Flat Emission Silicon Nitride Grating Couplers for Lidar Optical Antennas

Photonics 2025, 12, x https://doi.org/10.3390/xxxxx Article 1 Flat Emission Silicon Nitride Grating Couplers for Lidar Opti2 cal Antennas 3 Thenia Prousalidi 1,*, Georgios Syriopoulos 1, Evrydiki Kyriazi 1, Roel Botter 2, Charalampos Zervos 1, Giannis Pou4 lopoulos 1 and Dimitrios Apostolopoulos 5 1 School of Electrical and Computer Engineering, National Technical University of Athens, 15780 Zografou, 6 Greece 7 2 Lionix BV International, Hengelosestraat 500, 7521 AN Enschede, Netherlands 8 * Correspondence: [email protected]; 9 Abstract: Light detection and ranging (Lidar) is a key enabling technology for autono10 mous vehicles and drones. Its emerging implementations are based on photonic inte11 grated circuits (PICs) and optical phased arrays (OPAs). In this work we introduce a novel 12 approach to the design of OPA Lidar antennas based on Si3N4 grating couplers. The well13 established TriPleX platform and the asymmetric double stripe waveguide geometry with 14 full etching are employed, ensuring low complexity and simple fabrication, combined 15 with the low-loss advantages of the platform. The design study aims to optimize the per16 formance of the grating coupler based radiators as well as the OPA, thus enhancing the 17 overall capabilities of Si3N4 based Lidar. Uniform and non-uniform grating structures are 18 considered, achieving θ and φ angles divergence of 0.9° and 32°, and 0.54° and 25.41° 19 respectively. Also, wavelength sensitivity of 7°/100 nm is achieved. Last, the fundamental 20 OPA parameters are investigated, and 35 dBi of peak directivity is achieved for an 8-ele21 ment OPA. 22 Keywords: Grating Coupler, Lidar, Optical Phased Array, Optical Radiator, Silicon Ni23 tride 24 25 1. Introduction 26 Autonomous vehicles, terrestrial and airborne, have gained popularity in recent 27 years, with the automation use cases spreading across multiple sectors and industries. 28 Their existing and foreseen applications range from the automotive and mobility industry 29 with self-driving cars and automated taxis, to aerospace (automated urban air mobility 30 (UAM) scenarios) and drones [1], smart cities, logistics, manufacturing, industrial appli31 cations and healthcare [2]. Since autonomous vehicles operate in dynamic environments, 32 they rely on the use of advanced sensorial technologies for mapping [3], like the radio 33 detection and ranging (Radar) and light detection and ranging (Lidar). Extensive research 34 has taken place in recent years to advance the performance and co-integrate Radar and 35 Lidar sensors for autonomous vehicles [4-5]. 36 Lidar is a three-dimensional (3D) imaging, mapping and remote sensing technique 37 that relies on optical beam shaping and steering [6] and has emerged as a promising so38 lution for autonomous vehicles and drones. High performance Lidar systems compatible 39 with such applications need to enable long-range power transmission with low cost, low 40 power consumption, compact and robust implementations [7]. The high performance 41 Received: 31/01/2025 Revised: 21/02/2025 Accepted: date Published: date Citation: To be added by editorial staff during production. Copyright: © 2025 by the authors. Submitted for possible open access publication under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/license s/by/4.0/). Photonics 2025, 12, x FOR PEER REVIEW 2 of 16 relies on increased field of view (FOV), high angular resolution, small beam divergence 42 and increased scanning speed [8]. Traditionally, Lidar systems have been based on me43 chanical implementations with free-space optics and rotating parts [9] that however are 44 not compatible with the compact size, low cost and reliability requirements, and have 45 limited scanning speed [10]. Micro-electro-mechanical system (MEMS) Lidar is a more 46 compact alternative, showcasing, though, reduced FOV and vulnerability to mechanical 47 shocks. Solid-state Lidar has emerged in recent years as another technique that provides 48 a scalable and reliable solution that does not include any moving mechanical parts [11]. It 49 includes optical phased arrays (OPAs) where the beam is steered by waveguides instead 50 of moving parts [12], or Flash Lidar that works like a camera and captures the image by 51 illuminating the whole FOV [11]. However, a tradeoff exists also for these methods, as 52 they showcase limited steering angle and detection range respectively. 53 In recent years, photonic integrated circuit (PIC) based Lidar has been gaining mo54 mentum [13]. The typical choice for PIC based Lidar is the Silicon on insulator (SOI) plat55 form [10, 14-15] where the beam shaping and steering are performed by the silicon chip 56 with the help of integrated phase shifters and OPAs based on grating elements. A sche57 matic of an optical antenna in OPA configuration based on grating couplers is shown in 58 Figure 1. This platform offers many advantages. Being compatible with standard mature 59 complementary metal-oxide semiconductor (CMOS) fabrication processes it enables the 60 development of cost efficient, low power, reliable and robust Lidar systems, based on 61 highly integrated OPAs [16]. Compact grating antennas based on one-dimensional (1D) 62 OPAs can achieve two-dimensional (2D) beam steering by wavelength tuning along the 63 longitudinal direction, and by phase control along the lateral dimension [16-17]. The in64 herent ultra-high index contrast of the SOI platform allows for beam steering as high as 65 15o with 100 nm wavelength tuning [18]. However, Silicon (Si) PICs require precise control 66 of their components dimensions for optimal performance and are therefore susceptible to 67 fabrication process errors. Moreover, the SOI platform cannot support high input power 68 levels due to the appearance of non linear effects in Si, prohibiting its use in high power 69 systems [19]. This limits the application of SOI PIC based Lidar. 70 Capitalizing on the advances of SOI-based Lidar, the Silicon Nitride (Si3N4) platform 71 can enhance and further improve the capabilities and performance of PIC based Lidar. 72 The Si3N4 platform showcases very low propagation losses and is compatible with high 73 input optical power applications due to its low nonlinearities [20]. In combination with its 74 low index contrast, this platform is robust to fabrication-induced phase variations. More75 over, it is transparent at wavelengths below 1150 nm [21] and showcases reduced emis76 sion strength for increased emitter length and small divergence [22], making it an inter77 esting alternative to SOI for the development Lidar optical antennas [23]. The Lionix Tri78 PleX waveguide technology is one of the most well-established Si3N4 platforms. Among 79 the different waveguide geometries it offers [24], the asymmetric double stripe (ADS) 80 combines the ultra-low loss advantages of the platform with the lowest minimum bend 81 radii, requiring simpler fabrication processes and offering higher yield compared to the 82 other geometries [25]. Moreover, it is compatible with standard multi project wafer 83 (MPW) fabrication processes employing single etch depth, and is highly suitable for ap84 plications that require coupling to external components such as active materials. This is 85 especially important for Lidar applications, where the co-integration of the Si3N4 compo86 nents with other photonic or electronic chips and components (e.g. indium phosphide for 87 the realization of optical sources) can enable the development of complete Lidar and Ra88 dar systems. 89 However, the Si3N4 has its own limitations. Adequate beam steering by wavelength 90 tuning remains a challenge due to the limited material dispersion. Recent developments 91 have shown that steering angles of up to 7o for 100 nm tuning range can be achieved [22]. 92 Photonics 2025, 12, x FOR PEER REVIEW 3 of 16 Moreover, the Si3N4 based Lidar is not a mature technology. Although various studies 93 investigate Si3N4 optical antennas for Lidar [26-27] advances are still required to achieve 94 the desired performance combining low divergence, uniform emission profiles and long 95 length gratings with standard fabrication techniques. This is especially relevant for Si3N4 96 Lidar antennas employing the ADS waveguide geometry that have not yet been explored 97 in literature. However, many applications can benefit from such implementations, thanks 98 to their aforementioned advantages, making the research of ADS based Si3N4 optical ra99 diators highly desirable and impactful. 100 This paper introduces a new approach to the design of grating couplers (GC) for Li101 dar optical antennas in the TriPleX Si3N4 platform, employing the ADS geometry. The 102 effort focuses on achieving high performance of OPA based radiators for Lidar systems 103 that can be employed in autonomous vehicles and drones, meeting the demanding re104 quirements of such applications. More specifically, this design study targets high FOV of 105 the OPAs, increased resolution and low receiver losses. The resolution depends on the 106 beam divergence, and it can be improved by minimizing the beam divergence across the 107 longitudinal and lateral directions (theta (θ) and phi (φ) angles divergence respectively). 108 The beam divergence is determined by the optical aperture of the antenna, and also cor109 relates to the radiation uniformity throughout the antenna length. In general, θ angle di110 vergence depends on the design of the individual grating elements, while the φ angle 111 divergence depends on the aperture of the OPA. The receiver losses can be reduced by 112 targeting a flat emission profile in the longitudinal direction and achieve gratings with 113 long effective areas (long gratings) that can collect light with increased efficiency. At the 114 same time, this work targets Si3N4 optical radiators that are compatible with simple fabri115 cation processes and single etch depth, fabricable through MPWs, without requiring ac116 cess to more advanced techniques. Although this will signify limitations in the achieved 117 performance, it ensures that the designs are low cost and easily fabricable, a requirement 118 necessary in many applications. This is ensured with the use of the ADS geometry with 119 single full etch depth. The design process outlined in this paper consists of two parts. The 120 first concerns the design and optimization of the single grating element, to achieve uni121 form emission across long length and reduce the angles divergence. The second step tar122 gets the optimization of the OPA structure. 123 2. Layerstack and Antenna Design Considerations 124 The platform for the development of the optical radiators is the Lionix Si3N4 TriPleX 125 platform [24-25] and the ADS waveguide geometry. The multiple layers of Si3N4 and Sili126 con Dioxide (SiO2) allow to make low index contrast waveguides which is beneficial for 127 reduced sensitivity of the waveguided modes effective refractive index (neff) to the wave128 guide geometry variation [23]. This results in reduced emission strength and allows to 129 achieve long gratings. A cross-section of the ADS layerstack is shown in Figure 2(b). The 130 bottom and top Si3N4 layer thickness is 75 nm and 175 nm respectively, while the distance 131 between the two layers is 100 nm. The refractive index of the materials is nSiO2 = 1.44537 132 and nSi3N4= 1.98350 at 1550 nm. For realizing the grating teeth, full etching is employed 133 (removal of both layers of the Si3N4 ADS layerstack), a limitation imposed by the available 134 fabrication process. Adhering to this limitation ensures that the proposed design is fabri135 cable with standard fabrication processes and minimal fabrication risks. As a result, a 136 fixed etching depth is considered. The nominal waveguide width (w) is 1.1 μm, however 137 this value has been varied in the designed components. The design study was carried out 138 in the spectral range 1500 nm – 1600 nm, centered around 1550 nm. 139 Photonics 2025, 12, x FOR PEER REVIEW 4 of 16 140 Figure 1: Schematic of an optical antenna in OPA configuration based on grating couplers. The θ 141 and φ angles and the distance d between adjacent GC elements are noted. 142 143 Figure 2: (a) The OPA schematic to indicate the cross-sectional planes. (b) Schematic of the yz-plane 144 cross-section of the standard TriPleX ADS waveguide. The different regions (Si3N4 waveguide, SiO2 145 top oxide layer (TOX) and bottom oxide layer (BOX) and air top cladding) are marked with different 146 colors. (c) Schematic of the sideview (xz-plane cross-section) of a periodic grating structure. The 147 grating pitch is denoted with Λ and the filling factor with FF. The effective index of the etched part 148 is n0 and of the unetched part is n1. 149 The proposed GCs serve as the single radiator elements (building blocks) of the op150 tical antenna and act as the light emitters and receivers of the Lidar. They are arranged 151 linearly in series, one next to the other with distance d, in a linear 1D OPA configuration, 152 as shown in Figure 1. Figure 1 illustrates the θ and φ angles, along the longitudinal and 153 lateral direction of the OPA respectively. The 1D OPA can realize beam steering in both 154 directions, by tuning the wavelength in the longitudinal direction (θ) and by introducing 155 a phase shift across the grating antenna along the lateral direction (φ). The advantage of 156 this configuration is that 2D beam steering is ensured with an 1D structure, resulting in a 157 significant reduced footprint (orders of magnitude less) on the chip compared to an equiv158 alent 2D structure, considering that for a given aperture size 2D OPA requires N2 elements 159 instead of N elements for the 1D OPA. Moreover, in 2D OPAs, the on-chip real estate is 160 increased due to the overhead of the phase shifters required for beam steering in both 161 directions. 162 3. Design of Single Radiator Element 163 The building block of the proposed OPA Lidar optical antenna is the GC. The first 164 part of this study focuses on the design and optimization of the GC based emitter using 165 Photonics 2025, 12, x FOR PEER REVIEW 5 of 16 the Lumerical finite difference eigenmode (FDE) and finite-difference time-domain 166 (FDTD) solvers. Various configurations have been explored, including uniform and non167 uniform geometries, to optimize the performance of the single radiator element. 168 3.1. Principle of Operation 169 Among the grating parameters, the most relevant for this design study that will be 170 used in the design process are the effective refractive index of the grating period (neff-grating), 171 the emission angle (θ) and the coupling constant (k). 172 In a grating structure, the effective index of a grating period that consists of an 173 unetched part with effective refractive index of the supported mode n1 and an etched part 174 with effective refractive index of the supported mode n0, with filling factor FF, as shown 175 in Figure 2(c), is given by (1) 176 𝑛𝑒𝑓𝑓−𝑔𝑟𝑎𝑡𝑖𝑛𝑔 =𝐹𝐹 ∗ 𝑛1+(1 − 𝐹𝐹)∗ 𝑛0. (1) 177 For our structure, since the etched part consists only of SiO2 due to full etching, we assume 178 the effective refractive index in the etched part to be equal to the refractive index of SiO2 179 (n0 = nSiO2). 180 The emission angle θ of a grating is related to the pitch Λ, the operating wavelength 181 λ and the neff-grating, according to the Bragg condition expressed in (2) [28] 182 sin(𝜃)=𝑛𝑒𝑓𝑓−𝑔𝑟𝑎𝑡𝑖𝑛𝑔−𝜆 𝛬 𝑛𝑆𝑖𝑂2 . (2) 183 The coupling constant k expresses the effective refractive index contrast between the 184 high and low index sections of a grating period [27] and is given by (3). 185 𝑘 = 𝑛1 − 𝑛0 𝑛𝑒𝑓𝑓−𝑔𝑟𝑎𝑡𝑖𝑛𝑔∗ 𝛬. (3) 186 k relates to how fast (over how many periods) the power will be scattered outside of the 187 GC. For low loss waveguide platforms, having low k can help realize GCs with long ef188 fective lengths. One way to achieve low k is by varying the waveguide width along the 189 GC length [29]. In applications that target uniform emission profiles, apodization of k 190 along the length is desirable for engineering the emission profile. 191 3.2. Uniform Design 192 The simplest GC configuration in terms of design and fabrication complexity is a 193 uniform GC where the geometrical parameters (filling factor (FF), width, pitch (Λ)), cho194 sen for optimal performance, remain constant throughout the GC length. The effective 195 index of the grating (neff-grating), given by (1), is also constant across a uniform design. Uni196 form GC configurations were investigated and simulated in this study. A topview of the 197 uniform grating is shown in Figure 3. 198 199 Figure 3: Sideview of the uniform GC, showing the constant pitch, FF and width across the direction 200 of propagation. 201 Photonics 2025, 12, x FOR PEER REVIEW 6 of 16 202 Figure 4: Simulated θ and φ angles divergence (a) varying the grating width for fixed length of 50 203 μm and (b) varying the grating length for fixed width of 2 μm. 204 For the design proposed in this paper, FF was chosen equal to 0.5. The next parameter 205 that was investigated is the width. The nominal width in this platform is 1.1 μm, and its 206 minimum value for proper confinement of the mode in the waveguide is 700 nm. Hence, 207 width values greater than 1 μm were investigated. The waveguide width affects the light 208 confinement within the waveguide in the lateral dimension. At the same time, large width 209 values increase the required distance (d) between adjacent GCs (shown in Figure 1) in the 210 OPA to avoid evanescent filed coupling and cross-talk between the OPA channels. Last, 211 as the width increases, we enter the multi-mode regime, which should be avoided. There212 fore, a study of the width impact on the OPA performance (angles divergence) is neces213 sary to optimize its value. To this end, 3D-FDTD simulations were performed varying the 214 waveguide width between 1 μm and 4 μm with step of 0.5 μm, while keeping the other 215 parameters constant (length = 92.6 μm, FF = 0.5). The simulated θ and φ angles 3dB diver216 gence is shown in Figure 4(a). These results indicate that as the width increases, the φ 217 divergence will decrease (10.4% of decrease for width variation between 1 and 4 μm), 218 while θ divergence is monotonically affected (9% of increase for width variation between 219 1 and 4 μm). Since the θ divergence is close to 1o, this translates to 0.1o of change, that is 220 considered small in terms of absolute value for the application. Taking into account the 221 aforementioned limitations and to ensure that the spacing requirements in the OPA struc222 ture will remain reasonable, width up to 2 μm should be chosen. 223 As a second step, the effect of the GC length variation is studied, to optimize its value. 224 3D-FDTD were performed, this time varying the GC length between 25 μm and 200 μm, 225 while keeping the width constant to 2 μm and the FF to 0.5. The resulting simulated θ and 226 φ angles 3dB divergence is shown in Figure 4(b). We observe that both the θ and φ diver227 gence is reduced as the length increases. For length of 92.6 μm, the θ divergence is 0.94°, 228 while the φ divergence is 33.92°. For lengths longer than 160 μm the divergence values 229 start to converge. For length of 208 μm values for the θ and φ divergence as low as 0.63° 230 and 28° can be achieved. This signifies a θ and φ divergence reduction of 81% and 55.5% 231 respectively, for the investigated length increase from 25 μm to 200 μm. 232 233 Figure 5: Left: topview of the emission profile of the uniform grating for width of 2 μm and length 234 of 100 μm. The Ez component field distribution is shown with the color scale. Right: 1D plot of the 235 emission profile along the dashed line of the left figure. 236 Photonics 2025, 12, x FOR PEER REVIEW 7 of 16 237 Figure 6: Calculated emission angle θ of the farfield profile, varying the wavelength in the range 1.5 238 - 1.6 μm, for uniform teeth profile and pitch 926 nm. The electric field intensity is shown with the 239 colour scale. 240 Last, the pitch is freely chosen according to (2), to achieve the desired emission angle. 241 For emission angle of around -10o, which is a typical value for radiators, the pitch is set to 242 926 nm. These values result in optimal performance of the uniform grating, while mini243 mizing the θ and φ angle divergence. A 3D-FDTD simulation was repeated with the se244 lected geometrical parameters. The topview and sideview of the simulated emission pro245 file of the grating are shown in Figure 5. The extracted θ and φ angles divergence is 0.9o 246 and 32o respectively. It is worth noting that the φ divergence is large because these simu247 lations do not take into account the complete OPA structure, but only the single radiator 248 element. 249 Figure 6 illustrates the θ angle wavelength sensitivity as extracted from the farfield 250 data. With the uniform design, 7o of θ angle wavelength steering is achieved, with 100 nm 251 of wavelength shift, from 1500 nm to 1600 nm. However, it is evident from Figure 5 that 252 this component has an exponential emission profile and cannot meet the design target of 253 uniform emission and long effective length. This is expected because the uniform design 254 has a constant k, and given that every period receives less input light than the previous, 255 due to the light that is emitted upwards, the resulting emission profile will showcase an 256 exponential decay. Therefore, to achieve more uniform emission profiles, non-uniform 257 grating profiles have been investigated, as described in the next paragraph. 258 3.3. Non-uniform Design 259 Due to the performance limitations of the uniform design, non-uniform configura260 tions were also investigated to achieve more uniform emission profiles. The non-uni261 formity concerns the geometrical parameters of the design and suggests that along the 262 grating length some of the design parameters are being varied. Taking into account the 263 fabrication process limitations, and to ensure compatibility with Lionix TriPleX platform 264 fabrication processes that enable only full etching, the parameters that can be varied are 265 the waveguide width, the filling factor and the pitch. 266 The geometry variation of the non uniform grating is designed to ensure a constant 267 emission angle θ across the grating length, for low θ angle divergence and increased res268 olution in the longitudinal axis. According to (1) and (2), to ensure a constant emission 269 angle the induced geometry variation needs to be designed carefully. In this study we 270 vary the width and FF, keeping the pitch value constant, and considering only fully etched 271 waveguides. A topview and a sideview of the non-uniform design are shown in Figure 7 272 left and right respectively. Moreover, the geometry variation targets uniform emission 273 distributions. To achieve this, the k of every period should gradually increase across the 274 grating length. Increasing k is achieved by decreasing the FF (to approach FF of 0.5 that 275 Photonics 2025, 12, x FOR PEER REVIEW 8 of 16 results in stronger emission) and increasing the width across the grating length, which 276 leads to increasing emission rate [27]. 277 278 279 Figure 7: (Left) topview and (Right) sideview of the investigated non-uniform grating design with 280 varying width and FF. 281 282 Figure 8: Calculated effective refractive index of the TE0 mode varying the waveguide width. 283 The design process started with FDE simulations using Lumerical MODE solver. 284 First, the effective index of the fundamental TE mode of a waveguide cross-section (n1) 285 varying its width, is calculated, given the ADS TriPleX layerstack. The results are shown 286 in Figure 8. In the ADS TriPleX platform, to create the grating teeth the Si3N4 is fully 287 etched, removing both layers of the Si3N4 layerstack that are illustrated in Figure 2(b). 288 Therefore, the etched part of the grating geometry consists of SiO2. Since there is no wave289 guiding material in the etched part and the FDE solver cannot calculate any supported 290 modes, we assume the value of the effective refractive index of the etched part (n0) to be 291 equal to the value of the refractive index of SiO2 (nSiO2). Using (1) and the calculated n1 292 values, it is then possible to compute the neff-grating, for the different width and FF combina293 tions. This is shown in Figure 9. 294 From the results shown in Figure 9 it is evident that for specific pairs of waveguide 295 width and FF, the neff-grating remains constant. These contour lines, shown with black lines 296 in Figure 9, are the desired regions from which the width-FF pairs can be extracted. The 297 design methodology includes partitioning the available width range in N values to pro298 duce the width-FF pairs, as it will be described shortly. Thus, it is ensured that the emis299 sion angle θ is also constant. Moreover, the emission profile of the grating is no longer 300 exponential, but becomes more uniform. This is achieved because across the grating 301 length the FF decreases approaching 0.5, while the width increases, leading to an increas302 ing k. For a given contour line, the corresponding coupling constant k has been calculated 303 according to (3). Figure 10 illustrates k for three of the contour lines of Figure 9. It can be 304 observed that for all contour lines, k is similar, and it gradually increases as the width 305 increases, according to the design target. 306 Photonics 2025, 12, x FOR PEER REVIEW 9 of 16 307 Figure 9: neff-grating of the fundamental supported mode calculated via FDE simulations, varying the 308 waveguide width and FF. The black lines are the contour lines of the plots along which the neff-grating 309 has a constant value. The selected contour line is marked with the pink stars. 310 311 Figure 10: The calculated coupling constant k for the different width values across the grating, for 312 three of the contour lines. 313 It is noted that there are multiple contour lines one can work with. A choice between 314 the available options must therefore be made targeting optimal performance. In terms of 315 keeping the emission angle constant, all contour lines are equivalent. However, there are 316 two more criteria that can help make the best choice. 317 The first one concerns the compatibility of the design with the fabrication process 318 capabilities. The FF should remain below 0.9 for compatibility with the acceptable mini319 mum feature size of approximately 100 nm (for pitch around 1 μm, FF should be between 320 0.1 and 0.9). Moreover, the FF step should be as large as possible, leading to large steps in 321 the grating teeth length and compatibility with the minimum step size. This is satisfied 322 when choosing a contour line with large slope in the whole width range. The same is true 323 in the lateral direction where large width steps are desirable for compatibility with mini324 mum step size. For a given GC length (fixed amount of GC periods and partitioning steps 325 N), increasing the utilized width range will increase the width step. Note that we choose 326 to work with width larger than 1 μm for proper confinement of the fundamental mode 327 within the waveguide. Moreover, widths larger than 2 μm are not desirable since they 328 impose limitations in the spacing between the array elements and full OPA size, and also 329 signify operation in the multimode regime. 330 A second criterion for choosing the contour line is the k value. 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