Parameters contribute to the propagation profile change.
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).
[FIGURE 1 OMITTED]
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 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.
Figure 2 shows the propagation profile in two parallel waveguide with the coupling gap of 3 u.m. 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 u.m until 7 urn. From the graph we can see the signal start to shift when the gap is set after 6 urn. As a result few points that is suitably used to split the wavelength (see Figure 3).
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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.
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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.
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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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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.
Ab-Rahman, M.S. and H.F.A. Wahab, 2009. New Design of 1x3 Wavelength Demultiplexer Based on Tilted Grating in Glass Waveguide for First Window Operating Wavelength. Australian Journals of Basic Applied Science, 3(3): 2607-2613.
Ab-Rahman, M.S. and M.H.M. Zaman, 2009. The Measurement of Refractive Index and Thickness of Planar Waveguide Using Couple Mode Theory Method-The Programming Highlight. Australian Journals of Basic Applied Science, 3(3): 2876-2882.
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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.
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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|Title Annotation:||Original Article|
|Author:||Ab-Rahman, Mohammad Syuhaimi|
|Publication:||Advances in Natural and Applied Sciences|
|Date:||Nov 1, 2011|
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