  We report a 21-W mid-IR OPCPA that generates 131-{\mu}J and 97 fs
(sub-9-cycle) pulses at 160 kHz repetition rate and at a centre wavelength of
3.25 {\mu}m. Pulse-to-pulse stability of the CEP-stable output is excellent
with 0.33% rms over 288 million pulses (30 min) and compression close to a
single optical cycle was achieved through soliton self- compression inside a
gas-filled mid-IR anti-resonant-guiding photonic crystal fibre. Without any
additional compression device, stable generation of 14.5 fs
(1.35-optical-cycle) pulses was achieved at an average power of 9.6 W. The
resulting peak power of 3.9 GW in combination with the near-single-cycle
duration and intrinsic CEP stability, make our OPCPA a key-enabling technology
for the next generation of extreme photonics, strong-field attosecond research
and coherent X-ray science.
