  High-order harmonic generation (HHG) arising from the non-perturbative
interaction of intense light fields with matter constitutes a well-established
tabletop source of coherent extreme-ultraviolet and soft X-ray radiation, which
is typically emitted as attosecond pulse trains. However, ultrafast
applications increasingly demand isolated attosecond pulses (IAPs), which offer
great promise for advancing precision control of electron dynamics. Yet, the
direct generation of IAPs typically requires the synthesis of near-single-cycle
intense driving fields, which is technologically challenging. In this work, we
theoretically demonstrate a novel scheme for the straightforward and compact
generation of IAPs from multi-cycle infrared drivers using hollow capillary
fibers (HCFs). Starting from a standard, intense multi-cycle infrared pulse, a
light transient is generated by extreme soliton self-compression in a HCF with
decreasing pressure, and is subsequently used to drive HHG in a gas target.
Owing to the sub-cycle confinement of the HHG process, high-contrast IAPs are
continuously emitted almost independently of the carrier-envelope phase (CEP)
of the optimally self-compressed drivers. This results in a CEP-robust scheme
which is also stable under macroscopic propagation of the high harmonics in a
gas target. Our results open the way to a new generation of integrated
all-fiber IAP sources, overcoming the efficiency limitations of usual gating
techniques for multi-cycle drivers.
