# An Overview of Lidar Imaging Systems for Autonomous Vehicles
## 3 Sources and Detectors for Lidar Imaging Systems in Autonomous Vehicles
### Sources in Lidar Imaging Systems  
(a) Fiber lasers  
Fiber lasers use optical fibers as the active media for lasing. The optical fiber is doped with rare earth elements (such as Er\({}^{+3}\)) and one or several fiber-coupled DLs are used for pumping. In order to turn the fiber into a laser cavity, some kind of reflector (mirror) is needed to form a linear resonator or to build a fiber ring architecture (Figure 9). For commercial products, it is common to use a Bragg grating at the edge of the fiber which allows to reflect back a part of the photons. Although, in broad terms, the gain media of fiber lasers is similar to that of SS bulk lasers, the wave-guiding effect of the fiber and the small effective mode area usually lead to substantially different properties. For example, fiber lasers often operate with much higher laser gain and resonator losses than SSLs [90; 91; 92; 93]. Fiber lasers can have very long active regions, so they can provide very high optical gain. Currently, there are high-power fiber lasers with outputs of hundreds of watts, sometimes even several kilowatts (up to 100 kW in continuous-wave operation) from a single fiber [94]. This potential arises from two main reasons: on one side, from a high surface-to-volume ratio, which allows efficient cooling because of the low and distributed warming; on the other side, the guiding effect of the fiber avoids thermo-optical problems even under conditions of significant heating [95]. The fiber's waveguiding properties also permit the production of a diffraction-limited spot, that is, the smallest spot possible due to the laws of Physics, with very good beam quality, introducing a very small divergence of the beam linked only to the numerical aperture of the fibre, which stays usually within 0.1 deg values [96; 97]. This divergence is proportional to the footprint of the beam on the target, and thus is directly related to the spatial resolution of the lidar image. Furthermore, due to the fact that light is already coupled into a flexible fiber, it is easily sent to a movable focusing element allowing convenient power delivery and high stability to movement. Moreover, fiber lasers also enable configurable external trigger modes to ease the system synchronization and control and have a compact size because the fiber can be bent and coiled to save space. Some other advantages include its reliability, the availability of different wavelengths, their large PRR (\(>\)0.5 MHz is a typical value) and their small pulse width (\(<\)5 ns). Fiber lasers provide a good balance between peak power, pulse duration and pulse repetition rate, very well suited to the lidar specifications. Nowadays, its main drawback is its cost, which is much larger than other alternatives [45]. (b) Microchip lasers  
Microchip lasers are bulk SS lasers with a piece of doped crystal or glass working as the gain medium. Its design allows optical pump power to be transmitted into the material, as well as to spatially overlap the generated laser power with the region of the material which receives the pump energy (Figure 10). The ability to store large amounts of energy in the laser medium is one of the key advantages of bulk SS lasers and is unique to them. A saturable absorber (active or passive) induces high losses in the cavity until the energy storage saturates (passive) or is actively liberated by means of a shutter (active Q-switch) [98; 99]. The active medium of microchip lasers is a millimeter-thick, longitudinally pumped crystal (such as Nd:YAG) directly bonded to solid media that have saturable absorption at the laser wavelength. This transient condition results in the generation of a pulse containing a large fraction of the energy initially stored in the laser material, with a width that is related to the round-trip time of light in the laser cavity. Due to the bulk energy storage combined with their short laser resonator which leads to very short round-trip time, microchip lasers are well suited to the generation of high power short pulses, well beyond conventional lidar specs.