Spectral broadening of 4 mJ femtosecond pulses using a bulk multi-pass cell

Sammanfattning: Nonlinear optical effects can be used for spectral broadening, which the basis of the field of post-compression of laser pulses. When the intensity of light becomes high enough the refractive index becomes intensity dependent, and as the intensity often varies both with position and time so does the refractive index. The consequence of the position dependence is self focusing, an often unfavorable nonlinear effect, while the time dependence is what enables spectral broadening via self phase modulation (SPM). In this work a novel approach building on nonlinear Multi-Pass Cells (MPC),with the aim of broadening the spectrum of high energy pulses (4mJ) with a short pulse duration (25fs), is designed, implemented and characterized, both spatially and temporally. The obtained spectral broadening is from 50nm to 80nm, and the output beam is astigmatic. In parallel a numerical tool is implemented to study how self focusing affects the spatial beam profile in an MPC. The tool is based on the split step Fourier method for solving the time independent nonlinear Schr¨odinger equation. To further be able to study realistic beams a tool to apply aberrations via Zerenike polynomials is adapted. Results of the simulations show the importance of nonlinear mode matching for high power beams.