratio and detection distance of the OTDR system, the coherence technology was introduced into the system, and finally the phase information of the Rayleigh scattering was reflected in the form of interference intensity. The specific principle is as follows:

[0005] Figure 1 The working principle diagram of the traditional OTDR is shown in Figure 1. Figure 1 ​​​As shown, the swept frequency pulse laser emitted by the narrow linewidth laser source 51 is injected into the 98:2 fiber coupler 52 through the input channel, and is split by the fiber coupler, and is output from the first and second output channels 52A, 52B into the reference arm and the measurement arm, respectively, wherein the first output channel 52A outputs light power accounting for 98% of the total input light power, and the second output channel 52B outputs light power accounting for 2% of the total input light power. The output light of the first output channel 52A is injected into the acousto-optic modulator (AOM) 53, and the frequency shift of the acousto-optic modulator 53 generates a certain frequency difference with the reference light. The laser with a certain frequency shift is injected from the first channel 54A of the optical circulator 54 and is output from the second channel 54B, and is injected into the device under test 55. In the device under test 55, the signal light formed by Fresnel reflection or back Rayleigh scattering is input from the second channel 54B of the optical circulator 54 and is output from the third channel 54C. The reference light output from the second output channel 52B of the 98:2 fiber coupler and the signal light output from the third channel 54C of the optical circulator 54 are respectively injected into the first and second input channels (56A, 56B) of the 50:50 fiber coupler 56, and interference occurs in the 50:50 fiber coupler 56 to form an interference signal, which is respectively injected into the first and second output channels (56C, 56D) of the 50:50 fiber coupler 56. The light signal is converted into an electrical signal by the balanced detector 57, and is input into the signal demodulation circuit 58spu after demodulation, and is input into the computer 59.

[0006] As can be seen from the above, the two light signals input from the first and second input channels 56A, 56B of the 50:50 fiber coupler 56 come from the same narrow linewidth laser source 51, wherein one signal has an amplitude of E s (t), which comes from the Fresnel reflection or back Rayleigh scattering in the device under test 55 in the measurement arm, that is:

[0007]

[0008] wherein w0 is the center frequency of the output light of the narrow linewidth laser, Δw AOM is the frequency shift amount generated by the AOM, R is the backscattering coefficient of the optical fiber, is the phase information carried by the light source, τ d represents the time delay between the two light signals.

[0009] The other signal has an amplitude of E f (t), which comes from the reference light generated by the second output channel 52B of the 98:2 fiber coupler 52, and can be expressed as:

[0010]

[0011] wherein w0 and have the same physical meanings as in the measurement arm.

[0012] Therefore, the intensity of the AC component of the interference signal generated by the above two signals can be expressed as:

[0013]

[0014] In the formula, e(t)-e(t-τ) is the phase noise or nonlinear phase in the interference signal.

[0015] The phase noise or nonlinear phase generated by interference in the optical time domain reflection system will cause the backscattered light energy to spread, the reflection peak intensity to decrease, and the spatial resolution of the detection to decrease. Therefore, in order to ensure that the detection system has high spatial resolution, the phase noise and nonlinear phase in the system should be suppressed and eliminated.

[0016] The optical path structure of the CP-OTDR technology depends on the direct detection of the -OTDR, but unlike the -OTDR, this technology uses a series of linearly chirped optical pulses as probe light, greatly changing and simplifying the optical path structure, and having good robustness to the phase noise of the laser, but the spatial resolution is limited by the pulse width.

[0017] The TGD-OFDR technology is a distributed optical fiber sensing technology proposed by Liu et al. in 2015. Compared with the traditional OFDR system, the system uses the swept laser generated by the acousto-optic modulator for modulation as signal light, which is only selected in a narrow time window, and interferes with the wavelength-stable continuous laser in the reference path. Since the input pulse in the test path has a short pulse duration, the laser tuning rate can be made very high without affecting the measurement distance. However, this technology is currently limited by the nonlinear noise factors of frequency tuning.

[0018] At present, most of the designs of distributed vibration detection devices are mainly based on the phase-sensitive optical time domain reflection technology, i.e. the -OTDR method. However, this method has the problem that the detection result is limited by the phase noise in the system. The limitation of the phase noise will cause the Rayleigh scattering to be masked by the phase noise when the system performs long-distance vibration measurement, which limits the measurement distance of the -OTDR system. Therefore, solving this problem is also the top priority for technical breakthroughs in this field. In recent years, some scholars have proposed to partially improve the - Methods for testing distance using OTDR systems. In 2010, Zhang Dacheng et al. of Shanghai Huwei Fiber Optic Sensing Technology Co., Ltd. disclosed a high signal-to-noise ratio and large dynamic range optical acquisition system (CN101660944A). This system proposes a time-division receiving method from the perspective of detector detection mode. It divides the scattered light within a pulse period into time periods, uses detectors with different performance to detect the scattered light signals, and finally integrates the obtained optical signals to obtain the required measurement results.

[0019] In 2012, Lin Yanguo of Hangzhou Anyuan Technology Co., Ltd. proposed a phase-sensitive optical time-domain reflectometry sensing system and method (CN201210060043.X). This method is based on direct detection. The OTDR method, using a dual-light source approach, increases the detection range of a distributed fiber optic sensor by detecting different distances with beams of varying power, without increasing processing time.

[0020] In 2020, Wang Zhenwei and others from Shanghai University of Engineering Science disclosed a method based on - The composite fiber optic vibration sensing system of OTDR (CN202011420563.8) uses single-mode fiber as the sensing medium and simultaneously uses single-mode fiber and multimode fiber as sensing elements. It uses multimode fiber as a mode converter to inject high-power laser pulses into single-mode fiber, which effectively avoids the nonlinear effects caused by directly injecting high-power detection pulses into single-mode fiber, thereby increasing the detection distance without affecting the spatial resolution.

[0021] Based on the results proposed by researchers to date, while these methods have improved the performance of distributed vibration detection instruments to some extent, they have come at the cost of testing time or cost, or are limited by nonlinear effects caused by light source power, thus having shortcomings. Therefore, there is a need to propose a new distributed vibration detection method that can ensure both high spatial resolution and a long testing distance. Summary of the Invention

[0022] The purpose of this invention is to provide a dual-pulse long-distance distributed vibration detection device based on the OFDR principle. It uses two pulses from the same source for Rayleigh scattering detection and mixes them with an auxiliary interferometer to eliminate phase noise. This overcomes the limitation of coherence length on the measurement range and enables the detection of vibration signals with long range and high frequency response. Moreover, only two mixing operations are required in the optical path, the demodulation algorithm is simple, and the phase correction speed is fast.

[0023] In order to achieve the above object, the application provides a double-pulse long-distance distributed vibration detection device based on the OFDR principle, which comprises a narrow-line-width laser light source and a to-be-detected interferometer, and further comprises an auxiliary interferometer and a phase noise demodulation and elimination mechanism.

[0024] Preferably, the to-be-detected interferometer comprises a second optical fiber coupler connected with the first output channel of the first optical fiber coupler, and the first output channel and the second output channel of the second optical fiber coupler are connected with a homologous pulse modulation mechanism and a 90° optical bridge respectively.

[0025] The homologous pulse modulation mechanism comprises an acousto-optic modulator, the output channel of the acousto-optic modulator is connected with the input channel of a third optical fiber coupler, the first output channel and the second output channel of the third optical fiber coupler are connected with a first delay optical fiber and a single sideband modulator respectively, the first delay optical fiber and the single sideband modulator are connected with the first input channel and the second input channel of a fourth optical fiber coupler respectively, the output channel of the fourth optical fiber coupler is connected with the input port of an optical circulator, and the first output channel and the second output channel of the optical circulator are connected with a to-be-detected device and the 90° optical bridge respectively.

[0026] The first output channel and the second output channel of the 90° optical bridge are connected with the phase noise demodulation and elimination mechanism through a first balanced detector and a second balanced detector respectively.

[0027] The acousto-optic modulator and the single sideband modulator are connected with an arbitrary waveform generator.

[0028] Preferably, the auxiliary interferometer comprises a fifth optical fiber coupler connected with the second output channel of the first optical fiber coupler, the first output channel and the second output channel of the fifth optical fiber coupler are connected with a second delay optical fiber and the second input channel of a sixth optical fiber coupler respectively, the second delay optical fiber is connected with the first input channel of the sixth optical fiber coupler, and the first output channel and the second output channel of the sixth optical fiber coupler are connected with a third balanced detector, and the third balanced detector is connected with the phase noise demodulation and elimination mechanism.

[0029] Preferably, the phase noise demodulation and elimination mechanism comprises an optical-electric conversion and signal processing unit and an upper computer connected with the first balanced detector, the second balanced detector and the third balanced detector respectively.

[0030] Preferably, the splitting ratios of the first fiber coupler and the second fiber coupler are both 98:2.

[0031] The splitting ratios of the third fiber coupler, the fourth fiber coupler, the fifth fiber coupler and the sixth fiber coupler are 50:50.

[0032] Preferably, the first delay fiber and the second delay fiber have the same length.

[0033] The detection method of the double-pulse long-distance distributed vibration detection device based on the OFDR principle comprises the following steps:

[0034] S1, the output light of the narrow-bandwidth laser light source is split by the first fiber coupler according to a splitting ratio of 98:2 and is input into the second fiber coupler and the fifth fiber coupler respectively;

[0035] S2, after the laser signal enters the second fiber coupler, it is input into an acousto-optic modulator and a 90° optical bridge according to a splitting ratio of 98:2; the laser signal passing through the acousto-optic modulator is processed by wave cutting to form a rectangular pulse laser signal, which is uniformly split by the third fiber coupler and then injected into the first delay fiber and a single-sideband modulator respectively; the single-sideband modulator controlled by an arbitrary waveform generator converts the rectangular pulse laser signal into a sweep pulse signal, which is injected into the fourth fiber coupler together with another rectangular pulse laser signal that has been delayed in the first delay fiber; the output light of the fourth fiber coupler enters the partial fiber of the device to be measured after passing through an optical circulator; due to the piezoelectric effect caused by vibration, Rayleigh scattering occurs in the fiber, and the phase information generated by the Rayleigh scattering is carried by the output light; the output light enters the 90° optical bridge after passing through the optical circulator and is detected by the first balanced detector and the second balanced detector together with the laser signal with a proportion of 2% split by the second fiber coupler;

[0036] The laser signal entering the fifth fiber coupler is uniformly split and injected into the second delay fiber and the sixth fiber coupler respectively; the laser signal injected into the second delay fiber is delayed and also injected into the sixth fiber coupler; the two laser signals interfere with each other in the sixth fiber coupler and are uniformly split and injected into the third balanced detector;

[0037] S3, the laser signals output by the first balanced detector, the second balanced detector and the third balanced detector are processed by the phase noise elimination and signal demodulation algorithm in the phase noise demodulation and elimination mechanism, and the information generated by the vibration of the device to be measured can be obtained.

[0038] Preferably, in step S2:

[0039] The narrow bandwidth laser light source enters an acousto-optic modulator, and is controlled by an arbitrary waveform generator to perform wave cutting to form a rectangular pulse signal lasting for a certain time. The signal is uniformly split by a third optical fiber coupler to form a pair of homologous pulses with the same wavelength, frequency and phase, which enter the upper path of the first delay optical fiber and the lower path of the single sideband modulator respectively through the first output channel and the second output channel of the third optical fiber coupler.

[0040] In the upper path of the first delay optical fiber, the first delay optical fiber adds a microsecond-level delay to the pulse signal, so as to separate the pulse signal from the lower path of the single sideband modulator in the time domain.

[0041] In the lower path of the single sideband modulator, the single sideband modulator is controlled by the arbitrary waveform generator to perform waveform modulation on the input rectangular pulse signal to form a sweep pulse signal with a linearly changing pulse frequency in a period.

[0042] After the modulation, the upper and lower path pulse signals are input into the fourth optical fiber coupler through the first input channel and the second input channel of the fourth optical fiber coupler, so as to obtain homologous double pulses.

[0043] Preferably, in step S3:

[0044] Suppose that the delay between the homologous double pulses is τ D , and the delay generated in the device under test is τ i ; therefore, the result of the interference between the sweep pulse injected into the single sideband modulator and the reference arm should satisfy:

[0045]

[0046] And the result of the interference between the pulse injected into the first delay optical fiber and the reference arm should satisfy:

[0047]

[0048] The formula (1) and the formula (2) contain the same phase term After the delay of τ D , the mixing of the two formulas can eliminate to obtain the result:

[0049]

[0050] It can be seen that although the phase noise in the optical path is partially eliminated after the homologous double pulse structure, there are still two phase terms in the formula (3), one of which is the residual phase noise , that is, the phase noise accumulated by the narrow bandwidth laser light source in τ D , and the delay τ Dthe test result is mixed with the above formula for the second time, the phase noise in the test result can be completely eliminated; the other is Φ(τ i ,t+τ D )-Φ(τ i ,t), which can represent the change of the phase of Rayleigh scattering at the same point within time τ D , and further realize the demodulation of the frequency and amplitude of the vibration signal at the point.

[0051] Therefore, the present application has the following beneficial effects:

[0052] 1. The phase noise of the detection result is compensated by using the homologous double-pulse structure, the signal output by the auxiliary interferometer is mixed, the limitation of the coherence length on the range is broken, and the vibration signal outside the coherence length is obtained.

[0053] 2. The detection is performed by using the homologous double-pulse structure, the frequency response of the test is improved, the beat frequency signal in the double-pulse test result only needs to be separated by frequency domain filtering, and the frequency response only depends on the double-pulse injection interval (microsecond level).

[0054] 3. Only two mixings are needed, the demodulation algorithm has low complexity, the phase correction speed is fast, and the hardware adaptation is easy.

[0055] The technical solutions of the present application are further described in detail below by means of the drawings and examples. BRIEF DESCRIPTION OF DRAWINGS

[0056] Figure 1 is the working principle diagram of the traditional -OTDR;

[0057] Figure 2 is the principle diagram of the present application;

[0058] Figure 3 is the structure diagram of example 1 of the present application;

[0059] Figure 4 is the laser signal modulation diagram of the homologous pulse module of example 1 of the present application.

[0060] 1. narrow linewidth laser light source; 11, NLL laser; 12, input channel of the first optical fiber coupler; 13, first optical fiber coupler; 13A, first output channel of the first optical fiber coupler; 13B, second output channel of the first optical fiber coupler;

[0061] 2, the measured interferometer; 21, the input channel of the second fiber coupler; 22, the second fiber coupler; 22A, the first output channel of the second fiber coupler; 22B, the second output channel of the second fiber coupler; 23, the homodyne pulse modulation mechanism; 231, the acousto-optic modulator; 232, the output channel of the acousto-optic modulator; 233, the third fiber coupler; 233A, the first output channel of the third fiber coupler; 233B, the second output channel of the third fiber coupler; 234, the first delay fiber; 235, the single sideband modulator; 236, the arbitrary waveform generator; 237, the fourth fiber coupler; 237A, the first input channel of the fourth fiber coupler; 237B, the second input channel of the fourth fiber coupler; 238, the output channel of the fourth fiber coupler; 24, the optical circulator; 24A, the input channel of the optical circulator; 24B, the first output channel of the optical circulator; 24C, the second output channel of the optical circulator; 25, the device under test; 26, the 90° optical bridge; 27, the first balanced detector; 28, the second balanced detector;

[0062] 3, the auxiliary interferometer; 31, the input channel of the fifth fiber coupler; 32, the fifth fiber coupler; 32A, the first output channel of the fifth fiber coupler; 32B, the second output channel of the fifth fiber coupler; 33, the second delay fiber; 34, the sixth fiber coupler; 34A, the first input channel of the sixth fiber coupler; 34B, the second input channel of the sixth fiber coupler; 34C, the first output channel of the sixth fiber coupler; 34D, the first output channel of the sixth fiber coupler; 35, the third balanced detector;

[0063] 4, the phase noise demodulation and elimination mechanism; 41, the photoelectric conversion and signal processing unit; 42, the host computer. DETAILED DESCRIPTION

[0064] The present application will be further described below in conjunction with the drawings, and it should be noted that the present embodiment is based on the technical solution, and detailed implementation and specific operation process are given, but the protection scope of the present application is not limited to the present embodiment.

[0065] Figure 2 As shown in the schematic diagram of the present application, Figure 2 the structure of the present application includes a narrow linewidth laser light source 1 and a measured interferometer 2, and the present application further includes an auxiliary interferometer 3 and a phase noise demodulation and elimination mechanism 4, the narrow linewidth laser light source 1 includes an NLL laser 11 and a first fiber coupler 13 connected to the NLL laser 11, the first output channel 13A and the second output channel 13B of the first fiber coupler 13 are respectively connected to the measured interferometer 2 and the auxiliary interferometer 3, and the measured interferometer 2 and the auxiliary interferometer 3 are connected to the phase noise demodulation and elimination mechanism 4.

[0066] The auxiliary interferometer 3 comprises a fifth optical fiber coupler 32 connected with the second output channel 13B of the first optical fiber coupler 13, the first output channel 32A and the second output channel 32B of the fifth optical fiber coupler 32 are connected with a second delay optical fiber 33 and the second input channel 34B of a sixth optical fiber coupler 34 respectively, the second delay optical fiber 33 is connected with the first input channel 34A of the sixth optical fiber coupler 34, the first output channel 34C and the second output channel 34D of the sixth optical fiber coupler 34 are connected with a third balanced detector 35 respectively, and the third balanced detector 35 is connected with the phase noise demodulation and elimination mechanism 4.

[0067] The auxiliary interferometer 3 comprises a fifth optical fiber coupler 32 connected with the second output channel 13B of the first optical fiber coupler 13, the first output channel 32A and the second output channel 32B of the fifth optical fiber coupler 32 are connected with a second delay optical fiber 33 and the second input channel 34B of a sixth optical fiber coupler 34 respectively, the second delay optical fiber 33 is connected with the first input channel 34A of the sixth optical fiber coupler 34, the first output channel 34C and the second output channel 34D of the sixth optical fiber coupler 34 are connected with a third balanced detector 35 respectively, and the third balanced detector 35 is connected with the phase noise demodulation and elimination mechanism 4.

[0068] The auxiliary interferometer 3 comprises a fifth optical fiber coupler 32 connected with the second output channel 13B of the first optical fiber coupler 13, the first output channel 32A and the second output channel 32B of the fifth optical fiber coupler 32 are connected with a second delay optical fiber 33 and the second input channel 34B of a sixth optical fiber coupler 34 respectively, the second delay optical fiber 33 is connected with the first input channel 34A of the sixth optical fiber coupler 34, the first output channel 34C and the second output channel 34D of the sixth optical fiber coupler 34 are connected with a third balanced detector 35 respectively, and the third balanced detector 35 is connected with the phase noise demodulation and elimination mechanism 4.

[0069] Preferably, the splitting ratios of the first fiber coupler 13 and the second fiber coupler 22 are both 98:2; the splitting ratios of the third fiber coupler 233, the fourth fiber coupler 237, the fifth fiber coupler 32 and the sixth fiber coupler 34 are 50:50.

[0070] Preferably, the first delay fiber 234 and the second delay fiber 33 have the same length.

[0071] The detection method of the double-pulse long-distance distributed vibration detection device based on the OFDR principle comprises the following steps:

[0072] S1, the output light of the narrow-bandwidth laser light source 1 is split by the first fiber coupler 13 and input into the second fiber coupler 22 and the fifth fiber coupler 32 according to a splitting ratio of 98:2;

[0073] S2, after the laser signal enters the second fiber coupler 22, it is input into the acousto-optic modulator 231 and the 90° optical bridge 26 according to a splitting ratio of 98:2; the laser signal passing through the acousto-optic modulator 231 is processed by wave cutting to form a rectangular pulse laser signal, which is uniformly split by the third fiber coupler 233 and injected into the first delay fiber 234 and the single-sideband modulator 235; the arbitrary waveform generator 236 controls the single-sideband modulator 235 to convert the rectangular pulse laser signal into a sweep pulse signal, which is injected into the fourth fiber coupler 237 together with another rectangular pulse laser signal that has been delayed in the first delay fiber 234; the output light of the fourth fiber coupler 237 enters the part of the optical fiber of the device under test 25 after passing through the optical circulator 24; due to the piezoelectric effect caused by vibration, Rayleigh scattering occurs in the optical fiber, and the phase information generated by the Rayleigh scattering is carried by the output light, which enters the 90° optical bridge 26 after passing through the optical circulator 24 and is detected by the first balanced detector 27 and the second balanced detector 28 together with the laser signal with a proportion of 2% split by the second fiber coupler 22;

[0074] The laser signal entering the fifth fiber coupler 32 is uniformly split and injected into the second delay fiber 33 and the sixth fiber coupler 34; the laser signal injected into the second delay fiber 33 is delayed and also injected into the sixth fiber coupler 34; the two laser signals interfere with each other in the sixth fiber coupler 34 and are uniformly split and injected into the third balanced detector 35;

[0075] Preferably, in step S2:

[0076] The narrow bandwidth laser light source 1 enters the acousto-optic modulator 231, and the arbitrary waveform generator 236 controls the acousto-optic modulator 231 to perform wave cutting to form a rectangular pulse signal lasting for a certain time. The signal is uniformly split by the third optical fiber coupler 233 to form a pair of homologous pulses with the same wavelength, frequency and phase, which enter the upper path of the first delay optical fiber 234 and the lower path of the single sideband modulator respectively through the first output channel 233A and the second output channel 233B of the third optical fiber coupler 233.

[0077] In the upper path of the first delay optical fiber 234, the first delay optical fiber 234 adds a microsecond-level delay to the pulse signal, so that the pulse signal is separated from the pulse signal in the lower path of the single sideband modulator 235 in the time domain.

[0078] In the lower path of the single sideband modulator 235, the single sideband modulator 235 is controlled by the arbitrary waveform generator 236 to perform waveform modulation on the input rectangular pulse signal to form a sweep pulse signal with a linearly changing pulse frequency in a period.

[0079] After the upper and lower path pulses are modulated, they are input into the fourth optical fiber coupler through the first input channel 237A and the second input channel 237B of the fourth optical fiber coupler 237 respectively, so that the homologous double pulses are obtained.

[0080] S3, the laser signals output by the first balanced detector 27, the second balanced detector 28 and the third balanced detector 35 are processed by the phase noise elimination and signal demodulation algorithm in the phase noise demodulation and elimination mechanism, and information generated by the vibration of the device under test is obtained.

[0081] Preferably, in step S3:

[0082] Suppose the delay between the homologous double pulses is τ D , and the delay generated in the device under test is τ i ; therefore, the result of the interference between the sweep pulse injected into the single sideband modulator and the reference arm should satisfy:

[0083]

[0084] And the result of the interference between the pulse injected into the first delay optical fiber and the reference arm should satisfy:

[0085]

[0086] The formula (1) and the formula (2) contain the same phase term After a delay of τ D , the two formulas are mixed to eliminate and obtain the result:

[0087]

[0088] It can be seen that although the phase noise in the optical path is partially eliminated after passing through the co-originating double pulse structure, there are still two phase terms in equation (3), one of which is the residual phase noise. That is, a narrow bandwidth laser source at τ D The phase noise accumulated over a time period is represented here by a delay of τ. D The auxiliary interferometer, by mixing the test results with the above formula a second time, can completely eliminate the phase noise in the test results; the other term is Φ(τ) i ,t+τ D )-Φ(τ i The term ,t) can characterize the Rayleigh scattering phase at the same point in time τ. D The change within the signal is used to demodulate the frequency and amplitude of the vibration signal at a certain point.

[0089] Example 1 – Long-distance distributed vibration detection device using piezoelectric ceramics for dual-pulse detection.

[0090] Figure 3 This is a structural diagram of Embodiment 1 of the present invention; Figure 4 This is a diagram of the laser signal modulation of the homologous pulse module in Embodiment 1 of the present invention, as shown below. Figure 3 and Figure 4 As shown, piezoelectric ceramics are selected as the device under test. The device selection and parameters are as follows:

[0091] (1) The center wavelength of the narrow linewidth laser source is 1310nm and the linewidth is 3MHz;

[0092] (2) The 98 / 2 fiber optic coupler operates at a wavelength of 1310nm and has a splitting ratio of 98:2.

[0093] (3) The 50 / 50 fiber coupler operates at a wavelength of 1310nm and has a splitting ratio of 50:50.

[0094] (4) The operating band of the acousto-optic modulator (AOM) is 1300nm-1550nm, and the rise or fall time is 12ns;

[0095] (5) The first and second delay fibers are of equal length, both being 600m;

[0096] (6) The single-sideband modulator operates in the 780nm-2000nm band, with a half-wave voltage of <3V and a bandwidth of 3MHz / 10GHz;

[0097] (7) The optical circulator operates in the 1310±30nm band and has an insertion loss of less than 0.6dB;

[0098] (8) The selected piezoelectric ceramic has a piezoelectric charge constant of 450 PC / N, a piezoelectric voltage constant of 0.028 Vm / N, and a Young's modulus of 6 × 10⁻⁶.10 N / m;

[0099] (9) 90° optical bridge operating wavelength 1310nm, insertion loss should be less than 7dB;

[0100] (10) the photosensitive material of the first, second and third balanced detectors is InGaAs, and the light detection range is 800nm-1650nm.

[0101] The connection of the testing device is as follows:

[0102] The input channel of the first optical fiber coupler is connected with the narrow line width laser light source, and the first and second output channels are connected with the input channels of the second and fifth optical fiber couplers. The first and second output ports of the second optical fiber coupler are connected with the homologous pulse modulation mechanism and the 90° optical bridge respectively, the output channel of the acousto-optic modulator is connected with the input channel of the third optical fiber coupler, the first and second output channels of the third optical fiber coupler are connected with the first delay optical fiber and the single sideband modulator respectively, the arbitrary waveform generator is connected with the acousto-optic modulator and the single sideband modulator respectively to achieve the control purpose, the output of the single sideband modulator and the first delay optical fiber are connected with the first and second input channels of the fourth optical fiber coupler respectively, the output of the fourth optical fiber coupler is connected with the first input channel of the optical circulator, the second input channel of the optical circulator is connected with the selected piezoelectric ceramic to be tested, the third output channel of the optical circulator is connected with the 90° optical bridge, and the two output ports of the 90° optical bridge are connected with the first and second balanced detectors respectively. The first and second output channels of the fifth optical fiber coupler are connected with the second delay optical fiber and the second input channel of the sixth optical fiber coupler respectively, the output end of the second delay optical fiber is connected with the first input port of the sixth optical fiber coupler, and the first and second output channels of the sixth optical fiber coupler are connected with the third balanced detector. The output ports of the first, second and third balanced detectors are connected with the photoelectric signal conversion processing unit.

[0103] Therefore, the double-pulse long-distance distributed vibration detection device based on the OFDR principle with the above structure uses two homologous pulses for Rayleigh scattering detection, cooperates with an auxiliary interferometer for mixing, eliminates phase noise, breaks through the range limitation of coherence length, realizes long-range and high-frequency response vibration signal detection, only needs to mix twice in the optical path, the demodulation algorithm is simple, and the phase correction speed is fast.

[0104] Finally, it should be noted that: the above examples are only used to illustrate the technical solutions of the present application but not to limit them, although the present application has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present application can still be modified or replaced by equivalents, and these modifications or equivalent replacements cannot make the modified technical solutions deviate from the spirit and scope of the technical solutions of the present application.

Claims

1. A double-pulse long-distance distributed vibration detection device based on the principle of OFDR, comprising a narrow-linewidth laser light source and a to-be-measured interferometer, characterized in that: The narrow linewidth laser light source comprises an NLL laser and a first fiber coupler connected with the NLL laser, a first output channel and a second output channel of the first fiber coupler are connected with the to-be-tested interferometer and the auxiliary interferometer respectively, and the to-be-tested interferometer and the auxiliary interferometer are connected with the phase noise demodulation and elimination mechanism; The to-be-tested interferometer comprises a second fiber coupler connected with the first output channel of the first fiber coupler, and a first output channel and a second output channel of the second fiber coupler are connected with a homodyne pulse modulation mechanism and a 90° optical bridge respectively; The homodyne pulse modulation mechanism comprises an acousto-optic modulator, an output channel of the acousto-optic modulator is connected with an input channel of a third fiber coupler, a first output channel and a second output channel of the third fiber coupler are connected with a first delay fiber and a single sideband modulator respectively, the first delay fiber and the single sideband modulator are connected with a first input channel and a second input channel of a fourth fiber coupler respectively, and an output channel of the fourth fiber coupler is connected with an input port of an optical circulator, a first output channel and a second output channel of the optical circulator are connected with a to-be-tested device and the 90° optical bridge respectively; A first output channel and a second output channel of the 90° optical bridge are connected with the phase noise demodulation and elimination mechanism through a first balanced detector and a second balanced detector respectively; The acousto-optic modulator and the single sideband modulator are connected with an arbitrary waveform generator.

2. The dual-pulse long-range distributed vibration sensing apparatus based on the principle of OFDR according to claim 1, characterized in that: The auxiliary interferometer comprises a fifth fiber coupler connected with the second output channel of the first fiber coupler, a first output channel and a second output channel of the fifth fiber coupler are connected with a second delay fiber and a second input channel of a sixth fiber coupler respectively, the second delay fiber is connected with a first input channel of the sixth fiber coupler, and a first output channel and a second output channel of the sixth fiber coupler are connected with a third balanced detector, and the third balanced detector is connected with the phase noise demodulation and elimination mechanism.

3. The dual-pulse long-range distributed vibration sensing apparatus based on the principle of OFDR according to claim 2, characterized in that: The phase noise demodulation and elimination mechanism comprises an optical-electric conversion and signal processing unit and an upper computer connected with the first balanced detector, the second balanced detector and the third balanced detector respectively.

4. The dual-pulse long-range distributed vibration sensing apparatus based on the principle of OFDR according to claim 3, characterized in that: The splitting ratios of the first fiber coupler and the second fiber coupler are both 98:2; The splitting ratios of the third fiber coupler, the fourth fiber coupler, the fifth fiber coupler and the sixth fiber coupler are all 50:

50.

5. The dual-pulse long-range distributed vibration sensing apparatus based on the principle of OFDR according to claim 4, characterized in that: The first delay fiber and the second delay fiber have the same length.

6. The detection method of the double-pulse long-distance distributed vibration detection device based on the OFDR principle, characterized in that: The method comprises the following steps: S1, output light of the narrow bandwidth laser light source is split by the first fiber coupler, and is input into the second fiber coupler and the fifth fiber coupler according to a splitting ratio of 98:2 respectively; S2, the laser signal enters the second optical fiber coupler, and is input into the acousto-optic modulator and the 90° optical bridge according to the 98:2 light splitting ratio. The laser signal passing through the acousto-optic modulator is processed by wave cutting to form a rectangular pulse laser signal, which is uniformly split by the third optical fiber coupler and then injected into the first delay optical fiber and the single sideband modulator. The arbitrary waveform generator controls the single sideband modulator to convert the rectangular pulse laser signal into a sweep pulse signal. The sweep pulse signal is injected into the fourth optical fiber coupler together with another rectangular pulse laser signal that has been delayed in the first delay optical fiber. The output light of the fourth optical fiber coupler enters the part of the optical fiber of the device under test after passing through the optical circulator. The Rayleigh scattering occurring in the optical fiber due to the piezoelectric effect caused by vibration will carry the phase information generated by the output light, which enters the 90° optical bridge after passing through the optical circulator. The laser signal with a proportion of 2% split by the second optical fiber coupler is detected by the first and second balanced detectors. The laser signal entering the fifth optical fiber coupler is uniformly split and injected into the second delay optical fiber and the sixth optical fiber coupler. The laser signal injected into the second delay optical fiber is delayed and also injected into the sixth optical fiber coupler. The two laser signals interfere with each other in the sixth optical fiber coupler and are uniformly split and injected into the third balanced detector. In step S2: The narrow-bandwidth laser source enters the acousto-optic modulator, which is controlled by the arbitrary waveform generator to perform wave cutting and form a rectangular pulse signal that lasts for a certain time. This signal is uniformly split by the third optical fiber coupler to form a pair of homologous pulses with the same wavelength, frequency, and phase, which are injected into the upper path of the first delay optical fiber and the lower path of the single sideband modulator through the first and second output channels of the third optical fiber coupler. In the upper path of the first delay optical fiber, the first delay optical fiber adds a microsecond-level delay to the signal, which separates it from the lower path pulse signal of the single sideband modulator in the time domain. In the lower path of the single sideband modulator, the single sideband modulator is controlled by the arbitrary waveform generator to perform waveform modulation on the input rectangular pulse signal, forming a sweep pulse signal with a linearly varying pulse frequency within a period. After modulation, the upper and lower path pulses are input into the fourth optical fiber coupler through the first and second input channels of the fourth optical fiber coupler, respectively, to obtain homologous double pulses. S3, the laser signals output by the first, second, and third balanced detectors are processed by the phase noise elimination and signal demodulation algorithm in the phase noise demodulation and elimination mechanism to obtain the information generated by the vibration of the device under test.

7. The detection method of the dual-pulse long-range distributed vibration detection device based on the principle of OFDR according to claim 6, characterized in that: In step S3: Assume the delay between the homologous double pulses is τ D , the delay generated in the device under test is τ i ; therefore, the result of the interference between the swept-frequency pulse generated by the injection single-sideband modulator and the reference arm should satisfy: The result of the interference between the pulse injected into the first delay optical fiber and the reference arm should satisfy: The same phase term is contained in both equations (1) and (2) after a delay of τ D Mixing the two equations after a delay of τ The result is: It is known that although the phase noise in the optical path is partially eliminated after the homologous double pulse structure, there are still two phase terms in equation (3), one is the residual phase noise That is, the phase noise accumulated in τ D time by the narrow bandwidth laser source, and the test result is mixed with the second-order interference signal with the same delay τ D to completely eliminate the phase noise in the test result; the other is Φ(τ i ,t+τ D )-Φ(τ i ,t), which can represent the change of the phase of Rayleigh scattering at the same point in time τ D , and further demodulate the frequency and amplitude of the vibration signal at the point.

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