Laser (Light Amplification by the Stimulated Emission of Radiation) light is highly monochromatic, coherent, directional, and can be sharply focused.
Figure 1: Simulated emission
When a photon of energy hn (h is Plank’s constant and n the frequency of radiation) interacts with an atomic system ( Figure 1) which is in its upper state E2, the system is driven down to its lower state E1 (hn = E2 -E1) and two photons exit from the system. This process is called stimulated emission. The emitted photon is in every way identical with the triggering or stimulating photon. It has the same energy, direction, phase, and state of polarisation. Furthermore, each of these photons can cause another stimulated emission event and results in four photons emitted. Continuation of this process leads to a chain reaction. All photons emitted in this way have identical energies, directions, phases, and states of polarisation. This is how laser light acquires its characteristics.
The laser could be classified in many ways: pulsed and continuous; infrared, visible, and ultraviolet; high-power and low-power; and so on. The most important classification is into solid-state, gas, liquid, and semiconductor categories. For remote sensing purposes lasers capable of emitting high-power, short-duration, narrow-bandwidth pulse of radiant energy with a low degree of divergence are required. Lasers can be used for both spectral analysis and range measurement of a target. Altimetric LiDAR utilises the latter characteristic of the laser and discussions in the following sections will mostly concentrate on this. Therefore, the term LiDAR will, henceforth, generally mean range measurement or topographic LiDAR.