  We investigated the carrier-envelope phase (CEP) stability of a hollow-fiber
setup used for high-energy, few-cycle pulse generation. Saturation of the
output pulse energy is observed at 0.6 mJ for a 260 um inner-diameter, 1 m long
fiber, statically filled with neon, with the pressure adjusted to achieve an
output spectrum capable of supporting sub-4fs pulses. The maximum output pulse
energy can be increased to 0.8mJ by using either differential pumping, or
circularly polarized input pulses. We observe the onset of an
ionization-induced CEP instability, which does not increase beyond an input
pulse energy of 1.25 mJ due to losses in the fiber caused by ionization. There
is no significant difference in the CEP stability with differential pumping
compared to static-fill, demonstrating that gas flow in differentially pumped
fibers does not degrade the CEP stabilization.
