A dual-parameter sensor based on a photonic crystal fiber (PCF) concatenated with a fiber Bragg grating (FBG) is proposed and experimentally demonstrated for simultaneous measurement of magnetic field and temperature. Novel magnetic fluids (MF) with different concentration and surfactant are filled in the air holes of PCF. The magnetic field measurement property is only determined by PCF, while the temperature is co-determined by PCF and FBG. Experimental results show that the wavelength shift has a good linearity corresponding with temperature and magnetic field. Temperature and magnetic field sensitivity are proportional to concentration of MF and are affected by different surfactants. For PCF point, when polyethylene glycol is used as a surfactant and the magnetic fluid concentration is equal to 0.15, the highest magnetic field sensitivity is up to 924.63 pm/mT. The proposed sensor has a high sensitivity as well as cross-sensitivity resistance, which provides a promising candidate for dual-channel filtering or multi-parameter measurement applications.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
In recent years, photonic crystal fiber (PCF) has drawn increasing attention due to its high birefringence, unique spectral transmission band and highly flexible design. PCF is composed by solid substrate and air holes. Considering the air holes arranged in PCF, the air-hole filling structures are proposed and grows rapidly . Filling the tunable different sensitive materials in the air hole, the properties of the PCF can be modulated by changing the external parameters [2–4]. Especially, the liquid-filled photonic crystal fiber sensor has excellent sensing capability in temperature [5,6], strain , magnetic field  and refractive index  measurement due to its simple structure, small package size, high sensitivity and large wavelength tuning range [10,11]. Among them, the magnetic field is an external parameter that can not be ignored in practical applications. The fiber-optic magnetic field sensor is widely used in various complex magnetic field environments due to its strong anti-electromagnetic interference capability and high detection sensitivity.
Magnetic fluid (MF) is a nano-scale material whose optical properties change with the applied field. It has both the adjustable properties of solid magnetic properties and the flow characteristics of liquids. With the advent of MF, several applications of magneto-optical fiber device have been developed and demonstrated including modulators , optical switches , tunable slow light devices , and optical capacitors . Due to the easy manipulation of MF and the highly design flexiblility of PCF, magnetic field sensors have been proposed by filling MF into the air holed of PCF. By utilizing the tunable RI property of MF, a number of optical fiber magnetic field sensors have been extensively investigated based on Mach-Zehnder interferometer , Fabry-Perot interferometer , Michelson interferometer , multimode interferometers [12,13], an etched fiber Bragg grating , a tilted FBG , and a long period fiber grating . R. Gao et al. presented and experimentally demonstrated a method for measurement of a magnetic field by combining photonic crystal fibers (PCFs) and magnetic fluid. The experimental results show that a resolution of up to 0.09 Oe is achieved . Harneet V. et al. report a magnetic field sensor having advantages of both photonic crystal fiber and optofluidics, combining them on a single platform by infiltrating small amount of Fe3O4 magnetic optofluid in cladding holes of polarization-maintaining photonic crystal fiber. Magnetic field of few mT can be easily and very well detected with higher sensitivity of 242 pm/mT . Meanwhile, temperature-induced cross-sensitivity is an important factor that affects precise magnetic field detection. In order to over this problem, temperature-insensitive magnetic field sensors have been widely investigated. Although these kinds of sensors can resolve the problem of temperature-induced cross sensitivity and achieve good responses to the change of magnetic field, the measurement of single parameter cannot be satisfied in some practical applications which often need to acquire simultaneous informations of multi-parameters. In the past few years, diverse fiber-optical sensors for simultaneous measuremengt of magnetic field and temperature have been deeply studied. Chunran Sun et al. propose and experimentally demonstrate a dual-parameter sensor based on no-core fiber (NCF) cascaded with elliptical-core spun fiber (ECSF) in a Sagnac loop. Experimental results show that the dip of multimode interference is sensitive to the magnetic field (713.07 pm/mT), while the elliptical-core spun fiber loop mirror has almost no response to magnetic field. For the two dips monitored in the temperature measurement, the sensitivities of proposed sensor are −34.8 pm/°C and 304.55 pm/°C, respectively . Genghua Su et al. proposed a compact dual-parameter sensor for magnetic field intensity and temperature based on the multimode interference (MMI) by monitoring different interference dips with the magnetic field sensitivities of 74.3288pm/mT and 62.0037pm/mT, respectively, and the temperature sensitivities of −246.9 pm/°C and −286.67 pm/°C, respectively . Yinping Miao et al. reported a fiber magnetic field sensor based on concatenation of an LPFG with an etched multimode fiber (MMF), which achieved the sensitivities of −24 pm/mT and 39.29 pm/°C by LPFG and the sensitivities of 287.8 pm/mT and 40.48 pm/°C by MMF . Existing fiber-optic sensors measure magnetic field by placing the sensing element in a container with magnetic fluid solution. The flow of solution in the container will cause tension at the welding point. The container is generally made of plastic or glass, which will reduce the heat transfer speed of the magnetic fluid and the sensitivity to temperature. Considering the above problems, we inject magnetic fluid solution into the air hole of the photonic crystal fiber in the experiment. It is directly fused with the single-mode optical fiber to form an interference structure. The air holes in the PCF are natural containers for magnetic fluids, which decreases the complexity of the structure and become more sensitive to external temperature. The effect of tension on the welding point can also be ruled out. Therefore, we choose the method of filling magnetic fluid in the air holes.
In this paper, a dual-parameter sensor is demonstrated based on PCF cascaded with a FBG. Six kinds of MFs are divided into two groups and then filled in the air holes of PCF. The PCF-FBG structure can be used for detecting external magnetic field and temperature, which overcomes the cross-sensitivity problem. In experiment, according to the surfactant material, the magnetic fluid (MF) is divided into two groups: polyethylene glycol and oleic acid. The PCF-FBG approach is given to compare the sensitivity of several fluids. By analysis, results show that the wavelength shift has a good linearity corresponding with temperature and magnetic field. For PCF, the highest temperature sensitivity is 162.55 pm/°C, and the highest magnetic field sensitivity is up to 924.63 pm/mT. Corresponding to PCF, FBG is insensitive to changes in the magnetic field, while the temperature sensitivity remains essentially unchanged. Therefore, the reflection spectrum of the grating can be used as a reference point for the sensor to overcome the cross-sensitive problem. Temperature and magnetic field sensitivity are proportional to concentration and are affected by different surfactants.
2. Principle and experimental device
2.1 Magnetic fluid refractive index adjustable characteristics
The magnetic fluid is a stable gel-like liquid composed of a mixture of ferromagnetic particles, surfactant and base liquid. The concentration of ferromagnetic particles, the types of surfactants and base fluids all affect its optical properties. With the increase of the applied magnetic field, the magnetic fluid will excite the phenomenon of two-phase separation, which leads to the change of the dielectric constant of the magnetic fluid. Finally the effective refractive index of the magnetic fluid also changes with magnetic field. There are many factors that affect the refractive index change of the magnetic fluid. In addition to the strength of the external magnetic field, it is also related to the characteristics of the magnetic fluid itself (such as the type or diameter of the magnetic particles) and the temperature of the liquid.
The effective refractive index of the magnetic fluid is closely related to the distribution of the magnetic particles in the magnetic fluid. Then there are several factors that affect the distribution of magnetic solid particles in the magnetic fluid: the thermal motion of the particles associated with the temperature of the liquid, the dipole interaction between the magnetic particles, and light trapping. Y.F. Chen use the Langevin function to express the relationship between refractive index and the external environment :
2.2 Principle of photonic crystal fiber MZ structure and grating
The Fig. 1 shows schematic diagram of the sensing element, which is cascaded with a Bragg grating (FBG) and liquid filled PCF. The illustration shows the cross section of the photonic crystal fiber used in the experiment. The PCF have a solid silica core with a cladding diameter of 130 µm and an air pore diameter of 3 µm. When the SMF and PCF are welded, the air holes will collapse. Interference is produced by the collapse of the air holes of the photonic crystal. The fundamental model can be analyzed on the basis of the model interference theory. When interference occurs between the core mode and the cladding mode, the interference intensity can be expressed as follows :
According to formula (3), the interference fringe distance is related to the effective refractive index difference of the core and the cladding mode. If the amount of change in the effective refractive index is Δn, the interference peak will shift. The wavelength shift can be expressed as follows :
When the light is launched into the FBG, the wavelength meeting Bragg condition will be reflected, and the other wavelengths will propagate through the FBG. The central wavelength λB given by the following equation :25]:
2.3 Experimental device
Ends of the sensing device are connected to an ASE broadband source (KOHERAS, SuperK Uersa) and a spectrum analyzer (OSA, YOKOGAWA Q6375). The light source outputs super continuum light having a wavelength ranging from 600 nm to 1800 nm, and light emitted by the broadband light source is input to the single mode fiber and transmitted in the form of fundamental mode. At the first splice point, the cladding mode in the PCF is excited. Then, the cladding mode and a portion of the fundamental mode will transmit simultaneously in the PCF. The cladding mode is transmitted along the cladding of PCF, while the fundamental mode is transmitted in the core of PCF to form an MZ structure. At the second splice point, the fundamental mode and the cladding mode will be recoupled and transmitted in the SMF in the form of interference light. The interference spectrum is observed by a single-mode fiber at the other end into a spectrometer with a resolution of 0.02 nm. The refractive index of magnetic fluid solution is changed by external temperature or magnetic field. The main material of the core is silicon dioxide, which has extremely low thermo-optic coefficient and magneto-optical coefficient. It is not sensitive to temperature and magnetic fields. Therefore, cladding mode is greatly affected, while core mode is almost unaffected. Changes in core mode can be approximately ignored. In magnetic field sensing experiment, the ambient temperature is stable at room temperature of 20 ° C. The sensing area is placed flat on the stress frame to ensure that there is no axial strain. The position of the permanent magnet is changed to obtain different magnetic field strengths. In temperature sensing experiment, the external magnetic field B = 0 mT. The sensing portion is fixed in the glass plate to ensure that there is no axial strain, then temperature changed in the temperature control box.
During the welding process, the discharge of the electrodes causes the hole in the PCF to collapse. The proper parameter setting of the fusion splicer is important in view of the fact that sag collapse affects the transmission of light and loss of light transmission. Since the diameter of the PCF is different from that of a standard single-mode fiber, only the manual mode of fiber fusion splicer can be used to adjust the aligned PCF and single mode fiber. Proper spacing should be maintained and final soldering. In order to obtain the interference spectrum of stable and high extinction ratio, we optimized the core offset between SMF and PCF, the spacing of the two fibers, the alignment center, the welding current, the welding time and other parameters. Finally, the sensor structure is obtained. The two sides are ordinary single-mode fibers, and in the middle is photonic crystal fibers (about 5 cm) filled with the magnetic fluid solution cascaded with FBG.
3. Experimental results and discussion
3.1 Preparation and advantages of new magnetic fluid materials
Magnetic fluid is a composite magnetic material with liquid fluidity. The magnetic fluid can form a colloidal system due to the presence of a surfactant, so that the magnetic particles are continuously and stably dispersed in the base liquid. The magnetic particles cause the entire magnetic fluid to exhibit magnetism under specific conditions, and the constituent material may be magnetite. In order to keep the magnetic fluid from being affected by external influences, the base liquid may be a hydrocarbon or an organic solution. The molecular structure of the surfactant is amphiphilic and can effectively prevent precipitation or aggregation of magnetic particles. Therefore, the performance of the magnetic fluid is mainly affected by the above three factors. However, magnetic fluids inevitably have agglomeration during the preparation process. Conventional magnetic liquid particles agglomerate severely and usually appear large particles, resulting in a decrease in the stability and performance of the magnetic liquid. The study of high stability magnetic fluids is currently a hot topic in this field. Here, we prepare Fe3O4 nanoparticles by co-precipitation method, then coat them with oleic acid and polyethylene glycol respectively. It is found that the particle size distribution of the magnetic particles prepared by this method is relatively uniform. Furthermore, the double-layer surfactant can effectively reduce the agglomeration of the particles and greatly improve the stability of the magnetic liquid.
The magnetic fluid material is sensitive to the external magnetic field. When the magnetic field changes, the refractive index of the material will change, which leads to the change of the effective refractive index of the mode. The reaction to the spectrometer is the movement of the peak of the interference spectrum. It should be noted that the magnetic fluid material has a high light absorption coefficient, so the magnetic fluid concentration needs to be adjusted to reduce the light absorption coefficient to achieve a good experimental effect. In this experiment, the solution filled in the air hole is made up of six kinds of magnetic fluid solutions. The basic parameters are shown in the Table 1, and the samples are shown in the Fig. 2.
The six kinds of solutions can be divided into two groups, all of which are water-based magnetic fluid solutions and Fe3O4 nanoparticles is a magnetic particle. The first group differs from the second group in the presence of different surfactants. Within the group, the samples differ in the concentration of different magnetic particles. The use of polyethylene glycol as a surfactant allows the magnetic particles to be easily dispersed in the base carrier without large agglomeration, while the lower concentration ensures that the filler has a lower absorption coefficient. The saturation magnetic field of the magnetic fluid solution is 40 mT. When the external magnetic field is less than 40 mT, the sensitivity of the refractive index with the change of the magnetic field and temperature is relatively high, thus achieving a stable measurement experiment. A refractive index sensor is manufactured by fuse a single-mode fiber with a long-period grating. Glycerin with different refractive indices are dropped on the surface of the long-period grating. The relationship between the different refractive indices and the transmission peak wavelength of the long-period grating is obtained. The curve is fitted to obtain the equation between the refractive index and the wavelength. After the glycerin is cleaned, the magnetic fluid solution used in the experiments is dropped on the surface of the long-period grating in order. The refractive index of magnetic fluid solution is obtained by substituting the measured transmission peak wavelength into above equation. Long-period gratings are extremely sensitive to changes in surface refractive index and the change in transmission peaks is very stable. This method is very suitable for measuring the refractive index of liquid. After measurement, the refractive index of the six bottles of solution are 1.36813, 1.37142, 1.37653, 1.37418, 1.37807 and 1.37842. It can be seen that the refractive index of the solution in the same group increases with the increase of the concentration of the magnetic particles. Besides, the refractive index is smaller than the core of the photonic crystal fiber (approximately 1.44), which can ensure that the transmission condition of the refractive index-guided photonic crystal fiber is not destroyed.
3.2 Magnetic field and temperature sensing experiments
Using the magnetic fluid solution described above, a small amount of the solution is taken out into the glass container. After the coating layer is completely removed, one port of the photonic crystal fiber is placed in the container, and the other port is exposed to the air. Due to the capillary phenomenon, the magnetic fluid solution is sucked into the air holes, and the adsorption speed is less affected by the solution concentration. Once fully stabilized, place it under the microscope and as shown Fig. 3. It can be seen clearly that the solution has entered the fiber. After the air holes are filled, they are respectively fused with the single-mode fiber and the grating to form an MF-PCF structure. The six groups of solutions in the table numbered 1.1 to 1.3 and 2.1 to 2.3 are sequentially filled into the air holes of the photonic crystal fiber to obtain six experimental results, which are labeled according to the sample number.
The experiment is carried out after the number 1.1 solution is filled into the photonic crystal fiber. The Fig. 4(a) shows the interference spectrum of the photonic crystal fiber and the single mode fiber at zero magnetic field under room temperature conditions (about 20 ° C). The FBG point is the reflection of the grating, and the reflection wavelength is 1544.2 nm. It can be seen that the extinction ratio of the interference spectrum is not equal at different wavelengths, and the maximum is up to 10.01 dB. Compared with other special fiber optic interference sensors, such as mode-less fiber, dual-core fiber, multi-core fiber, it is easier to obtain a large extinction ratio formed by photonic crystal fiber and single-mode fiber. Therefore, there is no need for taper and offset, which can increase the stability of the sensor. The Fig. 4(b) shows the fast Fourier transform of the interference spectrum. The purpose of Fourier transform is to observe the number of interference modes directly. The number of peaks represents the number of modes participating in the interference. The existence of the peak is not obvious mainly due to the influence of welding loss, which causes the extinction of interference spectrum is relatively low. The extinction ratio determines the height of the peak. It can be seen from the figure that the core mode and multiple cladding modes are involved in the interference. There are two relatively obvious peaks, which corresponding to the two troughs at points FBG and MF-PCF in the spectrogram. Next, we select the FBG point and PCF point in the Fig. 4(a) as the measurement wavelength and study the sensing characteristics of the two points for the sensitivity of temperature and magnetic field.
When the magnetic field changes, the temperature is set to 20 °C. The sensing element is placed in a temperature-controlled box to keep the temperature constant. As can be seen in Figs. 5(a) and 5(b), as the magnetic field increases from 0 mT to 10 mT, the PCF point shifts 3.51 nm to long wavelength. It cause a red shift. In FBG point, the interference pattern keeps almost unchanged and the measured transmission dips under different temperatures are almost overlapped when the magnetic field changes. The ultra-low magnetic characteristic of grating is attributed to magnetic fluid insensitive. Therefore, as the magnetic field increases, the reflection peak of the grating hardly moves. Taking 0 mT as the reference point, the change of the two wavelength shifts with the magnetic field is obtained as shown in the Fig. 5(c). The magnetic field sensitivity of the PCF point is 348.51 pm/mT and the linearity is 0.990. The magnetic field sensitivity of the FBG point is 0.104 pm/mT and linearity is 0.999. The magnetic field sensitivity of the grating is almost negligible relative to a photonic crystal fiber filled with a magnetic fluid. In temperature experiment, as shown in the Figs. 5(d) and 5(e), when T changes from 20 to 70 °C, both FBG and PCF occur red shift with increasing temperature. Due to different sensitivity, the PCF point shifts 3.54 nm to long wavelength, while FBG point moves 0.541 nm to long wavelength. Taking 20 °C as the reference point, the change of the drift of the two wavelength points with temperature is shown in the Fig. 5(f). The temperature sensitivity of the PCF point is 68.23 pm/°C and the linearity is 0.990. The temperature sensitivity of the FBG point is 10.82. pm/°C and linearity is 0.997.
Under the same experimental conditions, only solution 1.1 is changed to 1.2 and 1.3. The magnetic field and temperature experiments are carried out respectively to compare the relationship between different concentration in same group. In order to ensure a single variable, the welding parameters, filling time and other data are consistent. The experimental results are shown in the Fig. 6. When the filling liquid is 1.2, the FBG point do not move in the magnetic field experiment, while the PCF point shift 6.16 nm to the long wavelength. In the temperature experiment, the FBG point shift to a long wavelength of 0.496 nm, while the PCF shift to a long wavelength of 4.94 nm. For linear fitting of the data in Figs. 6(c) and 6(f), the magnetic field sensitivity of the PCF point is 617.26 pm/mT and the magnetic field sensitivity of the FBG point is 0.116 pm/mT. The temperature sensitivity at the PCF point is 99.96 pm/°C, and the temperature sensitivity at the FBG point is 9.93 pm/°C. When the filling liquid is 1.3, the magnetic field sensitivity of the PCF point is 924.63 pm/mT, and the magnetic field sensitivity of the FBG point is 0.103 pm/mT. The temperature sensitivity of the PCF point is 123.07 pm /°C, and the temperature sensitivity of the FBG point is 9.97 pm /°C. The comparison of the three solutions is shown in Fig. 9(a).
Next, experiments are conducted on other three magnetic fluid solutions. The solution 2.1 is filled into the air hole of the photonic crystal fiber, and the FBG is fused to obtain the sensing element. The experimental conditions described above are maintained to measure the effect of the magnetic field on the interference spectrum. When the magnetic field B changes from 0 mT to 10 mT, the sensor interference spectrum of PCF point and FBG point are shown in Figs. 7(a) and 7(b). Similar to number 1.1 experimental results, the reflection peak of the grating hardly moved, while the PCF shifted to a long wavelength of 2.94 nm. It causing a red shift. The variation of the two wavelength shifts with the magnetic field is shown in the Fig. 7(c). The magnetic field sensitivity of the PCF point is 295.29 pm/mT and the linearity is 0.996. The magnetic field sensitivity of the FBG point is 0.121 pm/mT and the linearity is 0.999. The experiment of temperature sensing is shown in the Figs. 7(d) and 7(e). When the external magnetic field B is 0 mT, the temperature is changed in the temperature control box. When T changes from 20 to 70 °C, as the temperature increases, both FBG and PCF points are red-shifted. The FBG point shifts to a long wavelength of 0.488 nm and the PCF point shifts to a long wavelength of 7.39 nm. The change of the drift of the two wavelength points with temperature is shown in the Fig. 7(f). The temperature sensitivity of the PCF point is 145.59 pm/°C and the linearity is 0.985. The temperature sensitivity of the FBG point is 9.76 pm/°C and the linearity is 0.999.
After changing the solution 2.1 to 2.2 and 2.3, the magnetic field and temperature experiments are carried out respectively to compare the relationship between the concentration change. In order to ensure a single variable, the welding parameters, filling time and other data are consistent. The experimental results are shown in the Fig. 8. When the filling liquid is 2.2, the FBG point did not move in the magnetic field experiment, and the PCF point shifted to the long wavelength by 4.15 nm. In the temperature experiment, the FBG point shifted to a long wavelength of 0.50 nm, and the PCF point shifted to a long wavelength of 7.77 nm. Linear fitting of the data yielded a magnetic field sensitivity of 428.28 pm/mT at the PCF point and a magnetic field sensitivity of 0.117 pm/mT at the FBG point. The temperature sensitivity of the PCF point is 154.58 pm/°C, and the temperature sensitivity of the FBG point is 10.05 pm/°C. When the filling liquid is 2.3, the magnetic field sensitivity of the PCF point is 541.85 pm/mT, and the magnetic field sensitivity of the FBG point is 0.113 pm/mT. The temperature sensitivity of the PCF point is 162.55 pm/°C, and the temperature sensitivity of the FBG point is 10.03 pm/°C. The comparison of the three solutions is shown in Fig. 9(b).
The Table 2 summarizes the relationship between the magnetic solution and the PCF point sensitivity. Figure 9 shows the magnetic field sensitivity comparison of the six solutions. It can be seen that for the solution in the same group, the PCF magnetic field and temperature sensitivity rise with the increase of the magnetic particle concentration, while the magnetic field sensitivity changes more obviously. The changes of the magnetic field sensitivity in first group is significantly higher than that of the second group. This is because surfactants is polyethylene glycol in first group and surfactants is oleic acid in second group. Polyethylene glycol can make magnetic particles easy to disperse in base liquid so that there is no large agglomeration. When the magnetic field changes, the diffusion of the magnetic particles is much stronger than that of oleic acid. Therefore, as the concentration of the magnetic particles increases, the sensitivity of the magnetic field changes greatly. It is also obtain that oleic acid as a surfactant, the temperature sensitivity change is low at different concentrations. It can be used in an environment where temperature sensitivity is required to be stable. The results obtained by multiple experiments are similar, indicating that the sensor has good stability.
According to the above analysis, the drift of the PCF interference spectrum and the FBG reflection spectrum has a good linear relationship with the magnetic field and temperature. The demodulation of the two-parameter matrix algorithm is carried out using 1.3 sets of solutions with the highest magnetic field sensitivity. The PCF magnetic field sensitivity is 924.63 pm/mT and the temperature sensitivity is 123.07 pm/°C. The FBG magnetic field sensitivity can be approximated to 0, and the temperature sensitivity is 9.97 pm/°C. When magnetic field and temperature are simultaneously applied to the sensor, a matrix can be used to represent the offset of different wavelengths:
3.3 Analysis for the measurement error
When the fiber-optic sensor measures composite parameters, the measurement error is caused by the sensitivity coefficient matrix error and the characteristic parameter error. The error of the sensitivity coefficient matrix is mainly caused by the sensitivity measurement in the experiment, and it changes with the value of the parameter to be measured. In practical applications, we can try to reduce the sensitivity coefficient matrix error as much as possible through repeated experiments. Therefore, the measurement error of the composite parameter sensor generally refers to the characteristic parameter error, which can be expressed as:
A fiber-optic sensor capable of simultaneously measuring the magnetic field and temperature is proposed. Six kinds of magnetic fluid solution with different surfactants and different magnetic particle concentrations is filled into the photonic crystal fiber and fused with the Bragg grating to form a sensing structure. Magnetic field and temperature experiment is performed six times. The experimental results show that the sensitivity of sensor increases with the increase of the concentration of magnetic fluid solution. For PCF point, the highest temperature sensitivity is 162.55 pm/°C, and the highest magnetic field sensitivity is up to 924.63 pm/mT. The effect of different surfactants on the increment of sensitivity is different. The sensor has a good magnetic field and temperature response, simple structure and high sensitivity. At the same time, the dual-parameter demodulation matrix is used to effectively overcome the crosstalk problem.
National Natural Science Foundation of China (61525501, 61827817); Fundamental Research Funds for the Central Universities (2018YJS006).
The authors declare no conflicts of interest.
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