  Dual-comb spectroscopy is a rapidly developing technique enabling
ultraprecise broadband optical diagnostics of atoms and molecules. This
powerful tool typically requires two phase-locked femtosecond lasers, yet it
has been shown that it can be realized without any stabilization if the combs
are generated from a single laser cavity. Still, unavoidable intrinsic relative
phase-fluctuations always set a limit on the precision of any spectroscopic
measurements, hitherto limiting the applicability of bulk dual-comb lasers for
mode-resolved studies. Here, we demonstrate a versatile concept for low-noise
dual-comb generation from a single-cavity femtosecond solid-state laser based
on intrinsic polarization-multiplexing inside an optically anisotropic gain
crystal. Due to intracavity spatial separation of the orthogonally-polarized
beams, two sub-100 fs pulse trains are simultaneously generated from a 1.05
$\mu$m Yb:CNGS oscillator with a repetition rate difference of 4.7 kHz. The
laser exhibits the lowest relative noise ever demonstrated for a bulk dual-comb
source, supporting free-running mode-resolved spectroscopic measurements over a
second. Moreover, the developed dual-comb generation technique can be applied
to any solid-state laser exploiting a birefringent active crystal, paving the
way towards a new class of highly-coherent, single-cavity, dual-comb laser
sources operating in various spectral regions.
