Editing Lidar (section)

=== Laser ===

600–1,000&nbsp;[[nanometre|nm]] [[laser]]s are most common for non-scientific applications. The maximum power of the laser is limited, or an automatic shut-off system which turns the laser off at specific altitudes is used in order to make it eye-safe for the people on the ground.

One common alternative, 1,550&nbsp;nm lasers, are eye-safe at relatively high power levels since this wavelength is not strongly absorbed by the eye. A trade-off though is that current detector technology is less advanced, so these wavelengths are generally used at longer ranges with lower accuracies. They are also used for military applications because 1,550&nbsp;nm is not visible in [[Night vision device|night vision goggles]], unlike the shorter 1,000&nbsp;nm infrared laser.

Airborne topographic mapping lidars generally use 1,064&nbsp;nm diode-pumped [[Yttrium aluminium garnet|YAG]] lasers, while [[bathymetric]] (underwater depth research) systems generally use 532&nbsp;nm [[Second-harmonic generation|frequency-doubled]] diode pumped YAG lasers because 532&nbsp;nm penetrates water with much less [[attenuation]] than 1,064&nbsp;nm. Laser settings include the laser repetition rate (which controls the data collection speed). Pulse length is generally an attribute of the laser cavity length, the number of passes required through the gain material (YAG, [[YLF]], etc.), and [[Q-switch]] (pulsing) speed. Better target resolution is achieved with shorter pulses, provided the lidar receiver detectors and electronics have sufficient bandwidth.<ref name="cracknell" />

A [[phased array]] can illuminate any direction by using a microscopic array of individual antennas. Controlling the timing (phase) of each antenna steers a cohesive signal in a specific direction. Phased arrays have been used in radar since the 1940s. On the order of a million optical antennas are used to see a radiation pattern of a certain size in a certain direction. To achieve this the phase of each individual antenna (emitter) are precisely controlled. It is very difficult, if possible at all, to use the same technique in a lidar. The main problems are that all individual emitters must be coherent (technically coming from the same "master" oscillator or laser source), have dimensions about the wavelength of the emitted light (1 micron range) to act as a point source with their phases being controlled with high accuracy.

[[MEMS|Microelectromechanical mirrors (MEMS)]] are not entirely solid-state. However, their tiny form factor provides many of the same cost benefits. A single laser is directed to a single mirror that can be reoriented to view any part of the target field. The mirror spins at a rapid rate. However, MEMS systems generally operate in a single plane (left to right). To add a second dimension generally requires a second mirror that moves up and down. Alternatively, another laser can hit the same mirror from another angle. MEMS systems can be disrupted by shock/vibration and may require repeated calibration.<ref name=":11">{{Cite news |last=Mokey |first=Nick |date=2018-03-15 |title=A self-driving car in every driveway? Solid-state lidar is the key |work=Digital Trends |url=https://www.digitaltrends.com/cars/solid-state-lidar-for-self-driving-cars/ |access-date=2018-06-15}}</ref>