Proceedings of the International Conference on Computer and Communication Engineering 2008 May 13-15, 2008 Kuala Lumpur, Malaysia

978-1-4244-1692-9/08/$25.00 ©2008 IEEE

Analysis and Design of Optimal Demultiplexer Based on Mach-Zehnder Interferometer for CWDM Application

Noran Azizan Cholan1, Rahmat Talib1, Maisara Othman1, Jiwa Abdullah1, Teo Chun Siang1 and Nor Hafizah Ngajikin2

1 Universiti Tun Hussein Onn Malaysia(UTHM), 86400 Parit Raja, Batu Pahat, Johor, Malaysia

2 Universiti Teknologi Malaysia (UTM) 81310 UTM Skudai, Johor, Malaysia

noran@uthm.edu.my

Abstract

Optimal design of planar circuit

demultiplexer based on Mach-Zehnder interferometers and their cascaded forms are presented. A demultiplexer is designed for each two and four channels (wavelengths) respectively for Coarse Wavelength Division Multiplexing (CWDM) application which follows ITU-T G.694.2 standard. Optimized demultiplexer should have high transmission to get high power efficiency and low cross talk to avoid signal degradation. An analytical equation is presented to calculate the length difference (∆L) between the two arms of a Mach-Zehnder interferometer to produce different phase shift. Based on the length difference (∆L) calculation, Mach-Zehnder interferometer for different wavelengths is designed. Directional coupler is analyzed in such a way that it can split the power equally. Coupled Mode Theory (CMT) and Beam Propagation Method (BPM) is used as an analytical and numerical method respectively. Performance comparisons in terms of transmission, cross talk and 3 dB bandwidth are implemented for the demultiplexer. Transmission (dB) as high as -0.74 dB and cross talk as low as -21.06 dB have been obtained for four channel demultiplexer at wavelength, 1470=λ nm. A 3 dB bandwidth of 14nm has been recorded as well, which satisfies the requirement set by ITU-T G.694.2 which is 20 nm. Qualitative agreement between Beam Propagation Method (BPM) and calculation based on Coupled Mode Theory (CMT) has been achieved for each two and four channels demultiplexer respectively.

I. INTRODUCTION Wavelength Division Multiplexing (WDM) system

is currently taking over as the leading technology in the communication networks. It is a technology of combining a number of wavelengths into the same fiber. WDM system requires several devices to distribute, isolate and amplify optical power. One of those components is a demultiplexer which is used to separate the optical signals into appropriate channels. Several optical demultiplexers have been demonstrated by number of approaches including Mach-Zehnder interferometer[1][2][3], Fabry-Perot filter[4], thin film[5], fiber bragg grating[6] and array waveguide grating[7]. Of these approaches, Mach-Zehnder interferometer structure is considered superior due to its advantages such as low insertion loss, low cross talk and simple to fabricate.

This paper is organized as follows. In section II, an analytical equation to calculate the length difference (∆L) between the two arms of a Mach-Zehnder interferometer is presented as well as the output power of the demultiplexer. Simulation results are described in section III and summary is discussed in section IV.

Figure 1. Basic 2x2 Mach-Zehnder demultiplexer structure

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II. ANALYSIS OF MACH-ZEHNDER INTERFEROMETER Basic Mach-Zehnder structure consist of three main

parts, which are input directional coupler, phase shifter and output directional coupler. The function of Mach-Zehnder demultiplexer is based on the interference of the input signals on the directional coupler. From figure 1, consider an input fiber with two wavelengths λ1 and λ2. The optical power of both wavelengths is equally split on the input directional coupler, and each half is coupled into a waveguide. In the central section one of the waveguide is longer than the other by ∆L. The two halves arrive at a second directional coupler or combiner at difference phase and based on the phase variation, each wavelength interferes constructively on one of the two output fibers and destructively on the other.

Based on the Coupled Mode Theory (CMT) analysis[8], the output power of the demultiplexer is calculated as

11

1 ,2

, 2sin inout P

LkP ⎟⎟

⎠

⎞⎜⎜⎝

⎛ Δ= (1.1)

11

2 ,2

, 2cos inout P

LkP ⎟⎟

⎠

⎞⎜⎜⎝

⎛ Δ= (1.2)

where

fncnL

effeff Δ

=⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛λ

−λ

=Δ−

2112

1

21 (1.3)

and i

effi

nk

λ

π=

2

III. SIMULATION RESULTS AND ANALYSIS

In this research, 2D waveguide model is used with air as a cladding layer with refractive index, nclad = 1, silica as substrate layer with refractive index, nsub =1.480 and silica oxide as waveguide layer with refractive index, nwg =1.489. Equation (1.3) is used to calculate the length difference of the Mach-Zehnder Interferometer arms. To analyse the performance of the designed CWDM demultiplexer, a few parameters have been recorded which are

( )nin

mout

PPdBonTransmissi

,

,log10= (1.4)

( )nout

mout

PPdBcrosstalkCenter

,

,log10= (1.5)

100×−

=BPM

BPMEQU

PPP

Difference (1.6)

A Waveguide Directional Coupler

To form the Mach-Zehnder Interferometer structure, a directional coupler which can split the input power equally and have flat wavelength response over a particular spectral range is needed. As such, an analysis to determine the optimized structure for the directional coupler needs to be done. As shown in Figure 2, a directional coupler is designed. The input power is set to 1 Watt, Pin = 1 W and the Transmission is calculated by using equation (1.4). For d = 2 μm and S = 5.86 μm, the splitting ratio for the coupler is shown in Figure 3. The splitting ratio of the directional coupler is not constantly 50% when the input wavelengths vary from 1470nm to 1610nm. At wavelength 1530 nm, the splitting ratio is almost 50%. For each stage, the splitting ratio of directional coupler for each MZI needs to be optimized corresponding to central wavelength individually. This can be done by changing the parameter d and S for each directional coupler.

Figure 2. Directional Coupler structure

-5-4.5

-4-3.5

-3-2.5

-21450 1500 1550 1600

Wavelength (nm)Tran

smiss

ion

(dB

)

Pout 1 Pout 2

Figure 3. Transmission of Directional Coupler

424

(a)

-30-25-20-15-10-50

1510 1530 1550 1570

Wavelength (nm)

Tran

smis

sion

(dB)

BPM Analytical

(b)

-30

-25

-20

-15

-10

-5

01510 1520 1530 1540 1550 1560 1570

Wavelength (nm)

Tran

smis

sion

(dB

)

Port1 (1550nm) Port2 (1530nm)

(c)

-30

-25

-20

-15

-10

-5

01510 1520 1530 1540 1550 1560 1570

Wavelength (nm)

Tran

smis

sion

(dB

)

Port1 (1550nm) Port2 (1530nm)

(d) Figure 4. (a) Schematic structure of two channel demultiplexer. (b) Transmission spectrum at Port 1(1550 nm). (c) BPM simulation transmission spectrum. (d) Analytically calculated transmission spectrum

(a)

-40

-30

-20

-10

01450 1460 1470 1480 1490 1500 1510 1520 1530 1540 1550

Wavelength (nm)

Tran

smis

sion

(dB

)

BPM Analytical

a

(b)

-40-35-30-25-20-15-10

-501450 1460 1470 1480 1490 1500 1510 1520 1530 1540 1550

Wavelength (nm)

Tran

smis

sion

(dB

)

Port1(1530nm) Port2(1490nm)

Port3(1510nm) Port4(1470nm)

(c)

-40-35-30-25-20-15-10

-501450 1460 1470 1480 1490 1500 1510 1520 1530 1540 1550

Wavelength (nm)

Tran

smis

sion

(dB

)

Port1(1530nm) Port2(1490nm)

Port3(1510nm) Port4(1470nm)

(d)

Figure 5. (a) Schematic structure of four channel demultiplexer. (b) Transmission spectrum at Port 3(1510 nm). (c) BPM simulation transmission spectrum. (d) Analytically calculated transmission spectrum

425

B Two Channels Demultiplexer

The schematic structure of a two channels demultiplexer is shown in Figure 4(a). Through the MZI structure, the input sequence is separated at output Port 1and output Port 2. In this design, the input sequence [1530 nm, 1550 nm] is chosen. Using refractive index of waveguide, nwg = 1.489, the length difference in the interferometer arms ∆L in meter for the demultiplexer is L1 = 700 x 10-5 m and ∆L1 = 3.98 x 10-5 m. Figure 4(b) shows the transmission spectrum at Port 1 for wavelength λ=1550 nm. There is a qualitative agreement between numerical method (BPM) and analytical method. The simulation results shown in Figure 4(c) and 4(d) indicate that the designed two-channel CWDM demultiplexer has maximum transmission power at the wavelength 1550 nm and 1530 nm for output Port 1 and Port 2 respectively. The output power at those wavelengths is not more than 0.4 dB and 0.1 dB obtained by BPM simulation and analytical calculation respectively. The center crosstalks are below -15 dB for BPM and -22 dB for analytical calculation. C Four Channels Demultiplexer

The schematic structure of a four channels demultiplexer is shown in Figure 5(a). Through the MZI structure, the input sequence is separated at output Port 1, Port 2, Port 3 and Port 4. In this design, the input sequence [1470nm, 1490nm, 1530 nm, 1550 nm] is chosen. Using refractive index of waveguide, nwg = 1.489, the length difference in the interferometer arms ∆L in meter for the demultiplexer is L1 = 700 x 10-5 m, ∆L1 = 3.7846 x 10-5 m, L2 = L3 = 500 x 10-5 m, ∆L2 = 1.9138 x 10-5 m, ∆L3 = 1.8634 x 10-5 m. Figure 5 (b) shows the transmission spectrum at Port 3 for wavelength λ=1510 nm. There is a qualitative agreement between numerical method (BPM) and analytical method. The simulation results shown in Figure 5(c) and 5(d) indicate that the designed four-channel CWDM demultiplexer has maximum transmission power at the wavelengths 1530 nm, 1490

nm, 1510 nm and 1470 nm for output Port 1, Port 2, Port 3 and Port 4 respectively. The output power at those wavelengths is not more than 0.9 dB and 0.04 dB obtained by BPM simulation and analytical calculation respectively. This four-channel demultiplexer obtained maximum crosstalk of -16.06 dB at Port 2 (1490nm).

IV. SUMMARY A CWDM Mach-Zehnder demultiplexer has been

successfully designed for each two and four channels. Qualitative agreement between numerical method (BPM) and analytical method has been achieved as well. Performance parameters like transmission, cross talk and bandwidth meet the minimum requirements set by the ITU-T G.694.2 standard.

REFERENCES [1] Verbeek B.H., Henry, C.H., Olsson N.A., “Integrated four-

channel Mach-Zehnder multi/demultiplexer fabricated with phosphorous doped SiO2 waveguides on Si”,Journal of Lightwave Technology, IEEE, June 1988, pp. 1011-1263.

[2] M. Kolesik;,M. Matus, J.V. Moloney, “All-optical Mach-Zehnder-Interferometer-based demultiplexer-a computer simulation study”, Photonics Technology Letter, IEEE, Jan 2003, pp. 78-80.

[3] Takato N., Kominato T., Sugita A., Jinguji K., Toba, H., Kawachi M., “Silica-based integrated optic Mach-Zehnder multi/demultiplexer family with channel spacing of 0.01-250 nm”, Journal on Selected Areas in Communications, IEEE, Aug. 1990, pp. 1120-1127.

[4] M.M. Freire, M.F. De Carvalho, H.J.A. Da Silva, “Performance implications of three-mirror Fabry-Perot demultiplexers for 10-Gb/s WDM dispersion-supported transmission with 0.5-nm channel spacing”, Photonics Technology Letter, IEEE, Sept 1996, pp. 1261-1263.

[5] M. Gerken, D.A.B. Miller, “Wavelength demultiplexer using the spatial dispersion of multilayer thin-film structures”, Photonics Technology Letter, IEEE, Aug 2003, pp. 1097-1099.

[6] Sommart Sang-Ngern, Roeksabutr, A., “DWDM Demultiplexer Using Compound Optical Ring Resonator with Fiber Bragg Grating”, Asia Pacific conference on circuits and systems, IEEE, Dec. 2006, pp. 1907-1910.

[7] Kemiao Jia, Wenhui Wang, Yanzhe Tang, “Silicon-on-insulator-based optical demultiplexer employing turning-mirror-integrated arrayed-waveguide grating”, Photonics Technology Letter, IEEE, Feb. 2005, pp. 378-380.

[8] Gerd Keiser, Optical Fiber Communicationse, McGraw-Hilr, Boston, 2000.

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