Ultra Highly Efficient 1×3 and 1×6 Splitters for Terahertz Communication Applications

Abstract

In this letter, a novel and original configurations of highly efficient 1×3 and 1×6 beam splitters based on a photonic crystal with wide operating frequency band over the wavelength range, respectively (1.519-1.569) μm and (1.544-1.564) μm for terahertz communication applications are proposed. This is the most important results found in terms of transmission ever seen from these two devices in comparison with the literature. It is found that the efficient total transmissions of ∼99.9% and 96% for the 1 ×3 and 1 ×6 splitters, respectively, with very low insertion losses at output ports are obtained around the wavelength of 1.55 μm. The simulation results confirm that the current designs can provide an efficient splitting over a broad bandwidth of ∼50 nm for 1 ?3 splitter and 20 nm for the 1 ×6 splitter, respectively.

Full text

1434 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 28, NO. 13, JULY 1, 2016
Ultra-Highly Efficient 1 ×3and1×6 Splitters for
Terahertz Communication Applications
Fedaouche Amal, Abri Badaoui Hadjira, and Abri Mehadji
Abstract— In this letter, a novel and original configurations of
highly efficient 1×3and1×6 beam splitters based on a photonic
crystal with wide operating frequency band over the wavelength
range, respectively (1.519–1.569) µm and (1.544–1.564) µmfor
terahertz communication applications are proposed. This is the
most important results found in terms of transmission ever seen
from these two devices in comparison with the literature. It is
found that the efficient total transmissions of ∼99.9% and 96%
for the 1 ×3and1×6 splitters, respectively, with very low
insertion losses at output ports are obtained around the wave-
length of 1.55 µm. The simulation results confirm that the current
designs can provide an efficient splitting over a broad bandwidth
of ∼50 nm for 1 ×3 splitter and 20 nm for the 1 ×6 splitter,
respectively.
Index Terms—Two-dimensional photonic crystals, 2D-FDTD
method, 1 ×3 splitter, 1 ×6 splitter power splitter, optimization.
I. INTRODUCTION
THE control of light at periodic media has attracted
great research importance during the last years. This
interest comes from the ability to create periodic structures
that present a band gaps in their spectrum [1]. Band-gaps
occur in many wave propagation phenomena, including elec-
tromagnetic, acoustic and elastic waves. In the photonic crystal
structures, a periodic variation of the dielectric constant is
similar to the periodic potential of an electron in an atomic
lattice of solid state physics. Due to this analogy, the periodic
dielectric constant of the photon motion in a photonic crystal
leads to the division of the photonic modes into pass and stop
band frequency states. The modes at pass band frequencies
can propagate through the photonic crystal, while modes at
stop band frequencies do not propagate through the photonic
crystal. Here, the stop band, more popularly known as a
photonic band gap PBG which is the most essential property
that determines the practical significance of the photonic
crystal since it allows us to block and confine certain stop
band frequency modes.
Optical power splitter OPS is one of the most cru-
cial components in the field of photonic integrated circuits
PICs [2], [3], where one single input channel is divided into
two or more output channels. The power divider should divide
Manuscript received December 23, 2015; revised March 16, 2016; accepted
April 12, 2016. Date of publication April 20, 2016; date of current version
May 3, 2016.
The authors are with the Telecommunication Department, Faculty of
Technology, University of Tlemcen, Tlemcen 13000, Algeria (e-mail:
fa13000@live.fr; elnbh@yahoo.fr; abrim2002@yahoo.fr).
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.2016.2554103
an input power equally into output ports without significant
losses, and should be compact in size. There are multiple
manners in literature by which the amount of power of
injecting signals can be separated into two, three and four
output paths, for example, using directional coupling [4], [5],
multimode interference MMI [6], [7], multiple line defect
PHC waveguides MLPCW and Y-junction which has been
studied and invented by many researchers with several
configurations [8]–[12].
The 1×2 is the simplest power dividers; currently, there
have been enlarging attentions on designing 1 ×3power
splitters. In this work, we propose an alternative structure
to create a 1 ×6 beam splitter, using a new conception of
1×3 PhCs slab based power divider with low reflection,
high transmission, equal power splitting, and a wide operating
frequency range. This design formed by a Y-branch with
120 degree angle and one single row waveguide connected
in the middle of the junction, where the output channels
have an additional 60 degree bend which is parallel to the
input port [13]. Due to the 120° junction and 60° bend,
single mode operation typically suffers in these junctions that
results a reflection and a large transmission loss, including
the bending losses. To minimize these losses and increase
the performance of this device, an optimization method is
needed to modify the structural distribution at the splitting
region in order to obtain an equal output power transmission
for each output branch; using the technique of the reflection
mirror employed in [9] and [14]. A variety of numerical
methods have been applied to analyze photonic crystal topolo-
gies. The FDTD method has high capability of modeling the
periodic structures [15], [16]. However, the finite difference
time domain FDTD method is a powerful numerical tech-
nique widely used by researchers in various fields providing
accurate results. The performance of the proposed devices
is simulated by the 2D-FDTD for Terahertz communication
applications. The terahertz band gives the access to a diversity
of applications such as spectroscopy, imaging and sensing
which require ultra-high data rates and permit the development
of novel applications in the new nanoscale communication
paradigms.
The contents of this letter are summarized below.
In section II, we describe the design and parameters used for
the proposed 1×3 power splitter and simulation with optimiza-
tion. A mechanism is necessary in this case to equally divide
the output power with the simulation results and discussion.
A comparison with the other dividers is presented.
In section III, simulation results and discussion of
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AMAL et al.: ULTRA-HIGHLY EFFICIENT 1×3AND 1×6SPLITTERS FOR TERAHERTZ COMMUNICATION APPLICATIONS 1435
the 1 ×6 power splitter is provided. Finally, we end with a
conclusion in the last part mentioned in section IV.
II. 1 ×3S
PLITTER DESIGN AND SIMULATION
The device proposed here is composed of a triangular
lattice of air-holes in a dielectric slab InP/GaInAsP/InP. The
implementation of this PC slab is a two-dimensional problem
with an effective refractive index neff =3.24 embedded
in the air. The normalized radius of air holes r=0.36a,
where ais the lattice constant, and an air filling factor of
about 47%. In our simulations, we are interested only in the TE
polarization with its magnetic field only in the z direction Hz,
and the electric field in both xand ydirections Exand Ey.
These parameters allow opening a photonic band gap for the
normalized frequency a/λranging from 0.241 to 0.311, where
λis 1.55 μmcorresponding to the light wavelength.
The basic 1 ×3 power splitter which is realized by
single input photonic crystal waveguide PCW and tree output
channels. This device is composed of an 1 ×2 Y junction
by creating two mono rows directed toward M vector to get
two output ports and a second defect through K direction by
removing one row of air-holes to obtain the third output port,
where each output port form an angle of 60 degrees with each
of them. The dimensions of this structure are: Sx=21 μmin
length, Sy=14 μmin width, the 2-D FDTD mesh size are:
x=y=0.05, and the perfectly matched layers are used
to absorb the outgoing waves.
Let us concentrate in this section on the enhancing of the
transmission and reducing the reflection by optimizing the
1×3 junction, to get a broad bandwidth at the output ports
of the power splitter; the optimization is done by introducing
modifications, where we use two techniques to modify the
reference geometry, the first is to try to bring the expelled
lobes of the curve of the junction to the inside of the turn,
combining a reflecting mirror and cavity, by adding a trench of
air, oriented along the second neighbors, at the junction point
of the bend. The second technique is based on the elimination
of holes in front of the reflective mirrors; defect hole with
radii added in the middle of the junction, and including two
triangles so that the circle already added, is in the middle of the
distance between them, as is shown in the zoom in Fig. 1,
the latter therefore functions as a power reducer for output 2.
The main parameter of the added defect are L1,L2and L
where Ldenotes the length between the centers of the holes in
symmetry, the layout of the triangle is shown in Fig. 1. In our
simulations, we are interested only in the TE polarized light.
We have used 50000 iterations for the FDTD computation.
Fig. 2 represents the output performances of the transmission
and reflection power for chosen optimum parameters with
different wavelengths ranging from 1.50 μmto 1.58 μm.
The graph shows that the power splitter can effectively
operate between wavelengths of 1.519 μmto 1.569 μmwith
an operating bandwidth of about 50 nm which is in the C band
of optical communication spectrum.
On the other hand, we note that there is an efficient distrib-
ution of power in the three output ports to the 1.55 μmwave-
length that is close to the ideal case where the transmission
is 33.8% at output 1, 33.4% at output 2, and 32.7% at
Fig. 1. Design of the optimized 1 ×3 beam splitter excited in TE-polarized
light based on two-dimensional photonic crystal waveguides, with a trian-
gular lattice of air holes radii (r=0.36a)suspended in a dielectric slab
(nef f =3.24). The parameters are set as: Sx=21 μmSy=14 μm
x=y=0.05 μm, and the zoom of the defect in the junction area
where parameters are set as: r1=0.16a,L=3.4641μm,L1=1.56μm,and
L2=1.04 μm.
Fig. 2. Spectral response in transmission and reflection at the output ports
of the optimized 1 ×3 splitter. Power is divided almost equally between the
three branches. The total transmission recorded for the working wavelength
1.55 μmis equal to 99.9%, obtained by 2D-FDTD.
output 3, with a total of 99.9% for different output ports on
the same wavelength, while the reflection loss is as 0.1%,
this reflection due to losses between the wave guiding and
the wave splitting section and bending losses in the corners,
this results seem a good performance. In order to show the
performance of our structure in terms of transmission and
splitting, a comparative study has been made with some works
from literature. The recorded results are done around the
wavelength of interest 1.55 μm, or around the bandwidth.
By comparison with other works from the literature mentioned
in the table I, we can see that a high transmittance and an equal
multi output channels are achieved in our 2D photonic crystal
waveguide 1 ×3 power splitter.

1436 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 28, NO. 13, JULY 1, 2016
TAB L E I
1×3P
OWER SPLITTER TRANS MISSION PERFORMANCES
COMPARISON WITH OTHERS WORKS
Fig. 3. Design of the proposed 1 ×6 beam splitter excited in TE mode.
The PC has a triangular grid array of air holes radii (r=0.36a)etched in
a dielectric medium (nef f =3.24). The PC structure parameters are set as:
Sx=26 μm,Sy=24 μm,x=y=0.05 μmand the layout of the
defect with parameters r3=0.5aand L3=1.879 μm.
III. 1 ×6SPLITTER DESIGN AND SIMULATION
In order to design the 1 ×6 beam splitter structure,
we keep the same topology of the 1 ×3 splitter seen pre-
viously and divide each output in two output ports as inspired
from [9] by using 1×2 splitters S1,S
2and S3. Consequently,
we obtain six output ports for the 1 ×6 splitter. To reduce the
reflection losses, we introduce modifications in the 1 ×6 beam
splitter structure. For the optimization process, we use the
same techniques described previously used for optimizing the
transmission of the 1 ×3 junction to achieve an even splitting.
A top view of the modified structure of the 1×6 beam splitter
is shown in Fig. 3.
Fig. 4 presents respectively the spectral response results
in transmission and reflection obtained by the 2D-FDTD
simulation of the invented 1 ×6 beam splitter in the optical
C-band wavelengths.
The results show that the normalized output transmission is
divided equally on six output channels; it is clearly shown that
the performances of the beam splitter are greatly improved.
Fig. 4. Spectral response in transmission and reflection at the output ports
of the optimized 1 ×6 beam splitter. Power is divided almost equally between
the six branches. The total transmission recorded for 1.55 μmwavelength is
equal to 96%.
TAB L E II
1×2P
OWER SPLITTERS TRANS MISSION PERFORMANCES
TABLE III
1×6P
OWER SPLITTER TRANS MISSION PERFORMANCES
COMPARISON WITH OTHER WORKS
The total transmission recorded at 1.55 μm is equal to 96%,
where the values are approximately 16.3%, 15.9%, 15.9%,
15.8%, 15.8%, and 16.3% for output 1, output 2, output 3,
output 4, output 5, and output 6, respectively. On the
other hand, the beam splitter can operate in the ranges
from 1.544 μm to 1.564 μm with an operating bandwidth
of about 20 nm, and an effective splitting power. It is
demonstrated that the optimized device presents good perfor-
mance with negligible reflection loss over a wide bandwidth.
The 1×2and1×6 splitters performances are summarized
in table II and III.
The same thing, to show the performance of our proposed
1×6 beam splitter, a comparative study has been done with
some work of literature; we can see that a large transmission
with high bandwidth and equal multi output ports are produced
on our design 1 ×6 beam splitter.
The table IV summarizes the insertion loss IL performances
of the 1×3, 1×6 beam splitters. Let us notice that the IL is cal-
culated by the formula: IL =10log(Pout/Pin ). The related ILs
of the 1 ×3 splitter are respectively, 4.71, 4.76 and 4.85 with

AMAL et al.: ULTRA-HIGHLY EFFICIENT 1×3AND 1×6SPLITTERS FOR TERAHERTZ COMMUNICATION APPLICATIONS 1437
TAB L E IV
1×3AND 1×6POWER SPLITTER INS ERTION LOSSES PERFORMANCES
respect to injected power. While the related ILs of the 1 ×6
splitter are, respectively, 7.87, 7.98, 7.98, 8.01, 8.01, and 7.87
for port 1, port 2, port 3, port 4, port 5, and port 6. These
results are significant.
IV. CONCLUSION
In summary, in this letter we have investigated the trans-
mission in the C-band of the optical communication spectrum
for Terahertz communication applications with 1 ×3and
1×6 beam splitters. The first proposed 1 ×3 power splitter
is able to divide the input power equally into three out-
puts at a wide bandwidth; which the range wavelength is
[1.519-1.569] μm, whereas second conception 1 ×6 beam
splitter, transmits more than 15.8% power at each output
channel at target wavelength of 1.55 μm. By optimization
of junctions geometry, our designs demonstrate a highly effi-
ciently for power transmission, where the total transmissions
obtained at the out ports are 99.9% and 96% in the 1 ×3
and 1 ×6 beam splitters, respectively. These devices with
high efficiency and wide bandwidth more than, respectively
50 nm and 20 nm for 1 ×3and1×6 splitters, may
have practical applications in future terahertz communication
applications.
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