Page 316 - Tunable Lasers Handbook

276 Norman P. Barnes

such. the lower laser manifold is the AA, manifold, and the upper laser manifold is
the 3T,. For V, the ratio of Dq/B is lower than Dq/B values usually associated with
Cr. Adirect result of the lower value of Dq/B is that the IT, manifold is lower
than the 2E manifold. In doubly ionized Ni the lower laser manifold is the IT, and
the upper laser manifold is the IT... In doubly ionized Co the lower laser manifold
is the 3A, and the upper laser manifold is the jT,.
Co:MgF,, Ni:MgF, and VMgF2 can haveboth electronic and vibronic tran-
sitions. For example, consider the energy levels of Co:MgF,. Here the lower
laser manifold is split into six levels by the spin orbit interaction and the crystal
field effects. Each of these levels has Kramer’s degeneracy. Splitting of the lower
laser manifold is quite large, about 1400 cm-1 for Co:MgF,. - Because of the large
splitting. electronic transitions can occur between the upper and lower laser
manifolds. These electronic transitions produce relatively narrow and strong
peaks in the fluorescence spectrum. If a transition occurs in the vicinity of these
peaks, the lower laser level can have a significant population density, leading to
three-level-like operation. On the other hand, far from these peaks, the transi-
tions are vibronic. leading to four-level-like operation.
Laser materials are produced by replacing some of the Mg with the proper
active atom-Co, Ni. or V. Although there is some size discrepancy between Mg
and the active atoms, laser materials having concentrations above 0.01 atomic
and high optical quality can be produced. Because of the size discrepancy and
the strong interaction between the active atom and the crystal field, a shift in the
position of some of the spectral features can occur at higher concentrations. At
concentrations of 0.01 atomic. these effects are minimal.
MgF, is a good material from which to make a laser, primarily because of
its relatively high thermal conductivity. MgF, has a wide range of transparency,
extending from the ultraviolet through the midinfrared, about 6.5 pm. As such,
the losses at the laser wavelength can be low. Its wide range of transparency has
led to its use as a window material; consequently, polishing techniques have
been developed for this material. Thermal conductivity is high, approaching that
of YAG. This, coupled with the refractive index properties. allows the use of the
high power densities often associated with laser pumping. The physical proper-
ties oT MgF, are listed on Table 6.
MgF, is a birefringent material with a relatively low refractive index. This
crystal is- uniaxial, producing differences in the refractive index and spectra
depending on the polarization. The refractive index is only about 1.38. whereas
the difference between the ordinary and extraordinary indices of refraction is
about 0.01 1. A refractive index this low makes a conventional single-layer broad-
band antireflection coating impractical. As such, Brewster’s angle laser materials
are often employed where operation over a broad spectral band is desired.
Of the three laser materials. VMgF, has, to date, not been developed
because of relatively inefficient performance. The lifetime of the upper laser
level of this laser material is 2.3 ms at 77 K. However. relatively poor perfor-