Nearly transform-limited sub-20-fs pulses at 1065 nm and >10 nJ enabled by a flat field ultrafast pulse shaper.

Using a low-nonlinearity fiber and pulses from a nonlinear fiber amplifier seeded by a modelocked Yb-doped fiber oscillator, we generate 19-fs pulses centered at 1065 nm with 11.5 nJ of pulse energy (700 mW average power). The short (<15  cm) 10-μm core, polarization maintaining fiber minimizes deleterious nonlinear effects and eliminates fiber damage, while still producing pulse bandwidths well beyond the Yb gain bandwidth limit. A flat-field pulse shaper, utilizing a Plössl lens, compresses the pulse to within 92% of the transform-limited peak power. The total power transmission efficiency is as high as 67%, including fiber coupling losses and pulse shaper transmission, due to the novel pulse shaper design allowing the incorporation of a high-efficiency transmission grating.