Page 298 - Tunable Lasers Handbook

258 Norman P. Barnes

Since the Cr:BeA1204 laser does not operate like a three-level laser, the
thresholds can be modest at room temperature. Modest thresholds for this device
are associated with the relatively low effective stimulated emission cross section.
Threshold will. of course, depend on the reflectivity of the output mirror and the
losses. Using relatively high reflectivity mirrors, in excess of 0.8. normal mode
thresholds are on the order of 20 J. While output mirror reflectivities this high
are satisfactory for normal mode operation. they can lead to high-peak-power
densities within the laser resonator for Q-switched operation. Thresholds can be
decreased by operating the laser at elevated temperatures where the effective
stimulated emission cross section is higher.
Slope efficiencies of Cr:BeAl,O, laser can be relatively high, primarily due
to the efficient absorption of the flashlamp radiation. Slope efficiencies for nor-
mal mode operation can be on the order of 0.02. Slope efficiencies with Q-
switched operation are usually lower due to the loss associated with the insertion
of the Q-switch into the resonator and the less than unity storage efficiency. Stor-
age efficiency in this case is the fraction of Cr atoms pumped to the upper laser
manifold, which remains in the upper laser manifold at the time of the opening
of the Q-switch. Since the pump pulse is a fair fraction of the upper laser level
lifetime, some of the energy stored in the upper laser manifold decays during the
pump pulse. Losses associated with the insertion of the Q-switch are especially
significant for low-pain lasers. Because of the relatively low gain, components
selected for spectral or spatial mode control must be selected carefully in order
to minimize loss.
Even though the gain of Cr:BeAl,04 is relatively low, this material can be
made into an amplifier. A small-signalgain of about 4.5 has been achieved [20].
However, to achieve this gain, the operating temperature of the laser rod was
maintained at about 270 K and the laser rod was pumped very hard. about 1.9
MJ/m3. In this case the pump level refers to the electrical energy supplied to the
flashlamps divided by the volume of the laser rod. To achieve this pump level,
380 J/pulse was supplied to each of two flashlamps. Higher amplifier efficiency
can be achieved by using multiple passes through the amplifier. However. this
raises the optical energy density on the laser material.
Continuous wave oscillation of Cr:BeAl,O, has been achieved around the
peak gain wavelength of this laser material [26]. As in the case of Cr:A1,0,,
mercury-arc lamps were employed. Threshold was high, somewhat over 2006 W,
but the slope efficiency was also reasonably high. about 0.01. In this case, the
laser could be tuned from less than 0.74 pm to beyond 0.78 pm.

6. Ti:AI2O3

Ti:A1,0, is a laser material. tunable over much of the near infrared, which
has both a high gain and freedom from excited state absorption. Because Ti has