CN107782696A - The sensor-based system and method for distributed liquid refractivity are measured using tapered fiber - Google Patents

Abstract

A sensing system and method for measuring the refractive index of a distributed liquid by using a tapered optical fiber, which relates to the field of optical fiber sensing technology, based on the interference between the back Rayleigh scattering modes in the tapered region of the tapered optical fiber, the refractive index transmission is carried out. sense, through the strong evanescent wave generated in the tapered region to sense the change of the external refractive index, and change the effective mode refractive index of the back Rayleigh scattering propagation in the tapered fiber; place the tapered fiber in the liquid, and pass through the distributed fiber The sensing device detects the wavelength shift of the Rayleigh scattering spectrum of the tapered optical fiber, analyzes the wavelength shift, and obtains the continuously distributed refractive index distribution of the liquid. Realized the distributed liquid refractive index measurement with high spatial resolution reaching millimeter level.

Description

Sensing system and method for measuring distributed liquid refractive index using tapered optical 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 a 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 the fields of basic research, chemical analysis, environmental pollution assessment, medical diagnosis and food industry.

The optical fiber refractive index sensor has the characteristics of sensor intrinsic insulation, anti-electromagnetic interference, high sensitivity, high precision, high integration, high bandwidth, and reusability, which has become a research hotspot of refractive index sensor. Traditional optical fiber refractive index sensors include fiber Bragg gratings, long-period gratings, microbent fibers, photonic crystal fibers, fiber surface plasmon resonance, F-P sensors, online MZ sensors, microspheres, microring oscillators, and single-mode multi-mode optical fiber serial connections and other structures.

The above-mentioned traditional optical fiber refractive index sensors generally use broadband light sources and spectrometers for demodulation or tunable laser scanning detection. Using the transmitted light method, only single-point sensing can be performed. It is a discrete sensor and cannot accurately measure 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, and any point on a certain length of the optical fiber is a sensitive point to realize multi-point continuous sensing.

Contents of the invention

The invention provides a sensing system and method for measuring the refractive index of distributed liquids using tapered optical fiber. The invention realizes the measurement of the refractive index of distributed liquids 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 precise positioning of liquid layers, etc., see the following description for details:

A sensing system for measuring the refractive index of a distributed liquid using a tapered optical fiber, the sensing system includes: a distributed optical fiber sensing device for optical frequency domain reflection, and the sensing system is used to achieve high spatial resolution up to millimeters Level distributed liquid refractive index measurement;

The sensing system includes: tapered optical fiber,

Refractive index sensing is performed based on the back Rayleigh scattering intermode interference that occurs in the tapered region of the tapered fiber, and the change in the external refractive index is sensed through the strong evanescent wave generated in the tapered region to change the pulled The effective mode refractive index for back Rayleigh scattering propagation in the tapered fiber;

placing the tapered optical fiber in the 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 of the liquid distribution status.

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 the 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 cone area is 4 μm.

A kind of sensing method utilizing tapered optical fiber to measure distributed liquid refractive index, described sensing method comprises the following steps:

In the main interferometer, the beat frequency interference signal is formed by Rayleigh scattering of the tapered fiber back, and the fast Fourier transform is performed on the beat frequency interference signal respectively, and the optical frequency domain information is converted to the distance of each position in the corresponding tapered fiber For the distance domain information, each position of the tapered optical fiber is sequentially selected through a moving window of a certain width to form local distance domain information;

Both the reference signal and the measurement signal use the moving window to select the local distance domain information of the tapered fiber, and fill the local distance domain information with zeros. The number of zero padding can be several times the length of the local distance domain data before zero padding. The domain information uses the inverse complex Fourier transform and then converts 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 in the refractive index of the liquid. The amount of movement of the cross-correlation peak reflects the amount of change in the refractive index of the liquid.

The beneficial effects of the technical solution provided by the invention are:

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

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

3. It has been 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

Fig. 1 is a kind of structural representation of the sensor system that utilizes tapered optical fiber to measure distributed liquid refractive index;

Fig. 2 is another schematic structural diagram of a sensing system utilizing a tapered optical fiber to measure the refractive index of a distributed liquid;

Fig. 3 is a flow chart of a sensing method utilizing a tapered optical fiber to measure the refractive index of a distributed liquid;

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 parts represented by each label is as follows:

A: Distributed optical fiber sensing device for 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: polarization beam splitter;

17: polarizing beam splitter; 18: balanced 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 optical fiber 15;

27: the fiber core of the tapered optical fiber 15.

Detailed ways

In order to make the purpose, technical solution and advantages of the present invention clearer, the implementation manners of the present invention will be 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, several centimeters to several meters. If professional wire drawing equipment is used, it can be longer to tens of meters or even several kilometers. This opens up the possibility of 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, which is expected to realize distributed refractive index sensing.

Example 1

In order to solve technical problems such as dense distributed measurement of liquid refractive index, monitoring of liquid diffusion, and precise positioning of liquid layers in the existing technology, the embodiment of the present invention provides a sensor system for measuring distributed liquid refractive index using a tapered optical fiber , see Figure 1, see the description below for details:

The design principle of the embodiment of the present invention is as follows: based on the interference between the back Rayleigh scattering modes in the tapered region of the tapered optical fiber 15, the refractive index sensing is performed through the stronger evanescent wave ( The specific strength is determined according to actual empirical values, which is not limited in the embodiment of the present invention) Perceive the change of the external refractive index, thereby changing the effective mode refractive index of back Rayleigh scattering propagation in the tapered fiber 15 .

The change in the effective mode refractive index of the back Rayleigh scattering causes a change in the interference phase difference, manifested as a wavelength shift in the Rayleigh scattering spectrum. The distributed optical fiber sensing device A through optical frequency domain reflection detects the wavelength shift on the Rayleigh scattering spectrum in the region of the tapered optical fiber 15 . When the tapered optical fiber 15 is placed in the liquid, due to the effects of the above process, the continuously distributed refractive index distribution of the liquid can be obtained by analyzing the wavelength shift of the distributed Rayleigh scattering spectrum.

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

Example 2

Below in conjunction with Fig. 1, the sensing system in Embodiment 1 is further introduced, see the following description for details: 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 on a taper machine with a thin diameter. , including: a cladding 26 and a core 27 (both drawn to a tapered shape).

In the embodiment of the present invention, the drawing speed in the tapering process is 200 μm/s, the reciprocating distance of the 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 cone is 4 μm.

The distributed optical fiber sensing device of optical frequency domain reflection includes: tunable laser 1, 1:99 optical beam splitter 4, computer 11, GPIB (general interface bus) control module 21, clock trigger device 24 based on auxiliary interferometer, Master interferometer 25.

Wherein, the clock trigger device 24 based on the auxiliary interferometer includes: a detector 2, a first 50:50 coupler 5, a clock frequency multiplication circuit module 6, a delay fiber 7, a first Faraday rotating mirror 8, and a second Faraday rotating mirror 9 and isolator 10. The clock trigger device 24 based on the auxiliary interferometer is used to realize equal optical frequency spacing sampling, and its purpose is to suppress the nonlinear 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 balance detector 18 , a second balance detector 19 , an acquisition device 20 , a reference arm 22 and a 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 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 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 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 5 is connected; the a port of the first 50:50 coupler 5 is connected with one end of the detector 2; the c port of the first 50:50 coupler 5 is connected with the first Faraday rotating mirror 8; the first 50:50 The d port of the coupler 5 is connected with the second Faraday rotating mirror 9 through the delay optical fiber 7; the other end of the detector 2 is connected with the input end of the clock frequency multiplication circuit module 6; the output end of the clock shaping circuit module 6 is connected with the acquisition device 20 The input end is connected; 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 c port of the 50:50 beam splitter 3 is connected with 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 Tapered fiber 15 is connected; the c port of the second 50:50 coupler 14 is connected with the input end of the first polarization beam splitter 16; the d port of the second 50:50 coupler 14 is connected with the input of the second polarization beam splitter 17 The output end of the first polarization beam splitter 16 is connected with the input end of the first balanced detector 18 and the input end of the second balanced detector 19 respectively; the output end of the second polarization beam splitter 17 is respectively connected with the first The input end of the balance detector 18 and the input end of the second balance detector 19 are connected; 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 acquisition device 20 is connected to the computer 11.

When the device is working, the computer 11 controls the tunable laser 1 through the GPIB control module 21 to control the tuning speed, center wavelength, tuning start, etc.; The ratio of 1:99 enters the b port of the first 50:50 coupler 5 from the b port of the 1:99 optical beam splitter 4 through the isolator 10, and the light enters from the b port of the first 50:50 coupler 5, from The c and d ports of the first 50:50 coupler 5 exit, are respectively reflected by the first Faraday rotating mirror 8 and the second Faraday rotating 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 are 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, and 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 the acquisition device 20 external clock signal.

The outgoing light of the tunable laser 1 enters the a port of the 1:99 optical beam splitter 4, and enters the a port of the 50:50 beam splitter 3 from the c port of the 1:99 optical beam splitter 4; after 50:50 splitting 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; the light enters from the a port of the circulator 13, and enters from the c port of the circulator 13 Enter the tapered fiber 15, and the backscattered light of the tapered fiber 15 enters from the circulator 13 port c port, and outputs from the circulator 13 port b port; 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 through the b-port of the second 50:50 coupler 14 to form beat frequency interference and transmit light from the second 50:50 coupler 14 c port and d port output to the first polarizing beam splitter 16 and the first polarizing beam splitter 17, the first polarizing beam splitter 16 and the first polarizing beam splitter 17 pass through the first balanced detector 18 and the second balanced The detector 19 corresponds to collecting the signal light in the orthogonal direction output by the two polarization beam splitters, and the first balanced detector 18 and the second balanced detector 19 transmit the output analog electrical signal to the collecting device 20, and the collecting device 20 clocks 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 for the computer 11 to control the tunable laser 1 through it.

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

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 light interference.

The delay fiber 7 is used to realize non-equal arm beat frequency interference, and the optical frequency can be obtained according to the beat frequency and the length of the delay fiber.

The first Faraday rotating mirror 8 and the second Faraday rotating 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 intensity in the two orthogonal directions is basically the same during polarization beam 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: performs data processing on the interference signal collected by the collection device 20, and realizes optical fiber sensing based on optical frequency domain reflection using tapered optical fiber to measure the refractive index of distributed liquid.

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

Example 3

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

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

102: Both the reference signal and the measurement signal use the moving window to select the local distance domain information of the tapered fiber, and pad the local distance domain information with zeros. The number of zero padding can be several times the length of the local distance domain data before zero padding. The local distance domain information uses the inverse complex Fourier transform, and then converts 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 calculation 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 in the refractive index of the liquid. Proportional, the amount of movement of the cross-correlation peak reflects the amount of change in the refractive index of the liquid.

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

Example 4

The feasibility verification of the sensing system and sensing method in Examples 1-3 will be carried out in conjunction with specific experiments below, see Figure 3 and Figure 4, see the description below for details:

The verification experiment of the embodiment of the present invention is to use a narrow-diameter optical fiber taper, and use the cone of the narrow-diameter optical fiber to measure the distributed change of the refractive index. According to the refractive index sensing coefficient of the tapered optical fiber 15 measured in the previous period, as shown in Figure 3, It is K=68.52nm/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 its refractive index for measurement.

The real refractive index change can be obtained through calculation and look-up table. Using the sensing system and sensing method in Examples 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 this application. See 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 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 , as an example of the detection results, when the tapered optical fiber 15 is located within the range of 3.51 m to 3.54 m of the entire optical fiber, it is detected that there is a change in the refractive index of the liquid, and the spatial resolution of each point reaches 5 mm. The abscissa is the distance, and the ordinate is the back Rayleigh scattering wavelength shift. Different refractive index changes correspond to different curves. It can be seen that different refractive indices correspond to different wavelength shifts.

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

In the embodiments of the present invention, unless otherwise specified, the models of the devices are not limited, as long as they can complete 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 serial numbers of the above-mentioned embodiments of the present invention are for description only, and do not represent the advantages and disadvantages of the embodiments.

The above descriptions 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 range.

Claims (5)

1. A sensing system utilizing a tapered optical fiber to measure distributed liquid refractive index, said 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: tapered optical fiber, Refractive index sensing is performed based on the back Rayleigh scattering intermode interference that occurs in the tapered region of the tapered fiber, and the change in the external refractive index is sensed through the strong evanescent wave generated in the tapered region to change the pulled The effective mode refractive index for back Rayleigh scattering propagation in the tapered fiber; placing the tapered optical fiber in the 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 of the liquid distribution status. 2. A sensor system for measuring the refractive index of distributed liquids by using a tapered optical fiber according to claim 1, wherein the tapered optical fiber is a thin-diameter optical fiber drawn by a tapered optical fiber machine. 3. A kind of sensing system utilizing tapered optical fiber to measure distributed liquid refractive index according to claim 2, is characterized in that, described tapered machine drawing is specifically; Drawing speed is 200 μ m/s, hydrogen oxygen The reciprocating distance of the flame is 2000 μm, the reciprocating speed is 360 μm/s, the fiber elongation is 100000 μm, and the cone diameter is 4 μm. 4. A kind of sensing system utilizing tapered optical fiber to measure distributed liquid refractive index according to any one of claims 1-3, characterized in that, the high spatial resolution reaches 5mm distributed liquid refractive index Rate measurement, the sensitivity reaches 68.52nm/RIU. 5. A sensing method utilizing tapered optical fiber to measure distributed liquid refractive index, it is characterized in that, described sensing method comprises the following steps: In the main interferometer, the beat frequency interference signal is formed by Rayleigh scattering of the tapered fiber back, and the fast Fourier transform is performed on the beat frequency interference signal respectively, and the optical frequency domain information is converted to the distance of each position in the corresponding tapered fiber For the distance domain information, each position of the tapered optical fiber is sequentially selected through a moving window of a certain width to form local distance domain information; Both the reference signal and the measurement signal use the moving window to select the local distance domain information of the tapered fiber, and fill the local distance domain information with zeros. The information is converted to the optical frequency domain by complex Fourier inverse 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 in the refractive index of the liquid. The amount of movement of the cross-correlation peak reflects the amount of change in the refractive index of the liquid.