Summer 2016 – Q.3

3. (a) With neat energy level diagram, explain principle, construction & working of Nd-YAG laster? [8M]

Nd-YAG laser is a four level solid state laser. The active medium is YAG (Yttrium Aluminium Garnet, Y₃Al₅O₁₂) crystal wherein some Y³⁺ ions are replaced by Nd³⁺ ions, which acts as active centres.

Construction: The basic construction of Nd-YAG laser is shown in figure. It consists of an cylindrical
reflector housing the laser rod along one side of its focus lines and a flash lamp along the other focus line. The light leaving one focus of the ellipse passes through the other focus after reflection from the silvered surface of the reflector. Thus, the entire flash lamp radiation gets focused on the laser rod. The two ends of the rods are gets polished and silvered and constitute optical resonator.

Working: The energy level of Nd ion in YAG crystal is shown in figure. The energy level structure of Nd atom is preserved to a certain extent because of its relatively low concentration. However the energy levels are split and the structure is complex. The pumping of the Nd ions to the upper states is done by krypton arc lamp. The optical pumping with light of wavelength 5000 to 8000 Å excites the ground state Nd ions to higher states.

The metastable state E₃ is upper lasing level whereas E₂ forms the lower lasing level. The upper level E₃ will be rapidly populated as the excited Nd ions quickly make downward transition from the upper energy bands. The lower energy level E₂ is far above the ground level and hence it cannot be populated by Nd ions through the thermal transitions from the ground state. Therefore, the population inversion is readily achieved between the E₂ and E₃ levels. The laser transition occurs in the infrared (IR) region at a wavelength of about 10600 Å. As the laser is four level laser the population inversion can be maintained for continuous laser emission.

Thus, Nd-YAG laser can be operated in CW mode with efficiency better than 1%.

Given,

$latex l=10\;cm,\;\lambda=6\times10^{-5}cm,\;p=10,\;x_{n+p}-x_n=1\;cm,\;d=?$

We have, diameter of wire,

$latex \begin{array}{l}d=t\\\\t=\frac{lp\lambda}{2\left(x_{n+p}-x_n\right)}\\\\\;\;\;=\frac{10\times10\times\times6\times10^{-5}}{2\times1}\\\\\;\;\;=3\times10^{-3}cm\\\\\;\;\;=0.03\;mm\end{array}$

or

$latex \begin{array}{l}d=t\\\\L\theta=\frac{100\beta\times\lambda}{2\beta}\\\\\;\;\;\;\;=50\lambda\\\\\;\;\;\;\;=50\times6000\;\overset\circ A\\\\\;\;\;\;\;=30,0000\;\overset\circ A\\\\\;\;\;\;\;=3\times10^{-5}m\\\\\;\;\;\;\;=0.03\;mm\end{array}$