# Si Photonics for Practical LiDAR Solutions
## 2 An Overview of LiDAR Technologies
### Mapping Methods  
In terms of scene mapping, mechanical scanning and microelectromechanical system (MEMS) mirror scanning share a common feature that using macro or micro moving parts to steer an optical beam to different directions. Mechanical scanning spins the whole or the part of an emitter subsystem along one or two axes for up to 360\({}^{\circ}\) to scan a scene in one or two dimensions. It may use multiple pairs of emitters and receivers for mapping the other dimension when the spinning scans a single dimension. In MEMS mirror scanning, light is reflected by a Si based micro mirror vibrating in one or two directions, often with limited deflection angles.  
Mechanical scanning is the first commercialized and currently dominant LiDAR technology for automotive applications though it is often criticized for poor scalability, high cost and frequent re-calibration. MEMS scanning presents a potentially lower cost alternative as the MEMS mirror could follow a cost reduction path like other widely used MEMS chips. MEMS mirrors have been used for optical projection and switching applications for a long time, however, the MEMS mirror for LiDAR applications present many new challenges and its design must carefully balance the intrinsic trade-off among mirror size (for beam divergence), deflection angle (for steering range) and mirror weight (for frame rate). Many companies are diligently working toward bringing MEMS mirror-based LiDAR products to automotive market. The automotive grade reliability of MEMS mirrors, a great concern for many, is being field-tested at the time of writing. Both mechanical and MEMS scanning can handle high optical power so they both are popular choices for pulsed ToF with tens to hundreds of watts peak power for ranging hundreds of meters. Mechanically scanned multi-channel direct ToF products have already been adopted in the ADAS of luxury passenger vehicles since 2017 [10], in many robotic vehicles, and in autonomous test vehicle fleets [11].  
Another way of mapping is to image a scene with a camera lens system and an array of SPAD or photodiode (PD)-based depth sensor pixels. It is a true solid-state approach with a straightforward concept as well as support from a mature camera ecosystem. ToF cameras combining this approach with indirect phase-shift ToF are recently penetrating smartphone market for artificial reality (AR) and other applications. Most applications in automotive or consumer 3D sensing require hundreds or more pixels in at least one dimension. The requirements of large pixel area for good SNR and/or pixel-wise ToF read-out circuits currently restrict the pixel size to about 10-20 \(\upmu\)m which is an order of magnitude larger than today's color camera sensor pixel sizes. Nevertheless, the industry is continuously improving the sensor technology to enable automotive LiDAR companies to deploy such so-called flash LiDAR. For automotive applications, such imaging approach requires high peak optical power as it must illuminate a whole scene at far distances with sufficient reflected light at each pixel. Ultra-short pulses, on the order of nanosecond, must be used in such cases to comply with eye-safety requirement.  
The optical phased array (OPA) approach is another true solid-state method for scene mapping. It generally resembles a phased array radar that uses an array of phase tunable antennae to form the shape of an output beam and steer its direction, while operating at optical frequency instead. In theory, with a large number of antennae, an OPA can generate arbitrary beam shapes and steer a beam in 2D without a lens system. In practice, however, design complexity, power consumption and cost often limit the number of phase shifters and antennae thus the optical performance. As an OPA comprises a large number of optical elements so it only returns to people's attention as the rise of Si photonics which promises manufacturing such complex optical system on a chip at low cost.