A Novel Optical Fiber Magnetic Sensor Based
on Electroforming Long-Period Fiber Grating
Chia-Chin Chiang and Zheng-Jie Chen
Abstract—In this study, we present a novel optical fiber magnetic sensor based on electroforming long-period optical fiber grating (ELPFG) with a periodic polymer-metal (SU-8 photoresist and nickel) structure that is fabricated via a LIGA-like process. We measure the magnetic field through the nickel grating on the opti-cal fiber, and an ELPFG magnetic sensor opti-calibration experiment is also described. The coil produces a magnetic field to modulate the ELPFG, and we use an optical spectrum analyzer to analyze the resonant wavelength and changes in transmission loss. It is dis-covered from the experimental results that the optimal sensitivity of the ELPFG is −0.465 dB/mT, the linearity is 0.965, and the transmission loss will gradually increase following magnetic field enhancement. We infer that this phenomenon is due to the magne-tostriction materials of nickel, which cause the nickel structure to impose a slight stretching strain on the ELPFG with the applied magnetic field.
Index Terms—Electroforming long-period fiber grating, mag-netic sensor, optical fiber sensor.
I. INTRODUCTION
L
ONG period fiber grating (LPFG) has long been an im-portant passive optical component in fiber communication and sensor systems [1]. Its period lasts between 100 μm and 1 mm, and it promotes the coupling between propagating core and cladding modes. When a broadband light source is emitted at LPFG, a certain portion of the light will be coupled from the core mode to the cladding mode according to the phase-matching condition, while the resonant wavelengths within the spectrum will be attenuated, which may result in a dip in the transmitted spectrum. The attenuation loss band of LPFG is quite sensitive to the influence of external environmental fac-tors and conditions, which makes LPFG suitable for use as a sensor [2]. In fact, due to the high sensitivity of LPFG, as well as various advantages it provides in terms of operation in harsh environments, multi-point measurements, and immediate response, it is often used for the measurement of physical quan-tities such as strain [3], temperature [4], curve radius [5], and refractive index [6]–[8].There are several LPFG fabrication processes, such as ex-cimer laser writing [9], CO2 laser writing [10]–[13], and arc
discharged fabrication [14]. However, each of these methods Manuscript received June 11, 2014; revised July 17, 2014; accepted July 25, 2014. Date of publication August 5, 2014; date of current version August 27, 2014. This work was supported by the National Science Council, Taiwan, under Grants MOST-103-2221-E-151-009-MY3 and NSC-103-2623-E-151-002-D.
The authors are with the Department of Mechanical Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 807, Taiwan (e-mail: [email protected]).
Color versions of one or more of the figures in this paper are available at htpp://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/JLT.2014.2343979
has certain disadvantages. The conventional method for manu-facturing LPFG involves the application of UV excimer laser [9] to photosensitive fiber or hydrogen-soaked fiber to ob-tain the required variation in the periodic refractive indices in the core. However, this method is complicated and expensive. LPFG can also be made by CO2 laser machining or arc
dis-charge. These two production methods, however, involve “point by point” scribing processes, thus limiting the production rate. The studies [9]–[14] cited above applied different functions and principles to achieve the characteristics of LPFG. However, no studies thus far have reported direct fabrication of LPFG by sandwiching the optical fiber in periodic metal grating. There-fore, in this study, we propose a novel fabrication process for electroformed LPFG (ELPFG) that has the potential for mass production and compact application in all optical fiber magnetic sensing systems.
The following text discusses our survey of past optical fiber magnetic research. Previous studies of the topic have utilized the optical fiber sensor for magnetic field measurement by using magnetic fluid (MF) [15], [16], the Faraday effect [17], the polarization effect [18], and the magnetostriction effect [19].
In 2007, Liu et al. [15] used a layer of MF outside of the LPFG for magnetic sensing. They concluded that after the external magnetic field passes by the MF, the MF is likely to change its refraction. At the same time, such a refraction index could grad-ually diminish following the expansion of the external magnetic field. In 2012, Gao et al. [16] reported a highly sensitive mag-netic sensor utilizing D-shaped LPFG immersed in MF. They proposed that the change in depth of the attenuation dip under different magnetic field intensities is approximately sinusoidal with the applied magnetic field. Based on this phenomenon, we designed and developed the ELPFG magnetic sensor proposed in this paper.
In 2008, studies by Peng et al. [18] and Su et al. [20] proposed optical fiber sensors for magnetic field measure-ment using the polarization effect and the Faraday effect of fiber grating, respectively. The proposed sensors are interro-gated by polarization dependent loss [18] and different group delay (DGD) [20].
In 2009, Konstantaki et al. [21] proposed using MFs to demodulate magnetic field changes on LPFG. In that study, spectral tuning of optical fiber long period gratings was demon-strated by utilizing ferrofluids as outcladdings with an applied magnetic field. The spectra of the sensors are changed with the spatial magneto-displacement and through the magneto-optical refractive effect. Therefore, the transmission loss of the LPFG is tuned with the refractive index of the MF through the application of various magnetic fields.
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3332 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 32, NO. 19, OCTOBER 1, 2014
In 2013, Chen et al. [22] proposed the single mode-multimode-single mode (SMS) structure optical fiber compo-nent with a layer of MF outside of the SMS sensor for magnetic sensing. The magnetic field sensor can be interrogated by mon-itoring the wavelength shift and transmission loss. In the same year, Zheng et al. [23] proposed a magnetic field sensor us-ing tilted fiber Bragg gratus-ing (TFBG) with MF for which the refractive index changes with the external magnetic field. The guiding properties of cladding modes of the TFBG are therefore modulated by the external magnetic field.
Yariv and Winsor [24] first reported in 1980 on a measurement method involving magnetic fields passing through magnetostric-tive coating on optical fibers. A magnetostricmagnetostric-tive coating is one that undergoes a longitudinal strain (magnetostriction) when immersed in a magnetic field. Therefore, a magnetostriction-induced strain is applied to the optical fibers by a magnetic field. This strain affects the phase delay of a laser light beam propagating in the fiber.
In 2010, Thomas et al. [19] reported their investigation of the magnetostrictive characteristics of metallic glass alloy based amorphous magnetic thin films by using optical fiber sensor. This report pointed out the potential applications of the mag-netic field-dependent magnetostrictive material in magmag-netic field sensor devices.
The aforementioned studies utilized the optical fiber sensor for magnetic field measurement by using MF, the magneto-optic effect, and the magnetostriction effect. These methods cannot be used in mass production, however, and the stability of optical fiber magnetic sensors that utilize MF is questionable. Moreover, no study thus far has mentioned the possibility of using optical fiber sandwiched in periodic nickel magnetostric-tion coating for magnetic field measurement. In 2014, a study by Chiang et al. [25] was the first to report on the use of an electroforming technique for the fabrication of an ELPFG tun-able filter with which the optical properties of the ELPFG with thermal treatment were investigated. Only preliminary results were presented, without details of the process or a discussion of the effect of the periodic nickel magnetostriction coating on the magnetic field measurements.
In this paper, we propose a novel and robust ELPFG mag-netic sensor with a nickel periodical structure. The magmag-netic field causes the periodical nickel structure striction, and thereby modulates the transmission loss of the ELPFG sensor in vari-ous magnetic fields. The proposed ELPFG is made of an SU-8 photoresist (PR) and nickel structure. Because of the magne-tostriction behavior of the nickel material, we can obtain the res-onance attenuation dips of the ELPFG by applying a magnetic field. Therefore, the ELPFG can be used as a loss-adjustable magnetic-control optical filter or all-fiber magnetic field sensor. II. OPERATINGPRINCIPLE OF THEELPFG MAGNETICSENSOR
LPFG consists of periodic refractive index variations with pe-riods of 100 μm 1000 μm. The LPFG promotes the coupling between propagating core modes and propagating cladding modes to provide an attenuation loss band for sensing appli-cations [2]–[8]. The proposed ELPFG is made of optical fiber
sandwiched by a periodic polymer-metal (SU-8 PR and nickel) structure. According to the magnetostriction behavior of the nickel material and the strain-optics effect [26], when an exter-nal magnetic field is applied to the ELPFG, the periodic nickel structure on the optical fiber will induce the magnetostriction strain field in the longitudinal direction of the ELPFG, and the refractive index of the ELPFG will be modulated as a square wave. Based on the strain-optics effect, the refractive indices change linearly proportional to the strain field. Hence, the re-fractive index of the ELPFG will be modulated as a periodic square wave distribution along the optical fiber. According to the phase matching condition of coupled mode theory, the wave-length of an ELPFG under phase matching conditions can be written as (1) [26], [27]
λ = Λ(neff
core− neffcladding) (1)
whereλ is the resonant wavelength, Λ is the grating period, neffcore
is the effective refractive index of the core mode, and neffcladding is the effective refractive index of the cladding mode.
The transmittance of an ELPFG can be expressed with the AC component of the coupling coefficient (Kac
co−cl) between the core and the cladding. The transmittance of an ELPFG has a cosine-squared relationship and is defined as follows [26], [27]: T = cos2(κacco−clL) (2) where L indicates the length of the ELPFG. The transmittance is a function of Kcoac−cl, which is related to the amplitude of the refractive index change, owing to the periodic variation of the magnetostriction-induced strain field. When the magnetic field is applied, the periodic nickel structure of the ELPFG will induce the periodic variation of the magnetostriction strain on the optical fiber. Therefore, Kac
co−cl will change according to the strain-optics effect. Accordingly, the transmittance can be tuned by changing the magnetic field. The sensing principle of the ELPFG magnetic sensor is based on the monitoring of the transmittance of the ELPFG modulated by the magnetic field. Hence, the induced magnetostriction strain on the ELPFG causes changes of resonant attenuation loss in the ELPFG. The present study employs this principle to analyze the character of the magnetic field sensor.
III. PRODUCTIONPROCESS ANDEXPERIMENTALSETUP A. The Fabrication Process of ELPFG Magnetic Field Sensors
The novel ELPFG adopted in this paper is fabricated by the LIGA-like MEMS process [28] and electroforming process [29] that are utilized to produce symmetrical metal sandwiched long period fiber grating. In the LIGA-like MEMS process, the wafer is pre-coated with a copper conducting layer while the copper layer plays the role of the electroforming conductivity layer as well as the role of the sensor device releasing sacrificed layer. After coating the wafer with the copper layer, the wafer surface is cleaned with acetone and isopropanol. Then the Spin Coater is used to evenly distribute PR to the surface of the wafer in order to reach the PR thickness needed. The coated wafer is then placed on the heating plate for baking. The heating process will aid in the removal and evaporation of the organic solvent in
Fig. 1. Flowchart diagram of the ELPFG process.
the PR. After the baking process, the wafer is then exposed to an ultraviolet light with a mask, the purpose of which is to obtain the patterned structure needed. Upon completion, the wafer is then immersed in liquid developer to remove unexposed PR, leaving only the necessary first layer of structural patterns. Upon completing all the aforementioned steps, the etched optical fiber is adhered to the first structure of the wafer; the spin PR, baking, exposure, post-exposure baking, and developer procedures are then repeated to produce the etched optical fiber with periodic structure and complete processes before electroforming.
The Electroforming Process: The wafer completed with the MEMS process is fastened on the conducting plate so that the copper layer on the wafer can conduct with the conducting plate. The conducting plate is then placed on the nickel elec-troforming solution, namely, nickel sulfamate. Next, the anode is electroformed from the nickel ingot with a purity of 99.9%. The cathode is the copper conductivity layer. After completing all the preparation work, the electroforming is conducted until the needed thickness (130 μm) is reached. The entire wafer is then immersed into the mixed solution of hydrogen peroxide, ammonium water, and de-ionized water after completing the electroforming structure in order to carry out copper layer etch-ing (sacrificial layer) until the copper layer has all been etched, which, in turn, will release the ELPFG sensor. The process flowchart is shown in Fig. 1. Through this PR mass production process, we can obtain sixteen ELPFG devices per wafer. Fig. 2 shows the ELPFG structure diagram.
Fig. 2. Schematics of the ELPFG structure.
Fig. 3. Experimental setup of the magnetic field calibration test of the ELPFG sensor.
B. Experimental Setup
The purpose of the ELPFG magnetic field calibration test is to analyze the changes in optical characteristics when the ELPFG is affected by a magnetic field. The experimental setup for ELPFG magnetic modulation is shown in Fig. 3. The ex-periment equipment consists of a superluminescent diode, an optical spectrum analyzer (OSA), a power supply, a coil, a pre-cision stage, a thermal coupler, a data acquisition device, and an ELPFG sensor.
Firstly, the ELPFG is placed into the coil, and the two ends are fastened on the precision stage and load cell. The magnetic field is controlled by current, and for every increase of 1 A in current, the magnetic field is increased by 15.85 mT. The current is exported by the power supply based on increments of 0.3 A, increasing from 0 A (0 mT) to 3 A (47.6 mT). In order to reduce the effects of temperature on the ELPFG, we used a large diameter coil to decrease the rise in temperature when generating the magnetic field and controlled the temperature variation by air cooling and by waiting for the coil to cool down. The OSA is used to monitor the optical spectra and the ELPFG spectra change under the magnetic field being analyzed.
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Fig. 4. OM of the ELPFGs.
Fig. 5. SEM of the ELPFG.
IV. RESULTS ANDDISCUSSION A. Dimension and Appearance of the ELPFG Device
In this study, a novel ELPFG is fabricated by the LIGA-like MEMS process and electroforming. The ELPFG is composed of a polymer/metal periodic structure (consisting of SU-8 50 PR and electroformed nickel grating). Fig. 4 shows an optical microscopy (OM) photo of the ELPFG. The periodic rectan-gular structure shown there is the electroformed nickel grating. The material surrounding the nickel grating is the PR mold for electroforming, which has a gold color in the image in Fig. 4. The period of the ELPFG is 620 μm, and the total grating length is 2.5 cm in Fig. 4. Fig. 5 is an ELPFG SEM image. The red arrow in Fig. 5 points to an example of slight damage to the nickel gratings. This damage is produced by the ELPFG device releasing step of the fabrication process for separation of the electroformed nickel gratings and the copper sacrificial layer. The depth of these slight abrasions is very thin, less than 1 um. The thickness of the ELPFG itself, meanwhile, is about 130 um. Therefore, the effects of the damage to the nickel grating are insignificant. Furthermore, such damage can be reduced by slowing down the speed of the releasing process.
B. Characterization of ELPFG Sensor Under the Magnetic Field Modulation
The ELPFG magnetic sensor calibration test is presented in this paper to analyze the characterization of the ELPFG mag-netic sensor under magmag-netic field modulation. The magmag-netic
Fig. 6. The spectra of the ELPFGs (Λ: 620, 630, and 640 μm) under various applied magnetic fields (0 mT 47.6 mT).
field is applied to the ELPFG through the coil with a current. When the current is applied and passed through the coil, it is converted into a magnetic field, and the applied magnetic field intensity of the ELPFG ranges from 0 mT47.6 mT. For ev-ery increase of 1 A in current, the magnetic field is increased by 15.85 mT. The experiment adopts three parameters with different grating periods of 620, 630, and 640 μm of the ELPFG. When the magnetic field applied to the ELPFG was increased from 0 mT to 47.6 mT, the spectra of the ELPFG changed as the applied magnetic field changed.
Fig. 6 shows the optical spectra of the ELPFG magnetic sen-sors with the grating periods of 620, 630, and 640 μm during
Fig. 7. Resonant wavelengths of the ELPFGs (Λ: 620, 630, and 640 μm) under various applied magnetic fields.
the magnetic calibration test. The insertion loss of the ELPFG magnetic sensor is about 4 dB. The insertion loss is induced by the residual stress of the process because the proposed ELPFG is made of the etched optical fiber sandwiched by a periodic polymer-metal (SU-8 PR and nickel) structure. Before the mag-netic field is applied to the ELPFG, the transmission loss of the resonant dip is too low to observe so that the resonant wave-length is difficult to determine. As the applied magnetic field is increased, the transmission loss of the resonant dip is gradually increased. The wavelength and transmission loss of the ELPFG vary with magnetic change. This phenomenon is related to the magnetostriction effect and the induced magnetostriction strain of the periodic nickel structure. The refractive index variation of the entire element is given according to the induced magne-tostriction strain on the nickel structures of the ELPFG. As a result, the induced magnetostriction strain caused by the applied magnetic field of the coil causes the long period grating effect, according to Eq. (2). As shown in Fig. 6(a)–(c), the maximum transmission losses of the ELPFG resonant dip occur while the ELPFG is under the maximum magnetic force. At the same time, the wavelengths of the ELPFGs are 1505 nm, 1517 nm, and 1529 nm, respectively. The resonant wavelength becomes large as the grating period increases.
Fig. 7 shows the changes in the resonant wavelength of the ELPFG (Λ: 620, 630 and 640 μm) under various applied mag-netic fields between 0 mT 47.6 mT. It was discovered from the experiment that when the applied magnetic field is increasing (0 mT 47.6 mT), the resonant wavelengths of the ELPFG (Λ: 620 μm, 630 μm, and 640 μm) are red-shifted by 2.5, 7.0, and 8.5 nm, respectively. The reason for this phenomenon is that the changes in the ELPFG period and the effective refractive index are proportional to the change in the applied magnetic field. According to Eq. (1), a slight resonant wavelength shift is induced with the applied magnetic field.
Fig. 8 shows the corresponding transmission loss of the ELPFG under magnetic field modulation. When the applied magnetic field changes (0 mT 47.6 mT), the resonance at-tenuation dips of the ELPFGs are gradually increased, and the maximum resonance attenuation dips of the ELPFGs with the
Fig. 8. Resonant attenuation losses of the ELPFGs (Λ: 620, 630, and 640 μm) under various applied magnetic fields.
Fig. 9. Sensitivity and linearity analysis chart of three sets of magnetic field modulations.
maximum applied magnetic field intensity of 47.6 mT are −22.9 dB, −28.8 dB, and −21.7 dB, respectively. This phe-nomenon results from the induced magnetostriction strain on the ELPFG caused by the applied magnetic field.
C. Sensitivity and Linearity of ELPFGs With Applied Magnetic Field
With respect to the sensibility and linearity analysis of the magnetic field calibration data of the ELPFGs (Λ: 620, 630, and 640 μm), Fig. 9 shows that the sensitivities of the three experimental groups are similar, with the best sensitivity being −0.465 dB/mT and the worst sensitivity being −0.306 dB/mT. These sensitivities were compared, in turn, with the magnetic field sensitivities of the magnetic field sensors developed by Liu et al. [15] and Gao et al. [16], which were reported to be 0.0382 dB/mT and−0.1233 dB/mT, respectively. The magnetic field sensitivity of the ELPFG in this paper outperforms all of the abovementioned sensitivities. The experimental results also show that when the ELPFG sensor is at the magnetic field inten-sity of 47.6 mT, the attenuation dip loss reaches −28.8 dB, the optimal sensitivity is −0.465 dB/mT, and the linearity is 0.965.
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V. CONCLUSION
In the current work, a novel and robust ELPFG magnetic field sensor involving magnetostriction material and the LIGA-like process has been developed. The ELPFG magnetic field modu-lation of this paper refers to placing ELPFG into the coil, and then using the power supply to provide the current and drive the coil to generate a magnetic field that can modulate the ELPFG. An OSA was then used to analyze the changes in spectra, res-onant wavelength, and transmission loss. The experimental re-sults show that when the ELPFG sensor is at the magnetic field intensity of 47.6 mT, the attenuation dip loss reaches−28.8 dB, the optimal sensitivity is −0.465 dB/mT, and the linearity is 0.965. Therefore, the ELPFG has the potential to be applied in highly sensitive magnetic field sensors and loss tunable filters.
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Chia-Chin Chiang received the B.S. and the M.S. degree from the Department
of Mechanical Engineering and Mechatronic Engineering, Ocean University, Qingdao, China, in 1998 and 2000, respectively, and the Ph.D. degree from the National Taiwan University, Taipei, Taiwan, in 2005. He began as an assistant professor with the Department of Mechanical Engineering, Kaohsiung Univer-sity of Applied Sciences, Kaohsiung, Taiwan, in 2006, and became an Associate Professor in 2011. His research interests include fiber Bragg gratings, optical fiber sensors, and smart materials and structures.
Zheng-Jie Chen received the M.S. degree from the Department of Mechanical
Engineering, Kaohsiung University of Applied Sciences, Kaohsiung, Taiwan, in 2013. His research interests include fiber-optic sensors and microelectrome-chanical systems.
