IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 27, NO. 16, AUGUST 15, 2015 1717
Femtosecond Laser Ablated FBG Multitrenches
for Magnetic Field Sensor Application
Joseph Muna Karanja, Yutang Dai, Xian Zhou, Bin Liu, Minghong Yang, and Jixiang Dai
Abstract—Three-dimensional multitrench microstructures,
femtosecond laser ablated in fiber Bragg grating (FBG) cladding,
TbDyFe sputtered is proposed and demonstrated for magnetic
field sensing probe. Parameters such as the number of straight
microtrenches, translation speed (feed rate), and laser pulse
power of laser beam have been systematically varied and opti-
mized. A 5-µ m-thick giant Terfenol-D magnetostrictive film is
sputtered on to FBG microtrenches, and acts as magnetic sensing
transducer. Eight microtrench samples produced the highest
central wavelength shift of 120 pm, nearly fivefold more sensitive
compared with nonmicrostructured standard FBG. An increase
in laser pulse power to 20 mW generated the magnetic sensitivity
of 0.58 pm/mT. Interestingly, reduction in translational speed
contributed dramatically to the rise in the magnetic sensitivity
of the samples. These sensor samples show magnetic response
reversibility and have great potential in the magnetic field sensing
domain.
Index Terms— Optical fiber transducers, magnetic field
measurement, laser ablation, micromachining, thin films.
I. INTRODUCTION
R
ECENTLY, optical fiber magnetic field sensors have
been widely studied owing to diverse applications in
medicine, electric power and military. Fiber optical sensors
offers the best alternative choice due to its miniaturized
size, multiplexing capabilities, low cost, compact spatial
requirements, electromagnetic radiation interference immunity
and thus can be used remotely in many hostile environment as
compared to conventional electrical sensors [1]–[3]. Despite
the fact that FBG sensors have inherently good physical prop-
erties for strain, pressure and temperature, its magneto-optical
Faraday effect is too low for practical applications in magnetic
sensing [4]. Thus the idea to integrate a magnetostrictive
material with optical fiber sensors to improve weak magnetic
fields has become attractive research area [5], [6].
A giant TbDyFe (Terfenol-D) thin films which have
low-field magnetostriction with compensated anisotropy
has exhibited large magnetostrictive strains and wireless
driving power at low magnetic fields and is one of the most
promising magnetostrictive materials [7], [8]. When this giant
Manuscript received April 17, 2015; revised May 15, 2015; accepted
May 24, 2015. Date of publication June 1, 2015; date of current version
July 10, 2015. This work was supported in part by the National Natural Sci-
ence Foundation of China under Project 51175393 and Project 61475121 and
in part by the Natural Science Foundation of Hubei Provincial Government
under Project 2014CFB260.
The authors are with the National Engineering Laboratory for Fiber Optic
Sensing Technology, Wuhan University of Technology, Wuhan 430070,
China (e-mail: kamuna2007@yahoo.com; daiyt6688@whut.edu.cn;
chinazhoul@163.com; lb199108@126.com; minghong.yang@whut.edu.cn;
djx409081947@163.com).
Color versions of one or more of the figures in this letter are available
online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LPT.2015.2438228
TbDyFe magnetostrictive film is coated on to a FBG cladding,
its linear elongation due to magnetic field exposure will cause
fiber to undergo variation in its grating period. In turn, change
in grating period will generate a shift of FBG central wave-
length which can be correlated to the magnetic field response.
Optical fiber sensors sensitivity can be enhanced by
enabling interaction between guided lights and sensing
material. This can be achieved by partially removing the fiber
cladding diameter by polishing to form a D-shape fiber in
order expose evanescent fields to the external materials [9].
Removal of the fiber cladding through HF chemical etching
have been demonstrated [1], [10]–[12]. However, the
misgivings associated with this technique lies in numerous
safety and health issues, besides producing mechanically weak
fiber with limited practical applications [13]. Femtosecond
laser ablation possessed with short pulse, high peak power and
precise micromachining is a promising efficient fabrication
technique for industrial applications of hard and brittle
materials. Additionally, because of its non-contact nature, it
allows micromachining and surface patterning of materials
with minimal mechanical and thermal deformation [14].
In this letter, we propose the idea of depositing
magnetostrictive TbDyFe thin films on 3D multi-trenches
fiber microstructures manufactured by a femtosecond laser
on FBG cladding.
II. P
RINCIPLE
Fiber Bragg grating (FBG) is a permanent or semi-
permanent regular perturbation of the refractive index along
the fiber length which is formed when an intense optical
light is subjected to the fiber core. This strong radiation
subsequently forms a very narrow band called Bragg grating.
This narrow band is reflected back at only specific wavelength
called Bragg wavelength.
FBG sensor strain can be as a result of the following
underlying principles. Firstly, the physical expansion of the
FBG resulting from thermal effect, presence of a sensitive
coating that reacts with the surrounding environment leading
to strain and ultimately corresponding change in the grating
pitch. Secondly, change in fiber refractive index due to photo
elastic effects.
When a FBG is subjected to an applied strain (ε) at
constant temperature, both effective refractive index and
periodic change, results to wavelength shift.
λ
λ
=
(
1 − P
ε
)
ε (1)
Where λ is the original central wavelength, P
ε
is the
elasto-optical coefficient for typical silica fiber ε is the
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