Electrons in atoms can absorb energy from light or heat only if there are transitions between energy levels that match the energy carried by photons or phonons. For light, this means that any given transition will absorb only one specific wavelength of light. Photons with the correct wavelength can make electrons jump from lower energy levels to higher energy levels. Photons are consumed in the process.
When an electron is excited from one state to a higher energy level with an energy difference ΔE, it doesn't stay in that state forever. Eventually, a photon will spontaneously emerge from a vacuum with energy ΔE. Energy is conserved, electrons transition to unoccupied lower energy levels, to different energy levels with different time constants. This process is called "spontaneous emission". Spontaneous emission is a quantum mechanical effect and a direct physical manifestation of Heisenberg's uncertainty principle. The emitted photons have random directions, but their wavelengths match the absorption wavelengths of the transitions. This is the mechanism of fluorescence and thermal emission.
Absorption of photons with the correct wavelength by the transition can also cause electrons to drop from higher to lower levels, emitting new photons. The emitted photons are perfectly matched to the original photons in wavelength, phase and direction. This process is called stimulated emission.
Lasers do not exist in nature. However, we have found ways to artificially create this particular type of light. Lasers produce a narrow beam of light where all light waves have very similar wavelengths. The light waves of a laser travel with their crests, or in phase. That's why the laser beam is very narrow, very bright, and can be focused on a very small spot. Because the laser remains focused and doesn't spread too much, the laser beam can travel great distances. They can also focus a lot of energy on a very small area.
So we already know that lasers are powerful because laser beams can be focused to very small points, to very high irradiance levels, and laser beams can be kept narrow over great distances, with these properties, lasers can be used in a variety of Applications such as laser cutting, laser engraving, lithography, laser pointers and lidar.
The pulsed operation of a laser refers to any laser that is not a continuous wave, so the optical power appears in pulses of a certain duration at a certain repetition rate. This includes a wide range of techniques that address many different motivations. This application requires the generation of pulses with as large an energy as possible. Since the pulse energy is equal to the average power divided by the repetition rate, this can sometimes be achieved by reducing the pulse frequency so that more energy is built up between pulses. For example, in laser ablation, if a small amount of material on the surface of the workpiece is heated for a short period of time, it can be evaporated, while providing energy gradually will cause the heat to be absorbed into the bulk of the workpiece. A piece, never reaches a high enough temperature at a particular point.
How is the laser beam generated?
The laser cavity or resonator is the heart of the system. In some high-gain devices, a single transmission through a group of excited atoms or molecules is sufficient to initiate lasing; however, for most lasers, multiple passes through the lasing medium are required to further increase the gain. This is achieved along an optical axis defined by a set of cavity mirrors that generate feedback. The lasing medium is placed along the optical axis of the resonator. This unique axis with very high optical gain also becomes the propagation direction of the laser beam. A slightly different example of a uniquely long gain axis is a fiber laser.
The simplest cavity is defined by two mirrors facing each other—a total reflector and a partial reflector, whose reflectivity can vary between 30% and close to 100%. Light bounces back and forth between these mirrors, increasing in intensity each time it passes through the gain medium. Photons that are spontaneously emitted in directions other than the axis are simply lost and do not contribute to laser operation. As the laser is amplified, some of the light exits the cavity or oscillator through the partial reflector; however, in equilibrium, these "optical losses" are perfectly compensated by the optical gain experienced by the continuous round-trip of photons within the cavity.
