Double-core PCF-SPR ultra-wide-range high-refractive-index sensor based on multi-field coupling synergistic effect

Abstract

The invention discloses a dual-core PCF-SPR (Photonic Crystal Fiber-Surface Plasmon Resonance) ultra-wide-range high-refractive-index sensor based on a multi-field coupling synergistic effect. According to the invention, a multilayer air hole array arranged in a triangular lattice is used as a cladding, the air holes have uniform diameters, and part of the air holes are removed at the symmetrical positions of the cladding to form a double-fiber-core symmetrical structure; symmetrical polishing is carried out on the two sides in the x-axis direction, and an open type external analyte channel is constructed; a through-type large air hole is introduced into the center of the optical fiber to serve as a built-in sensing channel; self-driven fluid intercommunication of the internal sensing channel and the external analyte channel is realized through the vertical micro-channel; metal films are deposited on the side parabolic surface and the inner wall of the central large hole to serve as plasma excitation layers, and a dual-channel SPP excitation interface is formed; according to the sensor, through a dual-channel SPR excitation mechanism and dual-fiber core mode coupling enhancement, the refractive index detection upper limit is improved to 1.66, and the detection range is expanded to 1.40-1.66.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical fiber sensing, and particularly relates to a dual-core PCF-SPR ultra-wideband high refractive index sensor based on the multi-field coupling synergistic effect, which is applicable to the real-time detection of high refractive index, high sensitivity, and wide dynamic range refractive index sensing in the fields of biomedical detection, environmental monitoring, and organic solvent analysis Background Art

[0002] Surface Plasmon Resonance (SPR) technology is based on the collective oscillation characteristics of free electrons at the metal-dielectric interface, and can produce a sub-wavelength response to the change of the interface refractive index (RI). It has become the core technology for realizing ultrasensitive detection in the field of biochemical sensing.

[0003] Traditional SPR sensors usually adopt prism coupling or planar waveguide structures (such as the Kretschmann configuration), but they have inherent defects such as large volume, complex optical path calibration, and low evanescent field coupling efficiency, and it is difficult to meet the requirements of miniaturization and high-throughput detection.

[0004] In recent years, Photonic Crystal Fiber (PCF) has become a revolutionary carrier for SPR sensing due to its unique advantages: First, the optical field mode and evanescent field distribution of PCF can be flexibly regulated through structural parameters such as air hole arrangement and core size; Second, its all-fiber integrated characteristics can greatly simplify the complexity of the sensing system; Third, the SPR excitation region can be accurately positioned through selective metallization (such as gold and silver nano-coatings). Based on the above characteristics, researchers deposited metal thin films on the inner wall of the PCF air holes or the outer cladding surface, and successfully developed a variety of PCF-SPR sensors, and realized the high-sensitivity detection of low refractive index analytes (RI<1.45).

[0005] However, with the expansion of biochemical detection scenarios, the sensing demand for high refractive index analytes (RI>1.45) has increased sharply. Existing SPR-PCF sensors face significant technical bottlenecks in such applications: First, the intrinsic refractive index (n≈1.45) of silicon-based PCF materials forms an optical barrier. When the RI of the analyte approaches or exceeds 1.45, the penetration depth of the evanescent field decays sharply, resulting in a sharp decline in the light-analyte coupling efficiency; Second, there are problems such as a single excitation path and limited mode matching in the existing SPR excitation mechanism. Specifically: 1) The single-core single-channel design (only relying on internal or external SPR) is limited by the fundamental mode cut-off effect, resulting in a limited upper limit for RI detection; 2) Although the dual-core structure expands the detection range through external or internal channel SPR, a single excitation path still cannot break through the sharp drop in sensitivity in the high RI region; 3) Non-synergistic excitation of the dual channels will cause resonance peak overlap and lead to detection signal crosstalk.

[0006] The root cause of the above problems is that the traditional SPR-PCF design has not established a collaborative optimization model for the multi-path SPR excitation mechanism and has not systematically solved the problem of optical impedance mismatch between high-RI analytes and silicon-based materials. Therefore, it is urgent to innovate the sensor structure and excitation mechanism to achieve the collaborative improvement of high sensitivity and operational convenience while increasing the detection upper limit and expanding the detection range. Summary of the Invention

[0007] Aiming at the problems of existing PCF-SPR sensors, such as low detection upper limit for high refractive index, narrow detection range, low sensitivity, complex manufacturing process, and poor real-time performance, the present invention proposes a dual-core PCF-SPR ultra-wideband high refractive index sensor based on the collaborative effect of multi-field coupling, which breaks through the refractive index barrier of silica, realizes ultra-wideband detection in the range of 1.40 - 1.66 and accurate linear response, and significantly improves sensitivity and operational convenience.

[0008] To achieve the above object, the present invention provides the following technical solutions:

[0009] A dual-core PCF-SPR ultra-wideband high refractive index sensor based on the collaborative effect of multi-field coupling, with a multi-layer air hole array arranged in a triangular lattice as the cladding, the air holes having a uniform diameter, and removing some air holes at the symmetric positions of the cladding to form a dual-core symmetric structure;

[0010] Symmetrically polish both sides along the x-axis to construct an open external analyte channel; introduce a through-hole large air hole in the center of the optical fiber as an internal sensing channel; realize self-driven fluid intercommunication between the internal sensing channel and the external analyte channel through a vertical microchannel; deposit a metal film on the side polished surface and the inner wall of the central large hole as a plasma excitation layer to form a dual-channel SPP excitation interface;

[0011] The sensor improves the refractive index detection upper limit to 1.66 and expands the detection range to 1.40 - 1.66 through a dual-channel SPR excitation mechanism and dual-core mode coupling enhancement.

[0012] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0013] 1. Ultra-high and ultra-wide refractive index detection range breaks through material limitations

[0014] Traditional PCF-SPR sensors are limited by the refractive index of the silica substrate material (about 1.45) and can only detect materials with an analyte refractive index lower than that of the substrate. Through a dual-channel SPR excitation mechanism (the synergistic effect of the side-polished outer surface and the gold-plated central air hole) and a dual-core mode coupling enhancement design, the upper limit of refractive index detection in this invention is increased to 1.66, and the detection range is extended to 1.40–1.66, covering low-refractive-index biomolecules (RI 1.33 - 1.40) and high-refractive-index organic compounds (RI > 1.55).

[0015] 2. Enhancement of sensitivity and resolution by the synergistic effect of multi-physical field coupling

[0016] For the first time, the synergistic effect of multi-physical field coupling is used, including the phase matching between SPR and the guided mode of PCF, the mode coupling between the local SPP excited by the gold-plated central air hole and the extended SPP in the outer polished area, and the modulation effect of the dual-core structure on the mode coupling, which significantly enhances the interaction strength between the evanescent field and the analyte and improves the sensor performance. The sensor achieves a wavelength sensitivity of 6150 nm / RIU in the range of 1.40–1.66, and the resolution reaches 1.63×10 -5 RIU, and the polynomial fitting coefficient R 2 = 0.99978, showing excellent linear response and prediction accuracy.

[0017] 3. Open vertical microchannels achieve a balance between efficient penetration and structural robustness

[0018] The vertical open microchannels constructed by focused ion beam etching technology enable the synchronous and rapid infiltration of the analyte into the central air hole and the side-polished surface while ensuring the mechanical strength of the optical fiber, solving the problems of low penetration efficiency and easy structural damage caused by complex microfluidic channels in traditional closed PCF sensors, and providing a hardware basis for real-time on-line detection.

[0019] 4. Compatibility of manufacturing processes and application scalability

[0020] Based on standard stack-and-draw processes and post-processing techniques (wheel polishing, CVD gold plating), large-scale production can be achieved. The sensor has stable performance in the wide range of 1.40–1.66, making it adaptable to high-refractive-index media common in biochemical reactions (such as glycerol solutions, protein complexes, etc.), and providing highly robust solutions for scenarios such as cancer biomarker detection and industrial solvent concentration monitoring. Description of the Drawings

[0021] Figure 1 is a schematic cross-sectional structure diagram of the PCF-SPR sensor of the present invention.

[0022] Figure 2It is a corresponding relationship diagram of the loss of the PCF-SPR sensor of the present invention and the refractive index of the solution to be measured (1.40 - 1.66).

[0023] Figure 3 It is a corresponding relationship diagram of the resonance wavelength of the PCF-SPR sensor of the present invention and the refractive index of the solution to be measured (1.40 - 1.66). Detailed implementation manners

[0024] Next, the technical solutions in the embodiments of the present invention will be described in detail with reference to the accompanying drawings. It should be understood that the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0025] The present application provides a dual-core PCF-SPR ultra-wideband high-refractive-index sensor based on the multi-field coupling synergy effect. The sensor uses a three-layer air hole array arranged in a triangular lattice as the cladding, and all air holes maintain a uniform diameter. By precisely removing two air holes at the y-axis symmetric positions in the second layer, a dual-core symmetric structure is formed.

[0026] Symmetric polishing is performed on both sides of the PCF along the x-axis to construct an open external analyte channel; at the same time, a through-hole large air hole is introduced into the center of the optical fiber as an internal sensing channel, and a vertical micro-channel is fabricated using micro-nano processing technology to achieve self-driven fluid intercommunication between the internal and external channels.

[0027] A gold film is deposited on the side-polished surface and the inner wall of the central large hole as a plasma excitation layer. The entire sensor is based on a high-purity quartz glass substrate to achieve dual-channel surface plasmon resonance and dual-core coupling synergy effect to improve the detection performance.

[0028] Furthermore, a large air hole is arranged in the center of the PCF to form an internal channel, and the two symmetric outer surfaces are polished to form two external channels. This innovative design of the internal and external analyte channels achieves performance breakthroughs through the following mechanisms: First, a three-dimensional coupling system is constructed through the central channel and the gold-plated side-polished surfaces on both sides - the analyte contacts the radial gold layer of the central hole and the axial gold layer of the polished surface simultaneously, forming a sandwich-type SPR excitation structure. Combining the strong evanescent field hybridization effect generated by the dual solid-core guided modes through the channels, multi-dimensional sensitivity enhancement is achieved; Second, based on the composite resonance cavity formed by the open boundary of the central channel, the constraint of the traditional photonic crystal fiber medium is broken, enabling the surface plasmon resonance wavelength to continuously redshift with the refractive index of the analyte, broadening the refractive index detection range. If the radius R of the central air hole is too large, the manufacturing process tolerance will be reduced; if it is too small, the SPR effect of the central local field will be weakened, reducing the sensitivity of the sensor. The radius R of the central large air hole is 1.25 - 1.35 μm.

[0029] In addition, if the polishing depth L is too large, it will lead to the deterioration of the double-core coupling efficiency, resulting in asymmetric mode field leakage. At the same time, it will also deteriorate the optical performance of the PCF, such as a sharp increase in leakage loss and multi-mode interference. If the polishing depth L is too small, it will lead to a decrease in the evanescent field intensity on the surface of the gold film and the failure of the dual-channel synergistic effect. At the same time, it will also reduce the manufacturing tolerance of the PCF-SPR sensor. The polishing depth L is 5.9 - 6.0 μm.

[0030] Preferably, after comprehensively considering the manufacturing tolerance of the process and the comprehensive performance evaluation of the sensor, the embodiment of the present application finally sets the structural parameter R of the PCF to 1.30 μm and L to 6.00 μm.

[0031] Furthermore, a vertical open microchannel is designed on the side of the central air hole of the PCF. This channel realizes the spontaneous penetration of the analyte through capillary action, avoiding the defect of repeated perfusion required by the traditional closed channel and realizing real-time sensing. The design of the channel thickness needs to balance the hydrodynamic resistance and the optical mode constraint, and t s = 0.20 μm can avoid causing excessive transmission loss while ensuring rapid infiltration.

[0032] Furthermore, gold has become the preferred material due to its excellent chemical stability in the aqueous environment and excellent resonance peak shift characteristics, and its thickness will directly affect the performance of the sensor. In the structural design scheme of the PCF-SPR sensor in this embodiment, if the gold layer is too large, it will increase the difficulty of electric field penetration, weaken the peak of the confinement loss while enhancing the damping loss, and reduce the sensitivity. If the gold layer thickness is too small, it will gradually widen the full width at half maximum of the SPR resonance peak, which is not conducive to improving the resolution of the sensor. It is necessary to realize the coordinated regulation of sensitivity and resolution by optimizing the gold layer thickness. The gold film thickness m is 25 - 40 nm.

[0033] Preferably, considering the deposition process tolerance of the gold film and the overall performance index, the embodiment finally selects the gold film thickness m of the PCF to be 30 nm.

[0034] Furthermore, the cladding has three layers of air holes arranged in a triangular lattice. The cladding air hole pitch Λ is 1.98 - 2.02 μm, and the sizes of all air holes are the same, with a diameter D of 1.18 - 1.22 μm. Preferably, the diameter D of the cladding air holes is 1.20 μm, and the air hole pitch Λ is 2.00 μm.

[0035] Figure 1 It is a cross-sectional view of a dual-core PCF-SPR ultra-wideband high refractive index sensor based on the multi-field coupling synergistic effect, including: a cladding air hole array 1, the air holes are arranged in a triangular lattice, with a total of three layers, the air hole diameter D = 1.20 μm, and the distance Λ between the air holes = 2.00 μm.

[0036] Remove two air holes at the y-axis symmetric positions of the second layer to form a double-core symmetric structure. Symmetrically polish both sides of the PCF along the x-axis to construct an open external analyte channel with a polishing depth L = 6.00 μm. At the same time, introduce a through-hole large air hole at the center of the optical fiber as an internal sensing channel. The radius R of the central large air hole is 1.30 μm, and a vertical microchannel is fabricated using micro-nano processing technology to achieve self-driven fluid intercommunication between the internal and external channels. The width t of the vertical microchannel s = 0.20 μm.

[0037] Deposit a gold film on the side-polished surface and the inner wall of the central large hole as the plasmon excitation layer 3. The thickness m of the gold film is 30 nm. The entire sensor is based on a high-purity silica glass substrate 2.

[0038] The above is the description of the structure and parameters of the optical fiber. Next, the working principle of the optical fiber will be described:

[0039] When the PCF-SPR sensor is immersed in the analyte solution 4, the analyte will simultaneously wet the outside of the cladding polished surface and the inside of the central channel. When light waves pass through the double-core optical fiber, the polished symmetric cladding gold-plated surface and the gold layer on the inner wall of the central air-core channel form a double SPP excitation interface. The analyte will promote the phase-matching coupling between the core guided mode and the surface plasmon wave on the gold film surface, resulting in a large amount of light energy transferring to the metal surface through the evanescent field, forming a resonance peak loss spectrum. The resonance peak loss is related to the wavelength.

[0040] When the refractive index of the analyte changes, the SPP propagation constant and the double-core energy coupling efficiency will also change, which will break the original phase-matching condition and cause a resonance wavelength shift (red shift / blue shift). By observing the change of the resonance wavelength of the PCF-SPR sensor, the refractive index value of the analyte can be detected, thus achieving the purpose of sensing.

[0041] Verification example:

[0042] Establish a sensing model of this embodiment by the finite element method, and use a computer to simulate the above sensing situation. The effective refractive index of the transmission fundamental mode of the PCF can be calculated, and the transmission loss of the optical fiber can be obtained from the imaginary part of the effective refractive index:

[0043]

[0044] In the above formula, λ represents the wavelength of the incident light, with the unit of μm; Im(n eff ) represents the imaginary part of the effective refractive index.

[0045] When the refractive index of the analyte changes from 1.40 to 1.66 with a step size of 0.02, the loss spectra under different analyte refractive indices can be obtained, as shown in Figure 2As shown. Each curve has a peak, and the wavelength corresponding to the peak is the resonance wavelength. When the refractive index of the analyte is different, the resonance wavelength will shift.

[0046] The resonance wavelengths calculated for refractive indices of 1.40, 1.42, 1.44, 1.46, 1.48, 1.50, 1.52, 1.54, 1.56, 1.58, 1.60, 1.62, 1.64, 1.66 are 1414 nm, 1438 nm, 1480 nm, 1545 nm, 1635 nm, 1744 nm, 1865 nm, 1988 nm, 2110 nm, 2228 nm, 2341 nm, 2459 nm, 2567 nm, and 2660 nm respectively. Based on the above data, the relationship curve between the resonance wavelength and the refractive index of the analyte solution to be measured is obtained, as Figure 3 shown. The fitting formula after polynomial fitting is:

[0047] λ = 283609.68 - 554020.03na + 359232.64na 2 - 76773.10na 3

[0048] Here, λ represents the resonance wavelength, with the unit of nm, and na represents the refractive index of the analyte solution to be measured, with the unit of RIU. The polynomial fitting coefficient R 2 = 0.99978. The sensitivity of the PCF - SPR sensor in this example is calculated by the following formula:

[0049]

[0050] Here, Δλ represents the change in the resonance wavelength, and Δna represents the change in the refractive index of the solution to be measured. Thus, the maximum sensitivity of the PCF - SPR sensor can be calculated as 6150 nm / RIU. The resolution of the PCF - SPR sensor in this example can be calculated by the following formula:

[0051]

[0052] Here, Δλ min represents the minimum wavelength resolution of the spectrum, which is set to 0.1 nm. Thus, the maximum resolution of the PCF - SPR sensor can be calculated as 1.63×10 -5 RIU.

[0053] In summary, the sensor adopts a co - design of a dual - core symmetric side - polished structure and a vertical micro - channel with a central air hole. The gold - plated layers on the inner and outer surfaces achieve bidirectional SPR excitation and bidirectional dual - core coupling modulation, breaking through the detection bottleneck caused by the refractive index limitation of the silica substrate in traditional sensors. The refractive index (RI) detection range of the analyte is extended to 1.40 - 1.66, with a maximum sensitivity of 6150 nm / RIU and a resolution better than 1.63×10 -5 RIU. Through the optimization of the dual - core polarization mode coupling and the polynomial fitting algorithm, the resonance wavelength of the sensor in the ultra - wide refractive index range (1.40 - 1.66) has a high - precision correlation with the RI change (R 2 > 0.99978), and it supports real - time dynamic detection. It solves the sensing problems of high - refractive - index biochemical samples (such as concentrated DNA solutions, high - concentration biomarkers, organic solvents, etc.), and has the characteristics of controllable manufacturing process, ultra - wide detection range, high sensitivity, and excellent response linearity.

[0054] Although the embodiments of the present invention have been shown and described, for those of ordinary skill in the art, it can be understood that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principle and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A dual-core PCF-SPR ultra-wideband high refractive index sensor based on the multi-field coupling synergy effect, characterized in that: A multi-layer air hole array arranged in a triangular lattice serves as the cladding. The air holes have a normalized diameter, and partial air holes are removed at symmetric positions of the cladding to form a dual-core symmetric structure; Symmetric polishing is performed on both sides along the x-axis to construct an open external analyte channel; a through-type large air hole is introduced into the center of the optical fiber as an internal sensing channel; self-driven fluid intercommunication between the internal sensing channel and the external analyte channel is achieved through a vertical microchannel; A metal film is deposited on the side-polished surface and the inner wall of the central large hole as a plasma excitation layer to form a dual-channel SPP excitation interface; The sensor enhances through a dual-channel SPR excitation mechanism and dual-core mode coupling, raising the upper limit of refractive index detection to 1.66 and expanding the detection range to 1.40 - 1.

66.

2. The dual-core PCF-SPR ultra-wideband high refractive index sensor based on the multi-field coupling synergy effect according to claim 1, wherein The cladding has three layers of air holes and is arranged in a triangular lattice. The diameter of the cladding air holes is 1.18 - 1.22 μm, and the air hole pitch is 1.98 - 2.02 μm.

3. The dual-core PCF-SPR ultra-wideband high refractive index sensor based on the multi-field coupling synergistic effect according to claim 2, characterized in that, Two air holes are precisely removed at the y-axis symmetric positions of the second layer to form the dual-core symmetric structure.

4. The dual-core PCF-SPR ultra-wideband high refractive index sensor based on the multi-field coupling synergy effect according to claim 1, characterized in that, The thickness of the metal film is 25 - 40 nm.

5. The dual-core PCF-SPR ultra-wideband high refractive index sensor based on the multi-field coupling synergistic effect according to claim 4, characterized in that, The metal film is a gold film with a thickness of 30 nm.

6. The dual-core PCF-SPR ultra-wideband high refractive index sensor based on the multi-field coupling synergy effect according to claim 1 or 5, characterized in that, The vertical microchannel enables spontaneous penetration of the analyte through capillary action, and the width of this channel is 0.10 - 0.30 μm.

7. The dual-core PCF-SPR ultra-wideband high refractive index sensor based on the multi-field coupling synergistic effect according to claim 6, wherein, The radius of the central large air hole is 1.25 - 1.35 μm, and the polishing depth is 5.9 - 6.0 μm.

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