  Lasers capable of generating attosecond X-ray pulses in the water window (282
to 533 eV) through high-order harmonic generation are normally based on
inefficient, multi-stage optical parametric amplifiers or optical parametric
chirped pulse amplifiers pumped by femtosecond or picosecond lasers. Here we
report a very efficient single amplification stage laser based on traditional
chirped pulse amplification capable of producing 3 mJ, near-transform limited
44 fs (<6 cycles), 1 kHz pulses centered at 2.5 micrometer. The 68 GW peak
power is the highest value ever reached at this wavelength. In order to fully
compress the laser pulses our system is built in a nitrogen box. Our system
utilizes water cooled chromium doped zinc selenide (Cr2+:ZnSe) as the gain
medium and is pumped by a commercial nanosecond holmium doped
yttrium-aluminum-garnet (Ho:YAG) laser.
