  Mid-infrared frequency comb spectroscopy enables measurement of molecular at
megahertz spectral resolution, sub-hertz frequency accuracy and microsecond
acquisition speed. However, the widespread adoption of this technique has been
hindered by the complexity and alignment sensitivity of mid-infrared frequency
comb sources. Leveraging the underexplored mid-infrared window of silica fibers
presents a promising approach to address these challenges. In this study, we
present the first experimental demonstration and quantitative numerical
description of mid-infrared frequency comb generation in silica fibers. Our
all-silica-fiber frequency comb spans over two octaves (0.8 $\mu$m to 3.5
$\mu$m) with a power output of 100 mW in the mid-infrared region. The amplified
quantum noise is suppressed using four-cycle (25 fs) driving pulses, with the
carrier-envelope offset frequency exhibiting a signal-to-noise ratio of 40 dB
and a free-running bandwidth of 90 kHz. Our developed model provides
quantitative guidelines for mid-infrared frequency comb generation in silica
fibers, enabling all-fiber frequency comb spectroscopy in diverse fields such
as organic synthesis, pharmacokinetics processes, and environmental monitoring.
