Page 299 - Tunable Lasers Handbook

6 Transition Metal Solid-state Lasers 259

only one d electron, to first-order approximation. there are only two levels. In a
strict sense, a two-level laser would not lase since a population inversion could
not be achieved. However. because the Ti:AI,O, laser operates on a vibronic
transition. it can operate much like a four-level laser. In addition. an extremely
wide tuning range is available with this material. Full width at half-maximum
(FWHM) spectral bandwidth is roughly one-third of the peak emission ~ave-
length. Such a wide spectral bandwidth gives this laser material one of the
widest tuning ranges of any laser.
Many of the desirable properties of Ti:A1,0, result from the single electron
in the 3d level. Since there is only one electron available. the mutual repulsion
effects are zero. Consequently. only the spin orbit and crystal field effects
remain. Crystal field effects split the degenerate levels associated with the cen-
tral field approximation into two levels. With only two levels, deleterious excited
state absorption is negligible. Crystal field effects are the same as those associ-
ated with the Cr:A1,03, namely. a strong octahedral field with a tr-iginol distor-
tion. Since there is only a single 3d electron, rather than the three 3d electrons
associated with Cr, the levels are labeled differently. Neglecting the triginol dis-
tortion, the triply degenerate ground level is labeled as T, and the doubly degen-
erate esciied state is labeled as 'E. Triginol distortioil further splits the T,
grmnd level into two relatively closely spaced levels and the spin orbif interac-
tion further splits the lower of these into two levels [27],
Ti:A1,0; is produced by replacing a small fraction of the A1 atoms with Ti
atoms. Concentrations of Ti are relatively low, often less than 0.0015 by weight.
Although higher concentrations are possible, considerations such as optical qual-
ity tend to limit the Ti concentration. Ti sees the same symmetry as the A1 atoms
in A1,O;.
Since only a small fraction of the A1 is replaced with either Cr for Cr:AI,03
or Ti for Ti:AI,O,, the optical and mechanical properties of Ti:AI,O, are very
nearly the same as Cr:AI,O,. However, addition of Ti tends to produce a harder
crystal than undoped 41,O,.
Ti:Al,03 displays polarization-dependent absorption bands that peak about
0.49 pm, Absorption of radiation polarized along the optic axis, x polarized, is
more than twice as strong as absorption of radiation polarized perpendicular to
the optic axis, o polarized [28]. For both polarizations, a shoulder in the absorp-
tion spectra appears at about 0.51 ym. Because absorption occurs from wave-
lengths shorter than 0.45 pan to wavelengths longer than 0.60 pm, a wide range
of pump wavelengths is possible. However. even with relatively lorn Ti concen-
trations, peak absorption coefficients on the order of 200 m-1 are common.
Quantum efficiency can be deduced from the measurement of the lifetime of
the upper laser level as a function of temperature. At cryogenic temperatures, the
upper laser level lifetime has been measured to be 3.87 ys [29]. Lifetime is nom-
inally independent of temperature to about 200 K. At room temperature the life-
time is nominally 3.2 ps and it decreases rapidly as the temperature decreases.