Optimization of a 2.3 μm Yb:Tm:YLF laser, pumped at 685 nm and 960 nm simultaneously

For the first time, a unique pumping scheme for the thulium 2.3 micrometer transition is proposed that is capable of enhancing the output power by almost a factor three when compared to the traditional pumping scheme at 960 nm. The best relation between 685 nm pump power and 960 nm pump power is shown. The achieved output power of 620 mW is the highest reported so far. Introduction Tunable lasers emitting around 2.3 μm region are important in many application areas. They are especially of interest in gas detection [1,2] systems, because of the presence of strong absorption lines of atmospheric pollutants in the spectral region around 2.3 μm such as CO, CH4 and HF. The 2.3 μm laser is used for sensing carbon monoxide and hydrocarbon gases in combustion experiments and LIDAR applications [3], in part because of its relatively weak water vapor absorption. Lasers in the region 2.0-2.5 μm play also an important role in noninvasive blood glucose measurements [4]. Thulium has a large emission spectra around 2.3 μm with demonstrated tuning range of 2.2-2.45 μm using the YLF host [5]. Tm:YLF has strong absorption lines at 685 and 780 nm that are accessible with diode lasers. The absorption at 685 nm is three times larger than that at 780 nm [6]. With the commercial availability of high power (up to 5 W) diode lasers emitting at 685 nm, this pumping wavelength is of interest because it permits much more effective pump arrangements due to the higher absorption coefficient. Due to a highly concentration dependent cross-relaxation process that leads to a reduction of the 2.3 micron emission from the upper laser level, the thulium concentration should be kept below 2 mol % [5]. For efficient pump absorption, a high concentration sensitizer like ytterbium can be used. Yb can be diode-pumped at 960 nm where high-power diodes are available. In the 960 nm Yb:Tm pumping scheme occur three energy transfer up-conversion processes (ETU) as illustrated in Figure 1. After pump excitation from the F7/2 level to the F5/2 level, the ytterbium transfers its energy to the H5 Tm level, followed by a fast multi-phonon relaxation down to the metastable F4 level. A second ETU process to the F2 energy level of Tm 3+ followed by a rapid relaxation populates the upper laser level (H4). The next ETU causes losses to the system because it transfers population from the upper laser level into the G4 level of Tm. Other processes deplete the upper laser level: the non-radiative transfer via crossrelaxation originating from the H4 and H6 levels to the F4 level and the back-transfer from the H4 thulium level to F5/2 ytterbium level. 0 500