Q-Switched Ho:YAG Laser Produces 50-mJ Pulses

Breck Hitz

Eye-safe pulsed lasers operating in the 2-µm spectral region are useful in ranging, remote sensing and laser radar applications. Moreover, if they have high peak powers, their outputs can be efficiently converted in optical parametric oscillators into the 3- to 5-µm and the 8- to 12-µm regions that are important for many remote sensing applications. A team of scientists at BAE Systems North America in Nashua, N.H., has developed a resonantly pumped Ho:YAG laser that produces more than 50 mJ at 2.09 µm in a 14-ns, Q-switched pulse.

The laser is a "quasi two-level system" because it is optically pumped directly to the ⁵I₇ manifold, which serves as the upper laser level, and because the lower laser level is very close to the ground level. Thus, there essentially are only two levels: the upper and the "quasi" single level comprising the lower and the ground levels. (The lower is close enough to the ground level to cause significant thermal population but distinct enough to ensure that the pumping radiation and laser radiation are at different wavelengths.)

Researchers at BAE Systems North America configured their end-pumped Ho:YAG laser as a folded resonator. The laser produced 14-ns pulses of 2.09-µm radiation with energies of approximately 50 mJ.

The Ho:YAG laser was end-pumped by a diode-pumped Tm:YLF laser at 1.9 µm (see figure). Holmium laser rods often are co-doped with thulium as a sensitizing agent because it readily absorbs the pump radiation of diode lasers and transfers the energy to the holmium. There are disadvantages of the co-doping scheme, however, when it is scaled to higher pump powers. Up-conversion losses cause harmful thermal loading of the laser rod, and energy transfer from the holmium atoms to the thulium depletes the population inversion and adds to the thermal loading.

To avoid these problems, the scientists put the thulium atoms in a separate laser and transferred their energy to the holmium laser by directly optically pumping the holmium's upper laser level. They coupled the thulium laser's 60-Hz pulsed radiation into the holmium's cavity with a thin-film polarizer that was highly transmissive to the 1.9-µm radiation in the p-polarization and highly reflective to the 2.09-µm radiation in the s-polarization.

The relatively high quality of the thulium laser beam (M² ~ 3) ensured that its beam diameter was constant over the 35-mm length of the Ho:YAG rod, even when the pump radiation was reflected by the back mirror for a second pass through the rod. The rod was doped with 0.5 percent holmium and mounted on a thermoelectrically cooled copper finger. A Brewster-cut, acousto-optic Q-switch (represented in the figure by its rectangular housing), driven with 100 W of peak RF power at 27 MHz, Q-switched the laser at 60 Hz.

The Ho:YAG laser produced slightly more than 50 mJ of near-TEM₀₀ (M² ~ 1.2) output at 2.09 µm from 269 mJ of pump power from the Tm:YLF laser. The Q-switched pulse duration was 14 ns, indicating a peak power in excess of 3.5 MW.

However, because the physical mode match between the intracavity 2.09-µm laser light and the 1.9-µm pump light was poor -- the overlap was approximately 48 percent -- the researchers speculate that up to twice the output energy could be achieved by improving the pumping geometry.