Contrast enhancement by nonlinear ellipse rotation in gas-filled multipass cells

The temporal contrast of ultrashort pulses has been a major concern in applications as the light preceding the main pulse can significantly affect interaction properties. Consequently, nonlinear temporal filtering techniques such as cross-polarized phase generation [1], nonlinear ellipse rotation (NER) [2,3] or plasma mirrors [4] are often applied in high peak power laser systems. NER is an efficient post-compression technique to generate high contrast ultrashort pulses as it allows the simultaneous temporal cleaning and nonlinear spectral broadening. It also exhibits the highest conversion efficiency and best spatial beam characteristics. So far, this phenomenon has been used mostly in hollow core fibers. Recently, post-compression of ultrashort pulses in multipass cells are widely and efficiently used as they provide high transmission and excellent output beam quality. Cavity based setups could be also suitable for temporal cleaning by exploiting NER [5]. On the other hand, the possibilities for temporal cleaning in multipass cells have not been investigated in detail. Here, we report on the contrast enhancement of ultrashort pulses in gas-filled multipass cells by combining the post-compression technique with NER. The nonlinear spectral broadening and temporal cleaning were examined with the help of a 3-dimensional numerical model. Nonlinear propagation of pulses at mJ energy level with 800 nm or 1030 nm central wavelengths were simulated. We propose possible arrangements with optimized parameters for both cases. The results suggest that the proposed scheme is suitable to obtain a compression factor higher than five with 50% efficiency, meanwhile 10^3 contrast enhancement can be achieved. By customizing the cavity geometry in a way that the accumulation of B integral is slow and continuous during the propagation, the beam quality is preserved at the output and no space-time coupling could be observed. Pulses were also tested at 20 mJ energy, in that range where capillary-based setups are difficult to implement, and the results indicate that the technique allows the generation of high contrast, few-cycle pulses even with TW peak power. [1] Jullien A, Albert O, Burgy F, Hamoniaux G, Rousseau J P, Chambaret J P, Auge-Rochereau F, Cheriaux G, Etchepare J, Minkovski N and Saltiel S M, 2005 Opt. Lett. 30 920–922 [2] Khodakovskiy NG, Kalashnikov MP, Pajer V, Blumenstein A, Simon P, Toktamis MM, Lozano M, Mercier B, Cheng Z, Nagy T and Lopez-Martens R, 2019 Laser Phys Lett. 16 095001 [3] Mijesh NS, Hang XZ, Ischer PF, Uschet AAM, Salh RS, Tajalli A, Orgner UM and Veisz L, 2019 Opt Lett. 44 4028-4031. [4] Kapteyn H C, Murnane M M, Szoke A and Falcone R W, 1991 Opt. Lett. 16 490–492 [5] Kalashnikov M P, Risse E, Schonnagel H, Husakou A, Herrmann J and Sandner W, 2004 Opt. Express 12 5088–5097