  Directly accessing the middle infrared, the molecular functional group
spectral region, via supercontinuum generation processes based on turn-key
fiber lasers offers the undeniable advantage of simplicity and robustness.
Recently, the assessment of the coherence of the mid-IR dispersive wave in
silicon nitride waveguides, pumped at telecom wavelength, established an
important first step towards mid-IR frequency comb generation based on such
compact systems. Yet, the spectral reach and efficiency still fall short for
practical implementation. Here, we experimentally demonstrate for the first
time to our knowledge, that fs-fiber laser driven systems based on large-cross
section silicon nitride waveguides can reach, with powers sufficient to drive
dual-comb spectroscopy, the important greenhouse gases spectral region near 4
micron, typically accessed through different frequency generation or more
complex approaches. We show, from a 2 micron femtosecond fiber laser, up to 30%
power conversion and milliwatt-level output powers, proving that such sources
are suitable candidate for compact, chip-integrated spectroscopic and sensing
applications.
