A simple method for pulse contrast enhancement via self-focusing

Here we report on a simple-to-implement and cost-effective approach for laser pulse contrast enhancement, based on the nonlinear self-focusing effect. An intentionally induced and gently controlled self-focusing in a thin glass transforms the time-dependent intensity into variation in beam divergence. Followed by a spatial discriminating filter, only the strongly focused fraction traverses the setup, at the expense of efficiency. A numerical model, accounting for the pulse and material parameters via a Gaussian ABCD matrix, provides an estimate for the instantaneous beam waist and transmission efficiency, which enables us to evaluate the resulting contrast enhancement. The estimated contrast enhancement spans between 0.5 and 2.5 orders of magnitude, in conjunction with approximately 25%-90% estimated efficiency, depending on the pulse parameters. In a preliminary experiment we demonstrated the effect with 10s-J sub GW regime with approximately 40 efficiency and a contrast improvement of more than or equal to 20 dB.