Novel phi-OTDR system with low sampling rate requirement and polarization suppression

Abstract

The invention discloses a novel phi-OTDR (optical time domain reflectometry) system with low sampling rate requirement and polarization suppression, which belongs to the field of distributed optical fiber sensing and consists of a laser, three optical splitters, two acoustic optical modulators, three 3dB couplers, three balance detectors, a polarization controller, two polarization beam splitters, a data acquisition card and a computer. On the basis of a traditional phi-OTDR, a double-path AOM structure is introduced, and one path of optical signal is led out as a reference signal to participate in signal demodulation after light splitting and coupling, so that the requirement of a system on a DAQ sampling rate is reduced. And meanwhile, a polarization fading suppression structure is added in the system to adjust the optical signal, so that the output of two paths of cross-polarization signals is realized, and the influence of polarization fading is suppressed in combination with a phase optimal selection algorithm based on a time domain window. While the requirement of a traditional phi-OTDR system for the DAQ sampling rate is effectively reduced, the influence of polarization fading on phase extraction is suppressed, and the precision of system disturbance monitoring is improved.

Description

Technical Field

[0001] The invention relates to the field of distributed optical fiber sensing, and in particular to a novel Φ-OTDR system with low sampling rate requirements and polarization suppression. Background Art

[0002] Phase-sensitive optical time-domain reflectometry (Φ-OTDR) system is a distributed fiber optic sensing technology with advantages such as simple structure and wide dynamic range. It has been widely used in perimeter security, rail transit and structural health monitoring. Usually, the coherent detection type Φ-OTDR system uses the interference of input signal light and backward Rayleigh scattered light (RBS) to locate and extract disturbance information. In the coherent detection structure, the acousto-optic modulator (AOM) shifts the frequency of the input laser signal according to the frequency of the applied driving signal, and then obtains a fixed intermediate frequency carrier signal. The frequency of the intermediate frequency signal is determined by the AOM, and the typical value is 50MHz. This requires the data acquisition card (DAQ) to have a higher sampling rate to ensure distortion-free data acquisition. However, high-speed DAQ is expensive, which increases the hardware cost of the system. At the same time, high-speed DAQ will greatly increase the amount of collected data. Especially in the face of long-distance online monitoring, the massive amount of data will put forward higher requirements on the system's data storage capacity, transmission speed and processing speed.

[0003] In addition, since the optical pulse generated by the RBS in the optical fiber to be tested needs to beat with the local light after returning, and the birefringence effect in the optical fiber will cause the polarization state between the local light and the RBS to change randomly, there is a polarization fading problem. Therefore, the signal amplitude at some positions will be weakened or even submerged in the noise due to polarization fading, resulting in errors or even mistakes in the phase results. Summary of the invention

[0004] The main purpose of the present invention is to provide a novel Φ-OTDR system with low sampling rate requirements and polarization suppression, which can effectively solve the problems mentioned in the background technology.

[0005] To achieve the above object, the technical solution adopted by the present invention is:

[0006] A novel Φ-OTDR system with low sampling rate requirements and polarization suppression includes a laser, three optical splitters, two acousto-optic modulators (AOMs), three 3dB couplers, three balanced detectors (BPDs), a polarization controller (FPC), two polarization beam splitters (PBSs), a data acquisition card (DAQ) and a computer; the laser is used as a light source, the upper branch light output by optical splitter 1 is frequency-shifted by AOM1, and then outputs a light and b light through optical splitter 2; the lower branch light output by optical splitter 1 is frequency-shifted by AOM2, and then outputs c light and d light through optical splitter 3. After the beat frequency of a light and c light, a local signal u is output through balanced detector 1. LO; d light is injected into the sensing optical fiber as the detection light to return the backward Rayleigh scattered light (RBS); the polarization fading suppression structure is mainly composed of FPC and PBS. The polarization state of the optical signal can be adjusted by adjusting the FPC, so that the polarization state of b light and RBS is basically the same. The two PBSs decompose the RBS and b light into two groups of light beams with orthogonal polarization states (P light and S light), and the light in the same polarization state direction is received by the balanced detector 2 and the balanced detector 3 through the 3dB coupler, and two orthogonal polarization state signals u are output. p and u s , the digital signal output by DAQ is input into the computer for subsequent signal processing.

[0007] After the laser output light passes through the spectrometer, the AOM shifts the laser frequency according to the applied driving signal frequency, and outputs a light and b light (AOM1), c light and d light (AOM2) through the spectrometer. The expression is as follows:

[0008]

[0009]

[0010]

[0011]

[0012] In the above formula, f c Indicates the center frequency of the laser light source, f 1 、f 2 Respectively represent the frequency shift of AOM1 and AOM2, A a , A b , A c , A d Respectively represent the amplitudes of light a, light b, light c, and light d, represent random initial phases respectively.

[0013] The d light is injected into the sensing fiber through the fiber circulator as the detection light and returns to the RBS. Its expression is as follows:

[0014]

[0015] In the above formula, A dr represents the amplitude of RBS, It is the phase change produced when the sensing fiber is affected by external disturbances, and its value reflects the external disturbance information. After the a and c lights beat in the 3dB coupler, they pass through BPD1 to output the reference electrical signal u LO .

[0016] In the fading suppression structure, the b light and RBS regulated by FPC make their average intensities basically the same, and then they are decomposed by PBS and beat in the 3dB coupler, and then received by BPD to obtain two orthogonal electrical signals u p 、u s . Where u LO 、u p 、u s The expression is as follows:

[0017]

[0018]

[0019]

[0020] In the above formula, U LO is the reference electrical signal u LO The amplitude, U p , U s are the electrical signals u p 、u s The magnitude of LO 、u p 、u s The frequency is |f 2 -f 1 |, and |f 2 -f 1 |<f 1 ,|f 2 -f| 1 <f 2 .

[0021] DAQ with sampling rate f s Collect electrical signal u LO 、u p 、u s , and output the corresponding digital signal u LO (n),u p (n),u s (n) to, as follows:

[0022]

[0023]

[0024]

[0025] In the above formula, Δw n =2π(f 2 -f 1 ) / f s , where n = 1, 2, 3, ..., N, represents the sampling point number, and N is the total number of sampling points for each sampling.

[0026] The signal processing method of this system includes two processes, specifically:

[0027] Process 1: Phase extraction algorithm

[0028] Step 1: Place u LO (n) Shift the phase by 90° to obtain u LO-90° (n);

[0029]

[0030] Step 2: Put u LO (n),u LO-90° (n) respectively with u p (n) is mixed to obtain I p and Q p ;

[0031]

[0032]

[0033] Step 3: Use a low-pass filter with a cutoff frequency of 1MHz to remove the I p and Q p The sum frequency component of I p 'and Q p ';

[0034]

[0035]

[0036] In the above formula, filter(·) represents the low-pass filtering operation;

[0037] Step 4: Calculate u using the following formula: p (n) Amplitude information A p (n) and phase information

[0038]

[0039]

[0040] In the above formula, unwrap(·) represents the phase unwrapping operation. Similarly, after steps 1 to 4, we can get u s (n) Amplitude information A s (n) and

[0041] Process 2: Phase optimal selection method based on time domain window

[0042] Step 1: Select the time window T and transform the phase signals obtained in the two polarization states along the time axis and Divide into Z time intervals of equal width ( in is rounded down);

[0043] Step 2: Calculate the two-state amplitude signal A in the Zth time interval p , A s The average value A mean , z=1,2,...,Z;

[0044]

[0045]

[0046] Step 3: Select A in the Zth time interval mean The larger polarization state data corresponds to As the phase in the current interval

[0047]

[0048] Step 4: Steps 2 to 3 can obtain the phase information in Z time intervals. Connect the ends and splice them to get the real phase of the position changing with time.

[0049]

[0050] Compared with the prior art, the present invention has the following beneficial effects:

[0051] 1. Low sampling rate requirement: Compared with the traditional Φ-OTDR which requires high-speed DAQ to ensure accuracy, the system of the present invention significantly reduces the DAQ sampling rate requirement, which can greatly reduce the amount of data collected, especially in the face of long-distance online monitoring, which can effectively reduce the pressure of massive data processing;

[0052] 2. Polarization fading suppression: Adding a fading suppression structure to the proposed system and combining it with the proposed phase optimal selection method based on the time domain window can effectively suppress the influence of polarization fading and improve the accuracy of system disturbance monitoring. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Figure 1 It is a schematic diagram of the system composition of the present invention;

[0054] Figure 2 It is a schematic diagram of the phase extraction algorithm of the present invention;

[0055] Figure 3This is a flow chart of the phase optimal selection method based on the time domain window of the present invention. DETAILED DESCRIPTION

[0056] In order to make the technical means, creative features, objectives and effects achieved by the present invention easy to understand, the present invention is further explained below in conjunction with specific implementation methods.

[0057] In the novel coherent Φ-OTDR system provided by the present invention: the laser output power is 13dBm, the central wavelength is 1550nm, and the line width is 1.5kHz; the optical splitter 1 divides the input light into two beams of equal power, which are input into AOM1 and AOM2 respectively; AOM1 modulates the continuous light into pulse light and generates a 50MHz shift frequency; AOM2 generates a 55MHz shift frequency pulse light. The present invention mainly includes two parts: system structure and signal processing method.

[0058] like Figure 1 As shown in the figure, the system structure includes a laser, three optical splitters, two acousto-optic modulators (AOMs), three 3dB couplers, three balanced detectors (BPDs), a polarization controller (FPC), two polarization beam splitters (PBSs), a data acquisition card (DAQ) and a computer; the laser is used as the light source, the upper branch light output by optical splitter 1 is frequency-shifted by AOM1, and then outputs a light and b light through optical splitter 2; the lower branch light output by optical splitter 1 is frequency-shifted by AOM2, and then outputs c light and d light through optical splitter 3. After the beat frequency of a light and c light, the local signal u is output through balanced detector 1 LO ; d light is injected into the sensing fiber as the detection light to return the backward Rayleigh scattered light (RBS); the polarization fading suppression structure is mainly composed of FPC and PBS. The polarization state of the optical signal can be adjusted by adjusting the FPC, so that the average intensity of b light and RBS is basically the same. The two PBSs decompose RBS and b light into two groups of light beams with orthogonal polarization states (P light and S light), and the light in the same polarization state direction is received by balanced detector 2 and balanced detector 3 through 3dB couplers, and two orthogonal polarization state signals u are output. p and u s , the digital signal output by DAQ is input into the computer for subsequent signal processing. LO 、u p and u s The main frequency is the difference between the shift frequencies of the two AOMs, which is typically several MHz, thus reducing the system's sampling rate requirements for the DAQ.

[0059] like Figure 2-Figure 3 As shown, the signal processing method LO 、u p and u sThey are all M×N matrices (M represents the number of DAQ acquisitions, and N represents the total number of single DAQ samplings). Each row of data corresponds to the distribution of the distance along the optical fiber during each pulse, and each column of data corresponds to the value of each distance point during each laser pulse scan, characterizing the change law over time. The collected data is sequentially subjected to orthogonal demodulation and fading suppression processing, and the signal processing process is implemented according to the following algorithm:

[0060] 1) Phase extraction algorithm

[0061] First, u LO After 90° phase shift, we get u LO-90° , then u p 、u s Respectively with u LO and u LO-90° Perform mixing operation to obtain two sets of orthogonal signals (I p , Q p )、(I s , Q s ), and remove (I p , Q p )、(I s , Q s ) in the high frequency component, and obtain the difference frequency component (I p ', Q p ')、(I s ', Q s '), then perform square and root taking and inverse tangent operation on the difference frequency component to obtain u p 、u s The amplitude of the signal is A p , A s and Phase The phase It reflects the disturbance signal law along the fiber distance. They are all M×N matrices, each row of data represents the distribution of amplitude and phase along the distance, and each column of data represents the change of amplitude and phase over time.

[0062] 2) Phase optimal selection algorithm based on time domain window

[0063] Due to the influence of polarization fading, u p 、u s The signal strength distribution of u is inconsistent, while p 、u s The weaker amplitude position Distortion will occur, resulting in errors in the phase extraction results. In order to suppress the influence of polarization fading on phase extraction, the present invention proposes a phase optimal selection algorithm based on a time domain window. The algorithm selects a time domain window T and uses the window as a template to extract the phase of the phase optimal selection algorithm along the time axis. The two-state amplitude signal A is calculated for the disturbance at a certain position in the optical fiber to be tested. p , A s The average value A mean , preferably A mean The polarization state with a larger value As the disturbance signal at that location, Z time domain interval disturbance signals are obtained, and the final result can be obtained by splicing them.

[0064] The phase optimal selection method proposed in the present invention is obtained in process 1 Switching between the two options can effectively reduce the impact of polarization fading and reconstruct the vibration signal with high fidelity.

[0065] The above shows and describes the basic principles and main features of the present invention and the advantages of the present invention. It should be understood by those skilled in the art that the present invention is not limited to the above embodiments. The above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, which fall within the scope of the present invention to be protected. The scope of protection of the present invention is defined by the attached claims and their equivalents.

Claims

1. A novel Φ-OTDR system with low sampling rate requirement and polarization suppression, characterized by: The invention comprises a laser, three optical splitters, two acousto-optic modulators (AOMs), three 3dB couplers, three balanced detectors (BPDs), a polarization controller (FPC), two polarization beam splitters (PBSs), a data acquisition card (DAQ) and a computer. The laser is used as a light source. The upper branch light outputted by the optical splitter 1 is frequency-shifted by AOM1 and then outputs a light and b light through the optical splitter 2. The lower branch light outputted by the optical splitter 1 is frequency-shifted by AOM2 and then outputs c light and d light through the optical splitter 3. The a light and the c light beat each other and then pass through the balanced detector 1 to output a local signal u. LO ; d light is injected into the sensing fiber as the detection light to return the backward Rayleigh scattered light (RBS); the polarization fading suppression structure is mainly composed of FPC and PBS. Adjusting the FPC can adjust the polarization state of the optical signal so that the polarization state of b light and RBS are basically the same. The two PBSs decompose the RBS and b light into two groups of light beams P and S light with orthogonal polarization states respectively. The light with the same polarization direction is received by the balanced detector 2 and the balanced detector 3 through the 3dB coupler respectively, and outputs two orthogonal polarization state signals u p and u s , the digital signal output by DAQ is input into the computer for subsequent signal processing.

2. A novel Φ-OTDR system with low sampling rate requirement and polarization suppression according to claim 1, characterized in that: After the laser output light passes through the spectrometer, the AOM shifts the laser frequency according to the applied driving signal frequency, and outputs a light and b light, c light and d light through the spectrometer. The expression is as follows: In the above formula, f c represents the center frequency of the laser light source, f1 and f2 represent the frequency shift of AOM1 and AOM2 respectively, a , A b , A c , A d Respectively represent the amplitudes of light a, light b, light c, and light d, represent random initial phases respectively.

3. A novel Φ-OTDR system with low sampling rate requirement and polarization suppression according to claim 2, characterized in that: The d light is injected into the sensing fiber through the fiber circulator as the detection light and returns to the RBS. Its expression is as follows: In the above formula, A dr represents the amplitude of RBS, It is the phase change produced when the sensing fiber is affected by external disturbances, and its value reflects the external disturbance information; after the a and c lights beat in the 3dB coupler, they pass through BPD1 to output the reference electrical signal u LO .

4. A novel Φ-OTDR system with low sampling rate requirement and polarization suppression according to claim 3, characterized in that: In the fading suppression structure, the b light and RBS regulated by FPC make their polarization states basically equivalent, and then after being decomposed by PBS, they beat in the 3dB coupler and received by BPD to obtain two orthogonal electrical signals u. p 、u s ; where u LO 、u p 、u s The expression is as follows: In the above formula, U LO is the reference electrical signal u LO The amplitude, U p , U s are the electrical signals u p 、u s The magnitude of LO 、u p 、u s The frequency is |f2-f1|, and |f2-f1|<f1, |f2-f1|<f2.

5. A novel Φ-OTDR system with low sampling rate requirement and polarization suppression according to claim 4, characterized in that: DAQ with sampling rate f s Collect electrical signal u LO 、u p 、u s , and output the corresponding digital signal u LO (n),u p (n),u s (n) to, as follows: In the above formula, Δw n =2π(f2-f1) / f s , where n = 1, 2, 3, ..., N, represents the sampling point number, and N is the total number of sampling points.

6. The novel Φ-OTDR system with low sampling rate requirement and polarization suppression according to claim 1, characterized in that: The signal processing method of this system includes two processes, specifically: Process 1: Phase extraction algorithm Step 1: Place u LO (n) Shift the phase by 90° to obtain u LO-90° (n); Step 2: Put u LO (n),u LO-90° (n) respectively with u p (n) is mixed to obtain I p and Q p ; Step 3: Use a low-pass filter with a cutoff frequency of 1MHz to remove the I p and Q p The sum frequency component of I p 'and Q p '; In the above formula, filter(·) represents the low-pass filtering operation; Step 4: Calculate u using the following formula: p (n) Amplitude information A p (n) and phase information In the above formula, unwrap(·) represents the phase unwrapping operation; similarly, after steps 1 to 4, we can get u s (n) Amplitude information A s (n) and Process 2: Phase optimal selection method based on time domain window Step 1: Select the time window T and transform the phase signals obtained in the two polarization states along the time axis and Divide into Z time intervals of equal width ( in is rounded down); Step 2: Calculate the two-state amplitude signal A in the Zth time interval p , A s The average value A mean , z=1,2,...,Z; Step 3: Select A in the Zth time interval mean The larger polarization state data corresponds to As the phase in the current interval Step 4: Steps 2 to 3 can obtain the phase information in Z time intervals. Connect the ends and splice them to get the real phase of the position changing with time.

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