  We demonstrate loss-free generation of 3 mJ, 1 kHz, few-cycle (5 fs at 750 nm
central wavelength) double pulses with separation from 10 fs to 100 fs, using a
helium filled hollow core fiber (HCF) and chirped mirror compressor. Crucial to
our scheme are simulation-based modifications to the spectral phase and
amplitude of the oscillator seed pulse to eliminate the deleterious effects of
self-focusing and nonlinear phase pickup in the chirped pulse amplifier. The
shortest pulse separations are enabled by spectral reshaping and pulse
splitting in the HCF compressor.
