Characterization of 3x3 and 4x4 multimode interference couplers in InP generic photonic integration technology

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1 Chrcteriztion of x nd x multimode interference couplers in InP generic photonic integrtion technology Cittion for pulished version (APA): Pustkhod, D., Jing, X., vn Vliet, E. M., Willims, K. A., & Leijtens, X. J. M. (0). Chrcteriztion of x nd x multimode interference couplers in InP generic photonic integrtion technology. In P. Kockert, P. Emplit, S-P. Gorz, & S. Mssr (Eds.), Proceedings of the 0th Annul Symposium of the IEEE Photonics Benelux Chpter, -7 Novemer 0, Brussels, Belgium (pp. -8). Brussels: OPERA-photonics, Brussels School of Engineering. Document sttus nd dte: Pulished: 0/0/0 Document Version: Pulisher s PDF, lso known s Version of Record (includes finl pge, issue nd volume numers) Plese check the document version of this puliction: A sumitted mnuscript is the version of the rticle upon sumission nd efore peer-review. There cn e importnt differences etween the sumitted version nd the officil pulished version of record. People interested in the reserch re dvised to contct the uthor for the finl version of the puliction, or visit the DOI to the pulisher's wesite. The finl uthor version nd the glley proof re versions of the puliction fter peer review. The finl pulished version fetures the finl lyout of the pper including the volume, issue nd pge numers. Link to puliction Generl rights Copyright nd morl rights for the pulictions mde ccessile in the pulic portl re retined y the uthors nd/or other copyright owners nd it is condition of ccessing pulictions tht users recognise nd ide y the legl requirements ssocited with these rights. Users my downlod nd print one copy of ny puliction from the pulic portl for the purpose of privte study or reserch. You my not further distriute the mteril or use it for ny profit-mking ctivity or commercil gin You my freely distriute the URL identifying the puliction in the pulic portl. If the puliction is distriuted under the terms of Article f of the Dutch Copyright Act, indicted y the Tverne license ove, plese follow elow link for the End User Agreement: Tke down policy If you elieve tht this document reches copyright plese contct us t: openccess@tue.nl providing detils nd we will investigte your clim. Downlod dte: 9. Mr. 09

2 Proceedings Symposium IEEE Photonics Society Benelux, 0, Brussels, Belgium Chrcteriztion of nd multimode interference couplers in InP generic photonic integrtion technology D. I. Pustkhod, X. Hung, E. M. vn Vliet, K. A. Willims nd X. J. M. Leijtens Eindhoven University of Technology, Dept. of Electricl Engineering P. O. Box, 00 MB Eindhoven, The Netherlnds MMI couplers with multiple inputs re importnt uilding locks for generic photonic integrtion technology. Previously we hve developed design pproch for these components, which uses three key performnce prmeters, excess loss, imlnce, nd phse error. The optimized MMI couplers were mnufctured in dedicted run with pssive components only. Test results of these devices hve een pulished. In the current work we chrcterize MMI couplers tht were mnufctured in the Smrt Photonics generic ctive-pssive integrtion process. Integrted PIN detectors re included in the test circuits nd their output is red out electriclly. We chrcterized the test circuits nd compre the results with the mesured in the pssive run vlues. Introduction MMI couplers hve wide rnge of pplictions, such s power-splitting nd comining [], (de)multiplexing [] nd key elements in 90 nd 0 hyrids [, ]. The performnce of MMI couplers is rther sensitive to the wveguide width, which is criticl prmeter in the design []. The mesurements tht we crried out for the devices tht were fricted in the dedicted pssive run show low imlnce nd good tolernce to the friction errors. In order to include those uilding locks into the component lirry used for designing in the multi-project wfer runs, chrcteriztion of the MMI couplers fricted in such full MPW run ws mde. This pper focuses on the chrcteriztion results of those devices. The dt nlysis is descried nd the results for the properties of MMI couplers re evluted. MMI Design Figure shows the generl geometry of n MMI coupler. Typicl prmeters re indicted in Figure. There re two cross-sections ville in the Smrt Photonics MPW run lyer stck: shllow nd deeply etched wveguides with different index contrst. The MMI coupler nd ccess wveguides re fricted in the deep-etched cross-section. This llows to reduce the seprtion etween the ccess wveguides nd mke them wider nd them while keeping opticl coupling etween them low. It is the influence of W MMI, indicted in Figure, on the coupler performnce tht we wnt to nlyze. Performnce prmeters. According to the design pproch, three prmeters re used to mesure the performnce of n MMI coupler, the excess loss L ex, the imlnce χ mx nd the phse error φ mx. The definition of these quntities is given in eqs. (). The power which is not coupled out from the coupler is ssessed y excess loss. Imlnce mesures the devition from the idel coupling rtio. The phse error is the devition from the idel phse difference etween two djcent output ports, which equls π/n

3 Chrcteriztion of x nd x multimode interference couplers in InP generic... Air (.0) InP (.7) 00 nm 00 nm Q(.) InGAsP (.) InP (.7) 00 nm Figure : MMI coupler geometry. Cross-section of the deep etched wveguide in Smrt Photonics technology for N N MMI coupler. L ex [db] = 0log(P out /P in ) χ mx [db] = mx χ i j, i j χ i j = 0log(P oi /P oj ) () ϕ mx = mx ϕ i j, i j ϕ i j = ϕ oi ϕ oj π/n For imlnce nd phse error, ccording to eqution (), there re N(N )/ comintions of the output ports oi nd oj. Anlysing them individully is not necessry. We tke the mximum vlue of these two prmeters to mesure coupler performnce. Finl design. The geometry of MMI couplers ws optimized numericlly for the est performnce in the wvelength interrogtor device. This requires the lowest imlnce etween the output chnnels. Although the phse error is importnt, the effect cn e reduced significntly y using compenstion lgorithm []. Compred to pssive run, we expect slight degrdtion of the MMI coupler performnce due to lrger numer of friction steps in the full MPW run. Performnce mesurement An unlnced Mch Zehnder interferometer (MZI) is used s test structure for the designed MMI coupler. Figure shows the components nd lyout of the test structure. The design rm difference of L =. µm corresponds to free spectrl rnge of λ FSR 80 pm. Given the wvelength setting ccurcy of the tunle lser of pm nd temperture stiliztion of 0.00 C, this gives n ccurcy of phse mesurement of 0.7.The outputs of the MZI re terminted with PIN photodetectors, mesured photocurrent is proportionl to the intensity of light t the corresponding output of the MMI coupler. The mnufcturing of the test structure ws crried out s prt of Multi-Project Wfer (MPW) run, following generic integrtion methodology. Besides our custom-designed nd MMI couplers, the test structure contins set of stndrd uilding locks such s wveguides, -db MMI coupler nd PIN detectors ws used. The chrcteristics of the uilding locks re provided y the foundry in the design mnul, nd no detiled knowledge on the mnufcturing process is required y the designer [7]. The mesurement setup consists of tunle lser source nd polriztion mintining lensed fier to couple TE-polrized light to the chip into the input of the MZI. The electricl signl from the PIN photodetectors is trnsferred through proe crd to the current meter module. For etter responsivity t longer wvelength reverse is V = V

4 Proceedings Symposium IEEE Photonics Society Benelux, 0, Brussels, Belgium TE-polrized lser input Chip Proe crd ADC Figure : Schemtic () nd msk lyout () of the MZI test structure for coupler: () - db MMI coupler; () symmetric rms of the interferometer; () MMI coupler; () output wveguides; () PIN detectors; () metl proe pds is pplied to the photodetectors. The corresponding drk current for ech photodetector is mesured seprtely efore the wvelength sweep nd is lter sutrcted from the mesured spectrl response. During the mesurement the tunle lser is swept in the 0 80 nm rnge, nd the photocurrent from ll output detectors is recorded t ech wvelength. The cquired spectrl responses re then ech fitted to the expected signls to extrct opticl power P oi nd the phse ϕ oi from eqution (). The detils of fitting procedure could e found in []. The use of the photodetectors in the test structure mkes it possile to otin the wvelength response from ll coupler outputs in one sweep, contrry to the pssive run, where light from the output should e coupled out from the chip nd therefore seprte sweeps re required. This my reduce the mesurement error ssocited with coupling or temperture vritions from sweep to sweep. Mesurement results There re three chips mesured in totl, ech contining nd MMI couplers. For couplers, the nominl widths of nd µm re chosen. For MMI couplers, the widths of nd 0 µm re chosen. Vritions of ±00 nd ±00 nm from the design width were lso included to simulte the friction error. Figure shows the width dependency of types of couplers close to the centrl design wvelength λ = 0 nm. In generl, minimum of the curves is shifted towrds smller nominl design. This is most proly due to increse of n ctul wveguide width with respect to the designed. Also noticele is tht the -µm-wide coupler performs worse thn the -µm-wide, while -output couplers of oth width perform similrly. In further result discussion we refer only to the nrrow coupler vrition. In figure wvelength dependency of MMI couplers of selected width is shown. Similr to the pssive run, the vlues re constnt over the C-nd. Compred to the pssive run, ll devices hve dditionl excess loss of to db, imlnce is similr nd is pproximtely 0. db, nd the phse error is incresed y. Conclusions We presented nd MMI coupler chrcteriztion in complete MPW run with ctive-pssive integrtion. For µm-wide MMI coupler experimentl results t the centrl wvelength λ = 0 nm show worst-cse imlnce of less thn 0. db, phse error of nd excess loss of db. Performnce in the mesured wvelength spn of 0 80 nm is very constnt. Using the correction lgorithm [] the phse error cn 7

5 Chrcteriztion of x nd x multimode interference couplers in InP generic... Imlnce [db] Excess loss [db] Phse error [ ] µm µm µm 0 µm µm µm µm 0 µm µm µm µm 0 µm Excess loss [db] Imlnce [db] Phse error [ ] µm.8 µm.8 µm 9.8 µm Wvelength [nm] µm.8 µm.8 µm 9.8 µm Wvelength [nm] 8.8 µm.8 µm.8 µm 9.8 µm Wvelength [nm] Figure : Mesured performnce of the ( ) nd ( ) MMI couplers. design MMI width dependency. The vlues re verge over nm intervl. wvelength dependency. e reduced to, which for the given FSR gives wvelength mesurement sensitivity of less thn pm. Acknowledgements This reserch is crried out in the ProCon Project 9, supported y the Dutch Technology Foundtion STW, which is prt of the Netherlnds Orgniztion for Scientific Reserch (NWO), nd which is prtly funded y the Dutch Ministry of Economic Affirs. References [] L. Soldno nd E. Pennings, Opticl multi-mode interference devices sed on self-imging: Principles nd pplictions, Journl of Lightwve Technology, vol., pp. 7, Apr. 99. [] C. vn Dm, M. Amersfoort, G. ten Kte, et l., Novel InP-sed phsed-rry wvelength demultiplexer using generlized MMI-MZI configurtion, in Proc. 7th Eur. Conf. on Integr. Opt. (ECIO 9), pp. 7 78, Delft, The Netherlnds, April 99. [] L. Zimmermnn, K. Voigt, G. Winzer, K. Petermnn, nd C. Weinert, C -nd opticl 90 -hyrids sed on silicon-on-insultor wveguide couplers, Photonics Technology Letters, IEEE, vol., pp., Fe 009. [] P. Reyes-Iglesis, A. Orteg-Moñux, nd I. Molin-Fernández, Enhnced monolithiclly integrted coherent 0 downconverter with high friction yield, Opt. Express, vol. 0, pp. 0 08, Oct 0. [] E. M. vn Vliet, D. Pustkhod, M. K. Smit, nd X. J. M. Leijtens, Tolernt nd high performnce nd multimode interference couplers, in Proceedings of the 9th Annul Symposium of the IEEE Photonics Society Benelux Chpter, pp. 99 0, 0. [] E. Kleijn, E. M. vn Vliet, D. Pustkhod, M. K. Smit, nd X. J. M. Leijtens, Amplitude nd phse error correction lgorithm for MMI sed mch zehnder interferometers, Journl of Lightwve Technology, vol., pp. 9, June 0. [7] M. Smit, X. Leijtens, H. Amrosius, et l., An introduction to InP-sed generic integrtion technology, Semiconductor Science nd Technology, vol. 9, no. 8, p. 0800, 0. 8