# An Overview of Lidar Imaging Systems for Autonomous Vehicles
## 3 Sources and Detectors for Lidar Imaging Systems in Autonomous Vehicles
### Single-photon avalanche photodiodes  
APDs working in Geiger-mode are known as single-photon avalanche diodes (SPADs). SPADs are operated slightly above the breakdown threshold voltage (Figure 12), the electric field being so high that a single electron-hole pair injected into the depletion layer can trigger a strong, self-sustained avalanche. The current rises swiftly to a macroscopic steady level and it keeps flowing until the avalanche can be _quenched_, meaning it is stopped and the SPAD is operative again. Under these circumstances, the photocurrent is not linearly amplified, but rather a standard final current value is reached regardless if it has been triggered by only one or by several incident photons. The design of the device architecture needs to be prepared for repeated avalanches without compromising the response of the detector. The structure of SPADs is different from those of linear mode APDs in order to withstand repeated avalanches and have efficient and fast quenching mechanisms. For effective Geiger-mode operation, the avalanche process must be stopped and the photodetector must be brought back into its original quiescent state. This is the role of the quenching circuit. Once the photocurrent is triggered, the quenching circuit reduces the voltage at the photodiode below the breakdown voltage for a short time, so the avalanche is stopped. After some recovery time, the detector restores its sensitivity and is ready for the reception of further photons. Such a dead-time constitutes a substantial limitation of these devices, as it limits the count rate and leaves the device useless for times in the 100 ns scale, severely limiting its bandwidth. This is being tackled through improved quenching circuits. Currently, two types of quenching are in use: passive, in which avalanche is interrupted by lowering the bias voltage below breakdown using a high-value resistor; and active, based on active current feedback loops. Active quenching was specifically devoted to overcoming the slow recovery times characteristic of passive quenching. The rise of the avalanche is sensed through a low impedance and a reaction back on the device is triggered by controlling the bias voltage using active components (pulse generators or fast active switches) that force the quenching and reset transitions in shorter times [129]. Currently, active quenching is a very active research line, due to its relevance in low light detection in several imaging applications [130]. The macroscopic current generated due to the avalanche is discernible using electronic threshold detection. Since threshold detection is digital, it is essentially noiseless, although there exist different mechanisms that can also fire the avalanche process generating noise. The main sources of false counts in SPADs are thermally-generated carriers and afterpulsing [131]. The first case is due to the generation-recombination processes within the semiconductor as a result of thermal fluctuations, which may induce the avalanche and produce a false alarm. In the second case, during the avalanche, some carriers are captured by deep energy levels in the junction depletion layer and subsequently released with a statistically fluctuating delay. Released delayed carriers can retrigger the avalanche generating these after-pulses, an effect that increases with the delay of avalanche quenching and with the current intensity. SPADs are, however, extremely efficient in low light detection, and can be used when an extremely high sensitivity at the single-photon detection level is required. Devices with optimized amplifier electronics are also available in the CMOS integrated form, even as large detector arrays, in applications from quantum optics to low-light biomedical imaging. The intensity of the signal may be obtained by repeated illumination cycles counting the number of output pulses received within a measurement time slot.