# An Overview of Lidar Imaging Systems for Autonomous Vehicles
## 3 Sources and Detectors for Lidar Imaging Systems in Autonomous Vehicles
### (c) Diode lasers  
There are two major types of DLs: interband lasers, which use transitions from one electronic band to another; and quantum cascade lasers (QCL), which operate on intraband transitions relying on an artificial gain medium made possible by quantum-well structures, built using band structure engineering. DLs are by far the most widespread in use, but recently, QCL have emerged as an important source of mid and long-wave infrared emission [4; 45; 106].  
Interband DLs are electrically pumped semiconductor lasers in which the gain is generated by an electrical current flowing through a PN junction or (more frequently) with a P doped-Insulator-N doped (PIN) structure to improve performance. A PIN junction properly polarized makes the I-region the active media expanding the depletion zone, where carriers can recombine releasing energy as photons, and at the same time acting as a waveguide for the generated photons. The goal is to recombine all electrons and holes to create light confined by edge mirrors of the I region, one of them partially reflective so the radiation can escape from the cavity. This process can be spontaneous, but it can also be stimulated by incident photons effectively leading to optical amplification and to laser emission [89; 107; 108; 109].  
Interband lasers can be presented in a number of geometries that correspond to systems operating in very different regimes of optical output power, wavelength, bandwidth, and other properties. Some examples include edge-emitting lasers (EEL, Figure 11), vertical-cavity surface-emitting lasers (VCSEL), distributed feedback lasers (DFB), high-power stacked diode bars or external cavity DL. Many excellent reviews exist discussing the characteristics and applications of each of these types of laser [110; 111; 112; 113; 45; 114].  
Regarding lidar applications, DLs have the advantage of being cheap and compact, two key properties in autonomous vehicle applications. In particular, DLs are extremely good regarding their cost against peak power figure of merit. Although they provide less peak power than fiber or microchip lasers, it is still enough for several lidar applications (maximum peak power may be up to the 10 kW range in optimal conditions) [115]. However, they become limited by a reduced pulse repetition rate (\(\sim\)100 kHz) and an increased pulse width (\(\sim\)100 ns). Furthermore, high-power lasers are usually in the EEL configuration, yielding a degraded beam quality with a fast and a slow axis diverging differently, which negatively affects the laser footprint and the spatial resolution. However, they are currently used in almost every lidar, either as a direct source or as optical pumps in SSLs [116].  
Figure 10: Schematic diagram of a microchip laser.  
Table 3 presents a brief summary of the key characteristics of the different laser sources described when used in lidar applications.  
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).