Neodymium Doped Yttrium Aluminium Garnet

The Nd-YAG (Neodymium doped Yttrium Aluminium Garnet) is a four level solid state laser $(Y3Al5O12)$ . Neodymium $(Nd3+)$ is one of the rare earth element. The Nd-YAG laser can be send through flexible quartz fibre and they are easy to integrate into the production system.

Characteristics

Type	Solid state laser
Active	Yttrium Aluminium Garnet ( Y₃ Al₅ O₁₂ )
Active centre	Neodymium ( Nd³⁺ ions)
Pumping method	Optical pumping
Pumping source	Xenon flash lamp
Optical resonator	Ends of the rods polished with silver and two mirrors, one of them is totally reflecting and other is partially reflecting.
Power output	70 W
Power output	Pulsed (or) continuous wave.
Frequency and wavelength of the output	2.82 x 10⁴ Hz and 1.064 μm

Principle

The active medium is Yttrium Aluminium Garnet doped with Neodymium $(Nd3+)$ . It is optically pumped by xenon flash lamps and Neodymium ions are raised to excited levels. During the transition from metastable state to ground state, laser beam of wavelength $1.064μm$ is emitted.

Construction

It consists of a laser rod and Xenon (or) Krypton flash lamps. Both are placed inside a highly elliptical reflector cavity. The crystal which acts as the active element, Yttrium ions $(Y 3+)$ is replaced by Neodymium ions $(Nd 3+).$

The optical resonator is formed by grinding the ends of the rods and coated with silver accompanied by two mirrors, one is $100%$ reflecting and other is partially reflecting. The flash tube may be switched on and controlled with the help of a capacitor.

Fig. 1.1 Nd-YAG laser

Working

The xenon flash lamp is switched on and the light is allowed to fall on the laser rod.
Then the $Nd 3+$ ions excites from ground level $E 0$ to higher levels $E 4$ and $E 3$ by the absorption of radiation of wavelength $0.73μm$ and $0.80μm$ respectively.
From these energy levels the ions make non-radiative decay and reach the metastable state $(E 2)$
The neodymium ions are collected in the level $E 2$ . Population inversion is achieved between $E 2$ and $E 1$ .
Once the population inversion is achieved the stimulated emission builds up rapidly.
This results in a laser beam of wavelength $1.06μm(10600Å)$ is emitted. Then non-radiative transition takes place from the $E 1$ to $E 0$ .
A large amount of heat is produced by the flash tube during the working. Hence, cooling arrangement is made either by blowing air (or) circulating water over the elliptical cavity.

Fig. 1.2 Energy level diagram of Nd-YAG laser

Advantages

It has high output energy.
It has very high repetition rate operation.
It is much easier to achieve population inversion.

Disadvantages

The electron energy level structure of $Nd 3+$ in YAG is complicated.

Applications

It is used in remote sensing applications.
It is used in long haul communication system.
It is used in precision welding, drilling and continuous cutting in industry.
It is used in medical fields like endoscopy, ENT, neurology and general surgery.

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