Page 293 - Tunable Lasers Handbook

6 Transition Metal Solid-state Lcsers 53

laser manifold. For four-level laser operation, the lower laser manifold is well
above the ground manifold. Thus. the lower laser level has virtually no population
density at its operating temperature. A virtually empty lower laser level makes
threshold much easier to achieve since a high lower laser level population density
does not have to be overcome. Cr:BeAl,O,, on the other hand, operates on a
vibronic transition (see Fig. 13). As such. the population density of the ground
level does not have to be overcome in order to reach threshold, In short, since the
population density of the ground level does not have to be overcome, Cr:BeAl,Q,
operating on a vibronic transition resembles the operation of a four-level laser.
Even though the overall symmetry of the BeA1,0, crystal is considerably
different than the A1,0, crystal, the approximate octahedral symmetry for the
active atom prevails. As in the case of Cr:AI,O,, the Cr in Cr:Be4120, substitutes
€or the Al. Typical concentrations of Cr are in the range from 0.0005 to 0.003
atomic. That is. between 0.0005 and 0.003 of the A1 atoms are replaced by Cr
atoms. However, there are two different A1 sites in this material. One site has mir-
ror symmetry: the other has inversion symmetry. Most of the Cr substitutes for A1
in the slightly larger mirror site. about 0.78 of the Cr is found in chis site [20].
This is fortunate because this site is by far the dominant site for laser action. Both
Al sites are approximated as being octahedral. That is. the Cr atom is surrounded
by six 0 atoms forming an approximate octahedron. However, distortions to the
approximate octahedron provide for different optical properties along three axes.
BeA1,0,, like A1,Oj, has excellent mechanical and thermal properties for a
laser material [21]. Thermal conductivity is about half that of AY,O, but still
larger than the thermal conductivity of most other laser materials. It also a hard
material, conducive to taking a good optical polish. The laser induced damage
threshold for this material is very high. Excellent thermal and optical damage
thresholds are important since this material is generally subjected to higher ther-
mal and optical energy densities than higher gain materials. Germane physical
properties are listed in Table 2.
BeA1,0, is a birefringent material; however, it is a biaxial material rather
than an uniaxial material. That is. there are two directions in this material for
which the index of refraction is independent of the polarization. The refractive
indices of this material are about 1.74. Difference between the refractive indices
along the a and c axes is relatively small, about 0.002, whereas the difference
between the a and b axes is significantly larger, about 0.005.
Because of its biaxial nature, there are three absorption and emission spec-
tra, associated with the a, b, and c axes of the laser material. In general. absorp-
tion along any of these directions displays two broad absorption features.
Absorption peaks occur at approximately 0.42 and 0.56 pm as shown in Fig. 11.
The second absorption peak for radiation polarized along the b axis occurs at a
somewhat longer wavelength, about 0.59 pm. Linewidths for the absorption fea-
tures are about 0.05 and 0.08 pm, respectively. Absorption peaks are associated
with the transitions berween the -TI and IT, levels and the ,AAz ground level
Even for lightly doped laser material, the absorption coefficients at the peak are