C S S R 0 8’ 0 9 14 - 15 March 2009

C O N F E R E N C E ON S C I E N T I F I C & S O C I A L R E S E A R C H

Paper number: 6714645

DESIGN OF ASYMMETRIC MULTIMODE 1 X N OPTICAL PLANAR

WAVEGUIDE COUPLERS

N. Syafiqah Mohamed-Kassim, M. Hanapiah Mohd-Yusoff and M. Kamil Abd-Rahman

Faculty of Applied Science, Universiti Teknologi MARA, Shah Alam, MALAYSIA

norsyafiqahmk@yahoo.com, drkamil@salam.uitm.edu.my

ABSTRACT

In this paper, we demonstrate the designing of asymmetrical 1xN multimode optical planar-waveguide

couplers as optical component for optical power splitter/combiner with on-demand output ratios. The

1xN planar waveguide couplers can have N-number of output ports and only one input port. The

device splits the original optical power into N distinct optical power values. The output power will

depend on the design of the device itself. Asymmetrical 1x2 and 1x4 planar waveguide couplers were

designed using Optiwave Finite Difference Time Domain (FDTD) program and the outputs were

compared to Beam Propagation Method (BPM) and Tap-of-Ratio (TOFR) waveguide-design published

by other researchers. In this design, the main bus-line width was fixed at 1.0 mm whereas the tap-line

width varies depending on the TOFR requirements. Several different tap-line sizes for 1x2 planar

waveguide couplers are presented, which these correspond to various tap-width of 1.0 mm to 0.25 mm

with 0.05 mm interval, and the percent output power ratio obtained are 71% to 3%, respectively. The

construction of a 1x4 asymmetric planar waveguide coupler was design by utilizing the preceding 1x2

waveguide coupler design. The 1x2 Y-junction splitter is cascaded with two 1x2 waveguide couplers.

For a 1x4 planar-waveguide coupler, the output coupler ratios for port 1 and 4 are 14%, 23%, 29% and

33% ± 2% and for port 2 and 3 are 35%, 28%, 18% and 11% ± 2%, respectively.

Keywords: Optical Planar Waveguide, Multimode Optical Waveguide, Asymmetric Waveguide

Design

1. INTRODUCTION

Multimode polymer optical planar-waveguides are different class of device which has high potential

applications in data communications and optical sensors. However, the research are limited due to the fact

that highly multimode devices have smaller market demand due to high attenuation of polymer material

compare to that of silica-based glass material. The attenuation of polymer-based material at 650-nm

wavelength is about 200 dB/km whereas for a single mode glass optical fiber, the attenuation is only 0.2

dB/km at 1550 nm.

Multimode-based components such as Polymer/plastic Optical Fiber (POF) have been employed for

short-distance data communication and established wide application in multimode intensity-based sensor.

In sensor application, POF fiber sensors are normally used for physical or linear displacement sensors.

However, the uses of multimode optical planar waveguides as optical components for intra-

communication system and optical sensors have yet to be explored.

2. LITERATURE REVIEW

There are a number of reports and studies conducted on asymmetric couplers with their own specialty.

According to R.Griffin et al, they had presented the performance of a linear bus of identical asymmetric

couplers from measurement on a single coupler. It focused on all coupler parameters and monitored them

such as tap-on fraction, tap-off fraction, and tap launch excess loss. While B. -H. Vogers et al had done in

analysis of highly asymmetric couplers as a potential component for monolithic integration which

consisted of polymeric waveguide fabricated atop an asymmetric semiconductor waveguide. The

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C O N F E R E N C E ON S C I E N T I F I C & S O C I A L R E S E A R C H

Paper number: 6714645

simulations predicted a maximum coupling efficiency about 19% from semiconductor to polymer

waveguide.

Y-branch splitter is one of popular asymmetric couplers that had been done in researching. N. M.

Lyndin et al had discussed on asymmetric Y-coupler made from bent channel K+ -ion waveguide in glass.

The effect of the geometric parameters of the asymmetric Y-coupler and of the diffusion parameters

during the fabrication on the efficiency of optical power transfer to the branching channel had studied.

Asymmetric Y-couplers with transfer efficiencies of 2-56% had been developed. Besides, a low-loss Y-

branch with a multimode waveguide transition section had been presented by Qian Wang et al. They

considered both asymmetric and symmetric Y-branch to introduce and designed. For symmetric case it

can provide a low excess loss while for asymmetric case, a wide range of ratio has been monitored while

keeping low excess loss. Both of the designs have been verified by simulations and experimental results.

Recently, A. A. Ehsan et al had designed 1X2 and 1X4 hollow waveguide structure to be used as an

optical code generation device. The asymmetrical coupler and linear taper were designed and simulated

using ray tracing. The results presented enable the designer to obtain a unique relationship between the

tap-of ratio and waveguide tap width. Moreover, Hisaharu Yanagawa et al had proposed a broad-band

guided wave optical star coupler with asymmetric directional couplers. The wavelength characteristics of

uniform and alternating change of propagation constant directional couplers were investigated, and their

broad-band operation was shown theoretically.

According to Akihiro Takagi et al, they successfully found that any desired wavelength flattened

response almost in the center of the desired wavelength range could be theoretically obtained by the Beam

Propagation Method (BPM) by taking S-shaped waveguide coupling regions in the interactive region into

consideration. A range of wavelength was obtained by selecting optimum asymmetric guide parameters.

The coupling characteristics of two-core multimode fiber coupler had studied by A. A. Abou El-Fadl

et al. A theoretical model based on solution of the wave equation for a multimode optical fiber for

computing its propagation characteristic is analyzed. In addition, the dependence of the coupling

efficiency on the wavelength, the separation between the two cores and the refractive index was studied.

The effect of the length of the coupling region along the two-multimode core fiber coupler and the

refractive index on the coupling characteristics was also investigated. The obtained results were agreed

with the physical performance of the multimode optical fiber coupler.

Waveguide couplers and fulfillment had been investigated by Zhao Zhenfeng. It has detail discussion

in the single waveguide coupler which realizes the basic principle of two-outputs. They had deduced the

design formula of the waveguide coupler with broad-to-narrow cross wall single slot aperture. When the

inclination is zero degree, the coupler can be used as a power divider which has good balanced

attenuation.

Z.J. Cheng et al had studied on polymer-waveguide-based vertical coupler. A 2x2 polymer-

waveguide-based vertical coupler was designed and fabricated. The relationships between the coupling

efficiency and the key optical design parameters of vertical coupler (such as the crossing angle, gap

thickness between two waveguide layers and the waveguide dimensions) were investigated by using BPM.

Based on the simulations, a fully polymer vertical coupler has been demonstrated.

3. METHODOLOGY

The designing and simulation of the waveguide coupler was performed using finite-difference time-

domain (FDTD) technique. This approach was based on a direct numerical solution of the time-dependent

Maxwell’s curl equations. The FDTD was done using the 2D design. The waveguide was laid out in the

X-Z plane. The propagation is along Z. The Y-direction was assumed to be infinite. This assumption

removes all the ∂/∂y derivatives from Maxwell’s equations and splits them into two (TE and TM)

independent sets of equations. The wavelength used in this simulation was 1550 nm, with modal input

field transverse. While the refractive index for cladding and core were 1.3 and 1.5, respectively.

These designs have been constructed using a combination structure of Y-splitter and linear taper.

Several different tap-line for 1x2 planar waveguide couplers were presented, which these correspond to

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C O N F E R E N C E ON S C I E N T I F I C & S O C I A L R E S E A R C H

Paper number: 6714645

various tap-width of 1.0 mm to 0.5 mm with 0.05 mm interval. The use of the linear taper is to let large

core to be coupled and a waveguide design of 1.0 mm core was used in the simulation.

The design, as shown in Figure 1, started from basic 1x2 waveguide coupler, followed by 1x4

waveguide coupler design. The observation points are located at each output and input ports of the

waveguide as shown in Fig. 1. These observation points will record all the time domain response in each

single point so that spectrum analysis can be performed. To detect the pure scattering field, three

observation line detectors were placed at three locations to observe the frequency domain response,

compute power, and normalized power versus length.

Figure 1: 1x2 waveguide coupler design for 0.5 mm tap-width.

As in BPM tools, the modes used are depended on waveguide’s thickness. The modes can be

determined from this equation,

π)( 2

2

2

1 nnkdM

−≅ (1)

where, λπ2=k is the propagation constant, d is the thickness and n1 and n2 are the index of refraction

of the core and cladding, respectively. Based on the equation, the mode increases with the shorter

wavelength of the guided light, λ. And the number of modes reduced with thinner layer d of the

waveguide and smaller ∆n. Generally, a waveguide is characterized by the normalized frequency, 1

2 2 21 2( )V kd n n= − (2)

The number of modes in normalized frequency,

πVM ≅ (3)

In the simulation work, the modes used were 965 based on the waveguide thickness of 1,000 µm, 1.55 µm

wavelength and 1.5 and 1.3 refractive index of the core and cladding, respectively. The input light source

used was slab mode. The fraction of power output at port 2 of Figure 2 to the sum of input and output

powers was calculated and this is the tap-off ratio (TOFR), as given by (J. D. Love,1999).

tb

tTOFRρρ

ρ

+= (4)

Where tρ is the power output at port 2 and bρ is power input at port 1. The asymmetric Y-junction

splitter with bus line and linear taper is shown in Fig. 2. An input beam ray enters the device at port 1

where at the tap-off region or tap line, portion of this beam are tap-off and the remaining stay in the bus

line. So there is no excess loss as the light entered from port 1 and then propagates through either port 2 or

port 3.

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C O N F E R E N C E ON S C I E N T I F I C & S O C I A L R E S E A R C H

Paper number: 6714645

Figure 2: Asymmetric multimode Y-junction splitter with bus and tap lines.

4. RESULTS AND DISCUSSIONS

The diagrams of 2D simulation of the waveguide coupler performed by FDTD are shown in Fig. 3 for a

1X2 waveguide coupler design. The simulations show the design for (a) 0.9 mm and (b) 1.0 mm tap

width, where the modes at port 2 are believed to illustrate multimode interference with distinct peaks.

Light entering the waveguide that propagates to the port 2 and 3 are coupled at the intersection between

them. The output reading is depended on the size of the tap width.

Figure 3: Plots of FDTD simulation for (a) 0.9 mm and (b) 1.0 mm tap width 1x2 waveguide coupler.

From BPM simulation, the reading of TOFR is referred to as the intensity of the output spectra which

is shown in the Fig. 4(a). The output at the port 2 is divided to the sum of that output with the input which

is exactly equal to 1, resulted the TOFR. While for FDTD, the reading is taken from the power spectrum

which is shown in the Fig. 4(b)

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C O N F E R E N C E ON S C I E N T I F I C & S O C I A L R E S E A R C H

Paper number: 6714645

(a) (b)

Figure 4: (a) Intensity graph for 0.9 mm tap width at port 2 and 3 by using BPM. (b) Power spectrum

performed by FDTD.

Table 1 shows the results for 1X2 waveguide coupler design with power input 1.64x10-9 W/m (FDTD)

from 0.5 mm to 1.0 mm. It shows that there is some difference between FDTD and BPM percentage

readings at the range of 0.50 to 0.55 mm tap width, which could probably due to the mode shifting

towards one side of the waveguide. The TOFR of BPM has an estimated error of about ±5% due to the

uncertainty of measuring the intensity of the output spectra.

Table 1: TOFR of FDTD and BPM results for 1X2 waveguide coupler design.

Tap Width (mm) TOFR/FDTD (%) TOFR/BPM (±5%)

0.50 18 29

0.55 22 29

0.60 26 29

0.65 29 31

0.70 32 33

0.75 33 35

0.80 36 41

0.85 37 43

0.90 39 44

0.95 40 45

1.00 41 46

The results for the TOFR of FDTD and BPM against tap-width are shown in Fig. 5. The various tap width

of 1.00 mm to 0.50 mm with 0.05 mm interval is used for TOFR/FDTD of 18% to 41% while for

TOFR/BPM of 29% to 46%. Generally, the graph shown linear relationship between TOFR/FDTD and

TOFR/BPM with the tap width enables a higher-level 1XN waveguide coupler to be made.

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C O N F E R E N C E ON S C I E N T I F I C & S O C I A L R E S E A R C H

Paper number: 6714645

Figure 5: TOFR/FDTD and TOFR/BPM versus tap width for a 1x2 waveguide coupler.

The designing and simulating of a 1x4 waveguide coupler was performed by combining Y-junction and

asymmetric coupler. Fig. 7 showed the partitioning of the designs. The structure was divided into 6

channels and labeled as A, B, C, D, E, and F in the Fig. 7. Firstly, 50% power splitting was achieved using

Y-splitter as in Figure 6. Once the ~50% split has been achieved as shown in Table 2, resulted from FDTD

simulation (50%±3%), the asymmetric coupler design in section C, D, E and F was cascaded into the

system. The results are shown in Table 3 below.

Table 2: Simulation results for Y-junction splitter with power input 1.99x10-9 W/m.

Power Output (W/m) Output Ratio (±3%)

Port 2 9.17x10-10 46

Port 3 9.48x10-10 48

Figure 6: 1x2 Y-junction splitter.

Figure 7: Combining 1x2 Y-junction for a 1x4 waveguide coupler with 0.6 mm tap-width.

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In Table 3, the TOFR of FDTD results for 6 sets of 1x4 waveguide couplers are shown. The input power

used was 1.67x10-9 W/m. The simulation readings were based on FDTD software and then followed by

TOFR readings which are based on equation (4). The error was accumulated from the Y-splitter to the

1X4 waveguide coupler. We assumed that the error was twice of the reading from the Y-splitter.

Table 3: Results for TOFR of FDTD for 1X4 asymmetric waveguide coupler design.

Tap

Width TOFR of FDTD (±6%)

(mm) C D E F

0.5 12 26 25 12

0.6 16 23 21 17

0.7 19 19 22 20

0.8 22 17 14 22

0.9 23 14 12 24

1.0 25 10 11 25

The results demonstrated that the Y-junction splitter splits the power symmetrically. In addition, each of

the 1x2 waveguide coupler splits the power from the Y-splitter appropriately based on TOFR and tap

width design rule, as shown in Table 3. This table shows the design and simulation effective power values

at the output ports. As per design, each of the simulated output port corresponds close to the required

TOFR value. The comparison results indicate that a higher and more complex waveguide coupler design

can be constructed using this simple design. Fig. 8 below shows the taper section caused the modes splits

into 3 sections, which might be due to multimode interference and mode coupling that brings about the

obvious peaks at the output ports.

(a) (b)

Figure 8: 1x4 waveguide coupler simulations in magnetic fields view for (a) 0.6 mm and (b) 0.9 mm tap

width.

5. CONCLUSIONS

The designing of asymmetrical 1XN multimode optical planar waveguide couplers as optical component

for optical power splitter with on-demand output ratios was demonstrated. These designs have been

constructed using a combination structure of Y-splitter and linear taper. Several different tap-line for 1X2

planar waveguide couplers were presented, which these correspond to various tap-width of 1.0 mm to 0.5

mm with 0.05 mm interval. Starting from a basic 1x2 Y-junction waveguide coupler, the 1X4 waveguide

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Paper number: 6714645

coupler was designed. Finite-difference time-domain simulation has been performed to predict the output

performances of the model device which closely match the results from BPM model. The simulation

results have shown a linear relationship between the tap-off ratio and the waveguide tap-width.

ACKNOWLEDGEMENT

We acknowledge the Faculty of Applied Science, Universiti Teknologi MARA, Shah Alam for the

facilities and support provided.

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