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Abstract
Fiber SPR micro displacement sensor cannot be used for two-dimensional displacement sensing at present. In this paper, we proposed and demonstrated a fiber SPR two-dimensional micro displacement sensor based on the coaxial double waveguide with a conical structure. The coaxial double waveguide is fused into a cone as the light injection fiber, and two different forms of outgoing light fields can be obtained through two cores of the fiber. The horn shaped light field emitted by the ring core of the coaxial double waveguide can cooperate with the sensing fiber to realize the micro displacement sensing in the x-axis direction. And the straight beam emitted by the middle core of the coaxial double waveguide can cooperate with the sensing fiber to realize the micro displacement sensing in the y-axis direction. Through simulation analysis and experimental test, its average wavelength sensitivity and light intensity sensitivity of the x-axis displacement are 0.0537nm/µm and 0.000124a.u./µm, respectively. And that of the y-axis displacement are 0.315nm/µm and 0.00277a.u./µm, respectively. The proposed fiber sensor realizes the two-dimensional displacement sensing based on SPR, which can be widely used in the fields of two-dimensional micro displacement measurement and two-dimensional position precision positioning.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Fiber micro displacement sensor plays an important role in structural health monitoring, landslide monitoring, foundation settlement monitoring and other fields. Surface plasmon resonance (SPR) sensor has the advantages of high precision and high sensitivity in refractive index measurement [1–7], which has been used for the sensing of physical quantities such as temperature [8–11], bending [12,13], strain [14,15], pressure [16–18] and magnetic force [19]. And the scheme of micro displacement sensors based on SPR has been put forward one after another in recent years. In 2011, Lin et al. combined spatial prism SPR and heterodyne interferometry to realize small displacement measurement [20], but the sensor based on prism is too large and difficult to integrate. In 2016, Wang et al. proposed a fiber micro displacement sensor based on Otto structure [21]. Due to the limitation of Otto structure, the detection range is narrow, which is only 0-10nm, and the sensor is vulnerable to environmental interference. Compared with the Otto structure fiber SPR sensor, the fiber SPR sensor based on Kretschmann structure has the advantages of easy fabrication and integration on fiber and wide detection range. In 2017, Zhu et al. proposed to grind the end face of the graded multimode optical fiber, and coated 50nm gold film on the grinding inclined plane to construct Kretschmann structure, so as to realize fiber micro displacement sensing [22]. In 2018, our team welded the graded index multimode fiber and plastic clad multimode fiber, and constructed the Kretschmann structure sensing area on the plastic clad multimode fiber to realize micro displacement sensing [23]. In 2021, Liu et al. proposed a dual parameter SPR sensor for micro angle and micro displacement measurement by using graded index multimode fiber. By corroding and coating graded index multimode fiber, they realized the Kretschmann structure and measured micro angle and micro displacement [24]. To sum up, the proposed fiber SPR micro displacement sensor has not yet realized two-dimensional micro displacement sensing or two-dimensional precise positioning of the plane. If two sets of one-dimensional displacement sensors are superimposed when two-dimensional micro displacement sensing is required, there is a problem of high cost, and it is difficult to place the sensing probe in the narrow space measurement field where fiber sensors have advantages. For the demand of two-dimensional micro displacement sensor and plane two-dimensional precision positioning, it is urgent to carry out the research on fiber SPR two-dimensional micro displacement sensor.
The mechanism of SPR micro displacement sensing based on graded index multimode fiber is: when there is a longitudinal micro displacement between the displacement fiber and the graded multimode SPR photosensitive fiber, that is, the light injection position at the end face of the graded multimode fiber is offset. Due to the self-focusing effect of the transmitted beam in the graded multimode fiber core [25], the amplitude of the cosine path of the transmitted light in the graded multimode fiber changes, and the total reflection angle of the transmitted light changes, that is, the SPR resonance angle changes. The SPR resonance wavelength changes, and then the relationship between the micro displacement of the displacement fiber and the SPR resonance wavelength is established. Therefore, in order to construct two-dimensional sensing, the position of injection light must be changed when moving in the transverse direction, and the displacement fiber light field needs to be a non-direct beam.
In this paper, we proposed a fiber SPR two-dimensional displacement sensor based on conical coaxial double waveguide. We selected coaxial double waveguide fiber as the displacement light injection fiber. By fused biconical taper technology, the end face of the coaxial double waveguide fiber was fabricated to a taper. Two different forms of outgoing light fields are constructed, which are the horn light field from the ring core and the straight beam from the middle core. The sensing probe is a hetero-core structure constituted by graded multimode fiber, single-mode fiber and graded multimode fiber. By the hetero-core structure, the beam modulated by the core of the graded multimode fiber was leaked into the cladding of the single-mode fiber. A 50nm gold film was coated outside the cladding of the single-mode fiber to realize the Kretschmann SPR sensing structure. When the conical coaxial double waveguide ring core is illuminated and displaced in the x-axis direction with the sensing fiber, the x-axis direction movement of the horn light field leads to the change of the light injection position at the end face of the sensing fiber. After the multi-mode modulation by the graded multimode fiber, the SPR resonance angle and evanescent field intensity change, so as to realize the dual parameter sensing of SPR resonance wavelength and resonance valley depth on the x-axis position. When the middle core of the conical coaxial double waveguide passes light and has displacement in the y-axis direction with the sensing fiber, the movement of the straight beam in the y-axis direction leads to the change of the light injection position at the end face of the sensing fiber. After the multi-mode modulation by the graded multimode fiber, the SPR resonance angle and evanescent field intensity change, so as to realize the sensing of the y-axis position by the two parameters of SPR resonance wavelength and resonance valley depth. In addition, the influence of the numerical aperture of the graded multimode fiber in the sensing probe on the performance of the sensor is studied. With the decrease of the numerical aperture, the average sensitivity of the resonance valley depth of the sensor increases and the average sensitivity of the resonance wavelength decreases. The fiber SPR two-dimensional displacement sensor based on conical coaxial double waveguides proposed in this paper can be used in the field of precision two-dimensional displacement measurement and plane two-dimensional positioning in narrow fields.
2. Sensor structure and performance simulation
2.1. Sensor structure
The structure of the proposed fiber SPR two-dimensional micro displacement sensor based on coaxial double waveguide with a conical structure is shown in Fig. 1, the left side is a displacement probe, which is composed of a conical structure processed by a dual concentric-core Fiber (DCCF). The displacement probe generates a micro displacement relative to the right sensing probe by moving along the x-axis (left and right) or y-axis (up and down). The right side is the sensing probe, and the left side of the sensing probe is the modulation area, which is composed of a graded multimode fiber with a length of the beam cosine path period T/2. The middle is the single-mode fiber sensing area coated with a clad gold film, which is composed of a single-mode fiber stripped of the coating layer, coated with a 50nm gold film and then coated with UV curing adhesive outside the sensing gold film. The right side is a light receiving graded multimode fiber. The light from the cladding of the single-mode fiber cladding is collected and sent to the spectrometer for subsequent processing.
Fig. 1. Structural diagram of fiber SPR two-dimensional micro displacement sensor based on conical coaxial double waveguide.
2.2. Analysis of sensing principle
The sensing principle schematic diagram of the proposed sensor is shown in Fig. 2. Figure 2(a) shows the micro displacement sensing principle schematic diagram of the sensor in x-axis direction. When the light is injected into the ring core of coaxial double waveguide fiber by a wide-spectrum light source, the outgoing light field is horn shaped and the outgoing cone angle is $2\alpha$. The coaxial double waveguide with a conical structure is coaxially placed with the sensing fiber. When the transverse distance ${x_i}$ between two fibers gradually increases, the cone horn beam is incident from different longitudinal positions of the modulation graded multimode fiber area of the sensing fiber. Due to the self-focusing effect of graded multimode fiber, the amplitude of the transmitted beam with cosine trajectory increases gradually, after entering the cladding of single-mode fiber, the total reflection angle ${\theta _i}\; $ of beam transmission decreases. The evanescent field of single-mode fiber cladding mode contacts with the gold film on the surface of single-mode fiber cladding, resulting in the SPR effect, and the total reflection angle ${\theta _i}$ is the SPR incident angle. When the transverse distance ${x_i}$ increases, the SPR incident angle decreases, and the evanescent field strengthens, which will lead to the shift of the resonance wavelength and the change of the resonance intensity, so as to realize the dual parameter sensing of SPR resonance wavelength and resonance valley depth to the position of x-axis. The cladding light carrying SPR signal in the single-mode fiber continues to transmit to the right, enters the right graded multimode fiber core for light reception, and then sent to the spectrometer for subsequent processing.
Fig. 2. Sensing principle schematic diagram of fiber SPR two-dimensional displacement sensor based on conical coaxial double waveguide, (a) x-axis, (b) y-axis.
Figure 2(b) shows the schematic diagram of y-axis micro displacement sensing principle. When the light is injected into the middle core of coaxial double waveguide by the wide spectrum light source, the outgoing light field is a horizontal straight beam. The straight beam enters the modulated multimode end face of the sensing fiber horizontally. After being modulated by the self-focusing effect of the graded multimode fiber, the beam propagates to the right with a cosine path. After entering the cladding of the single-mode fiber, the total reflection angle of transmission light is $\theta _i^,$. Similarly, SPR occurs on the interface between the cladding of the single-mode fiber and the sensing gold film. When the longitudinal offset $y_i^,$ increases, the amplitude of the cosine beam in the modulated graded multimode fiber decreases, the SPR incident angle of the sensing zone decreases and the evanescent field strengthens, which will lead to the shift of the resonance wavelength and the change of the resonance intensity, so as to realize the dual parameter sensing of SPR resonance wavelength and resonance valley depth to the position of y-axis.
2.3. Optical field distribution of sensor
By the BeamPROP of Rsoft, the beam propagation path changes in the modulation area and the sensing area with the micro displacement of displacement fiber are simulated and calculated as shown in Fig. 3. The inner diameter of the coaxial double waveguide fiber ring core is 61µm. The thickness is 8.4µm. The diameter of the intermediate core is 8.4µm. Cladding diameter 125µm. The refractive indexes of the ring core layer, intermediate core layer and cladding are 1.4645, 1.4645, and 1.4613, respectively, and the light source type is set as Gaussian light source. The core and cladding diameters of the sensing single-mode fiber are 8.5µm and 125µm, and the core and cladding refractive index are set to be 1.4502 and 1.445. The core and cladding diameters of the graded multimode fiber are 105µm and 125µm, the maximum refractive index is 1.4807, the refractive index type is diffused, and the cladding refractive index is 1.4621. The simulating results of the light field distribution changes of the sensor with micro displacement in x-axis are shown in Fig. 3(a), in which when transverse distance$\; {x_i}$ increases from 100µm to 350µm, the total reflection angle ${\theta _i}$ (SPR incident angle) decreases from 88.7° to 83.9°, and the amplitude of the cosine path beam in the modulation region increases. As shown in Fig. 3(b), when the longitudinal offset $y_i^,$ increases from 25µm to 45µm, the total reflection angle ${\theta _i}$ (SPR incident angle) decreases from 84.2° to 81.7°, and the amplitude of the cosine path beam in the modulation region also increases.
Fig. 3. Light field distribution changes of the sensor with micro displacement in (a)x-axis, (b) y-axis.
2.4. Simulation of SPR two-dimensional displacement sensing
According to Fig. 3, with the increase of the transverse distance$\; {x_i}$, the SPR incident angle decreases, and with the increase of the longitudinal offset $y_i^,$, the SPR incident angle also decreases. In order to further verify the influence of SPR incident angle reduction on the resonance valley wavelength and depth of SPR sensing spectrum, the p-light reflectivity spectrum at different SPR incident angles is calculated with MATLAB software, and the sensing gold film thickness is set to 50nm, the sensing ambient refractive index is 1.35. The angles according to different transverse distances$\; {x_i}$ and longitudinal offset $y_i^,$ were input the calculation program, and the SPR sensing spectrums were obtained. As shown in Fig. 4(a), with the increase of the spacing in x-axis, the resonance wavelength moves to long wavelength, and the depth of resonance valley gradually deepens. Also as shown in Fig. 4(b), with the increase of the offset in y-axis, the resonance wavelength moves to long wavelength, and the depth of resonance valley gradually deepens. The simulating results indicate that the proposed two-dimensional fiber SPR sensor based on conical coaxial double waveguides can realize the sensing of micro displacement in the two directions of x and y axes with two parameters of SPR resonance wavelength and resonance valley depth.
Fig. 4. Simulating resonance spectrum changes of the sensor with micro displacement in (a)x-axis, (b) y-axis.
3. Probe fabrication and experimental testing
3.1. Fabrication of tapered coaxial double guided wave displacement fiber
In order to further verify and test the performance of optical fiber SPR displacement sensor based on conical coaxial double waveguide, the sensing probe was made and tested. Firstly, a displacement fiber is fabricated by conical coaxial double waveguide. Taking a section of coaxial dual waveguide with a length of 100cm (the diameter of the middle core is 8.4µm, inner diameter and outer diameter of the annular core are 61µm and 77.8µm. The cladding diameter is 125µm. The fiber is drawn by Jiangsu Fasheng Photoelectric Technology Co., Ltd.), and the coating layer of coaxial double waveguide fiber is mechanically stripped from the middle, with a length of 1cm. Placing the coaxial double waveguide fiber section with exposed cladding in the fiber fusion splicer FL-116, FIBERLINK) equipped with fiber electrofusion taper program. The left and right clamps of the fusion splicer clamp the left and right areas of the fiber without stripping the coating layer, and the stripped coating area of the fiber is facing the upper and lower electrodes, as shown in Fig. 5(a). When the electrode is discharged, the left and right clamps respectively clamp the fiber to move to both sides, and the middle of the fiber gradually becomes thinner until the middle of the fiber is fused to form a conical end face structure, as shown in Fig. 5(b) (the taper of the fiber can be adjusted by controlling the discharge power, times and the moving distance of the fiber clamp. In this paper, the taper probe parameter selects the discharge power of 30W for 10 times, and the moving distance of each discharge of the fiber clamp is 50µm). As shown in Fig. 5(c), the single-mode fiber core was aligned with the middle core of the coaxial double waveguide, a wide spectrum light was injected into the fiber core of the single-mode fiber to observe the light field emitted from the conical end face under the microscope, and the emitted beam is a parallel straight beam. Similarly, as shown in Fig. 5(d), when the single-mode fiber core was aligned with the annular core of the coaxial double waveguide, the outgoing light field is horn shaped, the exit angle of about 10.9°, which is consistent with the simulation analysis.
Fig. 5. Fabrication drawing of tapered coaxial double waveguide displacement optical fiber (a) schematic diagram of fiber clamping and discharge, (b) schematic diagram of discharge taper, (c) light field from the middle core of conical coaxial double waveguide, (d) light field from ring core of conical coaxial double waveguide.
3.2. Fabrication of sensing probe
The fabrication process of sensing probe is shown in Fig. 6. A single-mode fiber (SMF-28e, CORNING) and a graded multimode fiber (GI105/125-24/250, YOFC) with core diameter of 105µm were spliced coaxially as shown in Fig. 6(a). The left of the single-mode fiber was cut to keep the length of the single-mode fiber at 2cm, as shown in Fig. 6(b). Another graded multimode fiber with core diameter of 105µm was coaxially spliced with the single-mode fiber on the left as shown in Fig. 6(c). By the fixed length cutting device, the left graded multimode fiber is cut to keep the length as 900µm (T/2, T is the period length of cosine beam transmission path), and the heterogeneous core structure of graded multimode fiber-single-mode fiber-graded multimode fiber as shown in Fig. 6(d) was obtained. The heterogeneous core structure was placed in the magnetron sputtering (ETD-650MS, Vision Precision Instruments), and the single-mode fiber segment was directly below the gold target. The left graded multimode fiber was covered with a quartz tube to prevent the end face of the left end graded multimode optical fiber from being coated. Under the condition of on-line monitoring by the film thickness monitor, a 50nm gold film was coated on the outer surface of the single-mode fiber cladding, as shown in Fig. 6(e). Finally, a layer of UV curable adhesive (NOA135,Norland) with refractive index of 1.35 was coated outside the sensing gold film (according to the previous verification experimental data of the y-axis micro displacement, when the external refractive index environment of sensing probe is 1.34, 1.35 and 1.36 respectively, the micro displacement sensitivity increases in turn. The sensing probe with external refractive index of 1.35 has the highest light intensity sensitivity and medium wavelength sensitivity. Therefore, in this paper, UV curable adhesive with refractive index of 1.35 is selected to provide refractive index environment for the sensing area of the sensing probe.) and solidified. Then the quartz tube head was unplugged, and the fabrication and packaging of sensing probe was completed as shown in Fig. 6(f).
Fig. 6. Manufacturing flow chart of sensing probe, (a) the single-mode fiber was spliced with the graded multimode fiber, (b) cutting at 2cm to the left of single-mode optical fiber solder joint, (c) the left side of the single-mode fiber was spliced with the graded multimode fiber, (d) fixed length cutting at 900µm to the left of graded multimode fiber solder joint, (e) coating 50nm sensing gold film, (f) coating UV curing adhesive.
3.3. Construction of two-dimensional micro displacement sensing test device
The experiment test platform is shown in Fig. 7, the light from wide spectrum light source (HL-2000, Ocean Optics) was injected into the single-mode fiber. By a fiber coupling micro motion table, the single-mode fiber was adjusted to inject light for the coaxial double waveguide from the middle core or ring core under the electron microscope. The tapered end of the coaxial dual waveguide and the graded multimode modulation end of the sensing fiber were clamped on the left and right of the three-dimensional precision micro displacement adjustment table (MP-225, Sutter) respectively. By the left three-dimensional precision micro displacement adjustment table clamped with the tapered coaxial dual waveguide, the displacement along the x-axis or y-axis was generated. The light from the displacement fiber was injected into the left graded multimode fiber, the beam was modulated into a cosine path beam through the modulation region, and which was injected into the cladding of the single-mode fiber with an incident angle. The cladding mode evanescent field contacted with the 50nm gold film coated on the cladding surface. After the SPR effect occurred, the sensing optical signal continued to be transmitted to the graded multimode light receiving area on the right. The graded multimode light receiving area collected the photosensitive signal transmitted by the single-mode fiber cladding and sent it to the spectrometer (USB2000+, Ocean Optics). The spectrometer was connected to the computer to collect data and process the SPR attenuation spectrum.
3.4. Testing results
Firstly, by the fiber two-dimensional micro displacement sensing test device, the micro displacement sensing performance of the SPR two-dimensional micro displacement sensor based on conical coaxial double waveguide in the x-axis direction is tested. The fiber coupling micro motion stage was adjusted to make the single-mode fiber inject light for the ring core of the coaxial double waveguide. And by adjusting the three-dimensional precision micro displacement adjustment table, the displacement fiber and the sensing probe were coaxial alignment, and the clearance in the x-axis direction to was adjusted to 10µm. Then the clearance increased with the stepping of 40µm, up to 310µm. The corresponding SPR transmission spectrum was collected by the computer with the spectrometer every time the displacement was increased, the testing results are shown in Fig. 8(a). In which, with the increase of the displacement in x-axis, the depth of SPR resonance valley increases. In order to more clearly observe the movement of the resonance wavelength, we normalized the data in Fig. 8(a) to align the bottom of the SPR resonance trough to obtain Fig. 8(b). It can be seen that with the increase of the displacement in the x-axis direction, the SPR resonance wavelength moves to long wavelength.
Fig. 8. Testing results of the fiber SPR displacement sensor in (a) x-axis, (c) y-axis. (b) Normalization results of (a), (d) normalization results of (c).
Next, the sensing performance of fiber SPR two-dimensional micro displacement sensor in y-axis direction is tested. The fiber coupling micro motion stage was adjusted to make the single-mode fiber inject light for the middle core of the coaxial double waveguide. And by adjusting the three-dimensional precision micro displacement adjustment table, the displacement fiber and the sensing probe were coaxial alignment, and the clearance in the x-axis direction to was adjusted to 10µm. The displacement in y-axis direction was adjusted to 15µm (for good SPR resonance valley), then the displacement in y-axis direction increased with the stepping of 3µm, up to 45µm. The corresponding SPR transmission spectrum was collected by the computer with the spectrometer every time the displacement was increased, the testing results are shown in Fig. 8(c). In which, with the increase of the displacement in y-axis, the depth of SPR resonance valley increases. Also, we normalized the data in Fig. 8(c) to align the bottom of the SPR resonance trough to obtain Fig. 8(d). It can be seen that with the increase of the displacement in the y-axis direction, the SPR resonance wavelength moves to long wavelength.
The above experimental results indicate that the fiber SPR two-dimensional micro displacement sensor can realize the two-dimensional displacement sensing of SPR resonance valley depth and resonance wavelength in the x-axis and y-axis directions.
According to the data in Fig. 8(a), we took the micro displacement in the x-axis direction (transverse) as the abscissa and the SPR resonance valley depth corresponding to different micro displacements in the x-axis direction as the ordinate to plot the figure and carry out linear fitting according to the data trend, and the Fig. 9(a) was obtained. The SPR resonance valley depth sensitivity of the displacement in the x-axis direction of the two-dimensional micro displacement sensor is 0.000124a.u./µm.
Fig. 9. Relationship curves between relationship curves between, (a)micro displacement and resonance valley depth, (b) micro displacement and resonance wavelength in x-axis direction, (c) and (d) in y-axis direction.
According to the data in Fig. 8(b), we took the micro displacement in the x-axis direction (transverse) as the abscissa and the SPR resonance wavelength corresponding to different micro displacements in the x-axis direction as the ordinate to plot the figure and carry out linear fitting according to the data trend, and Fig. 9(b) was obtained. The SPR resonance wavelength sensitivity of the displacement in the x-axis direction of the two-dimensional micro displacement sensor is 0.0537nm/µm. Its x-axis displacement detection range can reach 10-310µm. Similarly, Fig. 9(c) and Fig. 9(d) show the relationship curves between micro displacement in the y-axis direction and resonance valley depth, micro displacement in the y-axis direction and resonance wavelength, The SPR resonance valley depth and wavelength sensitivity of the displacement in the y-axis direction of the two-dimensional micro displacement sensor are 0.00277a.u./µm and 0.315nm/µm, and the y-axis displacement detection range is 15-45µm.
According to Fig. 8 and Fig. 9, the displacement sensing of SPR resonance valley depth and resonance wavelength in the x-axis and y-axis directions can be realized by injecting light into the ring core and middle core of conical coaxial dual waveguide fiber respectively. The range of micro displacement in x-axis direction is 10-310µm. The wavelength sensitivity and valley depth sensitivity are 0.0537nm/µm and 0.000124a.u./µm, respectively. The range of micro displacement in y-axis direction is 15-45µm, and the wavelength sensitivity and valley depth sensitivity are 0.315 nm/µm and 0.00277a.u./µm, respectively.
4. Discussion
The numerical aperture NA of the fiber will affect its ability to collect and transmit light. In this paper, the modulation area of the sensing probe is a graded multimode fiber. When the NA is different, it will have different light receiving abilities, and the transmission mode maintained after self-focusing will also be different, resulting in different performance of two-dimensional displacement sensing. In this paper, three kinds of graded multimode fiber with different numerical aperture (NA is 0.3, 0.24, and 0.14, respectively) were employed to fabricate the sensing probe. Taking the micro displacement in y-axis direction as an example, the influence of graded multimode fiber NA in modulation region on displacement sensing performance was experimentally studied.
We first employed the graded multimode fiber with NA of 0.3 as the modulation fiber to fabricate the sensing probe. The sensing probe was clamped on the right table of the three-dimensional precision micro displacement adjustment table, under the electron microscope, the fiber core of the single-mode fiber was aligned with the core of the coaxial double waveguide middle through the fiber coupling micro motion table, so as to realize the light transmission to the coaxial double waveguide middle core. The center of the displacement fiber and sensing probe were aligned to the axis through the 3D precision displacement stage, and the x-axis clearance was adjusted to 10µm. The displacement in y-axis direction was adjusted to 15µm. Increased the displacement in the y-axis direction in turn, increasing by 5µm each time, up to 45µm. Each time the displacement increased the corresponding SPR transmission spectrum shall be collected by spectrometer. As shown in Fig. 10(a), with the increase of the displacement in y-axis direction, the depth of the SPR resonance valley increases. As the normalization results of Fig. 10(a) shown in Fig. 10(b), with the increase of the displacement in y-axis direction, the SPR resonance wavelength moves to long wavelength. Similarly, we fabricated the sensing probes by the graded multimode fibers with NA of 0.24 and 0.14 as the modulation fibers and tested them. The testing results are shown in Fig. 10(c) and 10(e), and the normalization results are shown in Fig. 10(d) and 10(f). According to Fig. 10, with the decrease of the NA of the graded multimode fiber used as modulation fiber, the depth of the resonance valley deepens.
Fig. 10. Testing results of the sensing probe fabricated by the graded multimode fiber with NA of (a) 0.3, (c) 0.24, (e) 0.14 as modulation fiber. (b), (d) and (f) normalization results of (a), (c) and (e).
The relationship curves between displacement and resonance intensity, displacement and resonance wavelength are shown in Fig. 11. According to Fig. 11(a), with the decrease of the NA of the graded multimode fiber, the resonance intensity sensitivity of sensor the increases. And according to Fig. 11(b), with the decrease of the NA of the graded multimode fiber, the resonance wavelength sensitivity of the sensor decreases.
Fig. 11. (a)The relationship curves of the displacement and the depth of the resonance valley, (b) the displacement and the resonance wavelength.
Based on the above analysis, the smaller the graded multimode numerical aperture in the modulation area of the sensing probe, the greater the SPR resonance valley depth sensitivity, while the smaller the SPR resonance wavelength sensitivity. When actually fabricating a fiber SPR two-dimensional micro displacement sensor based on conical coaxial double waveguide, the graded multimode fiber with appropriate numerical aperture can be selected according to whether SPR resonance valley depth parameter sensing or SPR resonance wavelength parameter sensing is used.
5. Conclusion
The fiber SPR micro displacement sensor cannot carry out two-dimensional displacement sensing and plane position sensing. According to the fiber SPR micro displacement sensing mechanism, in order to construct two-dimensional sensing, the displacement probe must be able to move in both x-axis and y-axis, and the light injection horizontal position opposite the sensing probe can be changed. The y-axis can be realized by using the straight beam emitted from the fiber core, and the oblique beam must be constructed in the x-axis direction. In this paper, the coaxial double waveguide fiber was used as the displacement fiber, and its end face was fabricated into a cone by electrofusion micromachining technology. When the ring core propagates light, the horn shaped oblique light field is constructed. When the intermediate core propagates light, the outgoing light field is a straight beam. The sensing probe was constructed by a hetero-core structure of graded multimode fiber-single-mode fiber-graded multimode fiber. A 50nm gold film was coated outside the cladding of the single-mode fiber to realize the Kretschmann SPR structure. The horn shaped light field emitted by the ring core of the coaxial double waveguide can cooperate with the sensing fiber to realize the micro displacement sensing in the x-axis direction. And the straight beam emitted by the middle core of the coaxial double waveguide can cooperate with the sensing fiber to realize the micro displacement sensing in the y-axis direction. The average wavelength and intensity sensitivity of x-axis displacement are 0.0537nm/µm and 0.000124a.u./µm, respectively. Its x-axis displacement detection range can reach 10-310µm. The average wavelength and intensity sensitivity of y-axis displacement are 0.315nm/µm and 0.00277a.u./µm, respectively. Its y-axis displacement detection range is 15-45µm. We also study the effect of the numerical aperture of the graded multimode fiber in the modulation region on the sensing performance of the sensor. The testing results indicate that the smaller the numerical aperture is, the greater the sensitivity of SPR resonance intensity is, while the smaller the sensitivity of SPR resonance wavelength is. When the fiber SPR two-dimensional micro displacement sensor based on conical coaxial double waveguide is actually manufactured, the appropriate graded multimode fiber numerical aperture can be selected according to whether SPR resonance valley depth parameter sensing or SPR resonance wavelength parameter sensing is used. The fiber SPR two-dimensional displacement sensor based on conical coaxial double waveguide realizes the two-dimensional displacement sensing of fiber SPR micro displacement sensor, which can be widely used in the field of two-dimensional micro displacement measurement and two-dimensional position precision positioning.
Funding
Science and Technology Planning Project of Guangdong Province (2018A01006); Fundamental Research Funds for Chongqing Three Gorges University of China (19ZDPY08); Chongqing Key Laboratory of Geological Environment Monitoring and Disaster Early-Warning in Three Gorges Reservoir Area (ZD2020A0102, ZD2020A0103); Science and Technology Project Affiliated to the Education Department of Chongqing Municipality (KJ1710247, KJQN201801217, KJQN201901226, KJQN202001214); Natural Science Foundation of Chongqing (cstc2019jcyj-msxmX0431); National Natural Science Foundation of China (61705025).
Disclosures
The authors declare no conflict of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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