  Single-cycle pulses with deterministic carrier-envelope phase enable the
study and control of light-matter interactions at the sub-cycle timescale, as
well as the efficient generation of low-noise multi-octave frequency combs.
However, current single-cycle light sources are difficult to implement and
operate, hindering their application and accessibility in a wider range of
research. In this paper, we present a single-cycle 100 MHz frequency comb in a
compact, turn-key, and reliable all-silica-fiber format. This is achieved by
amplifying 2 $\mu$m seed pulses in heavily-doped Tm:fiber, followed by cascaded
self-compression to yield 6.8 fs pulses with 215 kW peak power and 374 mW
average power. The corresponding spectrum covers more than two octaves, from
below 700 nm up to 3500 nm. Driven by this single-cycle pump, supercontinuum
with 180 mW of integrated power and a smooth spectral amplitude between 2100
and 2700 nm is generated directly in silica fibers. To broaden
applications,few-cycle pulses extending from 6 $\mu$m to beyond 22 $\mu$m with
long-term stable carrier-envelope phase are created using intra-pulse
difference frequency, and electro-optic sampling yields comb-tooth-resolved
spectra. Our work demonstrates the first all-fiber configuration that generates
single-cycle pulses, and provides a practical source to study nonlinear optics
on the same timescale.
