CN107782696B - Sensing system and method for measuring the refractive index of distributed liquid using a taper fiber - Google Patents

Abstract

A sensing system and a method for measuring a distributed liquid refractive index by using a tapered optical fiber relate to the technical field of optical fiber sensing, refractive index sensing is carried out based on back Rayleigh scattering intermode interference generated in a tapered area of the tapered optical fiber, and the effective mode refractive index of back Rayleigh scattering propagation in the tapered optical fiber is changed by sensing the change of an external refractive index through stronger evanescent waves generated in the tapered area; and placing the tapered optical fiber in liquid, detecting the wavelength shift of Rayleigh scattering spectrum of the tapered optical fiber through a distributed optical fiber sensing device, analyzing the wavelength shift, and obtaining the continuously distributed refractive index distribution condition of the liquid. The refractive index measurement of the distributed liquid with high spatial resolution reaching millimeter level is realized.

Description

Sensing system and method for measuring the refractive index of distributed liquid using a taper fiber

technical field

The invention relates to the technical field of optical fiber sensing, in particular to a sensing system and method for measuring the refractive index of distributed liquid by using a tapered optical fiber, which is applied to optical frequency domain reflection.

Background technique

The refractive index of a substance is an important physical quantity that reflects the internal information of a substance. The measurement of the refractive index of substances has a wide range of applications in basic research, chemical analysis, environmental pollution assessment, medical diagnosis, and the food industry.

Optical fiber refractive index sensors have the characteristics of intrinsic insulation, anti-electromagnetic interference, high sensitivity, high precision, high integration, high bandwidth, and reusability, which have become the research hotspot of refractive index sensors. Traditional fiber optic refractive index sensors include fiber Bragg gratings, long period gratings, microbend fibers, photonic crystal fibers, fiber surface plasmon resonance, F-P sensors, online MZ sensors, microspheres, microring oscillators, and single-mode multi-mode fiber tandem etc. structure.

The above-mentioned traditional optical fiber refractive index sensors generally use broadband light source and spectrometer demodulation or tunable laser scanning detection. Using transmitted light method, only single-point sensing can be performed, which is a discrete sensing, and cannot accurately measure a certain length. Refractive index distribution.

At present, there is an urgent need for an optical fiber refractive index sensor that can realize distributed refractive index sensing along the optical fiber. Any point on a certain length of the optical fiber is a sensitive point to realize multi-point continuous sensing.

SUMMARY OF THE INVENTION

The invention provides a sensing system and method for measuring the refractive index of distributed liquid by using a tapered optical fiber. The invention realizes the measurement of the refractive index of distributed liquid with high spatial resolution reaching mm (millimeter) level, and can be successfully applied to dense For distributed measurement of liquid refractive index, monitoring liquid diffusion, and precise positioning of liquid layers, please refer to the following descriptions:

A sensing system for measuring the refractive index of distributed liquid by using a tapered optical fiber, the sensing system comprises: a distributed optical fiber sensing device of optical frequency domain reflection, and the sensing system is used to achieve high spatial resolution up to millimeters Distributed Refractive Index Measurement of Levels of Liquids;

The sensing system includes: a tapered optical fiber,

Based on the interference between the back Rayleigh scattering modes in the tapered region of the tapered fiber, the refractive index sensing is performed, and the change of the external refractive index is sensed by the strong evanescent wave generated in the tapered region, and the change of the tapered fiber is changed. The effective mode index of back-Rayleigh scattering propagation in the tapered fiber;

The tapered fiber is placed in the liquid, and the distributed optical fiber sensing device detects the wavelength shift of the Rayleigh scattering spectrum of the tapered fiber, analyzes the wavelength shift, and obtains the continuously distributed refractive index of the liquid distribution.

The tapered optical fiber is drawn by a thin-diameter optical fiber on a tapered machine.

During the taper drawing, the drawing speed was 200 μm/s, the reciprocating distance of the hydrogen-oxygen flame was 2000 μm, the reciprocating speed was 360 μm/s, the fiber elongation was 100000 μm, and the diameter of the cone region was 4 μm.

A sensing method for measuring the refractive index of a distributed liquid by using a tapered optical fiber, the sensing method comprises the following steps:

In the main interferometer, the back-rayleigh scattering of the taper fiber is used to form a beat-frequency interference signal, and the beat-frequency interference signal is respectively subjected to fast Fourier transform to convert the optical frequency domain information to the distance corresponding to each position in the taper fiber. Domain information, for the distance domain information, each position of the taper fiber is sequentially selected through a moving window of a certain width to form the local distance domain information;

Both the reference signal and the measurement signal use the moving window to select the local distance field information of the tapered fiber, and fill the local distance field information with zeros. The domain information uses the complex inverse Fourier transform, and then converts it to the optical frequency domain to obtain the local optical frequency domain information of the reference signal and the measurement signal;

The cross-correlation operation is used to estimate the spectral wavelength shift of the local optical frequency domain information of the reference signal and the measurement signal. The shift of the cross-correlation peak reflects the wavelength shift of the Rayleigh scattering spectrum. The wavelength shift of the Rayleigh scattering spectrum is proportional to the change of the refractive index of the liquid. The amount of change in the refractive index of the liquid is reflected by the amount of cross-correlation peak shift.

The beneficial effects of the technical scheme provided by the present invention are:

1. The present invention can be successfully applied to dense distributed measurement of liquid refractive index, monitoring of liquid diffusion, and precise positioning of liquid layers;

2. Realize distributed liquid refractive index measurement with high spatial resolution of 5mm and sensitivity of 68.52nm/RIU;

3. It is verified by experiments that the maximum measurement error of temperature change is 0.0002RI, which verifies the effectiveness of the present invention.

Description of drawings

1 is a schematic structural diagram of a sensing system for measuring the refractive index of a distributed liquid by utilizing a taper fiber;

FIG. 2 is another schematic structural diagram of a sensing system for measuring the refractive index of a distributed liquid by utilizing a tapered optical fiber;

3 is a flow chart of a sensing method for measuring the refractive index of a distributed liquid by utilizing a taper optical fiber;

Fig. 4 is the schematic diagram of calibration curve;

FIG. 5 is a schematic diagram of an example of detection results.

In the accompanying drawings, the list of components represented by each number is as follows:

A: Distributed optical fiber sensing device of optical frequency domain reflection;

1: Tunable laser; 2: Detector;

3: 50:50 beam splitter; 4: 1:99 beam splitter;

5: 50:50 coupler; 6: clock shaping circuit module;

7: Delay fiber; 8: First Faraday mirror;

9: Second Faraday mirror; 10: Isolator;

11: Computer; 12: Polarization Controller;

13: Circulator; 14 50:50 Coupler;

15: Tapered fiber; 16: Polarizing beam splitter;

17: Polarizing beam splitter; 18: Balance detector;

19: balance detector; 20: acquisition device;

21: GPIB control module; 22: Reference arm;

23: Test arm; 24: Clock trigger device based on auxiliary interferometer;

25: main interferometer; 26: cladding of tapered fiber 15;

27: The core of the tapered optical fiber 15.

Detailed ways

In order to make the objectives, technical solutions and advantages of the present invention clearer, the embodiments of the present invention are further described in detail below.

The traditional refractive index sensing based on tapered fiber is single-point sensing, and the tapered region of the tapered fiber can maintain a certain length, that is, a few centimeters to several meters. This provides the possibility for distributed optical fiber refractive index sensing. In addition, combining the optical frequency domain reflectometry technology with the tapered fiber, the optical frequency domain reflectometry demodulates the back Rayleigh scattering in the tapered region of the tapered fiber, and is expected to realize distributed refractive index sensing.

Example 1

In order to solve the technical problems such as the inability to achieve dense distributed measurement of liquid refractive index, monitor liquid diffusion, and precise positioning of liquid layers in the prior art, the embodiment of the present invention provides a sensing system for measuring distributed liquid refractive index by using a taper optical fiber , see Figure 1, and details are described below:

The design principle of the embodiment of the present invention is as follows: the refractive index sensing is performed based on the interference between the back Rayleigh scattering modes in the tapered region of the tapered fiber 15, which is a strong evanescent wave ( The specific strength is determined according to the actual experience value, which is not limited in this embodiment of the present invention) to sense the change of the external refractive index, thereby changing the effective mode refractive index of back Rayleigh scattering propagation in the taper fiber 15 .

Changes in the index of refraction in the effective mode of back-Rayleigh scattering cause changes in the interference retardation, manifested as wavelength shifts on the Rayleigh scattering spectrum. The wavelength shift on the Rayleigh scattering spectrum on the region of the tapered fiber 15 is detected by the distributed optical fiber sensing device A of optical frequency domain reflection. When the tapered optical fiber 15 is placed in the liquid, the continuous distributed refractive index distribution of the liquid is obtained by analyzing the wavelength shift of the distributed Rayleigh scattering spectrum due to the effect of the above process.

To sum up, the embodiment of the present invention realizes distributed liquid refractive index measurement with high spatial resolution reaching mm (millimeter) level, and can be successfully applied to dense distributed measurement of liquid refractive index, monitoring of liquid diffusion, and accurate liquid layer measurement. positioning, etc.

Example 2

The sensing system in Embodiment 1 will be further introduced below with reference to FIG. 1, and the details will be described below: the sensing system includes: a tapered optical fiber 15, and a distributed optical fiber sensing device for optical frequency domain reflection, wherein the pulling The tapered optical fiber 15 is drawn by a tapered machine with a small diameter. , including: cladding 26 and core 27 (both drawn to a tapered shape).

In the embodiment of the present invention, the drawing speed during the taper drawing process is 200 μm/s, the oxyhydrogen flame reciprocating distance is 2000 μm, the reciprocating speed is 360 μm/s, the fiber elongation is 100000 μm, and the diameter of the cone region is 4 μm.

The distributed optical fiber sensing device of optical frequency domain reflection includes: a tunable laser 1, a 1:99 optical beam splitter 4, a computer 11, a GPIB (General Purpose Interface Bus) control module 21, a clock trigger device 24 based on an auxiliary interferometer, Main interferometer 25.

The clock triggering device 24 based on the auxiliary interferometer includes: a detector 2 , a first 50:50 coupler 5 , a clock frequency doubling circuit module 6 , a delay fiber 7 , a first Faraday turning mirror 8 , and a second Faraday turning mirror 9 and isolator 10. The clock triggering device 24 based on the auxiliary interferometer is used to realize sampling with equal optical frequency interval, and its purpose is to suppress the nonlinear scanning of the light source.

The main interferometer 25 includes: a 50:50 beam splitter 3, a polarization controller 12, a circulator 13, a second 50:50 coupler 14, a first polarization beam splitter 16, a second polarization beam splitter 17, The first balance probe 18 , the second balance probe 19 , the acquisition device 20 , the reference arm 22 and the test arm 23 . The main interferometer 25 is the core of the optical frequency domain reflection distributed optical fiber sensing device A, which is an improved Mach-Zehnder interferometer.

The input end of the GPIB control module 21 is connected to the computer 11; the output end of the GPIB control module 21 is connected to the tunable laser 1; the tunable laser 1 is connected to the a port of the 1:99 optical beam splitter 4; the 1:99 optical beam splitter 4 The b port of the isolator 10 is connected to one end; the c port of the 1:99 optical beam splitter 4 is connected to the a port of the 50:50 beam splitter 3; the other end of the isolator 10 is connected to the first 50:50 coupler The b port of the first 50:50 coupler 5 is connected to one end of the detector 2; the c port of the first 50:50 coupler 5 is connected to the first Faraday turning mirror 8; the first 50:50 The d port of the coupler 5 is connected with the second Faraday turning mirror 9 through the delay fiber 7; the other end of the detector 2 is connected with the input end of the clock frequency multiplying circuit module 6; the output end of the clock shaping circuit module 6 is connected with the output end of the acquisition device 20. The input end is connected; the b port of the 50:50 beam splitter 3 is connected to the input end of the polarization controller 12 through the reference arm 22; the c port of the 50:50 beam splitter 3 is connected to the a port of the circulator 13 through the test arm 23 The output end of the polarization controller 12 is connected with the a port of the second 50:50 coupler 14; the b port of the circulator 13 is connected with the b port of the second 50:50 coupler 14; the c port of the circulator 13 is connected with the pull The tapered fiber 15 is connected; the c port of the second 50:50 coupler 14 is connected to the input end of the first polarization beam splitter 16; the d port of the second 50:50 coupler 14 is connected to the input of the second polarization beam splitter 17 The output ends of the first polarization beam splitter 16 are respectively connected with the input end of the first balanced detector 18 and the input end of the second balanced detector 19; the output ends of the second polarization beam splitter 17 are respectively connected with the first balanced detector 18 and the second balanced detector 19. The input end of the balance detector 18 is connected with the input end of the second balance detector 19; the output end of the first balance detector 18 is connected with the input end of the acquisition device 20; the output end of the second balance detector 19 is connected with the acquisition device 20 The input end of the collecting device 20 is connected to the computer 11 .

When the device is working, the computer 11 controls the tunable laser 1 to control the tuning speed, center wavelength, tuning start, etc. through the GPIB control module 21; The ratio of 1:99 goes from the b port of the 1:99 optical beam splitter 4 through the isolator 10 into the b port of the first 50:50 coupler 5, and the light enters the b port of the first 50:50 coupler 5 from the b port of the first 50:50 coupler 5. The c and d ports of the first 50:50 coupler 5 exit, are respectively reflected by the first Faraday turning mirror 8 and the second Faraday turning mirror 9 of the two arms, and return to the c and d ports of the first 50:50 coupler 5 , the two beams of light interfere in the first 50:50 coupler 5 and output from the a port of the first 50:50 coupler 5; the outgoing light from the a port of the first 50:50 coupler 5 enters the detector 2, The detector 2 converts the detected optical signal into an interference beat frequency signal and transmits it to the clock shaping module 6 . The clock shaping module 6 shapes the interference beat frequency signal into a square wave, and the shaped signal is transmitted to the acquisition device 20 as a external clock signal.

The output light of tunable laser 1 enters from port a of 1:99 optical beam splitter 4, and enters port a of 50:50 beam splitter 3 from port c of 1:99 optical beam splitter 4; after 50:50 minutes The beamer 3 enters the polarization controller 12 in the reference arm 22 from the b port, and enters the a port of the circulator 13 on the test arm 23 from the c port; Enter the tapered fiber 15, and the backscattered light of the tapered fiber 15 enters from the port c of the circulator 13 and outputs from the port b of the circulator 13; the reference light output by the polarization controller 12 in the reference arm 22 passes through the second The a-port of the 50:50 coupler 14 and the backscattered light on the circulator 13 are combined by the b-port of the second 50:50 coupler 14 to form beat interference and transmit from the second 50:50 coupler 14 The c port and the d port are output to the first polarization beam splitter 16 and the first polarization beam splitter 17, and the first polarization beam splitter 16 and the first polarization beam splitter 17 pass through the first balance detector 18 and the second balance The detector 19 correspondingly collects the signal light in the orthogonal direction output by the two polarization beam splitters, the first balanced detector 18 and the second balanced detector 19 transmit the output analog electrical signal to the acquisition device 20, and the acquisition device 20 is clocked. The collected analog electrical signal is transmitted to the computer 11 under the action of the external clock signal formed by the shaping module 6 .

The GPIB control module 21 is used by the computer 11 to control the tunable laser 1 therethrough.

The tunable laser 1 is used to provide a light source for the optical frequency domain reflectometry system, and its optical frequency can be scanned linearly.

The isolator 10 prevents the reflected light from the b port of the first 50:50 coupler 5 in the auxiliary interferometer from entering the laser.

The first 50:50 coupler 5 is used for optical interference.

The delay fiber 7 is used to realize the beat frequency interference of the anisotropic arms, and the optical frequency can be obtained according to the beat frequency and the length of the delay fiber.

The first Faraday turning mirror 8 and the second Faraday turning mirror 9 are used to provide reflection for the interferometer, and can eliminate the polarization fading phenomenon of the interferometer.

The function of the polarization controller 12 is to adjust the polarization state of the reference light so that the light intensities in the two orthogonal directions are basically the same during polarization splitting.

The second 50:50 coupler 14 completes the polarization beam splitting of the signal to eliminate the influence of polarization fading noise.

Computer 11 : performs data processing on the interference signal collected by the collecting device 20 , and realizes optical fiber sensing based on optical frequency domain reflection using a taper fiber to measure the refractive index of a distributed liquid.

To sum up, the embodiment of the present invention realizes distributed liquid refractive index measurement with high spatial resolution reaching mm (millimeter) level, and can be successfully applied to dense distributed measurement of liquid refractive index, monitoring of liquid diffusion, and accurate liquid layer measurement. positioning, etc.

Example 3

An embodiment of the present invention provides a sensing method for measuring the refractive index of a distributed liquid by using a tapered optical fiber. The sensing method corresponds to the sensing systems in Embodiments 1 and 2. As shown in FIG. The steps of the sensing method are:

101: In the main interferometer 25, a beat-frequency interference signal is formed by back Rayleigh scattering from the taper fiber 15, and fast Fourier transform is performed on the beat-frequency interference signal respectively, and the optical frequency domain information is converted to the corresponding taper fiber. The distance domain information of each position in 15, the distance domain information is successively selected through a moving window of a certain width to form the local distance domain information of each position of the tapered optical fiber 15;

102: Both the reference signal and the measurement signal use the moving window to select the local distance field information of the tapered fiber, and zero-fill the local distance field information. The number of zero-filling can be several times the length of the local distance field data before zero-filling. The local distance domain information uses the complex inverse Fourier transform, and then converts it to the optical frequency domain to obtain the local optical frequency domain information of the reference signal and the measurement signal;

103: Use the cross-correlation operation to estimate the spectral wavelength shift of the local optical frequency domain information of the reference signal and the measurement signal. The shift of the cross-correlation peak reflects the wavelength shift of the Rayleigh scattering spectrum, and the wavelength shift of the Rayleigh scattering spectrum is proportional to the change of the refractive index of the liquid. It is proportional to the amount of change in the refractive index of the liquid through the amount of cross-correlation peak shift.

To sum up, the embodiment of the present invention realizes distributed liquid refractive index measurement with high spatial resolution reaching mm (millimeter) level, and can be successfully applied to dense distributed measurement of liquid refractive index, monitoring of liquid diffusion, and accurate liquid layer measurement. positioning, etc.

Example 4

The feasibility verification of the sensing system and sensing method in Embodiments 1-3 is carried out below in conjunction with specific tests, referring to FIG. 3 and FIG. 4 , and details are described below:

The verification experiment of the embodiment of the present invention adopts the taper of a thin-diameter optical fiber, and uses the cone of the thin-diameter optical fiber to measure the distributed change of the refractive index. is K=68.52 nm/RIU.

Put the tapered optical fiber 15 into the water tank for measurement, measure the refractive index of the glycerin aqueous solution, gradually change the concentration of the glycerin aqueous solution, and then change the refractive index for measurement.

The real refractive index change can be obtained by calculation and look-up table. Using the sensing system and sensing method in Embodiments 1-3 of the present invention, the demodulated refractive index change is compared with the real refractive index change value to verify the validity of the present application as shown in Table 1.

Table 1 Comparison of measured refractive index change and real refractive index change

True Refractive Index Change/RI Measuring Refractive Index Change/RI Error (measured value - true value)/RI 1.3574 1.3576 0.0002 1.3585 1.3585 0 1.3595 1.3595 0 1.3604 1.3604 0 1.3614 1.3613 -0.0001 1.3624 1.3623 -0.0001 1.3633 1.3632 -0.0001 1.3642 1.3641 -0.0001 1.3651 1.3651 0 1.3660 1.3660 0 1.3669 1.3669 0 1.3678 1.3679 0.0001 1.3686 1.3688 0.0002

It can be seen from Table 1 that the maximum measurement error of the temperature change is 0.0002RI, which verifies the effectiveness of the sensing system and method designed in the embodiment of the present invention.

As shown in FIG. 4 , to illustrate the detection results, when the tapered optical fiber 15 is located in the range of 3.51m to 3.54m of the entire fiber, it is detected that there is a refractive index change in the liquid, and the spatial resolution of each point reaches 5mm. The abscissa is the distance, and the ordinate is the wavelength shift of the back Rayleigh scattering. Different refractive index changes correspond to different curves. It can be seen that the wavelength shift corresponding to different refractive indices is different.

To sum up, the embodiment of the present invention realizes distributed liquid refractive index measurement with high spatial resolution reaching mm (millimeter) level, and can be successfully applied to dense distributed measurement of liquid refractive index, monitoring of liquid diffusion, and accurate liquid layer measurement. positioning, etc.

In the embodiment of the present invention, the models of each device are not limited unless otherwise specified, as long as the device can perform the above functions.

Those skilled in the art can understand that the accompanying drawing is only a schematic diagram of a preferred embodiment, and the above-mentioned serial numbers of the embodiments of the present invention are only for description, and do not represent the advantages or disadvantages of the embodiments.

The above are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included in the protection of the present invention. within the range.

Claims (5)

1. a sensing system utilizing a tapered optical fiber to measure the refractive index of a distributed liquid, the sensing system comprising: a distributed optical fiber sensing device of optical frequency domain reflection, characterized in that, The sensing system is used to realize distributed liquid refractive index measurement with high spatial resolution reaching millimeter level; The sensing system includes: a tapered optical fiber, Based on the interference between the back Rayleigh scattering modes in the tapered region of the tapered fiber, the refractive index sensing is performed, and the change of the external refractive index is sensed by the strong evanescent wave generated in the tapered region, and the change of the tapered fiber is changed. The effective mode index of back-Rayleigh scattering propagation in the tapered fiber; The tapered fiber is placed in the liquid, and the distributed optical fiber sensing device detects the wavelength shift of the Rayleigh scattering spectrum of the tapered fiber, analyzes the wavelength shift, and obtains the continuously distributed refractive index of the liquid distribution; The distributed optical fiber sensing device for optical frequency domain reflection includes: a tunable laser, a 1:99 optical beam splitter, a computer, a GPIB control module, a clock trigger device based on an auxiliary interferometer, and a main interferometer, The clock triggering device includes: a detector, a first 50:50 coupler, a clock frequency doubling circuit module, a delay fiber, a first Faraday turning mirror, a second Faraday turning mirror and an isolator; the clock triggering device is used to realize isooptical Frequency spacing sampling, the purpose of which is to suppress nonlinear scanning of the light source; The main interferometer includes: 50:50 beam splitter, polarization controller, circulator, second 50:50 coupler, first polarization beam splitter, second polarization beam splitter, first balance detector, second balance detectors, acquisition devices, reference arms and test arms; The input end of the GPIB control module is connected to the computer; the output end of the GPIB control module is connected to the tunable laser; the tunable laser is connected to the a port of the 1:99 optical beam splitter; the b port of the 1:99 optical beam splitter is connected to the isolator One end is connected; the c port of the 1:99 beam splitter is connected to the a port of the 50:50 beam splitter; the other end of the isolator is connected to the b port of the first 50:50 coupler; the first 50:50 coupling The a port of the first 50:50 coupler is connected to one end of the detector 2; the c port of the first 50:50 coupler is connected to the first Faraday turning mirror; the d port of the first 50:50 coupler is connected to the second Faraday turning mirror through a delay fiber ; The other end of the detector is connected to the input end of the clock frequency doubling circuit module; the output end of the clock shaping circuit module is connected to the input end of the acquisition device; the b port of the 50:50 beam splitter is connected to the input end of the polarization controller through the reference arm The c port of the 50:50 beam splitter is connected to the a port of the circulator through the test arm; the output of the polarization controller is connected to the a port of the second 50:50 coupler; the b port of the circulator is connected to the second The b port of the 50:50 coupler is connected to the b port; the c port of the circulator is connected to the taper fiber; the c port of the second 50:50 coupler is connected to the input of the first polarization beam splitter; the second 50:50 coupler The d port is connected with the input end of the second polarization beam splitter; the output end of the first polarization beam splitter is respectively connected with the input end of the first balanced detector and the input end of the second balanced detector; the second polarization beam splitter The output end of the detector is respectively connected with the input end of the first balanced detector and the input end of the second balanced detector; the output end of the first balanced detector is connected with the input end of the acquisition device; the output end of the second balanced detector is connected with The input end of the collection device is connected; the output end of the collection device is connected with the computer; The tunable laser is used to provide the light source for the optical frequency domain reflectometry system, and its optical frequency is linearly scanned; the isolator prevents the reflected light from the b port of the first 50:50 coupler in the auxiliary interferometer from entering the laser; the delay fiber is used to realize non- The beat frequency interference of the equal arms, the optical frequency is obtained according to the beat frequency and the length of the delay fiber; the first Faraday turning mirror and the second Faraday turning mirror are used to provide reflection for the interferometer and eliminate the polarization fading phenomenon of the interferometer; the second 50: The 50 coupler completes the polarization beam splitting of the signal and eliminates the influence of polarization fading noise; the computer processes the data of the interference signal collected by the acquisition device, and realizes the optical fiber transmission based on the optical frequency domain reflection using the taper fiber to measure the refractive index of the distributed liquid. sense. 2 . The sensing system for measuring the refractive index of distributed liquid by using a tapered optical fiber according to claim 1 , wherein the tapered optical fiber is a thin-diameter optical fiber drawn by a taper machine. 3 . 3 . The sensing system for measuring the refractive index of distributed liquid by using a taper fiber according to claim 2 , wherein the drawing of the taper machine is as follows: the drawing speed is 200 μm/s, the hydrogen-oxygen The flame reciprocating distance was 2000 μm, the reciprocating speed was 360 μm/s, the fiber elongation was 100000 μm, and the diameter of the cone region was 4 μm. 4. A sensing system for measuring distributed liquid refractive index using tapered optical fiber according to any one of claims 1-3, characterized in that the high spatial resolution reaches a distributed liquid refractive index of 5 mm Rate measurement, the sensitivity reaches 68.52nm/RIU. 5. A sensing method for a sensing system using a tapered optical fiber to measure the refractive index of a distributed liquid according to claim 1, wherein the sensing method comprises the following steps: In the main interferometer, the back-rayleigh scattering of the taper fiber is used to form a beat-frequency interference signal, and the beat-frequency interference signal is respectively subjected to fast Fourier transform to convert the optical frequency domain information to the distance corresponding to each position in the taper fiber. Domain information, for the distance domain information, each position of the taper fiber is sequentially selected through a moving window of a certain width to form the local distance domain information; Both the reference signal and the measurement signal use the moving window to select the local distance field information of the tapered fiber, and fill the local distance field information with zeros. The information is transformed into the optical frequency domain by inverse complex Fourier transform to obtain the local optical frequency domain information of the reference signal and the measurement signal; The cross-correlation operation is used to estimate the spectral wavelength shift of the local optical frequency domain information of the reference signal and the measurement signal. The shift of the cross-correlation peak reflects the wavelength shift of the Rayleigh scattering spectrum. The wavelength shift of the Rayleigh scattering spectrum is proportional to the change of the refractive index of the liquid. The amount of change in the refractive index of the liquid is reflected by the amount of cross-correlation peak shift.

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