# Si Photonics for Practical LiDAR Solutions
## 6 Summary  
In this article, the authors discuss Si photonics LiDAR focusing on practical automotive applications. The authors categorize a LiDAR by two key design dimensions, e.g., distance calculation method and scene mapping method, and analyze the pros and cons of the design choices commonly adopted in the industry. The authors analyze the system design considerations of automotive LiDAR relevant to Si photonics implementations. The authors review the published research works on Si photonics OPA in the past decade and discuss the system and component designs in details. The discussions attempt to emphasis on design tradeoffs and practicality instead of scientific novelty or exceptional performance of a single aspect. The commercialization status of certain technologies or components is briefly mentioned. The potential uses of Si photonics in coherent receivers and narrow linewidth tunable lasers for certain types of LiDAR solutions are briefly discussed.  
There are many challenges to make a Si photonics LiDAR product, particularly an OPA based product. Some challenges relate to the fundamental limitations in the mechanism or the materials and some are engineering difficulties. The large non-linearity effects of Si significantly limit the measurable range in direct ToF or phase shift indirect ToF. The use of SiN allows more power, but the poor thermo-optic phase tuning efficiency makes it less competitive in power consumption, if not a thermal management nightmare. The phase tuning in Si relies on either thermo-optic or free carrier effect. The former requires a careful balance between speed and power consumption, and the latter demands large footprint. Both methods present scaling challenges while large number of antennae and phase shifters is generally desired in an OPA for narrow beamwidth and wide steering angle. Nevertheless, among integrated photonics or other novel technologies eye for LiDAR applications, Si photonics has the best ecosystem, including materials, design software, foundry processes, packaging tools and testing automation, as well as talents and product experience accumulated from the past two decades. These are key factors to successfully form a business and develop products. Si photonics OPA, like any other technologies, may exhibit many trade-offs in designs and its commercial success depends on careful choice of designs and associated applications.  
To the best of the authors' knowledge, Si photonics based LiDAR have yet been mass-produced or field-deployed in meaningful quantity at the time of writing. The authors expect that some upcoming LiDAR products labeled with Si photonics may adopt a Si photonics coherent receiver to realize FMCW demodulation paired with a more traditional beam steering method. It may not be a true Si photonics LiDAR in many people's opinion, but certainly constitutes good progress. A LiDAR product using one or more Si photonics OPAs for 1D steering, and a more traditional steering method for other FOV dimension, may also enter the market soon for properly picked applications. The products using a 2D antenna array or wavelength tuning for 2D steering may take longer to develop and commercialize.  
Figure 9: (**a**) Schematic drawing of a wavelength tunable laser using Si photonics as a tunable external cavity; (**b**) Superimposed spectra of wavelength tunable laser lasing at different wavelengths. Adapted with permission from [87] © The Optical Society.  
**Author Contributions:** Analysis and writing, X.S.; data preparation, L.Z. and Q.Z.; funding acquisition, W.Z.  
**Funding:** This work was supported by the National Natural Science Foundation of China (NSFC) (Grant No. 61635013, 61675231, 61705257, and 61805277), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB24030600) and the Youth Innovation Promotion Association, CAS (2016353).  
**Conflicts of Interest:** The authors declare no conflict of interest.