Spectral Broadening in Concave-Convex Multipass Cells in Gas and Solid Medias

The demand for optical technologies to deliver shorter pulses and higher peak powers is ever growing. Spectral broadening in a multipass cell (MPC) in combination with a highly dispersive mirror compressor is a recent technology that can provide such pulses. Typical MPCs have a concave-concave geometry (cav-cav), which faces three design limitations: excessive self-focusing, laser-induced mirror damage and gas ionisation [1]. A different MPC-geometry comprising a concave and convex cell mirror (cav-vex) only has a virtual focus outside the MPC, thus, it mitigates the gas ionisation. Results with the cav-vex MPC geometry using ambient air and solid medium as the nonlinear medium were published [2], [3]. We extend on this concept and show in this work that spectral broadening can be achieved within a solid-state material as well as within a pressurised gas environment. This allows for finer control of the process since the medium's position within the cell or the gas pressure can be tuned and paves the way towards further energy scaling of the concept.