Question

Explain the processes of lasing in the Nd: YAG laser. What is the function of Nitrogen, and Helium in the gas mixture of the Nd: YAG laser?

Answer 1

Working of Nd:YAG Laser:   

One of the most widely used laser sources for moderate to high power uses a neodymium doped crystal Yttrium Aluminum Garnet (YAG), commonly designated Nd:YAG. In addition, other hosts can be used with Nd, such as calcium tungstate and glass. The Nd:YAG laser is optically pumped either by tungsten or krypton pump lamps and is capable of CW outputs approaching 2000 W at the 1.06 µm wavelength. The ends of the crystal, which is usually in the form of a rod, are lapped, polished, and may be coated to provide the cavity mirrors.

Nd:YAG lasers belong to the class of solid state lasers. Solid state lasers occupy a unique place in laser development. The first operational laser medium was a crystal of pink ruby (a sapphire crystal doped with chromium); since that time, the term “solid state laser” usually has been used to describe a laser whose active medium is a crystal doped with an impurity ion. Solid state lasers are rugged, simple to maintain, and capable of generating high powers.

Although solid state lasers offer some unique advantages over gas lasers, crystals are not ideal cavities or perfect laser media. Real crystals contain refractive index variations that distort the wavefront and mode structure of the laser. High power operation causes thermal expansion of the crystal that alters the effective cavity dimensions and thus changes the modes. The laser crystals are cooled by forced air or liquids, particularly for high repetition rates.

The most striking aspect of solid state lasers is that the output is usually not continuous, but consists of a large number of often separated power bursts. Normal mode and Q-Switched solid-state lasers are often designed for a high repetition-rate operation. Usually the specific parameters of operation are dictated by the application.

For example, pulsed YAG lasers operating 1 Hz at 150 Joules per pulse are used in metal removal applications. As the repetition rate increases, the allowable exit energy per pulse necessarily decreases. Systems are in operation, for example, which produce up to ten Joules per pulse at a repetition rate of 10 Hz. A similar laser, operated in the Q-Switched mode, could produce a one megawatt per pulse at a rate up to ten pulses a minute.

Commonly, the shielding gases used for surface treatment are the same used for laser welding.

Neutral gases, such as argon and helium, ensure full protection against ambient air. Sometimes, nitrogen can be also used. Argon-hydrogen mixtures ensure good surface appearance due to a blanketing effect and prevention of plasma generation in the presence of hydrogen.