April 1, 2014 / Vol. 39, No. 7 / OPTICS LETTERS
1905
Femtosecond laser ablation of microstructures in fiber and application in magnetic field sensing Qiancheng Zhao,1,3 Yutang Dai,1,4 Tao Li,1 Bin Liu,1 Minghong Yang,2 and Guanglin Yin1 1
National Engineering Laboratory for Fiber Optic Sensing Technology, Wuhan University of Technology, Wuhan 430070, China 2 Key Laboratory of Fiber Optic Sensing Technology and Information Processing, Wuhan University of Technology, Wuhan 430070, China 3
e-mail: [email protected] 4 e-mail: [email protected]
Received February 17, 2014; revised February 22, 2014; accepted February 22, 2014; posted February 24, 2014 (Doc. ID 206510); published March 21, 2014 A novel (to our knowledge) 3D microstructure manufactured by a femtosecond laser in fiber Bragg grating (FBG) fiber cladding is proposed. The special spiral parameters including single thread and double thread with certain pitches of 60 and 80 μm are controlled by the feed and rotation speed of a rotary fixture. Moreover, supermagnetostrictive TbDyFe film with a thickness of nearly 6 μm is deposited on microgrooves of a FBG by magnetron sputtering technology to form the magnetic field sensing probe. Experimental results demonstrate that a FBG with a double-thread microstructure has a sensitivity of 1.1 pm∕mT responding to a magnetic field, and in a theoretical situation, it is approximately six times higher than the original optical fiber grating (approximately 0.2 pm∕mT). This new microstructure and method show great prospect in the magnetic field sensing domain. © 2014 Optical Society of America OCIS codes: (140.3510) Lasers, fiber; (060.2370) Fiber optics sensors; (220.4610) Optical fabrication. http://dx.doi.org/10.1364/OL.39.001905
Optical fiber sensing technology has attracted much research interest and attention for its superior characteristics over the common optical fiber, such as immunity to electromagnetic radiation, resistance to chemical corrosion, small size, high accuracy, and capability of remote operation [1–5]. Meanwhile, laser ablation is a promising technique for improving fabrication efficiency, and it is a promising tool for many hard and brittle materials due to its high peak power, short pulse, and other excellent properties. These unique properties have led many people to study laser processing for transparent materials [6,7]. As for magnetic field sensors, much research has been concentrated on the development of magnetic field or electric current sensors using magnetostrictive transducers or other mechanisms. A magnetostrictive sensor using TbDyFe and Ni65Cu33Fe2 epoxy bonded with fiber Bragg grating (FBG) fiber was proposed for DC current and temperature discrimination by Mora et al. [8]. In 2010, Quintero et al. developed a light and compact optical FBG sensor for DC and AC magnetic field measurements; the fiber is coated by a thick layer of a magnetostrictive composite (MC) consisting of particles of TbDyFe dispersed in a polymeric matrix. The resolution achieved for a magnetic field can be 0.4 mT [9]. In 2012, Liu et al. replaced magnetostrictive alloy with MC to propose a MC-FBG magnetic field sensor [10], which is based on the direct coupling of the magnetostrictive strain in an epoxy-bonded Terfenol-D particle pseudo1–3 MC actuator with a FBG strain sensor. Experiments from Shengyang University of Technology tried to transmit deformation by a changed magnetic field through mechanical structure; the sensitivity concerning the magnetic field could be 0–40 pm [11]. However, all these methods are based on bulk magnetostrictive materials, which have proven problematic for miniature application. In 2011, Smith et al. developed a sensor based on a single-layer magnetostrictive film of Terfenol-D 0146-9592/14/071905-04$15.00/0
sputtered onto a single-mode fiber that had a femtosecond-laser-inscribed FBG and slot micromachined into it [12]. However, the sensitivity of that sensor is comparatively low. Chemical methods are also applied in the magnetic field domain. Yang et al. used an etched side circle of a FBG as a sensing element with TbDyFe thin films deposited on it. There exists more than a 45 pm change in the FBG wavelength when the magnet field increases up to 50 mT [13]. However, the use of the chemical etching process is relatively slow, and the etched fiber is rather fragile. In this Letter, a novel 3D microstructure is proposed and manufactured in a FBG by a femtosecond laser. In addition, supermagnetostrictive material TbDyFe thin film (6 μm thickness) is also sputtered in the microgrooves of the cladding as well as the surface of the cladding (10 mm in length). With the combination of these two technologies, a magnetic field sensing probe is successfully developed, and experimental results show high sensitivity in detecting magnetic field change. Because of its miniature size, easy fabrication, convenient operation, and repeatable testing, this new method shows great potential for future magnetic field sensing. Generally speaking, the magnetostrictive coefficient ξ is used to describe the extent of magnetostrictive effect. When magnetic field strength is increasing uniformly, the deformation of magnetostrictive material will increase linearly. If the strength reaches a certain value, the deformation will reach its maximum. The relationship
Fig. 1. Relationship between the magnetic field and deformation of a magnetostrictive material. © 2014 Optical Society of America
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OPTICS LETTERS / Vol. 39, No. 7 / April 1, 2014
between the magnetic field and deformation of magnetostrictive materials is shown in Fig. 1. On the other hand, it has been widely accepted that when the temperature stays constant, the center wavelength of the FBG will shift due to axial strain; the formula is given as follows: Δλ 1 − P ε ε: λ
(1)
λ represents the original center wavelength, while P ε is the elasto-optical coefficient for typical silica fiber; the general value is 0.22. ε is the strain produced from the outside, usually at the με level. At the same time, the magnetostrictive material will have a deformation of ε by a certain magnetic field, which we assume it to be H. The relationship between ε and H is ε f H:
(2)
f is the function of the two parameters and is determined by different types of magnetic field; we think that if the TbDyFe thin film is sputtered directly in the microgrooves of the cladding of the optical fiber grating, the elongation of the magnetostrictive film due to the magnetic field will result in a wavelength shift of the FBG. Combining formula (1) and formula (2), we can deduce that Δλ 1 − P ε ε 1 − P ε f H: λ
(3)
From Eq. (3), we can finally determine that the value of the magnetic field would be calculated as follows: Δλ : λ1 − P ε
Hf
−1
(4)
It is clear that as long as a shift in the center wavelength is detected, the outside magnetic field can be calculated and determined directly. In this Letter, we propose a 3D microstructure in the cladding of a FBG. Femtosecond laser processing is employed to ablate the microgrooves. The schematic of the microstructure is shown in Fig. 2. In Fig. 2, we can see that a certain thread pitch S is manufactured by a femtosecond laser. We make two kinds of pitches, namely 60 and 80 μm. TbDyFe film is directly coated in the microgrooves as well as on the surface of the cladding, and thus a primary new type of magnetic sensing probe is successfully made. In order to achieve a 3D microstructure in the cladding, a femtosecond laser (IFRIT, Cyber Laser, Tokyo, Japan) was used to fabricate such a microstructure.
Fig. 2. Schematic of the FBG magnetic field sensing probe.
The laser system is based on a 180 fs titanium–sapphire regenerative amplifier system. The fundamental wavelength of the laser is 780 nm. The pulse energy is controlled by the attenuation; 100%, 11%, or 2% of the total energy can be selected by adjusting to mode 1, 2, or 3, respectively. The repeat frequency of the pulse ranges from 1 to 1000 Hz. The maximum pulse energy is 1.08 mJ. In the experiment, pattern 2 was chosen for the appropriate energy acting on the optical fiber. The 3D moving stage has a movement range of 100, 100, and 25 mm (X, Y, and Z directions respectively). All three stages are driven by linear DC motors. The laser beam, which is in Gaussian mode, will finally be focused on the targets by the objective lens (Sigama, Koki, Japan) with a focal length of 60 mm. A high-resolution CCD camera connecting to a computer is also fixed to monitor the real-time process of the fabrication. The schematic of the femtosecond laser is shown in Fig. 3. As shown in Fig. 4, before fabrication, the Bragg grating was written in a standard-single mode fiber (SMF-28) by the phase mask technique with an ultraviolet (UV) excimer laser emitting at a 248 nm wavelength. The length of the FBG L is 10 mm, and the diameter of the FBG core is 9 μm, and when we remove the FBG’s protection layer (namely the polyimide layer, 10 mm in total), the diameter of the cladding is 125 μm. In the experiment, two types of 3D microstructures were ablated in the cladding of the optical fiber, namely single thread and double thread. The thread pitch is controlled by a special rotation fixture and the feed speed of the Y axial direction stage. The rotary speed R was set as 15 rpm, and F is the feed speed, which is decided by a program set in the operation panel. As for double thread, thread pitches of 60 and 80 μm were made. Moreover, the 3D microstructures were made under 18, 20, and 22 mW, respectively. Finally, nine samples were successfully made; we named the single thread ones S-1, S-2, and S-3, with SS-1 to SS-6 for the double thread ones.
Fig. 3.
Schematic of the femtosecond laser.
Fig. 4. Illustration of the fabricated FBG (SMF-28).
April 1, 2014 / Vol. 39, No. 7 / OPTICS LETTERS
Fig. 5.
SEM image of a fabricated single-thread microstructure.
Meanwhile, the original standard optical fiber grating was also used for comparison (O-1). The depth of the spiral microgrooves h can be measured by a scanning electron microscope (SEM). Figure 5 is a sample of a single-thread microstrcuture shown in a SEM image. h varies according to the energy of the laser ablation; all the microstructural samples are listed in Table 1. Finally, the magnetostrictive material TbDyFe was prepared for coating. The coating process was accomplished in a magnetron sputtering machine. We put the nine samples (plus O-1) in the special fixture of the sputtering chamber, the fabricated part (10 mm in length) was exposed for sputtering, and the other parts of the optical fiber were covered with aluminized paper to avoid deposition. During the sputtering process, the rotation fixture was applied to obtain a uniform thin TbDyFe film in the microgrooves as well as on the cladding (10 mm in length), the start power was set at 50 W, and the coating power stabilized at a level of 90 W. As a result, the average speed of sputtering was 0.15 nm∕s. The TbDyFe target and the microstructural optical fiber were kept at a distance of 50 mm. The depth of the target film was detected by an electronic module of the sputtering machine. We noted the thickness of the TbDyFe through the electronic module; it was 6 mm in thickness. The samples of single thread and double thread (coated with TbDyFe) are shown in Fig. 6. Table 1. Parameters for Microstructural Samples Sample O-1 S-1 S-2 S-3 SS-1 SS-2 SS-3 SS-4 SS-5 SS-6
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Thread Pitch (μm)
Energy (mW)
Depth (μm)
∞ 60 60 60 60 60 60 80 80 80
0 18 20 22 18 20 22 18 20 22
0 14 16.5 18.5 14 16.5 18.5 14 16.5 18.5
Fig. 6. SEM of microstructure. (a) Double thread and (b) single thread.
Fig. 7.
Schematic of the magnetic field experiment.
Eventually, an experimental system for testing magnetic fields was set up and conducted; the schematic is shown in Fig. 7. The magnetic field was supplied by a permanent magnet instead of an electric spiral coil in order to eliminate the thermal effect. The value of the magnetic field could be measured by a Gaussmeter. It depended on the distance D from the magnetic source (magnetic field range: 0–150 mT) to the sensing probe. The magnet was fixed on a special nonmagnetic fixture. In addition, the demodulator (SM130, Micron Optics) was used to detect the change of the center wavelength of the optical fiber grating; it had a resolution of 1 pm. The magnetic field sensing probe was protected in a glass tube to avoid being broken, and the N-S orientation of the permanent magnets was set as parallel to the fiber axis in order to stay at the uniaxial stress state. Figure 8 shows the relationship between the center wavelength shift and the magnetic field. It shows clearly that the FBG magnetic field sensing probe with double thread of 80 μm pitch has far more notable wavelength shift than the standard one. The sensitivity of sample SS-6 can be calculated as about 1.1 pm∕mT, while the original sample is about 0.2 pm∕mT. At the same time, as the magnetic field force is strengthening to a certain value at about 150 mT, the center wavelength shift of the FBG will increase more and more slowly due to the limitation of the TbDyFe coating elongation. The center wavelength of the FBG magnetic field sensing probe can return to the original center wavelength when the magnetic field is removed. In Fig. 9, it is obvious that energy has a significant influence on the sensitivity according to the magnetic field [see Figs. 9(a) and 9(b), separately]. As the energy becomes greater, the microgrooves get deeper. As a result, the cross-sectional area of the optical fiber cladding tends to become smaller, and thus more magnetostrictive
Fig. 8. Wavelength shift of the microstructural sensing probe (Samples SS-6 and O-1).
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OPTICS LETTERS / Vol. 39, No. 7 / April 1, 2014
Fig. 9. Wavelength shift of double-thread magnetic sensing probe. (a) Thread pitch 80 μm and (b) thread pitch 60 μm.
Fig. 10. Center wavelength shift of the single-thread microstructure responding to the changed magnetic field.
Figure 11 plots all the types of microstructures under the same energy. Double thread is more sensitive to the magnetic field in comparison with single thread, because double thread has a greater axial component and a smaller cross-sectional area. As a whole, the doublethread microstructure is obviously the most effective and suitable microstructure manufactured in the cladding of optical fiber and shows great promise in the magnetic field sensing domain. In conclusion, a different new type of microstructure has been proposed and successfully manufactured in the cladding of a FBG with the appropriate parameters of a femtosecond laser and a special rotation fixture. A 6 μm thickness of supermagnetostrictive material TbDyFe was coated in the cladding of a fiber grating by magnetron sputtering technology to form the sensing probe. Two types of microstructure have been tested in the magnetic field experiment. The results show that a FBG with double thread of 80 μm has the strongest response of 1.1 pm∕mT to the magnetic field, which is followed by the double thread of 60 μm (0.81 pm∕mT) and the single thread (0.7 pm∕mT). These microstructures have greatly improved in magnetic field sensing compared with the original optical fiber. For its convenient fabrication and high sensitivity, this microstructure shows promise for application in the optical fiber sensing field for magnetic fields. This work is financially supported by the Project of National Science Foundation of China (NSFC) (No. 51175393).
Fig. 11. Wavelength shift of different types and pitches of microstructures under the magnetic field.
material (TbDyFe) particles will coat the surface as well as the groove. The magnetostrictive effect, therefore, will be more apparent and the strain ε along the axis will get larger under the same magnetic field. In addition, the double thread of 80 μm is more sensitive to the magnetic field than that of 60 μm. This is due to the different axial component contributing to ε. When the pitch is bigger, the axial component of the strain becomes larger along the optical fiber, so it will produce more deformation along the axis. According to Fig. 10, the magnetic field sensing probe with the single thread also shows priority over the original one in response to the magnetic field. The maximum wavelength shift can be approximately 70 pm under a magnetic field value of 150 mT.
References 1. V. Bhatia, D. Campbell, and R. O. Claus, Opt. Lett. 22, 648 (1997). 2. C. Liao, L. Xu, C. Wang, Y. Wang, and K. Li, Opt. Lett. 38, 2 (2013). 3. A. D. Kersey, T. A. Berkoff, and W. W. Morey, Opt. Lett. 18, 16 (1993). 4. G. Meltz, M. M. Morey, and W. H. Glenn, Opt. Lett. 14, 823 (1989). 5. A. P. Zhang, B. O. Guan, X. M. Tao, and H. Y. Tam, IEEE Photon. Technol. Lett. 14, 92 (2002). 6. E. N. Glezer and E. Mazur, Appl. Phys. Lett. 71, 882 (1997). 7. K. Miura, J. Qiu, H. Inouye, and T. Mitsuyu, Appl. Phys. Lett. 71, 3329 (1997). 8. J. Mora, A. Diez, J. L. Cruz, and M. V. Andres, IEEE Photon. Technol. Lett. 12, 1680 (2000). 9. S. M. M. Quintero, A. M. B. Braga, and H. I. Weber, Sensors 10, 8119 (2010). 10. H. Liu, S. W. Or, and H. Y. Tam, Sens. Actuators A 173, 1 (2012). 11. M. H. Yang, Y. Sun, D. S. Zhang, and D. S. Jiang, Sens. Lett. 7, 576 (2009). 12. G. N. Smith, T. Allsop, K. Kalli, C. Koutsides, R. Neal, K. Sugden, P. Culverhouse, and I. Bennion, Opt. Express 19, 363 (2011). 13. M. H. Yang, J. X. Dai, C. M. Zhou, and D. S. Jiang, Opt. Express 17, 20777 (2009).
1905
Femtosecond laser ablation of microstructures in fiber and application in magnetic field sensing Qiancheng Zhao,1,3 Yutang Dai,1,4 Tao Li,1 Bin Liu,1 Minghong Yang,2 and Guanglin Yin1 1
National Engineering Laboratory for Fiber Optic Sensing Technology, Wuhan University of Technology, Wuhan 430070, China 2 Key Laboratory of Fiber Optic Sensing Technology and Information Processing, Wuhan University of Technology, Wuhan 430070, China 3
e-mail: [email protected] 4 e-mail: [email protected]
Received February 17, 2014; revised February 22, 2014; accepted February 22, 2014; posted February 24, 2014 (Doc. ID 206510); published March 21, 2014 A novel (to our knowledge) 3D microstructure manufactured by a femtosecond laser in fiber Bragg grating (FBG) fiber cladding is proposed. The special spiral parameters including single thread and double thread with certain pitches of 60 and 80 μm are controlled by the feed and rotation speed of a rotary fixture. Moreover, supermagnetostrictive TbDyFe film with a thickness of nearly 6 μm is deposited on microgrooves of a FBG by magnetron sputtering technology to form the magnetic field sensing probe. Experimental results demonstrate that a FBG with a double-thread microstructure has a sensitivity of 1.1 pm∕mT responding to a magnetic field, and in a theoretical situation, it is approximately six times higher than the original optical fiber grating (approximately 0.2 pm∕mT). This new microstructure and method show great prospect in the magnetic field sensing domain. © 2014 Optical Society of America OCIS codes: (140.3510) Lasers, fiber; (060.2370) Fiber optics sensors; (220.4610) Optical fabrication. http://dx.doi.org/10.1364/OL.39.001905
Optical fiber sensing technology has attracted much research interest and attention for its superior characteristics over the common optical fiber, such as immunity to electromagnetic radiation, resistance to chemical corrosion, small size, high accuracy, and capability of remote operation [1–5]. Meanwhile, laser ablation is a promising technique for improving fabrication efficiency, and it is a promising tool for many hard and brittle materials due to its high peak power, short pulse, and other excellent properties. These unique properties have led many people to study laser processing for transparent materials [6,7]. As for magnetic field sensors, much research has been concentrated on the development of magnetic field or electric current sensors using magnetostrictive transducers or other mechanisms. A magnetostrictive sensor using TbDyFe and Ni65Cu33Fe2 epoxy bonded with fiber Bragg grating (FBG) fiber was proposed for DC current and temperature discrimination by Mora et al. [8]. In 2010, Quintero et al. developed a light and compact optical FBG sensor for DC and AC magnetic field measurements; the fiber is coated by a thick layer of a magnetostrictive composite (MC) consisting of particles of TbDyFe dispersed in a polymeric matrix. The resolution achieved for a magnetic field can be 0.4 mT [9]. In 2012, Liu et al. replaced magnetostrictive alloy with MC to propose a MC-FBG magnetic field sensor [10], which is based on the direct coupling of the magnetostrictive strain in an epoxy-bonded Terfenol-D particle pseudo1–3 MC actuator with a FBG strain sensor. Experiments from Shengyang University of Technology tried to transmit deformation by a changed magnetic field through mechanical structure; the sensitivity concerning the magnetic field could be 0–40 pm [11]. However, all these methods are based on bulk magnetostrictive materials, which have proven problematic for miniature application. In 2011, Smith et al. developed a sensor based on a single-layer magnetostrictive film of Terfenol-D 0146-9592/14/071905-04$15.00/0
sputtered onto a single-mode fiber that had a femtosecond-laser-inscribed FBG and slot micromachined into it [12]. However, the sensitivity of that sensor is comparatively low. Chemical methods are also applied in the magnetic field domain. Yang et al. used an etched side circle of a FBG as a sensing element with TbDyFe thin films deposited on it. There exists more than a 45 pm change in the FBG wavelength when the magnet field increases up to 50 mT [13]. However, the use of the chemical etching process is relatively slow, and the etched fiber is rather fragile. In this Letter, a novel 3D microstructure is proposed and manufactured in a FBG by a femtosecond laser. In addition, supermagnetostrictive material TbDyFe thin film (6 μm thickness) is also sputtered in the microgrooves of the cladding as well as the surface of the cladding (10 mm in length). With the combination of these two technologies, a magnetic field sensing probe is successfully developed, and experimental results show high sensitivity in detecting magnetic field change. Because of its miniature size, easy fabrication, convenient operation, and repeatable testing, this new method shows great potential for future magnetic field sensing. Generally speaking, the magnetostrictive coefficient ξ is used to describe the extent of magnetostrictive effect. When magnetic field strength is increasing uniformly, the deformation of magnetostrictive material will increase linearly. If the strength reaches a certain value, the deformation will reach its maximum. The relationship
Fig. 1. Relationship between the magnetic field and deformation of a magnetostrictive material. © 2014 Optical Society of America
1906
OPTICS LETTERS / Vol. 39, No. 7 / April 1, 2014
between the magnetic field and deformation of magnetostrictive materials is shown in Fig. 1. On the other hand, it has been widely accepted that when the temperature stays constant, the center wavelength of the FBG will shift due to axial strain; the formula is given as follows: Δλ 1 − P ε ε: λ
(1)
λ represents the original center wavelength, while P ε is the elasto-optical coefficient for typical silica fiber; the general value is 0.22. ε is the strain produced from the outside, usually at the με level. At the same time, the magnetostrictive material will have a deformation of ε by a certain magnetic field, which we assume it to be H. The relationship between ε and H is ε f H:
(2)
f is the function of the two parameters and is determined by different types of magnetic field; we think that if the TbDyFe thin film is sputtered directly in the microgrooves of the cladding of the optical fiber grating, the elongation of the magnetostrictive film due to the magnetic field will result in a wavelength shift of the FBG. Combining formula (1) and formula (2), we can deduce that Δλ 1 − P ε ε 1 − P ε f H: λ
(3)
From Eq. (3), we can finally determine that the value of the magnetic field would be calculated as follows: Δλ : λ1 − P ε
Hf
−1
(4)
It is clear that as long as a shift in the center wavelength is detected, the outside magnetic field can be calculated and determined directly. In this Letter, we propose a 3D microstructure in the cladding of a FBG. Femtosecond laser processing is employed to ablate the microgrooves. The schematic of the microstructure is shown in Fig. 2. In Fig. 2, we can see that a certain thread pitch S is manufactured by a femtosecond laser. We make two kinds of pitches, namely 60 and 80 μm. TbDyFe film is directly coated in the microgrooves as well as on the surface of the cladding, and thus a primary new type of magnetic sensing probe is successfully made. In order to achieve a 3D microstructure in the cladding, a femtosecond laser (IFRIT, Cyber Laser, Tokyo, Japan) was used to fabricate such a microstructure.
Fig. 2. Schematic of the FBG magnetic field sensing probe.
The laser system is based on a 180 fs titanium–sapphire regenerative amplifier system. The fundamental wavelength of the laser is 780 nm. The pulse energy is controlled by the attenuation; 100%, 11%, or 2% of the total energy can be selected by adjusting to mode 1, 2, or 3, respectively. The repeat frequency of the pulse ranges from 1 to 1000 Hz. The maximum pulse energy is 1.08 mJ. In the experiment, pattern 2 was chosen for the appropriate energy acting on the optical fiber. The 3D moving stage has a movement range of 100, 100, and 25 mm (X, Y, and Z directions respectively). All three stages are driven by linear DC motors. The laser beam, which is in Gaussian mode, will finally be focused on the targets by the objective lens (Sigama, Koki, Japan) with a focal length of 60 mm. A high-resolution CCD camera connecting to a computer is also fixed to monitor the real-time process of the fabrication. The schematic of the femtosecond laser is shown in Fig. 3. As shown in Fig. 4, before fabrication, the Bragg grating was written in a standard-single mode fiber (SMF-28) by the phase mask technique with an ultraviolet (UV) excimer laser emitting at a 248 nm wavelength. The length of the FBG L is 10 mm, and the diameter of the FBG core is 9 μm, and when we remove the FBG’s protection layer (namely the polyimide layer, 10 mm in total), the diameter of the cladding is 125 μm. In the experiment, two types of 3D microstructures were ablated in the cladding of the optical fiber, namely single thread and double thread. The thread pitch is controlled by a special rotation fixture and the feed speed of the Y axial direction stage. The rotary speed R was set as 15 rpm, and F is the feed speed, which is decided by a program set in the operation panel. As for double thread, thread pitches of 60 and 80 μm were made. Moreover, the 3D microstructures were made under 18, 20, and 22 mW, respectively. Finally, nine samples were successfully made; we named the single thread ones S-1, S-2, and S-3, with SS-1 to SS-6 for the double thread ones.
Fig. 3.
Schematic of the femtosecond laser.
Fig. 4. Illustration of the fabricated FBG (SMF-28).
April 1, 2014 / Vol. 39, No. 7 / OPTICS LETTERS
Fig. 5.
SEM image of a fabricated single-thread microstructure.
Meanwhile, the original standard optical fiber grating was also used for comparison (O-1). The depth of the spiral microgrooves h can be measured by a scanning electron microscope (SEM). Figure 5 is a sample of a single-thread microstrcuture shown in a SEM image. h varies according to the energy of the laser ablation; all the microstructural samples are listed in Table 1. Finally, the magnetostrictive material TbDyFe was prepared for coating. The coating process was accomplished in a magnetron sputtering machine. We put the nine samples (plus O-1) in the special fixture of the sputtering chamber, the fabricated part (10 mm in length) was exposed for sputtering, and the other parts of the optical fiber were covered with aluminized paper to avoid deposition. During the sputtering process, the rotation fixture was applied to obtain a uniform thin TbDyFe film in the microgrooves as well as on the cladding (10 mm in length), the start power was set at 50 W, and the coating power stabilized at a level of 90 W. As a result, the average speed of sputtering was 0.15 nm∕s. The TbDyFe target and the microstructural optical fiber were kept at a distance of 50 mm. The depth of the target film was detected by an electronic module of the sputtering machine. We noted the thickness of the TbDyFe through the electronic module; it was 6 mm in thickness. The samples of single thread and double thread (coated with TbDyFe) are shown in Fig. 6. Table 1. Parameters for Microstructural Samples Sample O-1 S-1 S-2 S-3 SS-1 SS-2 SS-3 SS-4 SS-5 SS-6
1907
Thread Pitch (μm)
Energy (mW)
Depth (μm)
∞ 60 60 60 60 60 60 80 80 80
0 18 20 22 18 20 22 18 20 22
0 14 16.5 18.5 14 16.5 18.5 14 16.5 18.5
Fig. 6. SEM of microstructure. (a) Double thread and (b) single thread.
Fig. 7.
Schematic of the magnetic field experiment.
Eventually, an experimental system for testing magnetic fields was set up and conducted; the schematic is shown in Fig. 7. The magnetic field was supplied by a permanent magnet instead of an electric spiral coil in order to eliminate the thermal effect. The value of the magnetic field could be measured by a Gaussmeter. It depended on the distance D from the magnetic source (magnetic field range: 0–150 mT) to the sensing probe. The magnet was fixed on a special nonmagnetic fixture. In addition, the demodulator (SM130, Micron Optics) was used to detect the change of the center wavelength of the optical fiber grating; it had a resolution of 1 pm. The magnetic field sensing probe was protected in a glass tube to avoid being broken, and the N-S orientation of the permanent magnets was set as parallel to the fiber axis in order to stay at the uniaxial stress state. Figure 8 shows the relationship between the center wavelength shift and the magnetic field. It shows clearly that the FBG magnetic field sensing probe with double thread of 80 μm pitch has far more notable wavelength shift than the standard one. The sensitivity of sample SS-6 can be calculated as about 1.1 pm∕mT, while the original sample is about 0.2 pm∕mT. At the same time, as the magnetic field force is strengthening to a certain value at about 150 mT, the center wavelength shift of the FBG will increase more and more slowly due to the limitation of the TbDyFe coating elongation. The center wavelength of the FBG magnetic field sensing probe can return to the original center wavelength when the magnetic field is removed. In Fig. 9, it is obvious that energy has a significant influence on the sensitivity according to the magnetic field [see Figs. 9(a) and 9(b), separately]. As the energy becomes greater, the microgrooves get deeper. As a result, the cross-sectional area of the optical fiber cladding tends to become smaller, and thus more magnetostrictive
Fig. 8. Wavelength shift of the microstructural sensing probe (Samples SS-6 and O-1).
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OPTICS LETTERS / Vol. 39, No. 7 / April 1, 2014
Fig. 9. Wavelength shift of double-thread magnetic sensing probe. (a) Thread pitch 80 μm and (b) thread pitch 60 μm.
Fig. 10. Center wavelength shift of the single-thread microstructure responding to the changed magnetic field.
Figure 11 plots all the types of microstructures under the same energy. Double thread is more sensitive to the magnetic field in comparison with single thread, because double thread has a greater axial component and a smaller cross-sectional area. As a whole, the doublethread microstructure is obviously the most effective and suitable microstructure manufactured in the cladding of optical fiber and shows great promise in the magnetic field sensing domain. In conclusion, a different new type of microstructure has been proposed and successfully manufactured in the cladding of a FBG with the appropriate parameters of a femtosecond laser and a special rotation fixture. A 6 μm thickness of supermagnetostrictive material TbDyFe was coated in the cladding of a fiber grating by magnetron sputtering technology to form the sensing probe. Two types of microstructure have been tested in the magnetic field experiment. The results show that a FBG with double thread of 80 μm has the strongest response of 1.1 pm∕mT to the magnetic field, which is followed by the double thread of 60 μm (0.81 pm∕mT) and the single thread (0.7 pm∕mT). These microstructures have greatly improved in magnetic field sensing compared with the original optical fiber. For its convenient fabrication and high sensitivity, this microstructure shows promise for application in the optical fiber sensing field for magnetic fields. This work is financially supported by the Project of National Science Foundation of China (NSFC) (No. 51175393).
Fig. 11. Wavelength shift of different types and pitches of microstructures under the magnetic field.
material (TbDyFe) particles will coat the surface as well as the groove. The magnetostrictive effect, therefore, will be more apparent and the strain ε along the axis will get larger under the same magnetic field. In addition, the double thread of 80 μm is more sensitive to the magnetic field than that of 60 μm. This is due to the different axial component contributing to ε. When the pitch is bigger, the axial component of the strain becomes larger along the optical fiber, so it will produce more deformation along the axis. According to Fig. 10, the magnetic field sensing probe with the single thread also shows priority over the original one in response to the magnetic field. The maximum wavelength shift can be approximately 70 pm under a magnetic field value of 150 mT.
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