Efficient, diode-pumped, high-power (>300W) cryogenic Yb:YLF laser with broad-tunability (995-10205 nm): investigation of E//a-axis for lasing

Abstract

We present, what is to our knowledge, the first detailed lasing investigation of cryogenic Yb:YLF gain media in the E//a-axis. Compared to the usually employed E//c-axis, the a-axis of Yb:YLF provides a much broader and smooth gain profile, but this comes at the expense of reduced gain product. We have shown that, despite the lower gain, which (i) increases susceptibility to cavity losses, (ii) raises lasing threshold, and (iii) inflates thermal load, efficient and high-power lasing could be achieved in the E//a axis as well. A record continuous-wave (cw) powers above 300 W, cw slope efficiencies of 73%, and a tuning range covering the 995-1020.5 nm region were demonstrated. In quasi-cw lasing experiments, via minimization of thermal effects, slope efficiencies can be scaled up to 85%. In gain-switched operation, sub-50-µs long pulses with a peak power exceeding 2.5 kW at multi-kHz repetition rate were attained. We measured a beam quality factor below 1.5 for laser average powers up to 100 W and below 3 for laser average powers up to 300 W. Power scaling limits due to thermal effects, laser dynamics in pulsed pumping, and multicolor lasing operation potential were also investigated. The detailed results presented in this manuscript will pave the way towards development of high-power and high-energy Yb:YLF oscillators and amplifiers with sub-500-fs pulse duration.

20-mJ, sub-ps pulses at up to 70 W average power from a cryogenic Yb:YLF regenerative amplifier

“…Figure 2(a) shows the continuous-wave lasing results at 20% effective output coupling, where the results of the regenerative laser cavity are compared with a simple short-cavity setup. The short cavity results have been described in detail in [1], where a flat-flat cavity with a total length of 20 cm was used. Here the regenerative laser cavity results are taken with the cavity shown in Fig.…”

“…Yb:YAG gain media possesses a room-temperature (RT) fluorescence lifetime (τ) of 940 µs, and an emission cross section (σ em ) of around 2.15 x10 −20 cm 2 , resulting in a relatively large σ em τ-product of 2 × 10 −23 cm 2 s [1]. Moreover, it is thermo-mechanically quite strong, enabling different laser gain geometries such as crystalline fibers [2], or thin disks [3,4].…”

“…Unfortunately, the main drawback of Yb:YAG media is its narrow and steep gain profile, for details see Fig. 1 in [1]. The gain bandwidth of Yb:YAG at room temperature (RT) has a full-width-half-maximum (FWHM) of 8 nm, which narrows down to a 1.5 nm at cryogenic temperatures.…”

“…Under pumping conditions (∼350 W absorbed power), and an estimated thermal lens of around + 10 m, the beam size shrinks to 2.08 mm and 1.92 mm, at the center of the Yb:YLF crystal and PC, respectively. We refer the readers to the experimental section of [1] for a more detailed discussion on properties of the pump diode used in this study.…”

“…The relatively narrow bandwidth of the current seed source and the moderate gain available from the single Yb:YLF crystal was the main limiting factor in this initial study. a gain bandwidth of 10 nm for the E//a axis at cryogenic temperatures [1]. Moreover, unlike Yb:YAG, the emission profile is rather smooth, which minimizes gain-narrowing effects, that could potentially enable amplification of sub-250 fs long pulses.…”