CN111579534B - Optical fiber sensing detection device for detecting refractive index of solution and refractive index detection method - Google Patents
Optical fiber sensing detection device for detecting refractive index of solution and refractive index detection method Download PDFInfo
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Abstract
The invention discloses an optical fiber sensing detection device for detecting the refractive index of a solution, which comprises a broadband light source device, an optical fiber refractive index sensor and a spectrum analyzer, wherein the optical fiber refractive index sensor comprises a thin core optical fiber, a coreless optical fiber and two single mode fibers, which are sequentially welded to form a structure of single mode fibers, the thin core optical fiber, the coreless optical fiber and the single mode fibers, and two ends of the structure are respectively connected with the broadband light source device and the spectrum analyzer. The invention also discloses a refractive index detection method, which comprises the steps of immersing the thin core optical fiber and the coreless optical fiber in the optical fiber refractive index sensor in a solution to obtain corresponding transmission spectrograms, selecting the center wavelength of corresponding characteristic peaks, and calculating according to a formula to obtain the refractive index of the solution. The optical fiber sensing detection device has the advantages of simple structure, low cost, high refractive index sensitivity for solution detection and larger refractive index detection range.
Description
Technical Field
The invention relates to the field of optical fiber sensing, in particular to an optical fiber sensing detection device and an optical fiber sensing detection method for detecting the refractive index of a solution.
Background
In recent years, the optical fiber sensor has the advantages of strong flexibility, convenient manufacture, electromagnetic interference resistance, high temperature resistance, adaptability to harsh environments and the like, and is widely applied to the fields of physics, biology, chemistry and the like. Because of its unique advantages, fiber optic sensors have been widely used in the field of measurement of physical parameters. Such as refractive index, strain, temperature, humidity, curvature, etc. Among the above parameters, refractive index, strain and temperature are important parameters in many living applications. To date, many fiber optic sensing structures have been proposed that measure strain, temperature and refractive index. In 2008, tian Zhaobing et al proposed a biconical single mode fiber Mach-Zehnder interferometer. It has the advantages of simple manufacture, low cost, etc. The refractive index of the measurement sensitivity is low. In the range of refractive index 1.315-1.3618, the refractive index sensitivity is only 17.1nm/RIU; in 2012, liu Ji proposed that a conical photonic crystal fiber Mach-Zehnder interferometer was fabricated by chemical etching with a sensitivity of 199nm/RIU. But has complex manufacturing process and low mechanical strength; 2013, shao Min et al devised a Mach-Zehnder interferometer based on a multimode fiber core sandwiched between two waist-large fiber cones for refractive index sensing. But are severely temperature cross-sensitive; 2018, zhang Na et al proposed a multi-parameter miniature fiber optic refractive index sensor with a refractive index sensitivity of 131.93nm/RIU. But the mechanical strength of the sensor is low and there is cross sensitivity to temperature. In the research, the existing sensing technology has the defects of complex manufacture, low refractive index sensitivity and small refractive index detection range, and limits the application of the optical fiber sensor.
Disclosure of Invention
Aiming at the defects in the prior art, the invention aims to solve the technical problems that: how to provide an optical fiber sensing detection device for detecting the solution refractive index, which has low cost, simple structure, high refractive index sensitivity and large refractive index detection range.
In order to solve the technical problems, the invention adopts the following technical scheme:
the utility model provides a solution refractive index detects optical fiber sensing detection device, including broadband light source device, optical fiber refractive index sensor and spectrum analyzer, optical fiber refractive index sensor includes thin core fiber, coreless fiber and two single mode fibers, the fiber core diameter of thin core fiber is less than the fiber core diameter of single mode fiber, the one end of thin core fiber and the one end butt fusion of coreless fiber together, one of them single mode fiber's one end and thin core fiber just keep away from the one end butt fusion of coreless fiber, the output of broadband light source device is inserted to the other end, the one end and the one end butt fusion of coreless fiber just keep away from thin core fiber of another single mode fiber, the other end is connected at spectrum analyzer's input, thin core fiber and coreless fiber are sharp state.
As an optimization, the optical fiber refractive index sensor is located the position between the two fixed rings of the thin core optical fiber and the coreless optical fiber, the optical fiber fixing device comprises a fixing base, two fixing rings which are arranged on the same central line are arranged on the upper surface of the fixing base in a spaced mode and fixedly connected with the upper surface of the fixing base, the central lines of the fixing rings are horizontally arranged, the optical fiber refractive index sensor penetrates through the two fixing rings, the thin core optical fiber, the coreless optical fiber and the fusion joint of the thin core optical fiber and the single-mode optical fiber are located at the position between the two fixing rings, the fixing base and the outer sides of the two fixing rings in the opposite directions are respectively provided with an optical fiber support, the tops of the optical fiber supports are relatively provided with two U-shaped supports which are located on the same horizontal direction, the planes of the U-shaped supports are mutually parallel, the horizontal heights of the U-shaped supports are higher than the horizontal heights of the fixing rings, the bottoms of the U-shaped supports are fixedly connected with struts and are fixed on the optical fiber supports through struts, the two U-shaped supports penetrate through the inner parts of the two U-shaped supports on one side of the optical fiber supports respectively, the two U-shaped supports are arranged on the bottom surfaces of the U-shaped supports on the one side of the optical fiber supports respectively, the U-shaped supports are arranged on the two optical fiber supports on the bottom surfaces of the two sides of the U-shaped supports on the bottom surfaces of the U-shaped supports, the two optical supports are arranged on the two sides of the U-shaped supports on the bottom supports, and the two optical fiber supports on the two sides of the U-shaped supports on the bottom supports and the one sides of the U supports and the single-shaped supports and the single-fiber supports.
The invention also discloses a method for detecting the refractive index of the solution, which comprises the following steps:
(1) The optical fiber sensing detection device for detecting the refractive index of the solution is obtained, and a plurality of solutions with different refractive indexes are configured;
(2) When the optical fiber refractive index sensor is placed in the solution, the thin core optical fiber, the coreless optical fiber, the welding parts of the thin core optical fiber and the coreless optical fiber in the optical fiber refractive index sensor and the single mode optical fiber are all in the solution, the thin core optical fiber and the coreless optical fiber are in a straight line state, and the transmission spectrum on the spectrum analyzer is recorded after the broadband light source device is opened;
(3) Repeating the step (2) to detect all the solutions and record the transmission spectrums thereof, selecting interference wave troughs in the same section of each transmission spectrum, and obtaining y=a+bx, namely x= (y-a)/b through linear fitting, wherein y is the central wavelength corresponding to the selected wave trough in the transmission spectrum, a is the central wavelength corresponding to the wave trough when the refractive index approaches 0, b is the sensitivity of the refractive index, and x is the refractive index;
(4) Obtaining an unknown refractive index solution, placing an optical fiber refractive index sensor in the unknown refractive index solution, wherein the thin core optical fiber, the coreless optical fiber, the fusion joint of the thin core optical fiber and the coreless optical fiber in the optical fiber refractive index sensor and a single mode optical fiber are all positioned in the unknown refractive index solution, the central lines of the thin core optical fiber and the coreless optical fiber are positioned on the same straight line, recording the transmission spectrum on a spectrum analyzer after a broadband light source device is turned on, selecting a trough corresponding to the same section of the transmission spectrum in the step (2), obtaining a central wavelength value for the optical fiber, and substituting the central wavelength value into a formula x= (y-a)/b to obtain the refractive index of the solution.
As optimization, the optical fiber fixing device and the two optical fiber brackets are obtained, in the step (2) and the step (4), when a transmission spectrum diagram of a solution is obtained, the optical fiber refractive index sensor is firstly penetrated through the two fixing rings, the welding positions of the thin core optical fiber, the coreless optical fiber, the thin core optical fiber and the coreless optical fiber and the single mode optical fiber are respectively positioned between the two fixing rings, then the fixing base is placed in the solution, then the two optical fiber brackets are respectively correspondingly erected at the two single mode optical fibers outside the solution, the body of the single mode optical fiber is arranged on the inner bottom surfaces of the two U-shaped brackets at the corresponding side, then the balancing weight is hung on the body of the single mode optical fiber and between the two U-shaped brackets at the same side, the thin core optical fiber and the coreless optical fiber are in a straightened state by utilizing the gravity of the balancing weight, and finally the transmission spectrum on the spectrum analyzer is recorded after the broadband light source device is opened.
In summary, the beneficial effects of the invention are as follows: the optical fiber sensing detection device has the advantages of simple structure, low cost, high refractive index sensitivity for solution detection and larger refractive index detection range.
Drawings
For the purpose of making the objects, technical solutions and advantages of the present invention more apparent, the present invention will be described in further detail with reference to the accompanying drawings, in which:
FIG. 1 is a schematic diagram of a fusion splice structure of an optical fiber refractive index sensor according to the present invention;
FIG. 2 is a graph showing the transmission spectrum of a solution in a combination mode of a 9.5cm length of a thin core optical fiber and an 8.5cm length of a coreless optical fiber in an embodiment of the present invention;
FIG. 3 is a graph showing the transmission spectrum of a solution in a combination mode of a thin core fiber having a length of 8.5cm and a coreless fiber having a length of 9.5cm in the examples of the present invention;
FIG. 4 is a graph showing the transmission spectrum of a solution in a combination mode of a thin core fiber having a length of 8.5cm and a coreless fiber having a length of 8.5cm in the examples of the present invention;
FIG. 5 is a graph showing the transmission spectra of corresponding troughs of the optical fiber refractive index sensors in solutions having refractive indices 1.3331, 1.3377, 1.3424, and 1.3470 according to an embodiment of the present invention;
FIG. 6 is a graph showing transmission spectra of corresponding troughs of optical fiber refractive index sensors in solutions having refractive indices of 1.3516, 1.3562, 1.3609 and 1.3655 according to an embodiment of the present invention;
FIG. 7 is a graph showing the transmission spectra of corresponding troughs of the optical fiber refractive index sensor in solutions having refractive indices of 1.3701, 1.3747 and 1.3794 according to an embodiment of the present invention;
FIG. 8 is a graph showing the transmission spectra of the optical fiber refractive index sensor in solutions at 15℃at 25℃at 35℃and at 45℃in the examples of the present invention;
FIG. 9 is a graph showing the transmission spectra of the optical fiber refractive index sensor in solutions at 55℃at 65℃at 75℃and at 85℃in the examples of the present invention;
FIG. 10 is a graph showing transmission spectra of sodium chloride solution, alcohol, sucrose solution and glycerin at the same refractive index of an optical fiber refractive index sensor according to an embodiment of the present invention.
Detailed Description
The present invention will be described in further detail with reference to the accompanying drawings.
The optical fiber sensing detection device for detecting the solution refractive index in the specific embodiment comprises a broadband light source device, an optical fiber refractive index sensor and a spectrum analyzer, wherein the optical fiber refractive index sensor comprises a thin core optical fiber 2, a coreless optical fiber 3 and two single-mode optical fibers 1, the fiber core diameter of the thin core optical fiber 2 is smaller than that of the single-mode optical fiber 1, one end of the thin core optical fiber 2 is welded with one end of the coreless optical fiber 3, one end of one single-mode optical fiber 1 is welded with one end of the thin core optical fiber 2, which is far away from the coreless optical fiber 3, the other end of the single-mode optical fiber 1 is connected with one end of the coreless optical fiber 3, which is far away from the coreless optical fiber 2, the other end of the single-mode optical fiber 1 is connected with the input end of the spectrum analyzer, and the thin core optical fiber 2 and the coreless optical fiber 3 are in a straight line state.
In this embodiment, the optical fiber refractive index sensor just is located the thin core optic fibre with coreless optic fibre department is provided with optical fiber fixing device, and optical fiber fixing device includes unable adjustment base, and unable adjustment base's upper surface separates and fixedly connected with two fixed rings that set up with the central line, and the central line of fixed ring is the level setting, optical fiber refractive index sensor passes in two fixed rings, thin core optic fibre, coreless optic fibre and coreless optic fibre respectively with the butt fusion department of single mode fiber all is located the position between two fixed rings, unable adjustment base just is located along the outside of two fixed rings looks direction respectively and is provided with the optical fiber support, and the top of optical fiber support is provided with two U-shaped brackets that are located same horizontal direction relatively, and the plane mutual parallel arrangement of U-shaped bracket place, and the level of U-shaped bracket is higher than the level of fixed ring, and the bottom fixedly connected with branch of U-shaped bracket just is fixed on its place optical fiber support through branch, two respectively pass through the inside two U-shaped brackets of its place on one side and place single mode fiber, two can draw the pull wire to carry out directly to the optical fiber between two on the same side of fiber.
The sensor head consists of three parts, and is formed by welding a single-mode fiber, a thin-core fiber and a coreless fiber, and can be expressed as follows assuming that only a core mode and one of main cladding modes are considered
Wherein I is core Is the intensity of light propagating in the core;
is the intensity of light propagating in the cladding. Phi (phi) k For light propagation in core and claddingPhase difference
Where L is the sensor response length, taking into account φ k ＝(2k+1)π，
From equation (2) it can be deduced
Assume that
Along with delta n eff Changing with variation, peak wavelength n th The attenuation dip is expressed as
The Mach-Zehnder interference free spectral range of the FSR is shown as follows
The effective index of the core remains unchanged as the surrounding index of refraction changes. The effective refractive index of the cladding changes, and the phase difference changes, thereby causing the wavelength of the characteristic peak to shift. Then by measuring the wavelength shift the change in the surrounding refraction can be determined.
For temperature measurement of the sensor, both the effective refractive index and the sensor response length L are independent of temperature due to thermal expansion response and thermal effects. The temperature change and the wavelength relationship are expressed as
Wherein the method comprises the steps of
Is a thermo-optic coefficient, namely xi; />
The thermal expansion coefficient is alpha f. Thus, equation (6) is reduced to
The transmission spectra of the sensing fiber structures of the combined modes of the thin core fiber and the coreless fiber of different lengths are shown in fig. 1 to 3, and it can be seen from fig. 1 to 3 that the free spectral range decreases with the increase of the length of the interference arm, and the extinction ratio is inversely proportional to the length of the interference arm. This structure is identical to theoretical formulas (3) and (5). Meanwhile, it can be presumed from the formula (2) that the sensitivity of the sensor is mainly based on the effective length of the sensor and the elapsed condition of the transmission mode. Generally, the longer the sensor length, the higher the number of cladding modes in the evanescent wave, the higher the sensor sensitivity. In this embodiment, an optical fiber structure is selected in which both the thin core optical fiber and the coreless optical fiber are 8.5 cm.
For the investigation of the refractive index characteristics of the liquid, the room temperature was first set at 28 ℃ and the temperature was controlled to be stable. The thin core optical fiber and the coreless optical fiber are ensured to be in a straight line placement state all the time, and bending loss is eliminated. By changing the refractive index value of the solution, the drift of the interference wavelength was recorded as shown in fig. 4 to 6, so as to analyze the sensing characteristics thereof. And selecting an interference trough with the wavelength of around 1543nm to realize refractive index measurement. The refractive index liquid is prepared by sodium chloride solutions with different concentrations, and the refractive index value of the solution is 1.3331 to 1.37935. After each measurement of the sample, the portion of the fiber refractive index sensor immersed in the solution was repeatedly washed with deionized water, and then dried to a constant weight. Experimental results show that the wavelength of the characteristic peak of 1543nm wavelength is red-shifted with the increase of the external refractive index and passes through the linearityThe fitting gave refractive index sensitivities of 101.94462nm/RIU, respectively. The refractive index sensitivity for the selected characteristic peaks is different because they are formed by the core mode and the different cladding modes, respectively, to form corresponding interference fringes, and the propagation coefficient and excitation coefficient of each cladding mode are different, so that each characteristic peak will be different. It is known that temperature changes affect refractive index measurement, and if both changes at the same time, larger deviation of experimental results can be caused, which affects the reliability of test data. As shown in FIG. 7 and FIG. 8, the transmission spectrum of the response of the experimental temperature to the sensing performance shows that the maximum wavelength shift of 0.3nm is only caused when the temperature change range is 15-85 ℃, and the characteristic peak temperature sensitivity of 1543nm wavelength is only-0.00348 nm/DEG C. Since the sensitivity of temperature is closely related to the thermo-optic coefficient of the fiber itself, the greater the thermo-optic coefficient, the more sensitive the fiber to temperature changes. The coreless fiber is an optical fiber composed of quartz, so that the thermooptical coefficient is low, the coreless fiber can interact with a single-mode fiber to excite the multi-order cladding modes, the multi-order cladding modes form multi-mode interference due to different propagation coefficients, and the multi-order cladding modes can be continuously re-excited and recombined, so that the coreless fiber is insensitive to external temperature response. Namely, when the temperature is changed by 1 ℃, the maximum change amount corresponding to the refractive index is 4.4186 multiplied by 10 -5 The measurement effect of temperature on refractive index can be ignored within the tolerance range.
The response of the optical fiber refractive index sensor to different substances is explored, solutions of different solutes under the same refractive index are prepared, solutions with the refractive index of 1.3516 are prepared by using alcohol, salt, sucrose and glycerin respectively, the obtained transmission spectra are shown in figure 9, the characteristic wavelengths of the transmission spectra of the sensor are consistent under the same refractive index, the energy of the interference minima is different due to the fact that light propagates in an interference arm, evanescent waves can be generated on the surface of the interference arm to enter the external solution, the absorption effect of the evanescent waves by different solutes is different, and the energy difference exists in the interference minima of the output light spectrum.
To verify the stability of the sensor, the thin core optical fiber and the coreless optical fiber in the optical fiber refractive index sensor were immersed in a sodium chloride aqueous solution with a mass concentration of 15% at room temperature for 1 hour, and the wavelength was recorded every 10 minutes. The standard deviation of the characteristic peak at 1543nm wavelength was calculated from the transmission spectrum within 1 hour and found to be 0.007666 nm, respectively. This deviation is mainly caused by the stability of the light source and the fiber refractive index sensor and temperature variations. Through optimizing the fusion parameters of the optical fibers, the optical fiber sensor can have better coupling points, and the stability of the sensor is improved.
A method for detecting the refractive index of a solution comprises the following steps:
(1) The optical fiber sensing detection device for detecting the refractive index of the solution is obtained, and a plurality of solutions with different refractive indexes are configured;
(2) When the optical fiber refractive index sensor is placed in the solution, the thin core optical fiber, the coreless optical fiber, the welding parts of the thin core optical fiber and the coreless optical fiber in the optical fiber refractive index sensor and the single mode optical fiber are all in the solution, the thin core optical fiber and the coreless optical fiber are in a straight line state, and the transmission spectrum on the spectrum analyzer is recorded after the broadband light source device is opened;
(3) Repeating the step (2) to detect all the solutions and record the transmission spectrums thereof, selecting interference wave troughs in the same section of each transmission spectrum, and obtaining y=a+bx, namely x= (y-a)/b through linear fitting, wherein y is the central wavelength corresponding to the selected wave trough in the transmission spectrum, a is the central wavelength corresponding to the wave trough when the refractive index approaches 0, b is the sensitivity of the refractive index, and x is the refractive index;
(4) Obtaining an unknown refractive index solution, placing an optical fiber refractive index sensor in the unknown refractive index solution, wherein the thin core optical fiber, the coreless optical fiber, the fusion joint of the thin core optical fiber and the coreless optical fiber in the optical fiber refractive index sensor and a single mode optical fiber are all positioned in the unknown refractive index solution, the central lines of the thin core optical fiber and the coreless optical fiber are positioned on the same straight line, recording the transmission spectrum on a spectrum analyzer after a broadband light source device is turned on, selecting a trough corresponding to the same section of the transmission spectrum in the step (2), obtaining a central wavelength value for the optical fiber, and substituting the central wavelength value into a formula x= (y-a)/b to obtain the refractive index of the solution.
In this embodiment, the optical fiber fixing device and the two optical fiber holders are obtained in the step (2) and the step (4), when the transmission spectrum of the solution is obtained, the optical fiber refractive index sensor is first passed through the two fixing rings, the fusion joints of the thin core optical fiber, the coreless optical fiber and the thin core optical fiber with the single mode optical fiber are respectively located between the two fixing rings, then the fixing base is placed in the solution, then the two optical fiber holders are respectively and correspondingly erected at the two single mode optical fibers outside the solution, the body of the single mode optical fiber is placed on the inner bottom surfaces of the two U-shaped holders at the corresponding side, then the balancing weight is hung on the body of the single mode optical fiber and between the two U-shaped holders at the same side, the thin core optical fiber and the coreless optical fiber are in a straightened state by using the gravity of the balancing weight, and finally the transmission spectrum on the analyzer is recorded after the broadband light source device is opened.
Corresponding transmission spectra were obtained by measuring sodium chloride solutions of different concentrations (refractive index values from 1.3331 to 1.37935), selecting the trough between 1551nm and 1557nm in the spectrogram, and obtaining y=a+bx by linear fitting using origin software, fitting coefficient R 2 Refractive index sensitivity was 101.94462nm/RIU, i.e. x= (y-1415.3405)/101.94462, = 0.98447.
And measuring sodium chloride solution with unknown refractive index to obtain a transmission spectrum, selecting the central wavelength of a corresponding characteristic peak to be 1552.19nm, and calculating according to a formula to obtain the refractive index of the solution to be 1.3424.
The transmission spectrum is measured on the sucrose solution with unknown refractive index, the central wavelength of the corresponding characteristic peak is selected to be 1554.55nm, and the refractive index of the solution is 1.3655 according to the formula.
The transmission spectrum is measured on the glycerin solution with unknown refractive index, the central wavelength of the corresponding characteristic peak is selected to be 1555.48nm, and the refractive index of the solution is 1.3747 according to the formula.
Finally, it is noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be understood that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.
Claims (3)
1. The utility model provides a fiber sensing detection device that solution refracting index detected which characterized in that: the optical fiber refractive index sensor comprises a thin core optical fiber, a coreless optical fiber and two single mode optical fibers, wherein the fiber core diameter of the thin core optical fiber is smaller than that of the single mode optical fiber, one end of the thin core optical fiber is welded with one end of the coreless optical fiber, one end of one single mode optical fiber is welded with one end of the thin core optical fiber far away from the coreless optical fiber, the other end of the single mode optical fiber is connected with the output end of the broadband light source device, one end of the other single mode optical fiber is welded with one end of the coreless optical fiber far away from the thin core optical fiber, the other end of the other single mode optical fiber is connected with the input end of the spectrum analyzer, and the thin core optical fiber and the coreless optical fiber are in a straight line state;
the optical fiber fixing device comprises a fixing base, two fixing rings which are arranged on the same central line are arranged on the upper surface of the fixing base in a spaced mode and fixedly connected with the fixing rings, the central line of each fixing ring is horizontally arranged, the optical fiber refractive index sensor penetrates through the two fixing rings, the positions, between the welding positions of the thin core optical fiber and the coreless optical fiber and the single-mode optical fiber, of the two fixing rings are respectively located at the positions between the two fixing rings, optical fiber supports are respectively arranged on the outer sides of the fixing base in the opposite directions of the two fixing rings, the tops of the optical fiber supports are relatively provided with two U-shaped supports which are located on the same horizontal direction, the planes of the two U-shaped supports are mutually parallel, the horizontal height of the U-shaped supports is higher than that of the fixing rings, the bottoms of the U-shaped supports are fixedly connected with supporting rods, the U-shaped supports are fixed on the optical fibers of the supports through the supporting rods, the two single-mode optical fibers penetrate through the two U-shaped supports respectively at one side of the supporting rods, and the two single-mode optical fibers are placed in the two U-shaped supports, and the two single-mode optical fibers are placed on the bottom surfaces of the two single-mode optical fibers in the body, and the single-mode optical fibers are placed on the two single-mode optical fibers and pulled on the two sides of the single-mode optical fibers.
2. A method for detecting the refractive index of a solution, the method using the optical fiber sensing detection device for detecting the refractive index of a solution according to claim 1, characterized in that: the method comprises the following steps:
(1) Acquiring an optical fiber sensing detection device for detecting the refractive index of the solution, and configuring a plurality of solutions with different refractive indexes;
(2) When the optical fiber refractive index sensor is placed in the solution, the thin core optical fiber, the coreless optical fiber, the welding parts of the thin core optical fiber and the coreless optical fiber in the optical fiber refractive index sensor and the single mode optical fiber are all in the solution, the thin core optical fiber and the coreless optical fiber are in a straight line state, and the transmission spectrum on the spectrum analyzer is recorded after the broadband light source device is opened;
(3) Repeating the step (2) to detect other solutions and record transmission spectrums thereof, selecting interference troughs in the same section of each transmission spectrum, and obtaining y=a+bx, namely x= (y-a)/b through linear fitting, wherein y is a central wavelength corresponding to the selected trough in the transmission spectrum, a is a central wavelength corresponding to the trough when the refractive index approaches 0, b is the sensitivity of the refractive index, and x is the refractive index;
(4) Obtaining an unknown refractive index solution, placing an optical fiber refractive index sensor in the unknown refractive index solution, wherein the thin core optical fiber, the coreless optical fiber, the fusion joint of the thin core optical fiber and the coreless optical fiber in the optical fiber refractive index sensor and a single mode optical fiber are all positioned in the unknown refractive index solution, enabling the central lines of the thin core optical fiber and the coreless optical fiber to be positioned on the same straight line, recording a transmission spectrum on a spectrum analyzer after a broadband light source device is turned on, selecting a trough corresponding to the same section of the transmission spectrum in the step (2), obtaining a corresponding central wavelength value, and substituting the central wavelength value into a formula x= (y-a)/b to obtain the refractive index of the solution.
3. The method for detecting refractive index of a solution according to claim 2, wherein: in the step (2) and the step (4), when the transmission spectrum of the solution is obtained, the optical fiber refractive index sensor is firstly passed through the two fixing rings, so that the welding positions of the thin core optical fiber, the coreless optical fiber, the thin core optical fiber and the coreless optical fiber are respectively positioned between the two fixing rings and the single-mode optical fiber, then the fixing base is placed in the solution, then the two optical fiber brackets are respectively and correspondingly erected at the two single-mode optical fibers outside the solution, the body of the single-mode optical fiber is arranged on the inner bottom surfaces of the two U-shaped brackets on the corresponding side, then the balancing weight is hung on the fiber body of the single-mode optical fiber positioned between the two U-shaped brackets on the same side, the thin core optical fiber and the coreless optical fiber are in a straightened state by utilizing the gravity of the balancing weight, and finally, the transmission spectrum on the spectrum analyzer is recorded after the broadband light source device is opened.
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