Abstract

Access to energetic µJ–multi-mJ ultra-short fs laser pulses is crucial for many research topics today and in the future. Typically, laser amplifier pulses are an order of magnitude longer that the single-cycle limit making pulse compression desirable. We use soliton compression using cascaded quadratic nonlinearities to compress energetic pulses towards few-cycle duration. Specifically, we obtain sub-20 fs solitons by launching 0.25 mJ 55 fs pulses at 1300 nm in a lithium niobate (LN) crystal as short as 1 mm. The solitons are formed by phase-mismatched (cascaded) second-harmonic generation (SHG): upon propagation little second harmonic (SH) is generated, but the fundamental wave (FW) will instead experience a strong Kerr-like nonlinear phase shift. The total FW refractive index is $n_1+n_{\text{cubic}}^I{I}_1$ where $n_{\text{cubic}}^I=n_{\text{SHG}}^I+n_{\text{Kerr}}^I$. It includes contributions from the material Kerr nonlinearity $n_{\text{Kerr}}^I>0$ and the cascaded quadratic nonlinearities $n_{\text{SHG}}^I\simeq -4\pi d_{\text{eff}}^2/c{\epsilon }_0{\lambda }_1{n}_1^2{n}_2\Delta k$. The goal is to achieve a self-defocusing nonlinearity $n_{\text{cubic}}^I<0$ through a large phase mismatch ∆k ≫ 0. In this case, solitons can be formed with normal dispersion, which means anywhere in the visible and near-IR. Moreover it is possible to compress multi-mJ pulse-energies because there are no self-focusing problems [1].

© 2011 Optical Society of America