Brillouin dynamic grating reflection spectrum measuring device and method based on spectrum demodulation

Abstract

The invention belongs to the technical field of distributed optical fiber sensing, and discloses a Brillouin dynamic grating reflection spectrum measuring device based on spectrum demodulation, which comprises a pump light source and a tunable laser, the pump light source is connected with the optical fiber coupler, a first output port and a second output port of the optical fiber coupler are respectively connected with input ports of the single side band modulator and the second polarizer, and an output port of the single side band modulator is connected with one end of the polarization maintaining optical fiber through the first polarizer; the output port of the second polarizer is connected with the first input port of the polarization beam splitter; an output port of the tunable laser is connected with a second input port of the polarization beam splitter through the third polarizer and the optical circulator, and a combined beam output port of the polarization beam splitter is connected with the other end of the polarization maintaining optical fiber; and a reflection port of the optical circulator is connected with an input port of the spectrograph. According to the invention, rapid, high-precision and low-cost measurement of the BDG reflectance spectrum is realized, and the method is especially suitable for dynamic distributed optical fiber sensing.

Description

A Brillouin dynamic grating reflectance spectrum measurement device and method based on spectral demodulation Technical Field

[0001] This invention belongs to the field of distributed optical fiber sensing technology, and specifically relates to a Brillouin dynamic grating reflectance spectrum measurement device and method based on spectral demodulation. Background Technology

[0002] Distributed fiber optic sensing technology enables long-distance, distributed measurement of physical quantities such as temperature, strain, and vibration by transmitting and sensing optical signals in optical fibers. The sensing mechanism based on fiber optic Brillouin scattering has become a focus of attention in industry and academia due to its ability to simultaneously sense temperature and strain. However, traditional Brillouin distributed fiber optic sensing technology suffers from the problem of cross-sensitivity between temperature and strain; that is, a single Brillouin frequency shift measurement cannot distinguish between changes in the two physical quantities, which severely limits its accurate measurement capability in complex environments (Applied Optics, 55(31), 8470–8476, 2016). To address the problem of "cross-sensitivity between temperature and strain", Song, KY, and others first proposed the concept of Brillouin Dynamic Grating (BDG). Essentially, it excites a Brillouin dynamic grating in an optical fiber through stimulated Brillouin scattering. By measuring the reflection spectrum of the BDG, a birefringence frequency shift that is linearly related to temperature / strain is extracted, and the birefringence frequency shift and the Brillouin frequency shift form a dual parameter to achieve decoupled measurement of temperature and strain (Optics Letters, 31(3), 352-354, 2006).

[0003] To achieve distributed sensing based on the decoupling of temperature and strain in BDG, the key lies in accurately measuring its reflection spectrum. Currently, the main method for measuring the reflection spectrum of BDG is to use a tunable laser scanning combined with synchronous detection. This method reads the reflection spectrum by scanning the frequency of the tunable laser, and at each frequency point, a lock-in amplifier is used to extract weak reflection signals or a power meter is used to directly measure the reflected light power. This traditional method has the following obvious limitations: although the lock-in amplifier can extract weak signals, its reliance on slow wavelength scanning and time integration mechanisms results in extremely slow measurement speed, making real-time sensing impossible. Moreover, the system is complex, costly, and prone to introducing phase errors (Proceedings of SPIE, vol. 5855, 558–565, 2005); although the scheme of directly measuring the reflected light power with a power meter has a simple structure, it cannot distinguish between signal and noise, resulting in an extremely low signal-to-noise ratio, limited dynamic range, and poor measurement accuracy (Journal of Lightwave Technology, 27(16), 3300-3306, 2009). Both are constrained by the serial point-by-point scanning working mode, which together create bottlenecks in the measurement speed, accuracy and efficiency of traditional methods.

[0004] Therefore, it is necessary to improve the Brillouin dynamic grating reflectance spectrum measurement method to solve the problems of slow speed, high system cost and low measurement accuracy of the traditional BDG reflectance spectrum measurement method. Summary of the Invention

[0005] To address the technical problems of low accuracy, low signal-to-noise ratio, and slow measurement speed in existing Brillouin dynamic grating reflectance spectrum measurements, this invention proposes a Brillouin dynamic grating reflectance spectrum measurement device and method based on spectral demodulation.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: a Brillouin dynamic grating reflection spectrum measurement device based on spectral demodulation, comprising a pump source, an optical fiber coupler, a single-sideband modulator, a first polarizer, a polarization-maintaining fiber, a second polarizer, a polarization beam splitter, a tunable laser, a third polarizer, an optical circulator, and a spectrometer; the output port of the pump source is connected to the input port of the optical fiber coupler, the first output port of the optical fiber coupler is connected to the input port of the single-sideband modulator, the output port of the single-sideband modulator is connected to the input port of the first polarizer, and the output port of the first polarizer is connected to one end of the polarization-maintaining fiber; the second output port of the optical fiber coupler is connected to the input port of the second polarizer, and the output port of the second polarizer is connected to the first input port of the polarization beam splitter; the output port of the tunable laser is connected to the input port of the third polarizer, the output port of the third polarizer is connected to the input port of the optical circulator, the output port of the optical circulator is connected to the second input port of the polarization beam splitter, and the combined output port of the polarization beam splitter is connected to the other end of the polarization-maintaining fiber; the reflection port of the optical circulator is connected to the input port of the spectrometer.

[0007] The first and second output ports of the fiber coupler are used to output the second pump light and the first pump light, respectively. The single-sideband modulator is used to perform low-frequency frequency shifting on the second pump beam so that the frequency difference between the first and second pump lights is equal to the Brillouin frequency shift, thereby forming a Brillouin dynamic grating when they meet in the polarization-maintaining fiber. The tunable laser is used to output a probe light whose frequency difference with the first pump light is equal to the positive and negative birefringence frequency shift. The second polarizer is used to adjust the polarization direction of the first pump light so that it is along the first polarization axis of the polarization beam splitter. The first polarizer is used to adjust the polarization direction of the second pump light so that it is the same as the polarization direction of the first pump light in the polarization-maintaining fiber. The third polarizer is used to adjust the polarization direction of the probe light so that it is along the second polarization axis of the polarization beam splitter.

[0008] The spectrometer is used to collect the reflected light spectrum formed after the probe light is reflected by the Brillouin dynamic grating.

[0009] The Brillouin dynamic grating reflectance spectrum measurement device based on spectral demodulation further includes a computer. The output port of the spectrometer is connected to the input port of the computer. The computer is used to extract the peak values ​​of the reflected light spectrum collected by the spectrometer at different probe light frequencies, and to perform smooth fitting and normalization operations to obtain the reflectance spectrum.

[0010] The computer is also used to calculate the length of the Brillouin dynamic grating based on the spectral width of the reflection spectrum, using the following formula: Where c represents the speed of light, n represents the refractive index of the optical fiber, and L... c Δf represents the length of the Brillouin dynamic grating, and Δf represents the spectral width of the reflection spectrum.

[0011] The Brillouin dynamic grating reflectance spectrum measurement device based on spectral demodulation further includes a first erbium-doped fiber amplifier, a second erbium-doped fiber amplifier, a third erbium-doped fiber amplifier, and a fourth erbium-doped fiber amplifier. The first erbium-doped fiber amplifier is disposed between the first output port of the fiber coupler and the input port of the single-sideband modulator. The second erbium-doped fiber amplifier is disposed between the output port of the single-sideband modulator and the input port of the first polarizer. The first and second erbium-doped fiber amplifiers are used to amplify the light intensity before entering the single-sideband modulator and the light intensity output by the single-sideband modulator, respectively. The third erbium-doped fiber amplifier is disposed between the second output port of the fiber coupler and the input port of the second polarizer, and is used to amplify the light intensity entering the second polarizer. The fourth erbium-doped fiber amplifier is disposed between the output port of the tunable laser and the input port of the third polarizer, and is used to amplify the light intensity entering the third polarizer.

[0012] The Brillouin dynamic grating reflection spectrum measurement device based on spectral demodulation further includes a microwave signal generator, which is electrically connected to the driving end of the single-sideband modulator and is used to output a microwave signal to drive the single-sideband modulator to output the low-frequency sideband of the second pump beam.

[0013] The pump light source is a continuous light source, a pulsed light source, a chaotic light source, or an ASE light source.

[0014] Furthermore, this invention also provides a Brillouin dynamic grating reflection spectrum measurement method based on spectral demodulation, implemented using the aforementioned Brillouin dynamic grating reflection spectrum measurement device based on spectral demodulation, comprising the following steps: Step 1: Adjusting the offset frequency of the single-sideband modulator to equal the Brillouin frequency shift, so that the frequency of the output second pump light decreases the Brillouin frequency shift; adjusting the polarization angle of the first pump light through the second polarizer, so that the intensity of the first pump light is maximized after passing through the polarization beam splitter; adjusting the polarization angle of the second pump light through the first polarizer, so that the second pump light is incident along the corresponding axis of the polarization-maintaining fiber; simultaneously, adjusting the angle of the third polarizer, so that the intensity of the probe light is maximized after passing through the polarization beam splitter; Step 2: Sweeping the probe light frequency, and collecting the reflection light spectrum at different probe light frequencies using a spectrometer; Step 3: Extracting the peak values ​​of the reflection light spectra collected at different probe light frequencies, performing smooth fitting and normalization operations to form a peak-frequency curve, which is the reflection spectrum.

[0015] The Brillouin dynamic grating reflectance spectrum measurement method based on spectral demodulation further includes the following steps: Step 4: Extract the full width at half maximum (FWHM) of the peak-frequency curve to obtain the reflectance spectral width of the Brillouin dynamic grating; calculate the length of the Brillouin dynamic grating based on the reflectance spectral width, using the following formula: Where c represents the speed of light, n represents the refractive index of the optical fiber, and L...c Δf represents the length of the Brillouin dynamic grating, and Δf represents the spectral width of the reflection spectrum.

[0016] Compared with existing technologies, this invention has the following advantages: First, this invention uses two pump light sources with double-ended incident light to generate a BDG (Diverterless Radiation Gamma). Combined with the single-scan spectral acquisition capability of a spectrometer, it transforms the traditional serial measurement mode, which relies on point-by-point scanning of a tunable laser, into a spectral analysis mode based on parallel acquisition by the spectrometer. The single measurement time is shortened from the second level to the millisecond level, significantly improving measurement efficiency and meeting the real-time dynamic monitoring requirements for parameters such as temperature and strain.

[0017] Secondly, by introducing a single-sideband modulator and a microwave signal generator, this invention precisely controls the frequency difference between the two pump beams, effectively avoiding frequency drift and reflection spectrum distortion caused by tuning nonlinearity in traditional tunable lasers, thereby significantly improving the accuracy of BDG center frequency measurement and the long-term operational stability of the system.

[0018] Third, by rationally designing the optical path structure and signal processing flow, this invention reduces the system's reliance on expensive discrete components (such as lock-in amplifiers), effectively controlling costs and improving system reliability and maintainability. In terms of data processing, the reflection spectrum envelope is reconstructed through multiple spectral acquisitions and peak extraction algorithms, effectively smoothing random noise and further improving the system's signal-to-noise ratio while ensuring measurement accuracy.

[0019] Fourth, this invention supports a variety of pump light sources (including continuous light, pulsed light, chaotic light, ASE light, etc.), and has strong system flexibility and environmental adaptability. The appropriate light source type can be selected according to the actual application scenario, which expands the applicability of BDG sensing technology in complex industrial environments.

[0020] In summary, this invention solves the problems of slow speed, system instability, and nonlinear error in traditional spectral scanning methods based on lock-in amplifiers and optical power meters. It achieves fast, high-precision, and low-cost measurement of BDG reflectance spectra. Moreover, it has good light source compatibility and system adaptability, and is particularly suitable for dynamic distributed fiber optic sensing. Attached Figure Description

[0021] Figure 1 is a schematic diagram of a Brillouin dynamic grating reflection spectrum measurement device based on spectral demodulation provided in an embodiment of the present invention; the reference numerals in the figure are: 1-pump source (including but not limited to continuous light source, pulsed light source, chaotic light source, ASE light source, etc.); 2-fiber coupler; 3-first erbium-doped fiber amplifier; 4-single sideband modulator; 5-microwave signal generator; 6-second erbium-doped fiber amplifier; 7-first polarizer; 8-polarization-maintaining fiber; 9-third erbium-doped fiber amplifier; 10-second polarizer; 11-polarization beam splitter; 12-tunable laser; 13-fourth erbium-doped fiber amplifier; 14-third polarizer; 15-optical circulator; 16-spectrometer; 17-computer; Figure 2 is a schematic diagram of Brillouin dynamic grating reflection spectrum measurement based on spectral demodulation. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are some embodiments of the present invention, but not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] As shown in Figure 1, Embodiment 1 of the present invention provides a Brillouin dynamic grating reflection spectrum measurement device based on spectral demodulation, comprising a pump source 1, an optical fiber coupler 2, a single-sideband modulator 4, a first polarizer 7, a polarization-maintaining fiber 8, a second polarizer 10, a polarization beam splitter 11, a tunable laser 12, a third polarizer 14, an optical circulator 15, and a spectrometer 16; the output port of the pump source 1 is connected to the input port of the optical fiber coupler 2, the first output port of the optical fiber coupler 2 is connected to the input port of the single-sideband modulator 4, and the output port of the single-sideband modulator 4 is connected to the input port of the first polarizer 7. The output port of fiber 7 is connected to one end of polarization-maintaining fiber 8; the second output port of fiber coupler 2 is connected to the input port of second polarizer 10, and the output port of second polarizer 10 is connected to the first input port of polarization beam splitter 11; the output port of tunable laser 12 is connected to the input port of third polarizer 14, the output port of third polarizer 14 is connected to the input port of optical circulator 15, the output port of optical circulator 15 is connected to the second input port of polarization beam splitter 11, and the beam combining output port of polarization beam splitter 11 is connected to the other end of polarization-maintaining fiber 8; the reflection port of optical circulator 15 is connected to the input port of spectrometer 16.

[0024] In this embodiment, the first output port and the second output port of the fiber coupler 2 are used to output the second pump light and the first pump light, respectively. The single sideband modulator 4 is used to perform low-frequency frequency shift on the second pump light so that the frequency difference between the first pump light and the second pump light is equal to the Brillouin frequency shift, and then they meet in the polarization-maintaining fiber 8 to form a Brillouin dynamic grating.

[0025] In this embodiment, the tunable laser 12 is used to output probe light with a frequency difference from the first pump light equal to the positive and negative birefringence frequency shift.

[0026] In this embodiment, the second polarizer 10 is used to adjust the polarization direction of the first pump light so that it is along the first polarization axis of the polarization beam splitter 11; the first polarizer 7 is used to adjust the polarization direction of the second pump light so that it is the same as the polarization direction of the first pump light in the polarization-maintaining fiber 8; the third polarizer 14 is used to adjust the polarization direction of the probe light so that it is along the second polarization axis of the polarization beam splitter 11. Specifically, the first polarization axis is the x-axis and the second polarization axis is the y-axis. Alternatively, the first polarization axis can also be set to the y-axis and the second polarization axis to the x-axis, in which case the connection ports of the first pump light and the probe light with the polarization beam splitter 11 need to be interchanged. Through the polarization beam splitter 11, the first pump light and the probe light with perpendicular polarization can be combined into a single beam.

[0027] Preferably, in this embodiment, the fast axis or slow axis direction of the polarization-maintaining fiber 8 is the same as the polarization direction of the transmitted light from the polarization beam splitter 11, which ensures that the polarization component of the first pump light entering the polarization-maintaining fiber 8 along the fast axis or slow axis direction of the polarization-maintaining fiber 8 is maximized; the polarization of the second pump light can be controlled to be the same as that of the first pump light through the first polarizer 7. Correspondingly, the polarization direction of the probe light entering the polarization-maintaining fiber 8 is perpendicular to the first pump light and along the slow axis or fast axis direction of the polarization-maintaining fiber 8; for the former, the frequency difference between the probe light and the first pump light is a birefringence frequency shift, and for the latter, the frequency difference between the probe light and the first pump light is a negative birefringence frequency shift.

[0028] In this embodiment, the spectrometer 16 is used to collect the reflected light spectrum formed after the probe light is reflected by the Brillouin dynamic grating.

[0029] Furthermore, in this embodiment, the first input port of the polarization beam splitter 11 is an x-axis input port and the second input port is a y-axis input port. In addition, in this embodiment, the first input port of the polarization beam splitter 11 can be a y-axis input port, in which case the second input port is an x-axis input port.

[0030] Furthermore, the Brillouin dynamic grating reflectance spectrum measurement device based on spectral demodulation in this embodiment also includes a computer 17. The output port of the spectrometer 16 is connected to the input port of the computer 17. The computer 17 is used to extract the peak values ​​of the reflected light spectrum collected by the spectrometer 16 at different probe light frequencies, and perform smooth fitting and normalization operations to obtain the reflectance spectrum.

[0031] Furthermore, in this embodiment, the computer 17 is also used to calculate the length of the Brillouin dynamic grating based on the spectral width of the reflection spectrum, using the following formula: (1) Where c represents the speed of light, n represents the refractive index of the optical fiber, and L c Δf represents the length of the Brillouin dynamic grating, and Δf represents the spectral width of the reflection spectrum.

[0032] Furthermore, the Brillouin dynamic grating reflectance spectrum measurement device based on spectral demodulation in this embodiment also includes a first erbium-doped fiber amplifier 3, a second erbium-doped fiber amplifier 6, a third erbium-doped fiber amplifier 9, and a fourth erbium-doped fiber amplifier 13. The first erbium-doped fiber amplifier 3 is disposed between the first output port of the fiber coupler 2 and the input port of the single-sideband modulator 4. The second erbium-doped fiber amplifier 6 is disposed between the output port of the single-sideband modulator 4 and the input port of the first polarizer 7. The first erbium-doped fiber amplifier 3 and the second erbium-doped fiber amplifier 6 are used to amplify the light intensity before entering the single-sideband modulator 4 and the light intensity output by the single-sideband modulator 4, respectively. The third erbium-doped fiber amplifier 9 is disposed between the second output port of the fiber coupler 2 and the input port of the second polarizer 10, and is used to amplify the light intensity entering the second polarizer 10. The fourth erbium-doped fiber amplifier 13 is disposed between the output port of the tunable laser 12 and the input port of the third polarizer 14, and is used to amplify the light intensity entering the third polarizer 14.

[0033] Furthermore, the Brillouin dynamic grating reflection spectrum measurement device based on spectral demodulation in this embodiment also includes a microwave signal generator 5, which is electrically connected to the driving end of the single-sideband modulator 4 and is used to output a microwave signal to drive the single-sideband modulator 4 to output the low-frequency sideband of the second pump beam.

[0034] Furthermore, in this embodiment, the pump light source 1 is a continuous light source, a pulsed light source, a chaotic light source, or an ASE light source.

[0035] Furthermore, in this embodiment, the fiber coupler 2 is a 90:10 fiber coupler, wherein 10% of the port is used to output the first pump light and 90% of the port is used to output the second pump light.

[0036] The working principle of this invention is as follows: (1) The pump light source 1 is turned on, and the pump light output by it is split into two paths by the fiber coupler 2: the first pump light and the second pump light.

[0037] (2) After the first pump light is amplified by the third erbium-doped fiber amplifier 9, the polarization direction is adjusted to p polarization by the second polarizer 10. Then the second pump light enters the polarization beam splitter 11 from the first input port of the polarization beam splitter 11 and is transmitted to one end of the polarization-maintaining fiber 8. When it is incident, the polarization is incident along the slow axis of the polarization-maintaining fiber 8.

[0038] (3) After the second pump light is amplified by the first erbium-doped fiber amplifier 3, it is modulated by the single-sideband modulator 4. The offset frequency of the single-sideband modulator 4 is controlled by the radio frequency signal output by the microwave signal generator 5. The first pump light is modulated into a low-frequency single-sideband light signal that differs from the second pump light by a Brillouin frequency shift. After being amplified by the second erbium-doped amplifier 6, the polarization direction is adjusted by the first polarizer 7 to be the same as the slow axis of the polarization-maintaining fiber 8. Finally, it is incident on the other end of the polarization-maintaining fiber 8. During incident, the polarization is maintained along the slow axis of the polarization-maintaining fiber 8.

[0039] (4) The first pump light and the second pump light meet in the slow axis of the polarization-maintaining fiber 8 and the stimulated Brillouin scattering effect occurs. The polarization-maintaining fiber 8 generates an acoustic field due to the electrostriction effect. This acoustic field then modulates the refractive index of the fiber, eventually forming a Brillouin dynamic grating.

[0040] (5) The probe light is output from the tunable laser 12 and its wavelength is swept. The sweep frequency range of the probe light is controlled. When the probe light and the second pump light meet the phase matching condition, that is, when the frequency difference is the birefringence frequency shift, the reflectivity of the Brillouin dynamic grating reaches its maximum value. After the probe light is amplified by the fourth erbium-doped fiber amplifier 13, its polarization direction is adjusted to s-polarization by the third polarizer 14. Then, it is output through the output port of the circulator 15 and incident on the second input port of the polarization beam splitter 11. After being reflected by the polarization beam splitter 11, it is output from the beam combining output port together with the first pump light and incident on one end of the polarization-maintaining fiber 8. When incident, the polarization of the probe light is along the fast axis of the polarization-maintaining optical axis 8. By making the polarization directions of the probe light and the pump light along the fast axis and slow axis of the polarization-maintaining fiber 8, respectively, crosstalk of the signal can be avoided and the signal-to-noise ratio of the reflection spectrum can be improved.

[0041] (6) The probe light is reflected by the Brillouin dynamic grating to form reflected light. The reflected light returns along the original path to the polarization beam splitter 11 and circulator 15, and is output through the reflection port of circulator 15. The reflected spectrum is then collected by spectrometer 16. As the wavelength of the probe light moves, the peak value of the reflected spectrum will go through a process from low to high and then back to low. The peak value of this process needs to be extracted according to the following steps: First, the computer 17 sends a data saving command to the spectrometer 16. The spectrometer 16 saves the spectral data as a CSV file and transmits the data between the spectrometer 16 and the development board via the USB interface. Then, the computer 17 reads the spectral data from the development board and extracts the peak points from the CSV file. Finally, the peak points are connected, and smoothing and normalization operations are performed. The resulting curve is the reflected spectrum. In this embodiment, more than 30 extractions are required. The larger the number of samples, the denser the extracted peaks, and the more accurate the generated reflected spectrum. This function is integrated into the data processing interface of the host computer. After the data is saved by the control buttons on the interface, the system automatically processes the data, extracts the full width at half maximum (FWHM) of the fitted curve, which is the reflection spectral width of the Brillouin dynamic grating; calculates the length of the Brillouin dynamic grating according to the formula, and finally draws the envelope diagram of the reflection spectrum on the computer 17 and displays the reflection spectral width and the length of the Brillouin dynamic grating.

[0042] Example 2 of the present invention provides a Brillouin dynamic grating reflection spectrum measurement method based on spectral demodulation, which is implemented based on the Brillouin dynamic grating reflection spectrum measurement device based on spectral demodulation shown in Figure 1, and includes the following steps: Step 1: Adjust the offset frequency of the single-sideband modulator 4 to be equal to the Brillouin frequency shift, so that the frequency of the output second pump light decreases the Brillouin frequency shift; adjust the polarization angle of the first pump light through the second polarizer 10 so that the intensity of the first pump light is maximized after passing through the polarization beam splitter 11. At this time, it can be ensured that the polarization direction of the first pump light is along the first polarization axis (p-polarization or s-polarization) of the polarization beam splitter 11. Step 1: The polarization angle of the second pump light is adjusted by the first polarizer 7 so that the second pump light is incident along the corresponding axis of the polarization-maintaining fiber 8; at the same time, the angle of the third polarizer 14 is adjusted so that the intensity of the probe light is maximized after passing through the polarization beam splitter 11. At this time, it can be ensured that the polarization direction of the probe light is along the second polarization axis (s polarization or p polarization) of the polarization beam splitter 11; Step 2: The probe light is frequency swept, and the reflected light spectrum at different probe light frequencies is collected by the spectrometer 16; Step 3: The peak values ​​of the reflected light spectra collected at different probe light frequencies are extracted, and smooth fitting and normalization operations are performed to form the peak-frequency curve, which is the reflection spectrum.

[0043] Furthermore, the Brillouin dynamic grating reflection spectrum measurement method based on spectral demodulation in this embodiment also includes the following steps: Step 4: Extract the full width at half maximum (FWHM) of the peak-frequency curve to obtain the reflection spectrum width of the Brillouin dynamic grating; calculate the length of the Brillouin dynamic grating based on the reflection spectrum width, using the formula (1) above.

[0044] A chaotic light source with a center wavelength of 1550 nm, a linewidth of 0.075 nm, and a power of 3 dBm was used as the pump source 1 to generate a Brillouin dynamic grating. A panda-type polarization-maintaining fiber was used as the polarization-maintaining fiber 8 for the experiment, with an average refractive index of 1.45 in the fiber core. The complete process of acquiring the reflection spectrum of the chaotic BDG was carried out through the above steps. The measurement results of the reflection spectrum of the chaotic Brillouin dynamic grating based on spectral demodulation are shown in Figure 2. By extracting the full width at half maximum (FWHM) of the fitted curve, the reflection spectral width of the BDG was obtained as 4.66 GHz. Substituting this into formula (1), the length of the BDG was calculated to be 2.22 cm.

[0045] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A Brillouin dynamic grating reflectance spectrum measurement device based on spectral demodulation, characterized in that, The system includes a pump source (1), an optical fiber coupler (2), a single-sideband modulator (4), a first polarizer (7), a polarization-maintaining fiber (8), a second polarizer (10), a polarization beam splitter (11), a tunable laser (12), a third polarizer (14), an optical circulator (15), and a spectrometer (16). The output port of the pump source (1) is connected to the input port of the optical fiber coupler (2), the first output port of the optical fiber coupler (2) is connected to the input port of the single-sideband modulator (4), the output port of the single-sideband modulator (4) is connected to the input port of the first polarizer (7), and the output port of the first polarizer (7) is connected to one end of the polarization-maintaining fiber (8). The second output port of the fiber coupler (2) is connected to the input port of the second polarizer (10), and the output port of the second polarizer (10) is connected to the first input port of the polarization beam splitter (11); the output port of the tunable laser (12) is connected to the input port of the third polarizer (14), the output port of the third polarizer (14) is connected to the input port of the optical circulator (15), the output port of the optical circulator (15) is connected to the second input port of the polarization beam splitter (11), and the beam combining output port of the polarization beam splitter (11) is connected to the other end of the polarization-maintaining fiber (8); the reflection port of the optical circulator (15) is connected to the input port of the spectrometer (16).

2. The Brillouin dynamic grating reflectance spectrum measurement device based on spectral demodulation according to claim 1, characterized in that, The first output port and the second output port of the fiber coupler (2) are used to output the second pump light and the first pump light, respectively. The single-sideband modulator (4) is used to perform low-frequency frequency shift on the second pump beam so that the frequency difference between the first pump light and the second pump light is equal to the Brillouin frequency shift, and then they meet in the polarization-maintaining fiber (8) to form a Brillouin dynamic grating. The tunable laser (12) is used to output the probe light whose frequency difference with the first pump light is equal to the positive and negative birefringence frequency shift. The second polarizer (10) is used to adjust the polarization direction of the first pump light so that it is along the first polarization axis of the polarization beam splitter (11). The first polarizer (7) is used to adjust the polarization direction of the second pump light so that it is the same as the polarization direction of the first pump light in the polarization-maintaining fiber (8). The third polarizer (14) is used to adjust the polarization direction of the probe light so that it is along the second polarization axis of the polarization beam splitter (11).

3. The Brillouin dynamic grating reflectance spectrum measurement device based on spectral demodulation according to claim 2, characterized in that, The spectrometer (16) is used to collect the reflected light spectrum formed after the probe light is reflected by the Brillouin dynamic grating.

4. The Brillouin dynamic grating reflectance spectrum measurement device based on spectral demodulation according to claim 3, characterized in that, It also includes a computer (17), the output port of the spectrometer (16) is connected to the input port of the computer (17), the computer (17) is used to extract the peak values ​​of the reflected light spectrum collected by the spectrometer (16) at different probe light frequencies, and perform smooth fitting and normalization operations to obtain the reflection spectrum.

5. The Brillouin dynamic grating reflectance spectrum measurement device based on spectral demodulation according to claim 4, characterized in that, The computer (17) is also used to calculate the length of the Brillouin dynamic grating based on the spectral width of the reflection spectrum, using the following formula: Where c represents the speed of light, n represents the refractive index of the optical fiber, and L... c Δf represents the length of the Brillouin dynamic grating, and Δf represents the spectral width of the reflection spectrum.

6. The Brillouin dynamic grating reflectance spectrum measurement device based on spectral demodulation according to claim 1, characterized in that, It also includes a first erbium-doped fiber amplifier (3), a second erbium-doped fiber amplifier (6), a third erbium-doped fiber amplifier (9), and a fourth erbium-doped fiber amplifier (13). The first erbium-doped fiber amplifier (3) is located between the first output port of the fiber coupler (2) and the input port of the single-sideband modulator (4). The second erbium-doped fiber amplifier (6) is located between the output port of the single-sideband modulator (4) and the input port of the first polarizer (7). The first erbium-doped fiber amplifier (3), the second erbium-doped fiber amplifier (6), the third erbium-doped fiber amplifier (9), and the fourth erbium-doped fiber amplifier (13) are all located between the first output port of the fiber coupler (2) and the input port of the single-sideband modulator (4). 6) The first erbium-doped fiber amplifier (9) is used to amplify the light intensity before entering the single-sideband modulator (4) and the light intensity output by the single-sideband modulator (4), respectively; the second erbium-doped fiber amplifier (9) is set between the second output port of the fiber coupler (2) and the input port of the second polarizer (10) to amplify the light intensity entering the second polarizer (10); the fourth erbium-doped fiber amplifier (13) is set between the output port of the tunable laser (12) and the input port of the third polarizer (14) to amplify the light intensity entering the third polarizer (14).

7. The Brillouin dynamic grating reflectance spectrum measurement device based on spectral demodulation according to claim 1, characterized in that, It also includes a microwave signal generator (5), which is electrically connected to the driving end of the single-sideband modulator (4) and is used to output a microwave signal to drive the single-sideband modulator (4) to output the low-frequency sideband of the second pump beam.

8. The Brillouin dynamic grating reflectance spectrum measurement device based on spectral demodulation according to claim 1, characterized in that, The pump light source (1) is a continuous light source, a pulsed light source, a chaotic light source, or an ASE light source.

9. A method for measuring the reflectance spectrum of a Brillouin dynamic grating based on spectral demodulation, implemented based on the Brillouin dynamic grating reflectance spectrum measurement device based on spectral demodulation as described in claim 1, characterized in that, Includes the following steps: Step 1: Adjust the offset frequency of the single-sideband modulator (4) to equal the Brillouin frequency shift, so that the frequency of the output second pump light decreases the Brillouin frequency shift; adjust the polarization angle of the first pump light through the second polarizer (10) so that the intensity of the first pump light is maximized after passing through the polarization beam splitter (11); adjust the polarization angle of the second pump light through the first polarizer (7) so that the second pump light is incident along the corresponding axis of the polarization-maintaining fiber (8); at the same time, adjust the angle of the third polarizer (14) so ​​that the intensity of the probe light is maximized after passing through the polarization beam splitter (11); Step 2: Sweep the probe light frequency and collect the reflected light spectrum at different probe light frequencies through the spectrometer (16); Step 3: Extract the peak values ​​of the reflected light spectra collected at different probe light frequencies, perform smooth fitting and normalization operations to form the peak-frequency curve, which is the reflection spectrum.

10. The Brillouin dynamic grating reflectance spectrum measurement method based on spectral demodulation according to claim 9, characterized in that, The following steps are also included: Step 4: Extract the full width at half maximum (FWHM) of the peak-frequency curve to obtain the reflection spectral width of the Brillouin dynamic grating; calculate the length of the Brillouin dynamic grating based on the reflection spectral width, using the following formula: Where c represents the speed of light, n represents the refractive index of the optical fiber, and L... c Δf represents the length of the Brillouin dynamic grating, and Δf represents the spectral width of the reflection spectrum.

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