CN107782696B - Sensing system and method for measuring refractive index of distributed liquid by using tapered optical fiber - Google Patents
Sensing system and method for measuring refractive index of distributed liquid by using tapered optical fiber Download PDFInfo
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- CN107782696B CN107782696B CN201710860221.XA CN201710860221A CN107782696B CN 107782696 B CN107782696 B CN 107782696B CN 201710860221 A CN201710860221 A CN 201710860221A CN 107782696 B CN107782696 B CN 107782696B
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- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/41—Refractivity; Phase-affecting properties, e.g. optical path length
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- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
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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
Technical Field
The invention relates to the technical field of optical fiber sensing, in particular to a sensing system and a sensing method for measuring the refractive index of distributed liquid by using tapered optical fibers, which are applied to optical frequency domain reflection.
Background
The refractive index of a substance is an important physical quantity reflecting information inside the substance. The measurement of the refractive index of a substance has wide application in the fields of basic research, chemical analysis, environmental pollution assessment, medical diagnosis, food industry and the like.
The optical fiber refractive index sensor has the characteristics of intrinsic insulation, electromagnetic interference resistance, high sensitivity, high precision, high integration level, high bandwidth, reusability and the like, and becomes a research hotspot of the refractive index sensor. The traditional optical fiber refractive index sensor has the structures of Bragg optical fiber grating, long-period grating, microbend optical fiber, photonic crystal fiber, optical fiber surface plasmon resonance, F-P sensor, online MZ sensor, microsphere, micro-ring oscillator, single-mode multimode fiber concatenation and the like.
The traditional optical fiber refractive index sensor generally adopts a broadband light source and a spectrometer for demodulation or tunable laser scanning detection, and only single-point sensing can be carried out by using a transmission light mode, and the refractive index distribution on a certain length cannot be accurately measured due to discrete sensing.
At present, an optical fiber refractive index sensor is urgently needed to realize distributed refractive index sensing along an optical fiber, any point on a certain length of the optical fiber is a sensitive point, and multipoint continuous sensing is realized.
Disclosure of Invention
The invention provides a sensing system and a method for measuring the refractive index of distributed liquid by using a tapered optical fiber, which realize the refractive index measurement of the distributed liquid with high spatial resolution reaching mm (millimeter) level, can be successfully applied to the occasions of dense distributed measurement of the refractive index of the liquid, monitoring the diffusion of the liquid, accurate positioning of a liquid layer and the like, and are described in detail as follows:
a sensing system for measuring the refractive index of a distributed liquid using a tapered optical fiber, the sensing system comprising: the optical frequency domain reflection distributed optical fiber sensing device is characterized in that the sensing system is used for realizing distributed liquid refractive index measurement with high spatial resolution reaching millimeter level;
the sensing system includes: the optical fiber is drawn to be a cone,
based on interference between backward Rayleigh scattering modes generated in the tapered area of the tapered optical fiber, refractive index sensing is carried out, and the effective mode refractive index of backward Rayleigh scattering propagation in the tapered optical fiber is changed by sensing the change of the 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 the Rayleigh scattering spectrum of the tapered optical fiber through the distributed optical fiber sensing device, analyzing the wavelength shift, and obtaining the continuously distributed refractive index distribution condition of the liquid.
The tapered optical fiber is formed by drawing a thin-diameter optical fiber on a tapered machine.
The drawing speed in the tapering process is 200 μm/s, the reciprocating distance of oxyhydrogen flame is 2000 μm, the reciprocating speed is 360 μm/s, the elongation of the optical fiber is 100000 μm, and the diameter of the tapered zone is 4 μm.
A sensing method for measuring the refractive index of a distributed liquid by using a tapered optical fiber, the sensing method comprising the steps of:
in the main interferometer, a Rayleigh scattering is carried out on the tapered optical fiber in a backward direction to form beat frequency interference signals, fast Fourier transform is carried out on the beat frequency interference signals respectively, optical frequency domain information is converted into distance domain information corresponding to each position in the tapered optical fiber, and each position of the tapered optical fiber is sequentially selected for the distance domain information through a moving window with a certain width to form local distance domain information;
the reference signal and the measurement signal both utilize a mobile window to select local distance domain information of the tapered optical fiber, zero filling is carried out on the local distance domain information, the zero filling quantity can be multiple times of the data length of the local distance domain before zero filling, and then the local distance domain information after zero filling is converted into an optical frequency domain by utilizing complex Fourier inverse transformation to obtain the local optical frequency domain information of the reference signal and the measurement signal;
and performing spectral wavelength shift estimation on the local optical frequency domain information of the reference signal and the measurement signal by utilizing cross-correlation operation, wherein cross-correlation peak shift quantity reflects the wavelength shift of a Rayleigh scattering spectrum, the wavelength shift of the Rayleigh scattering spectrum is in direct proportion to the liquid refractive index variation, and the liquid refractive index variation is reflected through the cross-correlation peak shift quantity.
The technical scheme provided by the invention has the beneficial effects that:
1. the invention can be successfully applied to the occasions of dense liquid refractive index distributed measurement, liquid diffusion monitoring, liquid layer accurate positioning and the like;
2. the refractive index measurement of the distributed liquid with high spatial resolution of 5mm is realized, and the sensitivity reaches 68.52 nm/RIU;
3. the test verifies that the maximum measurement error of the temperature change is 0.0002RI, and the effectiveness of the invention is verified.
Drawings
FIG. 1 is a schematic diagram of a sensing system for measuring the refractive index of a distributed liquid using tapered optical fibers;
FIG. 2 is another schematic diagram of a sensing system for measuring the refractive index of a distributed fluid using tapered optical fibers;
FIG. 3 is a flow chart of a sensing method for measuring the refractive index of a distributed liquid using tapered optical fibers;
FIG. 4 is a schematic illustration of a calibration curve;
FIG. 5 is a diagram illustrating an example of the detection result.
In the drawings, the components represented by the respective reference numerals are listed below:
a: distributed optical fiber sensing means with optical frequency domain reflection;
1: a tunable laser; 2: a detector;
3: a 50:50 beam splitter; 4: 1:99 optical beam splitter;
5: a 50:50 coupler; 6: a clock shaping circuit module;
7: a delay optical fiber; 8: a first Faraday rotator mirror;
9: a second Faraday rotator mirror; 10: an isolator;
11: a computer; 12: a polarization controller;
13: a circulator; 1450: 50 coupler;
15: tapering the optical fiber; 16: a polarizing beam splitter;
17: a polarizing beam splitter; 18: a balance detector;
19: a balance detector; 20: a collection device;
21: a GPIB control module; 22: a reference arm;
23: a test arm; 24: a clock trigger device based on an auxiliary interferometer;
25: a main interferometer; 26: the cladding of the tapered fiber 15;
27: the core of the tapered fiber 15.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, embodiments of the present invention are described in further detail below.
The traditional refractive index sensing based on the tapered optical fiber is single-point sensing, and the tapered area of the tapered optical fiber can keep a certain length, namely a few centimeters to a few meters, for example, the length can be longer to dozens of meters or even kilometers by adopting professional wire drawing equipment, so that the possibility is provided for distributed optical fiber refractive index sensing. In addition, the optical frequency domain reflection technology is combined with the tapered optical fiber, the optical frequency domain reflection technology demodulates the backward Rayleigh scattering of the tapered area of the tapered optical fiber, and the distributed refractive index sensing is hopeful to be realized.
Example 1
In order to solve the technical problems that the prior art cannot realize dense distributed measurement of the refractive index of liquid, monitoring liquid diffusion, accurate positioning of a liquid layer and the like, the embodiment of the invention provides a sensing system for measuring the refractive index of distributed liquid by using tapered optical fibers, and the sensing system is described in detail in the following description with reference to fig. 1:
the design principle of the embodiment of the invention is as follows: refractive index sensing is performed based on interference between backward rayleigh scattering modes occurring in a tapered region of the tapered optical fiber 15, and the change of the external refractive index is sensed through a strong evanescent wave (the specific strength is determined according to an actual empirical value, which is not limited by the embodiment of the present invention) generated in the tapered region, so that the effective mode refractive index of backward rayleigh scattering propagation in the tapered optical fiber 15 is changed.
Changes in the effective mode index of refraction in the back rayleigh scattering cause changes in the interference phase difference, manifested as wavelength shifts in the rayleigh scattering spectrum. The distributed fiber sensing device a, which is reflected by the optical frequency domain, detects the wavelength shift on the rayleigh scattering spectrum over the area of the tapered fiber 15. When the tapered optical fiber 15 is placed in the liquid, the wavelength shift of the distributed rayleigh scattering spectrum is analyzed to obtain the continuously distributed refractive index distribution of the liquid due to the above process.
In summary, the embodiment of the present invention realizes the distributed liquid refractive index measurement with high spatial resolution up to mm (millimeter) level, and can be successfully applied to the dense liquid refractive index distributed measurement, the monitoring of liquid diffusion, the precise positioning of the liquid layer, and other occasions.
Example 2
The sensing system of embodiment 1 is further described below with reference to fig. 1, and is described in detail below: the sensing system includes: the optical fiber sensing device comprises a tapered optical fiber 15 and a distributed optical fiber sensing device with optical frequency domain reflection, wherein the tapered optical fiber 15 is formed by drawing a thin diameter on a tapered machine. The method comprises the following steps: cladding 26 and core 27 (both drawn to taper).
In the tapering process, the drawing speed is 200 mu m/s, the reciprocating distance of oxyhydrogen flame is 2000 mu m, the reciprocating speed is 360 mu m/s, the elongation of the optical fiber is 100000 mu m, and the diameter of a tapered zone is 4 mu m.
The distributed optical fiber sensing device with optical frequency domain reflection comprises: the system comprises 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 and a main interferometer 25.
The clock trigger device 24 based on the auxiliary interferometer includes: the detector 2, a first 50:50 coupler 5, a clock frequency doubling circuit module 6, a delay optical fiber 7, a first Faraday rotator mirror 8, a second Faraday rotator mirror 9 and an isolator 10. The auxiliary interferometer based clock trigger 24 is used to achieve equal optical frequency spacing sampling with the aim of suppressing non-linear scanning of the light source.
Wherein 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, a first balanced detector 18, a second balanced detector 19, an acquisition device 20, a reference arm 22 and a test arm 23. The main interferometer 25 is the core of the distributed fiber optic sensing device a, which is a modified mach zehnder interferometer, for optical frequency domain reflection.
The input end of the GPIB control module 21 is connected with the computer 11; the output end of the GPIB control module 21 is connected with the tunable laser 1; the tunable laser 1 is connected with a port a of the 1:99 optical beam splitter 4; the port b of the 1:99 optical beam splitter 4 is connected with one end of an isolator 10; the port c of the 1:99 optical splitter 4 is connected with the port a of the 50:50 optical splitter 3; the other end of the isolator 10 is connected with a port b of the first 50:50 coupler 5; the port a of the first 50:50 coupler 5 is connected with one end of the detector 2; the port c of the first 50:50 coupler 5 is connected with a first Faraday rotator mirror 8; the d port of the first 50:50 coupler 5 is connected with a second Faraday rotator mirror 9 through a delay optical fiber 7; the other end of the detector 2 is connected with the input end of the clock frequency doubling circuit module 6; the output end of the clock shaping circuit module 6 is connected with the input end of the acquisition device 20; the b port of the 50:50 beam splitter 3 is connected with the input end of the polarization controller 12 through the reference arm 22; the port c of the 50:50 beam splitter 3 is connected with the port a 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 port c of the circulator 13 is connected with a tapered optical fiber 15; the c-port of the second 50:50 coupler 14 is connected to the input of the first polarizing beam splitter 16; the d-port of the second 50:50 coupler 14 is connected to the input of the second polarizing beam splitter 17; the output end of the first polarization beam splitter 16 is respectively connected with the input end of a first balanced detector 18 and the input end of a second balanced detector 19; the output end of the second polarization beam splitter 17 is respectively connected with the input end of the first balanced detector 18 and the input end of the second balanced 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 input end of the acquisition device 20; the output of the acquisition device 20 is connected to the computer 11.
When the device works, the computer 11 controls the tunable laser 1 to control the tuning speed, the central wavelength, the tuning start and the like through the GPIB control module 21; outgoing light of the tunable laser 1 enters from a port a of a 1:99 optical beam splitter 4, enters from a port b of the 1:99 optical beam splitter 4 into a port b of a first 50:50 coupler 5 through an isolator 10 in a ratio of 1:99, enters from the port b of the first 50:50 coupler 5, exits from ports c and d of the first 50:50 coupler 5, is reflected by a first Faraday rotator mirror 8 and a second Faraday rotator mirror 9 of two arms respectively, returns to ports c and d of the first 50:50 coupler 5, interferes in the first 50:50 coupler 5, and is output from the port a of the first 50:50 coupler 5; first 50: the emergent light of the port a enters the detector 2 through the coupler 5 50, the detector 2 converts the detected light signal into an interference beat frequency signal and transmits the interference beat frequency signal to the clock shaping module 6, the interference beat frequency signal of the clock shaping module 6 is shaped into square waves, and the shaped signal is transmitted to the acquisition device 20 and serves as an external clock signal of the acquisition device 20.
Emergent light of the tunable laser 1 enters from a port a of a 1:99 optical beam splitter 4 and enters from a port c of the 1:99 optical beam splitter 4 into a port a of a 50:50 optical beam splitter 3; passing through a 50:50 the beam splitter 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; light enters from the port a of the circulator 13 and enters the tapered optical fiber 15 from the port c of the circulator 13, and back scattered light of the tapered optical fiber 15 enters from the port c of the circulator 13 and is output from the port b of the circulator 13; the reference light output by the polarization controller 12 in the reference arm 22 and the back scattering light on the circulator 13 are combined through the a port of the second 50:50 coupler 14 and the b port of the second 50:50 coupler 14 to form beat frequency interference and output from the c port and the d port of the second 50:50 coupler 14 to the first polarization beam splitter 16 and the first polarization beam splitter 17, the first polarization beam splitter 16 and the first polarization beam splitter 17 correspondingly collect the signal light in the orthogonal direction output by the two polarization beam splitters through the first balanced detector 18 and the second balanced detector 19, the first balanced detector 18 and the second balanced detector 19 transmit the output analog electric signals to the collecting device 20, and the collecting device 20 transmits the collected analog electric signals to the computer 11 under the action of the external clock signal formed by the clock shaping module 6.
The tunable laser 1 is used to provide a light source for an optical frequency domain reflectometry system, the optical frequency of which can be scanned linearly.
The isolator 10 prevents 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 beat frequency interference of an unequal arm, and can obtain an optical frequency according to the beat frequency and the length of the delay fiber.
The first Faraday rotator mirror 8 and the second Faraday rotator mirror 9 are used for providing reflection for the interferometer and eliminating the polarization fading phenomenon of the interferometer.
The polarization controller 12 is operative to adjust the polarization state of the reference light such that the intensities of the light in two orthogonal directions are substantially the same during polarization splitting.
The second 50:50 coupler 14 performs polarization beam splitting on the signal to eliminate the influence of polarization fading noise.
The computer 11: and the interference signals acquired by the acquisition device 20 are subjected to data processing, so that optical fiber sensing based on measuring the refractive index of the distributed liquid by using tapered optical fibers in optical frequency domain reflection is realized.
In summary, the embodiment of the present invention realizes the distributed liquid refractive index measurement with high spatial resolution up to mm (millimeter) level, and can be successfully applied to the dense liquid refractive index distributed measurement, the monitoring of liquid diffusion, the precise positioning of the liquid layer, and other occasions.
Example 3
The embodiment of the invention provides a sensing method for measuring a distributed liquid refractive index by using a tapered optical fiber, which corresponds to the sensing systems in the embodiments 1 and 2, and as shown in fig. 2, the sensing method comprises the following steps:
101: in the main interferometer 25, a beat frequency interference signal is formed by backward Rayleigh scattering of the tapered optical fiber 15, fast Fourier transform is respectively carried out on the beat frequency interference signal, optical frequency domain information is converted into distance domain information corresponding to each position in the tapered optical fiber 15, and each position of the tapered optical fiber 15 is sequentially selected for the distance domain information through a moving window with a certain width to form local distance domain information;
102: the reference signal and the measurement signal both utilize a mobile window to select local distance domain information of the tapered optical fiber, zero filling is carried out on the local distance domain information, the zero filling quantity can be multiple times of the data length of the local distance domain before zero filling, and then the local distance domain information after zero filling is converted into an optical frequency domain by utilizing complex Fourier inverse transformation to obtain the local optical frequency domain information of the reference signal and the measurement signal;
103: and performing spectral wavelength shift estimation on the local optical frequency domain information of the reference signal and the measurement signal by utilizing cross-correlation operation, wherein cross-correlation peak shift quantity reflects the wavelength shift of a Rayleigh scattering spectrum, the wavelength shift of the Rayleigh scattering spectrum is in direct proportion to the liquid refractive index variation, and the liquid refractive index variation is reflected through the cross-correlation peak shift quantity.
In summary, the embodiment of the present invention realizes the distributed liquid refractive index measurement with high spatial resolution up to mm (millimeter) level, and can be successfully applied to the dense liquid refractive index distributed measurement, the monitoring of liquid diffusion, the precise positioning of the liquid layer, and other occasions.
Example 4
The sensing systems and sensing methods of examples 1-3 were validated in the following detailed experiments, with reference to fig. 3 and 4, described in detail below:
in the verification experiment of the embodiment of the invention, the tapered optical fiber with a small diameter is adopted, the cone of the optical fiber with the small diameter is used for measuring the distributed change of the refractive index, and the refractive index sensing coefficient of the tapered optical fiber 15 measured in the early stage is shown as K which is 68.52nm/RIU in figure 3.
The tapered optical fiber 15 is placed in a water tank for measurement, the refractive index of the glycerol aqueous solution is measured, the concentration of the glycerol aqueous solution is gradually changed, and the refractive index is further changed for measurement.
The true refractive index change can be calculated and looked up. The sensing system and the sensing method in the embodiments 1 to 3 of the present invention are used to demodulate the refractive index change and compare the refractive index change with the true refractive index change value, so as to verify the validity of the present application, which is shown in table 1.
TABLE 1 comparison of measured refractive index changes with true refractive index changes
| True refractive index change/RI | Measurement of 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 |
As can be seen from Table 1, the maximum measurement error of the temperature change is 0.0002RI, which verifies the effectiveness of the sensing system and method designed by the embodiment of the invention.
As shown in FIG. 4, for the purpose of illustrating the detection results, the refractive index change of the liquid is detected in the range of 3.51m to 3.54m of the tapered fiber 15 located over the entire fiber, and the spatial resolution of each point reaches 5 mm. The abscissa is distance, the ordinate is backward rayleigh scattering wavelength shift amount, different refractive index changes correspond to different curves, and it can be seen that the wavelength shift amounts corresponding to different refractive indexes are different.
In summary, the embodiment of the present invention realizes the distributed liquid refractive index measurement with high spatial resolution up to mm (millimeter) level, and can be successfully applied to the dense liquid refractive index distributed measurement, the monitoring of liquid diffusion, the precise positioning of the liquid layer, and other occasions.
In the embodiment of the present invention, except for the specific description of the model of each device, the model of other devices is not limited, as long as the device can perform the above functions.
Those skilled in the art will appreciate that the drawings are only schematic illustrations of preferred embodiments, and the above-described embodiments of the present invention are merely provided for description and do not represent the merits of the embodiments.
The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents, improvements and the like that fall within the spirit and principle of the present invention are intended to be included therein.
Claims (5)
1. A sensing system for measuring the refractive index of a distributed liquid using a tapered optical fiber, the sensing system comprising: a distributed optical fiber sensing apparatus with optical frequency domain reflection,
the sensing system is used for realizing distributed liquid refractive index measurement with high spatial resolution reaching millimeter level;
the sensing system includes: the optical fiber is drawn to be a cone,
based on interference between backward Rayleigh scattering modes generated in the tapered area of the tapered optical fiber, refractive index sensing is carried out, and the effective mode refractive index of backward Rayleigh scattering propagation in the tapered optical fiber is changed by sensing the change of the external refractive index through stronger evanescent waves generated in the tapered area;
placing the tapered optical fiber in liquid, detecting the wavelength shift of Rayleigh scattering spectrum of the tapered optical fiber through the distributed optical fiber sensing device, analyzing the wavelength shift, and obtaining the continuously distributed refractive index distribution condition of the liquid;
wherein, the distributed optical fiber sensing device of 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, a main interferometer,
the clock trigger device includes: the device comprises a detector, a first 50:50 coupler, a clock frequency doubling circuit module, a delay optical fiber, a first Faraday rotator mirror, a second Faraday rotator mirror and an isolator; the clock trigger device is used for realizing equal optical frequency interval sampling and aims to inhibit the nonlinear scanning of a light source;
the main interferometer includes: the device comprises a 50:50 beam splitter, a polarization controller, a circulator, a second 50:50 coupler, a first polarization beam splitter, a second polarization beam splitter, a first balance detector, a second balance detector, a collecting device, a reference arm and a test arm;
the input end of the GPIB control module is connected with a computer; the output end of the GPIB control module is connected with the tunable laser; the tunable laser is connected with the port a of the 1:99 optical beam splitter; the b port of the 1:99 optical splitter is connected with one end of the isolator; the port c of the 1:99 optical splitter is connected with the port a of the 50:50 optical splitter; the other end of the isolator is connected with a port b of the first 50:50 coupler; the a port of the first 50:50 coupler is connected with one end of the detector 2; the port c of the first 50:50 coupler is connected with a first Faraday rotator mirror; the d port of the first 50:50 coupler is connected with a second Faraday rotator mirror through a delay optical fiber; the other end of the detector is connected with the input end of the clock frequency doubling circuit module; the output end of the clock shaping circuit module is connected with the input end of the acquisition device; the port b of the 50:50 beam splitter is connected with the input end of the polarization controller through a reference arm; the port c of the 50:50 beam splitter is connected with the port a of the circulator through a test arm; the output end of the polarization controller is connected with the port a of the second 50:50 coupler; the b port of the circulator is connected with the b port of the second 50:50 coupler; the port c of the circulator is connected with the tapered optical fiber; the port c of the second 50:50 coupler is connected with the input end of the first polarization beam splitter; the d port of the second 50:50 coupler 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 output end of the second 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 output end of the first balance 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 acquisition device; the output end of the acquisition device is connected with the computer;
the tunable laser is used for providing a light source for the optical frequency domain reflection system, and the optical frequency of the tunable laser is linearly scanned; the isolator prevents reflected light from the b-port of the first 50:50 coupler in the auxiliary interferometer from entering the laser; the delay optical fiber is used for realizing unequal-arm beat frequency interference and obtaining optical frequency according to the beat frequency and the length of the delay optical fiber; the first Faraday rotator mirror and the second Faraday rotator mirror are used for providing reflection for the interferometer and eliminating the polarization fading phenomenon of the interferometer; the second 50:50 coupler completes polarization beam splitting on the signal, and eliminates the influence of polarization fading noise; and the computer performs data processing on the interference signals acquired by the acquisition device, and realizes optical fiber sensing based on measuring the refractive index of the distributed liquid by using the tapered optical fiber in optical frequency domain reflection.
2. The sensing system of claim 1, wherein the tapered fiber is a small diameter fiber drawn by a tapering machine.
3. The sensing system for measuring the refractive index of the distributed liquid by using the tapered optical fiber as claimed in claim 2, wherein the tapered machine is drawn specifically as; the drawing speed was 200 μm/s, the reciprocating distance of oxyhydrogen flame was 2000 μm, the reciprocating speed was 360 μm/s, the elongation of the optical fiber was 100000 μm, and the diameter of the taper region was 4 μm.
4. A sensing system for measuring the refractive index of a distributed liquid using a tapered optical fiber according to any one of claims 1 to 3, wherein the high spatial resolution is up to 5mm for distributed liquid refractive index measurement and the sensitivity is up to 68.52 nm/RIU.
5. A sensing method for a sensing system for measuring the refractive index of a distributed liquid using a tapered optical fiber according to claim 1, the sensing method comprising the steps of:
in the main interferometer, a Rayleigh scattering is carried out on the tapered optical fiber in a backward direction to form beat frequency interference signals, fast Fourier transform is carried out on the beat frequency interference signals respectively, optical frequency domain information is converted into distance domain information corresponding to each position in the tapered optical fiber, and each position of the tapered optical fiber is sequentially selected for the distance domain information through a moving window with a certain width to form local distance domain information;
selecting local distance domain information of the tapered optical fiber by using a moving window for the reference signal and the measurement signal, carrying out zero filling on the local distance domain information, wherein the zero filling quantity is multiple times of the data length of the local distance domain before zero filling, and then converting the local distance domain information after zero filling into an optical frequency domain by using complex Fourier inverse transformation to obtain the local optical frequency domain information of the reference signal and the measurement signal;
and performing spectral wavelength shift estimation on the local optical frequency domain information of the reference signal and the measurement signal by utilizing cross-correlation operation, wherein cross-correlation peak shift quantity reflects the wavelength shift of a Rayleigh scattering spectrum, the wavelength shift of the Rayleigh scattering spectrum is in direct proportion to the liquid refractive index variation, and the liquid refractive index variation is reflected through the cross-correlation peak shift quantity.
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