CN103364105B - Optical fiber refractive index and temperature sensor based on multiple-mode interference and measuring method thereof - Google Patents

Abstract

The invention discloses an optical fiber refractive index and temperature sensor based on multimode interference and a measurement method thereof. The sensor includes a broadband light source, an optical fiber circulator, a measuring sensor head, and a spectrometer. The broadband light source is connected to the input port of the optical fiber circulator, and the optical fiber circulator The first output port is connected to the measurement sensing head through an optical fiber, and the second output port is connected to a spectrometer (4) through an optical fiber. During measurement, the light undergoes multi-mode interference inside the measuring sensor head, and Fresnel reflection occurs at the interface between the measuring sensor head and the substance to be measured, and then returns to the inside of the measuring sensor head to continue to propagate and undergo multi-mode interference, finally It is transmitted to the spectrometer (4), and the loss peak power and loss peak wavelength of the interference fringes are measured by the spectrometer (4), and then the refractive index and temperature of the substance to be measured are obtained through calculation. The invention can realize high-precision and wide-range refraction index and temperature measurement, and has simple structure and convenient operation.

Description

Optical fiber refractive index and temperature sensor based on multimode interference and its measurement method

technical field

The invention relates to a refraction index and temperature sensor, in particular to an optical fiber refraction index and temperature sensor based on multimode interference and a measuring method thereof.

Background technique

Fiber optic sensors have been extensively studied in recent years, and they have many advantages, such as small size, high sensitivity, anti-electromagnetic interference, and so on. They have attracted great interest in the field of remote measurement and process control, and can be used to measure temperature, stress, refractive index, displacement and other physical quantities. In recent years, the multimode interference phenomenon has been widely used in the field of sensors, such as the use of single-mode-multimode-single-mode (SMS) fiber structure, single-mode-multimode-single-mode fiber structure cascaded fiber Bragg grating, 3° Slanted multimode fiber Bragg grating, multimode-coreless-multimode fiber structure. All of the above methods are based on the multimode interference phenomenon that occurs in the optical fiber, however, most of these proposed methods are single-parameter measurements and the used single-mode-multimode-single-mode fiber structure is mainly transmission type, due to the fusion splicing The fusion joint between the single-mode optical fiber and the multi-mode optical fiber is likely to be broken when the bending is too large, so the transmission type structure is not easy to handle. In addition, the traditional sensing method of measuring the refractive index by using the multimode interference phenomenon generally needs to completely or partially remove the cladding of the multimode fiber, or even corrode part of the core, so that the core of the multimode fiber can fully contact the substance to be measured , so that the substance to be measured acts as the cladding of the multimode fiber core, causing the shift of the multimode interference resonance wavelength to achieve the measurement of the refractive index. The disadvantage of this method is that the tolerable strength is weakened due to the removal of the cladding of the fiber. The stability is reduced, the application range is limited, the production is complicated, and the cost is increased. The traditional method of measuring temperature, the traditional method of measuring temperature generally uses a fiber Bragg grating, or covers a certain material whose refractive index changes with temperature at the end of the single-mode fiber pigtail to change the relationship between the end of the single-mode fiber and the The Fresnel reflectivity on the material interface is used to measure the temperature by measuring the movement of the Bragg wavelength or the change of the Fresnel reflectivity. The disadvantages of these methods are high cost, complicated fabrication, and inconvenient large-scale application.

Contents of the invention

The purpose of the present invention is to overcome the above-mentioned deficiencies in the prior art, and provide an optical fiber refractive index and temperature sensor based on multimode interference and a measurement method thereof. The specific technical solutions are as follows.

A fiber optic refractive index and temperature sensor based on multimode interference, including a broadband light source, a fiber optic circulator, a measuring sensor head and a spectrometer; the input port of the fiber optic circulator is connected to the broadband light source through an optical fiber, and the first fiber optic circulator The output port is connected to the measuring sensor head through an optical fiber, and the second output port is connected to the input end of the spectrometer through an optical fiber; the light undergoes multi-mode interference inside the measuring sensor head, and a phenanthrene occurs at the interface between the measuring sensor head and the substance to be measured. The Neel reflection returns to the inside of the measuring sensor head to continue to propagate and undergo multi-mode interference, and finally transmits to the spectrometer. The loss peak power and loss peak wavelength of the interference fringe are measured by the spectrometer, and then the refractive index and temperature.

For the optical fiber refractive index and temperature sensor based on multi-mode interference, the measuring sensor head is a step-type multi-mode optical fiber whose end face is perpendicular to the axis of the optical fiber without removing the cladding. The light enters the multimode fiber through the single-mode fiber, and Fresnel reflection occurs at the interface between the end of the multimode fiber and the substance to be measured, and then returns to the multimode fiber, and finally coupled into the single-mode fiber. During this process, the light When entering a multimode fiber from a single-mode fiber, multiple eigenmodes are excited on the end face of the multimode fiber, and the light of these multiple modes interferes when propagating in the multimode fiber, and finally recouples into the single-mode fiber and transmitted to the spectrometer.

In the above-mentioned optical fiber refractive index and temperature sensor based on multimode interference, the broadband light source is a C-band (1520nm-1570nm) optical fiber broadband light source, and the optical fibers used for connection are common single-mode optical fibers.

In the above-mentioned optical fiber refractive index and temperature sensor based on multimode interference, the refractive index of the substance to be measured is calculated according to the rule that the peak loss power of the interference fringe changes with the change of the refractive index of the substance to be measured; according to the peak loss wavelength of the interference fringe According to the law that changes with the temperature of the substance to be measured, the temperature of the substance to be measured can be calculated.

The method for measuring the refractive index and temperature using the above optical fiber refractive index and temperature sensor includes: inserting the measuring sensor head into the substance to be measured; Fresnel reflection occurs on the interface and returns to the multimode fiber, and finally couples into the single-mode fiber. During this process, when the light enters the multimode fiber from the single-mode fiber, it excites multiple The light of these multiple modes interferes when propagating in the multimode fiber, and finally recouples into the single-mode fiber and transmits to the spectrometer. The loss peak power of the interference fringe changes with the change of the refractive index of the substance to be measured where the sensor head is located. The power of the loss peak of the interference fringe is measured by the spectrometer, and then the refractive index of the substance to be measured is obtained through calculation; the loss peak of the interference fringe The wavelength changes with the temperature of the substance to be measured where the sensor head is located. The loss peak wavelength of the interference fringe is measured by the spectrometer, and then the temperature of the substance to be measured is obtained through calculation.

In the above measurement method, the loss peak power of the interference fringes is

$\mathrm{<span\; class="notranslate">\; I</span>}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; [</span>{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 2</span>}\sqrt{{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}\mathrm{<span\; class="notranslate">\; cos</span>}<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;\&Delta;nL</span>}}{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ]</span>{<span\; class="notranslate">\; (</span>\frac{{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; co</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; x</span>}}}{{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; co</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; x</span>}}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}$

Among them, I ₁ and I ₂ are the optical power distributed by eigenmodes 1 and 2 respectively, Δn is the refractive index difference between these two modes, L is the length of the double multimode fiber, λ is the optical wavelength, n _co is the refractive index of the multimode fiber core, n _x is the refractive index of the substance to be measured; the loss peak wavelength of the interference fringes is

${\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; min</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; d</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}\mathrm{<span\; class="notranslate">\; m</span>}}{\mathrm{<span\; class="notranslate">\; L</span>}}{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; co</span>}}$

where d is the core diameter of the multimode fiber, L is the length of the doubled multimode fiber, n _co is the refractive index of the multimode fiber core, and m is the order of the mode.

In the above measurement method, when the temperature changes ΔT, the core diameter, length, and core refractive index of the multimode fiber will change accordingly, which will eventually lead to a change in the peak wavelength of the interference fringe loss, expressed as

${\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}\mathrm{<span\; class="notranslate">\; min</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; \&Delta;</span>}{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; min</span>}}<span\; class="notranslate">\; =</span>\frac{{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; d</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; \&Delta;d</span>}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}\mathrm{<span\; class="notranslate">\; m</span>}}{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; L</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; \&Delta;\; L</span>}<span\; class="notranslate">\; )</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; co</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; \&Delta;</span>}{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; co</span>}}<span\; class="notranslate">\; )</span>$

Where Δd=k ₁ ΔT, ΔL=k ₁ ΔT, Δn _co =k ₂ ΔT, k ₁ and k ₂ are the thermal expansion coefficient and thermo-optic coefficient of the multimode fiber respectively, λ _0min is the initial loss peak wavelength, and the change of the loss peak wavelength Δλ _min is only related to the temperature change ΔT.

Compared with the prior art, the present invention has the following advantages and technical effects:

(1) The sensor of the present invention can effectively avoid cross-sensitivity caused by measuring different physical parameters and improve measurement accuracy.

(2) The sensor of the present invention is simple in structure, easy to manufacture, low in cost, does not need special treatment such as removing cladding on the optical fiber, and is easy to operate.

(3) In addition to being used for general liquid detection, the sensor of the present invention can also be used for remote measurement and real-time monitoring of industrial production processes.

The sensor can realize high-precision and wide-range refractive index and temperature measurement, and has simple structure and convenient operation.

Description of drawings

Figure 1 is a schematic diagram of the structure of a fiber optic refractive index and temperature sensor based on multimode interference.

Figure 2 is the measured spectrum of the sensing system when the sensing head is in a medium with different refractive indices.

Fig. 3 is when the refractive index of NaCl solution changes from 1.3148 to 1.3534, the peak power loss of interference fringes varies with the refractive index of the solution.

Fig. 4 is the spectrum of the sensing system measured when the sensing head is in a 5% NaCl solution at different temperatures.

Fig. 5 shows the variation of the interference fringe loss peak wavelength with the solution temperature when the temperature of the 5% NaCl solution is changed from 25°C to 95°C.

Detailed ways

The specific implementation of the present invention will be described in further detail below in conjunction with the accompanying drawings, but the implementation and protection scope of the present invention are not limited thereto, and equivalent replacements that are substantially the same as the present invention all belong to the protection scope of the present invention.

Referring to FIG. 1 , a fiber optic refractive index and temperature sensor based on multimode interference includes a broadband light source 1 , a fiber optic circulator 2 , a measurement sensing head 3 and a spectrometer 4 . Wherein, the broadband light source 1 is connected to the first input port of the fiber optic circulator 2 , the first output port of the fiber optic circulator 2 is connected to the measurement sensing head, and the second output port is connected to the spectrometer 4 . The specific measurement is to measure the interference spectrum when the measuring sensor head is inserted into the measured substance by the spectrometer, obtain the loss peak power and loss peak wavelength of the interference fringe, and obtain the refractive index and temperature of the measured solution according to formulas (1) and (2) . The measuring sensing head is composed of a step-type multimode fiber whose end face is perpendicular to the axis of the fiber.

In the invention, the broadband light source 1 is a C-band (1520nm-1570nm) broadband light source. The transmission fiber is a single-mode fiber.

When performing a measurement, the measuring sensor head is inserted into the substance to be measured (such as a solution). The principle of the change of the peak power of the interference fringe loss with the change of the refractive index of the material to be measured where the sensor head is located is as follows:

According to the law of Fresnel reflection, the Fresnel reflectance at the interface between the end of the measuring sensor head and the substance to be measured is:

${\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; f</span>}}<span\; class="notranslate">\; =</span>{<span\; class="notranslate">\; (</span>\frac{{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; co</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; x</span>}}}{{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; co</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; x</span>}}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

Among them, n _co is the refractive index of the multimode fiber core, and n _x is the refractive index of the substance to be measured.

When the light enters the multimode fiber through the single-mode fiber, multiple eigenmodes will be excited in the multimode fiber, and the light of these modes will interfere in the multimode fiber, then the optical power finally coupled into the single-mode fiber is

$\mathrm{<span\; class="notranslate">\; I</span>}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; [</span>{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 2</span>}\sqrt{{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}\mathrm{<span\; class="notranslate">\; cos</span>}<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;\&Delta;nL</span>}}{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ]</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

where _I1 and _I2 are the optical powers of eigenmodes 1 and 2, respectively, I is the optical power measured by the spectrometer, L is the length of the doubled multimode fiber, λ is the optical wavelength, and Δn is the Poor refractive index.

From formulas (1) and (2), the peak power loss of interference fringes can be obtained as

$\mathrm{<span\; class="notranslate">\; I</span>}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; [</span>{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 2</span>}\sqrt{{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}\mathrm{<span\; class="notranslate">\; cos</span>}<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;\&Delta;nL</span>}}{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ]</span>{<span\; class="notranslate">\; (</span>\frac{{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; co</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; x</span>}}}{{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; co</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; x</span>}}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>$

Equation (3) shows that the loss peak power of the interference fringe is related to the refractive index difference of the optical mode, the length of the multimode fiber, the optical wavelength, the refractive index of the core of the multimode fiber, and the refractive index of the substance to be measured. The length of the mode fiber is easy to measure, and the refractive index of the core of the multimode fiber can be obtained by referring to the relevant product parameters, so by measuring the loss peak power of the interference fringe, the refractive index of the solution to be tested can be obtained.

The principle that the loss peak wavelength of interference fringes changes with the temperature of the substance to be measured where the sensor head is located is as follows:

According to the circular symmetry and ideal collimation of the input field, when the input field enters the multimode fiber, only the LP _0m mode will be excited, assuming that the field distribution of LP _0m is F _m (r), then the field on the end face of the multimode fiber distributed as

$\mathrm{<span\; class="notranslate">\; E.</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; r</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; 0</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\underset{\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; m</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}{\mathrm{<span\; class="notranslate">\; c</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; r</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 4</span>}<span\; class="notranslate">\; )</span>$

where _cm is the excitation coefficient of each mode, which can be expressed as

${\mathrm{<span\; class="notranslate">\; c</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; =</span>\frac{{<span\; class="notranslate">\; \&Integral;</span>}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{<span\; class="notranslate">\; \&infin;</span>}\mathrm{<span\; class="notranslate">\; E.</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; r</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; 0</span>}<span\; class="notranslate">\; )</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; r</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; rdr</span>}}{{<span\; class="notranslate">\; \&Integral;</span>}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{<span\; class="notranslate">\; \&infin;</span>}{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; r</span>}<span\; class="notranslate">\; )</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; r</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; rdr</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 5</span>}<span\; class="notranslate">\; )</span>$

When light propagates in a multimode fiber, the field distribution at a distance z can be expressed as

$\mathrm{<span\; class="notranslate">\; E.</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; r</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; z</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\underset{\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; m</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}{\mathrm{<span\; class="notranslate">\; c</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; r</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; exp</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; i</span>}{\mathrm{<span\; class="notranslate">\; \&beta;</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}\mathrm{<span\; class="notranslate">\; z</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 6</span>}<span\; class="notranslate">\; )</span>$

Among them, β _m is the propagation constant of each eigenmode in the multimode fiber, and the multimode interference will occur when the light of these multiple modes propagates in the multimode fiber, and has the same field as the input field at the distance z=L _z distribution, which is the so-called self-image phenomenon, L _z can be expressed as

${\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; z</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 16</span>}{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; co</span>}}{\mathrm{<span\; class="notranslate">\; a</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 7</span>}<span\; class="notranslate">\; )</span>$

where a is the core radius of the multimode fiber.

In this way, when the light is recoupled from the multimode fiber into the single-mode fiber, the light of some wavelengths is very strong, but the light of some wavelengths is very weak or even zero, and the interference is extremely small, that is, the loss peak wavelength of the interference fringe is

${\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; min</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; d</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}\mathrm{<span\; class="notranslate">\; m</span>}}{\mathrm{<span\; class="notranslate">\; L</span>}}{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; co</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 8</span>}<span\; class="notranslate">\; )</span>$

where d is the diameter of the multimode fiber core, L is the double length of the multimode fiber, and n _co is the refractive index of the multimode fiber core.

When the temperature changes ΔT, the core diameter, length, and core refractive index of the multimode fiber will change accordingly, which will eventually lead to a change in the peak wavelength of the interference fringe loss, which can be expressed as

${\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}\mathrm{<span\; class="notranslate">\; min</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; \&Delta;</span>}{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; min</span>}}<span\; class="notranslate">\; =</span>\frac{{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; d</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; \&Delta;d</span>}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}\mathrm{<span\; class="notranslate">\; m</span>}}{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; L</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; \&Delta;\; L</span>}<span\; class="notranslate">\; )</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; co</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; \&Delta;</span>}{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; co</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 9</span>}<span\; class="notranslate">\; )</span>$

Where Δd=k ₁ ΔT, ΔL=k ₁ ΔT, Δn _co =k ₂ ΔT, k ₁ and k ₂ are the thermal expansion coefficient and thermo-optic coefficient of the multimode fiber respectively, and λ _0min is the initial loss peak wavelength. It can be seen from formula (9) that the change in the wavelength of the loss peak Δλ _min is only related to the temperature change ΔT.

For further checking feasibility of the present invention, especially carry out following experiment:

Experiment 1:

In the experiment, the optical fiber sensor of the present invention is used to measure the spectrograms of different refractive index substances, as shown in Figure 2, five curves in the figure correspond to the optical fiber sensing head being placed in air, pure water, 2.5% concentration NaCl solution, 12.5% Concentration NaCl solution, spectrum in 25% concentration NaCl solution. The core diameter of the multimode fiber is 105 μm, the cladding diameter is 125 μm, and the length is 60 mm. It can be seen from Figure 2 that the peak loss power of interference fringes decreases with the increase of the refractive index of the substance to be measured placed on the optical fiber sensing head (for example, the higher the solution concentration, the smaller the peak loss power).

Table 1 shows the relationship between the peak power of the interference fringe loss and the refractive index of the same liquid (NaCl solution) with different concentrations.

Table 1

NaCl solution concentration (WT%) Corresponding refractive index Loss peak power/dBm 2.5 1.3148 -66.93 5 1.3190 -67.47 7.5 1.3234 -67.61 10 1.3277 -68.17 12.5 1.3319 -68.59 15 1.3362 -68.93 17.5 1.3405 -69.23 20 1.3448 -69.56 22.5 1.3491 -70.19 25 1.3534 -70.58

Fig. 3 is the data result and the linear fitting result of measuring the refractive index of NaCl solution with different concentrations by using the sensor of the present invention. It can be seen from Figure 3 that the experimental measurement data shows a good linear trend.

Experiment 2

In the experiment, the optical fiber sensor of the present invention is used to measure the spectrograms of 5% NaCl solution at different temperatures, as shown in Figure 4, and the three curves in the figure correspond to the optical fiber sensing head being placed at 25°C, 60°C, and 95°C respectively. °C Spectrum in 5% NaCl solution. It can be seen from Figure 4 that the peak wavelength of the interference fringe loss increases as the temperature of the substance to be measured placed in the optical fiber sensor head increases.

Table 2 shows the relationship between the interference fringe loss peak wavelength and the temperature of the same liquid (5% NaCl solution) at different temperatures.

Table 2

NaCl solution temperature (°C) Loss peak wavelength/nm 25 1544.76 30 1544.80 35 1544.84 40 1544.88 45 1544.92 50 1544.96 55 1545.00 60 1545.04 65 1545.08 70 1545.12 75 1545.16 80 1545.24 85 1545.28 90 1545.32 95 1545.36

Fig. 5 is the data result and linear fitting result of temperature measurement of 5% NaCl solution at different temperatures using the sensor of the present invention. It can be seen from Figure 5 that the experimental measurement data shows a good linear trend.

It can be known from the above experiments that the sensor of the present invention is feasible.

Claims (1)

1. The refractive index and temperature measurement method of optical fiber refractive index and temperature sensor based on multimode interference, described optical fiber refractive index and temperature sensor based on multimode interference comprises broadband light source (1), optical fiber circulator (2), measuring sensor A sensing head (3) and a spectrometer (4); the input port of the optical fiber circulator (2) is connected to the broadband light source (1) through an optical fiber, and the first output port of the optical fiber circulator (2) is connected to the measuring sensing head (3 ) is connected through an optical fiber, and the second output port is connected with the input end of the spectrometer (4) through an optical fiber; light undergoes multimode interference inside the measuring sensor head, and Fresnel reflection occurs at the interface between the measuring sensor head and the substance to be measured Return to the inside of the measuring sensor head to continue to propagate and cause multi-mode interference, and finally transmit to the spectrometer (4). The loss peak power and loss peak wavelength of the interference fringes are measured by the spectrometer (4), and then the measured substance is obtained by calculation. Refractive index and temperature; It is characterized in that the measurement method includes: inserting the measuring sensor head into the substance to be measured; light enters the multimode fiber through the single mode fiber, and excites multiple eigenmodes on the end face of the multimode fiber, and the multiple eigenmodes The light of the multimode fiber interferes in the multimode fiber, and Fresnel reflection occurs at the interface between the other end of the multimode fiber and the substance to be measured, and multimode interference occurs again. The loss peak power of the interference fringes is refracted with the substance to be measured The loss peak wavelength of the interference fringe changes with the temperature of the substance to be measured. The peak loss power and peak wavelength of the interference fringe are measured by the spectrometer, and then the refractive index and temperature of the substance to be measured are obtained through analysis and calculation; The loss peak power of the interference fringes is $\mathrm{<span\; class="notranslate">\; I</span>}<span\; class="notranslate">\; =</span><span\; class="notranslate">\; [</span>{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 2</span>}\sqrt{{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}\mathrm{<span\; class="notranslate">\; cos</span>}<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;\&Delta;nL</span>}}{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ]</span>{<span\; class="notranslate">\; (</span>\frac{{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; co</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; x</span>}}}{{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; co</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; x</span>}}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}$ Among them, I ₁ and I ₂ are the optical power distributed by eigenmodes 1 and 2 respectively, Δn is the refractive index difference between these two modes, L is the length of the double multimode fiber, λ is the optical wavelength, n _co is the refractive index of the multimode fiber core, n _x is the refractive index of the substance to be measured; the loss peak wavelength of the interference fringes is ${\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; min</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; d</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}\mathrm{<span\; class="notranslate">\; m</span>}}{\mathrm{<span\; class="notranslate">\; L</span>}}{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; co</span>}}$ where d is the core diameter of the multimode fiber, L is the length of double the multimode fiber, _nco is the refractive index of the multimode fiber core, and m is the order of the mode; When the temperature changes ΔT, the core diameter, length, and core refractive index of the multimode fiber will change accordingly, which will eventually lead to a change in the peak wavelength of the interference fringe loss, expressed as ${\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}\mathrm{<span\; class="notranslate">\; min</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; \&Delta;\&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; min</span>}}<span\; class="notranslate">\; =</span>\frac{{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; d</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; \&Delta;d</span>}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}\mathrm{<span\; class="notranslate">\; m</span>}}{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; L</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; \&Delta;\; L</span>}<span\; class="notranslate">\; )</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; co</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; \&Delta;n</span>}}_{\mathrm{<span\; class="notranslate">\; co</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ,</span>$ Where Δd=k ₁ ΔT, ΔL=k ₁ ΔT, Δn _co =k ₂ ΔT, k ₁ and k ₂ are the thermal expansion coefficient and thermo-optic coefficient of the multimode fiber respectively, λ _0min is the initial loss peak wavelength, and the change of the loss peak wavelength Δλ _min is only related to the temperature change ΔT.