29 Mar 2019

Understanding pulse behavior for ultimate control of amplification

ELI-ALPS researchers studied the wavelength dependent changes of refractive index of Ti:Sapphire crystals caused by population inversion for both π- and σ-polarized pulses. In a recent article in Optics Express, Roland Nagymihaly and his colleagues report their findings, which suggest that for achieving extremely short pulse durations below ten femtoseconds, the inversion induced phase shifts along the two polarization directions need to be taken into account.

‘At the High Field Laser Group of ELI-ALPS, we develop technologies to achieve high peak and average powers with ultrashort laser pulses,’ explains first author of the publication Roland Nagymihaly. ‘The ultimate aim is to build a sub-10 femtosecond laser with a peak power of 50 terawatts. To be able to achieve these unprecedented specifications, we need to develop special kinds of amplifiers. And in order to do that, we first need to understand what happens inside the amplifying crystal.’

In the paper, the researchers describe experiments on refractive index changes caused by population inversion. The spectral phase shift induced by inversion for both π- (parallel to the optical axis) and σ- (perpendicular to the optical axis) polarized pulses was extracted at different pump fluence values. At room temperature, a zero-phase shift was found with a sign change at the peak of the gain spectrum, while for σ-polarized pulses no such behavior was observed in the investigated spectral range. By decreasing the temperature of the crystal to 30 K, similar behavior was found, however, the zero-phase crossing was found to be shifted to around 760 nm.

‘It was very striking to see how different the crystal operates for the two different cross-sections. The experimental results were compared with numerical calculations based on the Kramers-Kronig theory, which resulted in a good agreement.’

Combining two techniques
The findings are specifically important for the Polarization-Encoded Chirped Pulse Amplification (PE-CPA) technology recently developed by the ELI-ALPS research group. This amplification technique holds promise to support a gain bandwidth sufficient for compressed pulses in the few-cycle regime with the Ti:Sa gain medium. Since this technique utilizes both gain cross-sections of the Ti:Sa crystal, the performance of this technique is affected by the phase fluctuations along the two axes.

‘We want to combine the PE-CPA technique with the large aperture thin disk method, which is currently under development within our group as well. Our expectation is that this combination will enable the high repetition rates of up to about 100 Hz we are aiming for, while the high spectral bandwidth is also kept to obtain few-cycle pulses.’

At the moment, the group is planning the next steps. ‘We are preparing in house experiments specifically aimed at polarization encoded amplification to reach energetic pulses with a duration down to 10 femtoseconds. Furthermore, we are also preparing for thin disk experiments with a special laser head we developed for high average power operation. At ELI-ALPS, we are truly pushing the limits of laser physics, and that’s what makes our work so fascinating.’

Roland s. Nagymihaly et. al, Broadband spectral characterization of the phase shift induced by population inversion in Ti:Sapphire, Optics Express Vol. 27, Issue 2, pp. 1226-1235 (2019) https://doi.org/10.1364/OE.27.001226