HIGHER-SPEED data transmission capabilities are demanded

JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 35, NO. 11, JUNE 1, 2017 2273 An Optical Leaky Wave Antenna by a Waffled Structure Hiroshi Hashiguchi, Student Member, IEEE, Keisuke Kondo, Toshihiko Baba, Member, IEEE, Member, OSA, and Hiroyuki Arai, Fellow, IEEE Abstract This paper presents an optical leaky waveguide antenna, i.e., waffled waveguide (WWG). From both simulations and experiments, we evaluate the effectiveness of the WWG in terms of the antenna characteristics. The WWG radiates a narrow beam tilted by sweeping the incident wavelength, and consists of an array of small rectangular holes fabricated by silicon photonics. Compared to the conventional grating waveguide designed only with the longitudinal period and depth as free parameters, the lateral period of the rectangular holes in the WWG adds the design flexibility; consequently, the WWG achieves a large aperture size and high antenna gain. We reconstruct radiation pattern from aperture distribution and confirm the high antenna gain experimentally. Index Terms Grating wave guide, leaky waveguide, optical antenna, waffle waveguide. I. INTRODUCTION HIGHER-SPEED data transmission capabilities are demanded in future wireless communications. Optical wireless systems have been studied at wavelength λ of 1.064, 1.55 and 8.08 μm, aiming at carrier frequencies much higher than radio frequencies (RF) [1] [6]. Optical antennas including nano-antennas with plasmonics [7] [9], optical photonic phased arrays and optical leaky waveguide antennas (OLWAs) [10] [18] have been studied as key components. These antennas are also usable for the detection and ranging of optical beams. Our objective is to realize optical antennas with a high antenna gain, wide angle beam steering and low cost mass-production for mobile communications. The evaluation of antenna gain is important in designing free space optical communication systems [6]. In this study, we focus on OLWAs fabricated by silicon (Si) photonics CMOS technology, enabling advanced photonic integrated devices and their mass-production at λ = 1.55 μm. A conventional OLWA is the grating waveguide (GWG) that consists of shallow linear grating formed on a Si waveguide. It radiates leaky waves of the waveguide mode into the free space and produces a narrow optical beam. The beam angle-θ is scanned by sweeping λ. The wider range of the scanning is Manuscript received November 4, 2016; revised January 15, 2017 and January 23, 2017; accepted January 23, 2017. Date of publication January 26, 2017; date of current version April 20, 2017. This work was supported by JSPS KAKENHI under Grant 25420359. The authors are with the Graduate School of Engineering, Yokohama National University, Yokohama 240-8501, Japan (e-mail: hashigucti-hiroshi-cn@ ynu.ac.jp; keisuke-kondo-vs@ynu.jp; baba@ynu.ac.jp; arai@ynu.ac.jp). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JLT.2017.2660520 possible by switching GWGs with different periods. To obtain a more directive radiation pattern (in other words, higher antenna gain), extending effective antenna area (antenna aperture) by reducing the index perturbation, i.e., etch depth, is necessary. It is not technically straightforward to reduce uniformly in the CMOS processing because the etch depth is required for an aperture length longer than, for example, 1 mm will be less than 1 nm, which is almost the disordering level of the etching process. Besides, Si photonics devices aiming at research and development are often fabricated in a multi-project wafer (MPW). Here, free optimization of the etch depth d is not permitted because the cost is shared by many users. Common users employ d 70 nm to form gratings as fiber couplers. The d much smaller than this value requires the split of wafers, resulting in a high cost. We proposed the waffled waveguide (WWG) OLWA to overcome these issues [19]. In this study, we evaluate quantitatively the antenna characteristics of the WWG from both simulations and measurements. The WWG consists of a two-dimensional array of rectangular corrugations (i.e., waffle structure) instead of the linear grating. Similar waffled structures have often been used in chip-fibers [20] [28]. The structure in this study focuses its use as OLWAs for mobile communications for the first time. The waffled structure reduces the perturbation of the grating, moderately suppresses the radiation rate, and extends the propagation length of light, resulting in a large antenna aperture and high antenna gain. In this study, we designed and fabricated GWG and WWG using Si photonics process, and compared the performance between them to confirm the effectiveness of the WWG. We use this process to reconstruct the radiation pattern with antenna gain of these OLWAs [29]. In the case of RF antennas, the antenna gain is usually obtained by comparing with that for a standard antenna such as dipole one. Because there are no standard antennas for OLWAs, we have to measure the radiation pattern completely. To avoid this measurement, the radiation pattern is reconstructed from the aperture distribution measured at near field, including its amplitude and phase [30]. In this method, both amplitude and phase are obtained experimentally without direct phase measurements and the radiation pattern is reconstructed. In the following, we first describe the structural details and theoretical characteristics of the GWG and WWG in Sections II and III, respectively. The reconstruction of radiation pattern and the comparison of antenna performance between the fabricated GWG and WWG are presented in Section IV. 0733-8724 2017 IEEE. 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2274 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 35, NO. 11, JUNE 1, 2017 Fig. 2. Antenna gain of GWG simulated for etch depth d. Fig. 1. Schematic structure and parameters of GWG, (a) cross-section, (b) bird s eye view. II. GWG Fig. 1 shows the structure and design parameters of GWG. The Si waveguide (refractive index = 3.45, height h = 0.21 μm, width w = 10 μm) is buried by SiO 2 claddings (refractive index = 1.45). The GWG has a linear grating on top of the Si layer with the depth of d, the period of Λ x, the top-bottom ratio of 1:1, the number of periods of N x, and the total length of L =Λ x N x. The light of y-polarization is incident on the GWG and propagates along the x-axis. The grating produces the leaky wave radiation of the guided mode and forms an optical beam in the zx-direction. The beam tilting in the zx-plane is modified by changing Λ x and the beam steering is obtained by sweeping λ. This GWG radiates in the upper and lower directions (x direction), and the gain of lower is slightly higher than the upper. We only evaluate the upward beam due to the constraints in used optical setup. The antenna gain G [dbi] is defined as the beam intensity normalized by the intensity of the completely uniform, angle independent light radiation. It is expressed with the antenna aperture A [m 2 ] as follows: G = 4πA λ 2 (1) A high antenna gain is obtained by a large A. In the GWG, it is achieved by expanding w and extending the effective length L eff, which is not the physical length L but the propagation length of waveguide mode determined by the loss. Expanding w is not particularly discussed in this study; we just employ a standard width w = 10 μm whose single mode propagation is effectively obtained by connecting GWG with a completely single mode Si wire waveguide through an adiabatic taper. Extending the L eff is available by reducing the loss, which is the sum of the scattering loss at disordered sidewalls of fabricated GWG and the leaky wave radiation. The scattering loss was measured separately to be 2.7 db/cm for fabricated Si-wire waveguide, and that of the GWG of 10 μm width should be lower than this value [31]. As shown later, L eff is shorter than 1 mm in our current devices. The scattering loss is negligible as compared with the leaky wave radiation. The radiation rate is determined by the depth d. We numerically simulated the antenna gain G as a function of d, as shown in Fig. 2, where a commercial three-dimensional full-wave simulator, CST MW STUDIO [32], was used (the same simulator was also used for other numerical simulations in this paper). The number of periods N x at 2000 (L = 1.14 mm) as shown later. The gain G is increased by simply reducing d and extending L eff. As mentioned above, d much smaller than 10 nm is difficult to fabricate uniformly in the Si photonics. We employ d = 70 nm, which is standard as that of fiber couplers in Si photonics. To estimate the radiation angle θ, we use the Floquet spatial harmonics whose propagation constant is given by β 2π/Λ x where β is the propagation constant of the waveguide mode [15]. By considering the phase matching in the x-direction between the leaky wave and waveguide mode, we obtain the relation, β 2π/Λ x = nk 0 sin θ (2) where k 0 is the wave number in vacuum and n is refractive index of the free space (it is air in this case). It is possible to calculate β for given λ by the effective refractive index method. Then the optimal period Λ x is estimated for the desired range of θ using Eq. (2), as shown in Fig. 3(a); Λ x = 0.40 0.65 μm for θ =0 to 60 at λ swept from 1.5 to 1.6 μm. Considering the limitation of the measurement shown later, Λ x is set at 0.570 μm to observe optical beams of 10 < θ < 0. Fig. 3(b) shows simulated S-parameters, when N x = 100 (L = 0.54 μm). TheS 11 showing the ratio of reflected power to the incident power is desired to be less than 10 db as an efficient antenna. It is not satisfied on the short wavelength side at λ = 1.5 μm; the reflection is increased by the stopband of the grating at θ oriented perpendicular to the surface of the GWG. S 21 showing the ratio of transmitted power passing through the device to the incident power is lower than 30 db, meaning that

HASHIGUCHI et al.: OPTICAL LEAKY WAVE ANTENNA BY A WAFFLED STRUCTURE 2275 Fig. 3. (a) Radiation angle range and λ. (b) S-parameters of GWG using Eq. (2) for (b), Λ x = 0.570 μm. Fig. 5. (a) Radiation angle range and (b) S-parameters simulated for WWG. Fig. 6. Antenna gain calculated with the number of period. Fig. 4. Schematic structure and design parameters of WWG. the incident light is sufficiently radiated out during the propagation. As a result, L eff is shortened to approximately 10 μm, as shown later, which is comparable to the core size of single-mode fiber. It is too short to obtain a high gain G. III. WWG We propose the WWG, as shown in Fig. 4, to reduce the radiation rate without changing the intended etch depth d from 70 nm. The waffled structure moderately reduces the radiation rate resulting in a long L eff. The number of rectangular holes along the y-axis modifies the radiation pattern in the yz-plane. Through the numerical simulations, the number of waffles is optimized to N y =20to maximize G for w = 10 μm. The period Λ y is set at 0.488 μm so that the number of rectangular holes along the y-axis is 20. The period Λ x is optimized to obtain 60 <θ<0, as shown in Fig. 5(a). The WWG also gives a wider range of θ by changing Λ x. We employ Λ x = 0.570 μm, which is the same as that of GWG. Fig. 5(b) shows the simulated S-parameters for the WWG. The peak of S 11 shifts to longer wavelength than that of GWG due to the change of the effective modal index. Regarding S 21,the WWG shows 10 db, a much higher value than that of GWG. This indicates that the leaky wave radiation is moderately suppressed by the waffled structure. Fig. 6 compares the simulated G between GWG and WWG, when N x is freely extended in the simulation when Λ x = 0.570 μm and d = 70 nm. In the GWG, G is almost saturated at N x < 200, whereas in the WWG; it continues to increase up to N x < 500 due to the extension of L eff and finally reaches a value 2.5 db higher than that of GWG. N x = 200 and 500 are appropriate for the GWG and WWG, respectively. In the experiment shown next, we set a sufficiently large N x, i.e., 2000, to radiate all incident light without reflection from the opposite end of the waveguide. IV. COMPARISON OF RADIATION PATTERN This section describes the reconstruction method of radiation pattern by the phase-less measurement and comparison of the measured characteristics between the GWG and the WWG. The measurement setup and its picture are shown in Figs. 7 and 8, respectively, and the OLWAs fabricated by the 180 nm CMOS process is shown in Fig. 9. The incident light of y-polarization in the range of λ = 1.5 1.6 μm is output from tunable laser source, and coupled to the Si-wire waveguide of 0.21 μm thickness and 0.4 μm width through an inverse-taper-type spotsize converter (SSC) using a couple of objective lenses.

2276 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 35, NO. 11, JUNE 1, 2017 Fig. 9. Fabricated OLWA. (a) Total view and magnified top view of Siphotonics chip. (b) Fabricated GWG and (c) WWG. Fig. 7. Fig. 8. Measurement setup of OLWA. Picture of measurement setup of OLWA. The channel waveguide is connected to the GWG or the WWG of 10 μm width through the adiabatic taper. The leaky wave radiation is observed in the xy-plane by another objective lens and InGaAs near-infrared camera, and the light intensity in a target area is quantitatively measured by an optical power meter. In the observation, we use 5 and 100 lenses whose numerical apertures (NAs) are 0.14 (measurement range θ = ± 8 ) and 0.50. (θ = ± 30 ), respectively. Keeping the dynamic range of the used optical power meter, we measured the aperture distribution at the distance between the device and observation planes, h m =0and radiation pattern at h m =1and 3 mm. The observation radiation pattern might be considered as a quasi-far-field because its free space propagation distance is shorter than the required length 2L 2 /λ (h m = 1.14 mm in Fig. 7) for measuring strict far-field [33]. Actually, the radiation pattern is almost close to the far-field, since the effective length is much shorter than the physical length L. We measure this far-field and aperture distribution to reconstruct the radiation pattern by the aperture distribution method. The radiation pattern is expressed as E i (θ, ϕ) = w L 0 0 E i (x m,y n ) exp{jk(mδxsin θcosϕ + nδysin θsin ϕ)}dxdy (3) where, ϕ is the radiation angle in the polar coordinate. For discrete points of the measurement, E i (θ, ϕ) = M m =1 n=1 N E i (x m,y n ) exp{jk(mδxsin θcosϕ+nδysin θsin ϕ)}δxδy (4) M = L/Δx, N = w/δy (5) where E i shows the aperture distribution on the device surface. Δx, Δy = λ/10 are sufficient to obtain a high accuracy of reconstruction. The aperture distribution is equivalent to the waveguide mode expressed as [15] E y (x, y) =E(y)exp(jγx) γ = jα + β (6) where γ is the complex propagation constant, α and β are the attenuation and phase constants, respectively. Since w is much wider than λ and we excite the fundamental guided mode through the taper and approximate the distribution along the y-axis to be a simple cosine function, i.e., E y (x, y) =E 0 cos(πy/w)exp{( α + jβ)x} (7) β = θ/δx To estimate α, the radiation power is measured from the surface of the OLWAs at near field. Fig. 10 shows the radiation power Pz n at each position x, where each power is normalized by the total radiation power. The incident wavelength λ is 1.5 μm, the magnification of the objective lens is 100. We monitored the aperture distribution, and measured the power distributions at h m =0. The powers gradually attenuated due to the radiation (solid line in Fig. 10(b)), but the attenuation is much faster

HASHIGUCHI et al.: OPTICAL LEAKY WAVE ANTENNA BY A WAFFLED STRUCTURE 2277 Fig. 11. Tilted angle of each waveguide, (a) tilted angle (b) measuring method. Fig. 10. Distribution of radiation power on waveguide surface at near field, (a) schematic of radiation, (b) measured and simulated results for GWG, and (c) those for WWG. than the simulation (dashed line) for GWG, which might be due to fabrication errors. We conducted the simulation again to quantify the errors for the grating depth d and pitches Λ x and Λ y. We found that errors in Λ x and Λ y have no effects, and that, if d is modified from the original 70 nm to 100 nm in the GWG, the measured results show a better fit to the simulation (dashed-dotted line). On the contrary, the error looks to be much smaller for the WWG, as shown in Fig. 10(c). Because of the waffle structure, the etch depth is maintained to the intended value or even smaller. This means that the waffle structure is effective in reducing the radiation rate not only by its design but also by the reduced etch depth. Finally, the α is obtained by the curve fitting for the power distribution using the least mean square method (LSM). Fig. 12. Reconstructed radiation pattern of each waveguide, (a) GWG (b) WWG. To obtain β, we evaluate θ = tan 1 (l/h s ) of the tilted beam by moving the observation plane from A to B with the distance h s and by measuring the horizontal shift length of the beam spot, l = l 2 l 1, as shown in Fig. 11, and then apply Eq. (7). The shift lengths are measured using of 5 objective lens between h m =1and h m =3mm. In accordance with the CST-simulation results, the reflection of the downward radiation, which is mentioned in Section II, has no severe effects such as the interference with the upward radiation. The measurement error in θ due to beam broadening is evaluated to be

2278 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 35, NO. 11, JUNE 1, 2017 1 at maximum. The beam was tilted by approximately 10 for a wavelength variation of 100 nm for both GWG and WWG. Using these α and β, we obtain the experimental aperture distribution from Eq. (7) and reconstruct the radiation pattern from Eq. (4) with the antenna gain. Fig. 12 shows the so-obtained radiation patterns as well as the simulated ones. The fabrication errors are also taken into account. Both OLWAs show rough agreement between the reconstructed and simulated results. As compared to the simulated results in Fig. 6, we obtained much higher gain due to the etch depth d reduced by the fabrication error. The reconstructed one for the WWG well shows a higher antenna gain than that of the GWG. V. CONCLUSION We proposed and demonstrated an OLWA, i.e., the WWG. The WWG achieved a wider aperture and higher gain, compared to those of the same size GWG. 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Baets, Fabrication of photonic wire and crystal circuits in silicon-oninsulator using 193-nm optical lithography, J. Lightw. Technol., vol. 27, no. 18, pp. 4076 4083, Sep. 2009. [32] 2016. [Online]. Available: http://www.cst.com CST Microwave Studio [33] Q. Zhao, C. Guclu, Y. Huang, F. Capolino, and O. Boyraz, Experimental demonstration of directive Si 3 N 4 optical leaky wave antennas with semiconductor perturbations, J. Lightw. Technol., vol. 34, no. 21, pp. 4864 4871, Nov. 2016. Hiroshi Hashiguchi (S 16) was born in Kagoshima, Japan, on January 6, 1989. He received the B.S. degree in electrical and electronic engineering from the National Defense Academy, Yokosuka, Japan, in 2010, and the M.S. degree from the Graduated School of Engineering, Yokohama National University, Yokohama, Japan, from 2014 to 2016. He is working toward the D.E. degree at Yokohama National University. Keisuke Kondo received the B.E., M.E, and Ph.D. degrees from the Department of Electrical and Computer Engineering, Yokohama National University, Yokohama, Japan, in 2012, 2013, and 2016, respectively. During his Ph.D. degree, he studied co- and counterpropagating slow-light systems. He is currently working toward Si-photonic crystal slow-light devices fabricated by CMOScompatible process as a Postdoctoral Fellow in the same university, receiving the Research Fellowship from JSPS. He is a Member of the JSAP.

HASHIGUCHI et al.: OPTICAL LEAKY WAVE ANTENNA BY A WAFFLED STRUCTURE 2279 Toshihiko Baba (M 93) received the B.E. and Ph.D. degrees from the Division of Electrical and Computer Engineering, Yokohama National University, Yokohama, Japan, in 1985 and 1990, respectively. He became an Associate Professor and a Full Professor of this university in 1994 and 2005, respectively. He has studied ARROW waveguides, VCSELs, photonic crystals (PCs), and Si photonics. He has demonstrated PC-based slow-light waveguides, high-speed modulators, nanolasers, biosensors and LEDs, and various Si-photonics components including first AWG. He is the author or coauthor of 190 papers with 11 500 citations (Google Scholar). He is a Member of the JSAP, the IEICE, and the OSA, and an Associate Member of the Science Council of Japan. He received the JSPS Award in 2005, the IEEE/LEOS Distinguished Lecturer Award in 2006/2007, the Ichimura Academic Award in 2012, and the Education, Culture, Sports, Science Minister s Commendation in 2016. Hiroyuki Arai (F 13) received the B.E. degree in electrical and electronic engineering and the M.E. and D.E. degrees in physical electronics from Tokyo Institute of Technology, Tokyo, Japan, in 1982, 1984, and 1987, respectively. After a Research Associate with Tokyo Institute of Technology, he joined Yokohama National University, Yokohama, Japan, as a Lecturer, in 1989. He was a Visiting Scholar with the University of California, Los Angeles, USA, in 1997, and was a Visiting Professor in 2005 and an Adjunct Professor from 2012 to 2014, respectively, with Yonsei University, Seoul, South Korea. He was an Adjunct Professor from 2012 to 2016. He is currently a Professor in the Department of Electrical and Computer Engineering, Yokohama National University. He investigated microwave passive components for high-power handling applications, such as RF plasma heating in large Tokamaks. He developed a flat diversity antenna for mobile telephone terminal, a polarization diversity base station antenna for Japanese PDC systems, small-base station antennas of in-building microcellular system, and DOA estimation for cellular system. His current research interests include MIMO antennas, wireless power transmission, energy harvesting in EM waves, and EMC/EMI antennas. He received the Young Engineers Award from the IEICE of Japan in 1989 and the Meritorious Award on Radio by the Association of Radio Industries and Businesses in 1997 for the development of polarization diversity antenna, in 2006 for the development of DOA estimation system, and in 2011 for the development of light-weight phantom. He was an Editor-in-Chief of the IEICE Transactions on Communications from 2005 to 2007, and was the Chair of the IEEE AP-S Japan Chapter from 2009 to 2010. He was an Associate Editor of the IEEE TRANSACTIONS ON ANTENNAS PROPAGATION from 2011 to 2013, and the Chair of the Technical Group on Antennas and Propagation of the IEICE from 2013 to 2014. He is a Fellow of the IEICE. He is the author of five edited books, three research book chapters, more than 150 journal papers, and 1100 conference papers.