Page 288 - Tunable Lasers Handbook

248 Norman P. Barnes

OOxygen 0 Aluminum

FIGURE 10 Crystal structure of.i120,.

there is an elongation of the octahedron in the vertical direction. This elongation
gives rise to a trigonal distortion.
Cr:A1,03 is produced by replacing a small fraction of the A1 atoms with Cr.
Through this replacement, sapphire becomes ruby. Typically, the fraction of the
A1 atoms replaced is small. In the production of Cr:A120,, about 0.0005 by
weight of the A1,0, is replaced by Cr203. In the laser material, Cr takes the
place of some of the A1 atoms and therefore sees the same symmetry as the A1
atoms. Replacement is straightforward since the A1 and Cr have the same
valence and are roughly the same size, Cr being somewhat larger.
A1,0, is a good material from which to make a laser. It is transparent from
about 6.2 to about 6.0 pm. Good transparency in the visible and near ultraviolet
allows a wide spectral region for efficient pump bands. It is a hard material,
which permits it to take a good polish. and it has a relatively high laser-induced
damage threshold. It has a very high thermal conductivity for a crystalline mate-
rial. High thermal conductivity is important in the design of high-average-power
laser systems. Other physical properties of this material are listed in Table 1 [ 131.
A1,0, is a birefringent material with a relatively high refractive index. It is
also a uniaxial material, that is, it has an unique optical axis. For directions of
propagation other than along the optic axis, this material has two refractive
indices. One refractive index is associated with radiation polarized in the optic
plane, that is, the plane defined by the direction of the optic axis and the direc-
tion of propagation. Another refractive index is associated with the normal to the
optic plane. These refractive indices are referred to as the extraordinary and ordi-
nary refractive indices, respectively. Refractive indices of this material do not
change significantly when doped with Cr. Birefringence, the difference between
these two refractive indices, is relatively small, about 0.008. However, the differ-
ences in the optical properties of these two polarizations are sufficient to make
the Cr:A1,0, laser operate in polarized modes.
Cr:A1,03 has two strong absorption bands, which differ slightly depending
on the polarization [12,14]. One of these absorption bands lies in the blue region