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Abstract
To improve the sensing performance of optical fiber magnetic field sensor based on magneto-refractive effect, a D-shaped photonic crystal fiber-surface plasmon resonance (PCF-SPR) sensor based on magneto-refractive effect is proposed and its magnetic field sensing characteristics are investigated. The designed D-shaped PCF has a core-analyte-gold structure. Within the D-shaped PCF, the side polishing surface is coated with the gold film and the special hole is sandwiched between the core and the gold film. To realize the high magnetic field sensitivity for the fiber SPR magnetic field sensor, the special hole is filled with magnetic fluid (MF). In this paper, we analyze the mode transmission characteristics and magnetic field sensing characteristics of this fiber sensor by finite element method. We also obtain a general rule for the optimization of PCF-SPR sensors by analyzing the dispersion curves, the energy of the surface plasmon polariton mode and the core mode on the sensing performance of the designed fiber sensor. The maximum refractive index sensitivity and magnetic field sensitivity of the optimized fiber are 59714.3 nm/RIU and 21750 pm/mT (50-130 Oe), respectively. Compared with optical fiber magnetic field sensors based on magneto-refractive effect reported previously, the magnetic field sensitivity in this paper is nearly two orders of magnitude higher and it can initially achieve nT magnitude magnetic field resolution and testing capability. The proposed fiber sensor has the advantages of simple structure, easy production, high sensitivity, and strong environmental adaptability. It not only improves the sensing performance of optical fiber magnetic field sensors, but also provides an ideal alternative platform for biosensors like microfluidics because of its high refractive index sensitivity and the special structure.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Magnetic field is closely related to human life and it is an essential fundamental parameter to be measured in many fields, such as power grid monitoring [1], communication and navigation [2], resource development [3], earthquake early warning [4] and medical diagnosis [5], ect. The magnetic field sensors can be divided into electrical magnetic field sensors and optical fiber magnetic field sensors according to the sensing medium and mechanism. Among them, electrical magnetic field sensors have been widely studied and used practical early. But electrical magnetic field sensors have some disadvantages such as weak anti-electromagnetic interference ability, requirement for an additional temperature control system, high cost, weak environmental adaptability and limited dynamic range for testing [6]. For instance, super-conducting quantum interference devices (SQUIDs) need liquid helium coolant, and spin exchange relaxation-free (SERF) magnetometers only operate near zero field [7,8]. With evolution and advancement of new technology fields such as human-computer interaction, bio-imaging, unmanned monitoring, industrial manufacturing, the internet of things and so on, high sensitivity, high integration, wearable, low cost, miniaturization, vector sensing and many other new application requirements for magnetic field sensors have been put forward [9]. In addition, to meet the demand for underwater magnetic field detection, magnetic field sensors need to be powerless and easy to network [10]. To overcome the issue, an efficient way is to resort to optical fibers, which possess the abilities of small size, strong anti-electromagnetic interference ability, strong environmental adaptability, easy integration and multiplexing [11].
Currently, optical fiber magnetic field sensors can be divided into different types based on the sensing mechanisms, including Faraday effect [12,13], magnetostrictive effect [14], and magneto-refractive effect [15–19]. Among them, Faraday effect materials are very complex to produce, magnetostrictive effect materials are difficult to integrate with optical fiber. In contrast, the sensors based on magneto-refractive effect of magnetic fluid (MF) are widely studied because of their simple production process, low cost and high integration. By specially designing the fiber structure, the guided light inside fiber coupled into MF via the evanescent field, and the magnetic response property of MF is used to realize the optical field modulation and the magnetic field sensing. Those structures include fiber Bragg grating (FBG) [15,16], cascade fiber [17], tapered fiber [18], photonic crystal fiber (PCF) [19], and the sensitivities of most of these are in the range of 20-700 pm/mT . In terms of sensitivity, test dynamic range, resolution and other basic performance, the performance of this type of magnetic field sensors needs to be improved to meet the demands of magnetic field sensing applications.
It is of interest that optical fiber based on surface plasmon resonance (SPR) uses the optical phenomenon of the electromagnetic surface mode (i.e., surface plasmon polariton mode) on the metal-dielectric surface excited by the evanescent field in optical fiber, and it is extremely sensitive to changes in the refractive index of external analyte [20,21]. Those SPR based optical fiber sensors have the advantages of high sensitivity, no mark, fast response, low detection limit and have been widely used in fields of environmental monitoring, biochemistry, medical diagnosis [22–24]. In addition, the structure of PCF has flexibility and uniqueness, and its holey structure provides a good way for filling [25]. The combination of MF, PCF and SPR enables high performance of magnetic field sensing capabilities.
In this paper, a D-shaped PCF-SPR sensor based on magneto-refractive effect is proposed. Compared with PCFs-SPR reported previously [26,27], our D-shaped PCF-SPR sensor has a core-analyte-gold structure. We design the special hole is sandwiched between the fiber core and the gold film, it is also filled with MF, which can achieve highly sensitive magnetic field sensing and avoid the corrosion of MF on metal and reduces the crosstalk of temperature on MF, to some extent. In addition, we analyze the mode transmission characteristics and magnetic field sensing characteristics of this fiber sensor using finite element method. We also obtain a general rule for the optimization of PCF-SPR sensors by analyzing the dispersion curves, the energy of the surface plasmon polariton (SPP) mode and the core mode on the sensing performance of the designed fiber sensor. The results show that our fiber sensor has extremely high refractive index sensitivity (59714.3 nm/RIU) and magnetic field sensitivity (21750 pm/mT). The magnetic field sensitivity of our fiber sensor is two orders of magnitude higher than the results in existing published reports [15–17,28–32]. It can initially achieve nT magnitude magnetic field resolution and testing capability. Therefore, a core-analyte-metal based structure is expected to achieve high sensitivity sensing more easily. In addition, our fiber sensor has the advantages of simple structure, easy production, high sensitivity, and strong environmental adaptability. It is not only expected to meet the needs of geomagnetic sensing applications, but also to achieve weak magnetic sensing capabilities such as environmental magnetic anomalies and medical diagnosis due to its nT resolution.
2. Model and theory
The cross-section of the proposed D-shaped PCF-SPR sensor is shown in Fig. 1(a). The side polishing surface is coated with the gold film and then the special hole below the center of the gold film is filled with MF to realize the high magnetic field sensitivity for the fiber SPR magnetic field sensor. The rest of the air holes are arranged in a hexagonal shape (same diameter and equally spaced), and an air hole in the center of the second layer is missing, forming a core to transmit light. In Figs. 1(a) and 1(b), the structure parameters Lm, Λ, d1, d2, D and h represent the thickness of the gold film (Lm), the pitch of air hole (Λ), the diameter of the air holes (d1), the diameter of the special hole (d2), the distance between the special hole and side polishing surface (D), and the distance between the first layer of air holes and side polishing surface (h), respectively. Figure 1(c) shows the schematic diagram of the proposed fiber sensor in three-dimensional (3D) model. The PCF can be produced by the stake-and-draw method [33], and then using the wheel polishing method or the V-groove side polishing method to fabricate the D-shaped PCF [34,35]. The selective filling process can make it possible to fill any air hole of the PCF with the liquid, including microscopic gluing method [36], laser drilling method [37], femtosecond laser two-photon polymerization method [38], focused ion beam milling method [39], manually gluing in the fusion splicer method [40]. Among them, the manually gluing in the fusion splicer method is an inexpensive way to achieve a single air hole to be filled with the liquid by aligning the capillary with any air hole in the PCF with a fusion splicer. Finally, the structure of the proposed D-shaped PCF-SPR sensor can be prepared by coating the gold film on the side polishing surface using chemical vapor deposition (CVD) [41].
Fig. 1. (a) Cross-section of the proposed D-shaped PCF-SPR sensor; (b) detail of the structure of the surface of the proposed fiber sensor; (c) schematic diagram of the proposed fiber sensor in three-dimensional (3D) model.
In the simulation, the initial setting parameters are shown in Table 1.
Table 1. Initial setting parameters setting.
The refractive index of air is 1 and the refractive index of fiber can be defined according to Sellmier’s equation [42]:
The dielectric constant of the metal layer is expressed using the Drude–Lorentz model according to Eq. (2):
In this paper, the sensing performance of our fiber sensor with different structures is analyzed by the loss spectrum analysis method, the sensitivity and other characteristics of the designed fiber sensor can be obtained by calculating the SPR peak shift of the loss spectra with this method. The equation is as follows [44]:
where $\lambda $ is wavelength, $Im({{n_{eff}}} )$ is the imaginary part of the effective refractive index of the core mode.Since the fiber sensor structure designed in this paper does not have rotational symmetry, resulting in different distribution of the refractive index of the fiber sensor in x and y directions. Two orthogonal modes are not degeneracy and therefore the x-polarized core mode and the y-polarized core mode have different effective refractive indices [45]. In addition, by solving the Maxwell equations of metal-dielectric surface, it can be found that the SPP mode is mainly excited by the electric field which is orthogonal to the metal surface [46–49]. In the structure of this fiber sensor, the y-polarized core mode has a more prominent optical phenomenon of SPR and higher sensing performance than the x-polarized core mode. So this paper focuses on the transmission and sensing characteristics of the y-polarized core mode. Figure 2 plots the real part of the effective refractive index of the y-polarized core mode (Re(neff)), that of the first-order SPP mode (Re(nspp)) with different wavelengths, and the loss spectrum of the D-shaped PCF-SPR sensor. The SPR effect is based on the coupled mode theory (CMT), which can be express as follows [50]:
Fig. 2. Real part of effective refractive index of the y-polarized core mode and the SPP mode, and the loss spectrum of the proposed D-shaped PCF-SPR sensor.
In theory, when the refractive index of MF in the special hole changes, it leads the effective refractive indices of the core mode and the SPP mode to change. And the change of phase matching condition causes the SPR peak to shift [51–53]. The SPP mode is more sensitive to the change in the refractive index of the MF than the core mode. Therefore, in this paper the fiber sensor has the special hole filled with MF near the position where the SPP mode is excited. Such a way, which better exploits the sensitive property of the SPP mode, greatly improves the magnetic field sensitivity of the fiber sensor. The fiber sensor sensitivity can be evaluated by the variation of the SPR resonant wavelength with the refractive index of analyte, which is expressed by Eq. (5) [54]:
where $\Delta {\lambda _{peak}}$ is the distance of the SPR resonant wavelength shift and $\Delta {n_a}$ is the change value of the refractive index of the special hole.The figure of merit (FOM) of the fiber sensor, which affects the accuracy of detection. The FOM be calculated as follows according to Eq. (6) [55]:
where ${S_\lambda }$ is the refractive index sensitivity of the fiber sensor, $FWHM$ is the full width at half-maximum of the loss spectrum.3. Simulations and analysis
We use COMSOL Multiphysics 5.5 based on finite element analysis in order to simulate the proposed D-shaped PCF-SPR sensor structure. Figures 3(a) and (b) show the optical field distribution of the x-polarized core mode and the y-polarized core mode. Based on the previous discussion, we choose the y-polarized core mode for our study and optimize the structure parameters Lm, Λ, D, d2 to improve the sensing performance of the proposed D-shaped PCF-SPR sensor.
3.1 Thickness of the gold film
In this paper, we first analyze the effect of Lm on sensing characteristics of the designed fiber sensor. Figures 4(a)-(e) show the losses spectrum of y-polarized core mode in the refractive index range of 1.43-1.435 when Lm are selected as 50 nm, 60 nm, 70 nm, 80 nm and 90 nm, respectively. It can be found that, SPR resonant wavelengths blue shift with an increase in Lm at the same refractive index.
Fig. 4. Influence of Lm on the loss spectrum. (a) Lm = 50 nm, (b) Lm = 60 nm, (c) Lm = 70 nm, (d) Lm = 80 nm, (e) Lm = 90 nm, and (f) the fitted results of the SPR resonant wavelength with different Lm.
In general, the sensitivity of the sensor is higher in longer working wavelength because of non-linear variation of the SPP mode dispersion curve [56]. We use the Drude model of gold to make a schematic, as shown in Fig. 5.
In Fig. 5, $\beta $ represents the propagation constant of light. c represents the speed of light. ${\omega _p}$ represents the plasma frequency of gold. $\omega $ represents the frequency of light. Dispersion curves of the SPP mode in air and in silicon are plotted. The phase matching condition of SPR is determined by the intersection between the core mode and the SPP mode dispersion curves. The dispersion curve of the light in air has no intersection with that of the SPP mode, so the SPR cannot be excited. When light enters different media, its dispersion curve is different. According to Fig. 5, compared to light in medium 2, light in medium 1 excites SPR at the shorter wave vector (longer wavelength) and has a bigger slope, which means a higher sensitivity of light in medium 1. Therefore, in order to improve the sensitivity of the fiber sensor, we need to adjust the PCF structure and ensure that the SPR is excited in the long-wave direction as much as possible.
In addition, we also calculate average losses of the y-polarized core mode when Lm are set to 50 nm, 60 nm, 70 nm, 80 nm, and 90 nm, which are 168.5 dB/cm, 162.4 dB/cm, 159.2 dB/cm, 154.5 dB/cm, and 148.8 dB/cm, respectively. So Lm is inversely related to the average losses within a certain range, and it weakens the energy in the special hole, thus leading to the reduction of the fiber sensor sensitivity [57].
Figure 4(f) shows the refractive index sensitivities of the proposed fiber sensor in different Lm, which are 44714.4 nm/RIU, 43914.3 nm/RIU, 42142.9 nm/RIU, 40685.7 nm/RIU, and 39400 nm/RIU, respectively. FOMs also can be calculated, they are 327.6 RIU-1, 353.6 RIU-1, 356.6 RIU-1, 358.9 RIU-1 and 354.4 RIU-1, respectively. And we can find that the refractive index sensing response of this fiber sensor varies linearly and FOM changes relatively small. But the sensing sensitivity decreases when Lm increases. Therefore, Lm of the optimized fiber sensor is set to 50 nm.
3.2 Pitch of air hole
This paper then analyze the effect of different Λ on sensing performance of this fiber sensor and the results are shown in Fig. 6. As seen in Figs. 6(a)-(e), SPR resonant wavelengths blue shift as Λ decreases. This is because the reduction of Λ in this fiber sensor structure decreases the effective refractive indices of the y-polarized core mode and the SPP mode. However, the effective refractive index of the SPP mode changes more significant than that of the y-polarized core mode, resulting in the blue shift of the phase matching point of SPR. It can also find that average losses at different Λ are calculated as 168.5 dB/cm, 162.4 dB/cm, 148.22 dB/cm, 124.0 dB/cm, and 93.8 dB/cm, respectively. Average losses decrease sequentially with the decrease of Λ. This is mainly due to the fact that the reduction of Λ not only makes SPR resonant wavelengths blue shift, but also leads to the reduction of the confined energy in this fiber sensor, which eventually weakens coupling between the y-polarized core mode and the SPP mode. Finally, it can be judged that the decrease of Λ reduces the sensitivity of fiber sensor. This is confirmed by result of the fiber sensor sensitivity, as shown in Fig. 6(f). The refractive index sensitivities are 44714.4 nm/RIU, 40457.1 nm/RIU, 36257.1 nm/RIU, 33028.6 nm/RIU, and 30857.1 nm/RIU, respectively. FOMs are 327.6 RIU-1, 340.9 RIU-1, 318.9 RIU-1, 273.7 RIU-1 and 226.6 RIU-1, respectively. Therefore, we choose the optimized value of Λ to be 8 µm.
Fig. 6. Influence of Λ on the loss spectrum. (a) Λ=8 µm, (b) Λ=7.8 µm, (c) Λ=7.6 µm, (d) Λ=7.4 µm, (e) Λ=7.2 µm, and (f) the fitted results of the SPR resonant wavelength with different Λ.
3.3 Distance between the special hole and side polishing surface
The simulation results of the designed fiber sensor sensing characteristics for different D are shown in Fig. 7. In Figs. 7(a)-(e), average losses are 168.5 dB/cm, 172.3 dB/cm, 172.1 dB/cm, 171.3 dB/cm and 170.44 dB/cm when D are set to 0 µm, 0.1 µm, 0.2 µm, 0.3 µm, and 0.4 µm, respectively. It can be seen that average losses are approximately the same, so the change of D has little effect on the energy of the y-polarized core mode coupled to the SPP mode. But SPR resonant wavelengths red shift as D increases.
Fig. 7. Influence of D on the loss spectrum. (a) D = 0 µm, (b) D = 0.1 µm, (c) D = 0.2 µm, (d) D = 0.3 µm, (e) D = 0.4 µm, and (f) the fitted results of the SPR resonant wavelength with different D.
However, the special hole moves away from the energy field of the SPP mode when D increases. As shown in Fig. 8, the energy field of the SPP mode at the metal-dielectric interface has the highest electric field intensity (NormE) and decays exponentially in the direction of its oscillation.
Therefore, D becomes larger leading to a rapid decrease in the energy field of the SPP mode in the special hole. Figure 7(f) shows the analysis result of the fiber sensor sensitivity for different values of D. It can be seen that the fiber sensor sensitivities are 44714.4 nm/RIU, 39200.0 nm/RIU, 34514.3 nm/RIU, 30371.4 nm/RIU, 26914.3 nm/RIU, respectively, so its sensitivity decreases gradually when D increases. Even if the SPR resonant wavelengths red shift, which still makes the fiber sensor less sensitive. FOM is also decreasing sequentially, and they are 327.6 RIU-1, 305.0 RIU-1, 272.8 RIU-1, 242.0 RIU-1 and 216.4 RIU-1, respectively. So, D is chosen as 0 µm.
3.4 Diameter of the special hole
In addition, the simulation analysis in this paper is found that d2 has a great impact on the sensing characteristics of the fiber sensor. The analyte in the special hole is a low refractive index medium compared to the material of fiber and therefore an increase in d2 leads to a decrease in the effective refractive index of the SPP mode. In addition, the y-polarized core mode is unaffected basically because the core of PCF is far from the special hole. This eventually leads to the blue shift of the phase matching point of SPR. As shown in Figs. 9(a)-(e), SPR resonant wavelengths gradually blue shift with an increase in with d2. Meanwhile, average losses are calculated as 168.5 dB/cm, 129.7 dB/cm, 116.9 dB/cm, 107.3 dB/cm and 92.3 dB/cm, respectively and so an increase in d2 also reduces the energy of the y-polarized core mode coupled to the SPP mode.
Fig. 9. Influence of d2 on the loss spectrum. (a) d2 = 2 µm, (b) d2 = 2.2 µm, (c) d2 = 2.4 µm, (d) d2 = 2.6 µm, (e) d2 = 2.8 µm, and (f) the fitted results of the SPR resonant wavelength with different d2.
However, an increase in d2 leads to a quadratic increase in the area of the special hole, which increases the overlap area between the energy field of the SPP mode and the special hole. By calculating the value of NormE in the special hole at the refractive index of 1.435, it can be found that the value of NormE in the special hole is increasing with an increase in d2 from the overall view in Fig. 10. This indicates that the energy in the special hole is still rising with an increase in d2.
The result of the sensitivity analysis in Fig. 9(f) shows that when d2 are set to 2 µm, 2.2 µm, 2.4 µm, 2.6 µm, and 2.8 µm, the fiber sensor sensitivities are 44714.4 nm/RIU, 50028.6 nm/RIU, 54457.1 nm/RIU, 57628.6 nm/RIU, and 59714.3 nm/RIU, respectively. So an increase in the energy in the special hole not only makes up the decrease in sensitivity due to the blue shift of SPR resonant wavelengths, but also leads the fiber sensor sensitivity to increase. FOMs are also calculated, which are 327.6 RIU-1, 345.0 RIU-1, 337.5 RIU-1, 324.0 RIU-1 and 288.0 RIU-1, respectively. It can be found that FOM reaches to minimum when d2 is set to 2.8 µm, but considering the low sensitivity of the current optical fiber sensor based on magneto-refractive effect, we choose to sacrifice FOM to some extend and then select d2 set to 2.8 µm as the optimized parameter.
4. Magnetic field sensing
In this paper, we also simulate the magnetic field response characteristics of the optimized fiber sensor, and the experimental setup of this fiber sensor is shown in Fig. 11. The input light from a broadband light source passes through a polarizer controller, where the polarization state of the light is modulated. The SPR effect is excited afterwards when the polarized light enters into the designed D-shaped PCF-SPR sensor. A DC source can be used to control the Helmholtz coil to generate the magnetic field from 50 Oe to 130 Oe. The change in the refractive index of MF causes the SPR resonant wavelength to shift, and then the measurement of the magnetic field is achieved by detecting the shift of the SPR resonant wavelength with an optical spectrum analyzer (OSA) and PC.
Fig. 11. Schematical diagram of the experimental setup of the D-shaped PCF-SPR sensor for magnetic field detection.
The change of the refractive index of MF can be expressed by the Langevin model function as [58]:
Due to the magneto-refractive index adjustability of MF in the fiber sensor, the magnetic field can control phase matching condition. The SPR resonant wavelength will shift on the spectrum with different magnetic field. The magnetic field sensitivity of the fiber sensor can be obtained by calculating the shift of the SPR resonant wavelength, which is defined as:
in Eq. (8), $\Delta {\lambda _{peak}}$ is the distance of the SPR resonant wavelength shift and $\Delta H$ is the change of the external magnetic field.The optimized fiber sensor is tested in the magnetic field range of 50-130 Oe. As shown in Fig. 12(a), the refractive index of MF is increasing with an increase in the magnetic field, leading to an increase in the effective refractive index of the SPP mode. Therefore, SPR resonant wavelength red shifts. The magnetic field sensitivity of the optimized fiber sensor is calculated by Eq. (8) and it can reach a maximum magnetic field sensitivity of 21750 pm/mT in Fig. 12(b).
Fig. 12. (a) Influence of the optimized fiber sensor on the loss spectrum with different magnetic field from 50 Oe to 130 Oe; (b) the fitted results of the SPR resonant wavelength of the y-polarized core mode with different magnetic field; (c) influence of the optimized fiber sensor on the loss spectrum with different temperature from 24.3 °C to 104.3 °C; (d) the fitted results of the SPR resonant wavelength of the y-polarized core mode with different temperature.
According to Eq. (7) we can find that the refractive index of MF is affected by temperature, so we investigate the temperature response characteristic of this fiber sensor from 24.3 °C to 104.3 °C. As shown in Fig. 12(c), the change of the SPR resonant wavelength does not shift significantly with changed temperature compared to the magnetic field. It has a temperature sensitivity of 100 pm/°C as shown in Fig. 12(d). Notably, according to the previous analysis, the fiber sensor in a long wave is more sensitive. While the temperature causes SPR peak blue shift and the magnetic field causes SPR peak red shift, so the sensor based on SPR can suppress the effect of MF by temperature to some extent. The temperature sensitivity of the fiber sensor is also very low compared to the magnetic field sensitivity, which is expected to further reduce its temperature sensitivity by designing a thermal barrier in practical application or cascading FBG for temperature compensation. Therefore, our optical fiber magnetic field sensor has a strong resistance to temperature interference.
In addition, another important parameter is the resolution which reflects the ability to detect minute variations of the analyte refractive indices. The refractive index resolution is calculated by the following formula [59]:
where, $\Delta {\lambda _{peak}}$ is the distance of the SPR resonant wavelength shift, $\Delta {n_a}$ is the change value of the refractive index of the special hole and $\Delta {\lambda _{min}}$ represents the minimum spectral resolution. In Eq. (9), when minute variations of the analyte refractive indices become minute variations of the magnetic field, the ability of fiber sensor to detect minute variations of the magnetic field can be evaluated. We set $\Delta {\lambda _{min}}$ in Eq. (9) to 1 pm, and the remaining factor is the value related to the sensitivity of the fiber sensor. Taking the magnetic field sensitivity of the designed fiber sensor into the Eq. (9), we can get the magnetic field detection resolution of 46 nT. It can be found that our fiber sensor can respond to nT magnitude magnetic field changes to some extent.5. Discussion
We summarize the performance of other optical fiber magnetic field sensors based on magneto-refractive effect. As shown in Table 2, according to published reports, the magnetic field sensitivity of our fiber sensor is two orders of magnitude higher than most of previous related works. Because of its ability to respond to nT magnitude magnetic field, it not only meets the needs of geomagnetic sensing applications such as earth navigation, geological exploration and earthquake early warning, but also has the potential to be applied to environmental magnetic anomalies and medical diagnosis. This fiber sensor greatly expands the application range for optical fiber magnetic field sensor based on magneto-refractive effect.
Table 2. Comparison of property of optical fiber magnetic field sensor reported in the other literatures.
6. Conclusion
In conclusion, we designed a novel D-shaped PCF-SPR sensor based on magneto-refractive effect, and its magnetic field sensing characteristics are investigated by finite element method. We focus on the variations of the y-polarized core mode and the SPP mode of this fiber sensor, and discuss the effects of the structure parameters Lm, Λ, D, d2 on the sensing characteristics of the fiber sensor. By analyzing the dispersion curves and energy of the SPP mode and the core mode, we obtain a general rule for the optimization of PCF-SPR sensors. After optimizing the structural parameters, the refractive index sensitivity and the magnetic field sensitivity of our fiber sensor can reach 59714.3 nm/RIU and 21750 pm/mT, respectively. Compared to the results in existing published reports, the magnetic field sensitivity of our optical fiber magnetic field sensor is two orders of magnitude higher. In addition, the temperature sensitivity of our fiber sensor is only 100 pm/°C, which means that the crosstalk of temperature is small enough. Our fiber sensor is not only expected to meet the needs of geomagnetic sensing applications, but also has the potential to be applied to environmental magnetic anomalies and medical diagnosis because of its nT magnitude magnetic field detection resolution. It is worth mentioning that the ultra-high sensitivity and the special structure of the fiber make it the great potential for biomedical sensing applications, such as the detection of biomolecules like glucose, blood proteins, and igG antibodies using its microfluidic channel.
Funding
National Natural Science Foundation of China (Grant Nos. 61661004, Grant Nos. 61775238, Grant Nos. 61805278); Project of State Key Laboratory of Transducer Technology of China (No. SKT2001).
Acknowledgments
The authors would like to thank the support of the laboratory and university.
Disclosures
The authors declare no conflicts 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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