CN103697920B - A kind of optical fiber sensor head and based on this sensing head measure the optical fiber sensing system of liquid refractivity and method - Google Patents

Abstract

An optical fiber sensing head and an optical fiber sensing system and method for measuring liquid refractive index based on the sensing head belong to the field of optical fiber sensing systems. The purpose of the invention is to solve the problem that the distance between the FP cavity of the existing optical fiber sensing head and the end of the optical fiber is relatively short, thereby affecting the reflectivity. An optical fiber sensing head and an optical fiber sensing system and method based on the sensing head for measuring the refractive index of a liquid described in the present invention overcome the problem of changing the optical path by opening an air channel on the optical fiber in the prior art. direction, so that the fiber cladding is used as an FP cavity, which avoids the influence of the liquid to be measured on the reflectivity of the two interfaces of the FP cavity at the same time. When the light incident into the sensor head is at S-polarization, the maximum sensitivity for measuring the refractive index reaches ‑298.3dB/RIU, and when it is at P-polarization, the maximum sensitivity for measuring the refractive index reaches ‑597.1dB/RIU. The invention is suitable for measuring the refractive index of liquid.

Description

An optical fiber sensing head and an optical fiber sensing system and method for measuring liquid refractive index based on the sensing head

technical field

The invention belongs to the field of optical fiber sensing system.

Background technique

Due to the low cost, corrosion resistance, high sensitivity and other excellent characteristics of optical fiber, in recent years, optical fiber has received great attention in refractive index sensing and has been widely used in biology, chemistry and other fields. Optical fiber refractive index sensors can be mainly divided into two types according to the detection spectrum, transmission optical fiber sensors and reflective optical fiber sensors. Compared with reflective optical fiber sensors, the sensing system of transmissive optical fiber sensors occupies a relatively large space, which limits its direct detection of refractive index in vivo. For reflective fiber optic sensors, the optical fiber is usually combined with an external Fabry-Perot (FP) cavity or an internal FP cavity to detect changes in the refractive index; and the external FP cavity also faces the relatively large space occupied by the sensor head. question. Therefore, in comparison, the sensor based on the FP cavity inside the optical fiber has a wider range of sensing applications for the refractive index. However, although the traditional optical fiber sensor based on the internal FP cavity has a small size and a large distance, due to the short distance between the FP cavity and the end of the optical fiber, when testing liquid, the liquid not only affects the reflectivity of the FP reflective surface, but also affects the optical fiber end. Influenced by the reflectivity of the terminal, cross-sensing will occur between the two influences, so that the reflected light intensity measured by the spectrometer cannot monotonically change with respect to the refractive index of the liquid to be measured, but can only present a monotonous change within a narrow range of refractive index. And the monotonic region changes with the change of the refractive index of the liquid to be measured. Therefore, the application of fiber optic sensors based on internal FP cavity in refractive index measurement is greatly limited.

Contents of the invention

The present invention aims to solve the problem that the distance between the FP cavity of the existing optical fiber sensing head and the end of the optical fiber is relatively short, thereby affecting the reflectivity. Now, an optical fiber sensing head and a method for measuring the refractive index of liquid based on the sensing head are provided. Optical fiber sensing system and method.

An optical fiber sensing head, comprising: an optical fiber and a reflective material;

The optical fiber includes: an optical fiber cladding and an optical fiber core;

The fiber cladding is wrapped on the outside of the fiber core; there is a quadrangular air channel on the fiber core, the cross section of the air channel is an isosceles trapezoid, and the angle between the plane where the waist of the isosceles trapezoid is located and the fiber axis is is 45 degrees, the height of the isosceles trapezoid is equal to the diameter of the fiber core, the length of the short base of the isosceles trapezoid is between 10 μm and 20 μm, the length of the air channel is equal to the diameter of the optical fiber, the short of the isosceles trapezoid A reflective material is fixed on the outside of the fiber cladding corresponding to the bottom edge;

The material of the reflective material is the same as that of the fiber core.

An optical fiber sensing system for measuring the refractive index of liquid, which includes: a light source, a 3dB coupler, a polarizer, a sensing head, a spectrometer and a signal processing module;

The light emitted by the light source is transmitted to the polarizer through the 3dB coupler. The polarizer is adjusted by the light and transmitted to the sensor head. The sensor head reflects the light into the polarizer. Transmission to the 3dB coupler, the 3dB coupler transmits the light to the spectrometer, and the spectral signal output end of the spectrometer is connected to the spectral signal input end of the signal processing module;

The signal processing module includes the following units:

A unit for collecting the intensity of the reflection spectrum obtained in the spectrometer at 1550nm;

A unit for obtaining the reflectivity R of the interface between the air channel and the fiber core;

Obtain the unit of the refractive index n of the liquid to be measured _;

The sensing head includes: optical fiber and reflective material;

The optical fiber includes: an optical fiber cladding and an optical fiber core;

The fiber cladding is wrapped on the outside of the fiber core; there is a quadrangular air channel on the fiber core, the cross section of the air channel is an isosceles trapezoid, and the angle between the plane where the waist of the isosceles trapezoid is located and the fiber axis is is 45 degrees, the height of the isosceles trapezoid is equal to the diameter of the fiber core, the length of the short base of the isosceles trapezoid is between 10 μm and 20 μm, the length of the air channel is equal to the diameter of the optical fiber, the short of the isosceles trapezoid The outer side of the optical fiber cladding corresponding to the bottom edge is provided with a reflective material.

A method for measuring the refractive index of a liquid, the method is realized based on the following device,

The device includes: a light source, a 3dB coupler, a polarizer, a sensor head and a spectrometer;

The light emitted by the light source is transmitted to the polarizer through the 3dB coupler. The polarizer is adjusted by the light and transmitted to the sensor head. The sensor head reflects the light into the polarizer. Transmission to the 3dB coupler, the 3dB coupler transmits the light to the spectrometer;

The sensing head includes: optical fiber and reflective material;

The optical fiber includes: an optical fiber cladding and an optical fiber core;

The fiber cladding is wrapped on the outside of the fiber core; there is a quadrangular air channel on the fiber core, the cross section of the air channel is an isosceles trapezoid, and the angle between the plane where the waist of the isosceles trapezoid is located and the fiber axis is is 45 degrees, the height of the isosceles trapezoid is equal to the diameter of the fiber core, the length of the short base of the isosceles trapezoid is between 10 μm and 20 μm, the length of the air channel is equal to the diameter of the optical fiber, the short of the isosceles trapezoid A reflective material is provided on the outside of the fiber cladding corresponding to the bottom edge;

A method for measuring the refractive index of liquid based on the above-mentioned device comprises the following steps:

Step 1: Adjust the polarizer so that the light incident on the sensor head is light in one polarization state; then perform step 2;

Step 2: keep the sensing head not immersed in the liquid, use a spectrometer to obtain the reference value I ₀ of the reflection spectrum intensity at 1550nm, and then perform step 3;

Step 3: Immerse the sensor head in the liquid to be tested, let the liquid to be tested enter the air channel, use a spectrometer to obtain the reflection spectrum intensity value I ₁ at 1550nm, and then perform step 4;

Step 4: Using the reflection spectrum intensity reference value I ₀ and the reflection spectrum intensity value I ₁ to obtain the reflectivity R of the contact surface between the air channel filled with the liquid to be measured and the fiber core of the optical fiber, and then perform step 5;

Step 5: Obtain the refractive index n ₂ of the liquid to be measured by using the reflectivity R of the contact surface between the air channel filled with the liquid to be measured and the fiber core of the optical fiber.

An optical fiber sensing head of the present invention and an optical fiber sensing system and method based on the sensing head for measuring the refractive index of a liquid overcome the problem of using an existing optical fiber sensing head to set an FP cavity outside the optical fiber in the prior art Instead, by opening an air channel on the optical fiber, the direction of the optical path is changed, so that the optical fiber cladding is used as the FP cavity, which avoids the influence of the liquid to be measured on the reflectivity of the two interfaces of the FP cavity at the same time. When the light incident into the sensor head is at S polarization, the maximum sensitivity for measuring the refractive index can be as high as -298.3dB/RIU, and when it is at P polarization, the maximum sensitivity for measuring the refractive index can be as high as -597.1dB/RIU.

The optical fiber sensing head and the optical fiber sensing system and method for measuring the refractive index of liquid based on the sensing head described in the present invention are suitable for measuring the refractive index of liquid.

Description of drawings

Fig. 1 is a schematic cross-sectional structure diagram of an optical fiber sensing head;

Fig. 2 is a schematic diagram of the principle of an optical fiber sensing system for measuring the refractive index of a liquid;

Fig. 3 is a flow chart of an optical fiber sensing method for measuring liquid refractive index;

Fig. 4 is the spectral change diagram of the reflection spectrum of the sensor head for the refractive index during S polarization;

Fig. 5 is a graph showing the spectral change of the reflectance spectrum of the sensing head with respect to the refractive index in the case of P polarization.

detailed description

Specific embodiment 1: This embodiment is described in detail with reference to FIG. 1. An optical fiber sensing head described in this embodiment includes: an optical fiber and a reflective material 4-3;

The optical fiber includes: an optical fiber cladding 4-1 and an optical fiber core 4-2;

The optical fiber cladding 4-1 is wrapped on the outside of the optical fiber core 4-2; a quadrangular prism-shaped air channel 4-4 is opened on the optical fiber core 4-2, and the cross section of the air channel 4-5 is isosceles trapezoidal, And the angle between the plane where the waist of the isosceles trapezoid is located and the fiber axis is 45 degrees, the height of the isosceles trapezoid is equal to the diameter of the fiber core 4-2, and the length of the short base of the isosceles trapezoid is in the range of 10 μm to 20 μm Between, the length of the air channel 4-5 is equal to the diameter of the optical fiber, and the outside of the optical fiber cladding 4-1 corresponding to the short base of the isosceles trapezoid is fixed with a reflective material 4-3.

Embodiment 2: This embodiment further describes the optical fiber sensing head described in Embodiment 1. In this embodiment, the optical fiber is a single-mode optical fiber.

Embodiment 3: This embodiment further describes the optical fiber sensing head described in Embodiment 1. In this embodiment, the optical fiber is a multimode optical fiber.

Embodiment 4: This embodiment is a further description of the optical fiber sensing head described in Embodiment 1. In this embodiment, the material of the reflective material 4-3 and the material of the optical fiber core 4-2 same.

Specific embodiment five: this embodiment is described in detail with reference to Fig. 2, a kind of optical fiber sensing system for measuring liquid refractive index described in this embodiment, it comprises: light source 1, 3dB coupler 2, polarizer 3, sensing head 4. Spectrometer 5 and signal processing module 6;

The light emitted by the light source 1 is transmitted to the polarizer 3 through the 3dB coupler 2, the polarizer 3 is adjusted by the light and transmitted to the sensor head 4, the sensor head 4 reflects the light to the polarizer 3, and the polarizer 3 transmit the light reflected by the sensor head 4 to the 3dB coupler 2, and the 3dB coupler 2 transmits the light to the spectrometer 5, and the spectral signal output end of the spectrometer 5 is connected to the spectral signal input end of the signal processing module 6;

The signal processing module 6 includes the following units:

A unit for collecting the reflectance spectrum intensity at 1550nm of the reflectance spectrum obtained in the spectrometer 5;

A unit for obtaining the reflectivity R of the interface between the air channel 4-5 and the optical fiber core 4-2;

Obtain the unit of the refractive index n of the liquid to be measured _;

The sensing head 4 includes: an optical fiber and a reflective material 4-3;

The optical fiber includes: an optical fiber cladding 4-1 and an optical fiber core 4-2;

The optical fiber cladding 4-1 is wrapped on the outside of the optical fiber core 4-2; a quadrangular prism-shaped air channel 4-4 is opened on the optical fiber core 4-2, and the cross section of the air channel 4-5 is isosceles trapezoidal, And the angle between the plane where the waist of the isosceles trapezoid is located and the fiber axis is 45 degrees, the height of the isosceles trapezoid is equal to the diameter of the fiber core 4-2, and the length of the short base of the isosceles trapezoid is in the range of 10 μm to 20 μm Between, the length of the air channel 4-5 is equal to the diameter of the optical fiber, and the outer side of the optical fiber cladding 4-1 corresponding to the short base of the isosceles trapezoid is provided with a reflective material 4-3.

Embodiment 6: This embodiment is a further description of the optical fiber sensing system for measuring the refractive index of liquid described in Embodiment 5. In this embodiment, the light source 1 is a Gaussian light source.

Embodiment 7: This embodiment is specifically described with reference to FIG. 3 , a method for measuring the refractive index of a liquid described in this embodiment, which is implemented based on the following device,

The device includes: a light source 1, a 3dB coupler 2, a polarizer 3, a sensing head 4 and a spectrometer 5;

The light emitted by the light source 1 is transmitted to the polarizer 3 through the 3dB coupler 2, the polarizer 3 is adjusted by the light and transmitted to the sensor head 4, the sensor head 4 reflects the light to the polarizer 3, and the polarizer 3 transmit the light reflected by the sensor head 4 to the 3dB coupler 2, and the 3dB coupler 2 transmits the light to the spectrometer 5;

The sensing head 4 includes: an optical fiber and a reflective material 4-3;

The optical fiber includes: an optical fiber cladding 4-1 and an optical fiber core 4-2;

The optical fiber cladding 4-1 is wrapped on the outside of the optical fiber core 4-2; a quadrangular prism-shaped air channel 4-4 is opened on the optical fiber core 4-2, and the cross section of the air channel 4-5 is isosceles trapezoidal, And the angle between the plane where the waist of the isosceles trapezoid is located and the fiber axis is 45 degrees, the height of the isosceles trapezoid is equal to the diameter of the fiber core 4-2, and the length of the short base of the isosceles trapezoid is in the range of 10 μm to 20 μm Between, the length of the air channel 4-5 is equal to the diameter of the optical fiber, and the outer side of the optical fiber cladding 4-1 corresponding to the short base of the isosceles trapezoid is provided with a reflective material 4-3;

A method for measuring the refractive index of liquid based on the above-mentioned device comprises the following steps:

Step 1: adjust the polarizer 3 so that the light incident on the sensor head 4 is light in one polarization state; then perform step 2;

Step 2: keep the sensing head 4 not immersed in the liquid, use the spectrometer 5 to obtain the reference value I ₀ of the reflection spectrum intensity at 1550nm, and then perform step 3;

Step 3: Immerse the sensor head 4 in the liquid to be tested, make the liquid to be tested enter the air channel (4-5), use the spectrometer 5 to obtain the reflection spectrum intensity value I ₁ of the reflection spectrum at 1550nm, and then perform step 4;

Step 4: Using the reflection spectrum intensity reference value I ₀ and the reflection spectrum intensity value I ₁ to obtain the reflectance R of the contact surface between the air channel 4-5 and the optical fiber core 4-2, and then perform step 5;

Step 5: Obtain the refractive index n ₂ of the liquid to be measured by using the reflectivity R of the interface between the air channel 4-5 filled with the liquid to be measured and the optical fiber core 4-2.

Embodiment 8: This embodiment is a further description of a method for measuring the refractive index of liquid described in Embodiment 7. In this embodiment, the air channel 4-5 filled with the liquid to be measured is obtained according to the following formula: The reflectivity R of the contact surface of the fiber core 4-2,

lg(I ₁ /I ₀ )=lg(R).

Specific Embodiment Nine: This embodiment is a further description of a method for measuring the refractive index of a liquid described in Embodiment 7. In this embodiment, the polarizer 3 is adjusted in step 1 so that the incident When the light is S-polarized, the refractive index n ₂ of the liquid to be measured is obtained according to the following formula,

$\left\{\begin{array}{c}\mathrm{<span\; class="notranslate">\; R</span>}<span\; class="notranslate">\; =</span>{<span\; class="notranslate">\; (</span>\frac{{\mathrm{<span\; class="notranslate">\; ncos\&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; cos\&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; ncos\&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; cos\&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}\\ \mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; /</span>{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}\end{array}\right.<span\; class="notranslate">\; ,</span>$

Wherein, n ₁ is the refractive index of the optical fiber core 4-2, θ ₁ is the angle between the light incident in the sensing head 4 and the plane where the air channel 4-5 and the contact surface of the optical fiber core 4-2 are located, θ ₂ is the refraction angle when the light incident on the sensor head 4 is refracted on the interface between the air channel 4-5 and the fiber core 4-2.

Embodiment 10: This embodiment is a further description of a method for measuring the refractive index of a liquid described in Embodiment 7. In this embodiment, the polarizer 3 is adjusted in step 1 so that the incident light into the sensor head 4 When the light is P-polarized, the refractive index n ₂ of the liquid to be measured is obtained according to the following formula,

$\left\{\begin{array}{c}\mathrm{<span\; class="notranslate">\; R</span>}<span\; class="notranslate">\; =</span>{<span\; class="notranslate">\; (</span>\frac{<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; ncos\&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; cos\&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}{{\mathrm{<span\; class="notranslate">\; ncos\&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; cos\&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}\\ \mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; /</span>{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}\end{array}\right.<span\; class="notranslate">\; ,</span>$

Wherein, n ₁ is the refractive index of the optical fiber core 4-2, θ ₁ is the angle between the light incident in the sensing head 4 and the plane where the air channel 4-5 and the contact surface of the optical fiber core 4-2 are located, θ ₂ is the refraction angle when the light incident on the sensor head 4 is refracted on the interface between the air channel 4-5 and the fiber core 4-2.

The fiber cladding 4-1 can be regarded as a Fabry-Perot (FP) cavity in function, and the signal light will be continuously reflected in the FP cavity. Since the reflectivity of the light beam on both sides of the FP cavity is equal and low, the transmission The reflection spectrum of the sensing probe can be expressed directly and simply by two-beam interference. which is:

Among them, R is the reflectivity of the slope of the air channel 4-5; I ₁ is the light intensity of the reflected light on one reflective surface of the FP cavity, and I ₂ is the light intensity of the reflected light on the other reflective surface of the FP cavity; λ is the light emitted by the light source 1 The light intensity of the light, n ₃ is the refractive index of the fiber cladding 4-1; L is the cavity length of the FP cavity, which is the thickness of the fiber cladding 4-1, is the initial phase.

Set the incident light as a Gaussian light source, with a bandwidth of 200nm, a spectral range of 1200nm to 1600nm, and a central wavelength of 1500nm. The refractive index of the liquid to be measured is generally 1.33 (water)-1.44, and the polarizer is adjusted so that the signal light is at S In the case of polarization, the data results of the communication window 1550nm were extracted from the measured reflection spectrum, and the measured results after processing are shown in Figure 4. increasing, the reflected light decreases, showing a monotonous change. The refractive index sensitivity of the sensor head is at least -73.6dB/RIU, and the maximum sensitivity can be as high as -298.3dB/RIU, which is fully capable of refractive index sensing.

Similarly, when the polarizer is adjusted so that the signal light is in the P polarization, other parameter settings are the same, and the measurement results obtained are shown in Figure 5. As the refractive index increases and gets closer to the core refractive index, the reflected light continues to Decrease, monotonous change. Moreover, it is found through calculation that the refractive index sensitivity of the sensor head is at least -148.0dB/RIU, and the maximum sensitivity can be as high as -597.1dB/RIU, which is fully capable of refractive index sensing.

Claims (9)

1. An optical fiber sensor head, is characterized in that, it comprises: optical fiber and reflection material (4-3); The optical fiber comprises: an optical fiber cladding (4-1) and an optical fiber core (4-2); The optical fiber cladding (4-1) is wrapped on the outside of the optical fiber core (4-2); on the optical fiber core (4-2), there is a quadrangular prism-shaped air passage (4-4), and the air passage (4 -5) is an isosceles trapezoid in section, and the angle between the plane where the waist of the isosceles trapezoid is located and the fiber axis is 45 degrees, the height of the isosceles trapezoid is equal to the diameter of the optical fiber core (4-2), and the isosceles trapezoid is equal to the diameter of the optical fiber core (4-2). The length of the short base of the trapezoid is between 10 μm and 20 μm, the length of the air channel (4-5) is equal to the diameter of the optical fiber, and the outer side of the optical fiber cladding (4-1) corresponding to the short base of the isosceles trapezoid is fixed There is a reflective material (4-3); the material of the reflective material (4-3) is the same as that of the optical fiber core (4-2). 2. An optical fiber sensing head according to claim 1, wherein the optical fiber is a single-mode optical fiber. 3. An optical fiber sensing head according to claim 1, wherein the optical fiber is a multimode optical fiber. 4. An optical fiber sensing system for measuring the refractive index of a liquid, characterized in that it comprises: a light source (1), a 3dB coupler (2), a polarizer (3), a sensing head (4), and a spectrometer (5) and a signal processing module (6); The light emitted by the light source (1) is transmitted to the polarizer (3) through the 3dB coupler (2), and the polarizer (3) is adjusted by the light and transmitted to the sensor head (4), the sensor head (4) This light is reflected into the polarizer (3), the polarizer (3) transmits the light reflected by the sensing head (4) into the 3dB coupler (2), and the 3dB coupler (2) transmits the light to the spectrometer ( In 5), the spectral signal output end of the spectrometer (5) is connected to the spectral signal input end of the signal processing module (6); The signal processing module (6) includes the following units: A unit for collecting the reflection spectrum intensity at 1550nm of the reflection spectrum obtained in the spectrometer (5); A unit for obtaining the reflectivity R of the interface between the air channel (4-5) and the fiber core (4-2); Obtain the unit of the refractive index n of the liquid to be measured _; The sensor head (4) includes: an optical fiber and a reflective material (4-3); The optical fiber comprises: an optical fiber cladding (4-1) and an optical fiber core (4-2); The optical fiber cladding (4-1) is wrapped on the outside of the optical fiber core (4-2); on the optical fiber core (4-2), there is a quadrangular prism-shaped air passage (4-4), and the air passage (4 -5) is an isosceles trapezoid in section, and the angle between the plane where the waist of the isosceles trapezoid is located and the fiber axis is 45 degrees, the height of the isosceles trapezoid is equal to the diameter of the optical fiber core (4-2), and the isosceles trapezoid is equal to the diameter of the optical fiber core (4-2). The length of the short base of the trapezoid is between 10 μm and 20 μm, the length of the air channel (4-5) is equal to the diameter of the optical fiber, and the outer side of the optical fiber cladding (4-1) corresponding to the short base of the isosceles trapezoid With reflective material (4-3). 5. An optical fiber sensing system for measuring the refractive index of liquid according to claim 4, characterized in that the light source (1) is a Gaussian light source. 6. A method for measuring the refractive index of a liquid, characterized in that the method is realized based on the following devices, The device comprises: a light source (1), a 3dB coupler (2), a polarizer (3), a sensor head (4) and a spectrometer (5); The light emitted by the light source (1) is transmitted to the polarizer (3) through the 3dB coupler (2), and the polarizer (3) is adjusted by the light and transmitted to the sensor head (4), the sensor head (4) This light is reflected into the polarizer (3), the polarizer (3) transmits the light reflected by the sensing head (4) into the 3dB coupler (2), and the 3dB coupler (2) transmits the light to the spectrometer ( 5) in; The sensor head (4) includes: an optical fiber and a reflective material (4-3); The optical fiber comprises: an optical fiber cladding (4-1) and an optical fiber core (4-2); The optical fiber cladding (4-1) is wrapped on the outside of the optical fiber core (4-2); on the optical fiber core (4-2), there is a quadrangular prism-shaped air passage (4-4), and the air passage (4 -5) is an isosceles trapezoid in section, and the angle between the plane where the waist of the isosceles trapezoid is located and the fiber axis is 45 degrees, the height of the isosceles trapezoid is equal to the diameter of the optical fiber core (4-2), and the isosceles trapezoid is equal to the diameter of the optical fiber core (4-2). The length of the short base of the trapezoid is between 10 μm and 20 μm, the length of the air channel (4-5) is equal to the diameter of the optical fiber, and the outer side of the optical fiber cladding (4-1) corresponding to the short base of the isosceles trapezoid With reflective material (4-3); A method for measuring the refractive index of liquid based on the above-mentioned device comprises the following steps: Step 1: adjust the polarizer (3), so that the light incident on the sensor head (4) is light in one polarization state; then perform step 2; Step 2: Keep the sensor head (4) not immersed in the liquid, use the spectrometer (5) to obtain the reference value I ₀ of the reflection spectrum intensity at 1550nm, and then perform step 3; Step 3: Immerse the sensor head (4) in the liquid to be tested, let the liquid to be tested enter the air channel (4-5), use the spectrometer (5) to obtain the reflection spectrum intensity value I ₁ at 1550nm of the reflection spectrum, and then execute Step four; Step 4: Using the reflection spectrum intensity reference value I ₀ and the reflection spectrum intensity value I ₁ to obtain the reflectivity R of the contact surface between the air channel (4-5) and the optical fiber core (4-2) filled with the liquid to be measured, and then Execute step five; Step 5: Obtain the refractive index n ₂ of the liquid to be measured by using the reflectivity R of the contact surface between the air channel (4-5) filled with the liquid to be measured and the fiber core (4-2). 7. a kind of method for measuring liquid refractive index according to claim 6, is characterized in that, obtain the air channel (4-5) that is filled with liquid to be measured and the contact surface of optical fiber core (4-2) according to following formula The reflectivity R, lg(I ₁ /I ₀ )=lg(R). 8. a kind of method for measuring liquid refractive index according to claim 6 is characterized in that, in step 1, adjust polarizer (3), when making the light incident in the sensor head (4) be S polarization, according to The following formula obtains the refractive index n ₂ of the liquid to be measured, $\left\{\begin{array}{c}\mathrm{<span\; class="notranslate">\; R</span>}<span\; class="notranslate">\; =</span>{<span\; class="notranslate">\; (</span>\frac{{\mathrm{<span\; class="notranslate">\; ncos\&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; cos\&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; ncos\&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; cos\&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}\\ \mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; /</span>{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}\end{array}\right.<span\; class="notranslate">\; ,</span>$ Among them, n ₁ is the refractive index of the fiber core (4-2), θ ₁ is the contact surface between the light and air channel (4-5) and the fiber core (4-2) incident in the sensor head (4) The included angle of the plane, θ ₂ is the angle of refraction when the light incident on the sensing head (4) is refracted on the interface between the air channel (4-5) and the fiber core (4-2). 9. a kind of method for measuring liquid refractive index according to claim 6 is characterized in that, in step 1, adjust polarizer (3), when making the light incident in the sensing head (4) be P polarization, according to The following formula obtains the refractive index n ₂ of the liquid to be measured, $\left\{\begin{array}{c}\mathrm{<span\; class="notranslate">\; R</span>}<span\; class="notranslate">\; =</span>{<span\; class="notranslate">\; (</span>\frac{<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; ncos\&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; cos\&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}{{\mathrm{<span\; class="notranslate">\; ncos\&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; cos\&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}\\ \mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; /</span>{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}\end{array}\right.<span\; class="notranslate">\; ,</span>$ Among them, n ₁ is the refractive index of the fiber core (4-2), θ ₁ is the contact surface between the light and air channel (4-5) and the fiber core (4-2) incident in the sensor head (4) The included angle of the plane, θ ₂ is the angle of refraction when the light incident on the sensing head (4) is refracted on the interface between the air channel (4-5) and the fiber core (4-2).