One of the approaches to generate strong fields with femtosecond optical pulses is to use enhancement in a cavity . With intracavity-enhanced fields, one can ionize gases for high harmonic experiments in the MHz repetition rate regime . Here we present an optical setup where strong fields are produced by a nonlinear optical down-conversion within an OPO cavity. The cavity is doubly resonant for signal and idler (doubly resonant optical parametric oscillator, DROPO). Our DROPO is pumped by a home-built Yb:YAG Kerr-lens mode-locked thin-disk laser oscillator with up to 35 W of average power, 32.5 MHz repetition rate and 270 fs pulse duration; A Brewster cut BBO crystal is used as an intracavity nonlinear medium to drive the DROPO.
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