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The thin disk (TD) technique has been used in laser oscillators and amplifiers for almost three decades to increase the average power of ultrafast laser systems [1]. Different host materials have been tested mostly with Yb-doping, which resulted in a wide range of pulse durations from 1 ps to sub-100 fs in combination with high average power, good stability and reliability [2]. An even more industrial approach, the thin-slab laser geometry, uses slab-shaped gain media instead of disks, where the cooling is performed from both large surfaces of the crystal [3]. A great advantage of Yb-based TD and thin-slab systems is that the laser crystal can be pumped by continous wave laser diodes due to the long active lifetime of the gain medium. However, the small gain bandwidth supports only sub-ps pulse duration, if high energy pulses are required. Ti:Sapphire (Ti:Sa), on the other hand, offers a gain bandwidth that supports sub-20 fs pulse duration in combination with peak powers up to the petawatt regime without using any post compression techniques [4]. Amplification with the TD geometry has been already demonstrated to be efficient based combined with Ti:Sa, but only in the multi-J regime with a repetition rate of 10 Hz [5]. Scaling the TD geometry to mJ-class pulses leads to serious thermal issues in the gain medium due to the decreased cooled surface to volume ratio. Recently, laser diodes operating around 450 nm were also found to be a good pumping source for Ti:Sa amplifiers with repetition rates in the 100 kHz regime [6], while laser operation in Ti:Sa pumped by a stack of LEDs was also demonstrated and stated to be scalable for high pulse energies [7].
For this reason, thin-slab type amplification is suggested based on Ti:Sa to obtain mJ-class, kW level average power pulses in combination with sub-20 fs pulse duration. Numerical simulations were performed on the amplification and thermal performance for different pulse energies, repetition rates and so average powers. The results revealed the limitations of the thermal performance in the thin-slab geometry. Water-cooled amplifiers were found to be able to provide pulses with 2 kW average power with reasonably low temperature gradient in the crystal. Room temperature cooling enabled 0.3 J output energy and extraction efficiency greater than 50% with a repetition rate exceeding 10 kHz. An increase of one order of magnitude in the repetition rate is possible when cryogenic cooling is considered with 77 K coolant temperature. Laser diode pumping was also investigated in combination with 1 MHz repetition rate of the amplified pulses, where the wall-plug efficiency of the amplifier system is expected to be increased significantly compared to conventional solid-state laser pumping. Experimental demonstration of the thin-slab type Ti:Sa amplification is under work with repetition rates ranging from 2 to 10 kHz in combination with a pump average power of 50 W.
References
1. J. Speiser, J. Opt. Soc. Am. B, 26, 26-35 (2009).
2. F. Druon et al, IEEE Phot. Journ. 3(2), 268-273 (2011).
3. P. Russbueldt et al, IEEE J. of Sel. Top. in Q. El. 21(1), 3100117 (2015).
4. Sergei Kühn et al, J. Phys. B: At. Mol. Opt. Phys. 50 132002 (2017).
5. V. Chvykov et al, Opt. Lett. 41(13), 3017-3020 (2016).
6. S. Backus et al, Opt. Expr. 25, 3666-3674 (2017).
7. P. Pichon et al, Optica 5(10), 1236-1239 (2018).
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