Rami Arieli: "The
Laser Adventure" Section 6.2.2 page 1
6.2.2 Nd Laser
In Nd laser Nd+3 ions
(as impurities of up to a few percent by weight) are replacing the atoms
of the solid host in the active medium.
Three known solid hosts are used for Nd-YAG
laser where Nd+3 ions are added as impurities:
-
Glass.
-
YAG (Yttrium Aluminum Garnet) Crystal.
-
YLF (LiYF4) Crystal.
The choice between the three possible hosts is according to the intended
use of the laser:
-
Glass is used
as the host material when a pulsed laser
is needed, with each pulse at high power, and the pulse repetition rate
is slow.
The active medium of Nd-Glass Laser can be manufactured in a shape
of disk or rod, with diameters of up
to 0.5 meter (!) and length of up to several meters (!). Such dimensions
are possible because glass is isotropic material, cheap, and can be easily
worked to the right shape.
High percentage (up to about 6%) of Nd ions can be added to glass as
impurity.
The problem with glass as a host
is its poor thermal conductivity. Thus
cooling the laser when it operates continuously or at high repetition rate
is difficult.
YAG crystal is
used for high repetition rate pulses
(more than one pulse per second). In this case a large amount of heat need
to be transferred away from the laser, and the thermal conductivity
of the YAG crystal is much higher than that of glass.
YAG crystal with the high quality needed for lasers can be made with
diameters of 2-15 [mm] and at lengths of 2-30 [cm].
The price of a YAG laser rod is high, since growing crystals is a
slow and complicated process.
The percentage of Nd ions in the YAG host is 1-4% by weight.