# An Overview of Lidar Imaging Systems for Autonomous Vehicles
## 3 Sources and Detectors for Lidar Imaging Systems in Autonomous Vehicles
### Multipixel photon counters  
SPADs are very efficient in the detection of single photons, as they provide a digital output in the presence of one or more photons. The signal obtained when detecting one or several photons is thus equivalent, which is a drawback in several applications. Multipixel photon counters (MPPCs), also known as silicon photomultipliers (SiPMs), are essentially SPAD arrays with cells of variable size that recombine the output signal of each individual SPAD into a joint analog signal [132; 133]. In such a fashion, the analog signal is proportional to the number of SPADs triggered, enabling photon-counting beyond the digital on/off photon detection capability presented by SPADs. Each microcell in an MPPC consists of a SPAD sensor with its own quenching circuit. When a microcell triggers in response to an absorbed photon, the Geiger avalanche causes a photocurrent to flow through the microcell. The avalanche is confined to the single-pixel where it was initiated while all other microcells remain fully charged and ready to detect photons. When a photon is detected, the receiving unit in the array outputs a single pulse with a fixed amplitude that does not vary with the number of photons entering the unit at the same time. The output amplitude is equal for each of the pixels. Although the device works in digital mode pixel by pixel, MPPCs become analog devices as all the microcells are read in parallel and each of the pulses generated by multiple units is superimposed onto each other to obtain the final photocurrent. As a drawback, linearity gets worse as more photons are incident on the device, because the probability for more than one photon hitting the same microcell increases. Further, as an array, the potential for crosstalk and afterpulsing between cells may be significant depending on the application [134]. Optical crosstalk occurs when a primary avalanche in a microcell triggers secondary discharges in one or more adjacent microcells, i.e., the unit that actually detects photons affects other pixels making them produce pulses that make the output signal higher than that implied by the amount of the incident light. Its probability depends on fixed factors (like the size of the microcell and the layered architecture) and on variable ones (like the difference between the bias voltage being applied and the breakdown voltage). A typical MPPC has microcell densities of between 100 and several 1000 per mm\({}^{2}\), depending upon the size of the unit. Its characteristics are greatly connected with the operating voltage and ambient temperature. In general, raising the reverse voltage increases the electric field inside the device and so, improves the gain, photon detection efficiency and time resolution. On the other hand, it also increases undesired components that lower the SNR, such as false triggers due to thermal noise and afterpulsing. Thus, the operating voltage must be carefully set in order to obtain the desired characteristics. Despite these practicalities, MPPCs have many attractive features including very high gains (about \(10^{6}\)), analog photon-counting capabilities, a wide number of available commercial sizes (they are even modular so they can be attached next to each other), and lower operation voltage and power consumption values than the ones required for conventional PMTs. Due to its characteristics, it is a useful device for most low light applications, in particular for lidar or in any single photon application where solid-state detectors are an advantage related to PMTs. (e) Photomultiplier tubes  
As a final detector in the series, photomultiplier tubes (PMTs) have played, and still play, a relevant role in several applications, including atmospheric lidar for remote sensing. They have been compared to MPPC detectors in atmospheric lidar showing comparable performance [135]. PMTs are based on the external photoelectric effect. A photon incident onto a photosensitive area within a vacuum tube that extracts a photoelectron from the material.