# Si Photonics for Practical LiDAR Solutions
## 4 Si Photonics Optical Phased Array
### Key Components
#### 4.2.1 Antenna  
An antenna in an OPA is a waveguide-to-free space coupler which emits light to far field for beam forming. A waveguide based grating coupler is the most frequently used antenna in Si photonics OPA designs because it diffracts output beam nearly perpendicularly out of chip plane which eases testing and packaging. Grating couplers have been widely used for coupling between waveguides and fibers in many Si photonics products. A grating coupler has unparalleled benefit in wafer-level testing and densely arrayed coupling whereas it suffers from high coupling loss (at least 2-3 dB in practice) and  
Figure 3: Simulated far field intensity distribution and antenna diffraction envelope for an OPA of 8 antennae (**a**) with uniform spacing of 4\(\lambda\) and antenna full divergence angle (Gaussian beam approximation) of 60\({}^{\circ}\); and (**b**) with unequal spacing from 3\(\lambda\) to 5\(\lambda\) and antenna full divergence angle of 30\({}^{\circ}\). strong wavelength and polarization dependence. A waveguide edge coupler which emits light right through a waveguide facet can also be used as an antenna. It in principle presents much lower loss and wavelength dependence while limits antenna arrangement in 1D. The grating coupler antennae used in a 2D and a 1D OPA are often quite different in design. For a 2D OPA, the spacing between adjacent antenna is usually as large as 10-30 \(\upmu\)m [54; 62] due to the large footprint of beam splitters and phase shifters. If a large steering range is demanded, with careful unequally spaced array design, the ultimate limiting factor tends to be the antenna emission diffraction envelope as shown in Figure 3b. Therefore, a small focusing grating with size of 2-3 \(\upmu\)m [54; 62] with 40-50\({}^{\circ}\) FOV in both directions is often adopted and an example is shown in Figure 4a. In order to diffract most light out of plane within 2-3 \(\upmu\)m propagation, unlike the shallow-etched design for fiber couplers, the waveguide core is often completely etched for most grating teeth. These focusing gratings may achieve about 50% upward diffraction efficiency in theory and usually less in practice. For a 1D OPA, waveguide antennae may be arranged very tightly such that a very large aliasing angle can be achieved. In such cases, a long and shallowly etched linear grating is generally more preferred than a short grating as the former results in less divergence angle in the FOV plane parallel to the waveguides (the non-steering FOV). In principle a wide-angle optical collimation system can narrow the divergence for the full steering range, but it diminishes the benefit of OPA beam forming and adds cost, size and complexity. In addition, when wavelength tuning is used for beam steering, as mentioned in Section 4.1.3, the chromatic aberration of any additional optical system must also be dealt with. Therefore, a long grating, i.e., a large aperture size, is often more desirable. Such gratings generally require very weak coupling strength, i.e., small refractive index change along propagation, which may not be easily fabricated. Different designs and fabrication methods are used to achieve such gratings (refer to Table 5): the Massachusetts Institute of Technology (MIT) and the Columbia University groups use side modulated grating [63; 64; 68] with width variation as small as 10 nm [68], as shown in Figure 4b, to get around 0.2\({}^{\circ}\) divergence; the Intel group uses very shallow grating etch (16 nm out of 400 nm thick waveguide) to get down to 0.1\({}^{\circ}\)[65]; and the MIT group uses SiN waveguide, with weaker mode confinement, to achieve an outstanding 0.02\({}^{\circ}\) record [66] (although no steering capability presents). Both shallowly etched grating and side modulated grating present processing challenges and do not comply with current commercially available Si photonics processes. The performance of a shallowly etched grating is sensitive to etch accuracy as shown in Figure 5a.