High power diode bar pumped Tm:YAG laser and intracavity pumped Ho:YAG laser

Tm3+ and Ho3+ doped lasers operating in the 'eyesafe' 2µm region have recently attracted growing interest owing to their numerous applications, particularly in LIDAR. For many LIDAR applications the higher atmospheric transmission around 2.1µm favours the Ho3+ laser over the Tm3+ laser at 2.0µm, whereas the latter offers a more convenient wavelength for diode-pumping. The standard approach to overcoming this problem is to use laser crystals with Tm3+, Ho3+ co-doping, the Tm3+ being diode-pumped at 785nm followed by energy transfer to the Ho3+ ions upper laser level. This process works efficiently in cw-lasers, but upconversion effects substantially shorten the storage lifetime in Q-switched lasers. This problem can be avoided using an intracavity pumping scheme with Tm3+ and Ho3+ ions separated into two different rods in the same cavity. The Tm3+ material can be directly diode-pumped at 785nm, while the Ho3+ material is pumped by the 2µm laser emission of the Tm3+. The weak absorption of the Ho3+ (typically a few percent) acts as the 'output coupler' for the Tm3+ laser. Recently such an intra-cavity pumped laser has been reported, which had an output of 120mW with the Tm:YAG being pumped by a Ti:Sapphire laser. Here we report a diode-pumped system with multi-watt output. Our approach makes use of the two-mirror beam-shaping technique recently reported, to reconfigure the output beam from a high-power diode bar so as to allow intense end-pumping with an essentially circular beam. Using this pump source we have demonstrated efficient room temperature operation of a Tm:YAG laser with an output power of 4W for 13.5W of incident pump. The set-up for the intracavity- pumped Ho:YAG laser is shown in Fig.1 and is similar to that of the Tm:YAG laser. The pump input mirror has high reflectivity from 2.0 to 2.1µm, and output coupler is highly reflective from 2.00 to 2.02µm with 10% transmission around 2.1µm. For 9.2W of diode power incident on the Tm:YAG rod the maximum output was 2.1W at 2097nm corresponding to a slope efficiency of 28%. The Tm:YAG laser itself operated at 2012nm. The threshold for Tm:YAG lasing was 1.5W, while the Ho:YAG started lasing at the slightly higher diode power of 1.7W. Appropriate modifications to the resonator design and optimum choice of dopant concentrations should bring significant further improvements in performance