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ACOPS Report Summary

Project ID: 617173
Funded under: FP7-IDEAS-ERC
Country: Germany

Mid-Term Report Summary - ACOPS (Advanced Coherent Ultrafast Laser Pulse Stacking)

We demonstrated the ‘stack-and-dump’ (SND) scheme in a 30-m long EC, corresponding to a length increase of a factor of 10 over the state of the art. Towards tapping the full potential of ECs as stacking devices for ultrashort pulses, this constitutes a crucial design criterion relaxing the thermal stress in the switch and in the cavity optics and allowing for longer times between successive pulses. The EC supported a steady-state power enhancement factor exceeding 200 when seeded with a 10-MHz repetition-rate train of 3-µJ pulses. The cavity enabled the enhancement of strongly stretched pulses (~1.5 ns). A systematic investigation of different dumping rates was performed with an intracavity acousto-optic modulator (AOM). This way, pulses with the accumulated energy of up to 65 input pulses, i.e. 0.2 mJ, were extracted at 30 kHz. These pulses were recompressed to the initial duration of 800 fs, demonstrating the feasibility of SND with strongly stretched pulses and energies surpassing previous results by three orders of magnitude. These results, even if not stating new laser parameter records on their own, constitute the first milestone towards a power-scalable device and, thus, are a necessary step towards the first stack-and-dump system providing truly unprecedented laser parameters.
S. Breitkopf, T. Eidam, A. Klenke, L. von Grafenstein, H. Carstens, S. Holzberger, E. Fill, T. Schreiber, F. Krausz, A. Tünnermann, I. Pupeza, and J. Limpert, "A concept for multiterawatt fibre lasers based on coherent pulse stacking in passive cavities," Light Sci. Appl. 3, 1–7 (2014).
S. Breitkopf, S. Wunderlich, T. Eidam, E. Shestaev, S. Holzberger, T. Gottschall, H. Carstens, A. Tünnermann, I. Pupeza, and J. Limpert, "Extraction of enhanced, ultrashort laser pulses from a passive 10-MHz stack-and-dump cavity”, submitted (2016).
Possible limitations caused by phase fluctuations or thermal issues have been investigated and suitable solutions were found.
O. de Vries, T. Saule, M. Plötner, F. Lücking, T. Eidam, A. Hoffmann, A. Klenke, S. Hädrich, J. Limpert, S. Holzberger, T. Schreiber, R. Eberhardt, I. Pupeza, and A. Tünnermann, "Acousto-optic pulse picking scheme with carrier-frequency-to-pulse-repetition-rate synchronization," Opt. Express 23, 19586 (2015).
S. Holzberger, N. Lilienfein, M. Trubetskov, H. Carstens, F. Lücking, V. Pervak, F. Krausz, and I. Pupeza, "Enhancement cavities for zero-offset-frequency pulse trains," Opt. Lett. 40, 2165–2168 (2015).
N. Lilienfein, H. Carstens, S. Holzberger, C. Jocher, T. Eidam, J. Limpert, A. Tünnermann, A. Apolonski, F. Krausz, and I. Pupeza, "Balancing of thermal lenses in enhancement cavities with transmissive elements.," Opt. Lett. 40, 843–6 (2015).
Additionally, the fibre development necessary, in order to scale the available input power to achieve the project-goals was successfully driven forward. The reason for the onset of mode instabilities is the thermal heat load in the amplifier. Two effects can lead to heat development in a doped fiber. Firstly: the quantum defect and secondly photo-darkening. We found out, that the impact of photodarkening is equally large and can therefore not be neglected as it was done before. Hence, using longer, less doped fibres is a possibility to reduce the heat load per meter and allow for much higher output powers.
C. Jauregui, H.-J. Otto, S. Breitkopf, J. Limpert, and A. Tünnermann, "Optimizing high-power Yb-doped fiber amplifier systems in the presence of transverse mode instabilities," Opt. Express 24, 7879–7892 (2016).
In the next step, fibres with this improved design will be employed as main-amplifier to be able to seed the cavity with higher average power and therefor increase the extracted pulse energy to the multi-mJ range as proposed in the Grant Agreement.

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