Optical frequency domain reflection strain and temperature sensing system

Abstract

The invention provides an optical frequency domain reflection strain and temperature sensing system which can be applied to the technical field of optical fiber sensing. 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, beat frequency signals generated by a first auxiliary interferometer and a second auxiliary interferometer are used as external clock signals, and co-prime sampling is performed on the beat frequency signals of a main interferometer, so that the sampling frequency of the external clock does not depend on the bandwidth of the signals any more, but depends on the sparsity or compressibility of the signals; 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 meanwhile the requirement for the sampling rate of a data acquisition card is lowered.

Description

Technical Field

[0001] The present invention relates to the field of optical fiber sensing technology, and in particular to an optical frequency domain reflection strain and temperature sensing system. Background Art

[0002] At present, the optical frequency domain reflectometry (OFDR) system that can realize real-time demodulation usually adopts an external clock sampling scheme, which uses the zero-crossing point of the auxiliary interferometer signal as the clock for external sampling of the acquisition card to achieve uniform frequency sampling of the main interferometer beat signal, and uses hardware to directly correct the nonlinearity of the light source frequency, and the light source sweep and correction are completed synchronously. However, this scheme is limited by the Nyquist sampling theorem. The frequency of the auxiliary interferometer beat signal should be more than twice the maximum frequency of the main interferometer beat signal, that is, the maximum length of the main interferometer sensor fiber is one-fourth of the optical path difference length of the auxiliary interferometer fiber.

[0003] At present, Shandong University has proposed a long-distance high spatial resolution system based on time division multiplexing. This system segments the optical 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] 1. Technical issues to be resolved

[0005] In order to solve the problem in the prior art that the sensing distance is limited by the Nyquist sampling theorem and to improve the sensing distance of the OFDR system, an embodiment of the present invention provides an optical frequency domain reflection strain and temperature sensing system, which constructs a frequency domain sparse model by writing a weak grating array in the optical fiber under test and adopts a compressed sensing algorithm so that the external clock sampling frequency no longer depends on the bandwidth of the signal, but on the sparsity or compressibility of the signal itself.

[0006] (II) Technical solution

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

[0008] According to a first aspect of the present invention, there is provided an optical frequency domain reflection strain and temperature sensing system, 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, and the periodic linear frequency modulated optical signal is converted into a first optical signal and a second optical signal through the first coupler, wherein the first optical signal is converted into a first beat frequency signal through the main interferometer, and the first beat frequency signal is converted into a first electrical signal through the first photodetector; the second optical signal is converted into a third optical signal and a fourth optical signal through the second coupler. signal, wherein the third optical signal is converted into a second beat frequency signal through the first auxiliary interferometer, and the second beat frequency signal is converted into a second electrical signal through the second photodetector; the fourth optical signal is converted into a third beat frequency signal through the second auxiliary interferometer, and the third beat frequency signal is converted into a third electrical signal through the third photodetector; the first electrical signal, the second electrical signal and the third electrical signal are collected 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 collect the first digital signal at the same time to obtain a sampled digital signal; the sampled digital signal is processed by a large-scale processor to obtain the change in strain and temperature.

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

[0010] In some exemplary embodiments, the delay fiber of the main interferometer includes a measured fiber, wherein the measured fiber is inscribed with a weak grating array; and the first beat signal is a frequency domain sparse signal.

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

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

[0013] In some exemplary embodiments, the sampled digital signal is processed by a large-scale processor to obtain the variation of strain and temperature, including: the large-scale processor uses a sparse recovery algorithm to restore the sampled digital signal to obtain a restored signal; based on the restored signal, a windowed cross-correlation demodulation algorithm is used to demodulate the variation of strain and temperature.

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

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

[0016] In some exemplary embodiments, the second coupler comprises 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] It can be seen from the above technical solutions that 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) The beat frequency signal of the main interferometer is sampled mutually primely, so that the external clock sampling frequency 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 and increases the sensing length of the OFDR system.

[0021] (2) The beat frequency signal of the main interferometer is sampled in a mutually prime manner, so that the external clock sampling frequency no longer depends on the bandwidth of the signal, but on the sparsity or compressibility of the signal itself, thereby reducing the demand for the sampling rate of the data acquisition card. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The above contents and other objects, features and advantages of the present invention will become more apparent through the following description of the embodiments of the present invention with reference to the accompanying drawings, in which:

[0023] Figure 1 The structure diagram of the optical frequency domain reflection strain and temperature sensing system according to an embodiment of the present invention is schematically shown. Description of Reference Numerals

[0024] 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 DESCRIPTION

[0025] In order to make the purpose, technical scheme and advantages of the present invention more clearly understood, the present invention is further described in detail below in combination with specific embodiments and with reference to the accompanying drawings. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0026] Figure 1 The structure diagram of the optical frequency domain reflection strain and temperature sensing system according to an embodiment of the present invention is schematically shown.

[0027] 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, wherein the tunable laser 1 generates a periodic linear frequency modulated optical signal, and the periodic linear frequency modulated optical signal is converted into a first optical signal and a second optical signal through the first coupler 3, wherein the first optical signal is converted into a first beat frequency signal through the main interferometer 4, and the first beat frequency signal is converted into a first electrical signal through the first photodetector 8; the second optical signal is converted into a third optical signal and a second electrical signal through the second coupler 5. Four optical signals, wherein the third optical signal is converted into a second beat frequency signal through the first auxiliary interferometer 6, and the second beat frequency signal is converted into a second electrical signal through the second photodetector 9; the fourth optical signal is converted into a third beat frequency signal through the second auxiliary interferometer 7, and the third beat frequency signal is converted into a third electrical signal through the third photodetector 10; the first electrical signal, the second electrical signal and the third electrical signal are collected 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 digital signal and the third digital signal as the first external clock signal and the second external clock signal respectively to collect the first digital signal at the same time to obtain a sampled digital signal; the sampled digital signal is processed by the large-scale processor 13 to obtain the change in strain and temperature.

[0028] In an embodiment of the present 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 , and is used to protect the tunable laser 1 to prevent reflected light from affecting the tunable laser 1 .

[0029] In some exemplary embodiments, the first coupler 3 comprises a 99:1 coupler, wherein the intensity of the first optical signal accounts for 99% of the intensity of the periodic linear frequency modulation optical signal; and the intensity of the second optical signal accounts for 1% of the intensity of the periodic linear frequency modulation optical signal.

[0030] In some exemplary embodiments, the second coupler 5 comprises 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.

[0031] In some exemplary embodiments, the main interferometer 4, the first auxiliary interferometer 6 and the second auxiliary interferometer 7 are all Mach-Zehnder interferometers.

[0032] In some exemplary embodiments, the delay fiber of the main interferometer 4 is the measured fiber, wherein the measured fiber is engraved with a weak grating array; and the first beat frequency signal is a frequency domain sparse signal. Optionally, the reflected light intensity of the weak grating is 20 dB higher than the Rayleigh scattered light intensity, the length of the weak grating is 1 mm, the interval of the weak grating is 4 mm, and the central wavelength of the weak grating is 1550 nm.

[0033] In some exemplary embodiments, the digital processing chip 12 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 collect the first digital signal to obtain a sampled digital signal, specifically including using the second digital signal and the third digital signal as the first external clock signal and the second external clock signal of two different frequencies respectively to simultaneously collect the beat frequency signal of the main interferometer 4, and obtain the uniform sampling sequences of the two channels respectively. and , and then combine the above two sampling sequences into the observation signal . The ratio K1=N / M1 of the Nyquist sampling rate N of the first digital signal and the frequency M1 of the second beat signal is an integer; the ratio K2=N / M2 of the Nyquist sampling rate N and the frequency M2 of the third beat signal is an integer; and K1 and K2 are mutually prime. A set of non-uniform sampling sequences can be obtained by using the mutually prime sampling method. This sampling strategy can reduce the aliasing effect caused by a single sampling rate. The sampling formula is:

[0034] (1)

[0035] Among them, y is The observation vector, yT represents the transposed vector of vector y, yes The observation matrix, x is the original signal vector of N×1, is the Fourier basis matrix, is an N×1 sparse coefficient vector.

[0036] It can be expressed as ,in is the observation matrix corresponding to the first external clock sampling channel, is the observation matrix corresponding to the second external clock sampling channel, where The expression is:

[0037] (2)

[0038] The expression is:

[0039] (3)

[0040] Considering that there is a certain time delay between the two external sampling clocks, when combining two observation matrices, it is necessary to fill zeros before and after the observation matrices. The number of zero-filling points can be determined by obtaining the time of the start sampling point and the end sampling point of the two channels.

[0041] In the embodiment of the present invention, by performing coprime sampling on the beat frequency signal of the main interferometer 4, the external clock sampling frequency no longer depends on the bandwidth of the signal, but depends on the sparsity or compressibility of the signal itself, thereby solving the problem that the sensing distance is limited by the Nyquist sampling theorem, increasing the sensing length of the OFDR system, and reducing the demand for the sampling rate of the data acquisition card 11.

[0042] In some exemplary embodiments, the large-scale processor 13 processes the sampled digital signal to obtain the variation of strain and temperature, which includes: the large-scale processor 13 uses a sparse recovery algorithm to restore the sampled digital signal to obtain a restored signal; based on the restored signal, a windowed cross-correlation demodulation algorithm is used to demodulate the variation of strain and temperature. Optionally, the sparse recovery algorithm includes at least one of a convex optimization algorithm, a greedy pursuit algorithm, or a sparse Bayesian learning algorithm.

[0043] The embodiments of the present invention are described above. However, these embodiments are only for the purpose of illustration, and are not intended to limit the scope of the present invention. Although each embodiment is described above, it does not mean that the measures in each embodiment cannot be used in combination advantageously. Without departing from the scope of the present invention, those skilled in the art may make various substitutions and modifications, which should all fall within the scope of the present invention.

Claims

1. An optical frequency domain reflection strain and temperature sensing system, characterized in that: 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 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 optical signal, which is converted into a first optical signal and a second optical signal through the first coupler, wherein the first optical signal is converted into a first beat frequency signal through the main interferometer, and the first beat frequency signal is converted into a first electrical signal through the first photodetector; the second optical signal is converted into a third optical signal and a fourth optical signal through the second coupler, wherein the third optical signal is converted into a second beat frequency signal through the first auxiliary interferometer, and the second beat frequency signal is converted into a second electrical signal through the second photodetector; the fourth optical signal is converted into a third beat frequency signal through the second auxiliary interferometer, and the third beat frequency signal is converted into a third electrical signal through the third photodetector; the first electrical signal, the second electrical signal and the third electrical signal are collected 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 collect the first digital signal at the same time to obtain a sampled digital signal; the sampled digital signal is processed by the large-scale processor to obtain the variation of 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 and is used to protect the tunable laser to prevent reflected light from 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, Wherein, the optical 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 reflected light intensity of the weak grating is 20 dB higher than the Rayleigh scattered light intensity, the length of the weak grating is 1 mm, the interval of the weak grating is 4 mm, and the central wavelength of the weak grating is 1550 nm.

5. The system according to claim 1 or 2, characterized in that: A ratio K1 of a sampling rate for collecting the first digital signal and a frequency of the second beat frequency signal is an integer; A ratio K2 of the sampling rate to the frequency of the third beat frequency signal is an integer; and K1 and K2 are prime to each other.

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

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

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

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