Baets, Roel

Rogier, Hendrik

Van Acoleyen, Karel

Optics Express

English

Ghent University Academic Bibliography

Two-dimensional optical phased arrayantenna on silicon-on-insulatorKarel Van Acoleyen,1,∗ Hendrik Rogier,2 and Roel Baets11Department of Information Technology (INTEC) - Photonics Research Group,Ghent University - imec, Sint-Pietersnieuwstraat 41, B-9000 Ghent, Belgium2Department of Information Technology (INTEC) - Electromagnetics Group,Ghent University, Sint-Pietersnieuwstraat 41, B-9000 Ghent, Belgium*karel.vanacoleyen@intec.ugent.beAbstract: Optical wireless links can offer a very large bandwidth andcan act as a complementary technology to radio-frequency links. However,optical components nowadays are rather bulky. Therefore, we have investi-gated the potential of silicon photonics to fabricate integrated componentsfor wireless optical communication. This paper presents a two-dimensionalphased array antenna consisting of grating couplers that couple lightoff-chip. Wavelength steering of 0.24◦/nm is presented, reducing the needof active phase modulators. The required steering range is 1.5◦. The 3dBangular coverage range of these antennas is 0.007π sr with a directivity ofmore than 38dBi and antenna losses smaller than 3dB.© 2010 Optical Society of AmericaOCIS codes: (050.1950) Diffraction gratings; (060.2605) Free-space optical communication;(130.3120) Integrated optics devices.References and links1. J. M. Kahn and J. R. Barry, “Wireless infrared communications,” Proc. IEEE 85, 265–298 (1997).2. K. Van Acoleyen, W. Bogaerts, J. Ja´gerska´, N. Le Thomas, R. Houdre´, and R. Baets, “Off-chip beam steeringwith a one-dimensional optical phased array on silicon-on-insulator,” Opt. Lett. 34, 1477–1479 (2009).3. P. F. McManamon, T. A. Dorschner, D. L. Corkum, L. J. Friedman, D. S. Hobbs, M. Holz, S. Liberman, H. Q.Nguyen, D. P. Resler, R. C. Sharp, and E. A. Watson, “Optical phased array technology,” Proc. IEEE 84, 268–298(1996).4. R. J. Green, H. Joshi, M. D. Higgins, and M. S. Leeson, “Recent developments in indoor optical wireless sys-tems,” IET Commun. Mag. 2, 3–10 (2008).5. S. K. Selvaraja, P. Jaenen, W. Bogaerts, D. Van Thourhout, P. Dumon, and R. Baets, “Fabrication of PhotonicWire and Crystal Circuits in Silicon-on-Insulator Using 193-nm Optical Lithography,” J. Lightwave Technol. 27,4076–4083 (2009).6. ePIXfab, “The silicon photonics platform,” http://www.epixfab.eu/.7. G. Roelkens, D. Vermeulen, D. Van Thourhout, R. Baets, S. Brision, P. Lyan, P. Gautier, and J. M. Fedeli, “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).8. 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 Photon.Technol. Lett. 19, 1919–1921 (2007).9. N. Le Thomas, R. Houdre´, M. V. Kotlyar, D. O’Brien, and T. E. Krauss, “Exploring light propagating in photoniccrystals with Fourier optics,” J. Opt. Soc. Am. B 24, 2964–2971 (2007).10. D. Vermeulen, S. Selvaraja, G. Verheyen, P. Lepage, W. Bogaerts, and G. Roelkens, “High-efficiency Silicon-On-Insulator Fiber-to-Chip Grating Couplers Using a Silicon Overlay,” in “Group IV Photonics,” (United States,2009), p. FPd1.11. K. Van Acoleyen, H. Rogier, and R. Baets, “Feasibility of Integrated Optical Phased Arrays for Indoor WirelessOptical Links,” in “Optical Communication, 2009. ECOC ’09. 35th European Conference on,” (Vienna, Austria,2009), p. P4.18.#125958 - $15.00 USD Received 24 Mar 2010; revised 28 May 2010; accepted 2 Jun 2010; published 10 Jun 2010(C) 2010 OSA 21 June 2010 / Vol. 18,  No. 13 / OPTICS EXPRESS  136551. IntroductionOn the one hands, as the available radio spectrum is getting more and more congested, peopleare looking for solutions by either using a different carrier frequency (60GHz band), MIMO(Multiple-Input-Multiple-Output) communication or by applying UWB (UltraWide Band). Theoptical domain, on the other hand, has a virtually unlimited bandwidth available free from reg-ulations. Optical systems do not suffer from electromagnetic interference, provide an increasedsecurity and could serve either as a stand-alone technology or in hybrid optical-RF systems.Therefore, these systems are getting increased attention for indoor wireless links [1].As the present wireless optical systems consist of bulky components, we have investigatedthe use of an integrated technology to fabricate components for wireless optical links. For thatpurpose, we have made use of the CMOS (Complementary Metal-Oxide Semiconductor) com-patible silicon-on-insulator (SOI) platform allowing dense integration. Beam scanning is im-portant for directive links and can be done by wavelength scanning or active phase tuning.Whereas for an N×N array, N2 phase tuners are needed to perform full two-dimensional beamscanning, this number can be reduced to N by using wavelength scanning in one direction.This paper presents a two-dimensional optical phased array (OPA) fabricated on SOI. OPAsallow very stable, rapid and precise beam steering without mechanical motion, making themrobust and insensitive to external constraints such as acceleration [2,3]. When choosing a largeelement spacing, we will obtain multiple higher-order grating lobes in the far-field. This usu-ally unwanted effect can find applications in multipath optical links [4]. The beam direction iscontrolled by tuning the phase relationship between an array of transmitters. This is performedby wavelength tuning in one direction thanks to delay lines between the elements. Integrationwith N active phase tuners would result in full two-dimensional steering of an N ×N array.Furthermore, these OPAs could be used in coherent receiver schemes.Section 2 describes the design and fabrication of the component using the SOI platform. Sec-tion 3 discusses the Fourier imaging measurement setup to investigate the far-field properties ofthe component. Section 4 provides the measurement results and section 5 presents a conclusion.2. Design and FabricationThe component shown in Fig. 1 was fabricated on SOI with an oxide thickness of 2µm and asilicon top layer thickness of 220nm using standard CMOS processes in imec. This high indexcontrast platform allows a high level of integration with small, low-loss waveguides with bendradii down to 5µm˜ []Selvaraja2009, ePIXfab. Two etching steps are used, one of 220nm foretching the waveguides and the multimode interference (MMI) splitters and the second of 70nmto etch the grating couplers. The input waveguide in Fig. 1 is connected to a grating coupler tocouple light into the waveguide system. Such a grating coupler uses the principle of diffractionto couple light from a single mode optical fiber into the TE-like mode of a 10µm waveguide [7].The waveguide then tapers down to a 450nm wide photonic wire being the input waveguide inFig. 1. This waveguide is then split by a MMI tree into K output waveguides, with K being 4for a 2 by 2 array and 16 for a 4 by 4 array. The radiating elements consist of a focusing gratingcoupler that couples out light in a near vertical direction [8].3. Measurement SetupFigure 2 schematically shows the Fourier-space imaging technique that was used to investigatethe off-chip radiation pattern [9]. The far-field of the component is imaged on the back-focalplane of a microscope objective with numerical aperture NA=0.5. In this plane, one point cor-responds to a unique direction of off-chip emission and can be parameterized by (sinθ , sinφ ).This back-focal plane is then brought back to a CCD camera by means of two lenses. The pa-#125958 - $15.00 USD Received 24 Mar 2010; revised 28 May 2010; accepted 2 Jun 2010; published 10 Jun 2010(C) 2010 OSA 21 June 2010 / Vol. 18,  No. 13 / OPTICS EXPRESS  13656Fig. 1. Two-dimensional OPA on SOI: schematic view (left), top-view SEM picture of themeasured 4×4 OPA (right). Light coming from the input waveguide is split via a MMI treeand then coupled out-of-plane using focusing grating couplers.rameters θ and φ are defined as the out-coupling angles in the longitudinal plane and transverseplane (i.e. perpendicular to the waveguide axis) with respect to the normal of the sample surface(Fig. 1).Fig. 2. Schematic view of the Fourier imaging setup: the backfocal plane of the microscopeobjective (MO) is brought back to the CCD camera by means of two lenses.4. Measurement Results and Discussion4.1. Far-field patternFigure 3(a) presents a schematic view of the cross section of a focusing grating coupler. The far-field of this focusing grating coupler is shown in Fig. 3(b). This far-field determines the steeringrange of the OPA. The envelope will shift in the θ -direction when changing the wavelength dueto the grating equationsinθ = Λne f f ,gc−λ0nctΛ, (1)with Λ = 625nm being the period of the grating, λ0 the free-space wavelength, ne f f ,gc theeffective index of the guided mode of the coupler region (≈ 2.67 at λ0 = 1550nm) and nct therefractive index of the background, which is air in this case. The shift in θ angle as well as thechange in beam width can be found in Fig. 3(c). The latter does not change significantly.These grating couplers are placed in a N ×M array configuration with positions smn =mΛxux + nΛyuy with Λx and Λy the spacing of the elements in the x- and y-direction, respec-tively, and where m = 0 . . .M−1 and n = 0 . . .N−1. The far-field is calculated by multiplicationof the far-field of one grating coupler with the array factor T (θ ,φ):T (θ ,φ) =M−1∑m=0N−1∑n=0Amne− jβmne jk ·smn =M−1∑m=0N−1∑n=0Amne− jβmne jk0[mΛx sinθ+nΛy sinφ ], (2)where βmn is the phase delay between the elements, k is the wave vector (in air) with magnitudek0 and Amn is the amplitude of each element which is constant in our case and further assumed#125958 - $15.00 USD Received 24 Mar 2010; revised 28 May 2010; accepted 2 Jun 2010; published 10 Jun 2010(C) 2010 OSA 21 June 2010 / Vol. 18,  No. 13 / OPTICS EXPRESS  13657(a)(b) θ (o)φ (o )  5 10 15−15−10−5051015 00.20.40.60.81(c) 1530 1540 1550 1560 1570468101214λ (nm)angle (o )  θΔθΔφFig. 3. (a) Schematic view of a cross section of a grating coupler. (b) Far-field pattern of thefocusing grating coupler at a wavelength of 1550nm. The FWHM width in the θ -directionis 4.8◦ and in the φ -direction is 9.6◦. (c) Out-coupling angle θ (blue), FWHM θ (red) andFWHM φ (green) as a function of wavelength.to be 1. Having only a length difference ΔL between the elements in the x-direction, we obtain:βmn = ne f f 2πλ ΔLmn = mne f f2πλ ΔL, (3)with ne f f the effective index of the fundamental TE-like mode. The array factor is calculated inclosed form:T (θ ,φ) = e j[(k0Λx sinθ−k0ne f f ΔL) M−12 +(k0Λy sinφ) N−12] sin(M k0Λx sinθ−k0ne f f ΔL2)sin(k0Λx sinθ−k0ne f f ΔL2)sin(N k0Λy sinφ2)sin(k0Λy sinφ2) .(4)The array factor exhibits a maximum in the θ -direction forsinθ = q λΛx+ne f f ΔLΛx, (5)for integer q. Due to the delay lines, the absolute value of q will be large. The beams will shiftat a rate ofdθdλ ≈dsinθdλ =qΛx+dne f fdλΔLΛx, (6)where the angle θ is assumed to be relatively small. Note that q is negative so that the beam willshift in the negative θ -direction for increasing wavelength. The effect of dispersion cannot beneglected in this high index contrast platform. There are two main contributions to dispersion:material dispersion and waveguide dispersion. Whereas the former is relatively weak for thesmall wavelength range considered in this paper, the latter has a significant influence due to theextremely high confinement in the small photonic wires. The change of ne f f as a function ofwavelength is negative as well, being about -0.013/nm around λ = 1.55µm.Figure 4 shows the measured far-field pattern of a 2× 2 OPA together with the theoreticalfar-field. Figure 5 shows a cross section of the far-field along the θ -direction for a 2× 2 and4×4 OPA. The asymmetry of these patterns is due to the delay lines. The parameters of theseOPAs are found in Table 1. The larger discrepancy for the 4×4 OPA is due to the saturation of#125958 - $15.00 USD Received 24 Mar 2010; revised 28 May 2010; accepted 2 Jun 2010; published 10 Jun 2010(C) 2010 OSA 21 June 2010 / Vol. 18,  No. 13 / OPTICS EXPRESS  13658(a) (b)Fig. 4. Theoretical (a) and measured (b) far-field pattern of a 2×2 OPA at a wavelength of1550nm, the parameters are found in Table 1.the camera pixels at the higher intensities. When the OPA becomes large, the beams are only afew pixels wide resulting in less accurate measurements with our setup.(a) 5 10 1500.20.40.60.81θ (o)I (a.u.)  TheoryMeasurement(b) 5 10 1500.20.40.60.81θ (o)I (a.u.)  TheoryMeasurementFig. 5. Cross section at φ = 0◦ of the far-field pattern of a (a) 2× 2 and (b) 4× 4 OPA ata wavelength of 1550nm, the parameters of the OPA are found in Table 1. The indicatedpeak has a width of (a) 0.66◦ and (b) 0.36◦.4.2. Steering capabilityBy changing the wavelength, the envelope of the far-field will shift [Fig. 3(c)], but also thearray factor will shift due to the delay lines. Figure 6(a) shows the shift of the peak indicatedin Fig. 5 by an arrow. For the 2× 2 OPA this shift is about −0.24◦/nm whereas for the 4× 4OPA it is −0.20◦/nm. The latter is smaller since the ratio ΔL/Λx is smaller. The contribution ofdispersion is about −0.11◦/nm for the 2×2 OPA and −0.09◦/nm for the 4×4 OPA.(a) 1545 1550 15559101112λ (nm)θ (o )  2x2 OPA4x4 OPA(b) 1530 1540 1550 1560 15705101520λ (nm)θ(o)  FWHM grating couplerarray factorFig. 6. (a) Shift of the peak shown in Fig. 5 by changing the wavelength. (b) Schematicview of wavelength steering capability. The red region represents the shift of the FWHM ofthe envelope of our far-field pattern (i.e. the far-field pattern of one grating coupler), whilethe blue dashed lines represents the shift of the individual emission lobes with wavelength.#125958 - $15.00 USD Received 24 Mar 2010; revised 28 May 2010; accepted 2 Jun 2010; published 10 Jun 2010(C) 2010 OSA 21 June 2010 / Vol. 18,  No. 13 / OPTICS EXPRESS  13659Figure 6(b) shows the coverage range as a function of wavelength. Steering could be per-formed by first steering the envelope while fine tuning is done by steering the array factor. Asthe envelope shifts, the array factor peaks will shift out of the envelope while new peaks willshift in. Since the peaks are spaced about arcsin(λ/Λ) = 1.48◦ (at 1550nm for a 60µm spac-ing), the required wavelength shift for fine tuning is not more than 7nm. By making the delaylines longer, this number can be further decreased.Steering in the φ -direction can be performed by incorporating phase tuners. To steer the beamof an N×N array in the φ -direction, N modulators are needed where in principle we only needto define the phase difference between each arm.4.3. EfficiencyThe waveguide losses in the SOI platform were shown to be less than 3dB/cm while the bendingloss is 0.013dB/bend. Since the total waveguide length is in the order of mm, this loss is notmore than 1dB [5]. The grating couplers fabricated with one etch of 70nm in a 220nm silicontop layer have a mean efficiency of about -5.2dB. The use of a silicon overlay can enhancethe efficiency up to -1.6dB [10]. This is the efficiency of the grating coupler when it is usedto guide light between an optical fiber and a photonic integrated circuit. The efficiency to freespace, instead of to an optical fiber, will be better. The total loss from waveguide input to freespace can thus be limited to less than 3dB.As shown in [11], the directivity of our OPA does not depend on element spacing but onlyon the effective total radiating aperture. The theoretical and measured directivity, calculated byD(θ ,φ) = 4π I(θ ,φ)Ptot, (7)with Ptot the total power radiated by the OPA, for the peak indicated by an arrow in Fig. 5, isgiven in Table 1. A good agreement with the theory can be seen. For the 2×2 OPA, the gain iseven higher because the lower intensity levels are not measured very accurately.Table 1. Parameters of the measured OPAs.N = M Λx = Λy (µm) ΔL (µm) Dth (dBi) Dmeas (dBi)2 60 87.2 34.9 37.04 60 72.2 40.0 38.45. ConclusionA two-dimensional OPA on SOI has been presented. Steering in one direction has been shownby means of wavelength tuning. Thanks to delay lines between the elements and the dispersionof these delay lines, a steering speed of 0.24◦/nm is presented. This speed is easily increasedby increasing the delay line length between the elements. The lobes of the OPA are spaced1.5◦ apart and the 3dB coverage range is about 10◦ in both directions (≈ 0.007π sr), with adirectivity of more than 38dBi while the antenna losses are smaller than 3dB. Complete two-dimensional steering can be performed by adding phase tuners on the waveguides. In this way,full two-dimensional beam steering is possible with limited need of active phase tuning.AcknowledgmentKarel Van Acoleyen and Hendrik Rogier acknowledge the Research Foundation - Flan-ders (FWO) for a research grant. The authors acknowledge the support of Ghent University(Methusalem project ‘Smart Photonic ICs’).#125958 - $15.00 USD Received 24 Mar 2010; revised 28 May 2010; accepted 2 Jun 2010; published 10 Jun 2010(C) 2010 OSA 21 June 2010 / Vol. 18,  No. 13 / OPTICS EXPRESS  13660

Two-dimensional optical phased array antenna on silicon-on-insulator

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Two-dimensional optical phased array antenna on silicon-on-insulator

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