Nd-YAG [Neodymium-Yttrium Aluminium Garnet] Laser

ND‒YAG [NEODYMIUM‒YTTRIUM ALUMINIUM GARNET] LASER

Characteristics of Nd‒YAG laser

Type: Doped insulator laser [Solid state laser]

Active medium: Yttrium Aluminium Garnet [Y₃Al₅O₁₂]

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 the other is partially reflecting.

Power output: 2×10⁴ watts

Nature of output: Pulsed

Wavelength emitted: 1.064 μm.

Introduction

Nd‒YAG laser is a doped insulator laser. It is a four level system in which the active medium is taken in the form of a crystal. Here the crystal is intensionally doped during its growth. Those type of lasers has number of energy levels with same energy. The laser is used to generate high power intensity.

Principle

The term "Doped Insulator Laser" refers to the active medium, yttrium aluminium garnet doped with neodymium Nd³⁺. The neodymium ion has many energy levels. Due to optical pumping these ions are raised to excited levels. During the transition from metastable state to E₁ state, the laser beam of wavelength 1.064 μm is emitted.

Construction

The active medium is made as a rod which has yttrium aluminium garnet [Y₃ Al₅ O₁₂] doped with a rare earth metal ion neodymium Nd³⁺. The Nd³⁺ ions normally occupies the yttrium ions and provides the energy levels for both the lasing transitions and pumping. This rod is placed inside a highly reflecting elliptical cavity as shown in Fig.10.10.

A close optical coupling is made by placing the xenon flash lamp near by the laser rod, in such a way that most of the radiation from the flash tube passes through the laser rod due to the elliptical cavity. The flash tube may be switched ON and controlled with the help of a capacitor. The discharge of capacitor is initiated using a high voltage source.

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 the other is partially reflecting which is included to increase the efficiency of the output beam.

Working

1. The xenon flash lamp is switched ON and the light is allowed to fall on the laser rod.

2. The intense white light excites the neodymium (Nd³⁺) ions from the ground state to various energy levels above E₂. Hence the atoms are raised to group of higher levels in E₃ as illustrated in the energy level diagram Fig.10.11.

3. From these energy levels the ions make non‒radiative decay and is gathered in a state called as meta stable state, until the population inversion is achieved.

4. Once the population inversion is achieved, the stimulated emission builds up rapidly.⠀ 5. Hence, pulsed form of laser beam of wavelength 1.064 μm is emitted during the transition from E₄ to E₁ (lower).

6. 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 crystal.

Applications of Nd‒YAG Laser

1. It is used in transmitting signals to a longer distances.

2. It is used in long haul communication system.

3. It is also used in the endoscopic applications.

4. It plays a vital role in remote sensing applications.

NOTE: For continuous laser beam the xenon flash lamp may be replaced with quartz ‒ halogen lamps.