A long-distance OFDR system based on time division multiplexing and data processing method thereof

Abstract

The present invention relates to a long-distance OFDR system based on time division multiplexing and a data processing method thereof, comprising a tunable laser, a first coupler, a sensing unit, an auxiliary interferometer, and a data acquisition unit; the sensing unit comprises three sensing sub-units. By adding a delay unit for eliminating asynchronous errors in the auxiliary interferometer, the limitation of the sensing distance and the difference between the two arms of the auxiliary interferometer is broken. The combination of stress measurement results of multiple sensing sub-units is realized by a data processing method based on the principle of time division multiplexing. The present invention can realize long-distance sensing by segmented processing of the sensing unit without increasing the extra cost to upgrade the laser light source, so that it has greater advantages and broader applications in the field of large-scale equipment monitoring such as fiber optic hydrophone towed arrays and aerospace equipment.

Description

Technical Field

[0001] The present invention relates to a long-distance OFDR (Optical Frequency Domain Reflectometry) system based on time division multiplexing and a data processing method thereof, which improves the sensing distance of the OFDR system without reducing the sensing performance, and belongs to the technical field of optical fiber sensing detection. Background Art

[0002] Distributed fiber optic sensing systems have the advantages of high sensitivity, anti-electromagnetic interference, good electrical insulation and distributed indifferent measurement, and have attracted widespread attention in applications such as structural health monitoring and pipeline intrusion monitoring. Distributed fiber optic sensing systems based on the inherent optical effects in optical fibers, such as Rayleigh scattering, Brillouin scattering and Raman scattering, have been developed and applied to the measurement of vibration, temperature and strain information in the environment. Compared with distributed fiber optic sensing systems based on Brillouin scattering and Raman scattering, since the intensity of Rayleigh scattered light in optical fibers is much greater than that of Brillouin scattered light and Raman scattered light, distributed fiber optic sensors based on Rayleigh scattering have higher sensitivity. As a representative of them, optical frequency domain reflectometry (OFDR) has the advantages of high dynamic range, high sensitivity and high spatial resolution. Therefore, it has very important applications in the field of high-precision monitoring.

[0003] The principle of OFDR is as follows: the linear frequency sweep light emitted by the tunable laser light source is divided into two beams through a coupler, one of which enters the optical fiber to be tested, and the backward Rayleigh scattered light of the optical fiber to be tested returns to form a signal light, which interferes with the other reference light. By collecting the beat signal and performing fast Fourier transform processing, the distance domain information constructed along the sensing optical fiber can be obtained. In the measurement of the OFDR system, it is necessary to collect a reference signal without external influence and a test signal affected by the optical fiber, and perform cross-correlation calculation between the test signal and the reference signal to obtain the change of external information. The OFDR system has the characteristics of high spatial resolution, and its system spatial resolution can reach the millimeter level. Therefore, it has very important applications in high-precision monitoring fields such as aerospace. However, due to the nonlinear phenomenon of frequency tuning in the laser, the increase of non-uniform sampling interval of the optical frequency affects the expansion of the reflected peak energy, which greatly affects the performance of the OFDR system. Usually, an auxiliary interferometer is added, and its output is used as a clock signal to trigger data acquisition, so as to achieve equal frequency interval sampling and avoid the nonlinear influence of frequency tuning. However, in order to satisfy the Nyquist sampling theorem, the maximum measurable range of this method is limited by the path difference between the two arms of the auxiliary interferometer. Therefore, how to effectively improve the sensing distance of the OFDR system without reducing the sensing performance is a very important research direction. Summary of the invention

[0004] In view of the shortcomings of the prior art, the present invention provides a long-distance OFDR (Optical Frequency Domain Reflectometry) system based on time division multiplexing, which effectively improves the sensing distance of the OFDR system without reducing the sensing performance.

[0005] The present invention also provides a data processing method for the above-mentioned long-distance OFDR system based on time division multiplexing.

[0006] The technical solution of the present invention is:

[0007] A long-distance OFDR system based on time division multiplexing includes a tunable laser, a first coupler, a sensing unit, an auxiliary interferometer, and a data acquisition unit; the sensing unit includes three sensing sub-units;

[0008] The continuous light output by the tunable laser is divided into two parts by the first coupler, one part is incident on the auxiliary interferometer, and after being processed, a trigger signal is provided to the digital acquisition card in the data acquisition unit, and the other part of the light enters the sensing unit;

[0009] Each sensor subunit includes a second coupler, a circulator, a sensing optical fiber, and a third coupler; in each sensor subunit, light is divided into two parts through the second coupler, one part of the light is used as reference light, and the rest of the light is used as detection light and enters the sensing optical fiber after passing through the circulator; the circulator allows the detection light to be continuously transmitted in the forward direction in each sensor subunit, while the Rayleigh scattered light is divided into multiple sections; the Rayleigh scattered light returned in the sensing optical fiber interferes with the local reference light in the third coupler, and then is collected separately by the data acquisition unit.

[0010] According to the preferred embodiment of the present invention, the data acquisition unit includes an optical switch, a delay optical fiber, a polarization beam splitter, a photodetector and a data acquisition card;

[0011] In the data acquisition unit, by adding a delay fiber at the output position of each sensor subunit, the required signal is guided back to the same position. By controlling the opening channel and time of the optical switch, the mixing of the signals during the sampling process is avoided. The signal is divided into p and s components by a polarization beam splitter, detected by two balanced photodetectors, and collected by a digital acquisition card.

[0012] Preferably, according to the present invention, the auxiliary interferometer includes a delay unit and a Mach-Zehnder interferometer; light enters the delay unit and is controlled by an optical switch to enter three compensation channels with delay optical fibers of different lengths, the delay optical fiber lengths of the compensation channels respectively correspond to the starting positions of each sensor sub-unit, and after correctly compensating for the synchronization error, the Mach-Zehnder interferometer generates an equal-frequency interval trigger signal, which is output to the data acquisition card of the data acquisition unit.

[0013] Preferably according to the present invention, the first coupler is a 10 / 90 optical coupler; the second coupler is a 1 / 99 optical coupler; and the third coupler is a 50 / 50 optical coupler.

[0014] The data processing method of the long-distance OFDR system based on time division multiplexing comprises the following steps:

[0015] S1: Control the optical switch to collect signals from the three-segment sensor unit twice, one of which is a signal without strain information, which is a reference signal; the other is a signal with strain information, which is a test signal;

[0016] S2: Perform fast Fourier transform (FFT) on the reference signal and the test signal respectively, convert the frequency domain information into distance domain information, and then scale the distance domain information proportionally according to the actual length of the sensing optical fiber, and convert the distance domain information into the length information corresponding to the sensing optical fiber;

[0017] A sliding window with a length of ΔX is used to divide the total length of the sensing fiber into several local distance domain information, which includes N data points as spatially localized Rayleigh scattering signals;

[0018] S3: Performing a fast inverse Fourier transform on the local distance domain information of the reference signal and the test signal in the first sliding window of each segment to obtain their respective local frequency spectrum information;

[0019] S4: performing cross-correlation calculation on the local frequency spectrum information of the reference signal and the test signal after the fast inverse Fourier transform obtained in step S3 to obtain the offset of the main peak of the spectrum;

[0020] S5: Repeat steps S3-S4 to obtain the cross-correlation results of each corresponding position of the sensing optical fiber of each sensing subunit and the offset of the main spectral peak thereof, and recombine the offsets of all the main spectral peaks obtained by each segment of the sensing subunit according to the corresponding position information of the sensing optical fiber, and obtain the spectral offset curves of all positions, and obtain the strain measurement results of each sensing subunit according to the obtained spectral offset information;

[0021] S6. Recombining the strain measurement results obtained by each sensor subunit according to the time sequence of acquisition.

[0022] Further preferably, ΔX represents the distributed spatial resolution of the long-range OFDR system, n and Δυ represent the effective refractive index of the sensing fiber and the optical frequency tuning range of the tunable laser source, respectively, and c is the speed of light propagation in a vacuum.

[0023] Furthermore, in step S1, a trigger signal is generated by hardware compensation to achieve equal frequency interval acquisition.

[0024] The beneficial effects of the present invention are:

[0025] 1) The long-distance OFDR system based on time division multiplexing proposed in the present invention can achieve long-distance sensing by segmented processing of the sensing unit without increasing the extra cost of upgrading the laser light source, so that it has greater advantages and broader applications in the field of large-scale equipment monitoring such as fiber optic hydrophone towed arrays and aerospace equipment.

[0026] 2) The long-distance OFDR system based on time division multiplexing proposed in the present invention can realize independent crosstalk-free sensing between sensing sub-units, flexibly configure the sensing parameters of each sub-unit to improve the spatial resolution, and realize long-distance sensing with high spatial resolution. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a flow chart of the data processing method of the long-distance OFDR system based on time division multiplexing;

[0028] Figure 2 This is a schematic diagram of the structure of a long-distance OFDR system based on time division multiplexing;

[0029] Figure 3 is a schematic diagram of the auxiliary interferometer structure;

[0030] Figure 4 It is a schematic diagram of stress information located in three sensor sub-units respectively measured by using the long-distance OFDR system of the present invention.

[0031] 1. Tunable laser, 2. First coupler, 3. Second coupler, 4. Circulator, 5. Sensing fiber, 6. Auxiliary interferometer, 7. Third coupler, 8. Optical switch, 9. Delay fiber, 10. Polarization beam splitter, 11. Photodetector, 12. Digital acquisition card. DETAILED DESCRIPTION

[0032] The following is combined with Figure 1-4 The technology is further described with specific embodiments to help understand the content of the present invention, but is not limited thereto.

[0033] Example 1

[0034] A long-distance OFDR system based on time division multiplexing, such as Figure 2 As shown, it includes a tunable laser 1, a first coupler 2 (10 / 90 optical coupler), a sensing unit, an auxiliary interferometer 6, and a data acquisition unit; the sensing unit includes three sensing sub-units;

[0035] The continuous light output by the tunable laser 1 is divided into two parts by the first coupler 2, 10% of which is incident on the auxiliary interferometer 6, and after processing, a trigger signal is provided to the digital acquisition card 12 in the data acquisition unit to avoid resampling errors, and the remaining 90% of the light enters the sensor unit;

[0036] Each sensor subunit includes a second coupler 3 (1 / 99 optical coupler), a circulator 4, a sensing optical fiber 5, and a third coupler 7 (50 / 50 optical coupler); in each sensor subunit, light is divided into two parts through the second coupler 3, 1% of the light is used as reference light, and 99% of the light is used as detection light and enters the sensing optical fiber 5 after passing through the circulator; the circulator 4 allows the detection light to be continuously transmitted in the forward direction in each sensor subunit, while the Rayleigh scattered light is divided into multiple sections; the Rayleigh scattered light returned from the sensing optical fiber 5 interferes with the local reference light in the third coupler 7, and then is collected separately by the data acquisition unit.

[0037] The data acquisition unit includes an optical switch 8, a delay optical fiber 9, a polarization beam splitter 10, a photodetector 11 and a data acquisition card; in the data acquisition unit, by adding a delay optical fiber 9 at the output position of each sensor subunit, the required signal is guided back to the same position, and by controlling the opening channel and time of the optical switch 8, mixing during the signal sampling process is avoided. The signal is divided into p and s components by the polarization beam splitter 10, detected by two balanced photodetectors 11, and collected by a digital acquisition card 12.

[0038] Example 2

[0039] According to a time division multiplexing-based long-distance OFDR system described in Example 1, Figure 3 As shown, the auxiliary interferometer 6 includes a delay unit and a Mach-Zehnder interferometer; light enters the delay unit and is controlled by an optical switch 8 to enter three compensation channels with delay optical fibers 9 of different lengths. The lengths of the delay optical fibers 9 of the compensation channels correspond to the starting positions of the sensor sub-units. After correctly compensating for the synchronization error, the Mach-Zehnder interferometer generates an equal-frequency interval trigger signal, which is output to the data acquisition card of the data acquisition unit.

[0040] The delay unit is used to eliminate the problem of asynchronism between the trigger signal and the detection signal, and the Mach-Zehnder interferometer is used to generate equal-frequency interval sampling trigger.

[0041] Example 3

[0042] Embodiment 1 or 2 A data processing method for a long-distance OFDR system based on time division multiplexing, such as Figure 1 As shown, the following steps are included:

[0043] S1: Analyze the sensing parameters such as sensing distance and spatial resolution required by the application environment, and build an OFDR system based on the time division multiplexing principle based on the parameters. The effective sensing distance of the example of the present invention is 225m, and the spatial resolution is 5mm.

[0044] The optical switch 8 is controlled to collect signals from the three-segment sensor subunit twice, one is a signal without strain information, which is a reference signal; the other is a signal containing strain information, which is a test signal; a trigger signal is generated by hardware compensation to achieve equal frequency interval collection.

[0045] S2: Performing fast Fourier transform (FFT) on the reference signal and the test signal respectively, converting the frequency domain information into distance domain information, and then scaling the distance domain information proportionally according to the actual length of the sensing optical fiber 5, and converting the distance domain information into the length information corresponding to the sensing optical fiber 5;

[0046] Using a sliding window with a length of ΔX, the total length of the sensing optical fiber 5 is divided into a number of local distance domain information, including N data points as spatial local Rayleigh scattering signals;

[0047] ΔX represents the distributed spatial resolution of the long-range OFDR system, n and Δυ represent the effective refractive index of the sensing optical fiber 5 and the optical frequency tuning range of the tunable laser source respectively, and c is the speed of light propagation in a vacuum.

[0048] S3: Performing a fast inverse Fourier transform on the local distance domain information of the reference signal and the test signal in the first sliding window of each segment to obtain their respective local frequency spectrum information;

[0049] S4: performing cross-correlation calculation on the local frequency spectrum information of the reference signal and the test signal after the fast inverse Fourier transform obtained in step S3 to obtain the offset of the main peak of the spectrum;

[0050] S5: Repeat steps S3-S4 to obtain the cross-correlation results of each corresponding position of the sensing optical fiber 5 of each sensing subunit and the offset of the main spectral peak thereof, and recombine the offsets of all the main spectral peaks obtained by each segment of the sensing subunit according to the corresponding position information of the sensing optical fiber 5, and obtain the spectral offset curves of all positions. The spectral offset is in a one-to-one correspondence with the strain measurement result. Therefore, the strain measurement result of each sensing subunit is obtained according to the obtained spectral offset information;

[0051] S6. Recombining the strain measurement results obtained by each sensing subunit according to the time sequence of acquisition, thereby increasing the sensing distance without reducing the sensing performance of each subunit.

[0052] Figure 4Schematic diagram of stress information located at three sensor subunits respectively measured by the long-distance OFDR system of the present invention. The stress information measured by the OFDR system is as follows: Figure 4 In the figure, (a), (b), and (c) stresses are applied at effective sensing distances of 73.4 m, 148.4 m, and 222.6 m, respectively, with an applied length of 30 cm. Figure 4 The results show that the OFDR system of the present invention can realize distributed stress sensing over long distances without performance loss.

[0053] Of course, the above description is not a limitation of the present technology, and the present technology is not limited to the above examples. Changes, modifications, additions or substitutions made by ordinary technicians in this technical field within the essential scope of the invention also fall within the protection scope of the present technology.

Claims

1. A long-distance OFDR system based on time division multiplexing, characterized in that: It includes a tunable laser, a first coupler, a sensing unit, an auxiliary interferometer, and a data acquisition unit; the sensing unit includes three sensing sub-units; The continuous light output by the tunable laser is divided into two parts by the first coupler, one part is incident on the auxiliary interferometer, and after being processed, a trigger signal is provided to the digital acquisition card in the data acquisition unit, and the other part of the light enters the sensing unit; Each sensor subunit includes a second coupler, a circulator, a sensing optical fiber, and a third coupler; in each sensor subunit, light is divided into two parts through the second coupler, one part of the light is used as a reference light, and the rest of the light is used as a detection light and enters the sensing optical fiber after passing through the circulator; the circulator allows the detection light to be continuously transmitted in each sensor subunit in the forward direction, while the Rayleigh scattered light is divided into multiple sections; the Rayleigh scattered light returned from the sensing optical fiber interferes with the local reference light in the third coupler, and then is collected separately by the data acquisition unit; The data acquisition unit includes an optical switch, a delay optical fiber, a polarization beam splitter, a photodetector and a data acquisition card; In the data acquisition unit, by adding a delay fiber at the output position of each sensor subunit, the required signal is guided back to the same position. By controlling the opening channel and time of the optical switch, the mixing of the signals during the sampling process is avoided. The signal is divided into p and s components by a polarization beam splitter, detected by two balanced photodetectors, and collected by a digital acquisition card.

2. A time division multiplexing based long distance OFDR system according to claim 1, characterized in that: The auxiliary interferometer includes a delay unit and a Mach-Zehnder interferometer; light enters the delay unit and is controlled by an optical switch to enter three compensation channels with delay optical fibers of different lengths. The delay optical fiber lengths of the compensation channels correspond to the starting positions of each sensor sub-unit. After correctly compensating for the synchronization error, the Mach-Zehnder interferometer generates an equal-frequency interval trigger signal, which is output to the data acquisition card of the data acquisition unit.

3. A time division multiplexing based long distance OFDR system according to claim 1 or 2, characterized in that: The first coupler is a 10 / 90 optical coupler; the second coupler is a 1 / 99 optical coupler; and the third coupler is a 50 / 50 optical coupler.

4. The data processing method for a long-distance OFDR system based on time division multiplexing according to any one of claims 1 to 3, characterized in that: The following steps are involved: S1: Control the optical switch to collect signals from the three-segment sensor unit twice, one of which is a signal without strain information, which is a reference signal; the other is a signal with strain information, which is a test signal; S2: Perform fast Fourier transform on the reference signal and the test signal respectively, convert the frequency domain information into distance domain information, and then scale the distance domain information proportionally according to the actual length of the sensing optical fiber, and convert the distance domain information into the length information corresponding to the sensing optical fiber; A sliding window with a length of ΔX is used to divide the total length of the sensing fiber into several local distance domain information, which includes N data points as spatially localized Rayleigh scattering signals; S3: Performing a fast inverse Fourier transform on the local distance domain information of the reference signal and the test signal in the first sliding window of each segment to obtain their respective local frequency spectrum information; S4: performing cross-correlation calculation on the local frequency spectrum information of the reference signal and the test signal after the fast inverse Fourier transform obtained in step S3 to obtain the offset of the main peak of the spectrum; S5: Repeat steps S3-S4 to obtain the cross-correlation results of each corresponding position of the sensing optical fiber of each sensing subunit and the offset of the main spectral peak thereof, and recombine the offsets of all the main spectral peaks obtained by each segment of the sensing subunit according to the corresponding position information of the sensing optical fiber, and obtain the spectral offset curves of all positions, and obtain the strain measurement results of each sensing subunit according to the obtained spectral offset information; S6. Recombining the strain measurement results obtained by each sensor subunit according to the time sequence of acquisition.

5. The data processing method of the long-distance OFDR system based on time division multiplexing according to claim 4 is characterized in that: ΔX represents the distributed spatial resolution of the long-range OFDR system, n and Δυ represent the effective refractive index of the sensing fiber and the optical frequency tuning range of the tunable laser source, respectively, and c is the speed of light propagation in a vacuum.

6. The data processing method of the long-distance OFDR system based on time division multiplexing according to claim 4 is characterized in that: In step S1, a trigger signal is generated by hardware compensation to achieve equal frequency interval acquisition.

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