1. A preparation method of a long suspended fiber core in an optical fiber is characterized by comprising the following steps:

step one, chemically etching the photonic crystal fiber:

connecting the photonic crystal fiber with a micro sample injector, fixing an interface by using AB glue, and pushing a sample injector pipe to the bottom to discharge air after the AB glue is fixed; then the photonic crystal fiber is immersed in hydrofluoric acid solution, the needle tube is drawn out, the hydrofluoric acid solution is sucked into the air hole to corrode the cladding due to the internal and external pressure difference, an air groove with a hexagonal structure is formed on the end face of the photonic crystal fiber after a period of corrosion due to the porous structure of the photonic crystal fiber, and an independent fiber core with the diameter of 20 microns is arranged in the center of the air groove;

and step two, welding the non-corroded end of the photonic crystal fiber containing the independent fiber core structure with the common single-mode fiber to obtain the fiber inner long suspended fiber core.

2. The method for preparing the optical fiber inner-length suspension fiber core according to claim 1, wherein the photonic crystal fiber is a thick-wall shaddock-type photonic crystal fiber which has a single-mode non-cutoff characteristic, the fiber core is doped with germanium, and a cladding of the fiber core is of a special structure consisting of shaddock-type air holes which are arranged in a hexagon.

3. The method of claim 1, wherein the microsyringe is a model 5ul microsyringe from Shanghai Pigeon DomaxK.

4. The method as claimed in claim 1, wherein the controlling the length of the sample injector controls the depth of the hydrofluoric acid solution entering the photonic crystal fiber, thereby controlling the depth of the etching.

5. The utility model provides an optical fiber sensor based on long suspension fibre core structure in optic fibre, includes casing (1), its characterized in that, the optic fibre cladding of long suspension fibre core (3) after corroding is casing (1), and the cavity between casing (1) and long suspension fibre core (3) is air cladding (2).

6. An optical fiber sensor based on a long suspended core structure in an optical fiber according to claim 1, characterized in that the housing (1) is a cylinder.

7. An optical fiber sensor based on a long suspended fiber core structure in an optical fiber as claimed in claim 6, characterized in that the cross section of the long suspended fiber core (3) is a hexagonal prism.

8. The optical fiber sensor based on the optical fiber inner long suspension fiber core structure is characterized in that before corrosion of the long suspension fiber core (3), a femtosecond laser direct writing or mask method is used for writing a Fiber Bragg Grating (FBG) with the length of 3mm on the fiber core of the photonic crystal fiber, the corroded long suspension fiber core can be used as a micro cantilever beam, and the FBG on the independent fiber core is used as a sensing element.

9. The optical fiber sensor based on the structure of the long suspended fiber core in the optical fiber as claimed in claim 6, wherein the long suspended fiber core (3) can construct FPI by controlling the appropriate length, and has two sides, one side is the fusion-spliced side (4) and the other side is the end face (5) of the optical fiber.

10. The optical fiber sensor based on the structure of the long suspended fiber core in the optical fiber according to claim 6, wherein a reflecting surface (6) is connected behind the long suspended fiber core (3) to form an optical fiber vibration sensor of a light intensity modulation type.

Background

When the optical wave signal is transmitted in the optical fiber, the characteristic parameters of the optical wave signal, such as intensity, phase, frequency, etc., are changed due to small changes caused by external environments, such as temperature, pressure, electric field, etc., and the change of the external physical quantity can be inferred by measuring a certain characteristic parameter, which is the optical fiber sensing technology. The optical fiber sensor can be normally used in severe environments such as high temperature, high pressure and the like, and has the advantages of small volume, light weight, corrosion resistance, high sensitivity and the like. According to the working principle of the optical fiber sensor, the optical fiber sensor can be divided into types such as an optical fiber interferometer, an optical fiber grating, an optical time domain reflectometer and the like. Most of the optical fiber sensors need to be matched with a mechanical structure for use, signals to be detected are amplified and transmitted to optical fibers through a combined structure, and the sensors are not only complex in manufacturing process, but also cannot realize embedded sensing in a tiny space. The micro-nano optical fiber has the advantages of simple structure, small volume, excellent sensing performance and the like, and is widely applied to the fields of optical sensing, lasers and the like. The diameter of the micro-nano optical fiber is usually close to or smaller than the wavelength of transmission light, and the micro-nano optical fiber has novel optical transmission characteristics of strong optical field restriction, large evanescent field proportion, small bending loss and the like, so that the micro-nano optical fiber sensor not only inherits the advantages of an optical fiber sensor, but also shows huge advantages and potentials when developing an optical sensor with higher sensitivity, faster response time, smaller device size and lower power consumption. The microstructure in the optical fiber can be mainly classified into a structure deformation type, a refractive index modulation type and a material removal type according to the nature of the structure. The material removal method mainly removes local materials of the optical fiber by using methods such as mechanical polishing, chemical corrosion and laser ablation, wherein the chemical corrosion method mainly uses hydrofluoric acid to corrode the optical fiber to manufacture the micro-nano optical fiber sensor, and has attracted extensive attention in recent years due to simple operation and low manufacturing cost.

In 2014, an independent fiber core structure in an optical fiber is proposed for the first time by royal rhodamine and the like, the Photonic Crystal Fiber (PCF) is spliced on a Single Mode Fiber (SMF) to prepare the structure, then an air hole cladding of the photonic crystal fiber is etched by hydrofluoric acid, and the formed fiber core structure in the optical fiber has the Fabry-Perot interference characteristic. The suspended core of the structure is about-2 μm and the length is 106 μm. The structure was used for temperature sensing with an experimentally determined sensitivity of 14.3 pm/deg.C. In 2015, by adopting the structure for coupling the whispering gallery mode microcavity, by royal rhodamine and the like, the structure utilizes the strong evanescent wave characteristic of the suspended fiber core structure, combines the advantages of in-fiber integration on one hand, and realizes the in-fiber integration of the whispering gallery mode microcavity. In 2018, Shao Shi Hua et al developed a high spatial resolution ultrasonic sensor based on a structure of a suspended fiber core in an optical fiber. The structure reduces the core diameter (less than 10 μm) of the PCF through an etching process to obtain higher spatial resolution and response sensitivity, and the length of the core-suspended structure is 240.8 μm. The research can only control the length of the photonic crystal fiber (the length of the suspended fiber core after immediate etching) by welding the photonic crystal fiber through the single-mode fiber and then cutting, but the length of the suspended core etched through the capillary action is limited, the longest length can only be about 300 mu m, and the length limits the sensing application range of the suspended fiber core type sensor. Therefore, the preparation of the suspension fiber core with a long length and the realization of the controllability of the suspension fiber core structure have important significance for the application and popularization of the structure.

Disclosure of Invention

In order to overcome the defects of the prior art, the invention aims to provide an optical fiber sensor based on an optical fiber inner-length suspension fiber core structure and a fiber core preparation method, and the optical fiber sensor has the characteristics of simple structure and controllable suspension core length.

In order to achieve the purpose, the invention adopts the technical scheme that:

a preparation method of a long suspended fiber core in an optical fiber is characterized by comprising the following steps:

step one, chemically etching the photonic crystal fiber:

connecting the photonic crystal fiber with a micro sample injector, fixing an interface by using AB glue, and pushing a sample injector pipe to the bottom to discharge air after the AB glue is solidified; then the photonic crystal fiber is immersed in hydrofluoric acid solution, the needle tube is drawn out, the hydrofluoric acid solution is sucked into the air hole to corrode the cladding due to the internal and external pressure difference, an air groove with a hexagonal structure is formed on the end face of the photonic crystal fiber after a period of corrosion due to the porous structure of the photonic crystal fiber, and an independent fiber core with the diameter of 20 microns is arranged in the center of the air groove;

and step two, welding the non-corroded end of the photonic crystal fiber containing the independent fiber core structure with the common single-mode fiber to obtain the fiber inner long suspended fiber core.

The photonic crystal fiber is a thick-wall shaddock-type photonic crystal fiber and has a single-mode non-cutoff characteristic, the fiber core is doped with germanium, and the cladding of the photonic crystal fiber is of a special structure consisting of shaddock-type air holes which are arranged in a hexagon.

The micro sample injector is 5ul of a micro sample injector of Shanghai Pigeon worker trade Co Ltd.

The depth of the hydrofluoric acid solution entering the photonic crystal fiber can be controlled by controlling the pumping length of the sample injector, so that the corrosion depth can be controlled.

The utility model provides an optical fiber sensor based on long suspension fibre core structure in optic fibre, includes casing (1), its characterized in that, the optic fibre cladding of long suspension fibre core (3) after corroding is casing (1), and the cavity between casing (1) and long suspension fibre core (3) is air cladding (2).

The shell (1) is a cylinder.

The section of the long suspended fiber core (3) is a hexagonal prism.

Before the long suspended fiber core (3) is corroded, a Fiber Bragg Grating (FBG) with the length of 3mm is engraved on the photonic crystal fiber core in a femtosecond laser direct writing mode, the corroded long suspended fiber core can be used as a micro cantilever beam, and the FBG on the independent fiber core is used as a sensing element.

Long suspension fibre core (3) through controlling suitable length, hang core structure self and can construct FPI, it possesses two sides, one side is optical fiber splice face (4), one side is fiber end face (5).

And a reflecting surface (6) is connected behind the long suspended fiber core (3) to form the light intensity modulation type optical fiber vibration sensor.

The invention has the beneficial effects that:

the optical fiber device based on the independent fiber core is simple and convenient to manufacture, only comprises two steps of chemical corrosion and optical fiber fusion, and has the potential of large-scale mass production. The depth of the hydrofluoric acid solution entering the photonic crystal fiber can be controlled by controlling the length of the sample injector, so that the purpose of controlling the corrosion depth is achieved. The length of the suspended fiber core can reach over centimeter magnitude, and the suspended fiber core has reasonable design and simple structure.

Drawings

Fig. 1 is a schematic structural view of embodiment 1 of the present invention.

Fig. 2 is a schematic structural diagram of embodiment 2 of the present invention.

Fig. 3 is a frequency response diagram of embodiment 2 of the present invention.

FIG. 4 is a graph of the output response of example 2 of the present invention at an input sinusoidal acceleration of 15m/s2 frequency of 20 Hz.

FIG. 5 is a schematic view of a production tool of example 1 of the present invention.

Fig. 6 shows an optical fiber vibration sensor of a light intensity modulation type shown in example 3.

Wherein, 1 is a shell; 2 is an air cladding; 3 is a long suspended fiber core; 4 is an optical fiber fusion surface; 5 is an optical fiber end face; and 6 is a reflecting surface.

Detailed Description

The present invention will be further described with reference to the accompanying drawings, but the present invention is not limited to the following embodiments.

Example 1

As shown in fig. 1, the optical fiber sensor based on the long floating fiber core according to the present embodiment includes a housing 1, an air cladding 2, and a long floating fiber core 3.

The housing 1 of the present embodiment has an overall external shape of a cylinder and is directly formed by the fiber cladding after etching. The fiber optic housing 2 is a cavity formed by air within the housing. The overall shape of the long floating core 3 is a prism with a hexagonal cross section, and is formed by an etched floating fiber core. The diameter of the suspension core is 19um, and the length is 3 mm. The sensor is integrated in structure, and the shell formed after corrosion protects the long suspended fiber core from being damaged. The preparation method is shown in fig. 5, the optical fiber 2 is inserted into the sample injector 1, and the interface is sealed with the AB glue.

Example 2

In the present embodiment, a fiber grating vibration sensor based on a long suspended fiber core is realized. Based on the long suspended fiber core sensor structure in embodiment 1, a micro cantilever beam micro-nano fiber grating structure shown in fig. 2 is designed. In the structure, a Fiber Bragg Grating (FBG) with the length of 3mm is engraved on the core of the photonic crystal fiber in a femtosecond laser direct writing mode. The corroded long suspended fiber core can be used as a micro cantilever beam, and the FBG on the independent fiber core is used as a sensing element. When the sensor is placed in a vibration environment, the fiber core can be bent to cause the period of the FBG grating to change, so that the Bragg wavelength is drifted, and the sensing of a vibration signal can be realized by monitoring the drift of the central wavelength.

Example 3

In this embodiment, based on the structure of embodiment 1, a light intensity modulation type optical fiber vibration sensor is constructed by adding a reflection surface to the end surface of an optical fiber, and the structure is shown in fig. 6. The optical fiber end face 5 and the reflecting face 4 form an FP cavity to realize intensity detection.

In order to verify the beneficial effects of the present invention, the inventor carried out a laboratory research test using the fiber grating vibration sensor based on the long suspended fiber core prepared in example 2, and the experimental conditions were as follows:

an experimental instrument: a vibration table (WS-Z30-40); demodulation system, oscilloscope (SDS-1100X-E), tunable laser (CBDX1-1-C-HOI-FA), and photodetector (KG-P-200M-A-FC).

1. Performing a frequency response experiment;

the invention and a standard charge acceleration sensor (YD81D-V) are fixed on a vibration table by adopting a screw rod, and the acceleration of an input sine wave is controlled to be 15m/s²And (4) performing signal demodulation by adopting a sideband filtering method, increasing the signal frequency from 20Hz to 100Hz., measuring the sine output of the corresponding sensor at each input frequency by using an oscilloscope, and calculating the corresponding amplitude. The experimental results are shown in FIG. 3, wherein 20-50Hz is the resonance region, the attenuation region is after 50Hz, and the frequency response range is 0-50 Hz.