We propose a novel grating coupler design which is inherently reflectionless by focusing the reflected light away from the entrance waveguide. The grating coupler design is investigated by means of 3D FDTD simulations

Baets, Roel

Bogaerts, Wim

De Koninck, Yannick

Lambert, Emmanuel

Li, Yanlu

Roelkens, Günther

Vermeulen, Diedrik

English

D. Vermeulen

Y. De Koninck

Y. Li

E. Lambert

W. Bogaerts

R. Baets

G. Roelkens

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Reflectionless grating coupling for silicon-on-insulator integrated circuits

Ghent University Academic Bibliography

Reflectionless Grating Coupling forSilicon-on-Insulator Integrated CircuitsD. Vermeulen, Y. De Koninck, Y. Li, E. Lambert, W. Bogaerts, R. Baets, G. RoelkensPhotonics Research Group, Department of Information Technology,Ghent University - imec, B-9000 Ghent, BelgiumAbstract: We propose a novel grating coupler design which is inherently reflectionless byfocusing the reflected light away from the entrance waveguide. The grating coupler design isinvestigated by means of 3D FDTD simulations.1. IntroductionPhotonic integration is considered to be a key technology for future advancements in optical communication technolo-gies. Scaling down the optical building blocks enables complex and ultra-compact photonic circuits at a fraction of thecost of conventional systems consisting out of discrete components. Choosing an appropriate platform for developingthis miniaturization technology is guided by functionality, compatibility, performance, yield and cost. Regarding thelast two issues, Silicon-on-Insulator (SOI) is by far the leading technology for low-cost and high-volume photonicintegration since it can benefit from the processes developed in the mature electronics industry. Although the highrefractive index contrast of SOI reduces the footprint of the integrated photonic devices considerably, it becomes moredifficult to achieve a high-performance mode-size convertor between a single-mode optical fiber (100µm2) and anintegrated optical waveguide (0.1µm2). A possible solution is a grating coupler which is a periodic structure that cou-ples light out of the chip to free space or to an optical fiber [1]. Fiber-to-chip grating couplers with very high efficiencyhave been demonstrated [2]. However, as most papers focus on coupling efficiency, polarization dependency, ease offabrication and bandwidth, a solution for the back reflection of these grating couplers is never investigated. A typicalreflection back into the waveguide for a high-efficiency grating coupler is around −17dB [3]. Other reported highback reflections are −10dB [4],[5] and −8dB [6]. These amounts of back reflections are unacceptable for integratedcircuits which contain integrated lasers. Furthermore, back reflections are highly unwanted in interferometer baseddesigns.2. Theoretical OutlineFor grating couplers there are two main sources which contribute to the reflection back into the waveguide. The mostimportant source is the second order reflection of the grating. Typically one eliminates this second order reflection bytilting the optical fiber under a small angle of around 10◦ with respect to the surface normal [7]. The second source ofback reflection is due the Fresnel reflection at the grating coupler interface. This reflection is highly dependent on thegrating structure itself and is very difficult to eliminate. In the proposed grating coupler design, we will refocus thesetwo sources of reflection away from the entrance waveguide thereby extinguishing the reflection almost completely.In [8] a compact focusing grating coupler is presented which uses a curved grating to focus the coupled light onto asingle-mode waveguide, achieving an eight-fold length reduction as compared to a standard linear grating and adiabatictaper, without performance penalty. For the reflectionless grating coupler we follow the same design rules. The gratingis curved such that a plain wavefront originating from the tilted optical fiber is phase-matched to the TE-polarizedmodes of a broad SOI waveguide, so that the wavefront is curved cylindrically and focusing occurs in the center ofcurvature of the wavefront. When the top surface of the waveguide is chosen to be the (Y,Z) plane of a right handedCartesian coordinate system, with z along the waveguide axis and the origin chosen to be in the desired focal point weobtain grating lines that are confocal ellipses Eq0 ,Eq0+1, ...,Eq0+n with the common focal point f1,q0 = f1,q0+1 = ...=f1,q0+n at the origin (see Fig. 1a): (z− qλ0nt sinθn2e f f−n2t sin2 θ)2(qλ0ne f fn2e f f−n2t sin2 θ)2 + y2[qλ0(n2e f f−n2t sin2 θ)1/2]2 = 1 (1)where q is an integer number for each grating line, θ is the angle between the fiber and the chip surface normal, nt isthe refractive index of the environment, λ0 is the vacuum wavelength and ne f f is the effective refractive index felt bythe cylindrical wave in the slab and grating area. The position of the second focal points is given byf2,q(z,y) =(2qλ0nt sinθn2e f f −n2t sin2 θ,0)(2)The phase-matching condition is fulfilled for all parts of the confocal ellipses. We can thus choose which part of theellipse we use for designing a grating coupler because every part of the ellipse will couple out the light under thesame angle θ . Normally one takes the right part of the confocal ellipses as shown in Fig. 1b, such that the propagationvector ~β of the entrance waveguide is parallel to the projected wave vector~kin,z of the diffracted light (waveguide angleα = 0 ◦). Although this seems like the most logical choice, this design will introduce the largest back reflection sincethe reflection from the grating lines will be refocused to the second focal points and coupled back into the entrancewaveguide. This is solved by using another part of the confocal ellipses as grating coupler and rotating the waveguidewith an angle α of for example 45◦, see Fig. 1c. In this design the focal points and the grating are not inline hence thegrating reflection is focused away from the entrance waveguide into a slab region, thereby fully eliminating the backreflection.(a) Confocal ellipses (common focal point) (b) α = 0◦ (c) α = 45◦Fig. 1: Reflection from curved grating couplers for different waveguide angles α . For z= 0 the k-diagram is drawn (K = the localreciprocal lattice vector).3. Simulations1.45 1.50 1.55 1.60Wavelength [µm]5550454035302520Reflection [dB]0 ◦45 ◦90 ◦(a) Shallow etched grating couplers1.45 1.50 1.55 1.60Wavelength [µm]454035302520151050Reflection [dB]0 ◦45 ◦90 ◦(b) Fully etched grating couplersFig. 2: Reflection spectra for shallow and fully etched grating couplers, each time for different values of the waveguide angle α .The grating couplers were simulated using full-vectorial 3D FDTD software [9]. To calculate the reflection spectra,a pulse with a Gaussian time profile was launched in the single-mode access waveguide and the reflected flux wasmeasured in that same waveguide. Figure 2 shows the reflection spectra of the proposed grating coupler for differentwaveguide angles α . For the shallow etched gratings (70nm etch in a 220nm Si core), the back reflection decreasesaround 10dB to 15dB thereby achieving typical back reflections of optical fiber connectors (−40dB). For fully etchedgratings (220nm Si core), the back reflection decreases from −5dB to −25dB thereby making these gratings usefulfor integrated circuits. The coupling efficiency was calculated by exciting a single frequency continuous wave in theaccess waveguide and calculating the overlap between the optical field above the coupler and the mode profile of asingle-mode fiber (θ ≈ 15 ◦), normalized to the power in the access waveguide. We found that the substrateless shallowetched design has a coupling efficiency of 33% independent of the waveguide angle α .(a) α = 0◦ (b) α = 45◦ (c) α = 90◦Fig. 3: Cross-sections of the electric field for a shallow etched grating coupler for different waveguide angles α . The color scale issaturated to visualize the reflection4. ConclusionWe have investigated a reflectionless grating coupler design by means of 3D FDTD simulations. The reflection could bedecreased down to−45dB, thereby realizing the same back reflection performance of a typical optical fiber connector,without introducing a coupling efficiency penalty. Furthermore, we showed that this method can be used to reduce thereflection of highly reflective (−5dB) grating couplers to useful reflections of −25dB. These devices could becomekey components in interferometer based designs and circuits using integrated lasers.5. AcknowledgementsDiedrik Vermeulen thanks the Institute for the Promotion of Innovation by Science and Technology in Flanders (IWT)for a grant. Yannick De Koninck acknowledges the Research Foundation - Flanders (FWO) for a research grant.References and links1. D. Taillaert, W. Bogaerts, P. Bienstman, T. Krauss, P. Van Daele, I. Moerman, S. Verstuyft, K. De Mesel, and R. Baets, “An out-of-plane gratingcoupler for efficient butt-coupling between compact planar waveguides and single-mode fibers,” IEEE Journal of Quantum Electronics,vol. 38, no. 7, pp. 949–955, Jul. 2002. [Online]. Available: http://ieeexplore.ieee.org/lpdocs/epic03/wrapper.htm?arnumber=10176132. D. Vermeulen, S. Selvaraja, P. Verheyen, G. Lepage, W. Bogaerts, P. Absil, D. Van Thourhout, and G. Roelkens, “High-efficiencyfiber-to-chip grating couplers realized using an advanced CMOS-compatible silicon-on-insulator platform.” Optics express, vol. 18, no. 17,pp. 18 278–83, Aug. 2010. [Online]. Available: http://www.ncbi.nlm.nih.gov/pubmed/207212203. A. Mekis, S. Gloeckner, G. Masini, A. Narasimha, T. Pinguet, S. Sahni, and P. Dobbelaere, “A Grating-Coupler-Enabled CMOSPhotonics Platform,” Selected Topics in Quantum Electronics, IEEE Journal of, no. 99, pp. 1–12, 2010. [Online]. Available:http://ieeexplore.ieee.org/xpls/abs all.jsp?arnumber=56456524. L. Zhou, Z. Li, Y. Zhu, Y. Li, Z. Fan, W. Han, Y. Yu, and J. Yu, “A novel highly efficient grating coupler withlarge filling factor used for optoelectronic integration,” Chinese Physics B, vol. 19, p. 124214, 2010. [Online]. Available:http://iopscience.iop.org/1674-1056/19/12/1242145. X. Chen and H. Tsang, “Polarization-Independent Grating Couplers for Silicon-on-Insulator Nanophotonic Waveguides,” OPTICS LETTERS,vol. 36, no. 6, pp. 796–798, 2011. [Online]. Available: http://www.opticsinfobase.org/ol/upcoming pdf.cfm?id=1394646. H. Tsang, “Nanoholes Grating Couplers for Coupling Between Silicon-on-Insulator Waveguides and Optical Fibers,” IEEE PhotonicsJournal, vol. 1, no. 3, pp. 184–190, Sep. 2009. [Online]. Available: http://ieeexplore.ieee.org/lpdocs/epic03/wrapper.htm?arnumber=52328767. D. Taillaert, P. Bienstman, and R. Baets, “Compact efficient broadband grating coupler for silicon-on-insulator waveguides.” Optics letters,vol. 29, no. 23, pp. 2749–51, Dec. 2004. [Online]. Available: http://www.ncbi.nlm.nih.gov/pubmed/156054938. F. Van Laere, T. Claes, J. Schrauwen, S. Scheerlinck, W. Bogaerts, D. Taillaert, L. O’Faolain, D. Van Thourhout, and R. Baets, “CompactFocusing Grating Couplers for Silicon-on-Insulator Integrated Circuits,” IEEE Photonics Technology Letters, vol. 19, no. 23, pp. 1919–1921,Dec. 2007. [Online]. Available: http://ieeexplore.ieee.org/lpdocs/epic03/wrapper.htm?arnumber=43831989. A. F. Oskooi, D. Roundy, M. Ibanescu, P. Bermel, J. Joannopoulos, and S. G. Johnson, “Meep: A flexible free-software package forelectromagnetic simulations by the FDTD method,” Computer Physics Communications, vol. 181, no. 3, pp. 687–702, Mar. 2010. [Online].Available: http://linkinghub.elsevier.com/retrieve/pii/S001046550900383X

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Reflectionless grating coupling for silicon-on-insulator integrated circuits

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Ghent University Academic Bibliography