Summer 2016 - Q.3 - Grad Plus

# 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%.

(b)Two plane rectangular pieces of glass are in contact at one edge & are separated at the other end 10 cm away by a wire to form a wedge shaped film.When the film was illuminated by light of wavelength 6000A°, 10 fringes were observed per cm. Determine the diameter of the wire. [7M]

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

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

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