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Designing planar waveguide optical looper based on couple mode theory--coupling length determination.

Introduction

Fiber optic is a technology that uses glass or plastic fiber to transmit data. A fiber optic cable consists of a bundle of glass threads, each of which is capable of transmitting messages modulated onto light waves. Fiber optics has several advantages over traditional metal communications lines. It can carry more data and less susceptible than metal cables to interference. The main disadvantage is that the cables are expensive to install. Fiber-optic communication is a method of transmitting information from one place to another by sending pulses of light through an optical fiber. The light forms an electromagnetic carrier wave that is modulated to carry information Applications of fiber optics include: telecommunications, Local Area Networks (LANs) and Wide Area Networks (WANs), factory automation and premises wiring (Intelligent Telecommunication System. 2006).

An optical waveguide is a physical structure that guides electromagnetic waves in the optical spectrum. Waveguides are basic components used for distributing electromagnetic wave power from one point to many points. The waves can be guided by using mechanism of wave reflection in the transverse plane which is normal to the direction of propagation. Optical waveguides can be classified according to their geometry such as planar, strip, or fiber waveguides, mode structure such as single-mode or multi-mode, refractive index distribution such as step or gradient index and material such as glass, polymer or semiconductor (Syuhaimi Ab. Rahman, 2004; Syuhaimi Ab. Rahman, 2001; Syuhaimi Ab. Rahman, 2005).

Coupled Mode Theory (CMT) is a theory that uses to describe the propagations of light in some waveguide or optical cavities under the influence of additional effect. The basic idea of CMT is to decompose all propagating light into the known modes of the undisturbed device, and then to calculate how these modes are coupled with each other by some additional influence. The complex amplitude of an arbitrary optical field in the dielectric waveguide is generally a superposition of these modes:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.]

Where, [a.sub.m] = the amplitude of the mode m;

[u.sub.m] (y) = the transverse distribution of the mode m;

[[beta].sub.m] = the propagation constant of the mode m.

Optical Device Design:

To design the new optical device, couple mode theory is used as the basic waveguide. In this design, we want to build optical looper and develop in optical filter. The optical looper is a type of wavelength-selective elements that can be used to synthesize a wide class of filter functions. We can use this kind of design as to select the certain wavelength and used the concept of filter functions. The design is slightly different with the optical resonator that has been mentioned in (Tadashi Miyashita, 1990) and (Kirankumar R. Hiremath, 2011).

The filter functions such as high pass and low pass allow their own frequency and attenuate frequency that not needed. High pass filter is a filter that passes high frequencies but attenuates or reduces the amplitude of frequencies lower than the cutoff frequency of the filter.

For the low pass filter, it passes low-frequency signals but attenuates signals with frequencies higher than the cutoff frequency. So, we use the range of certain selective wavelength as to work the filter functions. The figure 1 and figure 2 below show the range of the wavelength allowed for high pass filter and low filter respectively.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

The input signal is in form of wavelength, [lambda]. The designed device is optical filter that use ring waveguide. The ring resonator device is made up of two identical couplers, two half-circle connectors, and three terminals. The terminals are consists of one transceiver ([Tx.sub.1]) and two receiver ([Rx.sub.1] and [Rx.sub.2]) station. The structural diagram of the device is just below (Tadashi Miyashita, 1990).

[FIGURE 3 OMITTED]

As shown in Figure 3, multiple wavelengths entered into Tx 1 will be partially coupled into the straight coupler waveguide through Coupler 1. The range of wavelength, [[lambda].sub.1], [[lambda].sub.22], [[lambda].sub.3] are entered into fiber optic 1 and pass through the Coupler 1 and the output wavelength of [[lambda].sub.1], [[lambda].sub.2] will dropped at the Rx1.

For [[lambda].sub.3], it will pass through the ring and partially coupled into the Coupler 2 and pass through fiber optic 2. As a result, [[lambda].sub.3] will be dropped at Rx2.

So, we consider wavelength [[lambda].sub.3] satisfies the resonant condition which is

[n.sub.eff] L = [[lambda].sub.3]

where [n.sub.eff] is effective index of the bending waveguide and L is the length of the ring.

The resonant condition states that wavelength [[lambda].sub.1], [[lambda].sub.2], [[lambda].sub.3] are used as control signal that changes the free carrier density of semiconductor resonant waveguide thus changing its refractive index. The changes of the waveguide refractive index will lead the resonator wavelength shift. Then, wavelength [[lambda].sub.3] that passed through the ring resonator will propagate along the waveguide and partially coupled into Coupler 2. The output will drop at Rx2 (Kirankumar R. Hiremath, 2011).

[FIGURE 4 OMITTED]

This optical filter usually uses in various types of filter applications such as high pass and low pass filter. High-pass and low-pass filters can be used in digital image processing technology as to perform image modifications, enhancements and noise reduction. As we know, digital image processing is the use of computer algorithms to perform image processing on digital images. It allows a wider range of algorithms to be applied to the input data and can avoid problems such as the build-up of noise and signal distortion during processing.

The Art of Design--Determination of Coupling Length:

Coupling Length is defined as the active area where the signals are alternately migrate from one waveguide to another waveguide. This can be achieved by designing a fundamental of coupling waveguide architecture as shown in Figure 5. Then the coupling length will determine according to the coupling profile that is generated automatically from the BPM software used. The detail of exact coupling length selection is shown in Figure 6.

Four wavelengths of CWDM are injected to the input port. Due to the evanescent field and two very closed waveguide the signal will be migrate to another waveguide alternately. Due to different of wavelength used, propagation constant and wave number has make the signal oscillation and distance of coupling has become different and finally the signal can be separated (Figure 7). In this case wavelength of 1510 nm and 1530 nm together exit at one output port while 1550 nm and 1570 nm exit at another output port. Thus the selection point of coupling length has been determined successfully.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

Conclusion:

We have designed an waveguide based optical looper by using optical looper and linear fiber waveguide combination. Coupled Mode Theory has been applied in this design to describe the propagations of light in some waveguide or optical cavities under the influence of additional effect. This design we do for application optical filter. Usage of optical filter is to transmit light having certain properties, while blocking the remainder wavelength.

References

Intelligent Telecommunication System. 2006. Fiber Optic Network Designing http://itsoptic.com/website/services-&-solutions/fibre-optic-network- designing.htm

Kirankumar R. Hiremath, Jens Niegemann and Kurt Busch. 2011. Analysis of light propagation in slotted resonator based systems via coupled-mode theory. Optics express, 19(9): 8641-8655.

Mohd Syuhaimi Bin Ab. Rahman and Sahbudin Shaari, 2001. Design and Characteristics of Wavelength Demultiplexer Based on Tilted in Glass Waveguide, Proceedings 2001 IEEE National Symposium on Microelectronics, pp. 370-374, 2001, pub. IEEE Malaysia Section.

Mohd Syuhaimi Bin Ab. Rahman and Sahbudin Shaari, 2005, Modeling of New Structure of CWDM Waveguide Based Multiplexer and Demultiplexer, Proceeding 2005 IEEE National Symposium on Microelectronics, pp. 361-366, 2005, pub. IEEE Malaysia Section.

Syuhaimi Ab. Rahman, M. and Sahbudin Shaari, 2004, Modeling of Planar Lightwave Circuit OADM for CWDM, Proceeding 2004 Postgraduate Conference, 1.

Tadashi Miyashita, 1990. Planar Waveguide Devices on Silicon. Electronic Components and Technology Conference, 1990. Proceedings., 40th. DOI 10.1109/ECTC.1990.122167, 1: 55-59.

Corresponding Author: Mohammad Syuhaimi Ab-Rahman, Spectrum Technology Research Group (SPECTECH) Department of Electrical, Electronics and Systems Engineering Faculty of Engineering and Built Environment 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 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
Article Type:Technical report
Geographic Code:9MALA
Date:Apr 1, 2011
Words:1409
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