ANSYS-based birefringence property analysis of side-hole fiber induced by pressure and temperature
ANSYS-Based Birefringence Property Analysis of Side-Hole Fiber Induced by Pressure and Temperature
Xinbang ZHOU 1
Zhenfeng GONG 0
0 College of Physics and Optoelectronics Engineering, Dalian University of Technology , Dalian, 116024 , China
1 School of Ocean Science and Technology, Dalian University of Technology. Panjin , Panjin, 124000 , China
In this paper, we theoretically investigate the influences of pressure and temperature on the birefringence property of side-hole fibers with different shapes of holes using the finite element analysis method. A physical mechanism of the birefringence of the side-hole fiber is discussed with the presence of different external pressures and temperatures. The strain field distribution and birefringence values of circular-core, rectangular-core, and triangular-core side-hole fibers are presented. Our analysis shows the triangular-core side-hole fiber has low temperature sensitivity which weakens the cross sensitivity of temperature and strain. Additionally, an optimized structure design of the side-hole fiber is presented which can be used for the sensing application.
Fiber optics; birefringence; pressure measurement; temperature
1. Introduction
Elasto-optic and thermo-optic effects exist in all
kinds of optical fibers, and both of them affect the
birefringence property of the fiber under different
pressures and temperatures. Nowadays, side-hole
fiber has been widely used to pressure sensing [
1‒6
]
because of its unique property such as high pressure
sensitivity, low temperature sensitivity, and good
temperature compensation capability [7]. However,
there is no analysis about the optimized structure of
the side-hole fibers. Most of these studies are
confined to the side-hole fibers with circular holes,
and the problem of cross sensitivity between the
temperature and strain still exists. In this paper, we
propose two novel side-hole fibers with rectangular
holes and triangular holes. We analyze the
birefringence property of the side-hole fibers under
different pressures and temperatures. Comparing the
influences of the shape, size, and position of the
holes on the birefringence property of these three
kinds of side-hole fibers, we demonstrate that the
side-hole fiber with triangular holes is more suitable
for strain sensing, because the triangular-core
side-hole fiber has a lower temperature sensitivity,
which solves the problem of cross sensitivity of
temperature and strain very well.
2. Principle of birefringence
The birefringence property of the optical fiber
mainly includes two aspects, which are inherent
birefringence and induced birefringence. Inherent
birefringence is formed during the fiber fabrication
process. Once the optical fiber is fabricated, it is
difficult to change the inherent birefringence. The
induced birefringence is caused by the change in
external environment conditions. An anisotropic
stress distribution is induced in the core of the fiber,
which can further generate fiber birefringence via
the photo-elastic effect. As a result, we can use the
induced birefringence of the optical fiber to measure
the external conditions such as strain, temperature,
and refractive index.
nx = nx0 + C1δ x + C2δ y (1)
ny = ny0 + C2δ x + C1δ y (2)
where nx0 and ny0 are the effective refractive indexes
in the x and y directions of the core without ambient
pressure, respectively, C1 and C2 refer to stress
elasto-optic coefficients of the fiber, and δ x and
δ y present the strains of the core in the x and y
directions, respectively. The birefringence of the
fiber is defined as follows [
10
]:
B = nx − ny = (nx0 − ny0 ) + (C1 − C2 )(δ x −δ y ) . (3)
From (3), we know that the relationship between
the birefringence of the fiber and the strain
difference in the x and y directions is linear.
In other words, the birefringence property
analysis of the optical fiber can be converted
into the strain difference analysis of the core of the
fiber.
3. Simulation results of ANSYS
To numerically illustrate the influences of
different pressures and temperatures on the
birefringence property of side-hole fibers, we use
the finite element analysis method [
11
] namely
ANSYS to simulate the strain field distribution over
the cross section of the fiber at a given pressure or
temperature.
3.1 Effect of strain
In this simulation, the side-hole fibers are in the
liquid environment of 20 ℃, and the pressure of
1 × 105 Pa acts uniformly on the fiber surface. The
birefringence properties of circular-hole,
rectangular-hole, and triangular-hole side-hole fibers
are compared. Through this simulation results, we
get the optimal size, shape, and location of the holes
of side-hole fiber, which exhibits a high strain
sensitivity and a low temperature sensitivity.
3.1.1 Circular holes
For the side-hole fiber with circular holes, we
hope to find the suitable radius and position of the
holes in order to realize the largest strain sensitivity.
We define δ as the strain difference betwe (...truncated)