Refractive index fiber sensor based on cascaded structure of double peanut-shaped and etched multimode fiber

Abstract

The application discloses a refractive index fiber sensor based on a double peanut-shaped and etched multimode fiber cascade structure, which comprises a multimode fiber, an incident end single-mode fiber and an emission end single-mode fiber, the spherical structure at one end of the multimode fiber is fused with the spherical structure of the incident end single-mode fiber to form a peanut-shaped structure I, the spherical structure at the other end of the multimode fiber is fused with the spherical structure of the emission end single-mode fiber to form a peanut-shaped structure II, and a corrosion tapered zone is arranged at the middle part of the length direction of the multimode fiber, and the refractive index fiber sensor is simple to manufacture, compact in structure and low in cost, realizes the stability of light transmission, and is lower in temperature cross sensitivity.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of optical fiber sensor, and particularly relates to a refractive index optical fiber sensor based on a double peanut-shaped and etched multimode optical fiber cascade structure. BACKGROUND

[0002] In recent years, scientists have invested a lot of energy and resources in the research of interferometric fiber optic sensors, and have made research and demonstration in the fields of biological analysis, medical diagnosis, chemical and environmental monitoring, microchip science, gas sensor, etc., and have also achieved good economic benefits and excellent performance. Refractive index is one of the basic parameters representing the performance of materials, and also represents important information of product quality, composition and content, and has important significance in refractive index measurement. In order to improve the sensitivity of the sensor, an effective method is to manufacture a novel sensing structure.

[0003] A Chinese patent with the application number of "2016203542995" discloses a temperature and refractive index sensor based on multimode fiber intermodal interference and FBG, which is characterized in that it comprises an ASE broadband light source (1), a first jumper wire (2), a refractive index and temperature sensor (3), a sensor fixing platform (4), a second jumper wire (5), and a spectrometer (6). The refractive index and temperature sensor (3) is a full-fiber intermodal interference sensor combined with FBG, which is composed of a single-mode optical fiber (3-1), a de-coated multimode optical fiber (3-2), an FBG (3-3), and an exit-end single-mode optical fiber (3-4). The refractive index and temperature sensor (3) is straightened and suspended in the inverted triangular groove of the sensor fixing platform (4), the exit-end single-mode optical fiber (3-4) and the incident-end single-mode optical fiber (3-1) are long enough to make the tail fibers exceed the length of the groove, and the groove is sealed at both ends by using epoxy resin glue. The first jumper wire (2) is connected to the incident-end single-mode optical fiber (3-1) of the refractive index and temperature sensor (3), and the second jumper wire (5) is connected to the exit-end single-mode optical fiber (3-4) of the refractive index and temperature sensor (3) and the spectrometer (6). The technical scheme improves the extinction ratio of the final output spectrum by staggered fusion, and improves the sensitivity to temperature and refractive index to a certain extent by using hydrofluoric acid to de-coat the multimode optical fiber. However, the structure of the fiber double core staggered fusion method is difficult to grasp in the splicing process, which increases the difficulty of production. The fiber grating is very sensitive to temperature changes, so the cross-sensitivity is a key defect. The grating sensor used for simultaneous measurement of temperature and refractive index has serious limitations, and requires expensive manufacturing equipment and strict procedures (including laser light source and phase mask) to manufacture the grating.

[0004] The Chinese patent with the application number "2018212362350" discloses a refractive index sensor, which is characterized by including a photonic crystal fiber and two single-mode fibers, the two ends of the photonic crystal fiber are respectively coupled with a single-mode fiber, and a waist expansion fiber thick taper is formed at the two coupling points respectively; the surface of the photonic crystal fiber is coated with a graphene layer. This technical solution uses the special structure of graphene to have adsorption on molecules in the solution, improving the sensitivity of the refractive index sensor. However, the double-fiber thick taper structure is not conducive to distinguishing spectral information and complex products, and the stability and reliability are poor. Moreover, the graphene layer increases the complexity of the optical fiber, making the installation and maintenance of the optical fiber more difficult.

[0005] The Chinese patent with the application number "2020107108506" discloses an optical fiber refractive index sensor, which is characterized by including: an incident optical fiber for receiving incident light; a first spherical structure arranged in front of the incident optical fiber for dividing the incident light into a first part of light and a second part of light; a connecting optical fiber arranged in front of the first spherical structure for transmitting the first part of light and the second part of light; a fiber side polishing structure arranged in front of the connecting optical fiber for modulating the second part of light in the cladding of the connecting optical fiber by the liquid to be measured; a second spherical structure arranged in front of the fiber side polishing structure for coupling the second part of light in the cladding of the connecting optical fiber back to the core; and an exit optical fiber arranged in front of the second spherical structure for transmitting the light emitted by the second spherical structure to a spectrum analyzer. This technical solution improves the sensitivity of the sensor to a certain extent, but the side polishing structure affects the tensile strength of the optical fiber, the stability of the optical fiber sensor is insufficient, the repeatability is low, and the transmission accuracy is affected.

[0006] The Chinese patent with the application number "2021114428170" discloses a "FBG cascade fiber composite structure temperature-sensitive current elimination sensing device, characterized in that it comprises an ASE (1), a circulator (2), a measurement system (3), a spectrum analyzer (4), demodulation software (5), a computer (6); the measurement system (3) comprises a heating table (3-1), a power supply (3-2), a resistance box (3-3), a conductor rod (3-4), a sensing unit (3-5), a water platform (3-6), wherein: the conductor rod (3-4) and the sensing unit (3-5) are fixedly placed on the heating table (3-1), the conductor rod (3-4) and the sensing unit (3-5) are placed in parallel, the water platform (3-6) is placed on the right side of the heating table (3-1), and the power supply (3-2) and the resistance box (3-3) are placed on the water platform (3-6) to provide and adjust the current for the conductor rod (3-4); the sensing unit (3-5) comprises a single-mode optical fiber (3-5-1) containing an optical fiber bag (3-5-2), a photonic crystal fiber (3-5-3), a multi-mode micro-nano optical fiber (3-5-4), and an FBG (3-5-5) cascade to form a fiber composite structure, wherein the air hole of the photonic crystal fiber (3-5-3) is filled with ethanol chloroform material (3-5-7), and the FBG (3-5-5) is partially attached to the GMM material (3-5-6) to form the sensing unit (3-5); the specific preparation process of the sensing unit (3-5) includes the preparation of the fiber composite structure and the coating of the sensitive material. The cascade PCF provided by the technical solution fills ethanol chloroform material in the PCF, which greatly improves the sensitivity, but also has the disadvantages of high brittleness and low strength. Due to the low refractive index of ethanol chloroform material, the loss of the optical fiber increases, thereby reducing the transmission efficiency of the optical fiber. In addition, the tensile performance of ethanol chloroform material is poor, which will cause the tensile deformation of the optical fiber, thereby affecting the performance of the optical fiber and the cascade photonic crystal fiber and the taper. The taper structure is divided into two kinds, mechanical heating method and chemical corrosion method. The existing taper method preferably uses mechanical heating taper to obtain excellent taper type, but the cost is high, so the production cost of the sensor is high, the structure is relatively complex, and the repeatability is low. SUMMARY

[0007] The purpose of the present application is to provide a refractive index fiber sensor based on a double peanut-shaped and etched multi-mode optical fiber cascade structure, which is simple to manufacture, compact in structure, and low in cost. It not only realizes the stability of optical transmission, but also has lower temperature cross-sensitivity, solving the problems in the prior art.

[0008] To achieve the above-mentioned purpose, the present application adopts the following technical solutions:

[0009] The refractive index fiber sensor based on double peanut-shaped and etched multimode fiber cascade structure comprises a multimode fiber, an incident end single-mode fiber arranged at one end of the multimode fiber, and an outgoing end single-mode fiber arranged at the other end of the multimode fiber, both ends of the multimode fiber are spherical structures, the incident end single-mode fiber and the outgoing end single-mode fiber are both spherical structures close to one end of the multimode fiber, the spherical structure at one end of the multimode fiber is fused with the spherical structure of the incident end single-mode fiber to form a peanut-shaped structure I, and the spherical structure at the other end is fused with the spherical structure of the outgoing end single-mode fiber to form a peanut-shaped structure II, and a corrosion taper region is arranged at the middle part of the length direction of the multimode fiber.

[0010] Further, the distance between the coupling point of the peanut-shaped structure I and the coupling point of the peanut-shaped structure II is 1.17-1.22 cm.

[0011] Further, the diameter of the spherical structure is 131.20-131.80 μm.

[0012] Further, the waist taper diameter of the corrosion taper region is 51-80 μm.

[0013] Further, the corrosion solution of the multimode fiber is hydrofluoric acid.

[0014] Further, the preparation steps of the refractive index fiber sensor are as follows:

[0015] Step S1, the two ends of the multimode fiber are pre-fused by a commercial fusion machine to create a spherical structure;

[0016] Step S2, the incident end single-mode fiber is pre-fused close to one end of the multimode fiber to create a spherical structure, and the spherical structure of the incident end single-mode fiber is fused with the spherical structure at one end of the multimode fiber to form a peanut-shaped structure I;

[0017] Step S3, the outgoing end single-mode fiber is pre-fused close to one end of the multimode fiber to create a spherical structure, and the spherical structure of the outgoing end single-mode fiber is fused with the spherical structure at the other end of the multimode fiber to form a peanut-shaped structure II, thereby obtaining a double peanut-shaped fiber sensor;

[0018] Step S4, the two ends of the double peanut-shaped fiber sensor in step S3 are fixed on a polytetrafluoroethylene (PTFE) plate, the double peanut-shaped fiber sensor is kept parallel to the polytetrafluoroethylene plate with a distance of 1.5 mm, 150 μL of hydrofluoric acid solution is dropped at the middle position of the multimode fiber, and the spectrum is detected online by connecting the spectrometer through the outgoing end single-mode fiber until the ideal interference spectrum is obtained, and the corrosion taper region is completed.

[0019] Step S5, the remaining hydrofluoric acid solution is removed from the double-peanut-shaped optical fiber sensor after etching in step S4, and the etched tapered area is repeatedly washed with deionized water for 1-2 min;

[0020] Step S6, the washed double-peanut-shaped optical fiber sensor in step S5 is soaked in alcohol for 2 min, and then washed with deionized water for 1-2 min to obtain a refractive index optical fiber sensor based on a double-peanut-shaped and etched multimode optical fiber cascade structure.

[0021] Further, the mass percentage of hydrofluoric acid in step S4 is 40%.

[0022] The beneficial effects of the present application are as follows:

[0023] 1. The technical solution provided by the present application realizes a simple and low-cost Mach-Zehnder interferometer (Mach-Zehnder interferometer), which is simple to manufacture, compact in structure, and low in cost. The double-peanut-shaped structure can excite high-order cladding modes and couple light between core modes and cladding modes, realizing the propagation of light in the core and the cladding. The double-peanut-shaped structure is more stable and has high repeatability.

[0024] 2. The middle part of the multimode optical fiber is provided with an etched tapered area which is etched by a hydrofluoric acid solution, so that high-order cladding modes can be better excited. The chemical etching method is easy to manufacture and simple in method. A new type of tapered structure is manufactured, and the leakage amount of evanescent waves is increased, so that the sensor is more stable and reliable. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 The present application provides a schematic diagram of the overall structure of a refractive index optical fiber sensor based on a double-peanut-shaped and etched multimode optical fiber cascade structure.

[0026] Figure 2 The present application provides a topographic map of the schematic diagram of the overall structure of a refractive index optical fiber sensor based on a double-peanut-shaped and etched multimode optical fiber cascade structure.

[0027] Figure 3 The present application provides a schematic diagram of the experimental equipment connection of a refractive index optical fiber sensor based on a double-peanut-shaped and etched multimode optical fiber cascade structure.

[0028] The figure annotation explanation: 10, multimode optical fiber; 20, incident end single-mode optical fiber; 30, outgoing end single-mode optical fiber; 40, peanut-shaped structure I; 50, peanut-shaped structure II; 60, etched tapered area. DETAILED DESCRIPTION

[0029] In order to make the above objectives, features and advantages of the present application more obvious and easy to understand, the specific embodiments of the present application are described in detail below with reference to the drawings. In the following description, a large number of specific details are set forth in order to provide a thorough understanding of the present application. However, the present application can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without departing from the concept of the present application, so the present application is not limited to the specific implementations disclosed below.

[0030] The implementation is as follows:

[0031] As shown in the refractive index fiber sensor based on double peanut-shaped and etched multimode fiber cascade structure, Figure 1 , Figure 2 The refractive index fiber sensor based on double peanut-shaped and etched multimode fiber cascade structure includes a multimode fiber 10, an incident end single-mode fiber 20 arranged at one end of the multimode fiber 10, and an exit end single-mode fiber 30 arranged at the other end of the multimode fiber 10. Both ends of the multimode fiber 10 are in a spherical structure. The incident end single-mode fiber 20 and the exit end single-mode fiber 30 are both in a spherical structure near one end of the multimode fiber 10. The diameters of the spherical structures are both 131.20 μm to 131.80 μm, and the dimensional error is less than 1.29%. The spherical structure at one end of the multimode fiber 10 is fused with the spherical structure of the incident end single-mode fiber 20 to form a peanut-shaped structure I 40. The spherical structure at the other end is fused with the spherical structure of the exit end single-mode fiber 30 to form a peanut-shaped structure II 50. The middle part of the multimode fiber 10 in the length direction is etched using hydrofluoric acid (HF) to obtain an etched tapered region 60. The distance between the coupling point of the peanut-shaped structure I 40 and the coupling point of the peanut-shaped structure II 50 is 1.2 cm. The waist diameter of the etched tapered region 60 is 51 μm to 80 μm.

[0032] Example 1:

[0033] The preparation steps of the refractive index fiber sensor based on double peanut-shaped and etched multimode fiber cascade structure are as follows:

[0034] Step S1, the ends of the multimode fiber are pre-fused by a commercial fusion machine to create a spherical structure. The diameters of the spherical structures are 131.45 μm and 131.66 μm, respectively.

[0035] Step S2, the incident end single-mode fiber is pre-fused near one end of the multimode fiber to create a spherical structure. The diameter of the spherical structure is 131.74 μm. The spherical structure of the incident end single-mode fiber is fused with the spherical structure at one end of the multimode fiber to form a peanut-shaped structure I.

[0036] Step S3, pre-fuse the end of the exit single-mode optical fiber close to the end of the multi-mode optical fiber to create a spherical structure with a diameter of 131.25 μm, fuse the spherical structure of the multi-mode optical fiber away from the end of the incident single-mode optical fiber in step 2 with the spherical structure of the exit single-mode optical fiber to form a peanut-shaped structure II, and obtain a double peanut-shaped optical fiber sensor;

[0037] Step S4, fix the double peanut-shaped optical fiber sensor in step S3 at both ends on a polytetrafluoroethylene (PTFE) plate with hot melt adhesive. Due to the hydrophobic nature of polytetrafluoroethylene, the hydrofluoric acid solution is in the form of a hemisphere on the experimental plate, the double peanut-shaped optical fiber sensor is kept parallel to the polytetrafluoroethylene plate with a distance of 1.5 mm, 150 μL of hydrofluoric acid solution with a mass percentage concentration of 40% is dropped at the middle position of the multi-mode optical fiber, and the spectrum is detected online by connecting the spectrometer with the exit single-mode optical fiber to detect the interference spectrum after the corrosion of the multi-mode optical fiber until an ideal interference spectrum is obtained, and the corrosion of the conical region is completed. The waist diameter of the conical region is 75.96 μm;

[0038] Step S5, remove the remaining hydrofluoric acid solution from the double peanut-shaped optical fiber sensor after corrosion in step S4, and repeatedly flush the conical region with deionized water for 1 min;

[0039] Step S6, immerse the washed double peanut-shaped optical fiber sensor in step S5 in alcohol for 2 min, and then wash it with deionized water for 2 min to obtain a refractive index optical fiber sensor based on a double peanut-shaped and etched multi-mode optical fiber cascade structure.

[0040] Example 2:

[0041] The preparation steps of the refractive index optical fiber sensor based on a double peanut-shaped and etched multi-mode optical fiber cascade structure are as follows:

[0042] Step S1, pre-fuse the ends of the multi-mode optical fiber by a commercial fusion machine to create a spherical structure with diameters of 131.34 μm and 131.69 μm, respectively;

[0043] Step S2, pre-fuse the end of the incident single-mode optical fiber close to the end of the multi-mode optical fiber by a commercial fusion machine to create a spherical structure with a diameter of 131.73 μm, and fuse the spherical structure of the incident single-mode optical fiber with the spherical structure at the end of the multi-mode optical fiber by a commercial fusion machine to form a peanut-shaped structure I;

[0044] Step S3, pre-fuse the end of the exit single-mode optical fiber close to the end of the multi-mode optical fiber by a commercial fusion machine to create a spherical structure with a diameter of 131.25 μm, fuse the spherical structure of the multi-mode optical fiber away from the end of the incident single-mode optical fiber in step 2 with the spherical structure of the exit single-mode optical fiber by a commercial fusion machine to form a peanut-shaped structure II, and obtain a double peanut-shaped optical fiber sensor;

[0045] Step S4, the double peanut-shaped optical fiber sensor in step S3 is fixed on a polytetrafluoroethylene (PTFE) plate at both ends with a hot melt adhesive. Due to the hydrophobic nature of polytetrafluoroethylene, the hydrofluoric acid solution forms a hemisphere on the experimental plate, keeping the double peanut-shaped optical fiber sensor parallel to the polytetrafluoroethylene plate at a distance of 1.5 mm. 150 μL of 40% hydrofluoric acid solution is dropped at the middle position of the multi-mode optical fiber, and the spectrum is detected online by connecting the spectrometer through the exit end single-mode optical fiber until the ideal interference spectrum is obtained. The corrosion cone diameter of the corrosion cone region is 61.54 μm;

[0046] Step S5, the remaining hydrofluoric acid solution is removed from the double peanut-shaped optical fiber sensor after corrosion in step S4, and the corrosion cone region is repeatedly washed with deionized water for 1 min;

[0047] Step S6, the washed double peanut-shaped optical fiber sensor in step S5 is immersed in alcohol for 2 min, then washed with deionized water for 2 min, to obtain a refractive index optical fiber sensor based on a double peanut-shaped and etched multi-mode optical fiber cascade structure.

[0048] Example 3:

[0049] The preparation steps of the refractive index optical fiber sensor based on a double peanut-shaped and etched multi-mode optical fiber cascade structure are as follows:

[0050] Step S1, the two ends of the multi-mode optical fiber are pre-fused by a commercial fusion machine to create a spherical structure, and the spherical structure diameters are 131.49 μm and 131.77 μm, respectively;

[0051] Step S2, the incident end single-mode optical fiber is pre-fused close to one end of the multi-mode optical fiber to create a spherical structure, and the spherical structure diameter is 131.73 μm. The spherical structure of the incident end single-mode optical fiber is fused with the spherical structure at one end of the multi-mode optical fiber to form a peanut-shaped structure I;

[0052] Step S3, the exit end single-mode optical fiber is pre-fused close to one end of the multi-mode optical fiber to create a spherical structure, and the spherical structure diameter is 131.25 μm. The spherical structure of the multi-mode optical fiber away from the incident end single-mode optical fiber in step 2 is fused with the spherical structure of the exit end single-mode optical fiber to form a peanut-shaped structure II, to obtain a double peanut-shaped optical fiber sensor;

[0053] Step S4: Fix both ends of the dual peanut-shaped fiber optic sensor from step S3 to a polytetrafluoroethylene (PTFE) plate with hot melt adhesive. Due to the hydrophobic properties of PTFE, the hydrofluoric acid solution forms a hemispherical shape on the experimental plate. Keep the dual peanut-shaped fiber optic sensor parallel to the PTFE plate and at a distance of 1.5 mm. Drop 150 μL of a 40% hydrofluoric acid solution at the middle position of the multimode fiber. At the same time, use a spectrometer connected to the single-mode fiber at the output end to detect the interference spectrum of the multimode fiber after corrosion online until the ideal interference spectrum is obtained. The corrosion cone region is then completed. The waist cone diameter of the corrosion cone region is 51.92 μm.

[0054] Step S5: Remove the remaining hydrofluoric acid solution from the double peanut-shaped fiber optic sensor after corrosion in step S4, and rinse the corroded cone area repeatedly with deionized water for 1 minute.

[0055] Step S6: Immerse the rinsed double peanut-shaped optical sensor from step S5 in alcohol for 2 minutes, then rinse with deionized water for 2 minutes to obtain a refractive index optical fiber sensor based on a double peanut-shaped and etched multimode fiber cascade structure.

[0056] Experimental demonstration

[0057] 1) Experimental equipment and instruments: semiconductor optical amplifier, spectrometer, refractive index fiber optic sensor based on double peanut-shaped and etched multimode fiber cascade structure, glycerol solutions of various concentrations, glycerol measuring cup, dropper, optical slide, temperature control box;

[0058] 2) Experimental Procedure: The test solutions were glycerol solutions of various concentrations with refractive indices ranging from 1.3395 to 1.4200, as measured using an Abbe refractometer. Four groups were prepared as follows: Figure 3 The experimental setup shown includes three sets of refractive index fiber sensors based on a double peanut-shaped and etched multimode fiber cascade structure. The waist cone diameters of the etched conical regions are 75.96 μm, 61.54 μm, and 51.92 μm, respectively. These three fiber sensors are named Sensor #2, Sensor #3, and Sensor #4. A control group, Sensor #1, is a refractive index fiber sensor made of unetched multimode fiber with a waist cone diameter of 125 μm. The purpose of this control group is to investigate the effect of the waist cone diameter in the etched conical region of the sensor on the refractive index, temperature cross-sensitivity, and stability.

[0059] Sensitivity experiment of refractive index fiber optic sensor

[0060] Prepare four sets as follows Figure 3The fiber optic sensor shown has a single-mode fiber optic input connected to a broadband light source and a single-mode fiber optic output connected to a spectrometer. The fiber optic sensor is horizontally fixed on an optical slide. At a stable room temperature of 25°C, glycerol solutions of the same concentration are added to the four fiber optic sensors using a dropper for five minutes each time. Because light will couple in the sensing structure and cause interference, interference peaks will appear in the spectrometer. Therefore, the spectrometer records the data information of the four fiber optic sensors with different waist cone diameters in glycerol solutions of different concentrations.

[0061] The sensitivity experimental data of the refractive index fiber optic sensor are shown in the table below:

[0062]

[0063] The experimental results are as follows:

[0064] The fiber optic sensor detected wavelength offsets of 8.8 nm to 16.8 nm at tilt angle A and 8.8 nm to 17.6 nm at tilt angle B for glycerol solutions ranging from 1.3395 to 1.3945. For glycerol solutions ranging from 1.3945 to 1.4200, the fiber optic sensor detected wavelength offsets of 8.0 nm (minimum offset) to 20.8 nm (maximum offset) at tilt angle A and 10.4 nm to 17.6 nm at tilt angle B. Sensors #2, #3, and #4 showed significantly higher sensitivity than the control group sensor #1.

[0065] Temperature cross-sensitivity experiment of refractive index fiber optic sensor

[0066] like Figure 3 The input end of the refractive index light sensor #4 is connected to a broadband light source via a single-mode fiber, and the output end is connected to a spectrometer via a single-mode fiber. The fiber sensor is horizontally fixed on an optical slide and placed in a high and low temperature control box. The temperature is increased from 30℃ to 100℃, and data is recorded every 10℃.

[0067] The experimental data on the temperature cross-sensitivity of the refractive index fiber optic sensor are shown in the table below:

[0068]

[0069] The experimental results are as follows:

[0070] The temperature sensitivity of the refractive index sensor structure at tilt angle A wavelength is 0.03 nm / ℃, and the temperature sensitivity at tilt angle B wavelength is 0.02 nm / ℃. Therefore, within the allowable error range, the influence of temperature cross-sensitivity on the measurement results of the refractive index sensor can be ignored.

[0071] Stability experiment of refractive index fiber optic sensor

[0072] like Figure 3 The input end of the refractive index light sensor #4 is connected to a broadband light source via a single-mode fiber, and the output end is connected to a spectrometer via a single-mode fiber. The fiber optic sensor is horizontally fixed on an optical slide. The optical slide with the fixed fiber optic sensor structure is placed in glycerol solutions with refractive indices of 1.3363 and 1.4008, respectively. The room temperature is stabilized at 25°C, and the dynamic response spectrum is collected every 10 minutes for 2 hours.

[0073] The experimental data on the stability of the refractive index fiber optic sensor are shown in the table below:

[0074]

[0075] The experimental results are as follows:

[0076] The standard deviations of the refractive index for wavelengths at tilt angle A and tilt angle B of sensor #4 at 1.3663 nm are 0.1377 nm and 0.1231 nm, respectively. The standard deviations for the refractive index for wavelengths at tilt angle A and tilt angle B of sensor #4 at 1.4008 nm are 0.1066 nm and 0.1323 nm, respectively. These slight deviations require consideration of the stability of the broadband light source and spectrometer; therefore, this fiber optic refractive index sensor structure exhibits high stability.

[0077] In summary, the refractive index fiber sensor based on a dual peanut-shaped and etched multimode fiber cascade structure provided by this invention is simple to manufacture, compact in structure, and low in cost. It not only achieves stable optical transmission but also has lower temperature cross-sensitivity.

[0078] The foregoing has shown and described the basic principles and main features of the present invention and its advantages. It will be apparent to those skilled in the art that the present invention is not limited to the details of the above exemplary embodiments, and that the present invention can be implemented in other specific forms without departing from the spirit or basic characteristics of the present invention. Therefore, the embodiments should be regarded as exemplary and non-limiting in all respects. The scope of the present invention is defined by the appended claims rather than the foregoing description. Therefore, it is intended that all changes falling within the meaning and scope of the equivalents of the claims be included within the present invention, and no reference numerals in the claims should be regarded as limiting the scope of the claims.

[0079] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A refractive index fiber sensor based on a cascaded structure of double peanut-shaped and etched multimode fiber, comprising a multimode fiber, an incident-end single-mode fiber arranged at one end of the multimode fiber, and an emission-end single-mode fiber arranged at the other end of the multimode fiber, characterized in that: Both ends of the multimode fiber are spherical structures, the incident end single-mode fiber and the exit end single-mode fiber are spherical structures near one end of the multimode fiber, the spherical structure of one end of the multimode fiber is fused with the spherical structure of the incident end single-mode fiber to form a peanut-shaped structure I, and the spherical structure of the other end is fused with the spherical structure of the exit end single-mode fiber to form a peanut-shaped structure II, and the middle part of the length direction of the multimode fiber is provided with an etched taper region. ​ 2. The refractive index fiber sensor based on the cascaded structure of bimodal and etched multimode fiber according to claim 1, characterized in that: The distance between the coupling point of the peanut-shaped structure I and the coupling point of the peanut-shaped structure II is 1.17-1.22 cm.

3. The refractive index fiber sensor based on the cascaded structure of bimodal and etched multimode fiber according to claim 1, characterized in that: The diameter of the spherical structure is 131.20-131.80 μm.

4. The refractive index fiber sensor based on the cascaded structure of bimodal and etched multimode fiber according to claim 1, characterized in that: The waist diameter of the etched taper region is 51-80 μm.

5. The refractive index fiber sensor based on the cascaded structure of bimodal and etched multimode fiber according to claim 4, characterized in that: The etching solution of the multimode fiber is hydrofluoric acid.

6. The refractive index fiber sensor based on the cascaded structure of bimodal and etched multimode fiber according to any one of claims 1 to 5, characterized in that: The preparation steps of the refractive index fiber sensor are as follows: Step S1, the two ends of the multimode fiber are pre-fused by a commercial fusion machine to create a spherical structure; Step S2, the incident end single-mode fiber is pre-fused near one end of the multimode fiber to create a spherical structure, and the spherical structure of the incident end single-mode fiber is fused with the spherical structure of one end of the multimode fiber to form a peanut-shaped structure I; Step S3, the exit end single-mode fiber is pre-fused near one end of the multimode fiber to create a spherical structure, and the spherical structure of the multimode fiber away from the spherical structure of the incident end single-mode fiber at one end is fused with the spherical structure of the exit end single-mode fiber to form a peanut-shaped structure II, to obtain a double peanut-shaped fiber sensor; Step S4, the two ends of the double peanut-shaped fiber sensor in step S3 are fixed on a polytetrafluoroethylene (PTFE) plate, the double peanut-shaped fiber sensor is kept parallel to the polytetrafluoroethylene plate with a distance of 1.5 mm, 150 μL of hydrofluoric acid solution is dropped at the middle position of the multimode fiber, and the spectrum is detected online by connecting the spectrometer through the exit end single-mode fiber until the ideal interference spectrum is obtained, and the etching taper region is completed; Step S5, the remaining hydrofluoric acid solution is removed from the double peanut-shaped fiber sensor after etching in step S4, and the etching taper region is repeatedly washed with deionized water for 1-2 min; Step S6, the washed double peanut-shaped fiber sensor in step S5 is soaked in alcohol for 2 min, and then washed with deionized water for 1-2 min to obtain a refractive index fiber sensor based on a double peanut-shaped and etched multimode fiber cascade structure.

7. The refractive index fiber sensor based on the cascaded structure of bimodal and etched multimode fiber according to claim 6, characterized in that: The mass concentration percentage of hydrofluoric acid in step S4 is 40%.

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