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Advances in Natural and Applied Sciences, 5(4): 372-378, 2011
ISSN 1995-0772
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ORIGINAL ARTICLE
Parameters Contribute to the Propagation Profile Change
Mohammad Syuhaimi Ab-Rahman
Spectrum Technology Research Group (SPECTECH), Department of Electrical, Electronics and Systems
Engineering, Faculty of Engineering and Built Environment, Institute of Space Science (ANGKASA), Universiti
Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia.
Mohammad Syuhaimi Ab-Rahman: Parameters Contribute to the Propagation Profile Change.
ABSTRACT
Propagation profile is the main reference and useful in designing optical devices such as demultiplexer,
WDM coupler and Directional Coupler. The exact point to split wavelength can be determined sharply by
defining the coupling length. In this paper we proposed a few methods that can be used to shift the propagation
profile. As a result the wavelengths propagate in the waveguide can be multiplexed and split. Parameters such as
coupling gap, number of parallel waveguide, interruption and thickness among the factors that is used to control
the propagation profile.
Key words:
Introduction
Planar waveguides are waveguides with a planar geometry, which guide light only in one dimension. They
are often fabricated in the form of a thin transparent film with increased refractive index on some substrate, or
possibly embedded between two substrate layers (Ab-Rahman and Shaari, 2004). For example, a silicon nitride
can be embedded in silicon dioxide with slightly lower refractive index (see Figure 1). The two layer with
different refractive index is important to agree the Snell Laws and as the result the light can be propagate along
the material. Such active planar waveguides are sometimes used e.g. for optical amplifiers with high gain
(compared with that of bulk amplifiers) and relatively high power (at least multiple watts). There are also planar
waveguide lasers. Some of these devices can be pumped with a proximity-coupled laser diode, not requiring any
pump optics restricting the spatial region in which light can propagate. Usually, a waveguide contains a region
of increased refractive index, compared with the surrounding medium (called cladding). However, guidance is
also possible, e.g., by the use of reflections, e.g. at metallic interfaces. Some waveguides also involve plasmonic
effects at metals. The characterization of the waveguide can be achieved by using prism coupling technique in
which refractive index and thickness can be determined (Ab-Rahman and Zaman, 2009).
Fig. 1: Two different kinds of waveguides. Planar waveguides guide light only in the vertical direction, whereas
channel waveguides guide in two dimensions.
Most waveguides exhibit two-dimensional guidance, thus restricting the extension of guided light in two
dimensions and permitting propagation essentially only in one dimension. An example is the channel waveguide
shown in Figure 1. The most important type of two-dimensional waveguide is the optical fiber. There are also
one-dimensional waveguides, often called planar waveguides.
There are two structure used in designing optical devices based on the waveguide. The first is coupling
length where the energy propagate will be divided according to different splitting ratio and split according to the
Corresponding Author: Mohammad Syuhaimi Ab-Rahman, Spectrum Technology Research Group (SPECTECH),
Department of Electrical, Electronics and Systems Engineering, Faculty of Engineering and Built
Environment, Institute of Space Science (ANGKASA), Universiti Kebangsaan Malaysia, 43600
UKM Bangi, Selangor, Malaysia.
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Adv. in Nat. Appl. Sci., 5(4): 372-378, 2011
wavelengths. Here the propagation profile is important to be analyzed and then determine the exact coupling
length. The second structure is branching the waveguide into number of N. The concept is used to split the
propagate energy equally to N number of output port. These two structure have been used extensively to
develop new optical devices (Ab-Rahman and Shaari, 2001) (Ab-Rahman and Shaari, 2005) (Ab-Rahman et al.,
2009) (Ab-Rahman, 2011) (Ab-Rahman et al., 2011) (Kirankumar, 2011). The other concept is reflecting and
refracting (using mirror) to perform the division and split the wavelength and such device is called optomechanical switching devices (Hu et al., 2011). Couple Mode Theory is the fundamental design of new optical
device development. The basic concept is wavelength separation point which evanescent field plays an
important to distribute the energy among the coupled waveguide and the desire function of device is determined
by coupling length. However the wavelength channel spacing will influence the length of coupling. The beam
propagation method (BPM) has been one of the most popular approaches used in the modeling and simulation of
electromagnetic wave propagation in guided-wave optoelectronic and fiber-optic devices (Neyer et al., 1985).
Propagation Profile Shifting Parameters:
In this paper we proposed a few methods that can be used to shift the propagation profile. As a result the
wavelengths propagate in the waveguide can be multiplexed and split. Parameters such as coupling gap, number
of parallel waveguide, interruption and thickness among the factors that is used to control the propagation
profile. The shifted profile will be used to determine the exact point of coupling to combine or split the signal.
The path of the signal can also be rerouted using this technique.
Coupling Gap:
Figure 2 shows the propagation profile in two parallel waveguide with the coupling gap of 3 µm. Four
CWDM wavelengths are injected to the waveguide and propagate along the both waveguide alternately. From
the Figure2, we can observe the signals closely propagate and no point can be used to split them. To achieve this
proposal we increase the coupling gap to few micrometers. Figure 3 shows the propagation profile for four
different coupling gap from 4 µm until 7 µm. From the graph we can see the signal start to shift when the gap is
set after 6 µm. As a result few points that is suitably used to split the wavelength (see Figure 3).
Fig. 2: Propagation profile in optical waveguide with Air Gap 3 um.
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Adv. in Nat. Appl. Sci., 5(4): 372-378, 2011
Fig. 3: Different in waveguide gap also contribute to different of propagation profile.
Waveguide Interruption:
Figure 4 shows the propagation profile in two coupled waveguide. Two CWDM wavelengths are injected
into the input port and the device is designed to separate each wavelength to different output port. Due to
spacing between two channel is big therefore it is challenging to separate two wavelengths. Therefore some
initiative has to be introduced the enable the propagation step to be interrupted and able to separate the
wavelength eventually (Figure 4 (b). The original propagation profile of two coupled waveguide without any
disturbance is shown in Figure 5. According to the graph, both signal exits at different output but there is
leakage about 20% which also exit together at other different port for wavelength 1550 nm. After observing the
other points as well, there is no suitable point to split the wavelengths. Therefore the interruption has to be
introduced to the waveguide to disturbance the signal propagation in the waveguide.
Fig. 4: (a) Two coupled waveguide with disturbance. (b) The signal propagation in waveguides.
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Adv. in Nat. Appl. Sci., 5(4): 372-378, 2011
Fig. 5: Propagation profile with no waveguide disturbance.
Fig. 6: Propagation profile with waveguide disturbance at 52500 µm.
Fig. 7: Propagation profile with waveguide disturbance at 53000 µm.
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Adv. in Nat. Appl. Sci., 5(4): 372-378, 2011
The interruption is inserted at point 52500 um and we can see the three cycles are existed. This is because
the signal has to couple to times over three coupled waveguide. By observing the profile we able to define the
potential area that can be used to separate these two wavelengths as depicted in Figure 5. When the interrupter
starts to move (approximately 500 um) the value is getting bigger. The area of valley is important to determine
the separate point with effectively to avoid any leakages. This can be shown from Figure 6 and Figure 7.
Waveguide Thickness and Number of Coupling Waveguide:
Four waveguides are coupled together with some parts have been modified. The thickness has been
decreased to a few micrometer to enable the coupling process accelerated. The signal propagates along the
waveguide and coupled out to different output port. Here the thickness plays an important role to shifting the
propagation profile and further be separated. Figure 8 shows CWDM demultiplexer which used couple mode
theory with some modification of waveguide thickness to enable separating wide channel spacing wavelength.
With this technique (by varying the waveguide diameter/thickness), we able to design small and short optical
device even the operating wavelengths have wide channel spacing.
Fig. 8: Art of new CWDM multiplexer based on the thickness variation.
Fig. 9: Propagation profile of CWDM demultiplexer which used the concept of waveguide width variation.
The design is simulated using Opti_BPM product of Optiwave Corporation. Figure 10 and Figure 11 shows
the wavelength separation device when four wavelengths which are 1510 nm, 1550 nm, 1570 nm and 1590 nm
are injected to input port but two graph are only shown in this article.
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Fig. 10: Beam propagation profile when the wavelength 1510 nm dropped.
Fig. 11: Beam propagation profile when the wavelength 1530 nm dropped.
Conclusion:
Propagation profile is the main reference and useful in designing optical devices such as demultiplxer,
WDM coupler and Directional Coupler. The exact point to slit wavelength can be determined sharply by
defining the coupling length. In this paper we proposed a few method that can be used to shift the propagation
profile. As a result the wavelengths propagate in the waveguide can be multiplexed and split. Parameters such as
coupling gap, number of parallel waveguide, interruption and thickness among the factors that is used to control
the propagation profile.
Reference
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Waveguide Using Couple Mode Theory Method-The Programming Highlight. Australian Journals of Basic
Applied Science, 3(3): 2876-2882.
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Adv. in Nat. Appl. Sci., 5(4): 372-378, 2011
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