Ultralow-Loss 1 8 Splitter Based on Field Matching

2060IEEE PHOTONICS TECHNOLOGY LETTERS,VOL.16,NO.9,SEPTEMBER 2004Ultralow-Loss18Splitter Based on Field Matching Y JunctionJ.Gamet and G.PandraudAbstract—A novel approach to anultralow-loss junction is introduced and designed.By adding a succession of low index regions with properly chosen parameters in the straight waveguide before the branching region,the field mismatch is improved with a power presplitting effect.In order to show that effect we fabri-cated,tested,and compared18splitters formed by cascadedconventional branches and the proposed fieldmatching branch.Index Terms—Optical planar waveguide components,powersplitters,branch.I.I NTRODUCTIONTHE junctions are fundamental elements for integrated optical devices.They are used as power divider–combiner in power splitters and in interferometric devices like modula-tors,switches,and attenuators.Recently,splitters have attracted a growing interest as fundamental building blocks of passive optical networks [1].Silica-on-silicon waveguides have superior advantages such as low propagation losses (typically lower then 0.1dB/cm)and are ideal to realize different functions on the same chip [2].The major limitation whendesigning junction on such a platform is the transmission losses due to the minimum width of the gap between the two branching waveguides limited by photolithog-raphy and etching processes.The excess losses then generated are generally not suitable for practical applications.In this letter,we propose a new way ofdesigningjunc-tions for high index contrast silica-on-silicon waveguides.Wewill show that theproposedjunction,while used in a18splitter,can meet the stringent requirements of low polariza-tion-dependent loss (PDL)and low insertion loss uniformity.II.D ESIGN AND F ABRICATIONSegmented waveguides have shown [3]that they can help re-duce the insertion loss of integrated optical circuits.The segmentation increases the propagating mode field allowing a perfect matching with standard optical fibers.Fig.1(a)shows aconventionaljunction.It is well known that thegap between the two branching waveguides should be no less than2m when a plasma-enhanced chemical vapor deposition (PECVD)process is used to deposit the cladding layer (otherwise there may exist air voids at the tip ofthe branch).This gap influences the coupling between the incomingManuscript received April 29,2004;revised May 25,2004.J.Gamet is with Opsitech,S.A.,38000Grenoble,France.G.Pandraud is with Electronic Instrumentation Laboratories,TU Delft,Delft 2628CD,The Netherlands (e-mail:g.pandraud@ewi.tudelft.nl).Digital Object Identifier10.1109/LPT.2004.833071Fig.1.(a)Conventional Y junction and (b)the proposed lowloss.Fig.2.Normalized simulated fields at the end of the segmented section for transverse-electric polarization.guided mode and the output “supermode”but by introducing a segmentation [Fig.1(b)]a pre-splitting occurs improving the matching with the double-peaked intensity profile of the two output ing the beam propagation method [4],we carefully adjusted theparametersand of the segmented section.In order to find the minimum excess loss,we calculated the excess loss for a range of parameters (,)and varying the number of periods.The lowest excess loss has a minimal valueof 0.09dB for 13periods,whenm,m.Fig.2shows the fields obtained by simulation before the branching waveguides for the proposed optimized transition (gray line),the two outputs (double-peak profile),and the input waveguides.Residual losses still come from the mismatch in the center but for a fixed number of periods,the excess loss remains lower than 0.15dB (for any combination (,)ensuring a good fabrication tolerance.The theoretical excess loss ofthe junction without segmentation is 0.4dB.In order to test the presented optimal design,we fabricated18splitters.A straight and a bent waveguide are added to the photolithographic die to provide references.The18splitters with theoptimized branch are 33mm long and 2.25mm wide.1041-1135/04$20.00©2004IEEEGAMET AND PANDRAUD:ULTRALOW-LOSS18SPLITTER BASED ON FIELDMATCHING JUNCTION2061parison between 128splitters with segmented junction and without.18splitters without segmentation have also been placed on the same mask and have the same size.The fabrication process uses a PECVD process to deposit thesilica layers.Atm,the claddingindex is 1.444and the coreindex1.454.The waveguide core layer height is4.5m and the width is de fined by lithography as6.5m.The cladding is deposited in three steps of6m (to ensure optical isolation)and then heated up at high temperature.This process is required as the PECVD tech-nique introduces hydrogen that causes losses around1.5m.When designing wide spectrum devices,such loss has to be avoided.To reduce losses due to the curved waveguides,the minimum bent radius is kept higher than 8mm.The other parameters are the same as the one for the numerical example depicted in Fig.2.The refractive indexes of the segmented sectionareand ,alternatively.Thegap for thenonsegmented branch is also2m.Optical paths are designed using P-curves family combining loss optimization and widening at the interface between curved and straight waveguides to minimize PDL and excess loss [5].III.R ESULTSAsconventional branch are described as wavelength insen-sitive and widely used for wavelength-dense-multiplexing com-munication systems [6],the wavelength responses of the18splitters were characterized for comparison.The18splitters have been pigtailed with standard SMF28fibers (an-eight fiber block is used at the output).The light from a white light source is then launched into the devices and the alignment performed (according to the energies collected in all the output waveguides).A scan in wavelength is performed for a random state of polarization.Fig.3shows,for the eight outputs,the output power versus the wavelength for the improved and the conventional18splitters.The wavelength window for optical communicationscovers -bandand -band,in the range of 1528–1620nm,but the light source used here allowed us to go down to 1200nm.Inthe -and -band,both splitters have a flat response and can be considered as wavelength insensitive,but the18splitters using theproposed branch outperform the conventionaloneFig.4.Loss comparison between 128splitters and reference waveguides.:nonsegmented 128splitters.:segmented 128splitters.4:curved waveguide. :straightwaveguide.Fig.5.PDL of the 128splitter with the optimized Y junction.by about 1dB.In Fig.3,the total losses are presented and in-clude coupling loss.Numerical simulations presented in the previous section showed that the “supermode ”matches the incoming beamfor theproposedbranch.That is not the case anymore for theconventionaljunction and,theoretically,the 1dB is con firmed.When moving toward 1200nm,the18splitters with fieldmatchingbranches outperform the conventional splitter by about 3dB.Numerical simulations show once again a shift of the “supermode ”while moving to lower wavelengths increasing then the mode mismatch.Furthermore,when comparing theproposed branch with those using multimode transition section [7],the same mode distortion is observed (as the multimode interference MMI is highly wavelength dependent).Here,we have obtained a flat-tened response other the entire scanned spectrum.We have then averaged the eight output powers and compare them with the losses given by the straight and curve reference waveguides.Fig.4gives the total loss including the coupling loss (the fiber-to-fiber loss has been here subtracted).The new18splitters provide an excess loss of 0.4dB from the straight waveguide and part of it can be attributed to the curved sections.We are currently investigating new designs to reduce that but,as we saw in Fig.2,there is a limit as the segmented section does not perfectly match the branching waveguides mode pro file.Finally,we concentrated on the PDL and measured it (Fig.5)for the eight outputs over a scanned wavelength range.The PDL2062IEEE PHOTONICS TECHNOLOGY LETTERS,VOL.16,NO.9,SEPTEMBER 2004is calculated based on four scans at orthogonal states of polar-ization,using Mueller matrix algorithms in the optical commu-nications wavelength window.The worst value over the scanned wavelength window is 0.25dB.IV .C ONCLUSIONWe have designed,fabricated,and tested a18splitter based on newly lowloss junction.A gain of about 3dB in the low wavelengths range has been observed as well as a very low ex-cess loss.Finally,the PDL remains lower than 0.5dB on the optical communications wavelength window satisfying network designer requirements.R EFERENCES[1] C.R.Giles,C.Doerr,M.Zirngibl,C.Joyner,U.Koren,K.F.Dryer,J.Zyskind,J.Sulhoff,L.Stulz,and L.C.Wolf,“1.8petabit/s downstream capacity WDM passive optical network,”in Proc.Optical Fiber Com-munication (OFC’97),vol.4,Sept.1997,pp.22–25.[2]Y .Yamada,S.Suzuki,K.Moriwaki,Y .Hibino,Y .Tohmori,Y .Akatsu,Y .Nakasuga,T.Hashimoto,H.Terui,M.Yanagisawa,Y .Inoue,Y .Aka-hori,and 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