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

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 TEM 00 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. 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. At the same time, parameters of Yb:YLF such as thermal conductivity, thermal expansion coefficient, thermo-optic coefficient (dn/dT) are better than for RT Yb:YAG. On the other hand, the emission cross section of Yb:YLF at 80 K is only around 0.7 × 10 −20 cm 2 for the E//a axis, which is around 3 times lower than RT-Yb:YAG and 14 times lower when compared to cryogenic Yb:YAG. However, the longer fluorescence lifetime, τ=1990 µs [17], partly balances for the reduced gain, resulting in a σ em τ-product of 1.4 x10 −23 cm 2 s in comparison with 2 × 10 −23 cm 2 s and 9.4 × 10 −23 cm 2 s of RT and cryogenic Yb:YAG, respectively. As a result of the low emission cross section, one also suffers from a rather high saturation fluence (14 J/cm 2 ) in Yb:YLF, which creates challenges in optimizing extraction from amplifiers. As an example, for a stretched pulsewidth of 1 ns, the cavity optics has an estimated laser induced damage threshold (LIDT) of around 6.3 J/cm 2 , and long-term damage free operation usually requires operating the amplifier at a lower/safer value of ∼3 J/cm 2 or below. Operating the amplifier at a fluence value much lower than the saturation fluence: (i) reduces the extraction efficiency of the circulating amplified pulse, (ii) increases output energy fluctuations upon undesired perturbations, and (iii) results in circulation of a Gaussian beam profile in the amplifier (which is known to have 2 times higher fluence compared to flattop beams). Note that, despite these disadvantages, if the am...

Power and energy scaling of rod-type cryogenic Yb:YLF regenerative amplifiers

“…The first one, shown in Fig. 2(a), is almost identical to what we have explored in a recent experimental campaign [15]. A state-of-theart laser diode module provides up to 2 kW of pump power at a central wavelength of 960 nm, from a 600 µm core diameter fiber, with an estimated M 2 of 220.…”

“…In our analysis, we first investigated the thermo-mechanical behavior of the Yb:YLF crystals under thermal load, since the temperature of the gain element could significantly influence effective gain. In our earlier work, we saw experimentally that the maximum average pump power that could be handled effectively by a single Yb:YLF rod laser/amplifier (P abs,max ) was roughly around [1,15] P abs,max = 350 W + 1 2…”

“…1). The emission spectrum is also rather smooth/flat, which minimizes gain narrowing upon amplification [15]. In carefully designed systems, amplified pulse widths below 250 fs should become feasible [16].…”

“…In carefully designed systems, amplified pulse widths below 250 fs should become feasible [16]. Furthermore, earlier experimental work indicates that Yb:YLF combines a negative thermo-optic coefficient with positive population and/or photo-elastic-effect-induced lensing, and when combined, this results in a relatively small positive thermal lens [15]. As a result, compared to Yb:YAG, this provides a lower amount of beam distortion per heat load.…”

“…The amplification potential of Yb:YLF has already been explored, but only in a few studies [19][20][21][22][23]. Using regenerative amplifiers, pulses with 30 mJ energy at 20 Hz [19], 20 mJ energy with up to 3.5 kHz [15], and up to 5 mJ energy at 10 kHz [15,20] have been demonstrated using the broadband E //a axis of Yb:YLF. In multipass amplification geometry, pulse energies up to 190 mJ have been reported at 10 Hz [23], and a pulse energy of 10 mJ was demonstrated at 10 kHz [21].…”