阅读说明：本技术 一种基于d型双金属涂层的双折射pcf折射率传感器 (Double refraction PCF refractive index sensor based on D-type double-metal coating ) 是由 杨宏艳 韦银玉 刘孟银 陈昱澎 梅梓洋 苑立波 于 2021-01-12 设计创作，主要内容包括：本发明设计了一种基于D型TiO-2-Au涂覆的双折射光子晶体光纤折射率传感器,该传感器由一段包层区域抛磨掉部分形成的D型PCF、二氧化钛和金薄膜组成。本发明利用SPR效应,通过准确测量共振波长的变化,就能计算出金薄膜表面未知溶液折射率的变化,来实现传感检测。本发明的优点：PCF结构的D型设计和空气孔的不对称排布使得纤芯产生的倏逝波向包层泄露更多,PCF-SPR效应更强烈。为了解决金与基地材料的粘附问题,在基地与金薄膜之间涂覆一层二氧化钛材料,二种等离子体激发材料。纤芯方向空气孔的不对称性产生双折射,增强金薄膜表面SPR现象,提高传感器的测量折射的灵敏度和测量精度。该传感器结构紧凑,能够实时检测,灵敏度高,损耗低,具有良好的传感特性。(The invention designs a TiO based on D type 2 An Au coated birefringent photonic crystal fiber refractive index sensor consisting of a section of cladding area polished away part of the formed D-type PCF, titanium dioxide and gold thin film. The invention can calculate the change of the unknown solution refractive index on the surface of the gold film by accurately measuring the change of the resonance wavelength by utilizing the SPR effect, thereby realizing sensing detection. The invention has the advantages that: the D-type design of the PCF structure and the asymmetric arrangement of the air holes enable evanescent waves generated by the fiber core to leak more to the cladding, and the PCF-SPR effect is stronger. In order to solve the problem of adhesion between gold and a base material, a layer of titanium dioxide material and two plasma excitation materials are coated between the base and a gold film. The asymmetry of the air holes in the fiber core direction generates double refraction, the surface SPR phenomenon of the gold film is enhanced, and the sensitivity and the measurement precision of the measurement refraction of the sensor are improved. The sensor has the advantages of compact structure, real-time detection, high sensitivity, low loss and good sensing characteristic.)

1. A birefringent photonic crystal fiber refractive index sensor based on D-shaped bimetal coating is shown in figure 1 and comprises a fiber sensor body, and is characterized in that: the Photonic Crystal Fiber (PCF) sensor body is composed of (1), a D-type photonic crystal fiber, (2), air holes, (3) and titanium dioxide TiO₂(4), gold film, and (5), unknown solution composition. Polishing and grinding the PCF cladding region to a certain depth by a side polishing and grinding technology to form a polishing and grinding surface, and depositing a layer of titanium dioxide TiO on the polishing and grinding surface₂The sensing area is plated with a layer of gold film, the diameter D of the large air hole of the PCF cladding area is 10um, the diameter D of the small air hole is 0.42A, the distance between the air holes is 8um, the distance from the axially extending smooth surface of the D-type PCF to the fiber core is 11um, and the thickness of the titanium dioxide is 11umAnd the thickness of the gold film is t_g＝40nm。

2. The refractive index sensor based on the D-shaped bimetal coated birefringent photonic crystal fiber according to claim 1, wherein: polishing and grinding cladding region of photonic crystal fiber to a certain depthForming D-type PCF, and depositing TiO on the polished surface₂And the Au material is used as a plasma material and is used for detecting the sensing characteristic of the refractive index of the medium to be detected by sensor excitation SPR.

3. The refractive index sensor based on the D-shaped bimetal coated birefringent photonic crystal fiber according to claim 1, wherein: the cladding region of the photonic crystal fiber is formed by three layers of air hole arrays, and the sizes of the air holes in the orthogonal direction of the fiber core are different, so that the birefringence effect can be obtained. The larger the difference between the diameter D of the large air hole and the diameter D of the air hole, the stronger the birefringence effect. The diameter D of the big air hole of the sensor is 10um, D is 0.42 lambda, and the distance lambda of the air hole is 8 um.

4. The refractive index sensor based on the D-shaped bimetal coated birefringent photonic crystal fiber according to claim 1, is characterized in that: the polishing surface is arranged right above the missing hole of the annular pore of the first layer, and the polishing surface is designed to better enhance SPR. The sensing area is in direct contact with the unknown solution, when the refractive index of the unknown solution to be measured changes, the position change of a loss peak can be influenced, the change of the refractive index of the medium to be measured is demodulated by measuring the position change of the loss peak, and therefore the sensitivity of the sensor is calculated.

(I) technical field

The invention relates to a birefringent Photonic Crystal Fiber (PCF) refractive index sensor based on a D-type bimetallic coating, belonging to the field of special optical fiber and optical fiber sensing.

(II) background of the invention

The core mission of optical fiber sensing is to acquire information, and plays an irreplaceable role in the fields of information acquisition and information transmission. The Photonic Crystal Fiber (PCF) is a new type of optical fiber, and is characterized by that in the cladding region several micropores parallel to the axial direction of optical fiber are set, and the air hole array in the cladding can reduce effective refractive index of the cladding so as to meet the condition of "total reflection", and the light can be bound in the core region and transmitted, and said light-guiding mechanism is similar to traditional optical fiber, and is called "improved" total reflection principle. Compared with the properties of the traditional optical fiber, the PCF has unique properties including a periodically distributed micro-pore structure infinitely extending along the axial direction of the optical fiber, so that the PCF has the characteristics of flexible structural layout, no cut-off single mode, high nonlinearity, high birefringence, large mode field size, easiness in filling working substances, low transmission loss, controllable dispersion and the like. SPR has been introduced into PCF in recent years and has become a focus of research.

Surface Plasmon Resonance (SPR) principle: when incident light enters the light thinning medium from the light dense medium, a total reflection phenomenon occurs when an incident angle is larger than a total reflection angle, evanescent waves generated during total reflection enter the metal film and interact with free electrons of the metal film to excite Surface Plasma Waves (SPW) transmitted along the surface of the metal film. When the incident wavelength satisfies a certain value, most of the energy of the incident light is converted into the energy of SPW, and the energy of the reflected light is rapidly decreased, and a resonance absorption peak appears in the reflection spectrum, and the incident wavelength at this time is referred to as the resonance wavelength of SPR. The invention utilizes SPR effect to accurately measure the change of resonance wavelength generated by unknown solution, and can obtain the change of the refractive index of the unknown solution on the surface of the metal film, thereby realizing sensing detection. Since the resonant wavelength is particularly sensitive to unknown solution refractive index changes, SPR sensors enable "real-time" detection and very high sensitivity refractive index measurements. The photonic crystal fiber surface plasma optical fiber sensor has important application in the fields of biological medicine and life safety because of high sensitivity in the field of detecting various biological and chemical substances, and is a hot spot for research of research groups of various countries in the world in recent years.

In recent years, research on PCF-SPR refractive index sensors has been increasing, and particularly D-type optical fibers, using gold thin films as plasma materials have also been widely used. The design provides a D-type PCF-SPR sensor based on the research background, and because of the adhesion problem of a gold film and a base material, a titanium dioxide material Tio is added between the base material and the gold film₂. In order to obtain higher sensitivity, the design is designed on the arrangement and the size of the air holes, the air holes are formed by three rings of air holes which are arranged in a hexagonal mode, the sizes of the air holes in the orthogonal direction of the fiber core are different, and the birefringence effect can be obtained. The design is very sensitive to external environment change due to the special structure, and the sensing area is arranged outside the optical fiber, so the high-sensitivity low-loss TiO with high sensitivity₂Au coated PCF-SPR sensors are better and more widely used in many fields.

The PCF-SPR sensor has the advantages of compact structure, high sensitivity, easy remote control, and capability of resisting magnetic interference, and is reported by people, including refractive index sensor, magnetic field sensor, temperature sensor and pressure sensor. In 2017, Rifat, A.A and the like propose a D-type photonic crystal fiber refractive index sensor, titanium dioxide and a gold film are used as plasma materials, the wavelength sensitivity reaches 9800nm/RIU within the refractive index range of 1.33-1.43, and the maximum loss peak value is 23.18 dB/cm. In 2018, Paul, A.K and the like, gold is used as a plasma material, the wavelength sensitivity reaches 9000nm/RIU within the refractive index range of 1.33-1.40, and the maximum loss peak value is 65 dB/cm. In 2019, W.S et al designed a symmetrical side-polished double-core photonic crystal fiber refractive index sensor, with a maximum sensitivity of 8000nm/RIU and a maximum loss peak of 248.9dB/cm within a refractive index range of 1.3-1.42.

A new structure is designed, and the structure has higher sensitivity to the measurement of the refractive index. Design a D-type Au-Tio₂The coated double-refraction photon crystal optical fiber refractive index sensor has the structure that a cladding layer part is polished to a certain depth, and a polished surface, namely a sensing area, is used for solving the problem of adhesion of gold and a base materialFirstly, a layer of Tio is deposited on the polished surface₂Plating a layer of gold film on the material, and using the two materials as plasma excitation materials. The cladding region is formed by three layers of air hole arrays, and the sizes of the air holes in the orthogonal direction of the fiber core are different, so that the birefringence effect can be obtained. In birefringent fibers, the propagation velocities of two orthogonal polarization modes transmitted by single-mode fibers are different, and thus can be used to maintain the polarization state of light in optical devices. Unlike conventional fibers, the birefringence characteristic obtained with photonic crystal fibers is extremely insensitive to temperature, a characteristic that is desirable in many applications. The sensing area of the optical fiber sensor is arranged outside the optical fiber, and can directly contact with unknown solution, so that the defects of coating in the hole and sensing test are avoided. By reasonably setting the numerical aperture of the PCF and the thicknesses of the titanium dioxide and the gold film, an optimal sensitivity physical parameter can be obtained, so that the sensor has high sensitivity and high sensing precision in the refractive index range.

The research of the high-sensitivity refractive index sensor based on birefringence is of great significance, the D-type design of the PCF structure and the asymmetric arrangement of the air holes enable evanescent waves generated by the fiber core to leak more to the cladding, and the PCF-SPR effect is stronger. Depositing a thickness of Tio between the gold film and the substrate material₂The material not only solves the adhesion problem of the gold film and the substrate material, but also plays a role in sensitization. The sensor is simple in structure and process, high in integration level and capable of accurately detecting in a measuring range.

Disclosure of the invention

In order to solve the problems, the invention mainly provides a refractive index sensor which is simple in structure, easy to realize in process, good in stability and high in resolution.

The invention is realized by the following technical scheme:

d type TiO₂The cladding region of the birefringent PCF refractive index sensor structure of the Au coating is composed of three air holes which are arranged in a ring hexagon, the orthogonal direction of the fiber core in the first air ring is composed of air holes with different diameters, the diameter of the large air hole is D, and the diameters of the residual air holes are the same and are D. In the PCF cladding region by side polishingPolishing a certain depth H to form a polishing surface, firstly depositing a layer of Tio2(3) on the polishing surface, then depositing a layer of gold film (4) as a sensing area, wherein the two materials are on the axially extending smooth surface of the PCF, and the sensing area is directly contacted with the unknown solution (5).

Furthermore, the diameter D of the large air hole of the D-type PCF is 10um, the diameter D of the small air hole is 0.42 inverted V, the distance between the air holes is 8um, the distance from the axially extending smooth surface of the D-type Au-Tio2 coating PCF to the fiber core is 11um, and the thickness t of the titanium dioxide is 11um_Tio26nm and a gold film thickness t_g＝40nm。

In the method, an unknown solution is loaded outside a sensing area of a PCF refractive index sensor, due to the fact that the SPR effect is very sensitive to the change of the refractive index of the unknown solution, when the refractive index of the unknown solution on the surface of a gold film slightly changes, the wavelength position corresponding to a loss absorption peak changes, the change of the refractive index of the unknown solution is demodulated by measuring the offset of the position corresponding to the resonance wavelength, and the sensitivity in a low refractive index range is calculated.

The D-type birefringence PCF refractive index sensor designed by the method is simple in structure, the complex process of coating in the hole is avoided, the air hole structures with different sizes are designed in the orthogonal direction of the fiber core of the PCF structure to form birefringence, and the D-type design enhances the leakage of evanescent waves generated by the fiber core to the cladding region. Using TiO₂The advantage of the Au coating is not only to solve the adhesion problem of the gold thin film to the substrate material, but also to improve the sensitivity. The D-type Au-Tio2 coating birefringence PCF refractive index sensor manufactured by the model structure can realize high-sensitivity detection within the effective refraction range of 1.37-1.42, and the average wavelength sensitivity is 7920 nm/RIU; the maximum wavelength sensitivity can reach 16000nm/RIU, and the maximum measurement precision value is 6.25 multiplied by 10^-6nm/RIU; the maximum amplitude sensitivity reaches 1029.69nm/RIU, and the maximum resonance loss peak value is 5.62 dB/cm.

(IV) description of the drawings

FIG. 1 is a two-dimensional cross-sectional schematic of the present invention.

FIG. 2 is a graph showing the loss in the case of a change in the diameter of a large air hole according to the present invention.

Figure 3 is the bookThe invention has the refractive index n of 1.40-1.41 and the thickness of titanium dioxide is changed along with the thickness of titanium dioxideLoss spectrum plot of change.

FIG. 4 is a graph of loss spectra of the gold film of the present invention with varying thickness.

FIG. 5 is a graph of the variation of different refractive indices with wavelength according to the present invention.

FIG. 6 is a graph of amplitude sensitivity as a function of wavelength for the present invention.

The reference numbers in the figures are: (1) d type photonic crystal fiber, (2), air holes, (3) titanium dioxide TiO₂(4), gold thin film, (5), unknown solution, (6) perfect matching layer PML.

(V) detailed description of the preferred embodiments

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is further described in detail below with reference to the accompanying drawings in conjunction with specific examples.

The invention relates to a D-type Au-Tio2 coating birefringent photonic crystal fiber sensor, which adopts a fiber core cladding structure and is made of silicon dioxide as shown in figure 1, and the sensor consists of a D-type PCF (1) formed by polishing and grinding part of a section of cladding region, titanium dioxide (3), a gold film (4) and an unknown solution (5). And (3) polishing and grinding the PCF cladding region by a side polishing and grinding technology, wherein the polishing and grinding depth H is 11um, and a polishing and grinding surface is formed, and H is the distance from the polishing and grinding surface to the fiber core. Depositing a layer of titanium dioxide on the polished surface to a thickness t_Tio2About 6nm, depositing a gold film with a thickness t_g40 nm. The cladding region consists of three rings of hexagonally arranged air holes, the orthogonal direction of the fiber core in the first layer of air ring consists of air holes with different diameters, the diameter of the large air hole is D, and the diameters of the residual air holes are the same and are D. In order to obtain the optimal sensing sensitivity, parameters such as the diameter of an air hole, the thickness of titanium dioxide, the thickness of a gold film and the like can be optimized, and the sensing measurement with high sensitivity in the refractive index range of the sensor is realized. The birefringence PCF refractive index sensor has the advantages of simple process, compact structure, small volume, no temperature influence and the like, and can be applied to biological monitoringChemical detection, marine environment test and the like.

The preparation process of the sensor based on the D-type birefringence PCF refractive index comprises the following steps: the method comprises the steps of preparing a photonic crystal fiber by using a stacking and drawing fiber technology, polishing and grinding a cladding region of a section of the photonic crystal fiber by a certain depth by using a side polishing and grinding technology, wherein the distance between a formed polished surface and a fiber core is H ═ 11 um.

The use of a layer of titanium dioxide between the gold film and the substrate material reduces gold adhesion problems and improves sensitivity, and also aids in SPR excitation. The dielectric constant of titanium dioxide can be calculated by the following formula:

wherein the content of the first and second substances,is the refractive index of titanium dioxide, and λ is the wavelength, in um.

Gold, as a commonly used plasma material, has a relative dielectric constant ε_AuCan be described by the Lorentz-Drude model:

ε_Auis the dielectric constant of gold,. epsilon_∝Omega is the dielectric constant at high frequencies, omega is the angular frequency and omega is 2 pi c/lambda, omega_DIs the plasma frequency, gamma_DIs the damping frequency, and ω_D/2π＝2113.6THz,γ_D15.92THz, weight coefficient Delta epsilon 1.09, and the spectrum width and the oscillator intensity of Lorentz oscillator are gamma_L/2π＝104.86THz andΩ_L/2π＝650.07THz。

In the embodiment of the invention, the D-type design of the PCF structure enhances more evanescent waves generated by the fiber core to leak to the cladding, the air hole structures with different sizes are designed in the orthogonal direction of the fiber core to form double refraction, and a layer of Au-TiO is deposited on the polishing surface₂Detection of unknown solution by using coating as sensing layerIs used as a refractive index of (1). TiO2₂The coating can effectively link the core guided mode with the SPR mode to help excite SPR and improve the sensitivity of the sensor. Compared with other sensors with high refractive indexes, the sensor for measuring the refractive indexes avoids air hole coating or pouring liquid to be measured, reduces the difficulty of process manufacturing, and is wider in application. The invention needs polishing a section of PCF fiber cladding area to a certain depth H, the thickness range of the deposited titanium dioxide is changed between 6nm and 14nm, and the thickness range of the gold film is changed between 30nm and 50nm when the layer of gold film is coated.

By changing the diameter of the large air hole, the thickness of titanium dioxide, the thickness of a gold film, the refractive index of an external medium to be measured and other structural parameters, the position of a resonance absorption peak can be changed; the sensitivity of the sensor at the moment can be demodulated by changing the offset of the resonance absorption peak of the refractive index detection of the unknown solution, so that the function of real-time monitoring is achieved; by reasonably optimizing the structural parameters, an optimal sensitivity can be obtained.

The formula for calculating the sensitivity is:

wherein, the delta lambda is the wavelength variation of the resonance absorption peak, and the delta n is the variation of the refractive index of the external medium to be measured.

A common expression for calculating the amplitude sensitivity is:

where α (λ, n) is the total loss at which the unknown solution refractive index equals n,is the difference between two adjacent loss peaks due to unknown small changes in the solution refractive index,is the change in refractive index of the unknown solution.

Adopting a wavelength modulation method, wherein the variation range of the wavelength is 650-1250nm, carrying out numerical simulation on the designed experimental model by using a full vector Finite Element Method (FEM) -based method, solving the effective refractive index of the mode field under the coordination of the boundary condition of the anisotropic Perfect Matching Layer (PML), and then changing the mode field loss formula alpha (dB/cm) to 8.686 xk₀Im(neff)×10⁴The loss of the mode field is calculated, im (neff) is the imaginary part of the refractive index of the effective mode field, the wave vector k₀The loss spectrum of the sensor was plotted using Origin software at 2 pi/λ.

Under the condition of the refractive index of 1.41, the peak value of the coupling loss of the sensor changes with the wavelength along with the change of the diameter D of the large air hole, and is shown in FIG. 2. When the large air hole diameter D was increased from 4um to 10um, the loss peak increased from 1.39dB/cm to 5.49 dB/cm. The reason is that the larger the difference between the diameter D of the air hole and the diameter D of the air hole is, the stronger the birefringence effect of the optical fiber structure is, the more evanescent waves leaking from the fiber core to the cladding are, the stronger the SPR effect generated by exciting the plasma material is, and the maximum loss peak value is when D is 10um in the figure.

At a refractive index of 1.40-1.41, with titanium dioxide thicknessThe loss peak varies with wavelength from 4nm to 14nm as shown in figure 3. Under the condition of refractive index of 1.40, the refractive index of the material is changed along withIncreasing from 6nm to 14nm, the loss peak decreased from 3.533dB/cm to 3.127 dB/cm. Under the condition of refractive index of 1.41, the refractive index of the glass is changed along withIncreasing from 6nm to 14nm, the loss peak decreased from 4.916dB/cm to 4.246 dB/cm. In the refractive index of 1.40-1.41 and the thickness of titanium dioxideThe change from 6nm to 14nm can be obtained according to a sensitivity formulaThe sensitivity of the instrument is 9600 nm/RIU. As can be seen from the figure, when the refractive index is 1.40-1.41, the refractive index is inThe peak loss is the largest, indicating that the SPR phenomenon is most pronounced at this time, so the optimum parameter for titanium dioxide thickness is 6 nm.

At a refractive index of 1.40-1.41,with the thickness t of the gold film_gThe loss peak varies with wavelength from 30nm to 50nm as shown in fig. 4. When the refractive index was 1.40 and the thickness of the gold thin film was increased from 30nm to 50nm, the peak loss was reduced from 6.486dB/cm to 1.940dB/cm as the wavelength was increased. When the refractive index was 1.41 and the thickness of the gold thin film was increased from 30nm to 50nm, the peak loss was reduced from 9.978dB/cm to 2.538dB/cm as the wavelength was increased. As can be seen from the figure, the thickness of the gold film increases, the loss peak α gradually decreases, the resonant wavelength is red-shifted, and the full width at half maximum (FWHM) of the corresponding resonant loss peak is widened. The reason is that the surface plasma wave is very sensitive to the thickness variation of the gold thin film, and the phase matching point is red-shifted as the thickness of the gold thin film increases. The gold film damping increases, which results in a decrease in coupling efficiency, and the loss peak is reduced in the figure. On the other hand, the loss peak value is relatively large due to the fact that the gold film is too thin, so that the optimal parameter of the gold film thickness of the designed sensor is 40 nm.

Other parameters were fixed, the unknown solution refractive index was changed, and the loss peak was varied with wavelength as shown in fig. 5. In the refractive index range of 1.37-1.42, the peak of the absorption resonance peak is red-shifted with increasing refractive index of the unknown solution. When the refraction n is changed from 1.37 to 1.38, 1.39, 1.40, 1.41 and 1.42, the peak shift of the loss peak is 32, 48, 64, 92 and 160nm, respectively. The sensitivity was 3200, 4800, 6400, 9200 and 16000nm/RIU, respectively. Calculated using the refractive index formula, the maximum wavelength sensitivity was 16000nm/RIU and the average sensitivity was 7920nm/RIU over the refractive index range (n ═ 1.37-1.42). Measuring the transmission by using a high-precision spectrometer with the resolution of 0.1nmMinimum resolution of the sensor is 6.25 × 10^-6RIU。

FIG. 6 shows Au-TiO form D₂The amplitude sensitivity of the coated PCF-SPR sensor varies with wavelength. Exhibit analyte refractive indices of 1.37 to 1.38, 1.38 to 1.39, 1.39 to 1.40, 1.40 to 1.41, and 1.41 to 1.42 with amplitude sensitivities at wavelengths of 750, 798, 854, 942, and 1110nm of 235.42, 331.36, 550.65, 851.69, and 1029.69RIU, respectively^-1。

It should be noted that, although the above-mentioned embodiments of the present invention are illustrative, the present invention is not limited thereto, and thus the present invention is not limited to the above-mentioned embodiments. Other embodiments, which can be made by those skilled in the art in light of the teachings of the present invention, are considered to be within the scope of the present invention without departing from its principles.