Abstract
We report, what is to our knowledge, the highest average power obtained directly from a Yb:YLF regenerative amplifier to date. A fiber front-end provided seed pulses with an energy of 10 nJ and stretched pulsewidth of around 1 ns. The bow-tie type Yb:YLF ring amplifier was pulse pumped by a kW power 960 nm fiber coupled diode-module. By employing a pump spot diameter of 2.1 mm, we could generate 20-mJ pulses at repetition rates between 1 Hz and 3.5 kHz, 10 mJ pulses at 5 kHz, 6.5 mJ pulses at 7.5 kHz and 5 mJ pulses at 10 kHz. The highest average power (70 W) was obtained at 3.5 kHz operation, at an absorbed pump power level of 460 W, corresponding to a conversion efficiency of 15.2%. Despite operating in the unsaturated regime, usage of a very stable seed source limited the power fluctuations below 2% rms in a 5 minute time interval. The output pulses were centered around 1018.6 nm with a FWHM bandwidth of 2.1 nm, and could be compressed to below 1-ps pulse duration. The output beam maintained a TEM00 beam profile at all power levels, and possesses a beam quality factor better than 1.05 in both axis. 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.
Highlights
Yb:YAG gain media possesses a room-temperature (RT) fluorescence lifetime (τ) of 940 μs, and an emission cross section of around 2.15 x10−20 cm2, resulting in a relatively large σemτ-product of 2 × 10−23 cm2s [1]
The crystal was indium bonded from the top side to a cold head, which was cooled to cryogenic temperatures by liquid nitrogen
Two thin-film polarizer’s (TFPs), a half-wave plate (HWP) and a Pockell cell (PC) with a rise time of 6 ns were used for seeding of the amplifier (TFPs and HWP were antireflection coated at 1030 nm, whereas the PC AR coating was optimized for 1020 nm)
Summary
Yb:YAG gain media possesses a room-temperature (RT) fluorescence lifetime (τ) of 940 μs, and an emission cross section (σem) of around 2.15 x10−20 cm, resulting in a relatively large σemτ-product of 2 × 10−23 cm2s [1]. It is thermo-mechanically quite strong, enabling different laser gain geometries such as crystalline fibers [2], or thin disks [3,4]. Gain-narrowing effects are hard to compensate, and the achievable pulsewidth upon amplification is usually limited around 1 ps at RT, and a few picoseconds at cryogenic temperatures
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