Components of LASER: active medium, Pumping system and Optical resonator

What are the main components of LASER?

Components of LASER

There are three basic components of a LASER:

(i) active medium, (ii) pumping system and (iii) optical resonator

(i) Active medium:

The active medium in LASERS may be a solid, liquid, or gas. Different active media emit different energies or wavelengths of light. The basic requirement for the active medium of a LASER is that it should have suitable energy levels to achieve the condition of population inversion or it must have a metastable energy states to support stimulated emission. Those atoms of active medium, which are responsible for LASER action are called active centers and rest of the medium is called as host.

(ii) Pumping System

The pumping system consists of an external source that supplies energy to active medium and helps in obtaining the population inversion. The excitation of atoms may occur directly or through atom-atom collision. It can be optical, electrical or thermal in nature.

(iii) Optical Resonator:

It consists of a pair of parallel mirrors enclosing the active medium in between them. The reflectivity of one of the mirrors near to 100% and the other is partially transparent. It is basically a feedback device that reflects undesirable (off-axis) photons out of the system and directs the desirable (on-axis) photons back and forth through the active medium and in the process the number of photon is multiplied due to stimulated emission causing thereby amplification.

Working of Optical Resonator:-

Atoms (active centers) of the lasing material normally reside in ground energy state.
These atoms can be excited to a higher energy state by supplying external energy. The atoms are unstable at this state so they drop spontaneously to a metastable state in which they can stay longer in compare to the ordinary excited state and hence the population inversion can be achieved at this state. Some of the atoms can be de-excited spontaneously from the metastable state to their ground state emitting photons in random directions. Each spontaneous photon can stimulate other excited atoms to fall to their ground state by emitting a photon that travel in phase and in the same direction as the incident photon. If the direction of emitted photons is parallel to the optical axis, the emitted photons travel back and forth in the optical cavity through the lasing material between the totally reflecting mirror and the partially reflecting mirror. The light energy is amplified in this manner until sufficient energy is built up for a burst of laser light to be transmitted through the partially reflecting mirror.

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