# An Overview of Lidar Imaging Systems for Autonomous Vehicles
## 3 Sources and Detectors for Lidar Imaging Systems in Autonomous Vehicles
### Sources  
Lidars usually employ lasers sources with wavelengths from the infrared region, typically from 0.80 to 1.55 \(\upmu\)m, to take advantage of the atmospheric transmission window, and in particular of water at those wavelengths [87], while enabling the use of beams not visible to the human eye. The sources are mainly used in three regions: a waveband from 0.8 \(\upmu\)m to 0.95 \(\upmu\)m, dominated by diode lasers which may be combined with silicon-based photodetectors; lasers at 1.06 \(\upmu\)m, still usable for Si detectors and usually based on fibre lasers, and lasers at 1.55 \(\upmu\)m, available from the telecom industry, which need InGaAs detectors. However, other wavelengths are possible as the lidar detection principle is general and works in most cases regardless of the wavelength selected. Wavelength is in general chosen taking into account considerations related to cost and eye safety. 1.55 \(\upmu\)m, for instance, enables much more power within the eye safety limit defined by Class 1 [88] but may become really expensive if detectors need to be above the conventional telecom size (200 \(\upmu\)m). Thus, sources are based either on lasers or on nonlinear optical systems driven by lasers, although other sources may be found (such as LEDs), usually for short-range applications. Different performance features must be taken into account in order to select the most suitable source according to the system purpose. This includes, as the most relevant for automotive, peak power, pulse repetition rate (PRR), pulse width, wavelength (including purity and thermal drift), emission (single-mode, beam quality, CW/pulsed), size, weight, power consumption, shock resistance and operating temperature. In one very typical feature of lidar imaging systems, such performance features involve trade-offs; for instance, a large peak power typically goes against a large PRR or spectral purity. Currently, the most popular sources for use in lidar technology are solid-state lasers (SSL) and diode lasers (DLs), with few exceptions. SS lasers employ insulating solids (crystals, ceramics or glasses) with elements added (dopants) that provide the energy levels needed for lasing. A process called optical pumping is used to provide the energy for exciting the energy levels and creating the population inversion required for lasing. Generally, SSLs use DLs as pumping sources to create the population inversion needed for lasing. The use of SSLs for performing active-sensing for range-finding started in 1960 with the SS ruby laser, thanks to the development of techniques to generate pulses with a duration of nanoseconds. Since then, almost every type of laser developed has been employed in demonstrations of rangefinders. SSLs can be divided into a wide range of categories, for example bulk or fiber, with this latter increasingly becoming prevalent in lidar for autonomous vehicles, not only because of its efficiency and capacity for generation of high average powers with high beam quality and PRR, but also because it provides an easy way to mount and align the lidar using free-space optics. Beyond fiber lasers, increasing attention has been devoted to microchip lasers. They are, perhaps, the ultimate in miniaturization of diode-pumped SSLs. Its robust and readily mass-produced structure is very attractive. Their single-frequency CW performance and their excellence at generating sub-nanosecond pulses make microchip lasers well suited for many applications, lidar being among them. They need free-space alignment, unlike fibers, but can also achieve large peak energies with excellent beam quality. A more detailed description of all these laser sources, and of some others less relevant to lidar, can be found in [45]. Finally, semiconductor DLs are far more popular in the industry due to their lower cost and their countless applications beyond lidar. They are considered to be a separate laser category, although they are also made from solid-state material.