Question

Explain construction and working of Nd:YAG LASER. 

Answer 1

Neodymium-doped Yttrium Aluminum Garnet (Nd: YAG) laser is a solid state laser in which Nd:YAG is used as a laser medium.
Construction of Nd:YAG LASER :

Nd:YAG laser consists of three important elements: an energy source, active medium, and optical resonator.
Energy source :
The energy source or pump source supplies energy to the active medium to achieve population inversion. In Nd: YAG laser, light energy sources such as flashtube or laser diodes are used as energy source to supply energy to the active medium.

Active medium :
The active medium or laser medium of the Nd:YAG laser is made up of a synthetic crystalline material (Yttrium Aluminum Garnet (YAG)) doped with a chemical element (neodymium (Nd)). The lower energy state electrons of the neodymium ions are excited to the higher energy state to provide lasing action in the active medium.

Optical resonator :

The Nd:YAG crystal is placed between two mirrors. These two mirrors are optically coated or silvered.One mirror is fully silvered whereas, another mirror is partially silvered. The mirror, which is fully silvered, will completely reflect the light.The mirror which is partially silvered will reflect most part of the light but allows a small portion of light through it to produce the laser beam. This mirror is known as a partially reflecting mirror.

1.Energy levels E₁,E₂ and E₃ are of Nd and other levels belong to YAG.E₁ is ground state and E₃ offers metastable state. 
2.Pumping takes place with light of 5000 A^o to 8000A^o which excites Nd⁺³ ions to higher states.
3.The metastable state E³ rapidly gets populated due to downward transitions from higher energy levels as none of them is metastable.
4.Population inversion takes place between E³ and E².
5.A continuous LASER of 10600 A^o in infrared region is given out due to stimulated smission between E³ and E².