IEEE TRANSACTIONS ON MAGNETICS, VOL. 42, NO. 10, OCTOBER 2006
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Magnetic Sensor Based on Side-Polished Fiber Bragg Grating Coated With Iron Film Chuen-Lin Tien1 , Chang-Chou Hwang1 , Hong-Wei Chen2 , Wen Fung Liu1 , and Shane-Wen Lin2 Department of Electrical Engineering, Feng Chia University, Seatwen, Taichung 40724, Taiwan, R.O.C. Graduate Institute of Electrical and Communications Engineering, Feng Chia University, Taichung 40724, Taiwan, R.O.C. This paper presents a novel fiber-optic magnetic sensor based on a side-polished fiber Bragg grating coated with thin iron film. The Bragg wavelength shift of 0.08 nm was measured at the distance of 0.38 mm between fiber sensor and Nd-Fe-B magnet with remanent flux density of 1.115 T. We demonstrate the ability to measure magnetic fields using such a device. Index Terms—Fiber Bragg grating, magnetic sensor, side-polished fiber.
I. INTRODUCTION
II. PRINCIPLE
HERE is a critical need for nondestructive test evaluation (NDE) sensors that can rapidly detect the sensing parameters. The sensor based on a fiber Bragg grating (FBG) can satisfy the requirement in sensing a number of physical parameters such as strain, temperature, pressure, magnetic field, etc. [1]. Thus, this technique has attracted numerous researchers for developing a wide range of applications due to their many inherent advantages. These are, for example, small size, immunity to electromagnetic interference, wavelength multiplexing, and distributed sensing possibilities [2]–[5]. Compared with conventional electrical sensors, the in-fiber grating sensor system has numerous advantages including inexpensive production, high sensitivity, simple structure, and self-referencing with a linear response. In addition, single-mode fiber sensors based on sidepolished fibers have been used as simple fiber-optic sensing elements [6]. Several different types of magnetic sensors have been reported with different sensitivities [7]–[9]. Okamura [7] designed a fiber-optic magnetic sensor based on the Lorentzian force with linear phase sensitivities of 0.67 and 0.11 rad/(A/m) for measuring both ac and dc magnetic fields. Kersey and Marrone [8] proposed an FBG for dynamic magnetic field detection with the Faraday’s effect to induce a slight change of the refractive index of the fiber experienced by left and right circularly polarized light. Radojevic et al. [9] demonstrated a magnetic sensor by using multi-mode fibers with magnetic composite coating. They investigated the influence of the applied external magnetic field on the change of intensity of the transmitting light signal. However, the above reports do not use the side-polishedfiber technique in the sensing structure design for improving the sensor performance. Hence, in this paper we proposed a novel sensing structure as an optical fiber magnetic sensor by using the side-polished FBG coated with Fe film for measuring the static magnetic field strength. A Nd-Fe-B magnet was chosen for this investigation.
A fiber Bragg grating consists of a periodic modulation of the refractive index in the core of a single-mode optical fiber. When the Bragg condition is satisfied, the contributions of reflected light from each grating plane add constructively in the backward direction to form a back-reflected peak with a center wavelength defined by the grating parameters. The first-order Bragg condition is given by
T
Digital Object Identifier 10.1109/TMAG.2006.881095
(1) , is the center wavelength of where the Bragg wavelength, the input light that will be back-reflected from the Bragg grating, is the effective refraction index of the fiber core, and is the period of fiber Bragg grating. As the fiber cladding is polished to the core, the effective refraction index of the fiber core is changed resulting in the shift of reflected Bragg wavelength and bandwidth expansion. Thus, the fiber grating can be considered as an intrinsic optical transducer which changes the spectrum of the reflected light. FBG sensors have been proposed for different sensing functions; for example strain, temperature, dynamic magnetic field, etc., which is dependent on the grating center wavelength shift when the sensing parameters cause grating effective index or grating period variation. The sensing mechanism in this device is based on magnetic force that is induced by the Fe film coated on the side-polished FBG with the nearby Nd-Fe-B magnet which has a remanent flux density of 1.115 T. The Fe film deposited on the Bragg grating polished-surface will create a radial magnetic force, which will stretch the fiber to cause the Bragg wavelength shift. When the Fe film is attracted by an Nd-Fe-B magnet, mechanical expansion in the Fe film physically stretches the FBG and causes the change of the effective refraction index. The amount of the Bragg wavelength shift is proportional to the radial magnetic force. From measuring the amount of Bragg wavelength shift by using an optical spectrum analyzer (OSA), the magnetic field strength can be determined. The resolution of the OSA was set to 0.01 nm, in order to obtain experimental results of high quality.
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Fig. 1.
IEEE TRANSACTIONS ON MAGNETICS, VOL. 42, NO. 10, OCTOBER 2006
Schematic diagram of magnetic sensing head.
Fig. 3. Reflection spectra of before and after side-polished, and with Fe-coated. Fig. 2.
Experimental setup.
III. EXPERIMENTS The proposed magnetic sensor consists of a side-polished FBG and thin iron films with a thickness of 300 nm deposited on an interacting section. In our experiment, the FBG was written in a hydrogen-loaded single-mode fiber (SMF-28) by using the phase mask writing technique of a KrF excimer laser (248 nm). The FBG is 10 mm in length with reflectivity of 90% and with a Bragg wavelength of 1545.5 nm. To achieve fiber polishing, it was glued into a silicon V-groove with the setup described in [10]. The interacting region, with the diameter to be polished down to 67.5 m, can be monitored by using an optical microscope. A sensing head is composed of iron films and fiber gratings for detecting magnetic field variation. The structure of the magnetic sensor head is shown in Fig. 1. The length of the side-polished interacting section is about 10 mm. A thin iron film was deposited by the electron-beam evaporation technique. In our high-vacuum evaporation chamber, the evaporation source was a resistance-heated crucible bombarded by electrons. The distance between the evaporation source and the substrate was 935 mm. Substrates were mounted onto a planetary rotation substrate holder with a 400 mm diameter that rotated at a speed of 36 rpm. A special fiber holder was designed in the electron-beam deposition equipment for obtaining a uniform coating thickness. The vacuum chamber was initially pumped down by a mechanical pump and a cryopump to a base presTorr before deposition. During the sure of less than deposition, iron of 99% purity was evaporated from a 7-KW Torr. A thin film electron beam gun at a pressure of was deposited on a side-polished FBG and a glass substrate was held at temperature of 70 C. The coating substrates, prior to the deposition, had undergone ultrasonic cleaning progressively in acetone and ethanol and then dried in a vacuum dryer. The thickness of the film was monitored during the deposition by a quartz crystal thickness monitor. The films were grown at a uniform rate of 1.5 Å/s. A thermocouple was placed near the sample holder to monitor the substrates temperature. The experimental setup used to measure the magnetic field is shown in Fig. 2. A
Fig. 4. Reflection spectra of the FBG magnetic sensor.
permanent ferromagnetic magnet of Nd-Fe-B was chosen for the magnetic sensing experiment. In Fig. 2, the light from a wide-band ASE light source was launched to a side-polished FBG fiber with a Fe magnetic material coating. Both the transmission and reflection spectra from the Fe-coated section of FBG were detected by an optical spectrum analyzer (OSA). The effective refractive index in the interaction section of an FBG was varied in response to an applied magnetic field. Fig. 3 shows the reflected spectra of both before and after side-polished FBG, and fiber with Fe-coated films, respectively. The peak reflection wavelengths were measured at 1545.5 nm for unpolished FBG, 1544.48 nm for side-polished FBG, and 1544.46 nm for Fe-film coatings, respectively. When the permanent magnet approached the sensor head, the center wavelength of the reflection spectrum was shifted relative into the shorter wavelength side relative to the one without the magnetic field, as shown in Fig. 4. We measured a Bragg wavelength shift of 0.08 nm in the case where the distance was 0.38 mm between the fiber sensor and the Nd-Fe-B magnet. A plot of the measured transmission loss as a function of the distance between fiber sensor head and magnet is presented in Fig. 5.
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ACKNOWLEDGMENT This work was supported in part by the National Council, Taiwan, R.O.C.
REFERENCES
Fig. 5. The curve of the relative transmission loss versus the distance between the fiber sensor and magnet.
IV. CONCLUSION A new type of magnetic sensor based on side-polished fiber Bragg grating with Fe-coated films is demonstrated experimentally to have an influence on the Bragg wavelength when an external magnetic field is applied to this sensor. The magnitude of interaction depends on the distance between the fiber sensor and the Nd-Fe-B magnet. This magnetic sensor can further be developed to improve the sensitivity of the magnetic field measurement. The interaction length, the diameter of side-polished fiber and thin iron film thickness are related to the sensor sensitivity respectively and can be controlled independently.
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Manuscript received March 12, 2006 (e-mail:
[email protected]).
