Re-evaluating the V-Parameter: Insights into spatio-spectral homogeneity in nonlinear post-compression

Nonlinear post-compression has become an indispensable tool to meet today's challenging applications demands, requiring high peak intensities and ultrashort pulse durations down to a few optical cycles [1]. While it's commonly agreed upon that a uniform distribution of nonlinearity and such spectral broadening is necessary for a high-quality pulse compression, it's not obvious that nonlinear interactions with Gaussian-like beams exhibiting non-uniform intensity distributions are able to fulfill this condition. For waveguiding structures such as fibers and multipass cells some level of homogenization along the spatial dimensions are observed effectively equalizing the spectral distribution [2,3]. Though it is not evident how this homogenization effects behave for different interaction geometries and levels of nonlinearities. To quantify the homogeneity of the spatio-spectral distribution of arising in nonlinear post-compression schemes the V-Parameter introduced in [4] was widely adopted as figure of merit in the ultrafast community [5-6]. While the V-parameter is experimentally easily accessible we believe it is flawed on multiple levels for its intended application.