Liquid refractive index measurement device based on chaotic Brillouin dynamic grating

Abstract

The present invention belongs to the field of fiber optic chemical sensing technology, specifically a liquid refractive index measurement device based on chaotic Brillouin dynamic gratings. This device solves the problem of existing inability to perform distributed liquid refractive index analysis and the requirement that traditional fiber gratings (FBGs) must be permanently inscribed in the optical fiber for chemical refractive index analysis. The device comprises a chaotic light source, a first optical splitter, an electro-optic modulator, a first optical amplifier, a tunable optical filter, a first circulator, a sensing optical fiber, a variable optical delay line, a second optical amplifier, a light combiner, a tunable laser, an acousto-optic modulator, a third optical amplifier, a photodetector, and a bandpass filter. The sensing optical fiber is disposed in the liquid to be measured. Compared with traditional fiber grating refractive index measurement devices, the present invention is ready for use and can perform distributed measurement, overcoming the shortcomings of traditional fiber grating refractive index measurement devices, which require permanent grating inscription in the optical fiber and are almost entirely limited to point measurements.

Description

Technical Field

[0001] The invention belongs to the technical field of optical fiber chemical sensing, and in particular is a liquid refractive index measuring device based on chaotic Brillouin dynamic grating. Background Art

[0002] Fiber-optic chemical sensing, the analysis and identification of chemical liquid substances outside the boundaries of optical fibers, has been a major challenge facing the fiber-optic sensing community for decades. This is because standard single-mode optical fibers are designed to guide laser light within the core and minimize leakage, while the substances being tested are usually located outside the boundaries of the fiber cladding or even outside the protective coating.

[0003] The most widely used principle of fiber-optic chemical sensing relies on light propagating in cladding modes. The refractive index of the material outside the fiber is measured by analyzing these modes to identify the chemical species. The transverse profile of these modes is characterized by an evanescent tail that overlaps with the outer cladding medium. The primary difficulty in measuring the refractive index of liquids using cladding mode analysis lies in the coupling of light into these cladding modes. A common solution relies on fiber Bragg gratings (FBGs). Long-period fiber Bragg gratings (LPFBGs) can couple light between core and cladding modes (Chinese invention patent, CN 103175793A), while short-period fiber Bragg gratings (SPBGs) can similarly couple counter-propagating cladding modes, and the coupling efficiency can be improved by varying the grating tilt angle (Optics & Laser Technology, 2016, 78:19-33). However, both approaches suffer from a fundamental drawback: they require permanent inscription in the fiber. Consequently, liquid refractive index measurement devices based on cladding mode analysis are almost entirely limited to point measurements, and their extension to spatial distribution analysis remains difficult.

[0004] Based on this, it is necessary to invent a new distributed liquid refractive index measurement device to solve the problem that the existing distributed liquid refractive index measurement cannot be performed and the traditional grating must be permanently engraved in the optical fiber when performing liquid refractive index measurement. Summary of the Invention

[0005] In order to solve the problem that the existing technology cannot perform distributed liquid refractive index measurement and that traditional gratings must be permanently engraved in the optical fiber when performing liquid refractive index measurement, the present invention proposes a liquid refractive index measurement device based on chaotic Brillouin dynamic grating to achieve distributed measurement of liquid refractive index.

[0006] In order to solve the above technical problems, the technical solution adopted by the present invention is as follows: a liquid refractive index measuring device based on chaotic Brillouin dynamic grating, comprising: a chaotic light source, a first spectrometer, an electro-optical modulator, a first optical amplifier, a tunable optical filter, a first circulator, a sensing optical fiber, a variable optical delay line, a second optical amplifier, a light combiner, a tunable laser, an acousto-optic modulator, a third optical amplifier, a photodetector, and a bandpass filter; the sensing optical fiber is arranged in the liquid to be measured;

[0007] The chaotic laser emitted by the chaotic light source is divided into two beams of pump light after passing through the first optical splitter. The first pump light is frequency-shifted by the electro-optical modulator, amplified by the first optical amplifier, filtered by the optical filter, and enters one end of the sensing optical fiber through the first circulator; the second pump light is incident on the light combiner after passing through the variable optical delay line and the second optical amplifier;

[0008] The swept-frequency laser output by the tunable laser is modulated into a pulsed probe light by an acousto-optic modulator. This is then amplified by a third optical amplifier and incident on a light combiner. From there, the light combiner and another pump beam enter the other end of the sensing fiber. The two pump beams interfere at the point where they meet in the sensing fiber, forming a chaotic Brillouin dynamic grating.

[0009] The pulsed detection light output from one end of the sensing optical fiber passes through a first circulator and a bandpass filter and is detected by a photodetector. The photodetector is used to detect the light intensity signal of the detection light, and the light intensity signal is used to calculate the refractive index of the liquid outside the sensing optical fiber.

[0010] The liquid refractive index measuring device based on chaotic Brillouin dynamic grating also includes a computer, a lock-in amplifier,

[0011] The lock-in amplifier is connected to the output end of the photodetector, and the computer is connected to the lock-in amplifier; the computer is used to control the acquisition time interval of the lock-in amplifier, and the lock-in amplifier is used to collect the transmitted light intensity under different detection light frequencies and send it to the computer. The computer processes the sampling signal to obtain the frequency difference between the detection light and the second pump light under phase matching conditions, and calculates the refractive index of the liquid outside the sensing optical fiber based on the frequency difference signal.

[0012] The refractive index of a liquid is calculated as:

[0013] Calculate the effective refractive index of the sensing optical fiber cladding using the following formula:

[0014] ;

[0015] in, It represents the frequency difference between the probe light and the second pump light under phase matching conditions when the probe light is coupled to the m-order cladding mode. is the effective refractive index of the cladding in the m-order cladding mode, is the effective refractive index of the fiber core, is the frequency of the second pump light;

[0016] Then according to the cladding refractive index The refractive index of the liquid material outside the cladding is calculated.

[0017] The chaotic light source includes a distributed feedback laser, a second circulator, a second beam splitter, a polarization controller, and a variable optical attenuator. The laser output by the distributed feedback laser is output to the second beam splitter after passing through the second circulator, and is split into two beams by the second beam splitter. One beam is fed back to the laser cavity after passing through the variable optical attenuator, the polarization controller, and the second circulator, so that the other beam outputs chaotic laser.

[0018] The liquid refractive index measuring device based on chaotic Brillouin dynamic grating further includes an optical isolator, which is arranged at the input end of the first spectrometer.

[0019] The sensing optical fiber is a single-mode optical fiber, the outer coating layer of which is wiped off and the cladding is in direct contact with the substance to be measured.

[0020] The liquid refractive index measuring device based on chaotic Brillouin dynamic grating further includes a microwave signal source and a waveform generator, wherein the microwave signal source is used to drive the electro-optical modulator to modulate the first pump light beam into a double-sideband optical signal having a frequency difference from the center frequency equal to the Brillouin frequency shift;

[0021] The waveform generator is used to send a pulse signal to drive the acousto-optic modulator, so that it modulates the laser output by the tunable laser into pulse detection light; the pulse reference signal sent by the waveform generator is connected to the lock-in amplifier.

[0022] The central wavelength of the chaotic light source is 1550 nm, and the modulation frequency of the electro-optical modulator is equal to the frequency of the acoustic wave in the sensing optical fiber.

[0023] The first optical splitter is a 1×2 optical fiber coupler, and the optical combiner is a 2×1 optical fiber coupler.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] 1. This invention proposes a liquid refractive index measurement device based on chaotic Brillouin dynamic grating (DBG). This device utilizes two opposing chaotic laser beams with different Brillouin frequency shifts to generate a Brillouin dynamic grating (DBG) in a single-mode optical fiber. Furthermore, the device utilizes the property of the Brillouin dynamic grating (DBG) to scatter part of the probe light into counter-propagating cladding modes. The device determines the refractive index of the liquid outside the cladding through changes in the local coupling spectrum, thereby providing a reference for identifying the type of liquid immersed outside the cladding. Because the generation of the Brillouin dynamic grating (DBG) is based on stimulated Brillouin scattering of two pump beams, it is ready for immediate use, overcoming the drawback of traditional fiber Bragg grating (FBG) liquid refractive index measurement devices, which require the grating to be permanently inscribed in the optical fiber.

[0026] 2. Compared with the traditional fiber Bragg grating liquid refractive index measurement device which is almost completely limited to point measurement, the modulation of the two pump beams in the present invention can confine the chaotic Brillouin dynamic grating to a discrete and narrow fiber cross-section, and by adjusting the variable optical delay line to scan their position along the fiber, distributed liquid refractive index measurement can be achieved, overcoming the disadvantage that the traditional fiber Bragg grating liquid refractive index measurement device is almost completely limited to point measurement.

[0027] 3. The liquid refractive index measurement device provided by this invention uses a chaotic laser as its light source. The "thumbnail" autocorrelation characteristics of chaotic lasers enable them to have more concentrated energy than conventional lasers, resulting in stronger modulation of the optical fiber's refractive index and more pronounced changes in the coupled spectrum of local cladding reflection light, leading to higher refractive index measurement accuracy. Furthermore, the spatial resolution of liquid refractive index measurements based on chaotic Brillouin dynamic gratings is determined solely by the coherence length of the chaotic laser signal, achieving centimeter-level spatial resolution. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 A schematic structural diagram of a liquid refractive index measuring device based on chaotic Brillouin dynamic grating provided in the first embodiment of the present invention;

[0029] Figure 2 Schematic diagram of the frequency offset between the probe light and the second pump light as the refractive index of the liquid outside the cladding changes;

[0030] Figure 3 A schematic structural diagram of a liquid refractive index measuring device based on chaotic Brillouin dynamic grating provided in the second embodiment of the present invention;

[0031] In the figure: 1-distributed feedback laser, 2-second circulator, 3-second optical splitter, 4-polarization controller, 5-variable optical attenuator, 6-optical isolator, 7-first optical splitter, 8-microwave signal source, 9-electro-optic modulator, 10-first optical amplifier, 11-tunable optical filter, 12-first circulator, 13-sensing fiber, 14-variable optical delay line, 15-second optical amplifier, 16-optical combiner, 17-tunable laser, 18-waveform generator, 19-acousto-optic modulator, 20-third optical amplifier, 21-computer, 22-lock-in amplifier, 23-photodetector, 24-bandpass filter. DETAILED DESCRIPTION

[0032] In order to make the purpose, 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 part of the embodiments of the present invention, not all the embodiments; based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0033] Example 1

[0034] like Figure 1 As shown, embodiment 1 of the present invention provides a liquid refractive index measuring device based on chaotic Brillouin dynamic grating, comprising: a chaotic light source, a first spectrometer 7, an electro-optical modulator 9, a first optical amplifier 10, a tunable optical filter 11, a first circulator 12, a sensing fiber 13, a variable optical delay line 14, a second optical amplifier 15, a light combiner 16, a tunable laser 17, an acousto-optic modulator 19, a third optical amplifier 20, a computer 21, a phase-locked amplifier 22, a photodetector 23, and a bandpass filter 24; the sensing fiber 13 is arranged in the liquid to be measured.

[0035] The chaotic laser emitted by the chaotic light source is divided into two beams of pump light after passing through the first optical splitter 7. The first pump light is frequency-shifted by the electro-optical modulator 9, amplified by the first optical amplifier 10, and filtered by the optical filter 11, and enters one end of the sensing optical fiber 13 through the first circulator 12; the second pump light passes through the variable optical delay line 14 and the second optical amplifier 15 and is incident on the light combiner 16.

[0036] The swept-frequency laser output by the tunable laser 17 is modulated into pulsed probe light by the acousto-optic modulator 19. It is then amplified by the third optical amplifier 20 and incident on the light combiner 16. After passing through the light combiner 16, it joins another pump beam and enters the other end of the sensing fiber 13. The two pump beams interfere at the point where they meet in the sensing fiber 13, forming a chaotic Brillouin dynamic grating. The variable optical delay line 14 adjusts the optical path of the second pump beam to alter the position of the chaotic Brillouin dynamic grating, enabling distributed liquid refractive index measurement.

[0037] The pulsed detection light output from one end of the sensing optical fiber 13 passes through the first circulator 12 and the bandpass filter 24 and is detected by the photodetector 23. The photodetector 23 is used to detect the transmitted light intensity signal of the detection light, and the light intensity signal is used to calculate the refractive index of the liquid along the outside of the sensing optical fiber 13.

[0038] Further, if Figure 1 As shown, the liquid refractive index measurement device based on chaotic Brillouin dynamic grating of this embodiment also includes a computer 21 and a lock-in amplifier 22; the lock-in amplifier 22 is connected to the output end of the photodetector 23, and the lock-in amplifier 22 is also connected to the computer 21. The computer 21 is used to control the acquisition time interval of the lock-in amplifier 22. The lock-in amplifier 22 is used to collect the transmitted light intensity at different detection light frequencies and send it to the computer 21. The computer 21 performs data processing based on the sampling signal to obtain the frequency difference between the detection light and the second pump light under phase matching conditions. Based on the frequency difference signal, the computer 21 calculates the refractive index of the liquid outside the sensing fiber 13. Specifically, the computer 21 performs data processing on the sampling signal by performing a Lorentz fitting on the sampling signal. The frequency difference corresponding to the peak value is the frequency difference between the detection light and the second pump light under phase matching conditions.

[0039] Further, if Figure 1 As shown, a liquid refractive index measuring device based on a chaotic Brillouin dynamic grating in this embodiment also includes a microwave signal source 8 and a waveform generator 18. The microwave signal source 8 is used to drive the electro-optical modulator 9 to modulate the first beam of pump light into a double-sideband optical signal with a frequency difference from the center frequency being the Brillouin frequency shift; the waveform generator 18 is used to emit a pulse signal to drive the acousto-optic modulator 19 to modulate the laser output by the tunable laser 17 into a pulse detection light; the pulse reference signal emitted by the waveform generator 18 is connected to the phase-locked amplifier 22.

[0040] In this embodiment, the waveform generator 18 is an arbitrary waveform generator, whose external trigger output terminal is connected to the external trigger input terminal of the phase-locked amplifier 22, and the output terminal of the phase-locked amplifier 22 is connected to the computer 21. In addition, the sampling frequency of the phase-locked amplifier is consistent with the sweep frequency of the tunable laser 17.

[0041] Specifically, in this embodiment, the central wavelength of the chaotic light source is 1550 nm. The modulation frequency of the electro-optic modulator 9 is equal to the acoustic frequency within the sensing fiber 13, close to the fiber's Brillouin frequency shift. The first optical splitter 7 is a 1×2 fiber coupler with a coupling ratio of 90:10, where 90% of the light output from the output port enters the electro-optic modulator 9 as the first pump light. The optical combiner 16 is a 2×1 fiber coupler.

[0042] In this embodiment, the chaotic laser light emitted by the chaotic light source is split into two pump light channels by a first optical splitter 7. The first chaotic pump light channel passes through an electro-optical modulator 9 and is modulated by the signal output by a microwave signal source 8 into a double-sideband optical signal with a frequency difference from the center frequency equal to the Brillouin frequency shift. After being amplified by a first optical amplifier 10, the upper sideband is filtered out by a tunable optical filter 11 and then input into a sensing fiber 13 via a first circulator 12. The second pump light channel passes through a variable optical delay line 14, a second optical amplifier 15, and an optical combiner 16 before being input into the other end of the sensing fiber 13. The two chaotic pump light channels interfere with each other at the point where they meet in the sensing fiber 13, causing the sensing fiber 13 to generate an acoustic wave field under electrostriction. This acoustic wave field periodically modulates the refractive index of the sensing fiber 13, forming a chaotic Brillouin dynamic grating.

[0043] The center frequencies of the two chaotic pump lights satisfy the following equation:

[0044] ; (1)

[0045] ω pump1 is the center frequency of the first pump light; ω pump2 is the center frequency of the second pump light; Ω is the acoustic frequency of the sensing fiber, which is close to the Brillouin frequency shift of the fiber. Therefore, in this embodiment, the modulation frequency of the electro-optical modulator 9 is equal to the acoustic frequency in the sensing fiber 13.

[0046] In this embodiment, the pulsed probe light is amplified by the third optical amplifier 20 and then input into the sensing fiber 13 via the optical combiner 16. Under the coupling effect of the chaotic Brillouin dynamic grating, the probe light in the core mode encounters the chaotic Brillouin dynamic grating, generating a partial reflection signal and entering the cladding mode.

[0047] The effective refractive index of the sensing fiber's cladding mode changes with the refractive index of the material outside the cladding, causing the frequency offset between the probe light and the second pump light to change. The reflection spectrum contains information about the frequency offset between the probe light and the second pump light. Therefore, measuring the reflection spectrum can indirectly measure the refractive index of the material outside the fiber cladding to determine the chemical composition of the external liquid. However, directly observing the reflected light in the cladding mode is difficult. Therefore, monitoring the transmission loss of the probe light in the core mode is used instead of directly measuring the reflected light. Stronger reflected light coupled to the cladding mode means weaker transmitted light in the core mode. Therefore, monitoring the transmission spectrum of the probe light can be used instead of directly measuring the reflection spectrum. In this embodiment, the frequency of the probe light is scanned, and changes in the core light intensity are detected using a photodetector 23 and a lock-in amplifier 22. Because the reflected light intensity varies at different probe light frequencies, a spectrum of intensity changes at different frequency offsets between the probe light and the second pump light can be obtained. The frequency shift corresponding to the peak value is the frequency offset that achieves optimal phase matching. This frequency offset changes with changes in the refractive index of the external liquid.

[0048] Therefore, in this embodiment, the frequency of the probe light is in a sweep mode. By sweeping the frequency of the probe light, when the detected light intensity changes the most, it indicates that the optimal phase matching has been achieved. At this time, the frequency difference between the probe light and the second pump light satisfies the following conditions:

[0049] ; (2)

[0050] in, It represents the frequency difference between the probe light and the second pump light under phase matching conditions when the probe light is coupled to the m-order cladding mode. is the frequency of the probe light coupled into the m-order cladding, is the effective refractive index of the cladding in the m-order cladding mode, is the effective refractive index of the fiber core, is the frequency of the second pump light. Therefore, when the frequency difference under the best phase matching is measured When , the effective refractive index of the cladding can be calculated by formula (2) Then, using the existing refractive index calculation method, the effective refractive index of the cladding can be The refractive index of the liquid material outside the cladding is calculated. The selection of the coupling order m between the probe light and the cladding mode can be determined through numerical simulation or specific measurement.

[0051] Specifically, in this embodiment, the sensing optical fiber 13 is a single-mode optical fiber, the outer coating layer of which is wiped off, and the cladding is in direct contact with the substance to be measured, so as to achieve better results.

[0052] In addition, if Figure 2FIG. 1 is a schematic diagram showing how the frequency offset between the probe light and the second pump light changes with the refractive index of the liquid outside the cladding. Therefore, in this embodiment, the relationship between the frequency offset and the refractive index of the liquid outside the cladding can also be calibrated in advance. Then, during actual measurement, the refractive index of the liquid outside the cladding can be directly obtained based on the frequency offset.

[0053] Example 2

[0054] like Figure 3 As shown, embodiment 2 of the present invention is a liquid refractive index measuring device based on chaotic Brillouin dynamic grating. Similar to embodiment 1, it includes a chaotic light source, a first spectrometer 7, an electro-optical modulator 9, a first optical amplifier 10, a tunable optical filter 11, a first circulator 12, a sensing fiber 13, a variable optical delay line 14, a second optical amplifier 15, a light combiner 16, a tunable laser 17, an acousto-optic modulator 19, a third optical amplifier 20, a computer 21, a phase-locked amplifier 22, a photodetector 23, and a bandpass filter 24; the sensing fiber 13 is arranged in the liquid to be measured.

[0055] Different from Example 1, in this embodiment, the chaotic light source is obtained by optical feedback of a distributed feedback laser 1. Specifically, the chaotic light source includes a distributed feedback laser 1, a second circulator 2, a second beam splitter 3, a polarization controller 4, and a variable optical attenuator 5. The laser output by the distributed feedback laser 1 is output to the second beam splitter 3 after passing through the second circulator 2, and is divided into two beams by the second beam splitter 3. One beam is fed back to the laser cavity after passing through the variable optical attenuator 5, the polarization controller 4, and the second circulator 2, so that the other beam outputs a chaotic laser.

[0056] Furthermore, in this embodiment, an optical isolator 6 is further included, and the optical isolator 6 is provided at the input end of the first optical splitter 7. The second optical splitter 3 is specifically a 1×2 optical fiber coupler with a coupling ratio of 50:50.

[0057] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements 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 liquid refractive index measuring device based on chaotic Brillouin dynamic grating, characterized in that: include: A chaotic light source, a first optical splitter (7), an electro-optical modulator (9), a first optical amplifier (10), a tunable optical filter (11), a first circulator (12), a sensing optical fiber (13), a variable optical delay line (14), a second optical amplifier (15), a light combiner (16), a tunable laser (17), an acousto-optic modulator (19), a third optical amplifier (20), a photodetector (23), and a bandpass filter (24); the sensing optical fiber (13) is arranged in a liquid to be measured; The chaotic laser light emitted by the chaotic light source is divided into two beams of pump light after passing through a first optical splitter (7), wherein the first pump light is frequency-shifted by an electro-optical modulator (9), amplified by a first optical amplifier (10), filtered by an optical filter (11), and enters one end of a sensing optical fiber (13) through a first circulator (12); the second pump light is incident on a light combiner (16) after passing through a variable optical delay line (14) and a second optical amplifier (15); The frequency-sweep laser output by the tunable laser (17) is modulated into a pulsed detection light by an acousto-optic modulator (19), amplified by a third optical amplifier (20), and then incident on a light combiner (16). The light combiner (16) and another pump light beam enter the other end of the sensing optical fiber (13); the two pump light beams interfere with each other at the point where they meet in the sensing optical fiber (13), forming a chaotic Brillouin dynamic grating. The pulsed detection light output from one end of the sensing optical fiber (13) passes through a first circulator (12) and a bandpass filter (24) and is then detected by a photodetector (23). The photodetector (23) is used to detect a transmitted light intensity signal of the detection light. The light intensity signal is used to calculate the refractive index of the liquid outside the sensing optical fiber (13).

2. The liquid refractive index measuring device based on chaotic Brillouin dynamic grating according to claim 1, characterized in that: It also includes a computer (21), a lock-in amplifier (22), The lock-in amplifier (22) is connected to the output end of the photodetector (23), and the computer (21) is connected to the lock-in amplifier (22); the computer (21) is used to control the acquisition time interval of the lock-in amplifier (22); the lock-in amplifier (22) is used to collect the reflected light intensity under different detection light frequencies and send it to the computer (21); the computer (21) performs data processing on the sampling signal to obtain the frequency difference between the detection light and the second pump light under the phase matching condition, and calculates the refractive index of the liquid outside the sensing optical fiber (13) according to the frequency difference signal.

3. The liquid refractive index measuring device based on chaotic Brillouin dynamic grating according to claim 1, characterized in that: The refractive index of a liquid is calculated as: Calculate the effective refractive index of the sensing optical fiber cladding using the following formula: Among them, Δω (m) It represents the frequency difference between the probe light and the second pump light under phase matching conditions when the probe light is coupled to the m-order cladding mode. is the effective refractive index of the cladding in the m-order cladding mode, n core is the effective refractive index of the fiber core, ω pump2 is the frequency of the second pump light; Then according to the cladding refractive index The refractive index of the liquid material outside the cladding is calculated.

4. The liquid refractive index measuring device based on chaotic Brillouin dynamic grating according to claim 1, characterized in that: The chaotic light source comprises a distributed feedback laser (1), a second circulator (2), a second optical splitter (3), a polarization controller (4), and a variable optical attenuator (5). The laser light output by the distributed feedback laser (1) passes through the second circulator (2) and is output to the second optical splitter (3). The laser light is split into two beams by the second optical splitter (3), one of which passes through the variable optical attenuator (5), the polarization controller (4), and the second circulator (2) and is fed back to the laser cavity of the distributed feedback laser (1), so that the other beam outputs chaotic laser light.

5. The liquid refractive index measuring device based on chaotic Brillouin dynamic grating according to claim 4, characterized in that: It also includes an optical isolator (6), which is arranged at the input end of the first optical splitter (7).

6. The liquid refractive index measuring device based on chaotic Brillouin dynamic grating according to claim 1, characterized in that: The sensing optical fiber (13) is a single-mode optical fiber, the outer coating of which is wiped off and the cladding is in direct contact with the substance to be measured.

7. The liquid refractive index measuring device based on chaotic Brillouin dynamic grating according to claim 2, characterized in that: The device also includes a microwave signal source (8) and a waveform generator (18), wherein the microwave signal source (8) is used to drive the electro-optical modulator (9) so as to modulate the first pump light beam into a double-sideband optical signal having a frequency difference from the center frequency equal to the Brillouin frequency shift; The waveform generator (18) is used to send a pulse signal to drive the acousto-optic modulator (19), so that the acousto-optic modulator (19) modulates the laser light output by the tunable laser (17) into pulsed detection light; the pulse reference signal sent by the waveform generator (18) is connected to the lock-in amplifier (22).

8. The liquid refractive index measuring device based on chaotic Brillouin dynamic grating according to claim 1, characterized in that: The central wavelength of the chaotic light source is 1550 nm, and the modulation frequency of the electro-optical modulator (9) is equal to the frequency of the sound waves in the sensing optical fiber (13).

9. The liquid refractive index measuring device based on chaotic Brillouin dynamic grating according to claim 1, characterized in that: The first optical splitter (7) is a 1×2 optical fiber coupler, and the optical combiner (16) is a 2×1 optical fiber coupler.

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