Q-switched and diode-pumped 2 μm solid state lasers are becoming of increasing interest for efficient pumping of mid-infrared emitting optical parametric oscillators (OPOs). In particular, Thulium and Holmium rare earths seem to be most suited for systems with high efficiency due to their long upper state lifetime. In addition, the Ho:YAG emission around 2.1 μm is not in the strong absorption spectral band of water and it is a suitable wavelength for non linear crystal pumping, such as ZnGeP2. Several works on Ho:YAG laser end-pumped by diode-pumped Tm:YLF laser have demonstrated high power operations1. To simplify the set-ups, experiments with Tm-Ho intracavity lasers have been done; they demonstrated a 36.5% slope efficiency2. Unfortunately these set-ups3 did not allow Q-switched operations and the thermal lens in the rods led to relatively poor beam quality (M2~5-6).
We designed an original intracavity configuration with a dichroic polarizing beamsplitter to decouple Tm:YLF and Ho:YAG cavities. This solution was to improve the beam quality and allow Q-switched operations. 1.9 W of 2.09 μm at the 17.3 W diodes pump level were obtained. The slope efficiency of the diode-pump to the Ho laser output and the optical-to-optical conversion efficiency achieved were respectively ~ 21.4 % and ~ 11 %. As anticipated 4, a shift of Tm:YLF emission was experimentally scaled from 1.908 to 1.953 μm which led to an efficiency decrease for the Tm laser. Unfortunately, in this intracavity geometry, Ho:YAG acted as a saturable absorber. Instead of a cw operation in free running, random Tm:YLF laser pulses of ~ 2.5 μs were observed that each resulted in an Ho pulse (~ 200-250 ns). When the acousto-optic modulator (AOM) worked, the Ho pulses did not follow the Q-switched frequency. In fact Ho emission depends on the Tm:YLF pump energy accumulated between two gates of the AOM.
Some possible ways to optimize the efficiency and to avoid the passive Q-switching behaviour of Ho:YAG are tested and first results are presented in this paper.