# An Overview of Lidar Imaging Systems for Autonomous Vehicles
## 2 Basics of Lidar Imaging
### Optical Phased Arrays  
An optical phased array (OPA) is a novel type of solid-state device that enables to us steer the beam using a multiplicity of micro-structured waveguides. Its operating principle is equivalent to that of microwave phased arrays, where the beam direction is controlled by tuning the phase relationship between arrays of transmitter antennas. By aligning the emitters' phases of several coherent emitters, the emitted light interferes constructively in the far-field at certain angles enabling to steer the beam (Figure 7). While phased arrays in radio were first explored more than a century ago, optical beam steering by phase modulation has been demonstrated first in the late 1980s [69; 70; 71; 72].  
In an OPA device, an optical phase modulator controls the speed of light passing through the device. Regulating the speed of light enables control of the shape and orientation of the wave-front resulting from the combination of the emission from the synced waveguides. For instance, the top beam is not delayed, while the middle and bottom beams are delayed by increasing amounts at will. This phenomenon effectively allows the deflection of a light beam, steering it in different directions. OPAs can achieve very stable, rapid, and precise beam steering. Since there are no mechanical moving parts at all, they are robust and insensitive to external constraints such as acceleration, allowing extremely high scanning speeds (over 100 kHz) over large angles. Moreover, they are highly compact and can be stored in a single chip. However, the insertion loss of the laser power in the OPA is still a drawback, [4; 73] as it is their current ability to handle the large power densities required for long-range lidar imaging.  
Figure 7: Schematic diagram of the working principle of an optical phased array (OPA): emitted fields from each antenna interfere to steer a far-field pattern.  
OPAs have gained interest in recent years as an alternative to traditional mechanical beam steering or MEMS-based techniques because they completely lack inertia, which limits the ability to reach a large steering range at high speed. Another relevant advantage is brought by the fact that steering elements may be integrated with an on-chip laser. As a developing technology with high potential, the interests on OPA for automotive lidar is growing in academia and industry, even though OPAs are still under test for long-range lidar. However, they are operative in some commercially available units targeting shorter and mid ranges [74]. Recently, sophisticated OPAs have been demonstrated with performance parameters that make them seem suitable for high power lidar applications [75; 76; 71]. The combination of OPAs and FMCW detection has got the theoretical potential to drive a lidar system fully solid-state and on-chip, which becomes one of the best potential combinations for a lidar unit. An on-chip lidar fabricated using combined CMOS and photonics manufacturing, if industrialized, has huge potential regarding reliability and reduced cost. However, both technologies, and in particular OPA, need further development from its current state.