Question

Compare the differences in the manufacture of Nd-YAG and Nd-glass lasers. Compare their laser power output characteristics and explain in terms of their construction features.

Answer 1

The most frequently employed solid-state laser material is yttrium aluminum garnet doped with neodymium ions to a concentration as high as 5%. The chemical formula for YAG is Y3Al5O12. YAG is a hard clear crystal with a thermal conductivity about ten times that of glass. It has a high melting point and is strong and durable. Pure YAG is a clear crystal similar in appearance to pure sapphire. The addition of the rare earth element neodymium results in a purple tint to the Nd:YAG laser rod. The neodymium atoms are in a triply-ionized state inside the crystal, just as chromium atoms are in a ruby laser rod.

Neodymium atoms are also used as the active elements in Nd:glass lasers. The doping level is usually 1% or less. The absorption spectrum and energy-level diagrams of Nd:glass are similar to those of Nd:YAG, but the glass absorption peaks are much broader and less distinct. The reason for this is that glass is not a crystalline structure as is YAG. Glass is a supercooled fluid and has a random amorphous structure. Neodymium ions in a YAG crystal all have the same spacing from neighboring atoms and very similar environments. In glass the atomic distances and distribution are random, and each ion has a different environment. This causes the energy levels of different ions to shift differently, broadening all the absorption and emission lines considerably. This also results in a somewhat longer lifetime for the upper lasing level. This means that Nd:glass has a higher efficiency than Nd:YAG in the pulsed mode and a broader output linewidth.