Absil, P

Bogaerts, Wim

Lepage, G

Roelkens, Günther

Selvaraja, shankar kumar

Van Thourhout, Dries

Verheyen, P

Vermeulen, Diedrik

Optics Express

English

Ghent University Academic Bibliography

High-Efficiency Fiber-to-Chip GratingCouplers realized using An AdvancedCMOS-Compatible Silicon-On-InsulatorPlatformD. Vermeulen 1, S. Selvaraja 1, P. Verheyen 2, G. Lepage 2W. Bogaerts 1, P. Absil 2 , D. Van Thourhout 1 and G. Roelkens 11 Photonics Research Group, Department of Information Technology,Ghent University - imec, B-9000 Ghent, Belgium2 imec,Kapeldreef 75, B-3001 Leuven, BelgiumDiedrik.Vermeulen@intec.ugent.behttp://photonics.intec.ugent.beAbstract: A new generation of Silicon-on-Insulator fiber-to-chip gratingcouplers which use a silicon overlay to enhance the directionality andthereby the coupling efficiency is presented. Devices are realized on a200mm wafer in a CMOS pilot line. The fabricated fiber couplers show acoupling efficiency of −1.6dB and a 3dB bandwidth of 80nm.© 2010 Optical Society of AmericaOCIS codes: (000.0000) General.References and links1. D. Taillaert, W. Bogaerts, P. Bienstman, T.F. Krauss, P. Van Dale, I. Moerman, S. Verstuyft, K. De Mesel, andR. Baets, “An out-of-plane grating coupler for efficient butt-coupling between compact planar waveguides andsingle-mode fibers,” IEEE J. Quantum Electron. 14, 949–955 (2002).2. D. Taillaert, P. Bienstman, and R. Baets, “Compact efficient broadband grating coupler for silicon-on-insulatorwaveguides,” Opt. Lett. 29, 2749–2751 (2004).3. G. Roelkens, D. Van Thourhout, and R. Baets, “High Efficiency grating coupler between silicon-on-insulatorwaveguides and perfectly vertical optical fibers,” Opt. Lett. 32, 1495–1497 (2007).4. X. Chen, C. Li, and H. K. Tsang, “Fabrication-tolerant waveguide chirped grating coupler for coupling to aperfectly vertical optical fiber,” IEEE Photonics Technology Letters, 20, 1914-1916 (2008).5. G. J. Liu, Q. Li, G. L. Jin, and B. M. Liang, “Transfer matrix method analysis of apodized grating couplers,”Opt. Commun. 235, 319–324 (2004).6. L. Vivien, D. Pascal, S. Lardenois, D. Marris-Morini, E. Cassan, F. Grillot, S. Laval, J.M. Fe´de´li, and L. ElMelhaoui, “Light injection in SOI microwaveguides using high-efficiency grating couplers,” J. Lightwave Tech-nology, 24, 3810–3815 (2006).7. S. K. Selvaraja, D. Vermeulen, M. Schaekers, E. Sleeckx, W. Bogaerts, G. Roelkens, P. Dumon, D. VanThourhout, and R. Baets, “Highly efficient grating coupler between optical fiber and silicon photonic circuit,” inLasers and Electro-Optics/International Quantum Electronics Conference, (2009).8. F. Van Laere, G. Roelkens, M. Ayre, J. Schrauwen, D. Taillaert, D. Van Thourhout, T. F. Krauss, and R. Baets,“Compact and highly efficient grating couplers between optical fiber and nanophotonic waveguides,” J. Light-wave Technology, 25, 151–156 (2007).9. J. Schrauwen, F. Van Laere, D. Van Thourhout, and R. Baets, “Focused-ion-beam fabrication of slanted gratingcouplers in silicon-on-insulator waveguides,” IEEE Photonics Technology Letters, 19, 816–818 (2007).10. G. Roelkens, D. Van Thourhout, and R. Baets, “High efficiency silicon-on-insulator grating coupler based on apoly-silicon overlay,” Opt. Express 14, 11622–11630 (2006).11. G. Roelkens, D. Vermeulen, D. Van Thourhout, R. Baets, S. Brision, P. Lyan, P. Gautier, and J. M. Fe´de´li, “Highefficiency diffractive grating couplers for interfacing a single mode optical fiber with a nanophotonic silicon-on-insulator waveguide circuit,” Appl. Phys. Lett. 92, 131101 (2008).12. X. Chen, C. Li, S. M. G. Lo, K. Fung, and H. K. Tsang, “Silicon Waveguide Grating Couplers with EngineeredCoupling Strength for Optimized Coupling,” in European Conference on Integrated Optics, (2010).13. T.K. Saha, and W. Zhou, “High efficiency diffractive grating coupler based on transferred silicon nanomembraneoverlay on photonic waveguide,” J. Phys. D: Appl. Phys. 42, 085115 (2009).14. F. Van Laere, T. Claes, J. Schrauwen, S. Scheerlinck, W. Bogaerts, D. Taillaert, L. O’Faolain, D. Van Thourhout,and R. Baets, “Compact focusing grating couplers for silicon-on-insulator integrated circuits,” IEEE PhotonicsTechnology Letters, 27, 612–618 (2009).15. D. Vermeulen, K. Van Acoleyen, S. Ghosh, S. Selvaraja, W.A.D. De Cort, N.A. Yebo, E. Hallynck, K. De Vos,P.P.P. Debackere, P. Dumon, W. Bogaerts, G. Roelkens, D. Van Thourhout, and R. Baets, “Efficient Tapering tothe Fundamental Quasi-TM Mode in Asymmetrical Waveguides,” in European Conference on Integrated Optics,(2010).16. C. R. Doerr, L. Chen, M. Rasras, Y.K. Chen, J. Weiner, and M. Earnshaw, “Diplexer With Integrated Filters andPhotodetector in Ge/Si Using Γ−X and Γ−M Directions in a Grating Coupler,” Photonics Technology Letters,21, 1698–1700 (2009).1. IntroductionIn the last decade much research has focused on silicon photonic integrated circuits, due to thescaling in size that can be accomplished because of the high refractive index contrast availableon the SOI material platform. However, the scaling of the device cross-section complicatesthe interfacing with a single mode optical fiber. Several schemes have been proposed to tacklethis problem. We focused particularly on fiber-to-chip coupling by means of subwavelengthstructured surfaces such as line gratings or photonic crystals. With these grating couplers onecan couple out-of-plane and thus test photonic circuits on a wafer-scale without the need ofany post-processing or cleaving. Furthermore because of a relaxed 1dB alignment sensitivityof 2µm, it is even possible to align several optical fibers at the same time by using fiber ar-rays for wafer-scale testing or packaging. Despite the fact that these grating couplers are a verygood candidate for fiber interfacing for the low cost and high volume oriented silicon photonicsframework, they suffer from relatively high coupling loss, making them only useful for researchpurposes or specific applications with a relaxed power budget.In 2002 we reported an SOI grating coupler with a fiber coupling efficiency of −7dB [1].Since then, a lot of attempts were made to improve the coupling efficiency significantly. In ge-neral there are three factors that contribute to the poor coupling efficiency of standard gratingcouplers. Most of the light 35%−45% is lost because of diffraction towards the substrate andaround 20% is lost because of poor mode-matching to the Gaussian shaped mode of the fiber.For perfectly vertical coupling another 30% is reflected back into the waveguide due to secondorder Bragg diffraction. This last issue can be easily solved by coupling under a small anglewhich breaks the symmetry and eliminates the reflection [2]. An alternative method is to etcha deep slit in front of the grating which acts together with the grating as a Fabry-Perot cavity[3]. By chirping the grating, back reflections [4] and losses due to mode-mismatch [5] can beminimized. Most attempts to increase the directionality of the grating all involve substrate engi-neering [6]. Other approaches are either based on amorphous silicon, which limits the thermalbudget [7], or require a lot of post-processing [8]. Alternatively, one can use exotic gratingswith for example slanted facets, in analogy to a blazed grating, to enhance the coupling to thefirst diffraction order in the superstrate [9]. In 2006 however, we proposed a rather simple andelegant way to adapt the grating and increase the coupling efficiency drastically, i.e. by defi-ning a silicon overlay prior to etching the grating [10]. Early attempts which involved epitaxialsilicon growth were promising and showed efficiencies up to −2.6dB [11]. In this paper wedemonstrate high efficiency grating couplers, fabricated in a CMOS pilot line, with −1.6dBcoupling efficiency, approaching the coupling efficiencies required in practical applications.Fig. 1. Schematic illustrations of fiber-to-chip high index contrast grating coupler optimiza-tion factors: the Bragg condition (a), upwards directionality (b), 2D fiber mode-matchingin z- and x-direction (c,d) and mode-matching between the waveguide and Bloch mode ofthe grating (e).2. Fiber Coupler Grating DesignFor designing a fiber-to-chip grating coupler, which is inherently highly efficient, several de-sign issues have to be taken into account. These are summarized in Figure 1. The first orderdiffraction of a grating is described by the Bragg condition β −K = kz where the propagationconstant of the guided mode β = 2piλ ne f f , K =2piΛ and the projected wavevector of the incidentmode kz = 2piλ sinθ with λ the wavelength of the light, Λ the period of the grating and θ theangle of the fiber with respect to the surface normal of the grating. In order to avoid a highsecond order reflection, it is convenient to couple in under a positive fiber angle as shown inFigure 1(a). In general the effective refractive index ne f f of the grating is a function of the gra-ting parameters shown in Figure 2(a). However, since for a given grating period and given fiberangle it is almost always possible to find a wavelength at which the bragg condition is fulfilled,this dependency will have a minor influence on the maximal coupling efficiency of the gratingcoupler but rather on the central wavelength of the coupling spectrum. In the rest of the analysiswe will assume that the Bragg condition is satisfied and that the fiber coupling angle θ is closeto zero.The physical principle behind a highly directional grating as shown in Figure 1(b) can beunderstood as follows. Considering the diffracted field pattern as the superposition of the fieldsemitted by an array of scattering centers (which have a pi phase shift with respect to eachother for a perfect vertically coupling grating), constructive interference towards the superstrate(and hence the optical fiber) can be achieved by realizing an additional pi phase shift duringthe propagation towards the superstrate, since light is propagating either in air (in the etchedslit) or in silicon (in the grating tooth). This directionality is thus a function of the waveguidethickness h, the etch depth e in the waveguide and the silicon overlay thickness o (see Figure2). Furthermore, in order to reach high fiber coupling efficiency, a highly directional grating isrequired while at the same time the grating strength needs to be optimized for maximal overlapwith the Gaussian fiber mode, shown in Figure 1(c). This mode-matching requirement is afunction of the same three parameters h, e and o as the high directionality condition. Threedifferent classes of grating cases shown in Figure 2 can be considered which can be favorable,depending on the considered photonics platform, robustness and cost considerations.Fig. 2. Parameterized highly directional grating couplers: the general case (a), without sil-icon overlay (b), with silicon overlay but no waveguide etch (c) and both silicon overlayand waveguide etch (d).The first case 2(b) is a grating without silicon overlay where high directionality and an op-timal grating strength is only achieved for a certain waveguide thickness h and etch depth e.In [12] this grating type is optimized and a coupling efficiency of −1.2dB is achieved for awaveguide thickness h of 340nm and a deep waveguide etch depth e of 200nm. Importantdrawbacks are the fixed waveguide thickness which is multimode and the deep waveguide etchdepth which introduces a large mode mismatch between the waveguide mode and the Blochmode of the grating resulting in high back reflections (see Figure 1(e)). The last issue is im-proved by apodizing the grating, leading to reduced reflections and a better mode matchingwith the Gaussian fiber mode. However, an apodized grating requires feature sizes which arebeyond the limit of current 193nm DUV lithography.For case 2(c), no waveguide etch e is required. This kind of grating coupler has been theo-retically reported in [13], where a transferred silicon nanomenbrane is proposed in order tofabricate the grating. For a waveguide thickness h of 220nm and a silicon overlay thicknesso of 240nm the simulated efficiency was 64%. We optimized this grating type and simulatedfor a uniform grating with h= 150nm and o= 220nm around 80% coupling efficiency, whichis the theoretical upper limit because of the mode mismatch between the exponential shapedelectrical field profile of the uniform grating and the gaussian shaped mode of the optical fiber.Unfortunately, a waveguide thickness of h = 150nm leads to a lower confinement of the TEoptical mode, and does not support any TM polarized waveguide modes. We also like to pointout that the coupling strength of a grating with no waveguide etch is solely dependent on thesilicon overlay and thus on the evanescent field of the waveguide mode. Because this evanes-cent field is different for orthogonal polarized waveguide modes, it is unlikely that there existsa grating which is highly efficient for both polarizations. Despite these drawbacks, this gratingcoupler type could be an ideal candidate for using in a single-polarization low-cost photonicsplatform. This grating can be made very robust and with only one combined grating/waveguideetch step by using an oxide etch stop layer between the waveguide and silicon overlay, therebyfollowing a similar fabrication process as will be described in the fabrication section.The most general fiber coupler grating using a silicon overlay is case 2(d) where both a sili-con overlay o and waveguide etch e is used to define the grating. This gives us the freedomto choose an appropriate waveguide thickness h of for example 220nm. This high efficiencygrating coupler type is theoretically described in [10] and shows around 80% of coupling ef-ficiency according to simulations. Although it requires more process steps than the previouscases, it is compatible with any high index contrast photonic platform. Moreover, the intro-duced additional thickness levels could be used to optimize other integrated photonic devices.An optimization issue that is unrelated to the previous discussion is the mode-matching betweenthe Gaussian fiber mode and the waveguide mode in the x-direction as is shown in Figure 1 (d).For a 220nm thick waveguide, the optimal waveguide width w is 15µm. Since the taper lengthscales quadratic with the waveguide width, a long taper section (≈ 500µm) would be needed toadiabatically transform the broad waveguide of the grating into a single-mode optical waveg-uide. This can be easily solved by curving the grating as is demonstrated in [14] or using afocusing taper [15], without decreasing the grating coupling efficiency.3. FabricationWe developed an advanced Silicon-on-Insulator process flow which is capable of realizing themost general case in Figure 2(d) of the high efficiency grating couplers. The complete fabrica-tion process is done in the 200mm imec CMOS pilot line with 193nm DUV lithography. Firsta very thin silicon oxide layer is deposited on top of a 200mm SOI wafer with a buried oxidelayer thickness of 2µm and a crystalline silicon layer thickness of 220nm. Simulations showthat the thickness of the buried oxide layer is of no importance for the grating directionality,since the grating intrinsically is very directional and does not have to rely on the oxide/siliconinterface reflection to achieve constructive interference towards the optical fiber. In a secondstep an amorphous silicon layer of 160nm is deposited and a deep etch is performed throughthe amorphous silicon and oxide layer reaching 70nm into the crystalline silicon layer. Thisis used to define the slits of the high efficiency grating couplers. Next, the amorphous siliconis removed where necessary and the deposited oxide layer in the first step will now act as anetch stop layer in order to not affect the underlying crystalline silicon. The width of the firsttooth will not affect the grating coupling efficiency if it is narrower than the subsequent teeth.By contrast, larger first tooth widths introduce a rapid decrease of the coupling efficiency. An-ticipating possible mask misalignment between the silicon overlay and the grating etch stepis done by deliberately designing the first tooth smaller. This can be clearly seen in Figure 3where a side view image of the fabricated silicon overlay grating is shown. In the final step thestrip waveguides are defined in the crystalline silicon layer. The measured progation loss fora straight single-mode waveguide of 450nm was 1.8dB/cm, demonstrating the high processquality of this advanced SOI fabrication process.4. Experimental ResultsThe measurement results are shown in Figure 4. All measurements were performed with asingle-mode fiber tilted under an angle of 13 degrees relative to the surface normal of the gra-ting. Index matching fluid was applied between the grating and fiber facet to reduce Fresnelreflections. TE polarized light is used in all experiments, since the gratings behave very pola-rization dependent. Figure 4(a) shows the central wavelength of the fiber coupling spectrum asa function of the grating period and this for several duty cycles. The central wavelength scalesalmost linearly with the grating period because the effective refractive index of the grating isonly weakly dependent upon the wavelength. In Figure 4(b), the coupling efficiency for diffe-Fig. 3. Bird’s eye view of a silicon overlay grating fiber coupler.rent grating periods is plotted versus the duty cycle. It seems that a duty cycle of 35% (refers tothe unetched silicon part) is optimal and we measured a maximum efficiency of −1.6dB witha 3dB bandwidth of 80nm for a grating period of 690nm and central wavelength of 1530nm.This is the highest fiber coupling efficiency reported to date for CMOS-compatible fiber-to-chipgrating couplers.Fig. 4. Measurement results of a sweep of 1D fiber couplers (a-Si overlay) for differentduty cycles (a) and different grating periods (b).5. ConclusionDifferent types of high efficiency grating couplers which are inherently highly directional arediscussed. The most general case of a grating coupler with silicon overlay and etch into theoptical waveguide is fabricated using 193nm DUV lithography. We have shown experimen-tally a coupling efficiency of −1.6dB which is the highest fiber coupling efficiency for gratingcouplers fabricated in a CMOS pilot line. The optimal duty cycle of the grating is found to be35%. A silicon overlay could also be beneficial for more advanced grating couplers whichincorporate polarization diversity and wavelength demultiplexing [16]. Besides the gratingcouplers, the silicon overlay can be used in other components on the SOI chip as an extradegree of design freedom.6. AcknowledgementsDiedrik Vermeulen thanks the Institute for the Promotion of Innovation by Science and Tech-nology in Flanders (IWT) for a grant. W. Bogaerts and G. Roelkens acknowledge the Fund forScientific Research Vlaanderen (FWO) for a postdoctoral grant. The research leading to theseresults has received funding from the European Community’s Seventh Framework Programme(FP7/2007-2013) under grant agreement no 224312 HELIOS.

High-efficiency fiber-to-chip grating couplers realized using an advanced CMOS-compatible silicon-on-insulator platform

https://biblio.ugent.be/publication/1147439/file/1147459

High-efficiency fiber-to-chip grating couplers realized using an advanced CMOS-compatible silicon-on-insulator platform

Abstract

Similar works

Full text

Available Versions

Ghent University Academic Bibliography