CN111929278A - Optical fiber refractive index sensor and refractive index detection method - Google Patents
Optical fiber refractive index sensor and refractive index detection method Download PDFInfo
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- CN111929278A CN111929278A CN202010710850.6A CN202010710850A CN111929278A CN 111929278 A CN111929278 A CN 111929278A CN 202010710850 A CN202010710850 A CN 202010710850A CN 111929278 A CN111929278 A CN 111929278A
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/41—Refractivity; Phase-affecting properties, e.g. optical path length
- G01N21/45—Refractivity; Phase-affecting properties, e.g. optical path length using interferometric methods; using Schlieren methods
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/41—Refractivity; Phase-affecting properties, e.g. optical path length
- G01N21/45—Refractivity; Phase-affecting properties, e.g. optical path length using interferometric methods; using Schlieren methods
- G01N2021/458—Refractivity; Phase-affecting properties, e.g. optical path length using interferometric methods; using Schlieren methods using interferential sensor, e.g. sensor fibre, possibly on optical waveguide
Abstract
The invention discloses an optical fiber refractive index sensor and a refractive index detection method, wherein the optical fiber refractive index sensor comprises: an incident optical fiber for receiving incident light; a first ball-type structure for dividing the incident light into a first portion of light and a second portion of light; a connecting optical fiber for transmitting the first portion of light and the second portion of light; the optical fiber side-polishing structure is used for modulating the second part of light of the cladding in the connecting optical fiber by liquid to be measured; a second spherical-type structure for optically coupling the second portion of cladding back into the core in the connecting fiber; and an exit optical fiber for transmitting the light emitted from the second spherical structure to a spectrum analyzer. The optical fiber refractive index sensor can convert the variable quantity of the refractive index to be detected into the spectral wavelength drift of the detection signal, has the advantages of simple and convenient manufacture, simple structure, low cost, high sensitivity, strong corrosion resistance and strong external electromagnetic interference resistance, and can be applied to various practical projects.
Description
Technical Field
The invention relates to the technical field of optical fiber sensing, in particular to an optical fiber refractive index sensor.
Background
The Mach-Zehnder interference structure is widely applied to an optical fiber sensing system. When one beam of light is divided into two beams by the coupler and respectively propagates along the two optical fibers, one of the two beams of light is used as a reference light path, and the other beam of light is used as a modulation light path. The two beams will recombine at the next coupler and interfere. When the modulation optical path is affected by the outside, the optical path of the light propagating in the modulation optical path changes, so that the optical path difference of the two paths of light changes, and the interference fringes also change correspondingly. By means of the corresponding relation between the interference fringe change and the environment change, the change of the environment parameter can be deduced through the change of the interference fringe in the measurement.
With the continuous development of optical fiber sensing technology, the way in which mach-zehnder interference structures are used in optical fiber sensors has also changed. The role of optical fibers has also evolved from a mere transmission function to a combined transmission and sensing function. After the optical fiber is processed, a special structure is formed, the special structure can be used as a coupler to couple a part of light propagating along the fiber core of the optical fiber into a cladding, and the light is coupled back to the fiber core again in the next similar fiber coupler, so that interference of two parts of light propagating along the fiber core and the cladding is realized, and the principle of Mach-Zehnder interference is realized. When the influence of the external environment on the fiber core cladding is different, the light propagating along the fiber core and the light propagating along the cladding generate interference change due to the difference of the reaction of the fiber core cladding on the external environment, and the interference change is reflected on the change of a spectrogram in the application of optical fiber sensing. After the relation between the spectral change and the external environment change is analyzed, the external environment can be monitored by monitoring the spectral change, and the effect of the sensor is realized. The side-polishing structure increases the influence of the external environment on the optical fiber light propagation so that the sensor is more sensitive to the external environment, and therefore the side-polishing structure is used as a part for enhancing the sensitivity of the sensor. The optical fiber sensor has obvious advantages in monitoring of a small-space environment with strong electromagnetic interference. The high-sensitivity fiber refractive index sensors currently studied generally use special fibers such as fiber gratings, photonic band gap fibers, etc., which are expensive.
Therefore, the prior art still needs to be improved and developed to address the above drawbacks.
Disclosure of Invention
The invention aims to solve the technical problems that a high-sensitivity optical fiber refractive index sensor is high in cost, and provides an optical fiber refractive index sensor and a refractive index detection method.
The technical scheme adopted by the invention for solving the technical problem is as follows:
an optical fiber refractive index sensor, wherein the optical fiber refractive index sensor comprises:
an incident optical fiber for receiving incident light;
the first ball-shaped structure is arranged in front of the incident optical fiber and is used for dividing the incident light into a first part of light and a second part of light;
the connecting optical fiber is arranged in front of the first ball-shaped structure and used for transmitting the first part of light and the second part of light;
the optical fiber side-polishing structure is arranged in front of the connecting optical fiber and is used for modulating the second part of light of the cladding in the connecting optical fiber by liquid to be measured;
a second spherical-type structure disposed in front of the fiber side-cast structure for optically coupling the second portion of the cladding back into the core in the stub fiber;
and the emergent optical fiber is arranged in front of the second spherical structure and is used for transmitting the light emitted by the second spherical structure to the optical spectrum analyzer.
The optical fiber refractive index sensor is characterized in that the first part of light is transmitted in an optical fiber core, and the second part of light is transmitted in an optical fiber cladding.
The optical fiber refractive index sensor, wherein the first spherical structure is respectively connected with the incident optical fiber and the connecting optical fiber;
the second spherical structure is connected with the connecting optical fiber and the emergent optical fiber respectively.
The optical fiber refractive index sensor is characterized in that the incident optical fiber, the first spherical structure, the connecting optical fiber, the optical fiber side-throwing structure, the second spherical structure and the emergent optical fiber form a linear structure.
The optical fiber refractive index sensor is characterized in that the incident optical fiber, the first spherical structure, the connecting optical fiber, the second spherical structure and the emergent optical fiber adopt single-mode optical fibers.
The optical fiber refractive index sensor is characterized in that the first spherical structure and the second spherical structure have the same structure.
The optical fiber refractive index sensor is characterized in that the diameter ranges of the first spherical structure and the second spherical structure are 240 um and 250 um.
The optical fiber refractive index sensor is characterized in that the length ranges of the incident optical fiber and the emergent optical fiber are 30-50 cm; the length range of the connecting optical fiber is 2-3 cm.
The optical fiber refractive index sensor is characterized in that the length range of the optical fiber side-polishing structure is 0.8-1.2cm, and the depth range is 66.5-67.5 um.
In addition, the invention also provides a method for realizing refractive index detection based on the optical fiber refractive index sensor, wherein the method comprises the following steps:
the incident light enters from the optical fiber core of the incident optical fiber, and after passing through the first spherical structure, the light in the optical fiber core is divided into a first part of light and a second part of light;
the first part of light is transmitted in a fiber core of the connecting fiber, and the second part of light is transmitted in a fiber cladding of the connecting fiber;
the second part of light transmitted in the optical fiber cladding of the connecting optical fiber is modulated by liquid to be measured when passing through the optical fiber side-throwing structure;
the second part of light in the optical fiber cladding layer passing through the second spherical structure is coupled back to the optical fiber core and is transmitted into the optical spectrum analyzer by the emergent optical fiber;
and the spectrum analyzer analyzes the wavelength information of the modulated light to obtain the refractive index of the measured liquid.
Has the advantages that: the invention provides an optical fiber refractive index sensor and a refractive index detection method, wherein the optical fiber refractive index sensor comprises: an incident optical fiber for receiving incident light; the first ball-shaped structure is arranged in front of the incident optical fiber and is used for dividing the incident light into a first part of light and a second part of light; the connecting optical fiber is arranged in front of the first ball-shaped structure and used for transmitting the first part of light and the second part of light; the optical fiber side-polishing structure is arranged in front of the connecting optical fiber and is used for modulating the second part of light of the cladding in the connecting optical fiber by liquid to be measured; a second spherical-type structure disposed in front of the fiber side-cast structure for optically coupling the second portion of the cladding back into the core in the stub fiber; and the emergent optical fiber is arranged in front of the second spherical structure and is used for transmitting the light emitted by the second spherical structure to the optical spectrum analyzer. The optical fiber refractive index sensor can convert the variable quantity of the refractive index to be detected into the spectral wavelength drift of the detection signal, has the advantages of simple and convenient manufacture, simple structure, low cost, high sensitivity, strong corrosion resistance and strong external electromagnetic interference resistance, and can be applied to various practical projects.
Drawings
FIG. 1 is a schematic structural diagram of a preferred embodiment of the optical fiber refractive index sensor of the present invention.
Fig. 2 is a schematic diagram of the principle of optical path transmission in the optical fiber refractive index sensor of the present invention.
FIG. 3 is a flow chart of a method for detecting refractive index based on an optical fiber refractive index sensor according to a preferred embodiment of the present invention.
FIG. 4 is an experimental graph of the change of the interference spectrum of the optical fiber refractive index sensor under different refractive indexes in the preferred embodiment of the optical fiber refractive index sensor of the present invention.
FIG. 5 is a line-fit plot of refractive index sensitivity for a preferred embodiment of the fiber optic refractive index sensor of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention clearer and clearer, the present invention is further described in detail below with reference to the accompanying drawings and examples. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
Referring to fig. 1, fig. 1 is a schematic structural diagram of an optical fiber refractive index sensor according to a preferred embodiment of the present invention.
As shown in fig. 1 and 2, an embodiment of the present invention provides an optical fiber refractive index sensor, including:
an incident optical fiber 1 for receiving incident light; a first ball-type structure 2 disposed in front of the incident optical fiber 1 (as shown in fig. 2, the front of the directional arrow indicates the front, that is, the transmission direction of light in the optical path indicates the front), for dividing the incident light into a first portion of light (as shown in the direction of a in fig. 2) and a second portion of light (as shown in the direction of b in fig. 2); the connecting optical fiber 3 is arranged in front of the first ball-type structure 2 and is used for transmitting the first part of light and the second part of light; the optical fiber side-polishing structure 4 is arranged in front of the connecting optical fiber 3 and is used for modulating the second part of light of the cladding in the connecting optical fiber 3 by liquid to be measured; a second spherical-shaped structure 5 disposed in front of said fiber side-cast structure 4 for optically coupling said second portion of the cladding of said connecting fiber 3 back to the core; and an exit optical fiber 6 arranged in front of the second spherical structure 5 for transmitting the light emitted from the second spherical structure 5 to a spectrum analyzer.
The optical fiber consists of an optical fiber core, an optical fiber cladding and a sheath, wherein the optical fiber core and the optical fiber cladding are two glass materials with different refractive indexes and are basic structures of the optical fiber, the refractive index of the optical fiber core is higher than that of the optical fiber cladding, and the principle of optical fiber transmission is that light is transmitted along the optical fiber in a lossless mode by total reflection between the optical fiber core and the optical fiber cladding; in the present invention, as shown in fig. 2, the first portion of light is transmitted through the fiber core 7 and the second portion of light is transmitted through the fiber cladding 8.
As shown in fig. 1 and 2, the first ball-type structure 2 is connected to the incident optical fiber 1 and the connecting optical fiber 3 respectively; the second spherical structure 5 is connected to the connecting fiber 3 and the exit fiber 6, respectively. The incident optical fiber 1, the first spherical structure 2, the connecting optical fiber 3, the optical fiber side-polishing structure 4, the second spherical structure 5 and the emergent optical fiber 6 form a linear structure (i.e. a Mach-Zehnder refractive index sensor which is in a linear design, has a cylindrical shape and is similar to a spherical-side-polishing-spherical structure).
Specifically, the incident optical fiber 1, the exit optical fiber 6, and the connection optical fiber 3 can be g.652 (the g.652 optical fiber is a widely used single-mode optical fiber, called 1310nm performance-optimized single-mode optical fiber, also called dispersion-unshifted optical fiber), g.653 (a dispersion-shifted optical fiber with zero dispersion wavelength shifted from 1.3 μm to 1.55 μm), and g.655 (an optical fiber with small positive dispersion or negative dispersion at 1550nm operating wavelength, called non-zero dispersion-shifted single-mode optical fiber), and the single-mode optical fiber is characterized in that the dispersion in 1530-1565 nm operating window is not zero, and a suitable dispersion system value capable of suppressing four-wave mixing is maintained, and since the dispersion coefficient of the optical fiber in 1530-1565 nm operating window is small and not zero, a small amount of dispersion compensation optical fiber is required only when a line is formed by the g.655 optical fiber for a long distance of 10Gbit/s or more, and thus the optical fiber is a fiber capable of realizing a long distance of 10Gbit/s or more at present, A preferred optical fiber type of a dense wavelength division multiplexing optical fiber communication system for large-capacity communication) single-mode optical fiber (a single-mode optical fiber has a thin central glass core, a core diameter of generally 9 or 10 μm, and can only transmit an optical fiber of one mode, so that intermodal dispersion is small, and the single-mode optical fiber is suitable for remote communication, and compared with a multimode optical fiber, the single-mode optical fiber can support a longer transmission distance, and in 100Mbps ethernet or 1G gigabit network, the single-mode optical fiber can support a transmission distance exceeding 5000 m); the lengths of the incident optical fiber 1 and the emergent optical fiber 6 are 30-50 cm; the length of the connecting optical fiber 3 is 2-3cm, preferably 2 cm.
Specifically, the first spherical structure and the second spherical structure have the same structure (i.e. the parameters and structures of both are the same), and g.652, g.653 and g.655 single-mode fibers can be adopted as the first spherical structure 2 and the second spherical structure 5; the diameter of the first spherical structure 2 and the second spherical structure 5 is 240 um and 250um, preferably 250 um.
The invention utilizes the cheap single-mode optical fiber to manufacture the refractive index sensor which can be compared with the sensitivity of a special optical fiber sensor through process modification.
Specifically, the length of the optical fiber side-polishing structure 4 is 0.8-1.2cm, preferably 10cm, and the depth is 66.5-67.5 um.
Further, the diameter of the first spherical structure 2 and the second spherical structure 5 is preferably 250um, and both are made by twice discharging in the "self-setting mode 1" of an optical fiber fusion splicer (such as nodesen NS-80); the specific parameters of the "self-setting mode 1" are: ' end face angle 3.0; the section spacing is 10; an overlap amount of 0; the premelting time is 100; the premelting force is 100; welding time 100, welding power 100; cleaning time 3; the re-discharge time was 100'. Then, the first spherical structure 2 and the second spherical structure 5 are respectively welded with the connecting optical fiber 3 by using a self-setting mode 2; the specific parameters of the "self-setting mode 2" are: ' end face angle 3.0; the section spacing is 15; the amount of overlap 150; a premelting time of 25; the premelting force is 30; welding time 40, welding power 100; cleaning time 3; the re-discharge time is 100' (wherein all parameters are relative quantities, and the unit of a specific parameter can be set according to actual operation).
Further, the length of the optical fiber side polishing structure 4 is preferably 10cm, the depth is 66.5-67.5um, and the polishing is completed by an optical fiber side polishing micro-machining platform (FSP-II-C). Setting the operation parameters as follows: the structure shown in FIG. 1 can be obtained by polishing the optical fiber at a side polishing speed of 60' with a polishing length of 10mm and a polishing depth of 58 mm. The linear structure formed by the first spherical structure 2, the connecting optical fiber 3, the optical fiber side-throwing structure 4 and the second spherical structure 5 is a sensing head of the sensor and is used for detecting the external refractive index.
Based on the above embodiments, the present invention further provides a refractive index detection method, please refer to fig. 3, and fig. 3 is a flowchart of a preferred embodiment of a method for implementing refractive index detection based on an optical fiber refractive index sensor according to the present invention.
The specific implementation process is shown in fig. 3, and the method for implementing refractive index detection based on the optical fiber refractive index sensor provided by the embodiment of the invention comprises the following steps:
step S100, the incident light enters from the fiber core of the incident optical fiber 1, and after passing through the first spherical structure 2, the light in the fiber core is divided into a first part of light and a second part of light;
step S200, the first part of light is transmitted in the fiber core 7 of the connection fiber 3, and the second part of light is transmitted in the fiber cladding 8 of the connection fiber 3;
step S300, modulating the second part of light transmitted in the optical fiber cladding 8 of the connecting optical fiber 3 by liquid to be measured when the second part of light passes through the optical fiber side-throwing structure 4;
step S400, the second part of light in the optical fiber cladding 8 passing through the second spherical structure 5 is coupled back to the optical fiber core 7 and is transmitted into a spectrum analyzer by the emergent optical fiber 6;
and step S500, analyzing the wavelength information of the modulated light by the spectrum analyzer to obtain the refractive index of the measured liquid.
That is, the incident light enters from the core of the incident optical fiber 1, after passing through the first spherical structure 2, the light in the fiber core 7 will be divided into two parts to be transmitted continuously in the connecting optical fiber 3, the first part of light (a direction) is transmitted in the fiber core 7, and the second part of light is optically coupled to the fiber cladding 8 to be transmitted; the light transmitted in the fiber cladding 8 of the connecting fiber 3 is modulated by the liquid to be measured when passing through the fiber side-polishing structure 4, and then the light passing through the fiber cladding 8 of the second spherical structure 5 is coupled back to the fiber core 7 and transmitted into the spectrum analyzer through the emergent fiber 6. The refractive index of the measured liquid can be obtained by analyzing the wavelength information of the modulated light.
Namely, the incident light is divided into two parts of light after passing through the first spherical structure 2, one part of light propagates along the fiber core, the other part of light propagates along the fiber cladding, and the light transmitted along the fiber core through the second spherical structure 5 interferes with the light transmitted along the fiber cladding; when the external refractive index changes, the influence on the fiber core and the fiber cladding is different, so that the effective refractive index difference of the fiber core and the fiber cladding is changed, the transmission condition of light in the fiber core and the fiber cladding is changed, and the wavelength of a spectrum is shifted and changed; the change condition of the refractive index can be obtained by monitoring the wavelength drift of the interference spectrum.
Further, when in use, the incident optical fiber 1 and the emergent optical fiber 6 are respectively fixed on a clamping table, a lifting table is placed below the sensing head, and the lifting table can be used for placing the breaking piece loaded with liquid with different refractive indexes; the stage is raised during the measurement so that the sensor head is in contact with the fluid, and the stage is lowered after the data is measured to replace the slide with another fluid of refractive index. FIG. 4 is a graph of the change in interference spectrum at different refractive indices at room temperature, showing that the peak at 1564.24nm of the interference spectrum corresponding to the sensor head shifts to 1593.4nm as the refractive index of the fluid applied to the sensor head changes in the range of 1.33269-1.42294. FIG. 5 is a plot of a refractive index sensitivity line fit of the present invention, the sensitivity of the sensor being 213.479nm per unit Refractive Index (RIU) when the refractive index is varied from 1.33269 to 1.39716 and 730.502nm/RIU when the refractive index is varied from 1.40734 to 1.42294.
In summary, the present invention discloses an optical fiber refractive index sensor and a refractive index detection method, wherein the optical fiber refractive index sensor includes: an incident optical fiber for receiving incident light; the first ball-shaped structure is arranged in front of the incident optical fiber and is used for dividing the incident light into a first part of light and a second part of light; the connecting optical fiber is arranged in front of the first ball-shaped structure and used for transmitting the first part of light and the second part of light; the optical fiber side-polishing structure is arranged in front of the connecting optical fiber and is used for modulating the second part of light of the cladding in the connecting optical fiber by liquid to be measured; a second spherical-type structure disposed in front of the fiber side-cast structure for optically coupling the second portion of the cladding back into the core in the stub fiber; and the emergent optical fiber is arranged in front of the second spherical structure and is used for transmitting the light emitted by the second spherical structure to the optical spectrum analyzer. The optical fiber refractive index sensor can convert the variable quantity of the refractive index to be detected into the spectral wavelength drift of the detection signal, has the advantages of simple and convenient manufacture, simple structure, low cost, high sensitivity, strong corrosion resistance and strong external electromagnetic interference resistance, and can be applied to various practical projects.
It is to be understood that the invention is not limited to the examples described above, but that modifications and variations may be effected thereto by those of ordinary skill in the art in light of the foregoing description, and that all such modifications and variations are intended to be within the scope of the invention as defined by the appended claims.
Claims (10)
1. An optical fiber refractive index sensor, comprising:
an incident optical fiber for receiving incident light;
the first ball-shaped structure is arranged in front of the incident optical fiber and is used for dividing the incident light into a first part of light and a second part of light;
the connecting optical fiber is arranged in front of the first ball-shaped structure and used for transmitting the first part of light and the second part of light;
the optical fiber side-polishing structure is arranged in front of the connecting optical fiber and is used for modulating the second part of light of the cladding in the connecting optical fiber by liquid to be measured;
a second spherical-type structure disposed in front of the fiber side-cast structure for optically coupling the second portion of the cladding back into the core in the stub fiber;
and the emergent optical fiber is arranged in front of the second spherical structure and is used for transmitting the light emitted by the second spherical structure to the optical spectrum analyzer.
2. The optical fiber refractive index sensor according to claim 1, wherein the first portion of light is transmitted in a fiber core and the second portion of light is transmitted in a fiber cladding.
3. The optical fiber refractive index sensor according to claim 1, wherein the first ball-type structure is connected to the incident optical fiber and the connection optical fiber, respectively;
the second spherical structure is connected with the connecting optical fiber and the emergent optical fiber respectively.
4. The optical fiber refractive index sensor according to claim 1, wherein the incident optical fiber, the first ball-type structure, the connection optical fiber, the optical fiber side-polishing structure, the second ball-type structure, and the exit optical fiber constitute a line-type structure.
5. The optical fiber refractive index sensor according to claim 1, wherein the incident optical fiber, the first spherical structure, the connecting optical fiber, the second spherical structure, and the exit optical fiber are single mode optical fibers.
6. The optical fiber refractive index sensor according to claim 1, wherein the first spherical-type structure and the second spherical-type structure are identical in structure.
7. The optical fiber refractive index sensor according to claim 6, wherein the first spherical structure and the second spherical structure have a diameter in the range of 240-250 μm.
8. The optical fiber refractive index sensor according to claim 1, wherein the length of the incident optical fiber and the exit optical fiber ranges from 30 to 50 cm; the length range of the connecting optical fiber is 2-3 cm.
9. The optical fiber refractive index sensor according to claim 1, wherein the optical fiber side-polishing structure has a length ranging from 0.8 to 1.2cm and a depth ranging from 66.5 to 67.5 um.
10. A method for realizing refractive index detection based on the optical fiber refractive index sensor according to any one of claims 1 to 9, comprising the steps of:
the incident light enters from the optical fiber core of the incident optical fiber, and after passing through the first spherical structure, the light in the optical fiber core is divided into a first part of light and a second part of light;
the first part of light is transmitted in a fiber core of the connecting fiber, and the second part of light is transmitted in a fiber cladding of the connecting fiber;
the second part of light transmitted in the optical fiber cladding of the connecting optical fiber is modulated by liquid to be measured when passing through the optical fiber side-throwing structure;
the second part of light in the optical fiber cladding layer passing through the second spherical structure is coupled back to the optical fiber core and is transmitted into the optical spectrum analyzer by the emergent optical fiber;
and the spectrum analyzer analyzes the wavelength information of the modulated light to obtain the refractive index of the measured liquid.
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