Intracavity Pumping of Ho:YAG Laser Boosts Efficiency
Both thulium and holmium ions make good infrared lasers when doped into solid-state hosts, thulium lasing at ~1.9 µm and holmium at ~2.1 µm. Thulium has absorption bands corresponding to the wavelengths of commercial diode lasers at 792 nm, but holmium lacks absorption bands at any common diode-laser wavelengths. Holmium does, however, absorb at the ~1.9-µm wavelength of the Tm laser, so one approach to an efficient Ho laser is to pump it with a Tm laser. Recently, researchers at the French-German Research Institute of Saint-Louis in France demonstrated an efficient Tm-pumped Ho:YAG laser, in which the Ho:YAG crystal was inside the Tm-laser resonator.
The Ho:YAG laser is efficiently pumped because it is inside the Tm:YLF resonator, where the approximately 1.9-µm pump power is much higher than it would be outside.
The advantage of placing the Ho:YAG crystal inside the Tm-laser resonator is that the 1.9-µm power is typically an order of magnitude greater there than in the Tm laser's output beam. A high reflector is substituted for the Tm laser's output mirror, and the absorption in the Ho:YAG acts as the output coupler for the Tm laser. The situation is analogous to intracavity frequency doubling, in which the nonlinear crystal is placed inside the resonator.
The Tm laser is a particularly attractive pump laser because a cross-relaxation process allows it to create a pair of 1.9-µm laser photons for each 792-nm pump photon. The researchers designed their two-in-one laser with an L-shaped resonator for the Tm:YLF laser and a straight resonator for the Ho:YAG laser (see figure). All three mirrors of the Tm:YLF resonator -- the folding mirror and the two end mirrors -- were maximum reflectors at the 1.9-µm Tm:YLF wavelength. The Tm:YLF crystal was pumped from both ends with a pair of 792-nm diode lasers to a maximum pump power of slightly more than 15 W.
The natural anisotropy of YLF forces it to lase in one polarization only, and the researchers oriented the Tm:YLF rod so that it lased in the s-polarization. The dichroic folding mirror transmitted more than 99.9 percent of the p-polarized 2.1-µm wavelength of the Ho:YAG laser, but only half of the s-polarization, thereby forcing the Ho:YAG laser to oscillate only in the p-polarization. The Ho:YAG output mirror at the top of the diagram transmitted 10 percent at 2.1 µm.
Intracavity loss at a laser's wavelength can force the device to shift to a different wavelength where the loss is lower. A Tm:YLF laser with the same laser rod and a configuration very similar to that in the diagram -- but without the Ho:YAG rod -- produced 4 W of output at 1.908 µm. But with the Ho:YAG rod in the resonator, the wavelength of the Tm:YLF laser shifted to 1.953 µm, where the Ho absorption was less than at 1.908 µm. Nonetheless, the absorption was enough for efficient Ho:YAG oscillation.
By measuring the output through one of the maximum reflectors, the researchers calculated that the intracavity circulating power of the Tm:YLF laser was approximately 70 W and that the power absorbed in the Ho:YAG was approximately 3.5 W. The Ho:YAG output at 2.09 µm was 1.6 W when the diodes pumped the Tm:YLF rod at 15.4 W.
They observed an unusual and irregular pulsing behavior of their laser in the 10-kHz range, but the period between the pulses was not constant. The duration of the Tm:YLF pulses was several microseconds, and the duration of the Ho:YAG pulses was considerably shorter, roughly 200 ns. The scientists attributed the pulsing to the Ho:YAG acting as a saturable absorber in the Tm:YLF resonator, Q-switching it on an irregular basis. The Q-switched Tm:YLF laser then gain-switches the Ho:YAG laser, resulting in the observed behavior.
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