# An Overview of Lidar Imaging Systems for Autonomous Vehicles
## 2 Basics of Lidar Imaging
### Optical Phased Arrays
#### 2.2.2 Detector Arrays  
Due to the lack of popularity in the automotive market of scanning lidar approaches based on moving elements, alternative imaging methods have been proposed to overcome their limitations beyond MEMS scanners and OPAs. These scannerless techniques typically combine specialized illumination strategies with arrays of receivers. Transmitting optical elements illuminate a whole scene and a linear array (or matrix) of detectors receives the signals of separate angular subsections in parallel, allowing to obtain range data of the target in a single-shot (Figure 8) making it easy to manage real-time applications. The illumination may be pulsed (flash imagers) or continuous (AMCW or FMCW lidars). With the exception of FMCW lidars, where coherent detection enables longer ranges, flash imagers or imagers based on AMCW principle (TOF cameras) are limited to medium to short ranges. In flash lidars the emitted light pulse is dispersed in all directions, significantly reducing the SNR, while in TOF cameras the phase ambiguity effect limits the measured ranges to a few meters. A brief description of the basic working principle is provided next. (a) Flash imagers  
One very successful architecture for lidar imaging systems in autonomous vehicles is flash lidar, which has progressed to a point where it is very close to commercial deployment in short and medium-range systems. In a flash lidar, imaging is obtained by flood-illuminating a target scene or a portion of a target scene using pulsed light. The backscattered light is collected by the receiver which is divided among multiple detectors. They respond to the schematics in Figure 8, considering a pulsed source and appropriate expanding optics to expand the beam to cover the scene of interest. Each detector captures the image distance, and sometimes the reflected intensity using the conventional  
Figure 8: Detector array-based lidar diagram. time-of-flight principle. Hence, both the optical power imaged onto a 2-D array of detectors and the 3D point cloud are directly obtained with a single laser blast on the target [45; 77; 78]. In a flash-lidar system, the FOV of the array of detectors needs to closely match the illuminated region on the scene. This is usually achieved by using an appropriate divergent optical system which expands the laser beam to illuminate the full FOV. The beam divergence of the laser is usually arranged to optically match the receiver FOV, allowing us to illuminate all the pixels in the array at once. Each detector in the array is individually triggered by the arrival of a pulse return, and measures both its intensity and the range. Thus, the spatial resolution strongly depends on the resolution of the camera, that is, on the density with which the detectors have been packed, usually limited by CMOS technology patterning. The resolution in z depends, usually, on the pulse width and the accuracy of the time counting device. Typically, the spatial resolution obtained is not very high due to the size and cost of the focal plane array used. Values around some tenths of Kilopixels are usual [42], limited by cost and size of the detector (as they usually work at 1.55 um, and are thus InGaAs based. Resolution in depth and angular resolution are comparable to scanning lidars, even better in some arrangements. Since the light intensity from the transmitter is dispersed with a relatively large angle to cover the full scene, and such a value is limited by eye-safety considerations, the measurement distance is dependent on sensing configurations including aspects like emitted power, sensor FOV and detector type and sensitivity. It can vary from tenths of meters to very long distances, although at present they are used in the 20 m to 150 m range in automotive. Range and spatial resolution of the measurement obviously depend on the FOV considered for the system, which limits the entrance pupil of the optics and the area where the illuminator needs to spread power enough to be detected afterwards.