DESIGN AND CHARACTERIZATION OF PHOTONIC CRYSTAL FIBER FOR SENSING APPLICATIONS
European Scientific Journal April 2015 edition vol.11
DESIGN AND CHARACTERIZATION OF PHOTONIC CRYSTAL FIBER FOR SENSING APPLICATIONS
Monir Morshed Md. Faizul Huq Arif Sayed Asaduzzaman Kawsar Ahmed 0
0 Department of Information and Communication Technology, MawlanaBhashani Science and Technology University , Santosh, Tangail , Bangladesh
A simple structure of Photonic Crystal Fiber (PCF) for gas sensing and chemical sensing has been proposed in this paper. Index guiding properties of proposed PCF have been numerically investigated by using finite element method (FEM). From the numerical result, it is shown that the relative sensitivity and confinement loss depend on geomatrical parameters and wavelength. The relative sensitivity is increased by a increase of the diameters of central hollow core and innermost ring holes and confinement loss is decreased with a increase of the diameters of outermost cladding holes. By optimize the parmeters, the relative sensitivity is improved to the value of 20.10%. In this case, the confinement loss of the fiber is 1.09×10-3 dB/m.
Confinement loss; Evanescent field; Gas sensor; Photonic crystal fiber; and Relative sensitivity
A new era for faster communication in the modern world has started
with the invention of PCFs. Now-a-days, PCF attracts the reasearchers due to
its flexible properties comparing to the conventional optical fiber to design
the sensors. PCF has been used for many sensing applications, such as gas
sensing (Yu et al. 2008; Olyaee et al. 2013; Olyaee et al. 2014; Dash et al.
2014; Akowuah et al. 2012), chemical sensing (Monro et al. 2010), bio
sensing (Sharan et al. 2014), cancer cell detection (Hossain et al. 2013),
Although, many papers have been reported on PCF based gas
sensors, a great challenge takes place to the researchers for the enhancement
of the sensitivity and reduction of confinement loss of PCF based gas sensors
(Yu et al. 2008). X. Yu et al. at 2008 experimentally investigated a
evanescent field absorption sensor where a Pure-Silica Defected-Core
Photonic Crystal Fiber was used. They obtained the relative sensitivity of
4.79% and in this case confinement loss was 32.4dB/m. But, the sensitivity
and confinement loss of this fiber was not significant. In 2011, Park et al.
introduced a hollow high index ring defect of [X. Yu et al.,2008] that
consists of the central air hole surrounded by a high index GeO2 doped SiO2
glass ring to improve sensing capability. The paper shown that the sensitivity
increases 10% amend proportionally by some attributes like decrease the
distance between center of two central cores, the increase of central core
diameter and increase of diameters of air holes of two outer most layers and
decreases the confinement loss 6×10-4 dB/m linearly. In 2013, D. Liangcheng
et al. reported that the relative sensitivity of a gas sensor increases with
respect to the increase of the diameters of the hollow core. In 2014, Olayee et
al proposed a modified structure of [J. Park et al., 2011] to get the better
relative sensitivity and confinement loss and also proposed a new structure
with hexagonal holes in the inner ring instead of circular ring to significantly
increase the sensitivity.
On the other hand, J. S. Chiang and T. L. Wu (2006) observed the
propagation characteristics of an octagonal photonic crystal fiber. They
showed that the confinement loss of O-PCF is lower than the H-PCF.
Comparing to conventional PCFs, Octagonal PCFs provided some major
attractive feature like bluer confinement loss, petty effective area and
wideband single mode operation (X. Yu et al. ,2008).
According to the overall discussion, to get the higher relative
sensitivity and lower confinement loss, a simple modified index guided
hollow core PCF for gas sensing and chemical sensing has been proposed
which consist three rings of air holes with three different formations. The
inner most air holes ring of the cladding are kept hexagonal, the middle air
holes ring and the outer most ring are made octagonal and decagonal,
respectively. Here, complexity of the structure is reduced by remove the
doping in the core and by decrease the number of air holes in the cladding
compare to prior PCFs (S. Olyaee, 2014; J. Park, 2011).
Geometries of the Proposed PCF:
Fig 1. Cross-sectional view of proposed structure
Fig. 1 shows the cross sectional view of the proposed M-PCF
(modified photonic crystal fiber). It consist of circular air holes which has
been arranged in three rings on the silica background. The inner most ring of
the air holes is hexagonal ,containing 6 air holes with the diameter d1. The
second ring is octagonal, containing 16 air holes with the diameter d2. The
third ring is decagonal, which comprises of 30 air holes with the diameter d3.
There is a central hollow core which has also been filled with air, and
diameter is dc. The air holes are arranged with two pitch values which have
been denoted as Ʌ1 and Ʌ2 in the Fig.1.
Numerical Method Analysis
Finite-element method [FEM] with perfectly matched boundary
layers (PML) was utilized to solve the Maxwell’s equations because of its
reliability (Saitoh et al.2002) and also investigated the optical properties of
the PCF and measured confinement loss and sensitivity, as the goal of
finding low confinement loss with high sensitivity.
Confinement loss: A small portion of power leakage can not avoid
due to finite number of air holes in the cladding when light energy passes
through a photonic crystal fiber. Confinement loss is the leaking of light
from core to exterior matrix material. Confinement loss can be varied
according to the number of layers, number of air holes, air hole diameter and
the pitch. The confinement loss (dB/m) Lc has been defined by equation 1.
(K. Kaneshima et al.,2006):
Lc=8.68k0Im[nef f] (1)
where, k0=2π/λ; λ is the light wave length and Im[nef f]is imaginary part of
the refractive index.
Sensitivity: From the Lambert Beers Law the relationship between gas
concentration and optical intensity can be expressed in equation (2)
where IT(ω) and I0(ω) are the intensity of light before the absorbed energy
and after the absorbed energy from light, L is the path length, C is the gas
concentration and r is a relative sensitivity respectively.
The relative sensitivity can be calculated using equation (3)
r = Re[nnreff ] f (3)
where, Re[nef f]= real part of the effective refractive index of the guided
mode; nr = refractive index of absorbing material and f is called the ratio of
the air hole power and the total power where f is also calculated as optical
power distribution function.
f = ∫sample Re ExHy−EyHx dx dy (4)
∫to tal Re ExHy−EyHx dx dy
where, Ex , Ey and Hx , Hy are the transverse electric fields and magnetic
fields of the mode respectively. Increment the value of f will show the better
sensitivity of the PCF. The mode field pattern Ex , Ey and Hx , Hy can be
generated by the COMSOL Multiphysics 4.2 utilizing finite-element method
(FEM) (Hoo et al.2003).
Result and Discussion
In this section, the dependance of the relative sensitivity and
confinement loss on wavelength, hollow core, diameter of holes in first ring,
diameter of holes in outer ring have been analyzed.
The variation of different hollow core diameters of dc=3 µm, dc=3.10
µm and dc=3.20 causes linearly decrement of refractive index with the
wavelength which have been shown in Fig. 2(b) .
Fig. 3(a) shows the impact of changing of hollow core diameter on
relative sensitivity. The relative sensitivity increases as a function of
wavelength according to the increasing of the core diameter which has been
shown in the figure when dc=3.00 µm, dc=3.10 µm and dc=3.20 µm. Fig. 3
(b) illustrates the impact of changing of hollow core diameter on
confinement loss. The behavior of changing of confinement loss is not
proper. From the Fig. 3(b) it is clear that confinement loss increases
according to the increasing of the core diameter. For dc=3.00 µm the
confinement loss is higher than the other diameters and for dc=3.10 µm the
confinement loss is lower.
Fig 3. (a) Relative sensitivity versus wavelength (b)Confinement loss versus wavelength for
different hollow core diameter dc=3 µm, dc=3.10 µm and dc=3.20 µm
Fig 4. (a) Relative sensitivity versus wavelength (b) Confinement loss versus wavelengthfor
different Inner Ring diameter d1=3.4 µm, and d1=3.2 µm.
Fig. 4(a) shows the effect of changing the diameter of holes in inner
ring on relative sensitivity. The relative sensitivity increases according to the
increment of the diameter of air holes in inner ring which has been shown in
the figure when d1=3.4 µm, d1=3.2 µm. The relative sensitivity increases
linearly with the wavelength and for d1=3.4 µm the relative sensitivity is
higher. From the figure Fig.4(b) it is clear that Confinement loss increases
with respect to wavelength. For d1=3.2 µm confinement loss is lower than
d1=3.4 µm. For d1=3.4 µm the increasing of confinement loss is sharp for the
wavelength from 1.1 µm to 1.8 µm.
Fig 5. (a) Relative sensitivity versus wavelength (b) Confinement loss versus wavelength for
different outer Ring diameter d3=2.2 µm, and d3=2.1 µm.
Fig.5(a) illustrates the influence of changing of the diameter of holes
in outer ring on relative sensitivity with respect to wavelength. The relative
sensitivity increases according to the increment of the diameter of holes
which has been shown in the figure. The relative sensitivity increases
linearly with the wavelength. And from Fig. 5(b) it is clear that the
confinement loss increases gradually with the different outer ring diameters.
For the diameter of holes in outer ring air d3=2.2 µm, confinement loss is
lower and almost sharply increases with the wavelength.
Fig 6. Comparison of Relative Sensitivity among proposed PCF and Prior PCFs.
The comparison of relative sensitivity among the proposed PCF and
the prior PCFs (S. Olyaee, 2014; J. Park, 2011) have been shown in Fig. 6
which represents that the proposed M-PCF shows higher sensitivity than the
prior PCFs from wavelength 1.1 µm to 2 µm.
The proposed simple M-PCF has been investigated by two guiding
properties ( the relative sensitivity and the confinement loss). It improves the
relative sensitivity comparing with the prior structure. It shows that the
relative sensitivity increases to the value of 20.10% at the wavelength of 1.33
µm and the confinement loss of this fiber is 1.09×10-3 dB/m which is much
better than the early proposed structures. In this structure, there has not been
used any doping. As it’s structural simplicity, it would be easy to fabricate.
Therefore, we could warrant that it can be used for both the gas and the
chemical sensing applications.
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