ICSE2004 Proc. 2004, Kuala Lumpur, Malaysia

Conventional Arrayed Waveguide Grating with 4 ChannelStructure Design for CWDM

Heng Cherng Woei*, Nazarina Abdul Rahman and Sahbudin Shaari, Member, IEEE, Member, SPIEPhotonics Technology Laboratory,

Institute of MicroEngineering and Nanoelectronics,Universiti Kebangsaan Malaysia,

43600 UKM Bangi, Selangor, Malaysia.*Email: hengcw(a)vlsi.eng.ukm.my

Abstract - The cost effective of CoarseWavelength Division Multiplexing (CWDM)become more interested topic in the platformfor metro networks compare to the alternativeDense-WDM (DWDM), which is wellestablished in long-haul application. Arrayedwaveguide grating (AWG) is a key element forWDM system in optical communication due toability to multiplex and demultiplex light ofdifferent wavelength. In the present work, twoconventional AWGs with 4 channel weredesigned to operate in CWDM, where theconventional AWG usually is designed tooperate in WDM or DWDM system. Thedevices were designed to operate at centralwavelength 1.51 gim and 1.55 ,Im with 20 nmwavelength spacing between the channels.The maximum insertion loss for both devicesis around 4.5 dB. Besides, the crosstalk forAWG with center wavelength 1.51 gm and1.55 jtm is found to be less than -32.50 dB and-30.52 dB respectively.

I. INTRODUCTION

The enormous grow in the demand of bandwidthis pushing the utilization of fibre infrastructuresto their limits. To fulfill this requirement theconstant technology evolution is substituting theactual single wavelength systems connected in apoint-to-point topology by WDM systems.WDM technology enabled optical multiplexingand demultiplexing which individual signalshave different light wavelengths can be separatedor combined to transmit in single fiber optic.

Coarse WDM (CWDM) allows thewavelengths to be spaced farther apart, whichallows for economical solutions in sparseapplications. Since DWDM utilizes very closelyspaced wavelengths (around 0.8nm), thewavelengths need to be very tightly controlledover temperature, which limits the operational

temperature range, and also incurs significantexpense that needed to maintain signal integrityof the tightly packed wavelengths [1-2].However, the broad standard channel wavelengthspacing (around 20 nm) of CWDM systemslimited number of channels per band. Therefore,CWDM is designed for short to medium rangenetworks that are fiber lean typically found inmetro access networks.

Arrayed waveguide grating (AWG)multiplexer is a key element for wavelengthdivision multiplexing (WDM) systems in opticaltelecommunication. Recently, a research workhad done by applying AWG in CWDMapplication. Leo et. al. have designed a polymerbased 2x8 AWG which can perform one half ofthe devices as a multiplexer and the other half asa demultiplexer [3]. A new structure ofAWGs tooperate in broadband environmental has alsobeen introduced. These AWGs have twochannels, which can operate at 1.00-1.55 ,um,1.31-1.53 jim and 1.47-1.55 ptm [4].

In our present work, we have designed twoconventional 4 channel AWG structures, whichable to operate at central wavelength 1.51 jimand 1.55 pim with 20 nm wavelength spacingbetween the channel. The AWGs for centralwavelength 1.51/1.55 gm is able to operate inC/C+L band windows. This is among our firstwork on CWDM AWGs by using similarcommercial software to design DWDM AWG[5].

II. PRINCIPLE OF AWG

Basically, an AWG is consisting Ninput/output waveguides, slab waveguide alsoknown as free propagation region, FPR andarrayed grating, as shown schematically inFigure 1. The path length difference, AL,between adjacent array waveguides is designed

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ICSE2004 Proc. 2004, Kuala Lumpur, Malaysia

to be equal to an integer number of the centralwavelength as given by

AL = m. (I)Ng

where m is an integer of diffraction order, A, isthe central wavelength and Ng is the effectiverefractive index of the channel waveguide [6].The relationship between AL with channelspacing of the WDM (AA) is given by

AL- nsdDc (2)NcFA(

where n, is the effective refractive index of theslab waveguide, NX is the group index of theeffective refractive index of the array waveguide[7]. D, d and F are coefficient parameter of theslab waveguide. Finally, the conversion of thechannel spacing in length, AA to frequency, Af asbelow

cAf=-AA (3)AC

where c is speed of light in vacuum.

Arrayedwaveguide

Input port Slab waveguide Output port

Figure I Schematic layout of the conventional AWGwith 16x 16 channel.

After the optical light is launched into aninput waveguide, it diverges while entering thefirst FPR. When it reaches the input aperture ofthe arrayed waveguide, it propagates through tothe output aperture at second FPR. The fields ofthe central wavelength will be in phase andreproduced in the second FPR. At the same time,the diverged light at first FPR transformed into aconvergent one with equal amplitude and phasedistribution, and an image of the input field at thefirst FPR will be formed at the central of the

second FPR. The linearly increasing path lengthof the arrayed waveguides, will cause otherwavelengths to induce a linearly phase change.As consequence, the outgoing light will be tiltedand the focal point will shift along the secondFPR. By placing receiver waveguides at properpositions along the second FPR, spatialseparation of the different wavelength channelcan be obtained.

III. DESIGN

In our design of AWG for CWDM, thelayout of the conventional AWG is stillmaintained. The position of input port and outputport is symmetrically formed, and the position ofthe FPR-arrayed waveguide-FPR is on the top ofI/O port. The schematic layout of the 4x4channel AWG for CWDM with centralwavelength 1.51 ,um is show in Figure 2.

Figure 2 Schematic layout of the 4x4 channel AWGwith central wavelength 1.51 jim.

Table I Design parameters for AWG with centralwavelength 1.55 gtm

Parameter ValueCenter Wavelength 1.55 jimChannel Spacing 20 nm (2496 GHz)Diffraction order 15Path length difference 16.001 gmFPR length 1434.50 jimNo. of arrayed waveguide 14Effective index of core 1.45492Free Spectral Range 13235.76 GHz

Table 2 Design parameters for AWG with centralwavelength 1.51 jim

Parameter ValueCenter Wavelength 1.51 gmChannel Spacing 20 nm (2630 GHz)Diffraction order 15Path length difference 15.588 gumFPR length 1394.77 gmNo. of arrayed waveguide 14Effective index of core 1.45488Free Spectral Range 13595.918 GHz

WDM_phasar design tool from Optiwave,has been used to design 4 channel AWGoperating at central wavelength 1.51 jim and 1.55jim with same material. The refractive index of

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ICSE2004 Proc. 2004, Kuala Lumpur, Malaysia

core/cladding at 1.55 ,um is 1.456/1.45. The coresize is 5 gm x 5 gm with buried type waveguide.The port separation of input/output is designed tobe 250 ptm for pigtailing to fiber ribbon. Alldesign parameters are listed in Table I for AWGwith central wavelength 1.55 pim and Table 2 forAWG with central wavelength 1.51 ,tmrespectively.

In the design, the index contrast betweencore and cladding is low thus the waveguidecannot be bent too much. At the same time, thepath length difference of arrayed waveguide islow thus the orientation angle at FPR can bedecreased. If the orientation angle at FPR ismaintained as for AWG DWDM, the arrayedwaveguide will mix together. Thus, the totaldevice size for both AWGs is maintained 42 mmx 1.8 mm.

IV. RESULTS AND DISCUSSION

The simulation result of AWG with centralwavelength 1.55 [tm is shown in Figure 3. Itshows the output distribution of the 4 channelexit at the output waveguides. The four outputwavelengths XI, X2, X3 and X4 are 1.592, 1.570,1.550 and 1.530 [tm respectively. The channelspacing is 20 nm except the spacing for channelbetween I and 2 is 22 nm, cause by the shiftedwavelength, XI due to phase error in the arrayedwaveguide. However, the maximum insertionloss of 4.56 dB is at channel 4 and the minimuminsertion loss of 3.39 dB is at channel 2 withoverall uniformity 1.17 dB. The crosstalk is lessthan -30.52 dB.

0o00- 4 3 2

000

2000-3 X ,/

-53000 . ;

0., e

t 5 0 I 4.54 150 1.5110 15.600Wavelength, rn

Figure 3 Output spectial responses of the AWG 4channel with centilal wavelenigth 1 55 tm.

Table 3 shows the computed outputparameters of AWG with central wavelength1.55 ,tm. These values have been computed atbandwidth level -30dB. The bandwidth level isused as the reference to define the bandwidths.

Table 3 Output parameter computed with bandwidthlevel -30dB for AWG 4 channel with centralwavelength 1.55 um.Channel Peak Width Cross Talk SpacingNo (dB) (nm) (dB) (nm)

1 -3.97 30.81 -31.90 222 -3.39 35.69 -30.52 203 -3.55 34.84 -34.77 204 -4.56 29.37 -37.60 NA

For AWG with central wavelength 1.51 ,um,the corresponding output response of the channelis shown in Figure 4. The four outputwavelengths Xi, X2, X3 and X4 are 1.550, 1.530,1.510 and 1.490 ,um respectively. The channelspacing obtained from simulation result isperfectly 20 nm. The maximum insertion loss ofchannel is 4.53 dB at channel 4 and minimuminsertion loss is 3.30 dB at channel 2. In addition,the crosstalk for this AWG is less than -32.50 dBand the insertion loss uniformity is 1.23 dB. Thedetail value of output parameter of AWG withcentral wavelength 1.51 gm is shows in Table 4.

0.00 4 3 2 1

-20.00

3-30.00

4 -40.00

-50.00 K?:7v > ' I K

1.480 1.500 1.520 1.540 1.560Wavelength, urm

Figure 4 Output spectral responses of the AWG 4channel with central wavelength 1.51 pm.

According from the simulation result, wefound that these AWGs can work properly inCWDM system. The size of the devices is muchlonger because of reducing the insertion losscause by waveguide banding and reducing theorientation angle at FPR. However, the size of

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ICSE2004 Proc. 2004, Kuala Lumpur, Malaysia

device can be reduced by increasing the indexcontrast between core and cladding asdemonstrate in AWG with 256 channel forDWDM system [8].

Table 4 Output parameter computed with bandwidthlevel -30dB for AWG 4 channel with centralwavelength 1.51 um.Channel Peak Width Cross Talk SpacingNo (dB) (nm) (dB) (nm) _

1 -3.94 29.60 -32.50 202 -3.30 31.97 -32.73 203 -3.49 31.86 -35.09 204 -4.53 28.42 -38.10 NA

[5] S. Shaari & S.K. Mah, "Design implementation of upto 20 channel silica-based arrayed waveguide WDM".ICSE Proc. 235-240, 2000.

[6] M.K. Smit & C.van Dam, "Phasar-based WDM-devices: Principles, design and applications". IEEE JSelect. Topics Quantum Electron. 2(2): 236-250, 1996.

[7] A. Kaneko, T. Goh, H. Yamada, T. Tanaka, & I.Ogawa, "Design and applications of silica-based planarlightwave circuits". IEEE J. Select. Topics QuantumElectron. 5(5): 1227-1236, 1999.

[8] Y. Hida & Y. Hibino, "Ultra-high density AWGscomposed of super-high A PLCs." Optical FiberComm. Conf '02 Th(C6): 399-401, 2002.

V. CONCLUSION

Two CWDM AWGs with 4 channel havebeen designed and simulated to study the spectralresponse of the device and their specification,working at central wavelength 1.55 gm and 1.51[im respectively. The maximum insertion forboth devices is around 4.5 dB. The crosstalk forAWG with center wavelength 1.51 lim and 1.55jim is found to be less than -32.50 dB and -30.52dB respectively. These results can be used asbenchmark when the real devices are developed.From this work, we conclude that with properdesign, AWG still can be used in CWDM systemwith channel spacing 20 nm.

ACKNOWLEDGEMENT

The authors would like to thank theMalaysian Ministry of Science, Technology andthe Environment for sponsoring this work underNational Photonics Top Down Research Project020202T00 1.

REFERENCE

[11 K. Kincade, "CWDM breathes life into metro, access,and enterprise applications", Laser focus world, 39(3):97-100, 2003.

[2] M. Schneider, "CWDM or DWDM in metro networks:Which platform makes economic sense?", Lightvave21(9): 2004.

[31 C.J. Leo, P.V. Ramana & K. Sudharsanam, "Design ofpolymer arrayed waveguide gratings for accessnetwork and CWDM applications", Electron-Pack.Technol. Conf 647-651, 2003.

[4] R. Adar, C.H. Henry, C. Dragone, R.C. Kistler & M.A.Milbrodt, "Broad-band array multiplexers made withsilica waveguides on silicon", J. Lightwave Technol,11(2): 212-218, 1993.

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