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Enhancement the service flexibility in passive optical network through 2x3 optical moderator based on planar waveguide device.


Fiber-to-the-home (FTTH) network is a network technology which uses fiber optic as a transmission medium to transmit Triple-play (data, voice and video) services. By using fiber optic, which has a very high broadband, FTTH network is capable to transmit data with higher capacity compare to the technology based on cuprum cable (Keiser 2000). FTTH network plays an important role in reducing and solving the final access Bottleneck problem in broadband access network specifically in future optical access network (Yeh 2005). Nowdays, FTTH network is recognized as the latest solution for various type of communication and multimedia services including telephone, high speed internet access, digital cable television (CATV) and video (Lee 2006). According to Hutcheson (2008), the number of FTTH user in the world is almost 40 thousand in the year 2008 and the number is said to be increased to 108 thousand by the year 2012. FTTH network which is based on passive optical network (PON) comprises of 3 main components which are optical line terminal (OLT), optical splitter and optical network unit (ONU). Optical splitter determines the size of network user it covers. Although FTTH-PON is an ideal network, a lot of new fiber has the potential to be developed. This paper discussed one of the latest devices which allows two services from two different network to be combined and benefit the users. In the application of the FTTH, optical moderator or the 2x3 optical moderator is designed to solve the problem when different data transmission is required in the same house which in turn requires combination of transmission power from a different bandwidth. Besides that, this optical moderator is developed to solve the problem when the house uses transmission power higher than the other houses, which the power is taken from other transmission power that is combined to obtain enough power.


One analysis approach which beam propagation method (BPM) in used solving scalar wave equation for the transverse electric (TE) and transverse magnetic (TM) polarization. Beam propagation method can be used to measure the amplitude of electric field inside and outside of diffraction structure influenced by an arbitrary input field. Beam propagation method is a step-by-step method to simulate the path of a light beam through a certain waveguide medium (Feit et al. 1978 and Song et al., 2003).

Waveguide is one of the important components to develop an integrated optical device. Y branch design has a simple structure and can be fabricated so that the device is packed with a stable divergence ratio using a planar light wave circuit (PLC) (Lin et al., 1999).

By taking into consideration the loss ability of optical material which is the diffusion loss, scattering loss and radiant loss, the suitable polymer is produce to obtain low losses characteristic (Daum 2002). The advantage of using polymer is to produce integrated optical circuit including reasonable fabrication cost based on the technology used in the microelectronics industry. Besides, polymers have a combination of non-linear electro-optic, photosensitive and thermo-optic characteristic (Norazan et al., 2004).

SU-8 polymer is used as a waveguide material to produce optical switch device. SU-8 based on the Epoxy structures, strengthen the resistance system chemically with sensitive characteristic and High Aspect Ratio. SU8 is used for the first time in the micro-electromechanical-system (MEMS) and microsystem structure which needs the non-conductive characteristic.

In this research, Y branch optical moderator was first design to obtain the desired result using the beam propagation method. The research was done on a symmetrical Y branch optical moderator to acquire an output power that is symmetry for 2x3 optical moderator. After the desired result is obtained, the research is preceded by analyzing the device to obtain fixed parameter value. This optical moderator is design using the commercialized SU-8 material. Parameters such as refraction index, length and width of the waveguide as well as the wide-angle Y-branch have to be analyzed using the BPM-CAD simulation for the purpose of getting the optimal design.

2. Proposed Application:

Various Applications to Premises:

In the FTTH-PON network, signal towards the user is normally sharing the same wavelength which is 1480nm (voice & data signal) and 1550nm (video signal). If the application is widen through the increase of broadband and also additional new wavelength, this will cause the system to be complex. Hence, the easiest way is to distribute each Optical Line Terminal (OLT) to coordinate the different wavelength. The user application is represented by certain wavelength function and for the purpose of commercialization, not all premises will subscribe to the entire service offered. Hence, this optical moderator is managing wavelength as signal facilitator where the OLT which have various different (model) application and can be enjoyed by the subscriber even if outside of the coverage area.

Figure 2 illustrates the function of optical moderator in the FTTH-PON network. OLT A provides different services from OLT B and the position of both of the OLT is far apart. Premises in area A will receive all of the services offer by OLT A and same goes with premises in area B which will only receive the services offers by OLT B. If there is another area say area C that will need the services offers by both OLT, it can be satisfied by the facilitator that is connected before the optical splitter. With the facilitator present, both services can be offered to area that requires it. The construction concept of this moderator is easy but its application in the optical network is broad. It is the component in the optical network that enable both signals in different network to be combined as well as maintaining the other signal in their original passageway (both OLT operates at different wavelength).


Collision Notification/Detection:

We have proposed, for the first time, 2x3 optical moderator to be used code-sense multiple access / collision detection to implement Ethernet (or packet) based services on wide area passive optical networks. A collision detection unit can be easily implemented by employing the reflectance of the customer transmitted signal to ONU with the assisting of 1x3 passive splitter. The passive splitter is modularly upgraded with directional coupling to reflect optical power to all fibers when the number of user/region increased. The received signal is been analyzed by each ONU and the presence of optical power (mixed signals) is sufficient to notify that a collision has taken place. The reflectivity can be achieved by combining the two ports to enable the signal be 'U' turn to the output port. The collision detection proposed in this application is purposely recommended for wide distribution area in PON. For example a single OLT can be connected to two distribution area. The configuration proposed is the first collision detection can be implemented after 3xN optical splitter as the collision detection for the specific distribution area. The application of 2x3 Optical Moderator as a collision detector in PON network is shown in Figure 3.


Optical Add and Drop Multiplexer (OADM) Device:

2x3 Optical Moderator is also been used as Optical Cross Add and Drop Multiplexer (OADM) when it is cascaded with fiber Bragg grating. Two type of grating used to reflected single wavelength and multi wavelength respectively. The signal enters the Port 3 and be reflected to Port 4. The dropped wavelength will be passed through and new injected signal will be entering at Port 5 and send out to Port 4 after be reflected by FBG at Port 2.


3. Device Design:

The design of 2x3 optical moderator have two input signal and divided into three output where at the second output, 50% of each input signal is combined. This moderator is used for two different input signals in term of wavelength. The usage of 2x3 optical moderator solve the problem to divide different input power where the network do not need to use different moderator in dividing different power. This shall reduce the cost by diminishing the use of two different moderator and replace it with one moderator which is able to combine the signal power. This moderator with the ability to combine power at both outputs also can be used to divide more power to one location compare to the other location.

The optical waveguide design depends on the characteristic of light signal propagation, waveguide mode, coupling mode, losses and gain. Because the waveguide modeling involved a lot of design parameter, the simulation technique is convenient and speeds up the design process and characterization. The user only needs to make sure that the design parameters and the computer software will take up the data to perform measurement and simulation. There are two entries for the parameter; first is for determining the geometry of waveguide, fabrication material and material constant. The second is for the measurement and simulation parameter (Anon 2000).

The 2x3 optical moderator is based on symmetric Y splitter that divide the output power equally which is 50% for each arm. Two inputs for this device is refer as 11 and ^ The 2x3 optical moderator divide 50% 11 power on the first arm, 50% 12 on the third arm and combine the input power 50% X1 and 50% 12 on arm two. Figure 5 shows the design layout of 2x3 optical moderator.

The 6Lim width waveguide is used in designing the optical-splitter where this width is the optimum waveguide width for commercialized SU-8 material. The length of the waveguide is also being emphasized so that it can be applied practically and obtained the most optimum value in terms of output power. Input Plane field that is used is Gaussian shaped and the design is simulated for TE and TM mode output power. Index of refraction for SU-8 material that is used is 1.599 with the thickness of 6Lim using the rotation coating technique with the speed of 4000rpm (Azrulnizam 2005).

The 2x3 optical moderator is simulated in four windows display which is the optical field window, effective index of refraction, Cut View and also output power. Both devices is being analyze on the effect of the design parameters such as the width of waveguide, the change of refraction index, wide-angle branching and main wavelength which is 1310 nm (Upstream), 1480 nm (Downstream), 1550 nm (Video) and 1625 nm (Test). The result of simulation from is analyzed from output power on TE and TM polarization point of view on each devices output terminal.


Result and discussion

The analysis of this research focuses on the effect of the studied parameter towards the output power. Figure 6 illustrates the propagation power along the 2x3 moderator waveguide. Optical field for the 2x3 moderator at the end of waveguide propagation is as shown in Figure 7.

The measured output power of each arm along the waveguide propagation of 2x3 optical moderato is illustrates in Figure 8. Discussed below is the simulation result from output power on TE and TM polarization point of view on each device output terminal. From the simulation result conducted using BPM_CAD, it was found that output power on each arm are 23 % (Output terminal 1), 45 % (Output terminal 2) and 23 % (Output terminal 3). The calculated total output power is 91 %. Then path loss, L is 0.36 dB. Insertion loss for 2x3 optical moderator is 6.29 dB (Output terminal 1), 3.47 dB (Output terminal 2) and 6.29 dB (Output terminal 3). Figure 9 and Figure 10 shows the percentage of output power and insertion loss measured at three output port respectively.




Most of the power loss happens during the waveguide bends, hence wide-angle branching waveguide needs to be designed as small as possible to avoid power loss. The wide-angle branching waveguide depends on the L value where L is the length of bend and M is width of the bend. Higher L value indicates smaller bend radius which ultimately means decrease in power loss. When the M value is increased, the wide-angle branching increases and causes the loss of power during the propagation mode. The design can't be too long because power loss can also occur during propagation in linear waveguide.



Output power of the waveguide was studied by changing the width of the waveguide, and the width used for this study is between 2 L m until 8 L m. From Figure 11, it was found that the maximum output power is at 5 L m and the minimum output power is at 2 L m width. The power loss increases with the increase of waveguide width but not at a great deal because the range is only 0.06 (equivalent to 6%). The loss of output power also decreases if the width of waveguide is small, this is due to Gaussian light that propagate in the waveguide contain Evanescent mode which is Gaussian tail mode that is out of the waveguide. This contributes to the loss of output power along the waveguide propagation.

The power loss increases with the increase of difference in relative refractive index. The different of relative refractive index can be calculated using the following formula:

[DELTA] = ([n.sub.1.sup.2] - [n.sub.2.sup.2]) / 2 [n.sub.1.sup.2] (1)

where [n.sub.1] is the core refractive index and n2 is the protective refractive index. The value of protective refractive index which is the substrate index of refraction is set to 1.522 where as the value of interval for the core refractive index was set to be 0.004.

From Figure 12, it can be seen that the output power decreases and power loss increases when the difference in relative refractive index increases. This exactly corresponds to the theory. There is huge difference between output power of TE and TM at 0.0424 refractive index difference.

The effect of change in arc-shaped waveguide branching angle towards output power at the end of waveguide was observed by changing the M value between 160 [micro]m until 260 [micro]m with interval of 20 [micro]m. The M value is the width of branch opening. From Figure 13, it can be seen that output power is in the form of sinusoidal where initially the power decreases and increase at M value of 240 [micro]m and decreases again afterwards. The maximum output power at wide-angle branching is at the M value of 260 [micro]m. When the wide-angle branching increases, the power loss during propagation also increases, but there is not much change in output power produce with the change in wide-angle branching. Figure 14 shows that minimum output power is at 1625 nm wavelength. Power loss increases when wavelength decrease. Hence, at wavelength value higher than 1550 nm, it can be seen that the power loss increases.

Table 1 shows the derivatives equation from the effect of observed parameters toward the output power for TE and TM plane. From the equation, the relationship between the parameters can be expressed. X parameter refers to the size of tested wavelength.






This research is conducted to introduce the new device that was designed which is the optical moderator. This device was characterized towards certain parameter and output power in TE as well as TM plane. Application of this moderator device was explained in the user's FTTH access network as a facilitator in increasing the number of the services to user premises. With this device, FTTH network is more flexible and quality of the service increased.

During characterization, although there were losses in output power on the optical moderator, it was still acceptable. Power loss in the design is due to waveguide bends. Loses also occurs in linear waveguide due to Evanescent mode.

Factors that need to be emphasized in the design process has been concluded from this study and points towards design proposal of an optimum 2x3 optical moderator. Result from the analysis of output power of the 2x3 optical splitter combine found that when the refractive index, wide-angle branching and width of the waveguide increases will lead to the decrease of output power. However, width of the waveguide that can be determined is limited to ensure the propagation mode that propagates in the waveguide is still in single-mode, but the width of the waveguide also can't be too small because the propagation mode can't propagates in a narrow waveguide. Based on the materials used to develop this device which is SU-8 polymer that operates at wavelength of 1550 nm and above, it was found that the maximum power is at the wavelength of 1625 nm. Output power increases with the increase of wavelength.


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Corresponding Author: Mohammad Syuhaimi Ab-Rahman, Computer and Network Security Research Group Department of Electrical, Electronics and Systems Engineering Faculty of Engineering and Built Environment Space Science Institute (ANGKASA) Universiti Kebangsaan Malaysia 43600 UKM Bangi, Selangor, Malaysia.

Mohammad Syuhaimi Ab-Rahman

Computer and Network Security Research Group Department of Electrical, Electronics and Systems Engineering Faculty of Engineering and Built Environment Space Science Institute (ANGKASA) Universiti Kebangsaan Malaysia 43600 UKM Bangi, Selangor, Malaysia.
Table 1: Derivatives equation from the effect of observed parameters
toward the output power for 2x3 optical moderator.

Parameters           Output power (TE)           Output power (TM)

Width of waveguide   y = -0.0128[x.sup.2]        y = -0.0081[x.sup.2]
([micro]m)           + 0.1175x + 0.6792          + 0.0613x + 0.8312
Wavelength (nm)      y = [0.8765e.sup.0.0177x]   y = [0.8702e.sup.
Refractive index     y = -0.0019[x.sup.3] +      y = -0.0027[x.sup.3] +
                     0.0197[x.sup.2] - 0.0674x   0.0259[x.sup.2] -
                     + 0.9819                    0.079x + 1.0072
Wide-angle           y = -0.0015[x.sup.2] +      y = -0.0031[x.sup.3] +
branching (m         0.0094x + 0.9094            0.0374[x.sup.2]
value)                                           -0.1292x + 1.0444
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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:4EXRO
Date:Apr 1, 2011
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