# An Overview of Lidar Imaging Systems for Autonomous Vehicles
## 3 Sources and Detectors for Lidar Imaging Systems in Autonomous Vehicles
### Photodetectors  
Along with light sources, photodetectors are the other main component of a lidar system that has dedicated features. Photodetectors are the critical photon sensing device in an active receiver which enables the TOF measurement. It needs to have a large sensitivity for direct detection of intensity, and it also has to be able to detect short pulses, thus requiring high bandwidth. A wide variety of detectors are available for lidar imaging, ranging from single-element detectors to arrays of 2D detectors, which may build an image with a single pulse [117]. The material of the detector defines its sensitivity to the wavelength of interest. Currently, Si-based detectors are used for the waveband between 0.3 \(\upmu\)m and 1.1 \(\upmu\)m, while InGaAs detectors are used above 1.1 \(\upmu\)m, although they have acceptable sensitivities from 0.7 \(\upmu\)m and beyond [118]. InP detectors and InGaAs/InP heterostructures have also been proposed as detectors in the mid-infrared [119], although their use in commercial lidar systems is rare due to their large cost if outside telecommunications standards and, eventually, the need for cooling them to reduce their noise figure. Figure 11: Schematic diagram of a edge-emitting (EEL) diode laser. \begin{table}
\begin{tabular}{c l l l} \hline \hline  & **Fibre Laser** & **Microchip Laser** & **Diode Laser** \\ \hline Amplifying media & Doped optical fibre & Semiconductor crystal & Semiconductor PN \\  & & & junction \\ \hline Peak power (typ) & \textgreater{}10 kW & \textgreater{}1 kW & 0.1 kW \\ \hline PRR & \textless{}1 MHz & \textless{}1 MHz & \textless{}100 KHz \\ \hline Pulse width & \textless{}5 ns & \textless{}5 ns & 100 ns \\ \hline Main advantage & Pulse peak power, & Pulse peak power, & Cost, compact \\  & PRR, beam quality. & PRR, beam quality & \\  & Beam delivery & & \\ \hline Main disadvantage & Cost & Cost, beam delivery & Max output power and \\  & & & PRR. Beam quality \\ \hline \hline \end{tabular} Note: these are typical values for reference when used for lidar. Individual components evolve continuously and may be different at the time of publishing or from specific providers. Further, several trade-offs related to laser performance need be considered (e.g., some DLs may have much larger peak power, but at smaller PRR values).  
\end{table} 
Table 3: Summary: main features of sources for lidar. Light detection in lidar imaging systems usually accounts for five different types of detectors: PIN diodes, avalanche photodiodes (APD), single-photon avalanche photodiodes (SPAD), multi-pixel photon counters (MPPC) and, eventually, photomultiplier tubes (PMT). Each one may be built from the material which addresses the wavelength of interest. The most used single-detector are PIN photodiodes, which may be very fast detecting events in light if they have enough sensitivity for the application, but do not provide any gain inside of their media, so in optimal efficiency conditions, each photon creates a single photoelectron. For applications that need moderate to high sensitivity and can tolerate bandwidths just below the GHz regime typical of PIN diodes, avalanche photodiodes (APDs) are the most useful receivers, since their structure provides a certain level of multiplication of the current generated by the incident light. APDs are able to internally increase the current generated by the photons incident in the sensitive area, providing a level of gain, usually around two orders of magnitude. In fact, their gain is proportional to the reverse bias applied, so they are linear devices with adjustable gain which provide a current output proportional to the optical power received. Single-photon avalanche diodes (SPAD) are essentially APDs biased beyond the breakdown voltage, and with their internal structure arranged to repetitively withstand large avalanche events.