Optical frequency domain reflectometry strain and temperature sensing system

Abstract

The application provides an optical frequency domain reflection strain and temperature sensing system, which can be applied to the field of optical fiber sensing technology. The sensing system comprises a tunable laser, a first coupler, a main interferometer, a second coupler, a first auxiliary interferometer, a second auxiliary interferometer, a first photoelectric detector, a second photoelectric detector, a third photoelectric detector, a data acquisition card, a digital processing chip and a large-scale processor. By taking beat frequency signals generated by the first auxiliary interferometer and the second auxiliary interferometer as external clock signals, the main interferometer beat frequency signals are sampled by using the method of coprime sampling, so that the external clock sampling frequency is no longer dependent on the bandwidth of the signal, but dependent on the sparsity or compressibility of the signal itself. The problem that the sensing distance is limited by the Nyquist sampling theorem is solved, the sensing length of the OFDR system is improved, and the demand for the sampling rate of the data acquisition card is reduced.

Description

Technical Field

[0001] This invention relates to the field of fiber optic sensing technology, and more particularly to an optical frequency domain reflection strain and temperature sensing system. Background Technology

[0002] Currently, optical frequency domain reflection (OFDR) systems capable of real-time demodulation typically employ an external clock sampling scheme. This scheme uses the zero-crossing point of the auxiliary interferometer signal as the clock for external sampling by the acquisition card, achieving uniform frequency sampling of the main interferometer's beat frequency signal. Hardware direct correction of the light source frequency nonlinearity is then performed, with the light source frequency sweep and correction completed synchronously. However, this scheme is limited by the Nyquist sampling theorem: the frequency of the auxiliary interferometer's beat frequency signal must be more than twice the maximum frequency of the main interferometer's beat frequency signal; that is, the maximum length of the main interferometer's sensing fiber must be one-quarter of the optical path difference length of the auxiliary interferometer's fiber.

[0003] Currently, Shandong University has proposed a long-distance, high spatial resolution system based on time-division multiplexing. This system segments the fiber under test by adding couplers and circulators, and performs time-division multiplexing processing on the Rayleigh scattering signal to improve the sensing distance of the OFDR system. However, the segmented fiber length of this method is still limited by the optical path difference of the auxiliary interferometer fiber, and the system structure is relatively complex. Summary of the Invention

[0004] (a) Technical problems to be solved

[0005] To address the limitation of sensing distance by the Nyquist sampling theorem in existing technologies and to improve the sensing distance of OFDR systems, embodiments of the present invention provide an optical frequency domain reflection strain and temperature sensing system. This system constructs a frequency domain sparse model by inscribing a weak grating array in the optical fiber under test and employs a compressed sensing algorithm, so that the external clock sampling frequency no longer depends on the bandwidth of the signal, but rather on the sparsity or compressibility of the signal itself.

[0006] (II) Technical Solution

[0007] In view of the above problems, embodiments of the present invention provide an optical frequency domain reflection strain and temperature sensing system.

[0008] According to a first aspect of the present invention, an optical frequency domain reflection strain and temperature sensing system is provided, comprising: a tunable laser, a first coupler, a main interferometer, a second coupler, a first auxiliary interferometer, a second auxiliary interferometer, a first photodetector, a second photodetector, a third photodetector, a data acquisition card, a digital processing chip, and a large-scale processor, wherein the tunable laser generates a periodic linear frequency modulated optical signal, the periodic linear frequency modulated optical signal is converted into a first optical signal and a second optical signal by the first coupler, wherein the first optical signal is converted into a first beat frequency signal by the main interferometer, the first beat frequency signal is converted into a first electrical signal by the first photodetector; the second optical signal is converted into a third optical signal and a fourth optical signal by the second coupler. The signals are as follows: the third optical signal is converted into a second beat frequency signal by the first auxiliary interferometer, and the second beat frequency signal is converted into a second electrical signal by the second photodetector; the fourth optical signal is converted into a third beat frequency signal by the second auxiliary interferometer, and the third beat frequency signal is converted into a third electrical signal by the third photodetector; the first electrical signal, the second electrical signal, and the third electrical signal are acquired by the data acquisition card and converted into a first digital signal, a second digital signal, and a third digital signal, respectively; the digital processing chip uses the second digital signal and the third digital signal as the first external clock signal and the second external clock signal, respectively, to simultaneously acquire the first digital signal and obtain the sampled digital signal; the sampled digital signal is processed by a large-scale processor to obtain the changes in strain and temperature.

[0009] In some exemplary embodiments, the sensing system further includes an isolator disposed between the tunable laser and the first coupler to protect the tunable laser from reflected light affecting it.

[0010] In some exemplary embodiments, the delay fiber of the master interferometer includes the fiber under test, wherein the fiber under test is etched with a weak grating array; and the first beat frequency signal is a frequency-domain sparse signal.

[0011] In some exemplary embodiments, the intensity of the reflected light from the weak grating is 20 dB higher than the intensity of the Rayleigh scattered light, the length of the weak grating is 1 mm, the spacing of the weak grating is 4 mm, and the center wavelength of the weak grating is 1550 nm.

[0012] In some exemplary embodiments, the ratio K1 of the sampling rate of the first digital signal to the frequency of the second beat frequency signal is an integer; the ratio K2 of the sampling rate to the frequency of the third beat frequency signal is an integer; and K1 and K2 are coprime numbers.

[0013] In some exemplary embodiments, the process of obtaining the strain and temperature changes by processing the sampled digital signal through a large-scale processor includes: the large-scale processor using a sparse recovery algorithm to restore the sampled digital signal to obtain a restored signal; and using a windowed cross-correlation demodulation algorithm to demodulate the strain and temperature changes based on the restored signal.

[0014] In some exemplary embodiments, the sparse recovery algorithm includes at least one of convex optimization algorithms, greedy tracing algorithms, or sparse Bayesian learning algorithms.

[0015] In some exemplary embodiments, the first coupler includes a 99:1 coupler, wherein the intensity of the first optical signal accounts for 99% of the intensity of the periodic linear frequency modulated optical signal; and the intensity of the second optical signal accounts for 1% of the intensity of the periodic linear frequency modulated optical signal.

[0016] In some exemplary embodiments, the second coupler includes a 50:50 coupler, wherein the intensity of the third optical signal accounts for 50% of the intensity of the second optical signal; and the intensity of the fourth optical signal accounts for 50% of the intensity of the second optical signal.

[0017] In some exemplary embodiments, the main interferometer, the first auxiliary interferometer, and the second auxiliary interferometer are all Mach-Zehnder interferometers.

[0018] (III) Beneficial Effects

[0019] As can be seen from the above technical solutions, the optical frequency domain reflection strain and temperature sensing system provided by the embodiments of the present invention has at least one of the following beneficial effects:

[0020] (1) Coprime sampling of the beat frequency signal of the main interferometer makes the sampling frequency of the external clock no longer depend on the bandwidth of the signal, but on the sparsity or compressibility of the signal itself, thus solving the problem that the sensing distance is limited by the Nyquist sampling theorem and improving the sensing length of the OFDR system.

[0021] (2) Coprime sampling is performed on the beat frequency signal of the main interferometer so that the sampling frequency of the external clock no longer depends on the bandwidth of the signal, but on the sparsity or compressibility of the signal itself, thus reducing the requirement for the sampling rate of the data acquisition card. Attached Figure Description

[0022] The above-described features, other objects, and advantages of the present invention will become clearer from the following description of embodiments of the invention with reference to the accompanying drawings, in which:

[0023] Figure 1 A schematic diagram of the structure of an optical frequency domain reflection strain and temperature sensing system according to an embodiment of the present invention is shown.

[0024] Explanation of reference numerals in the attached figures

[0025] 1-Tunable laser; 2-Isolator; 3-First coupler; 4-Main interferometer; 5-Second coupler; 6-First auxiliary interferometer; 7-Second auxiliary interferometer; 8-First photodetector; 9-Second photodetector; 10-Third photodetector; 11-Data acquisition card; 12-Digital processing chip; 13-Large-scale processor. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments and the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0027] Figure 1 A schematic diagram of the structure of an optical frequency domain reflection strain and temperature sensing system according to an embodiment of the present invention is shown.

[0028] like Figure 1 As shown, the optical frequency domain reflection strain and temperature sensing system according to an embodiment of the present invention includes: a tunable laser 1, a first coupler 3, a main interferometer 4, a second coupler 5, a first auxiliary interferometer 6, a second auxiliary interferometer 7, a first photodetector 8, a second photodetector 9, a third photodetector 10, a data acquisition card 11, a digital processing chip 12, and a large-scale processor 13. The tunable laser 1 generates a periodic linear frequency modulated optical signal, which is converted into a first optical signal and a second optical signal via the first coupler 3. The first optical signal is converted into a first beat frequency signal via the main interferometer 4, and the first beat frequency signal is converted into a first electrical signal via the first photodetector 8. The second optical signal is converted into a third optical signal and a second electrical signal via the second coupler 5. Four optical signals are generated. The third optical signal is converted into a second beat frequency signal by the first auxiliary interferometer 6, and the second beat frequency signal is converted into a second electrical signal by the second photodetector 9. The fourth optical signal is converted into a third beat frequency signal by the second auxiliary interferometer 7, and the third beat frequency signal is converted into a third electrical signal by the third photodetector 10. The first, second, and third electrical signals are acquired by the data acquisition card 11 and converted into a first digital signal, a second digital signal, and a third digital signal, respectively. The digital processing chip 12 uses the second and third digital signals as the first and second external clock signals, respectively, to simultaneously acquire the first digital signal and obtain a sampled digital signal. The sampled digital signal is processed by the large-scale processor 13 to obtain the changes in strain and temperature.

[0029] In this embodiment of the invention, the sensing system may further include an isolator 2, wherein the isolator 2 is disposed between the tunable laser 1 and the first coupler 3, for protecting the tunable laser 1 to prevent reflected light from affecting the tunable laser 1.

[0030] In some exemplary embodiments, the first coupler 3 includes a 99:1 coupler, wherein the intensity of the first optical signal accounts for 99% of the intensity of the periodic linear frequency modulated optical signal; and the intensity of the second optical signal accounts for 1% of the intensity of the periodic linear frequency modulated optical signal.

[0031] In some exemplary embodiments, the second coupler 5 includes a 50:50 coupler, wherein the intensity of the third optical signal accounts for 50% of the intensity of the second optical signal; and the intensity of the fourth optical signal accounts for 50% of the intensity of the second optical signal.

[0032] In some exemplary embodiments, the main interferometer 4, the first auxiliary interferometer 6, and the second auxiliary interferometer 7 are all Mach-Zehnder interferometers.

[0033] In some exemplary embodiments, the delay fiber of the master interferometer 4 is the fiber under test, wherein the fiber under test is etched with a weak grating array; and the first beat frequency signal is a frequency-domain sparse signal. Optionally, the intensity of the reflected light from the weak grating is 20 dB higher than the intensity of the Rayleigh scattered light, the length of the weak grating is 1 mm, the spacing of the weak grating is 4 mm, and the center wavelength of the weak grating is 1550 nm.

[0034] In some exemplary embodiments, the digital processing chip 12 simultaneously acquires the first digital signal by using the second digital signal and the third digital signal as the first external clock signal and the second external clock signal, respectively, to obtain sampled digital signals. Specifically, this includes simultaneously acquiring the beat frequency signal of the main interferometer 4 by using the second digital signal and the third digital signal as two different frequency first external clock signals and second external clock signals, respectively, to obtain uniform sampling sequences for the two channels. and The two sampling sequences were then combined to form the observed signal. The ratio of the Nyquist sampling rate N of the first digital signal to the frequency M1 of the second beat frequency signal, K1 = N / M1, is an integer; the ratio of the Nyquist sampling rate N to the frequency M2 of the third beat frequency signal, K2 = N / M2, is also an integer; and K1 and K2 are coprime numbers. Using coprime sampling, a non-uniform sampling sequence can be obtained. This sampling strategy can reduce the aliasing effect caused by a single sampling rate. The sampling formula is:

[0035] (1)

[0036] Where y is The observation vector, yT Let y be the transpose of vector y. yes The observation matrix, where x is the original signal vector of N×1. It is a Fourier basis matrix. It is an N×1 sparse coefficient vector.

[0037] It can be represented as ,in This is the observation matrix corresponding to the first external clock sampling channel. This is the observation matrix corresponding to the second external clock sampling channel, where, The expression is:

[0038] (2)

[0039] The expression is:

[0040] (3)

[0041] Considering the time delay between the two external sampling clocks, zeros need to be padded before and after the observation matrices when combining the two observation matrices. The number of zeros can be determined by obtaining the start and end sampling times of the two channels.

[0042] In this embodiment of the invention, by performing coprime sampling on the beat frequency signal of the main interferometer 4, the sampling frequency of the external clock no longer depends on the bandwidth of the signal, but on the sparsity or compressibility of the signal itself. This solves the problem that the sensing distance is limited by the Nyquist sampling theorem, increases the sensing length of the OFDR system, and reduces the requirement for the sampling rate of the data acquisition card 11.

[0043] In some exemplary embodiments, the process of obtaining strain and temperature changes by the large-scale processor 13 from the sampled digital signal includes: the large-scale processor 13 using a sparse recovery algorithm to reconstruct the sampled digital signal to obtain a reconstructed signal; and based on the reconstructed signal, using a windowed cross-correlation demodulation algorithm to demodulate the strain and temperature changes. Optionally, the sparse recovery algorithm includes at least one of a convex optimization algorithm, a greedy tracking algorithm, or a sparse Bayesian learning algorithm.

[0044] The embodiments of the present invention have been described above. However, these embodiments are merely illustrative and not intended to limit the scope of the invention. Although various embodiments have been described above, this does not mean that the measures in the various embodiments cannot be used advantageously in combination. Various substitutions and modifications can be made by those skilled in the art without departing from the scope of the invention, and all such substitutions and modifications should fall within the scope of the invention.

Claims

1. A light frequency domain reflection strain and temperature sensing system, characterized in that, The sensing system includes: a tunable laser, a first coupler, a main interferometer, a second coupler, a first auxiliary interferometer, a second auxiliary interferometer, a first photodetector, a second photodetector, a third photodetector, a data acquisition card, a digital processing chip, and a large-scale processor. The tunable laser generates a periodic linear frequency modulated (LFM) optical signal. This LFM optical signal is converted into a first optical signal and a second optical signal via the first coupler. The first optical signal is converted into a first beat frequency signal via the main interferometer, and then into a first electrical signal via the first photodetector. The second optical signal is converted into a third and a fourth optical signal via the second coupler. The third optical signal is converted into a second beat frequency signal via the first auxiliary interferometer, and then into a second electrical signal via the second photodetector. The fourth optical signal is converted into a third beat frequency signal via the second auxiliary interferometer, and then into a third electrical signal via the third photodetector. The first, second, and third electrical signals are acquired by the data acquisition card and converted into a first, second, and third digital signal, respectively. The digital processing chip uses the second and third digital signals as a first and a second external clock signal, respectively, to simultaneously acquire the first digital signal, obtaining a sampled digital signal. The sampled digital signal is processed by the large-scale processor to obtain the changes in strain and temperature.

2. The system according to claim 1, characterized in that, The sensing system further includes an isolator, wherein the isolator is disposed between the tunable laser and the first coupler to protect the tunable laser from reflected light affecting the tunable laser.

3. The system according to claim 1 or 2, characterized in that, The delay fiber of the main interferometer is the fiber under test. The fiber under test includes a weak grating array; and The first beat frequency signal is a frequency-domain sparse signal.

4. The system according to claim 3, characterized in that, The intensity of the reflected light from the weak grating is 20 dB higher than the intensity of the Rayleigh scattered light. The length of the weak grating is 1 mm, the spacing between the weak gratings is 4 mm, and the center wavelength of the weak grating is 1550 nm.

5. The system according to claim 1 or 2, characterized in that, The ratio K1 of the sampling rate of the first digital signal to the frequency of the second beat frequency signal is an integer; The ratio K2 of the sampling rate to the frequency of the third beat frequency signal is an integer; and K1 and K2 are coprime numbers.

6. The system according to claim 1 or 2, characterized in that, The sampled digital signal, after being processed by the large-scale processor, yields the strain and temperature changes, including: The large-scale processor uses a sparse recovery algorithm to restore the sampled digital signal to obtain the restored signal; Based on the restored signal, a windowed cross-correlation demodulation algorithm is used to demodulate the strain and temperature changes.

7. The system according to claim 6, characterized in that, The sparse recovery algorithm includes at least one of convex optimization algorithms, greedy tracing algorithms, or sparse Bayesian learning algorithms.

8. The system according to claim 1, characterized in that, The first coupler includes a 99:1 coupler, wherein the intensity of the first optical signal accounts for 99% of the intensity of the periodic linear frequency modulated optical signal; and the intensity of the second optical signal accounts for 1% of the intensity of the periodic linear frequency modulated optical signal.

9. The system according to claim 1, characterized in that, The second coupler includes a 50:50 coupler, wherein the intensity of the third optical signal accounts for 50% of the intensity of the second optical signal; and the intensity of the fourth optical signal accounts for 50% of the intensity of the second optical signal.

10. The system according to claim 1, characterized in that, The main interferometer, the first auxiliary interferometer, and the second auxiliary interferometer are all Mach-Zehnder interferometers.

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