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Ultrashort pulses with TW to multiple PW peak power are mostly generated by state-of-the-art Ti:Sapphire- (Ti:Sa) based chirped pulse amplification (CPA) systems [1,2]. The main reasons of using Ti:Sa crystals are the extremely large gain bandwidth and exceptional physical properties of the gain medium [3], the availability of the high energy pulsed pumping sources. Additionally, carrier-envelope phase (CEP) stabilization schemes can be easily implemented for Ti:Sa laser systems up to TW peak power [4]. Polarization-encoded CPA (PE-CPA) is a recently developed technique, which holds promise to support a gain bandwidth sufficient for compressed pulses in the few-cycle regime with Ti:Sa-based amplification [5,6]. The performance of the PE-CPA technique is however affected by the spectral amplitude and phase variations along both gain cross-sections.
The population inversion induced refractive index changes (RICs) were measured within the main emission band of Ti:Sa. By using spectrally resolved interferometry (SRI), spectral phase changes of amplified pulses were measured in a Jamin-type arrangement. The spectral phase shift induced by inversion for both π- and σ-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. The electronic RICs are explained by the polarizability difference between excited and unexcited Ti3+ ions in the crystal. Compression of PE-amplified pulses is currently under investigation by numerical simulations and will be experimentally tested with a spectral bandwidth corresponding to sub-10 fs pulse duration.
Carrier-envelope phase (CEP) fluctuations were also investigated in a PE amplifier stage by using the SRI technique. Temperature and inversion instabilities were found to be the major sources of CEP noise caused by the amplification process, accounting for 60 mrad of CEP noise in a four-pass amplifier stage with 1.2 J/cm2 absorbed pump fluence. CEP stability of the PE amplification was compared to the conventional process, which showed a sub-10 mrad noise increase in case of the PE amplifier under similar operating conditions. However, the degradation of the CEP stability in the PE amplifier compared to the conventional stage was found to be below 10 mrad within the same operational conditions [7].
References
1. Z. Gan, L. Yu, S. Li, C. Wang, X. Liang, Y. Liu, W. Li, Z. Guo, Z. Fan, X. Yuan, L. Xu, Z. Liu, Y. Xu, J. Lu, H. Lu, D. Yin, Y. Leng, R. Li, and Z. Xu, Opt. Express 25(5), 5169-5178 (2017).
2. A. Golinelli, X. Chen, E. Gontier, B. Bussière, O. Tcherbakoff, M. Natile, P. d’Oliveira, P.-M. Paul, and J.-F. Hergott, Opt. Letters 42(12), 2326-2329 (2017).
3. P. F. Moulton, “Spectroscopic and laser caracteristics of Ti:Al2O3,” J. Opt. Soc. Am. B 3(1), 125-133 (1986).
4. F. Lücking, V. Crozatier, N. Forget, A. Assion, and F. Krausz, Opt. Lett. 39(13), 3884–3887 (2014).
5. M. Kalashnikov, H. Cao, K. Osvay, and V. Chvykov, Opt. Letters 41(1), 25-28 (2016).
6. H. Cao, M. Kalashnikov, K. Osvay, N. Khodakovskiy, R. S. Nagymihaly, and V. Chvykov, Laser Phys. Lett. 15 045003 (2018).
7. R. S. Nagymihaly, H. Cao, P. Jojart, M. Kalashnikov, A. Borzsonyi, V. Chvykov, R. Flender, M. Kovacs, and K. Osvay, J. Opt. Soc. Am. B 35(4), A1-A5 (2018).
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