Study of Nonlinearities in Terahertz-Repetition-Rate mJ-CPA

We report on self-phase modulation (SPM) effects when generating multiple picosecond-spaced ultrashort pulses with chirped-pulse amplification (CPA) and high, hundreds of picoseconds, chirped pulse durations. For ultrashort pulses, a picosecond pulse spacing <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\Delta \mathrm{t}$</tex> is comparable to the sub-picosecond pulse duration <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\tau_{\mathrm{c}}$</tex>. In this case, the chirped waveform consists of only a few strong interference peaks whose spacing scales with the inverse pulse spacing <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$1/\Delta \mathrm{t}$</tex>. This leads to substantial Self-Phase Modulation (SPM) around the intensity maxima [1], leading to the generation of pre- and post-pulses in the recompressed waveform, as reported for petawatt-scale systems suffering from weak parasitic post-pulses generated prior amplification [2].