CN113959988A - Tandem SPR sensor based on capillary optical fiber - Google Patents
Tandem SPR sensor based on capillary optical fiber Download PDFInfo
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Abstract
The invention discloses a tandem SPR sensor based on a capillary optical fiber. The sensor consists of a single-core optical fiber 1, a square capillary optical fiber 2 and a metal film 3, wherein a common optical fiber and the square capillary optical fiber are sequentially welded together to form a structure of 'single-core optical fiber-square capillary optical fiber-single-core optical fiber', and any one rectangular surface of the square capillary optical fiber and the metal film plated on the surface can form an SPR sensing unit. According to the basic principle that the material and thickness of the metal thin film influence the SPR resonance wavelength, the four surfaces of the square capillary optical fiber are respectively plated with the metal thin films with different materials and different thicknesses, the positions of the four SPR resonance wavelengths are separated, and finally the series sensing of the four SPR sensors is realized. The sensor can coat corresponding sensitive substances on the SPR sensing unit according to the requirements of users, and can be used for online real-time measurement in multiple fields of biosensing, chemical analysis, drug research and development, environmental monitoring and the like.
Description
(I) technical field
The invention relates to a tandem SPR sensor based on a capillary optical fiber, belonging to the field of optical fiber sensing.
(II) background of the invention
Fiber Surface Plasmon Resonance (SPR) sensors are a product of high sensitivity combined with fiber sensing technology. Light propagates in the fiber core in a total reflection mode, and after the operations of side polishing, plating of a gold film and the like are performed on the fiber, a part of light can penetrate into the gold film to generate evanescent waves, so that free electrons in the gold film are triggered to oscillate, and Surface Plasmon Waves (SPW) are generated. When the evanescent wave and the SPW are matched with each other, the evanescent wave and the SPW resonate, total reflection is destroyed, light with a certain specific wavelength is absorbed by free electrons, energy of the light is reduced rapidly, and the light is reflected as a resonance valley on a resonance spectrogram. When the refractive index of the gold film surface substance or the SPR resonance excitation angle is changed, the resonance valley can drift, and the drift amount of the resonance valley corresponds to the change of the measured parameter one by one, so that the measurement of the parameter to be measured can be realized. At present, the SPR phenomenon is drawing attention due to its extremely sensitive property to the refractive index of the medium in contact with the metal thin film, and therefore, the SPR detection technique has been rapidly developed and widely used in a variety of fields such as biosensing, chemical analysis, drug development, food safety, environmental monitoring, medical diagnosis, etc.
Although the traditional prism type SPR sensor based on an Otto structure and a Kretschmann structure has higher detection resolution, the devices are bulky and not suitable for remote measurement, and the signals of the devices are easily influenced by mechanical vibration to cause unstable system test results. Compared with the traditional prism type SPR sensing technology, the optical fiber SPR sensing technology adopts optical fibers as a transmission medium, realizes the integration of 'transmission' and 'sensing', has the advantages of small volume, high sensitivity, high integration level, electromagnetic interference resistance, no need of marking, flexible structure, capability of realizing remote real-time monitoring, normal work under complex and severe environments and the like, is popular among a plurality of students, and becomes an important branch and hotspot research direction in the field of optical sensing.
In the field of optical fiber sensing, people are constantly pursuing and exploring a sensing technology which has high resolution, miniaturization and can realize simultaneous measurement of multiple parameters. As sensing technology advances, fiber SPR sensors of various structures and functions are beginning to emerge.
The utility model with publication number CN212568471U is a dual-channel fiber SPR sensor made by sequentially welding a multimode fiber, a corrosion single-mode fiber, and a multimode fiber, and plating a gold film and a silver film on the upper and lower surfaces of the corrosion single-mode fiber, but it is difficult to measure three parameters and more; the invention patent with publication number CN110308115A provides an interference type optical fiber SPR sensor by performing silver film coating on the surface of a single mode fiber-coreless fiber-single mode fiber structure, however, the refractive index sensing sensitivity of the interference type optical fiber SPR sensor is generally lower than that of SPR sensing; the application number CN202011230947.3 provides an optical fiber SPR sensor for measuring magnetic field and temperature based on STS structure, which realizes the simultaneous measurement of magnetic field and temperature by performing grating writing, thin core fiber corrosion, silver film coating, capillary magnetic fluid encapsulation and other operations on the thin core fiber in the structure of single mode fiber-thin core fiber-single mode fiber, and the sensor has complex preparation process and generally low temperature sensing sensitivity based on the thermal expansion effect of the optical fiber grating.
The invention provides a tandem SPR sensor based on capillary optical fibers for overcoming the defects of the prior art, and the tandem SPR sensor has the characteristics of multi-channel sensing, high sensitivity, simplicity in preparation and high integration level. The single-core optical fiber and the capillary optical fiber are sequentially welded together to form a structure of 'single-core optical fiber-capillary optical fiber-single-core optical fiber', and according to the basic principle that the material and thickness of a metal film influence the SPR resonance wavelength, the positions of a plurality of SPR resonance wavelengths are separated by controlling the material, thickness and number of layers of metal films plated on the surfaces of different areas of the capillary optical fiber, so that the series sensing of a plurality of SPR sensors is integrated; in addition, a plurality of layers of metal films and a multilayer film structure can be further plated on the metal film, the penetration depth of evanescent waves is deepened, a resonance absorption peak is narrowed, and the sensitivity and the precision of the sensor are further improved, so that the sensor provided by the invention has greater advantages compared with the prior art.
Disclosure of the invention
In view of the shortcomings of the prior art, it is an object of the present invention to provide an in-line SPR sensor based on a capillary fiber. The sensor forms a structure of 'single-core optical fiber-capillary optical fiber-single-core optical fiber' by welding the single-core optical fiber and the capillary optical fiber together in sequence, and realizes series sensing by controlling the material, thickness and number of layers of metal films plated on the surfaces of different areas of the capillary optical fiber and connecting a plurality of SPR sensing units in series.
The purpose of the invention is realized as follows:
as shown in figure 1, the sensor is prepared by sequentially welding a single-core optical fiber 1 and a square capillary optical fiber 2 together to form a structure of 'single-core optical fiber 1-square capillary optical fiber 2-single-core optical fiber 1', and respectively plating metal films 3 on four rectangular surfaces of the square capillary optical fiber 2. Any one rectangular surface of the square capillary optical fiber and the metal film plated on the surface form an SPR sensing unit. When two ends of a square capillary optical fiber 2 are sequentially welded with a single-core optical fiber 1 in a hot melting manner, a conical transition region 4 and a conical transition region 5 are formed near a welding point due to the fact that an air hole 202 is heated and collapsed, when wide-spectrum light 6 is transmitted to a left welding point from the single-core optical fiber 1, the wide-spectrum light is divided into divergent light beams 7 by the conical transition region 4, then the divergent light beams 7 are totally internally reflected at the interface of a square cladding 201 and a metal film 3 to form reflected light beams 8, evanescent waves 9 transmitted along the interface are generated at the same time, when the evanescent waves 9 are matched with surface plasma wave vectors of the metal film 3, the SPR phenomenon can be generated on four surfaces of the square cladding 201, at the moment, energy of the reflected light beams 8 is greatly consumed, and the reflected light beams 8 of the last four surfaces are collected by the single-core optical fiber 1 and then output sensing spectra 10, so that four series SPR sensors are formed; in addition, since the material, thickness and number of layers of the metal thin film all affect the SPR resonance wavelength, the multiple SPR resonance wavelengths in the sensing spectrum 11 can be separated by controlling the coating parameters of the metal thin film 3 on each outer surface of the square cladding 201, the sensing capability of the tandem SPR sensor is improved, and finally the refractive index of the substance on the surface of the metal thin film can be measured in real time by solving the relationship between the drift amount of the SPR resonance wavelength and the refractive index variation amount of the external substance.
The single-core optical fiber 1 playing a role in inputting light and collecting light in the tandem SPR sensor based on the capillary optical fiber can be one of a single-mode optical fiber, a few-mode optical fiber, a multi-mode optical fiber and a ring-core optical fiber, wherein the multi-mode optical fiber can play a better light collecting effect.
The outer contour shape of the square cladding of the square capillary fiber 2 used in the tandem SPR sensor based on the capillary fiber can also be one of a circle, a triangle, a pentagon or other polygons. If the polygonal capillary optical fiber is adopted, the single-core optical fiber, the polygonal capillary optical fiber and the single-core optical fiber are sequentially welded together to form a structure of 'single-core optical fiber-polygonal capillary optical fiber-single-core optical fiber', metal films are plated on the rectangular surfaces of the polygonal capillary optical fiber, and the purpose of separating a plurality of SPR resonance wavelengths is achieved by controlling the material, the thickness and the number of layers of the metal films plated on the rectangular surfaces of the square capillary optical fiber, so that the multi-series SPR sensing is realized; if the round capillary optical fiber is adopted, the single-core optical fiber, the round capillary optical fiber and the single-core optical fiber are sequentially welded together to form a structure of 'single-core optical fiber-round capillary optical fiber-single-core optical fiber', the outer surface of the round capillary optical fiber is divided into a plurality of parts, metal films are plated on the parts divided on the outer surface of the round capillary optical fiber to form a plurality of SPR sensing units, the purpose of separating a plurality of SPR resonance wavelengths is achieved by controlling the material, the thickness and the number of layers of the metal films plated in each SPR sensing unit, and series SPR sensing is finally achieved.
The metal film used in the tandem SPR sensor based on the capillary optical fiber can be a single-layer metal material capable of exciting surface plasma waves, such as gold, silver and aluminum; the structure can also be a composite film structure consisting of a medium and metal, and the structure can enhance the electric field at the interface of the metal film and the medium and enhance the penetration depth of evanescent waves, thereby improving the sensitivity of the sensor.
When a coating machine is used for coating metal films on the rectangular surfaces of the square capillary optical fiber, the adopted coating modes can be single-side coating, rotary coating and a mode of combining single-side coating and rotary coating.
The tandem SPR sensor based on the capillary optical fiber is simplified into a three-layer film structure of square cladding, metal film and medium to be measured, the reflectivity and the transmissivity of the three-layer film structure and the distribution condition of an electromagnetic field are analyzed by adopting a transmission matrix method, and corresponding mathematical description is given below.
When the incident light undergoes total internal reflection at the interface of the square cladding and the metal film, as shown in fig. 5, the square cladding 201 is represented by a C layer, the metal film is represented by a W layer, and the medium to be measured is represented by an S layer, because of the tangential continuity of the electromagnetic field, we can obtain:
in the formulae (1) and (2), 1 and 2 represent the upper and lower surfaces of the W layer, respectively. Considering the propagation of light in the metal thin film, there are:
in the formula (2), δ represents a phase change at both interfaces 1 and 2. By bringing formulae (3) and (4) into formulae (1) and (2), it is possible to obtain:
further, the following formula (5) and (6) may be substituted into the formula (1):
VC＝US(-γisinδ)+VS cosδ (8)
writing equations (7), (8) in matrix form, has:
wherein M in formula (10) is the feature matrix of the metal film. The reflection coefficient and the transmission coefficient of the whole three-layer film structure of the square cladding 201, the metal film 6 and the medium to be detected can be obtained by analyzing the distribution conditions of U and V, and are respectively as follows:
finally, the reflectivity and the transmissivity of the whole three-layer film structure of the square cladding layer, the metal film and the medium to be detected can be obtained, and are respectively as follows:
the invention has the beneficial effects that:
the invention provides a tandem SPR sensor based on capillary optical fibers according to the requirements of the modern sensing field and inherits the advantages of the existing sensor. The sensor can realize the separation of a plurality of SPR resonance wavelengths in a sensing spectrum by controlling the coating parameters of the metal films on the outer surfaces of the square cladding layers, achieves the purpose of series SPR sensing, realizes the analysis and detection of a plurality of different substances in a sample simultaneously, and overcomes the defects of complex operation, low automation degree, low detection efficiency and the like of the prior detection technology, so that the sensor provided by the invention has greater advantages compared with the prior art.
(IV) description of the drawings
FIG. 1 is a schematic diagram of the structure of a SPR sensor based on a square capillary fiber. The sensor consists of a single-core optical fiber 1, a square capillary optical fiber 2 and a metal film 3.
FIG. 2 is a schematic diagram of the preparation process of the structure of "single core fiber-square capillary fiber-single core fiber". In the figure, 1 is a single core optical fiber, 2 is a square capillary optical fiber, 203 is a fusion splicer clamping tool, and 204 is a fusion splicer discharge arc.
FIG. 3 is a schematic diagram of a sensor coating process. FIG. 3(a) is a schematic view of a single-sided gold film plating; FIG. 3(b) is a schematic view of spin-on gold film plating.
Fig. 4 is a cross-sectional view of a polygonal capillary fiber, 401 indicating a cladding of the polygonal capillary fiber, and 402 indicating air holes of the polygonal capillary fiber. Fig. 4(a) shows a triangular capillary fiber, fig. 4(b) shows a rectangular capillary fiber, fig. 4(c) shows a pentagonal capillary fiber, and fig. 4(d) shows a hexagonal capillary fiber.
FIG. 5 is a schematic diagram of a three-layer film structure of "square cladding-metal film-medium to be measured".
FIG. 6 is a system test chart of a fiber temperature and humidity SPR sensor based on a square capillary tube.
(V) detailed description of the preferred embodiments
The invention will be further elucidated by way of example with reference to the drawing.
The preparation process of the SPR sensor based on the square capillary optical fiber mainly comprises the following two steps:
and step 1, welding. As shown in fig. 2, after cutting the square capillary optical fiber 2 to a predetermined length, placing the cut square capillary optical fiber in a clamping tool 203 of a fusion splicer, and placing and fixing the cut end of the square capillary optical fiber 2 between the clamping tool 203 and the electric arcs 204 and 205; similarly, the single-core optical fiber 1 is placed in the clamping tool 203 at the other end of the fusion splicer, the cutting end of the single-core optical fiber 1 is placed between the clamping tool and the electric arcs 204 and 205 and fixed, after the parameters of the fusion splicer are set, the single-core optical fiber 1 and the square capillary optical fiber 2 are fused, and after the fusion splicing is completed, the structure of the single-core optical fiber 1-the square capillary optical fiber 2 can be obtained. Subsequently, in the same manner, a single core optical fiber 3 is fusion-spliced to the other end of the square capillary optical fiber 2, as shown in fig. 2(b), thereby obtaining a "single core optical fiber 1-square capillary optical fiber 2-single core optical fiber 3" structure.
By changing parameters of the welding machine, such as discharge intensity and discharge time, welding effects of the conical transition region and the non-conical transition region can be achieved, more specifically, conical transition regions with different shapes can be prepared by adjusting the discharge intensity and the discharge time, and the shape of the conical transition region determines the capacity of beam segmentation: when the length of the square capillary optical fiber 2 is fixed, the steeper the tapered transition region is, the more times the light wave is totally reflected in the cladding of the square capillary optical fiber 2 is, and after the metal film is plated, the more light wave energy is consumed by the SPR phenomenon, which causes difficulty in finishing light collection, so that the shape of the tapered transition region needs to be strictly controlled.
And 2, coating the film. A schematic diagram of coating of a single-sided coating is shown in FIG. 3 (a). A prepared sample 301 of 'single-core optical fiber-square capillary optical fiber-single-core optical fiber' is placed in a cavity of an ion sputtering instrument and fixed by a clamping tool 302, so that the sample 301 is horizontal to a table top, a vacuum pump is started to pump the environment in the cavity into a lower vacuum state, the potential difference between a negative electrode 303 and a positive electrode 304 is more than hundreds of volts, glow discharge is generated, positive ions are accelerated to bombard a target electrode 305 under the action of an electric field, and the target is sputtered by the atoms 306 to form a layer of metal film 307 on the sample 301. Wherein the negative electrode 303 under the sample is fixed with the probe 308 of the film thickness monitor, the probe 308 and the monitor 309 form a film thickness monitoring system together, the bombarded gold atoms 306 sequentially drop on the sample 301 and the probe 308 from top to bottom, the thickness of the gold film plated on the sample 301 can be converted by calculating the weight of the gold film on the probe 308 within a certain time, and the thickness can be displayed by the monitor 309.
Besides the method of single-sided plating, a method of spin-coating may also be employed. Referring to fig. 3(b), the principle of spin coating is the same as that of single-sided coating, except that the holding tool in the cavity of the ion sputtering apparatus is replaced by a rotating motor 310, and the rotating motor 310 drives the sample 301 to make uniform rotation motion clockwise or counterclockwise, so that the gold atoms 306 can fall on the entire outer surface of the sample 301, and after a certain time of accumulation, a thin film 307 is formed.
The invention is further illustrated below with reference to specific examples.
Example (b): an optical fiber temperature and humidity SPR sensor based on a square capillary tube is used for simultaneously measuring the ambient temperature and the ambient humidity.
As shown in fig. 6, the temperature and humidity SPR sensor based on the square capillary optical fiber is composed of a single-mode optical fiber 1, a square capillary optical fiber 2, a gold film 3, polydimethylsiloxane 4, polyvinyl alcohol 5, a broadband light source 6, and a signal processing unit 7, wherein the single-mode optical fiber 1 includes a cladding 101 and a fiber core 102, and the square capillary optical fiber 2 includes a square cladding 201 and an air hole 202. Referring to the steps 1-2 of the preparation process, the single mode optical fiber 1, the square capillary optical fiber 2 and the single mode optical fiber 1 are sequentially welded together to form a structure of the single mode optical fiber 1-the square capillary optical fiber 2 and the single mode optical fiber 1, gold films 3 with different thicknesses are respectively plated on the upper surface and the lower surface of the square capillary optical fiber 2 by adopting a method of coating a single surface of a gold film, so that resonance wavelengths corresponding to two SPR sensing units connected in series in the sensor are separated, the series connection of the two SPR sensing units is realized, polydimethylsiloxane 4 and polyvinyl alcohol 5 are respectively coated on the gold films 3 of the two SPR sensing units, and finally the preparation of the optical fiber temperature and humidity sensor based on the square capillary is completed. For an SPR temperature sensing unit in the sensor, as polydimethylsiloxane has a high negative thermo-optic coefficient, the refractive index of the polydimethylsiloxane is reduced due to the temperature rise, so that the resonance wavelength corresponding to the SPR temperature sensing unit is shifted, and the sensing of the ambient temperature can be realized by solving the relation between the shift amount of the resonance wavelength and the temperature variation; for the SPR humidity sensing unit in the sensor, the ambient humidity can affect the refractive index of polyvinyl alcohol, so that the resonance wavelength corresponding to the SPR humidity sensing unit is shifted, and the sensing of the ambient temperature can be realized by solving the relation between the shift amount of the resonance wavelength and the change amount of the humidity. The broadband light source 6 emits incident light waves to the sensor to excite the SPR phenomenon, sensing signals output from the sensor are transmitted to the signal processing unit 7, the signal processing unit 7 is composed of a spectrometer and a computer, and finally the displacement condition of the SPR resonance wavelength can be displayed on the computer to realize real-time measurement of temperature and humidity.
Claims (4)
1. The invention provides a tandem SPR sensor based on a capillary optical fiber, which is characterized in that: the sensor consists of a single-core optical fiber 1, a square capillary optical fiber 2 and a metal film 3, wherein the single-core optical fiber 1 comprises a cladding 101 and a fiber core 102, and the square capillary optical fiber 2 comprises a square cladding 201 and an air hole 202; when two ends of a square capillary optical fiber 2 are sequentially welded with a single-core optical fiber 1 in a hot melting way, conical transition regions 4 and 5 are formed near a welding point due to the fact that an air hole 202 is heated and collapsed, metal thin films 3 are additionally arranged on four outer surfaces of a square cladding 201, and four SPR sensing units can be constructed; when the broad spectrum light 6 is introduced to a left welding point from the single-core optical fiber 1, the broad spectrum light 6 can be scattered by the conical transition region 4 to form a divergent light beam 7, then the divergent light beam 7 is totally internally reflected at four interfaces of the square cladding 201 and the metal film 3 at the same time to form a reflected light beam 8 and generate an evanescent wave 9 transmitted along the interfaces, when the evanescent wave 9 is matched with the surface plasma wave vectors of the metal film 3, an SPR phenomenon can occur, at the moment, energy of the reflected light beam 8 is greatly consumed, the reflected light beams 8 of the four surfaces can be finally collected by the single-core optical fiber 1 and then output a sensing spectrum 10, and by controlling the coating parameters of the metal film 3 on each outer surface of the square cladding 201, separation of a plurality of SPR resonance wavelengths in the sensing spectrum 10 can be realized, so that the sensing capacity of the tandem type SPR sensor is improved.
2. The capillary fiber based in-line SPR sensor of claim 1 wherein: the single core optical fiber used may be one of a single mode optical fiber, a few mode optical fiber, a multi-mode optical fiber, or a ring core optical fiber.
3. The capillary fiber based in-line SPR sensor of claim 1 wherein: the outer contour shape of the square cladding of the square capillary fiber can be one of a circle, a triangle, a pentagon or other polygons.
4. The capillary fiber based in-line SPR sensor of claim 1 wherein: the metal film used can also be a composite film structure consisting of a medium and a metal.
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| CN114965379B (en) * | 2022-05-16 | 2024-05-03 | 桂林电子科技大学 | Indoor environment automatic monitoring and regulating system based on SPR |
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