Page 287 - Tunable Lasers Handbook

6 Transition Metal Solid-State Lasers 247

X primary disadvantage of Cr:A1,0, is its three-lekel operating scheme. In a
three-level scheme, the levels are the ground level, the pump level, and the upper
laser level. Figure 9 depicts the situation. In this scheme, the loner laser level is
the ground level. For lasing to occur, the population density of the upper laser
level has to be greater than the population density of the lower laser level. If the
population density of the upper laser level is higher than the population densit)
of the lower laser level. a populuiioii im,el-sion is said to exist. To achieve a pop-
ulation inversion, roughly half of the Cr atoms must be pumped to the upper
laser level, Pumping levels must be high in order for this to occur. If a popuia-
tion inversion is achieved, laser action can only be sustained as locg as the popu-
lation inversion is maintained. Consequently, when lasing terminates, all of the
remaining energy stored in the upper laser level is lost. A three-lecel laser is rel-
atively inefficient because of this. First, a great deal of pump energy is expended
to store enough energy in the upper laser level to achieve population inversion or
threshold. Second, of the energy stored in the upper laser level, only that portion
of it which is above threshold is available for laser output. Despite this limitation
on the efficiency of the Cr:AI,O,, the use of these lasers continues to this da:y.
A120,, or sapphire. is a c&tal composed of alternate hexagonal layers of A1
and 0 atoms, as shown in the Fig. 10. Oxygen atoms fill a layer in a close-packed
hexagonal arrangement. On top of this layer is a layer of A1 atoms. which nestle
in the depressions between three adjacent 0 atoms of the loner layer. In a filled
layer, one-third of the potential AI sites is left unfilled. 4 third layer is composed
of 0 atoms. again in a close-packed hexagonal arrangement. However, this layer
is displaced from the first layer. To first-order approximation, each A1 atom has
six 0 neighbors in an octahedral arrangement. However, since the distance
between the 0 layers is larger than the distance between 0 atoms within a layer.

Pump
Manifold