Re: How is Nd:YAG prepared?

Greetings:

The neodymium (Nd) laser is the most common member of a family of lasers 
called solid-state lasers. The Nd may be incorporated into various host 
materials, either synthetic crystals or glasses of different compositions. 
In a crystal, the Nd is essentially an impurity which takes the place of 
another element with roughly the same ion size (most often yttrium). In 
glasses and crystalline hosts, the typical Nd doping is about 1% by weight, 
giving a Nd concentration of 10^20 (1 followed by 20 zeros) atoms per cubic 
centimeter which is about the optimum concentration for laser action.

Many other crystals have been doped with Nd and several are available 
commercially. YAG (yttrium aluminum garnet) is the most common laser 
material with YLF (yttrium lithium fluoride) and YALO (yttrium aluminate-Y 
Al O3) also being well developed laser materials.

YAG is a hard, brittle material and its growth is characterized as a black 
art (a well kept secret). Generally YAG crystals are grown by the 
Czochralski (pulling from a melt) process. The powdered garnet material and 
the desired amount of Nd powder are placed in a platinum crucible which is 
then lowered into a cylindrical oven and melted. The liquid melt must be 
held at a constant temperature (a fraction of one degree) for many days or 
even weeks.  A small YAG "seed" crystal attached to a long rod is then 
lowered into the top of the melt and slowly rotated at about 10 revolutions 
per minute. The rod is then very slowly pulled upward from the melt a few 
millimeters per hour and the liquid on the seed solidifies (crystallizes) 
growing a longer crystal on the end of the seed crystal with the atoms 
oriented exactly to match the seed crystal. Thus a 10 cm (4 in) long YAG 
rod can take as much as 100 hours (4 days) to pull. Too high a rotation 
speed can produce temperature fluctuations from turbulence in the liquid 
melt spoiling the YAG crystal. However, more rapid rotation causes the melt 
to be better mixed producing better Nd distribution in the melt. The black 
art is knowing how to control the temperature, rotation rate and pulling 
speed to obtain the best YAG crystals.

Crystal growth problems limit the maximum length of YAG rods to about 10cm 
(4 in) with diameters between 6mm to 9 mm (.35 in). Glass can be made in 
much larger blocks to provide higher output laser power. The NOVA fusion 
laser at the Lawrence Livermore National Laboratory (LLNL) uses stacks of 
many elliptical Nd-glass disks 46cm by 85 cm (18 in by 33 in) for its final 
amplifier stage. YAG laser oscillators, as in NOVA,  are often followed by 
Nd-glass laser amplifiers, because YAG has a better beam quality, which can 
then be boosted in power by the amplifiers. This is called a master 
oscillator power amplifier (MOPA) configuration. You can find pictures of 
the giant NOVA laser at:
 http://hangar.llnl.gov/
tour/Tour.html

Recently our laboratory (http://www.hrl.com) set a new record for YAG 
output power by using indium gallium arsenide (InGaAs) diode lasers to pump 
a pencil sized YAG rod which generated 1000 watts of continuous laser 
power, all in a very tight beam of light. One of the technical problems 
with this laser was removing more the 3000 watts of waste heat produced by 
the diode pumps and light absorption within the YAG rod.


Best regards, Your Mad Scientist
Adrian Popa