A Brief Research on
Optical Fiber-Based Magnetic Field Sensors
By Tang Yiheng
Optical fiber-based magnetic field sensor is a kind of optical fiber sensor, which realizes its
measurements about magnetic field by modulating incident light under the effect of this field and
then demodulating it to get the corresponding parameters of this field. Compared to the traditional
magnetic field sensors, optical fiber-based ones have their strengths on light weight, compact size,
low cost, remote controllability, good security, wide dynamic range, strong adaptability to harsh
environments, and immunity to electromagnetic interference[1, 2].
Since the general insulating material of optical fiber can’t respond to the electric and magnetic
signals, specially designed optical structures and magnetically sensitive materials are necessary to
build the relation between the magnetic field and the light.
First, we focus on the specially desinged optical structures. These fiber configurations can be
classified as three branches: fiber gratings, fiber-based interferometry, and tailored fiber with
evanescent field. The first grating-based ones still have three types, which are fiber Bragg grating
(FBG), long-period fiber grating (LFBG) and tilted fiber Bragg grating (TFBG) (Fig. 1(a)). The
Bragg grating equation(�� = 2���� �, �� represents the refected Bragg wavelength, ���� is the
effective refractive index of the fiber core, � stands for the grating period) with different slight
changes can explain their principles respectively[3]. The magnetic field intensity will affect either
the ���� or the grating period which will lead to a wavelength shift then.
As for the interferometers, they also have several types, which are Mach-Zehnder interferometer
(MZI), Fabry-Perot interferometer (FPI) and Sagnac interferometer (SI) and Michelson
interferometer (MI) (Fig. 1(b))[4]. For this branch, we can observe the interference pattern or
spectra to measure. MZI has two optical paths, one for sensing the other for referencing. If there is
phase variation between the two paths caused by magnetic field, there will be corresponding
interference pattern on the detector. FPI is a multi-beam interferometer which has highly
sensitivity on its cavity changes. With different wavelengths, refractive indexes or cavity lengths
under the influence of magnetic field, it will present different spectra lines. Actually, since MZI, SI
and MI are two-beam interference systems, they almost share the same principle. For instance, a
magnetic field can change the position of the movable mirror in MI, resulting in different optical
paths and different interference patterns.
The third branch, tailored fiber with evanescent field, have three main forms: tapered fibers,
D-shaped fibers, U-shaped fibers(Fig. 1(c)). The concept of this part is to change the confinement
condition in a normal fiber so that the light leaks out will form the evanescent field to contact with
the enviroment. For tapered fibers, the taper waist can’t keep its original confinement and the light
will leak from here. For D-shaped fibers, by removing the upper part of fiber cladding, the light
can leak out. U-shaped fibers don’t need to remove any part of the cladding, with great bending,
the guided modes will unavoidably leak out and become radiation modes[4].
Figure 1. (a) the structures of three kinds of bragg gratings (b) the structures of four types of
interferometers (c) the structures of three forms of fiber that will produce evanescent field[2]
Magnetically sensitive materials are used to convert the change of magnetic field (intesity and
direction) into the change of other physical parameters like refractive index that can be detected
by an optical fiber. These materials can also be divided into three groups: magnetic fluid materials,
magnetic-strictive materials, and magneto-optical materials. For magnetic fluids (MFs), they have
optical abilities such as introducing the change of refractive index, birefringence and Faraday
effect when there is a magnetic field which can make the magnetic particles in the material to form
chains. Magneto-optical materials take advantage of magneto-striction effect which is a feature of
ferromagnets. Different magnitudes of a magnetic field will lead to different lengths of a
ferromagnet which can affect the shape of some optical structure. The third category is
magneto-optical materials that use the magneto-optical Faraday effect, which is also the most
often used ones in fact, rotating the polarization plane of a linearly polarized light during its
propagation. The rotation angle (is given by � = �
�
���
0
= ��� , where L is travelling distance
in the material, B is the magnetic flux density along the direction of propagation, V is a special
constant that strongly relies on the media we choose, environmental temperature and wavelength
of incident light[1]) can be realize by doping rare-earth ions into glasses[5]. Garnet-crystals have
been greatly researched these years.
Obviously, magnetically sensitive materials need appropriate optical structure and vice versa,
otherwise the sensors can’t operate.
Except for the magnetic field sensor, the whole system can be applied as current sensors,
geomagnetic monitoring, and quasi-distributed magnetic sensors. Let’s take current sensors for an
example. The reason why we use this kind of sensor is to monitor the working status of electric
power systems safely, reliably and sensitivly. Based on the Faraday magneto-optical effect, the
sensor determines the current magnitude by the change of current caused by the magnetic field[2].
It is apt to measure great current in trasnmission cable if with protection on insulating. Besides, we
can choose semiconductor laser diodes instead of He-Ne lasers instead to be the light source since
they are lighter, less sensitive to vibration, less needed of high starting voltage. The first
magneto-optical optical fiber current sensor came into being in 1980. There have been recent
researches on minimizing the influence of temperature, improvement of stability and sensitivity
and mitigating residual linear birefringence.
In order to meet higher criteria and more demands, optical fiber-based magnetic field sensors still
have a lot of aspects to optimize its performance. First, when it comes to manufacturing, the
device packaging needs to be solved, not mention to the stability of the whole system. Since this
system contains a lot of elements such as laser, prism, and polarizer, a little displacement of one
element is likely to introduce great deviation of measurement. The best solution is to integrate all
the parts of the whole optical path to fix relative position among elements. Second, low sensitivity
and anti-disturbance issues must be overcome which limit its precision and measuring range to
some extent[2]. The inevitable influence brought by temperature must be dealt with. Proper choice
and design on structure and material can help a lot. Imperfect fabrication process of optical fiber
must be considered as well. In addition, researches on multi-functional and multi-materials fiber
can shed light on its broader prospects on geology exploration, biomedical detection, and
aerospace engineering maybe even on national defense[4].
Reference
[1]
PENG J, JIA S, BIAN J, et al. Recent progress on electromagnetic field measurement based
on optical sensors [J]. Sensors, 2019, 19(13): 2860.
[2]
ZHANG J, WANG C, CHEN Y, et al. Fiber structures and material science in optical fiber
magnetic field sensors [J]. Frontiers of Optoelectronics, 2022, 15(1): 34.
[3]
OTHONOS A. Fiber bragg gratings [J]. Review of scientific instruments, 1997, 68(12):
4309-41.
[4]
ALBERTO N, DOMINGUES M F, MARQUES C, et al. Optical fiber magnetic field sensors
based on magnetic fluid: A review [J]. Sensors, 2018, 18(12): 4325.
[5]
SUN L, JIANG S, ZUEGEL J, et al. Effective Verdet constant in a terbium-doped-core
phosphate fiber [J]. Optics letters, 2009, 34(11): 1699-701.