A distributed optical fiber refractive index sensor and device

Abstract

The present invention provides a distributed optical fiber refractive index sensor and device. The sensor includes a single-mode optical fiber, a mode coupler, a multi-mode optical fiber, a mode multiplexer, and a reflection structure connected in sequence. The mode multiplexer has an input port and at least three output ports. The multi-mode optical fiber is connected to the input port. The reflection structure is connected to the three output ports to form three independently arranged optical channels. The multi-mode optical fiber includes a core and a cladding. The core is at least partially exposed outside the cladding to form an open area for contacting an external refractive index liquid. The scanning laser emitted by the light source is injected from the single-mode optical fiber and sequentially transmitted to the reflection structure. After being reflected by the reflection structure, it is transmitted in reverse to be emitted from the single-mode optical fiber. This scheme adopts optical frequency reflection technology, which can simultaneously meet the distributed measurement of large dynamic range and high spatial resolution, and is more sensitive to external refractive index changes. The open area makes the distributed optical fiber refractive index sensor more sensitive.

Description

Technical Field

[0001] The present invention relates to the technical field of optical fiber sensors, and in particular to a distributed optical fiber refractive index sensor and a device. Background Art

[0002] At present, most fiber optic refractive index sensors can only measure the refractive index of a local or fixed point, and have limitations in the simultaneous measurement of multiple target molecules, the microscopic dynamic diffusion of liquids, or the precise measurement of the refractive index gradient of liquids. At the same time, due to the complexity of the biological environment, the simultaneous detection of high-throughput and multi-component biological molecules, the refractive index-based fiber optic biological multi-component measurement is obviously difficult, and there is an urgent need for distributed measurement of the refractive index.

[0003] Among the related technologies, distributed fiber optic sensing technologies include optical time domain reflection technology and optical frequency reflection technology. For optical time domain reflection technology, it is limited by the width of the detector and the pulse light source, and it is difficult to simultaneously meet the requirements of large dynamic range and high spatial resolution for distributed measurement; for optical frequency reflection technology, it mainly uses an interferometer with two arms of unequal length to achieve distributed measurement, and senses the change in the effective refractive index of the fundamental mode in the fiber caused by the external refractive index, and the sensitivity is not high. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a distributed optical fiber refractive index sensor and device, which can at least solve the deficiencies in the related art to a certain extent.

[0005] In order to solve the above technical problems, the first aspect of the present invention provides a distributed optical fiber refractive index sensor, comprising a single-mode optical fiber, a mode coupler, a multi-mode optical fiber, a mode multiplexer, and a reflection structure connected in sequence, wherein the mode multiplexer has an input port and at least three output ports, the multi-mode optical fiber is connected to the input port, and the reflection structure is connected to the three output ports to form three independently arranged optical channels, the multi-mode optical fiber comprises a core and a cladding coated on the outside of the core, and the core is at least partially exposed on the outside of the cladding to form an open area for contacting an external refractive index liquid; the scanning laser emitted by the light source is injected from the single-mode optical fiber and sequentially transmitted to the reflection structure, and after being reflected by the reflection structure, it is reversely transmitted to be emitted from the single-mode optical fiber; wherein the three optical channels respectively transmit a reference optical signal, a base mode optical signal, and a high-order mode optical signal, and there is a difference in the transmission time of the base mode optical signal and the high-order mode optical signal in the corresponding optical channel, and the base mode optical signal and the high-order mode optical signal reflected by the reflection structure generate inter-mode interference in the mode coupler.

[0006] Preferably, the mode multiplexer has a first output port, a second output port and a third output port, and the reflection structure includes: a first optical fiber delay ring connected to the first output port and used to transmit the fundamental mode optical signal, a second optical fiber delay ring connected to the second output port and used to transmit the high-order mode optical signal, and three reflection devices connected to the third output port, the first optical fiber delay ring and the second optical fiber delay ring, respectively, the three reflection devices and the first output port, the second output port and the third output port form three optical channels, and the length of the first optical fiber delay ring is less than the length of the second optical fiber delay ring.

[0007] Preferably, the first fiber optic delay ring and the second fiber optic delay ring are both formed by continuous multiple turns of single-mode optical fiber, and the first fiber optic delay ring and the first output port, the second fiber optic delay ring and the second output port, the first fiber optic delay ring and the reflector, the second fiber optic delay ring and the reflector, and the third output port and the reflector are all connected by single-mode optical fiber.

[0008] Preferably, the open area is formed by performing a taper process or a side-throwing process on the multimode optical fiber.

[0009] Preferably, the multimode optical fiber is an open-core microstructure optical fiber.

[0010] Preferably, the mode coupler comprises any one of a coreless optical fiber and an unaligned fused hetero-core optical fiber, and the end face of the single-mode optical fiber is in contact with the end face of the coreless optical fiber or the end face of the single-mode optical fiber is in contact with the end face of the hetero-core optical fiber.

[0011] Preferably, the length of the coreless optical fiber ranges from 100 μm to 400 μm.

[0012] Preferably, the type of mode multiplexer comprises a photonic lantern.

[0013] A second aspect of the present invention provides a distributed optical fiber refractive index sensing device, comprising a distributed optical fiber refractive index sensor as described in any one of the above items.

[0014] Preferably, the distributed optical fiber refractive index sensing device further comprises a light source, a data acquisition device, and an optical fiber circulator, and the optical fiber circulator is respectively connected to the light source, the distributed optical fiber refractive index sensor, and the data acquisition device.

[0015] Compared with the prior art, the distributed optical fiber refractive index sensor and device of the present invention have the following beneficial effects: a mode coupler can excite a fundamental mode optical signal and a high-order mode optical signal in a multimode optical fiber, and the fundamental mode optical signal is transmitted in the core of the multimode optical fiber, and the high-order mode optical signal is transmitted in the cladding of the multimode optical fiber; a reference optical signal, a fundamental mode optical signal, and a high-order mode optical signal are transmitted in corresponding channels and reflected by a reflection structure, and there is a difference in the transmission time of the fundamental mode optical signal and the high-order mode optical signal in the corresponding optical channel, and the light emitted by the light source is a scanning laser, so that the fundamental mode optical signal and the high-order mode optical signal reflected by the reflection structure can be transmitted to the mode coupler and reflected in the mode coupler. The optical fiber refractive index sensor of the present invention adopts the optical frequency reflection technology, which can simultaneously meet the distributed measurement requirements of large dynamic range and high spatial resolution, and is more sensitive to the external refractive index change; at the same time, the fiber core is at least partially exposed outside the cladding to form an open area for contacting with the external refractive index liquid, so that the energy overlap factor between the distributed optical fiber refractive index sensor and the external refractive index liquid is high, thereby making the distributed optical fiber refractive index sensor more sensitive, and realizing the distributed optical fiber refractive index measurement with higher spatial resolution and higher refractive index sensitivity. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without paying any creative work.

[0017] Figure 1 is a partial structural schematic diagram of a distributed optical fiber refractive index sensor according to an embodiment of the present invention;

[0018] Figure 2 Schematic diagram of a distributed optical fiber refractive index sensing device according to an embodiment of the present invention.

[0019] In the accompanying drawings, each figure mark represents: 1. single-mode optical fiber; 2. mode coupler; 3. multimode optical fiber; 31. core; 32. cladding; 321. open area; 4. mode multiplexer; 5. reflection structure; 51. reflection device; 52. first optical fiber delay loop; 53. second optical fiber delay loop; 10. distributed optical fiber refractive index sensor; 20. light source; 30. data acquisition device; 40. optical fiber circulator. DETAILED DESCRIPTION

[0020] The embodiments of the present invention are described in detail below, and examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present invention, but are not to be construed as limiting the present invention. All other embodiments obtained by ordinary technicians in the field without creative work based on the embodiments of the present invention are within the scope of protection of the present invention.

[0021] In the description of the present invention, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", "axial", "circumferential", "radial" and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the referred device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as limiting the present invention.

[0022] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features. In the description of the present invention, the meaning of "plurality" is two or more, unless otherwise clearly and specifically defined.

[0023] Example:

[0024] See also Figure 1The embodiment of the present invention provides a distributed optical fiber refractive index sensing device, including a distributed optical fiber refractive index sensor 10. The distributed optical fiber refractive index sensing device also includes a light source 20, a data acquisition device 30, and an optical fiber circulator 40, and the optical fiber circulator 40 is respectively connected to the light source 20, the distributed optical fiber refractive index sensor 10, and the data acquisition device 30. Specifically, the optical fiber circulator 40 has three ports, and the light source 20, the distributed optical fiber refractive index sensor 10, and the data acquisition device 30 are respectively connected to the three ports of the optical fiber circulator 40. The light emitted by the light source 20 passes through the optical fiber circulator 40 and then enters the distributed optical fiber refractive index sensor 10, and then is transmitted from the distributed optical fiber refractive index sensor 10 to the optical fiber circulator 40, and finally is transmitted from the optical fiber circulator 40 to the data acquisition device 30. Among them, the light source 20 can be a swept-frequency laser, the sweeping range of the laser wavelength of the swept-frequency laser is in the range of 1500nm-1600nm, the scanning speed is 100nm / s or less, and the output power is above 10mW; the light emitted by the light source 20 enters the distributed optical fiber refractive index sensor 10 to produce Rayleigh scattering phenomenon; the data acquisition device 30 can be a data acquisition card or a spectrometer, and the acquisition speed of the acquisition card is greater than 10M.

[0025] It should be noted that the refractive index is an important physical property of liquid materials. By measuring the refractive index, the content of various components in the liquid can be analyzed. Therefore, the refractive index sensor has a wide range of applications in chemical processes, food production, and biomedicine. In recent years, due to the rapid development of fiber optic sensor technology with its advantages of small structure, light weight, anti-electromagnetic interference, and high sensitivity, various types of fiber optic refractive index sensors have emerged, such as fiber Bragg grating type, fiber interferometer type, fiber plasma resonance type refractive index sensor, etc., and have shown broad application potential.

[0026] See also Figure 1 and Figure 2The distributed optical fiber refractive index sensor 10 includes a single-mode optical fiber 1, a mode coupler 2, a multi-mode optical fiber 3, a mode multiplexer 4, and a reflection structure 5 which are connected in sequence. The mode multiplexer 4 has an input port and at least three output ports. The multi-mode optical fiber 3 is connected to the input port. The reflection structure 5 is connected to the three output ports to form three independently arranged optical channels. The multi-mode optical fiber 3 includes a core 31 and a cladding 32 coated on the outside of the core 31. The core 31 is at least partially exposed on the outside of the cladding 32 to form an open area 321 for contacting with an external refractive index liquid. The scanning laser emitted by the light source 20 is injected from the single-mode optical fiber 1 and sequentially transmitted to the reflection structure 5. After being reflected by the reflection structure 5, it is transmitted in reverse to be emitted from the single-mode optical fiber 1. Among them, the three optical channels transmit a reference optical signal, a base mode optical signal, and a high-order mode optical signal respectively, and there is a difference in the transmission time of the base mode optical signal and the high-order mode optical signal in the corresponding optical channel. The base mode optical signal and the high-order mode optical signal reflected by the reflection structure 5 generate inter-mode interference in the mode coupler 2.

[0027] It should be understood that the mode coupler 2 can excite the fundamental mode light signal and the high-order mode light signal in the multimode optical fiber 3, and the fundamental mode light signal is transmitted in the core 31 of the multimode optical fiber 3, and the high-order mode light signal is transmitted in the cladding 32 of the multimode optical fiber 3; the reference light signal, the fundamental mode light signal, and the high-order mode light signal are transmitted in the corresponding channels and reflected by the reflection structure 5, and there is a difference in the transmission time of the fundamental mode light signal and the high-order mode light signal in the corresponding optical channel, and the light emitted by the light source 20 is a scanning laser, so that the fundamental mode light signal and the high-order mode light signal reflected by the reflection structure 5 can be transmitted to the mode coupler 2 and generate inter-mode interference in the mode coupler 2, thereby forming An inter-mode interferometer for fundamental mode optical signals / high-order mode optical signals, namely, the distributed optical fiber refractive index sensor 10 in the present solution adopts optical frequency reflection technology, which can simultaneously meet the distributed measurement requirements of large dynamic range and high spatial resolution, and is more sensitive to external refractive index changes; at the same time, the fiber core 31 is at least partially exposed to the outside of the cladding 32 to form an open area 321 for contacting with the external refractive index liquid, so that the energy overlap factor of the distributed optical fiber refractive index sensor 10 and the external refractive index liquid is high, thereby making the distributed optical fiber refractive index sensor 10 more sensitive, and realizing distributed optical fiber refractive index measurement with higher spatial resolution and higher refractive index sensitivity.

[0028] It should be noted that the light emitted by the light source 20 is incident on the distributed optical fiber refractive index sensor 10 through the single-mode optical fiber 1, the fundamental mode optical signal and the high-order mode optical signal excited in the multimode optical fiber 3 are separated by the mode multiplexer 4, and the separated fundamental mode optical signal and the high-order mode optical signal are delayed, the fundamental mode optical signal and the high-order mode optical signal reflected by the reflection structure 5 generate inter-mode interference in the mode coupler 2, and the Rayleigh scattering interference signal related to the position is collected and processed by the data acquisition device 30 to realize the distributed measurement of the external refractive index, that is, the coherent frequency domain reflection technology is used to realize high-resolution and high-sensitivity distributed refractive index sensing; wherein, the fundamental mode optical signal and the high-order mode optical signal at different times generate inter-mode interference in the mode coupler 2.

[0029] See also Figure 1 and Figure 2 In one embodiment, the mode multiplexer 4 has a first output port, a second output port and a third output port, and the reflection structure 5 includes: a first optical fiber delay ring 52 connected to the first output port and used to transmit a fundamental mode optical signal, a second optical fiber delay ring 53 connected to the second output port and used to transmit a high-order mode optical signal, and three reflection devices 51 respectively connected to the third output port, the first optical fiber delay ring 52 and the second optical fiber delay ring 53, the three reflection devices 51 and the first output port, the second output port and the third output port form three optical channels, and the length of the first optical fiber delay ring 52 is less than the length of the second optical fiber delay ring 53. Specifically, the reflector 51 can be a reflector, the first optical fiber delay ring 52 delays the base mode optical signal, and the second optical fiber delay ring 53 delays the high-order mode optical signal. The transmission rate of the base mode optical signal in the corresponding optical channel is greater than the transmission rate of the high-order mode optical signal in the corresponding optical channel. Since the length of the first optical fiber delay ring 52 is less than the length of the second optical fiber delay ring 53, it can be ensured that there is a difference in the transmission time of the base mode optical signal and the high-order mode optical signal in the corresponding optical channel, so that the base mode optical signal and the high-order mode optical signal can be reflected and transmitted to the mode coupler 2 at the same time, so that during laser scanning, the base mode optical signal and the high-order mode optical signal can generate inter-mode interference when they are transmitted in the reverse direction to the mode coupler 2, so that the distributed optical fiber refractive index sensor 10 is an inter-mode interferometer of the base mode optical signal / high-order mode optical signal. According to actual needs, a single-mode optical fiber of sufficient length can also be directly set to delay the base mode optical signal and the high-order mode optical signal.

[0030] It should be understood that after delaying the fundamental mode optical signal and the high-order mode optical signal through the optical fiber delay ring 52, the spatial frequency in the frequency domain can be increased; moreover, the time domain interference signal formed by the inter-mode interference in the mode coupler 2 passes through the optical fiber circulator 40 and is collected by the data acquisition device 30. The interference signal in the time domain is transformed into the frequency domain through Fourier transform. The frequency domain signal includes the scattered signals at different positions of the multimode optical fiber 3. The scattered signals are sensitive to the external refractive index. The spatial resolution of the refractive index sensing can reach sub-millimeter or several hundred microns.

[0031] It should be noted that the transmission paths of the base mode optical signal, the higher-order mode optical signal and the reference optical signal in the distributed optical fiber refractive index sensor 10 are as follows: the base mode optical signal excited in the multimode optical fiber 3 passes through the first output port of the mode multiplexer 4, the first optical fiber delay loop 52, and the reflecting device 51, and is reflected by the reflecting device 51 and then transmitted back to the mode coupler 2; the higher-order mode optical signal excited in the multimode optical fiber 3 passes through the second output port of the mode multiplexer 4, the second optical fiber delay loop 53, and the reflecting device 51, and is reflected by the reflecting device 51 and then transmitted back to the mode coupler 2; the reference optical signal passes through the third output port of the mode multiplexer 4 and the reflecting device 51, and is reflected by the reflecting device 51 and then transmitted back to the mode coupler 2. Among them, since the fundamental mode optical signal and the high-order mode optical signal are delayed by the first optical fiber delay ring 52 and the second optical fiber delay ring 53 respectively, the reversely transmitted fundamental mode optical signal and the high-order mode optical signal will not be transmitted to the mode coupler 2 with the reference optical signal; the fundamental mode optical signal and the high-order mode optical signal generate inter-mode interference in the mode coupler 2 and then are emitted from the single-mode optical fiber 1, and the reference optical signal is directly transmitted from the mode coupler 2 to the single-mode optical fiber 1 and emitted from the single-mode optical fiber 1.

[0032] See also Figure 1 and Figure 2 Furthermore, the first optical fiber delay ring 52 and the second optical fiber delay ring 53 are both formed by continuous multiple turns of single-mode optical fiber, and the first optical fiber delay ring 52 and the first output port, the second optical fiber delay ring 53 and the second output port, the first optical fiber delay ring 52 and the reflector 51, the second optical fiber delay ring 53 and the reflector 51, and the third output port and the reflector 51 are all connected by single-mode optical fiber. Among them, the number of turns of the optical fiber delay ring 52 corresponding to the base mode optical signal is greater than the number of turns of the optical fiber delay ring 52 corresponding to the high-order mode optical signal, so as to ensure that the delay time of the first optical fiber delay ring 52 to the base mode optical signal is greater than the delay time of the second optical fiber delay ring 53 to the high-order mode optical signal; the specific number of turns of the first optical fiber delay ring 52 and the second optical fiber delay ring 53 can be set according to actual needs after ensuring the length of the first optical fiber delay ring 52 and the length of the second optical fiber delay ring 53, such as two turns, three turns and five turns, etc., and the corresponding length of the optical fiber delay ring 52 is 1 meter to several hundred meters.

[0033] See also Figure 1 and Figure 2 In some embodiments, the open area 321 is formed by performing a taper process or a side-casting process on the multi-mode optical fiber 3 to facilitate the forming of the open area 321 on the multi-mode optical fiber 3 .

[0034] In some embodiments, the multimode optical fiber 3 is a microstructure optical fiber with an open core. Specifically, the microstructure is a groove opened on the cladding 32 of the multimode optical fiber 3, and the core 31 of the multimode optical fiber 3 is exposed in the groove; the groove is arranged on the upper side and the lower side of the cladding 32, which is conducive to a wider contact area between the core 31 and the refractive index liquid.

[0035] See also Figure 1 and Figure 2 In some embodiments, the mode coupler 2 includes any one of a coreless optical fiber and a hetero-core optical fiber with misaligned fusion, and the end face of the single-mode optical fiber 1 is in contact with the end face of the coreless optical fiber or the end face of the single-mode optical fiber 1 is in contact with the end face of the hetero-core optical fiber. Specifically, the mode coupler 2 can be a coreless optical fiber, and the end face of the single-mode optical fiber 1 is in contact with the end face of the coreless optical fiber. When the light of the light source 20 is injected from the single-mode optical fiber 1, the end face of the single-mode optical fiber 1 and the end face of the coreless optical fiber are excited to generate a high-order mode light signal due to the mode coupling effect, thereby causing the core 31 mode of the coreless optical fiber to interfere with the evanescent mode, and the high-order mode light signal is transmitted in the cladding 32 of the multimode optical fiber 3, and the base mode light signal is transmitted in the core 31 of the multimode optical fiber 3. According to actual needs, the mode coupler 2 can also be a hetero-core optical fiber with misaligned fusion, and the end faces of the single-mode optical fiber 1 and the hetero-core optical fiber are in contact. Preferably, the coreless optical fiber and the single-mode optical fiber 1 are coaxially arranged, and the coreless optical fiber and the multimode optical fiber 3 are coaxially arranged.

[0036] Furthermore, grooves are provided on the circumferential side of the coreless optical fiber, and the grooves on the coreless optical fiber can make the wall surface of the coreless optical fiber present an irregular shape, which is conducive to the inter-mode interference between the base mode optical signal and the high-order mode optical signal in the mode coupler 2. And a plurality of grooves can be provided on the coreless optical fiber, and each groove is distributed along the circumferential direction. The length range of the coreless optical fiber is 100μm-400μm, such as 100μm, 200μm, 300μm, and 400μm, etc., which is conducive to exciting the high-order mode optical signal in the multimode optical fiber 3.

[0037] See also Figure 1 and Figure 2 In one embodiment, the type of the mode multiplexer 4 includes a photon lantern, which has one input port and three output ports to separate the base mode optical signal and the multimode in the multimode optical fiber 3 and output them through different output ports. According to actual needs, the photon lantern can also have one input port and six output ports.

[0038] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A distributed optical fiber refractive index sensor, characterized in that: The invention comprises a single-mode optical fiber, a mode coupler, a multi-mode optical fiber, a mode multiplexer, and a reflection structure connected in sequence, wherein the mode multiplexer has an input port and at least three output ports, the multi-mode optical fiber is connected to the input port, the reflection structure is connected to the three output ports to form three independently arranged optical channels, the multi-mode optical fiber comprises a core and a cladding coated on the outside of the core, the core is at least partially exposed on the outside of the cladding to form an open area for contacting with an external refractive index liquid; The scanning laser emitted by the light source is injected into the single-mode optical fiber and sequentially transmitted to the reflection structure, and then transmitted in reverse to be emitted from the single-mode optical fiber after being reflected by the reflection structure; wherein the three optical channels transmit the reference optical signal, the fundamental mode optical signal, and the high-order mode optical signal respectively, and there is a difference in the transmission time of the fundamental mode optical signal and the high-order mode optical signal in the corresponding optical channel, and the fundamental mode optical signal and the high-order mode optical signal reflected by the reflection structure generate inter-mode interference in the mode coupler; The mode multiplexer has a first output port, a second output port and a third output port, and the reflection structure includes: a first optical fiber delay ring connected to the first output port and used to transmit the fundamental mode optical signal, a second optical fiber delay ring connected to the second output port and used to transmit the high-order mode optical signal, and three reflection devices respectively connected to the third output port, the first optical fiber delay ring and the second optical fiber delay ring, the three reflection devices and the first output port, the second output port and the third output port form three optical channels, and the length of the first optical fiber delay ring is less than the length of the second optical fiber delay ring.

2. The distributed optical fiber refractive index sensor according to claim 1, characterized in that: The first fiber delay ring and the second fiber delay ring are both formed by continuous multiple turns of single-mode optical fiber, and the first fiber delay ring and the first output port, the second fiber delay ring and the second output port, the first fiber delay ring and the reflector, the second fiber delay ring and the reflector, and the third output port and the reflector are all connected by single-mode optical fiber.

3. The distributed optical fiber refractive index sensor according to claim 1, characterized in that: The open area is formed by performing a taper process or a side-throwing process on the multimode optical fiber.

4. The distributed optical fiber refractive index sensor according to claim 1, characterized in that: The multimode optical fiber is a microstructure optical fiber with an open core.

5. The distributed optical fiber refractive index sensor according to claim 1, characterized in that: The mode coupler comprises any one of a coreless optical fiber and an unaligned fused hetero-core optical fiber, and the end face of the single-mode optical fiber is in contact with the end face of the coreless optical fiber or the end face of the single-mode optical fiber is in contact with the end face of the hetero-core optical fiber.

6. The distributed optical fiber refractive index sensor according to claim 5, characterized in that: The length of the coreless optical fiber ranges from 100 μm to 400 μm.

7. The distributed optical fiber refractive index sensor according to claim 1, characterized in that: Types of mode multiplexers include photon lanterns.

8. A distributed optical fiber refractive index sensing device, characterized in that: It comprises the distributed optical fiber refractive index sensor as claimed in any one of claims 1 to 7.

9. The distributed optical fiber refractive index sensing device according to claim 8, characterized in that: The distributed optical fiber refractive index sensor device also includes a light source, a data acquisition device, and an optical fiber circulator, and the optical fiber circulator is respectively connected to the light source, the distributed optical fiber refractive index sensor, and the data acquisition device.

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