Nd - Yag Laser

Nd - YAG LASER

Nd - YAG laser is Neodymium based laser. Nd stands for Neodymium (rare earth element) and YAG for Yttrium Aluminium Garnet (Y3 A15O12).

It is a four-level solid state laser.

Principle

The active medium Nd-YAG rod is optically pumped by krypton flash tube. The neodymium ions (Nd3+) are raised to excited energy levels. During transition from metastable state to ground state, a laser beam of wavelength 1.064 um is emitted.

Construction

The construction of Nd-YAG laser is shown in fig. 5.17.

A small amount of yttrium ions (Y3+) is replaced with neodymium ions (Nd3+) in the active medium of Nd-YAG rod.

The active medium Nd-YAG crystal is cut into a cylindrical rod. The ends of this rod are highly polished and optically flat and parallel. The cylindrical rod (laser rod) and a pumping source (krypton flash tube) are kept in an elliptical reflector cavity in order to focus most of the light into Nd YAG rod.

The optical resonator is formed by using two external reflecting mirrors. One mirror M1 is fully reflecting while the other mirror M2 is partially reflecting.

Working

The energy level of Nd3+ ion in Nd - YAG laser is shown in fig 5.18.

• When krypton flash tube is switched on, the neodymium ions are excited from ground state E0 to upper energy levels E3 and E4 (pump bands) due to absorption of light radiation of wavelengths 0.73 μm and 0.80 μm. 

• The neodymium ions from these excited energy levels make a transition to energy level E2 by non-radiative transition. E2 is a metastable state.

• Now, the neodymium ions are collected in this energy level E2. Thus, the population inversion is achieved between E2 and E1.

• A neodymium ion makes a spontaneous transition from E1 to E2 by emitting a photon of energy hv. This emitted photon triggers a chain of stimulated photons between E2 and E1.

• The photons thus generated travel back and forth between two mirrors and grow in strength due to induced emission. Hence, the photon number multiplies more rapidly. 

• After enough strength is attained, an intense laser light of wavelength 1.06 μm (10600 Å) is emitted and flow through the partially reflecting mirror M2. This corresponds to transition of ions from E2 to E1.

Characteristics

• Type: It is a four-level solid state laser.

(E1, E2, E3 and E4) 

• Active medium: Nd-YAG rod. 

• Pumping method: Optical pumping

• Pumping source: Krypton flash tube.

• Optical resonator: Two ends of Nd-YAG rod polished with silver (one end is fully silvered and other is partially silvered).

• Power output: 20 kW.

• Nature of output: Pulsed or continuous beam of light. 

• Wavelength of output: 1.06 μm (infra-red).

Advantages

• This laser has high energy output. 

• It is much easier to achieve population inversion.

Disadvantages

The electron energy level structure of Nd3+ in Nd - YAG is complicated.

Applications

• Nd-YAG laser is used in range finders and illuminators.

• It is widely used in resistor trimming, scribing, micro-machining operations such as welding, drilling etc.

• It finds many medical applications such as endoscopy, urology, neurosurgery.