High power (>500W) cryogenically cooled Yb:YLF cw-oscillator operating at 995 nm and 1019 nm using E//c axis for lasing

Abstract

We present record continuous wave (cw) output power levels from cryogenically cooled Yb:YLiF4 (Yb:YLF) lasers in rod geometry. The laser system is pumped by a state-of-the-art 960 nm diode module, and vertically polarized lasing was employed using the E//c axis of Yb:YLF. Lasing performance was investigated at different output coupling levels in different cavity configurations and the laser crystal temperature was estimated via monitoring the emission spectrum of the gain media. We have obtained a cw output power up to 400 W at a wavelength of 995 nm. The absorbed pump power was around 720 W, and the laser output had a TEM00 beam profile with an M2 of 1.3 in both axes. At higher absorbed pump power levels with increasing laser crystal temperature, we observed a lasing wavelength shift from 995 nm to 1019 nm. In this regime cw output power levels above 500 W have been achieved at an absorbed pump power of 750 W. Further power scaling was limited by the onset of strong thermal lensing. We discuss underlying physical mechanisms for the wavelength shift and present detailed temperature measurements under lasing conditions.

Highly efficient cryogenic Yb:YLF regenerative amplifier with 250 W average power

“…A short cavity with better immunity to thermal lensing was required to achieve higher powers from Yb:YAG rod (365 W cw power at 480 W of absorbed power). Hence, the cw output power obtainable with cryogenic Yb:YLF from a single rod [6], is already significantly higher than what can be achieved with cryogenic Yb:YAG systems under similar conditions. First of all, as we also observed in cw laser experiments, for low absorbed pump power and hence low crystal temperatures, the gain at 995 nm is very strong [8], and the system started parasitic cavity-dumped lasing at 995 nm with sub-10-ns pulse width even when seeded with 30 nJ pulses at 1019 nm.…”

“…Benefits of cryogenic cooling were already discussed in this initial work [1], and lasing at cryogenic temperatures was shown only a year later [2]. Over the past 20 years, output power of cryogenic (Yb:LiYF4) laser systems evolved from 100 mW to 500 W level [2][3][4][5][6][7]. Interestingly, as it is underlined by Kawanaka et al in their pioneering work [1], the Yb:YLF gain media possess broad emission bands even at cryogenic temperatures [8][9][10].…”

“…For absorbed pump power levels above 500 W, the thermally induced lensing in the crystal gradually deteriorated the output beam profile as shown in Fig. 2 (b) [6]. Once the absorbed pump reached 750 W level, the thermal lens become rather dynamic and it was not possible to optimize the laser with further re-alignment (a reduction in output power is also observed).…”

“…2 (a) [20]. The free running cw laser wavelength was 995 nm for an absorbed pump just below 300 W. Then the wavelength switched to the 1019 nm line for pump powers above 300 W [6]. The change in lasing wavelength with absorbed power is due to the induced variation in crystal temperature: at higher temperatures due to Boltzmann redistribution of level populations, the gain cross section at 1019 nm line becomes larger than that of 995 nm transition (see Figs.…”

Error analysis of contactless optical temperature probing methods for cryogenic Yb:YAG

“…The variation of pump absorption at 1030 nm with temperature was employed in estimating Yb:YAG thin-disk temperatures in the 40-300 K range by Petrov et al [22]. We have recently investigated different spectral methods for temperature estimation of cryogenic Yb:YLF samples and demonstrated a temperature estimation accuracy below ± 1 K in the 78-300 K region [23], and used this for analyzing laser performance [24,25]. Similar methods had already been in use for temperature estimation of Yb:YLF and Yb:YAG by the cryogenic optical refrigeration community [26][27][28][29][30].…”