Highly sensitive magnetic field sensor based on

Highly sensitive magnetic field sensor based on microfiber coupler with magnetic fluid Longfeng Luo, Shengli Pu, Jiali Tang, Xianglong Zeng, and Mahieddine Lahoubi Citation: Applied Physics Letters 106, 193507 (2015); doi: 10.1063/1.4921267 View online: http://dx.doi.org/10.1063/1.4921267 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/106/19?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Microelectromechanical magnetic field sensor based on ΔE effect Appl. Phys. Lett. 105, 052414 (2014); 10.1063/1.4891540 Low temperature sensitive intensity-interrogated magnetic field sensor based on modal interference in thin-core fiber and magnetic fluid Appl. Phys. Lett. 104, 252402 (2014); 10.1063/1.4884896 Magnetic field sensing based on tilted fiber Bragg grating coated with nanoparticle magnetic fluid Appl. Phys. Lett. 104, 061903 (2014); 10.1063/1.4864649 Intensity-modulated magnetic field sensor based on magnetic fluid and optical fiber gratings Appl. Phys. Lett. 103, 183511 (2013); 10.1063/1.4828562 Fiber-optic in-line magnetic field sensor based on the magnetic fluid and multimode interference effects Appl. Phys. Lett. 103, 151101 (2013); 10.1063/1.4824470

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APPLIED PHYSICS LETTERS 106, 193507 (2015)

Highly sensitive magnetic field sensor based on microfiber coupler with magnetic fluid Longfeng Luo,1 Shengli Pu,1,a) Jiali Tang,1 Xianglong Zeng,2 and Mahieddine Lahoubi3 1

College of Science, University of Shanghai for Science and Technology, Shanghai 200093, China 2Key Laboratory of Specialty Fiber Optics and Optical Access Network, Shanghai University, Shanghai 200072, China 3 Department of Physics, Faculty of Sciences, Laboratory L.P.S., Badji Mokhtar-Annaba University, P. O. Box 12, 23000 Annaba, Algeria 2

(Received 13 April 2015; accepted 6 May 2015; published online 14 May 2015) A kind of magnetic field sensor using a microfiber coupler (MFC) surrounded with magnetic fluid (MF) is proposed and experimentally demonstrated. As the MFC is strongly sensitive to the surrounding refractive index (RI) and MF’s RI is sensitive to magnetic field, the magnetic field sensing function of the proposed structure is realized. Interrogation of magnetic field strength is achieved by measuring the dip wavelength shift and transmission loss change of the transmission spectrum. The experimental results show that the sensitivity of the sensor is wavelength-dependent. The maximum sensitivity of 191.8 pm/Oe is achieved at wavelength of around 1537 nm in this work. In addition, a sensitivity of 0.037 dB/Oe is achieved by monitoring variation of the fringe visibility. These suggest the potential applications of the proposed structure in tunable all-in-fiber photonic C 2015 AIP Publishing LLC. devices such as magneto-optical modulator, filter, and sensing. V [http://dx.doi.org/10.1063/1.4921267]

Optical microfiber has received a lot of attention owing to its unique geometry with low dimension, large evanescent field, strong light confinement, low loss, and robustness, since the pioneering work in 2003.1 Hitherto, many optical devices based on microfiber have been investigated, such as knot resonator,2 coil resonator,3 and microfiber refractometer.4 The outstanding properties make microfiber-based photonic devices promising to design sensors with high sensitivity, compact size, low cost, and fast response. Recently, the microfiber coupler (MFC) made from conventional singlemode fibers was proposed. MFC-based refractive index (RI) sensor,5 thermometers,6 micro-force sensor,7 and broadband 3 dB coupler8 have been demonstrated. In 2013, Bo et al. reported a highly sensitive refractometric sensor based on MFC with a maximum sensitivity circa 4155 nm/RIU within the RI range of 1.3340 to 1.3515.9 Magnetic fluid (MF) is a kind of stable colloidal system consisting of surfactant-coated magnetic nanoparticles (usually 3–15 nm in diameter) dispersed in a suitable liquid carrier.10 It possesses both the features of magnetic property of solid magnetic materials and fluidity of liquids. MF presents versatile magneto-optical properties including Faraday effect, tunable RI, field dependent transmission, and birefringence.11–14 Thus, many unique optical devices based on MF have been designed.15–18 Comparing with the traditional devices, MF-based corresponding optical devices possess the advantages of high sensitivity and small size.19 Recently, various optical fiber structures with MF have been proposed for magnetic field sensing. The employed structures include long period grating,20 fiber Bragg grating,21 multimode interference,22 Michelson interferometer,23 S-tapered fiber,24 Sagnac interferometer,25 microfiber a)

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0003-6951/2015/106(19)/193507/5/$30.00

mode interferometer,26 and photonic crystal fiber (PCF)based interferometers.27,28 However, fiber grating-based sensors usually have a low RI sensitivity. Higher sensitivity can only be achieved with complicated etching process. Though PCF-based sensor is promising, the cost is relatively high. In addition, infiltrating PCF with MF and fusion splicing the MF-filled PCF is not easy. Among various sensors, microfiber-based sensor has the distinctive advantage of easy fabrication, low cost, and high sensitivity.29 In this work, a kind of magnetic field sensor based on a MFC and MF is proposed and experimentally demonstrated. The MFC is extremely sensitive to surrounding RI due to the large evanescent field outside the fiber surface at the fused region. Since the RI of MF is sensitive to the magnetic field, the MFC surrounded with MF can be used as a magnetic field sensor. Furthermore, it also implies the potential applications of the proposed structure to optical power splitter, optical filter, and other optical devices.

FIG. 1. (a) Schematic of the proposed MFC and (b) and (c) microscope images of coupling region and cross-section of the MFC cutting at the uniform waist region.

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Fig. 1(a) shows the schematic of the proposed MFC. Using the flame brushing method,19 the MFC is manufactured by laterally fusing and tapering two twisted single mode optical fibers. The MFC comprises two conical transition regions, a central uniform waist region, and four input/output ports. Light is injected into port P1 or P2 and exits from port P3 and P4. Fig. 1(b) illustrates the microscope image of waist region of the as-fabricated MFC. Fig. 1(c) illustrates the microscope image of the cross-section of the MFC cutting at the uniform waist region. The diameter of each coupled microfiber is 1.8 lm. The length of the central uniform waist region is 10 mm. The shape of cross-section shown in Fig. 1(c) indicates that the microfibers are well fused together. Because of the strong fusion of microfibers, the MFC cannot be assumed as a weakly fused coupler. The strongly fused coupler theory must be employed.30,31 The interference between the lowest-order even and odd modes at the coupling region is considered. If light with power of P0 enters the input port P1, the power at the output port P4 can be described by P ¼ P0 sin2 ðCLÞ;

(1)

where L is the coupling length of the MFC and C is the coupling coefficient of the whole coupling region. The explicit expression for C is given as30 C¼

3pk 1 ; 2 32n1 a ð1 þ 1=V Þ2

(2)

where V ¼ ½ð2paÞ=kðn21 n20 Þ1=2 and k is the wavelength of incident light, a is the diameter of one of the microfibers, and n1 and n0 refer to the RIs of silica and environment, respectively. Because of the large modal size and relatively small overlap, the contribution of the transition regions is negligible. Equations (1) and (2) exposit that the output power depends on the environmental RI n0 , coupling length L, and coupler radial size 2a. When the environmental RI changes, P changes as well. This will lead to the change of output spectrum (i.e., spectral pattern and transmission loss). Considering the magnetically tunable RI and absorption of MF, the valley wavelength and transmission loss of the output spectrum may be sensitive to the external magnetic field when using MF as the cladding of the MFC structure. Thus, magnetic field measurement can be realized by detecting wavelength shift and transmission loss of the output spectrum. The schematic configuration of the proposed sensor is shown in Fig. 2. A MFC structure surrounded by MF is

FIG. 2. Schematic of the MFC-based magnetic field sensor with MF as the surrounding.

totally sealed in a capillary tube. Both ends of the capillary are sealed with UV glue to avoid MF leaking or evaporating. In our experiments, the water-based MF with density of 1.18 g/cm3 at 25 C and saturation magnetization of 200 Oe provided by Beijing Sunrise Ferrofluid Technological Co., Ltd. is employed. The diameter of the magnetic nanoparticles is around 10 nm. The MF is diluted with distilled water. The volume ratio of water-based MF to distilled water is 1:10. The experimental measurement setup is shown in Fig. 3. Light from a supercontinuum broadband source (SBS, Wuhan Yangtze Soton Laser Co., Ltd.) is launched into P1 and the output light is collected from P4 using an optical spectrum analyzer (AQ6370C). The sensing structure is placed between two poles of an electromagnet, which generates a uniform magnetic field with nonuniformity of less than 0.1% within the sample region. The strength of magnetic field is adjusted by tuning the magnitude of the supply current. The magnetic field direction is perpendicular to the optical fiber axis. A Tesla meter is used to measure the strength of the magnetic field. During our experiments, the ambient temperature is kept at 18 C. The transmission spectra of the MFC before and after being immersed into MF without external magnetic field are presented in Fig. 4. A distinct decrease in transmission can be observed in the spectrum. This is attributed to fractional mode power extending outside of the microfiber and the absorptive properties of MF. Besides, even lower transmission around 1450 nm is noticed, which is due to the strong absorption of aqueous liquid.32 In addition, the dip wavelength spacing is larger in MF compared to that in air. Equations (1) and (2) indicate that the output power P depends on k periodically. The corresponding period of output power variation with respect to k, i.e., dip wavelength spacing Dk can be given by30 Dk ¼

32n1 a2 ð1 þ 1=V Þ3 : ð1 1=V Þ 3L

(3)

Since V is dependent of n0 , the sensitivity of Dk to n0 , i.e., @ðDkÞ=@n0 can be derived as ! @ ðDkÞ 128n1 n0 a4 p2 3ð1 þ 1=V Þ2 ð1 þ 1=V Þ3 þ ¼ : (4) ð1 1=V Þ @n0 3Lk2 V 3 ð1 1=V Þ2 The value of V for the coupler fabricated in our work is larger than one, so @ðDkÞ=@n0 > 0. This means the dip

FIG. 3. Schematic of the experimental measurement setup.


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FIG. 4. Transmission spectra of the MFC immersed in air and MF, respectively.

wavelength spacing Dk will increase with the environmental RI n0 increase. As the RI of MF is larger than that of air, the observed experimental result for the dip wavelength spacing variation can be well explained. The transmission spectra of the as-fabricated structure at different magnetic field strengths are presented in Fig. 5. In view of the period-like feature of the transmission spectra (see Fig. 4), Fig. 5 only shows the typical transmission spectra for wavelength ranging from 1200 to 1300 nm for clarity. From Fig. 5, the red shift of wavelength and decrease of peak intensity with magnetic field are observed. The wavelength shift of different dips versus the magnetic field strength is explicitly plotted in Fig. 6. It is clear from Fig. 6 that all dips shift towards longer wavelength as magnetic field strength increases. The response of dip wavelength shift to H is generally nonlinear. At relatively low magnetic field, the dip wavelength variation with magnetic field is slight and unobvious. Nevertheless, at relatively high field regime, the response of dip wavelength to magnetic field tends to saturate. The measured results shown in Figs. 5 and 6 can be explained by the tunable RI and evanescent field absorption of MF, which are assigned to the agglomeration of magnetic particles under externally applied magnetic field.22 Without applying magnetic field, the nanoparticles in MF are

FIG. 5. Transmission spectral responses to the magnetic field strength.

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FIG. 6. Dip wavelength shift as a function of magnetic field strength.

dispersed homogeneously. As external magnetic field is applied beyond certain critical value, the nanoparticles start to agglomerate and the chain-like clusters are formed. In the meantime, phase separation (columns and liquid) occurs, which leads to the change of dielectric constant (i.e., RI) of MF. The change of MF’s RI results in the variation of coupling coefficients of MFC, which will cause the wavelength shift on the transmission spectrum. The transmission intensity attenuation is mainly due to the magnetic-field-dependent evanescent field absorption of MF. According to Eqs. (1) and (2), the dip wavelengths occur at CL ¼ mp, where m is an integer. For the as-fabricated MFC, it is obvious that CL (defining CL ¼ U) is a function of k and n0 . Therefore, the sensitivity of dip wavelength shift to the environmental RI change (dk=dn0 ) can be derived as follows: @Uðk; n0 Þ @Uðk; n0 Þ dk þ dn0 ¼ 0: @k @n0

(5)

So, dk=dn0 is given as dk @Uðk; n0 Þ=@n0 8p2 a2 n0 1 ¼ 2 ¼ : @Uðk; n0 Þ=@k k dn0 V ðV 1Þ

(6)

The value of V for the coupler fabricated in our work is larger than one, so dk=dn0 > 0. When the externally magnetic field increases, the environment RI n0 will increase as well. As a result, the dip wavelength will exhibit a red shift with magnetic field as shown in Figs. 5 and 6. The nonlinear dependence in Fig. 6 can be explained by the magnetic-field-dependent RI of MF satisfying Langevinlike function.24 The dip wavelengths change slowly as the magnetic field intensity is below 100 Oe. This is due to the initial magnetization of MF. The shift of dip wavelength gradually becomes constant for magnetic field strength beyond 275 Oe, which is due to the saturated magnetization of MF. At this stage, all of the magnetic nanoparticles within MF have already agglomerated to form magnetic clusters or columns, so the RI of MF will not markedly change with magnetic field further. When the magnetic field strength lies between 100 and 275 Oe, linear fitting is applied to the experimental data as shown in Fig. 6. The sensitivities of 58.7 pm/Oe, 74.9 pm/


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FIG. 7. Sensitivity as a function of dip wavelength.

Oe, 96 pm/Oe, 114.4 pm/Oe, 141.5 pm/Oe, 170.1 pm/Oe, and 191.8 pm/Oe are achieved for dip wavelength around 1000 nm, 1110 nm, 1201 nm, 1305 nm, 1396 nm, 1494 nm, and 1537 nm, respectively. Among these fittings, the R2 values all exceed 0.99, which show good linearity. Though the low concentration MF is utilized in our experiments, the obtained sensitivity of the proposed sensing structure is 10 times greater than that of long period grating-based structure (18.3 pm/Oe),20 twice larger than that of the structure based on multimode interference (90.5 pm/Oe),22 and 80 times higher than that using MF as the cladding of PCF (2.4 pm/ Oe).27 It is worth noting that the sensitivity of the proposed MFC sensor can be further enhanced by optimizing the RI of MF, decreasing taper diameter, and increasing taper length. Moreover, Fig. 6 implies that the sensitivity of wavelength shift increases with dip wavelength monotonously. The corresponding results are replotted in Fig. 7. Fig. 7 shows an almost linear dependence of the sensitivity on the dip wavelength. Thus, a linear fitting is applied to the sensitivity variation with respect to the dip wavelength. The slope of 0.2439 (pm/Oe)/nm with R2 value of 0.9721 is achieved. As the applied magnetic field increases from 0 to 400 Oe, a portion of light is attenuated because of the extinction of evanescent field. So the contrast ratio of the interference fringes decreases. We define the fringe visibility as

FIG. 8. Fringe visibility as a function of magnetic field.

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VF ¼ ðTpeakL þ TpeakR Þ=2 Tdip to assess the variation of contrast ratio of the interference fringes, where Tdip is transmission power of certain dip and TpeakL and TpeakR are transmission powers of the adjacent left and right peaks of the selected dip, respectively. For example, we choose a dip at around 1287 nm with an initial VF of 14 dB. The variation of VF with magnetic field is shown in Fig. 8. The response of VF to H is generally nonlinear, which is very similar to the dip wavelength shift with magnetic field (see Fig. 6). This is assigned to the same physical process dominating the wavelength shift and transmission loss. But when magnetic field strength lies between 50 and 275 Oe, the response has a fairly good linearity. The corresponding sensitivity is obtained to be 0.037 dB/Oe with R2 value of 0.9902. In conclusion, a kind of magnetic field sensor based on MF-clad MFC is proposed and experimentally demonstrated. The results show that both of the transmission loss and dip wavelength are sensitive to the magnetic field. The sensitivity of the wavelength shift is wavelength-dependent and a highest magnetic field sensing sensitivity of 191.8 pm/Oe has been achieved at around 1537 nm in our experiments. By monitoring the fringe visibility variation, a sensitivity of 0.037 dB/Oe is achieved. Considering the compact size, low cost, and high sensitivity of the proposed structure, the proposed MFC structure is also competent for tunable all-infiber photonic devices, such as filters and all-optical switching. This research was supported by the Shanghai Natural Science Fund (Grant No. 13ZR1427400), Shanghai Key Laboratory of Specialty Fiber Optics and Optical Access Networks (Grant No. SKLSFO2014-05), and the Hujiang Foundation of China (Grant No. B14004). 1

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