Optics Communications 258 (2006) 155–158

www.elsevier.com/locate/optcom

Compact SOI arrayed waveguide grating demultiplexerwith broad spectral response

Qing Fang *, Fang Li, Yuliang Liu

Research and Development Center for Optoelectronics, Institute of Semiconductors, The Chinese Academy of Sciences,

P.O. Box 912, Beijing 100083, P.R. China

Received 6 June 2005; accepted 25 July 2005

Abstract

A compact eight-channel flat spectral response arrayed waveguide grating (AWG) multiplexer based on silicon-on-insulator (SOI) materials has been fabricated on the planar lightwave circuit (PLC). The 1-dB bandwidth of48 GHz and 3-dB bandwidth of 69 GHz are obtained for the 100 GHz channel spacing. Not only non-adjacent cross-talk but also adjacent crosstalk are less than �25 dB. The on-chip propagation loss range is from 3.5 to 3.9 dB, and thetotal device size is 1.5 · 1.0 cm2.� 2005 Elsevier B.V. All rights reserved.

Keywords: Silicon-on-insulator; Arrayed waveguide grating; Broad spectral response; Compact

1. Introduction

The performance of wavelength division multi-plexing (WDM) optical networks greatly dependson the spectral characteristics of their components[1]. One key component of WDM networks is thearrayed waveguide grating (AWG) [2,3]. In orderto allow the concatenation of many such devicesand reduce the need for accurate wavelength con-trol, the filter device�s response must approximate

0030-4018/$ - see front matter � 2005 Elsevier B.V. All rights reserv

doi:10.1016/j.optcom.2005.07.058

* Corresponding author. Tel.: +860182304119; fax:+860182304016.

E-mail address: qingfang@red.semi.ac.cn (Q. Fang).

a broad spectral response. For an AWG multi-plexer/demultiplexer of Gaussian type, the ratio ofthe 1-dB bandwidth to channel spacing is small.Some methods have been reported to broaden thespectral response, such as asymmetric MZ filtermethod [4], multiple Roland circles method [5]and input MMI coupler method [6] in silica andInP based AWG, but never in the SOI basedAWG. Silicon-on-insulator (SOI) technology hasshown to be a promising technology for guidedwave photonic devices operating in the infrared. Anumber of SOI guided wave optical devices andcircuits with high performance have already beendemonstrated. SOI technology offers tremendous

ed.

156 Q. Fang et al. / Optics Communications 258 (2006) 155–158

potential for cost-effective monolithic integrationincluding the variable optical attenuator, wave-length demultiplexer, photodetectors and electroniccircuitry [7–9].

In this paper, we consider an optimal design ofMMI input waveguides and exponential multi-mode taper output waveguides for broadeningthe spectral response of an AWG demultiplexerbased on SOI materials. The measured resultsshow that the 3 dB bandwidth is about 70% of100 GHz channel spacing. The insertion loss uni-formity is 0.5 dB with FSR of 15 nm. Comparedto the silica AWG, The bend radius of the AWGbased on SOI is smaller because of the high rela-tive index difference. The device size is only1.5 · 1.0 cm2.

2. Theory and design

With the increase of the etching depth on theSOI chip, the refractive index difference becomeshigh. High refractive index difference can reducethe AWG chip size remarkably because of not onlyreduction of the minimum bend radius but alsoreduction of the slab focal length by the smallerlateral spacing of waveguides connected to theslabs. The different epitaxial layer thickness hasdifferent maximal etching depth for a single guidedmode. That is to say, the epitaxial layer thicknessis an important factor for the AWG chip size,shown in Fig. 1. In our design, the SOI wafer with

Fig. 1. AWG chip size vs. epitaxial layer thickness.

the epitaxial layer of 2.5 lm-thickness is chosen. Inorder to broaden the spectral response, an MMIcoupler is connected at the end of the input wave-guide and exponential multimode waveguides ischosen as the output waveguides at the end ofthe second free propagation region (FPR). A two-fold image can be obtained at the end of the MMIcoupler if the MMI coupler is designed reasonably.According to the design theory of AWG, the 1:1image of the twofold image can be produced atthe focal line of second FPR, which connects theexponential multimode output waveguides.The spectral response of AWG is determined bythe overlap between the 1:1 focused field of two-fold image and the output guide-mode. The pro-cess of field overlap is illustrated schematically inFig. 2. The spectral response of AWG can be given

T ðDf Þ ¼Z

uimageðy � Y ÞuoðyÞ dy� �2

; ð1Þ

where uimage is the 1:1 focused field of the twofoldimage; uo is the output guide-mode field and Y isthe peak separations of the twofold image. Theoutput guide-mode field uo and the input guide-mode field ui can be approximated by the follow-ing Gaussian distribution:

uiðyÞ ¼ C1 expð�y2=w2i Þ; ð2Þ

uoðyÞ ¼ C2 expð�y2=w2oÞ; ð3Þ

where C1 and C2 are constant; wi and wo are thewidth of input waveguide and output waveguide,respectively. The twofold image field uimage is de-scribed by the following sum of two Gaussiandistributions:

Fig. 2. Spectral response profile by overlap integral.

Fig. 3. The input waveguide of the fabricated SOI AWG.

Q. Fang et al. / Optics Communications 258 (2006) 155–158 157

uimageðy � Y Þ ¼ C exp �ðy � Y =2Þ2

w2i

" #(

þ exp �ðy þ Y =2Þ2

w2i

" #). ð4Þ

Substituting (3) and (4) into (1), then

T ðDf Þ / exp �ðy � Y =2Þ2

w2i

" #(

þ exp �ðy þ Y =2Þ2

w2i

" #). ð5Þ

At the same time

T ðDf Þ / expð�y2=w2oÞ. ð6Þ

So the spectral response of AWG is affiliated withthe peak separations Y of the twofold imageand the width of the input/output waveguide. Withthe increase of the width of output waveguide, thetop of the spectral response becomes flat. By opti-mizing the values of Y and wo, the ideal flat spectralresponse of AWG can be obtained.

We have designed an eight-channel 100-GHzarrayed waveguide grating (AWG1) with MMI in-put waveguide and exponential multimode taperoutput waveguides. The width of each array wave-guide with minimum radius of 1000 lm is 2.0 lm.The device is designed to operate at the grating or-der of 150, with a path length difference of67.38 lm, and the FPR focal length of 2763 lm.To reduce the insertion loss uniformity of the de-vice, the free spectral range of 15 nm is chosen.According to the self-image theory, the widthand length of MMI which connects input singlemode waveguide with first FPR are 5 and 35 lm,respectively. The peak separation of the twofoldimage is about 3.0 lm. The exponential multimodetaper is chosen as output waveguide. The begin-ning width and end width of the taper are 4 and2 lm, respectively. In order to obtain the idealspectral response, the value of the exponential ischosen as 3. In order to compare the flat spectralresponse with that of convertional AWG, anothernormal arrayed waveguide grating (AWG2) hav-ing the same device parameters is allocated in thechip.

3. Fabrication and results

The AWGs were fabricated on SOI wafer hav-ing a 2.5 lm-thick Si on the top of a 0.4 lm-thickSiO2 layer, shown in Fig. 3. After photolithogra-phy and pattern formation, the devices wereetched a rib height of 1.2 lm by the inductive cou-pled plasma (ICP) etching technology. In order toprotect the rib waveguides from being destroyed, a2.0 lm-thick SiO2 layer was grown by PECVDafter ICP etching process.

The AWG devices were measured by the combi-nation of an amplified spontaneous emission(ASE) resource and an optical spectrum analyzer.Light was coupled into the input waveguide of theAWG chip using the cone-type optical fiber for re-duce the coupling loss. The spectral responses ofthe device were shown in Figs. 4 and 5. TheFig. 4 shows the spectral response of conventionalAWG2 with the propagation loss of 1.6 dB and thecrosstalk of 28 dB. At the same time, the 1-dBbandwidth is 19 GHz and 3-dB bandwidth is30 GHz. Compared to the spectral response ofconvertional AWG2, the Fig. 5 shows the flat spec-tral response of AWG1 with the 1-dB bandwidthof 48 GHz and 3-dB bandwidth of 69 GHz. Cross-talk to neighboring and all other channels ofAWG1 are less than �25 dB. The on-chip propa-gation loss range is from 3.5 to 3.9 dB. Those re-sults display that AWG1 has a more than 100%of 1- and 3-dB bandwidth ratio, compared to

Fig. 5. Measured spectral response of the AWG with MMIinput waveguide and exponential multimode taper outputwaveguides.

Fig. 4. Measured spectral response of the conventional AWG.

158 Q. Fang et al. / Optics Communications 258 (2006) 155–158

AWG2. But the propagation loss of AWG1 is big-ger 2.0 dB than AWG2, because the MMI couplerand the exponential multimode taper output wave-guides are used in the AWG1.

4. Conclusion

A compact eight-channel flat spectral responsearrayed waveguide grating (AWG) multiplexer

based on silicon-on-insulator materials has beenreported. The total chip size is only 1.5 ·1.0 cm2. The 1-dB bandwidth of 48 GHz and 3-dB bandwidth of 69 GHz are obtained for the100 GHz channel spacing. Compared to the con-ventional AWG, the flat spectral response of theAWG with MMI input waveguide and exponen-tial multimode taper output waveguides has amore than 100% of 1- and 3-dB bandwidth ratio.Crosstalk to neighboring is less than �25 dB;and the on-chip propagation loss range is from3.5 to 3.9 dB.

Acknowledgment

This work was supported by National NaturalScience Foundation of China under Grant No.90104003.

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