Page 300 - Tunable Lasers Handbook

260 Norman P. Barnes

The experimental data of lifetime as a function of temperature can be well repre-
sented by using the Struck and Fonger theory as shown in Fig. 18. Ratioing the
lifetime at room temperature to the lifetime at cryogenic temperatures yields an
estimate of the quantum efficiency of 0.83. A short upper laser level lifetime
complicates flashlamp pumping of this material. Consequently. the majority of
the systems developed to date use laser pumping.
Polarized emission spectra of Ti:Al,O, display a single broad emission band
for both polarizations. As expected. the K polarization displays considerably more
intensity than the o polarization, approximately in the ratio of 3:l [28]. Again
using the Struck and Fonger theory, the emission spectra follows the expected
lineshape [30]. A curve fit of the 'II polarized experimental data to the expected
fluorescent spectrum. as shown in Fig. 19, yields a good fit with the zero phonon
line at about 15968 cm-1. Using the McCumber theory to predict the gain indi-
cates that gain exists well beyond 1 .O pm. Peak stimulated emission cross section
occurs at 0.795 pm and is about 4.3 x 10-23 m2. A large effective stimulated emis-
sion cross section makes Ti:A120, an extremely useful laser material.
Although excited state absorption is negligible, initially Ti:Al,O, suffered
from absorption losses at the lasing wavelengths. Experimental evidence indi-
cated this absorption was caused by quadruply ionized Ti [3 I]. Ti substitutes for
the Al, which is triply ionized. However, Ti has a predilection for the quadruply
ionized state. Consequently, some of the Ti in A1,03 was found in this state. To
overcome the loss associated with the quadruply ionized Ti, different growth
techniques were tried and postgrowth annealing was implemented. Both of these
techniques resulted in substantial decreases in the loss.
Loss at the lasing wavelength in Ti:Al,O, was characterized by a figure of
merit that related the loss to the Ti concentration. More than one figure of merit
has been proposed, but the figure of merit used here will be defined as the
absorption coefficient at the peak of the pump absorption, about 0.49 ym, to the
absorption coefficient at the peak of the gain, about 0.80 ym. Experimental evi-
dence indicated that the absorption at the lasing wavelength increased quadrati-
cally with the Ti concentration [31]. A log-log plot of the absorption coefficient
at the lasing wavelength versus the absorption coefficient at the pump wave-
length showed a linear dependence with a slope of 2.0. A quadratic dependence
was explained on the basis of Ti pair formation, one triply ionized and the other
quadruply ionized. As much as 0.03 of the Ti was found to occur in the quadru-
ply ionized state. Early samples of Ti:A1703 had figures of merit of about 5.
However with improvements in growth and annealing. Ti:Al20, with figures of
merit well in excess of 100 are now available.
Using laser pumping, arbitrarily low thresholds can be obtained. For pulsed
operation, the most commonly used pump laser is the kquency-doubled NdYAG
laser. Absorption coefficients on the order of 100 m-1 are common at 0.532 pm. To
obtain efficient absorption of the pump radiation. longitudinal pumping is often
employed. Because the beam quality of the pump can be relatively good, the pump