# Silicon Ribbon-Based Dual-Beam Optical Phased Array with Low Crosstalk and Large FoV
## 1 Introduction  
Light detection and ranging (LiDAR) is one of the most promising and high-potential technologies in scanning and detection nowadays. LiDAR systems use a C-band wavelength laser to provide both high resolution and long range on the swept surface [1, 2]. In general, traditional LiDAR beam steering methods are often based on mechanical rotation, such as micro-electro-mechanical systems (MEMS). However, automatic rotation control has limited the scanning speed, volume, and accuracy of LiDAR. Therefore, many investigations have been conducted in recent years toward the concept of \"LiDAR on-chip\" [3, 4], among which the phased optical array is demonstrated through a fiber-based 1550 nm wavelength range using silicon-based waveguides. In recent years, the optical phased array (OPA) has been shown to have no inertia phenomena, to reach large steering angles at high speed and to be capable of being integrated into a small footprint. Its main structure can be divided into four parts: a beam splitter array, a phase shifter array, an S-bend, and a transmitting antenna array. In a non-mechanic beam steering of LiDAR, a fully packaged one-dimensional beam steering system from a fully CMOS-compatible silicon photonics platform was experimentally demonstrated beside OPA [5]. This configuration enabled multi-wavelength operation in the 1477 nm wavelength range when employing a 1 \(\times\) 9 array of add/drop ring resonators. The divergent beams that exit the waveguides along the output facet at each resonance wavelength were then collimated and magnified using an array of micro-lenses. This one-dimensional OPA demonstrated a FoV of 33.7 degrees. OPAs can steer the beam through the longitudinal plane along the waveguide axis and transverse plane perpendicular to the waveguide. The non-phased array approaches to beam steering in [1] will be difficult to scale to two-dimensional scanning. In contrast,OPAs can solve this issue by using the wavelength as a degree of tuning freedom via the diffraction direction change of the grating. In OPAs, the presence of a grating lobe is not expected, because it will degrade the angle difference between the grating and main lobes, called the field of view (FoV), and limit the steering angle range. The spacing between transmitting antenna arrays must be reduced to have a larger beam steering angle in an OPA. However, the antenna array elements are too close to induce crosstalk between waveguides. Therefore, array decoupling is necessary when a dense array is considered. One example is the 180-degree lobe-free beam steering formation with one-dimensional gratings in the transverse plane perpendicular to the waveguide. In sequence, 3 widths, 300, 350, and 400 nm, are index-mismatched and successfully implemented on the emitter with the sub-wavelength pitch [6]. The optical phase compensation proposes another two index-mismatched silicon wires in the emitter array through the waveguide width adjustment to avoid the phase shifter's complex and addressable thermal control [7]. The FoV of an OPA depends on the spacing between transmitting antennas. Even though the two-dimensional antenna array spacing could be kept as \(\lambda/2\) for maximizing beam steering angles, where \(\lambda\) is the operating wavelength, the waveguide grating may still affect each and cause crosstalk. [PERSON]'s team [8] tried to enlarge the conventional OPA grating antenna gap to 1.2 \(\upmu\)m or more to avoid the crosstalk effect. Moreover, [PERSON]'s group [9] illustrated the FoV as 66 degrees in the \(\Phi\) horizontal direction for a 1.2 \(\upmu\)m array waveguide spacing. To make the waveguides more compact, [PERSON]'s team proposed to put two straight waveguides as a source barrier layer between the compact waveguides [10]. In recent years, [PERSON]'s team [11] used a particle swarm optimization algorithm to optimize source-stopping and resistive layer parameters through silicon strips with non-uniform subwavelength widths.