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

A kind of optical fiber sensor head and based on this sensing head measure the optical fiber sensing system of liquid refractivity and method, belongs to optical fiber sensing system field.It is to solve close together away from optical fiber tail-end of existing fiber sensing head FP chamber, and then the problem that reflectance is produced impact.A kind of optical fiber sensor head of the present invention and based on this sensing head measure the optical fiber sensing system of liquid refractivity and method, overcome in prior art, by opening an air duct on optical fiber, change the direction of light path, thus using fibre cladding as FP chamber, it is to avoid testing liquid impact on two interface reflectance in FP chamber simultaneously.When the light incided in sensing head is in S-polarization, the peak response measuring refractive index reaches 298.3dB/RIU, and when being in P polarization, the peak response measuring refractive index reaches 597.1dB/RIU.The present invention is applicable to measure the refractive index of liquid.

Description

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

Technical field

The invention belongs to optical fiber sensing system field.

Background technology

Due to good characteristics such as the low cost of optical fiber, anticorrosive, high sensitivities, optical fiber obtains in terms of refractive index sensing in recent years The biggest concern has also extensively applied to the fields such as biological, chemical.Optic fibre refractive index sensor be can be divided mainly into by detecting light spectrum Two classes, transmission-type Fibre Optical Sensor and reflective optical sensor.Relative to reflective optical sensor, transmission-type optical fiber passes The sensor-based system of sensor is taken up space relatively large, limits it and carries out direct refractive index detection in vivo.For instead Emitting Fibre Optical Sensor, generally uses optical fiber to combine outer fiber Fabry-Perot (FP) chamber or inside of optical fibre FP chamber and comes The change of detection refractive index；And outside FP chamber, it is faced with sensing head equally and is taken up space relatively large problem.Thus compare For, sensor based on inside of optical fibre FP chamber is wider in terms of the Application in Sensing of refractive index.But it is traditional based on interior Although the Fibre Optical Sensor size in FP chamber, portion is little, distance is big, due to close together away from optical fiber tail-end of FP chamber, works as test fluid During body, liquid is in addition to producing impact to the reflectance of FP reflecting surface, and the reflectance of optical fiber tail-end is produced impact, both Impact can occur intersection sensing, thus the reflective light intensity that derivative spectomstry instrument records cannot become in dullness for testing liquid refractive index Change, monotone variation can only be presented in one section of the narrowest ranges of indices of refraction, and monotone area is along with testing liquid refractive index Change and change.Thus greatly limit the Fibre Optical Sensor based on internal FP chamber application in terms of refractometry.

Summary of the invention

The present invention is to solve close together away from optical fiber tail-end of existing fiber sensing head FP chamber, and then reflectance is produced shadow The problem rung, now provides a kind of optical fiber sensor head and the optical fiber sensing system measuring liquid refractivity based on this sensing head and side Method.

A kind of optical fiber sensor head, it includes: optical fiber and reflecting material；

Described optical fiber includes: fibre cladding and fiber core；

Fibre cladding is wrapped in the outside of fiber core；Having the air duct of four prism type on fiber core, this air leads to The cross section in road is isosceles trapezoid, and the angle of the waist place plane of this isosceles trapezoid and fiber axis is 45 degree, this isosceles trapezoid High in the diameter of fiber core, the length range on the short base of this isosceles trapezoid is between 10 μm to 20 μm, and air leads to The length in road, equal to the diameter of optical fiber, is fixed with reflecting material outside the fibre cladding corresponding to short base of this isosceles trapezoid；

The material of described reflecting material is identical with the material of fiber core.

A kind of optical fiber sensing system measuring liquid refractivity, it includes: light source, three-dB coupler, polariser, sensing head, Spectrogrph and signal processing module；

The light that light source sends is in three-dB coupler transmission to polariser, and polariser is adjusted through this light and transmits to sensing head In, sensing head is by this luminous reflectance to polariser, in the optical transport that sensing head is reflected by polariser to three-dB coupler, and 3dB Bonder is by this optical transport to spectrogrph, and the spectral signal that the spectral signal outfan of spectrogrph connects signal processing module is defeated Enter end；

Described signal processing module includes with lower unit:

Gather the unit of the reflectance spectrum obtained in spectrogrph reflectance spectrum intensity at 1550nm；

Obtain the unit of air duct and the reflectance R of fiber core contact surface；

Obtain refractive index n of testing liquid₂Unit；

Described sensing head includes: optical fiber and reflecting material；

Described optical fiber includes: fibre cladding and fiber core；

Fibre cladding is wrapped in the outside of fiber core；Having the air duct of four prism type on fiber core, this air leads to The cross section in road is isosceles trapezoid, and the angle of the waist place plane of this isosceles trapezoid and fiber axis is 45 degree, this isosceles trapezoid High in the diameter of fiber core, the length range on the short base of this isosceles trapezoid is between 10 μm to 20 μm, and air leads to The length in road, equal to the diameter of optical fiber, is provided with reflecting material outside the fibre cladding corresponding to short base of this isosceles trapezoid.

A kind of method measuring liquid refractivity, the method realizes based on following apparatus,

Described device includes: light source, three-dB coupler, polariser, sensing head and spectrogrph；

The light that light source sends is in three-dB coupler transmission to polariser, and polariser is adjusted through this light and transmits to sensing head In, sensing head is by this luminous reflectance to polariser, in the optical transport that sensing head is reflected by polariser to three-dB coupler, and 3dB Bonder is by this optical transport to spectrogrph；

Described sensing head includes: optical fiber and reflecting material；

Described optical fiber includes: fibre cladding and fiber core；

Fibre cladding is wrapped in the outside of fiber core；Having the air duct of four prism type on fiber core, this air leads to The cross section in road is isosceles trapezoid, and the angle of the waist place plane of this isosceles trapezoid and fiber axis is 45 degree, this isosceles trapezoid High in the diameter of fiber core, the length range on the short base of this isosceles trapezoid is between 10 μm to 20 μm, and air leads to The length in road, equal to the diameter of optical fiber, is provided with reflecting material outside the fibre cladding corresponding to short base of this isosceles trapezoid；

A kind of method measuring liquid refractivity based on said apparatus comprises the following steps:

Step one: regulation polariser, making the light inciding in sensing head is the light under a kind of polarization state；Then step 2 is performed；

Step 2: keep sensing head not immerse in liquid, utilize spectrogrph to obtain reflectance spectrum reflectance spectrum at 1550nm strong Degree reference value I₀, then perform step 3；

Step 3: immersed by sensing head in testing liquid, makes testing liquid enter air duct, utilizes spectrogrph to obtain reflection Spectrum reflectance spectrum intensity level I at 1550nm₁, then perform step 4；

Step 4: utilize reflectance spectrum intensity reference value I₀With reflectance spectrum intensity level I₁, it is thus achieved that it is filled with the air duct of testing liquid With the reflectance R of fiber core contact surface, then perform step 5；

Step 5: utilize and be filled with the air duct of testing liquid and the reflectance R of fiber core contact surface, it is thus achieved that testing liquid Refractive index n₂。

A kind of optical fiber sensor head of the present invention and based on this sensing head measure the optical fiber sensing system of liquid refractivity and side Method, overcomes in prior art, uses existing fiber sensing head to arrange the intrinsic thinking in FP chamber in outer fiber, but passes through Optical fiber is opened an air duct, changes the direction of light path, thus using fibre cladding as FP chamber, it is to avoid testing liquid Impact on two interface reflectance in FP chamber simultaneously.When the light incided in sensing head is in S-polarization, measure refractive index Peak response can be up to-298.3dB/RIU, when being in P polarization, measure refractive index peak response can be up to -597.1dB/RIU。

A kind of optical fiber sensor head of the present invention and based on this sensing head measure the optical fiber sensing system of liquid refractivity and side Method, it is adaptable to the refractive index of liquid is measured.

Accompanying drawing explanation

Fig. 1 is the cross-sectional view of a kind of optical fiber sensor head；

Fig. 2 is the principle schematic of a kind of optical fiber sensing system measuring liquid refractivity；

Fig. 3 is the flow chart of a kind of optical fiber sensing method measuring liquid refractivity；

When Fig. 4 is S-polarization, sensing head reflectance spectrum is for the spectrum change figure of refractive index；

When Fig. 5 is P polarization, sensing head reflectance spectrum is for the spectrum change figure of refractive index.

Detailed description of the invention

Detailed description of the invention one: illustrate present embodiment with reference to Fig. 1, a kind of optical fiber sensor head described in present embodiment, It includes: optical fiber and reflecting material 4-3；

Described optical fiber includes: fibre cladding 4-1 and fiber core 4-2；

Fibre cladding 4-1 is wrapped in the outside of fiber core 4-2；Fiber core 4-2 has the air duct of four prism type 4-4, the cross section of this air duct 4-5 is isosceles trapezoid, and the angle of the waist place plane of this isosceles trapezoid and fiber axis is 45 Degree, this isosceles trapezoid high in the diameter of fiber core 4-2, the length range on the short base of this isosceles trapezoid is in 10 μm Between 20 μm, the length of air duct 4-5 is equal to the diameter of optical fiber, the optical fiber corresponding to short base of this isosceles trapezoid Reflecting material 4-3 it is fixed with outside covering 4-1.

Detailed description of the invention two: present embodiment is to make a kind of optical fiber sensor head described in detailed description of the invention one furtherly Bright, in present embodiment, described optical fiber is single-mode fiber.

Detailed description of the invention three: present embodiment is to make a kind of optical fiber sensor head described in detailed description of the invention one furtherly Bright, in present embodiment, described optical fiber is multimode fibre.

Detailed description of the invention four: present embodiment is to make a kind of optical fiber sensor head described in detailed description of the invention one furtherly Bright, in present embodiment, the material of described reflecting material 4-3 is identical with the material of fiber core 4-2.

Detailed description of the invention five: illustrate present embodiment with reference to Fig. 2, a kind of liquid of measuring described in present embodiment is rolled over Penetrating the optical fiber sensing system of rate, it includes: light source 1, three-dB coupler 2, polariser 3, sensing head 4, spectrogrph 5 and Signal processing module 6；

The light that light source 1 sends transmits to polariser 3 through three-dB coupler 2, and polariser 3 is adjusted through this light and transmits To sensing head 4, sensing head 4 is by this luminous reflectance to polariser 3, and the optical transport that sensing head 4 is reflected by polariser 3 is extremely In three-dB coupler 2, three-dB coupler 2 is by this optical transport to spectrogrph 5, and the spectral signal outfan of spectrogrph 5 is even Connect the spectral signal input of signal processing module 6；

Described signal processing module 6 includes with lower unit:

Gather the unit of the reflectance spectrum obtained in spectrogrph 5 reflectance spectrum intensity at 1550nm；

Obtain the unit of air duct 4-5 and the reflectance R of fiber core 4-2 contact surface；

Obtain refractive index n of testing liquid₂Unit；

Described sensing head 4 includes: optical fiber and reflecting material 4-3；

Described optical fiber includes: fibre cladding 4-1 and fiber core 4-2；

Fibre cladding 4-1 is wrapped in the outside of fiber core 4-2；Fiber core 4-2 has the air duct of four prism type 4-4, the cross section of this air duct 4-5 is isosceles trapezoid, and the angle of the waist place plane of this isosceles trapezoid and fiber axis is 45 Degree, this isosceles trapezoid high in the diameter of fiber core 4-2, the length range on the short base of this isosceles trapezoid is in 10 μm Between 20 μm, the length of air duct 4-5 is equal to the diameter of optical fiber, the optical fiber corresponding to short base of this isosceles trapezoid Reflecting material 4-3 it is provided with outside covering 4-1.

Detailed description of the invention six: present embodiment is to a kind of optical fiber measuring liquid refractivity described in detailed description of the invention five Sensor-based system is described further, and in present embodiment, described light source 1 is Gauss light source.

Detailed description of the invention seven: illustrate present embodiment with reference to Fig. 3, a kind of liquid of measuring described in present embodiment is rolled over The method penetrating rate, the method realizes based on following apparatus,

Described device includes: light source 1, three-dB coupler 2, polariser 3, sensing head 4 and spectrogrph 5；

The light that light source 1 sends transmits to polariser 3 through three-dB coupler 2, and polariser 3 is adjusted through this light and transmits To sensing head 4, sensing head 4 is by this luminous reflectance to polariser 3, and the optical transport that sensing head 4 is reflected by polariser 3 is extremely In three-dB coupler 2, three-dB coupler 2 is by this optical transport to spectrogrph 5；

Described sensing head 4 includes: optical fiber and reflecting material 4-3；

Described optical fiber includes: fibre cladding 4-1 and fiber core 4-2；

Fibre cladding 4-1 is wrapped in the outside of fiber core 4-2；Fiber core 4-2 has the air duct of four prism type 4-4, the cross section of this air duct 4-5 is isosceles trapezoid, and the angle of the waist place plane of this isosceles trapezoid and fiber axis is 45 Degree, this isosceles trapezoid high in the diameter of fiber core 4-2, the length range on the short base of this isosceles trapezoid is in 10 μm Between 20 μm, the length of air duct 4-5 is equal to the diameter of optical fiber, the optical fiber corresponding to short base of this isosceles trapezoid Reflecting material 4-3 it is provided with outside covering 4-1；

A kind of method measuring liquid refractivity based on said apparatus comprises the following steps:

Step one: regulation polariser 3, making the light inciding in sensing head 4 is the light under a kind of polarization state；Then step is performed Rapid two；

Step 2: keep sensing head 4 not immerse in liquid, utilize spectrogrph 5 to obtain reflectance spectrum reflection at 1550nm Spectral intensity reference value I₀, then perform step 3；

Step 3: immerse in testing liquid by sensing head 4, makes testing liquid enter air duct (4-5), utilizes spectrogrph 5 Obtain reflectance spectrum reflectance spectrum intensity level I at 1550nm₁, then perform step 4；

Step 4: utilize reflectance spectrum intensity reference value I₀With reflectance spectrum intensity level I₁, it is thus achieved that air duct 4-5 and fiber core The reflectance R of 4-2 contact surface, then performs step 5；

Step 5: utilize and be filled with the air duct 4-5 of testing liquid and the reflectance R of fiber core 4-2 contact surface, it is thus achieved that treat Survey refractive index n of liquid₂。

Detailed description of the invention eight: present embodiment is to a kind of method measuring liquid refractivity described in detailed description of the invention seven Being described further, in present embodiment, the air duct 4-5 being filled with testing liquid according to equation below acquisition is long and slender with light The reflectance R of core 4-2 contact surface,

lg(I₁/I₀)=lg (R).

Detailed description of the invention nine: present embodiment is to a kind of method measuring liquid refractivity described in detailed description of the invention seven Being described further, in present embodiment, regulate polariser 3 in step one, making the light inciding in sensing head 4 is S-polarization Time, refractive index n of testing liquid is obtained according to equation below₂,

$\left\{\begin{array}{c}R={\left(\frac{{\mathrm{ncos\θ}}_1-{\mathrm{cos\θ}}_2}{{\mathrm{ncos\θ}}_1+{\mathrm{cos\θ}}_2}\right)}^2\\ n=n_1/n_2\end{array}\right.,$

Wherein, n₁For the refractive index of fiber core 4-2, θ₁For inciding the light in sensing head 4 and air duct 4-5 and optical fiber The angle of fibre core 4-2 contact surface place plane, θ₂For inciding the light in sensing head 4 at air duct 4-5 and fiber core Refraction angle during refraction is there is on 4-2 contact surface.

Detailed description of the invention ten: present embodiment is to a kind of method measuring liquid refractivity described in detailed description of the invention seven Being described further, in present embodiment, regulate polariser 3 in step one, making the light inciding in sensing head 4 is that P is inclined When shaking, obtain refractive index n of testing liquid according to equation below₂,

$\left\{\begin{array}{c}R={\left(\frac{-{\mathrm{ncos\θ}}_2+{\mathrm{cos\θ}}_1}{{\mathrm{ncos\θ}}_2+{\mathrm{cos\θ}}_1}\right)}^2\\ n=n_1/n_2\end{array}\right.,$

Wherein, n₁For the refractive index of fiber core 4-2, θ₁For inciding the light in sensing head 4 and air duct 4-5 and optical fiber The angle of fibre core 4-2 contact surface place plane, θ₂For inciding the light in sensing head 4 at air duct 4-5 and fiber core Refraction angle during refraction is there is on 4-2 contact surface.

Fibre cladding 4-1 functionally can be considered Fabry-Perot (FP) chamber, and flashlight can be the most anti-in FP chamber Penetrating, owing to light beam reflectance on two sides on FP chamber is equal and relatively low, thus the reflectance spectrum of this sensor probe can be the simplest Represent with two-beam interference.That is:

Wherein, R is the reflectance on air duct 4-5 inclined-plane；I₁For reflecting the light intensity of light, I on the reflecting surface of one, FP chamber₂For The light intensity of light is reflected on another reflecting surface of FP chamber；λ is the light intensity of the light that light source 1 sends, n₃Folding for fibre cladding 4-1 Penetrate rate；L is that the chamber in FP chamber is long, is the thickness of fibre cladding 4-1,For initial phase.

Setting incident illumination as Gauss light source, carry a width of 200nm, spectral region is 1200nm to 1600nm, and centre wavelength is 1500nm, selected 1.33 generally conventional (water)-1.44 of testing liquid refractive index, regulate polariser, allow flashlight be in S During polarization, from measured reflectance spectrum, extract the data result of communication window 1550nm, the measurement result obtained after process As shown in Figure 4, along with refractive index constantly increases, becoming closer to fiber core refractive index, transmission light constantly increases, and reflection light is not Disconnected reduction, in monotone variation.The refractive index sensitivity of sensing head is minimum is-73.6dB/RIU, and peak response can be up to-298.3 DB/RIU, has the ability to carry out refractive index sensing completely.

In like manner, regulating polariser, when allowing flashlight be in P polarization, other parameters arrange identical, and the measurement result obtained is such as Shown in Fig. 5, along with refractive index constantly increases, becoming closer to fiber core refractive index, reflection light constantly reduces, in monotone variation. Further, being found by calculating, the refractive index sensitivity of this sensing head is minimum is-148.0dB/RIU, and peak response can be up to -597.1dB/RIU, has the ability to carry out refractive index sensing completely.

Claims (9)

1. an optical fiber sensor head, it is characterised in that it includes: optical fiber and reflecting material (4-3)；

Described optical fiber includes: fibre cladding (4-1) and fiber core (4-2)；

Fibre cladding (4-1) is wrapped in the outside of fiber core (4-2)；Fiber core (4-2) has four prism type Air duct (4-4), the cross section of this air duct (4-5) is isosceles trapezoid, and the waist place plane of this isosceles trapezoid with The angle of fiber axis is 45 degree, this isosceles trapezoid high in the diameter of fiber core (4-2), the short end of this isosceles trapezoid The length range on limit is between 10 μm to 20 μm, and the length of air duct (4-5) is equal to the diameter of optical fiber, these isosceles The trapezoidal outside of the fibre cladding (4-1) corresponding to short base is fixed with reflecting material (4-3)；Described reflecting material (4-3) Material identical with the material of fiber core (4-2).

A kind of optical fiber sensor head the most according to claim 1, it is characterised in that described optical fiber is single-mode fiber.

A kind of optical fiber sensor head the most according to claim 1, it is characterised in that described optical fiber is multimode fibre.

4. the optical fiber sensing system measuring liquid refractivity, it is characterised in that it includes: light source (1), 3dB couple Device (2), polariser (3), sensing head (4), spectrogrph (5) and signal processing module (6)；

The light that light source (1) sends is in three-dB coupler (2) transmission to polariser (3), and polariser (3) enters through this light Row regulation is also transmitted to sensing head (4), sensing head (4) by this luminous reflectance to polariser (3), polariser (3) The optical transport that sensing head (4) is reflected in three-dB coupler (2), three-dB coupler (2) by this optical transport to spectrogrph (5), in, the spectral signal outfan of spectrogrph (5) connects the spectral signal input of signal processing module (6)；

Described signal processing module (6) includes with lower unit:

Gather the unit of the reflectance spectrum obtained in spectrogrph (5) reflectance spectrum intensity at 1550nm；

Obtain the unit of air duct (4-5) and the reflectance R of fiber core (4-2) contact surface；

Obtain refractive index n of testing liquid₂Unit；

Described sensing head (4) including: optical fiber and reflecting material (4-3)；

Described optical fiber includes: fibre cladding (4-1) and fiber core (4-2)；

Fibre cladding (4-1) is wrapped in the outside of fiber core (4-2)；Fiber core (4-2) has four prism type Air duct (4-4), the cross section of this air duct (4-5) is isosceles trapezoid, and the waist place plane of this isosceles trapezoid with The angle of fiber axis is 45 degree, this isosceles trapezoid high in the diameter of fiber core (4-2), the short end of this isosceles trapezoid The length range on limit is between 10 μm to 20 μm, and the length of air duct (4-5) is equal to the diameter of optical fiber, these isosceles The trapezoidal outside of the fibre cladding (4-1) corresponding to short base is provided with reflecting material (4-3).

A kind of optical fiber sensing system measuring liquid refractivity the most according to claim 4, it is characterised in that described light Source (1) is Gauss light source.

6. the method measuring liquid refractivity, it is characterised in that the method realizes based on following apparatus,

Described device includes: light source (1), three-dB coupler (2), polariser (3), sensing head (4) and spectrogrph (5)；

The light that light source (1) sends is in three-dB coupler (2) transmission to polariser (3), and polariser (3) enters through this light Row regulation is also transmitted to sensing head (4), sensing head (4) by this luminous reflectance to polariser (3), polariser (3) The optical transport that sensing head (4) is reflected in three-dB coupler (2), three-dB coupler (2) by this optical transport to spectrogrph (5) in；

Described sensing head (4) including: optical fiber and reflecting material (4-3)；

Described optical fiber includes: fibre cladding (4-1) and fiber core (4-2)；

Fibre cladding (4-1) is wrapped in the outside of fiber core (4-2)；Fiber core (4-2) has four prism type Air duct (4-4), the cross section of this air duct (4-5) is isosceles trapezoid, and the waist place plane of this isosceles trapezoid with The angle of fiber axis is 45 degree, this isosceles trapezoid high in the diameter of fiber core (4-2), the short end of this isosceles trapezoid The length range on limit is between 10 μm to 20 μm, and the length of air duct (4-5) is equal to the diameter of optical fiber, these isosceles The trapezoidal outside of the fibre cladding (4-1) corresponding to short base is provided with reflecting material (4-3)；

A kind of method measuring liquid refractivity based on said apparatus comprises the following steps:

Step one: regulation polariser (3), making to incide the light in sensing head (4) is the light under a kind of polarization state；Then hold Row step 2；

Step 2: keep sensing head (4) not immerse in liquid, utilizes spectrogrph (5) to obtain reflectance spectrum at 1550nm Reflectance spectrum intensity reference value I₀, then perform step 3；

Step 3: immersed in testing liquid by sensing head (4), makes testing liquid enter air duct (4-5), utilizes spectrum Instrument (5) obtains reflectance spectrum reflectance spectrum intensity level I at 1550nm₁, then perform step 4；

Step 4: utilize reflectance spectrum intensity reference value I₀With reflectance spectrum intensity level I₁, it is thus achieved that it is filled with the air duct of testing liquid (4-5) with the reflectance R of fiber core (4-2) contact surface, step 5 is then performed；

Step 5: utilize and be filled with the air duct (4-5) of testing liquid and the reflectance R of fiber core (4-2) contact surface, Obtain refractive index n of testing liquid₂。

A kind of method measuring liquid refractivity the most according to claim 6, it is characterised in that obtain according to equation below The air duct (4-5) of testing liquid and the reflectance R of fiber core (4-2) contact surface must be filled with,

lg(I₁/I₀)=lg (R).

A kind of method measuring liquid refractivity the most according to claim 6, it is characterised in that in step one, regulation is partially Shake device (3), makes to incide the light in sensing head (4) when being S-polarization, obtains the refractive index of testing liquid according to equation below n₂,

$\left\{\begin{array}{c}R={\left(\frac{{\mathrm{ncos\θ}}_1-{\mathrm{cos\θ}}_2}{{\mathrm{ncos\θ}}_1+{\mathrm{cos\θ}}_2}\right)}^2\\ n=n_1/n_2\end{array}\right.,$

Wherein, n₁For the refractive index of fiber core (4-2), θ₁For inciding the light in sensing head (4) and air duct (4-5) With the angle of fiber core (4-2) contact surface place plane, θ₂For inciding the light in sensing head (4) at air duct (4-5) With the refraction angle occurred on fiber core (4-2) contact surface when reflecting.

A kind of method measuring liquid refractivity the most according to claim 6, it is characterised in that in step one, regulation is partially Shake device (3), makes to incide the light in sensing head (4) when being P polarization, obtains the refractive index of testing liquid according to equation below n₂,

$\left\{\begin{array}{c}R={\left(\frac{-{\mathrm{ncos\θ}}_2+{\mathrm{cos\θ}}_1}{{\mathrm{ncos\θ}}_2+{\mathrm{cos\θ}}_1}\right)}^2\\ n=n_1/n_2\end{array}\right.,$

Wherein, n₁For the refractive index of fiber core (4-2), θ₁For inciding the light in sensing head (4) and air duct (4-5) With the angle of fiber core (4-2) contact surface place plane, θ₂For inciding the light in sensing head (4) at air duct (4-5) With the refraction angle occurred on fiber core (4-2) contact surface when reflecting.