  Frequency combs, spectra of evenly-spaced narrow phase-coherent laser lines,
have revolutionized precision measurements. On-chip frequency comb generators
hold much promise for fully-integrated instruments of time and frequency
metrology. While outstanding developments are being reported with Kerr, quantum
cascade and microring electro-optic combs, the field of high-resolution
multiplexed gas-phase spectroscopy has remained inaccessible to such devices,
because of their large line spacing, their small number of usable comb lines,
the intensity variations between their comb lines, and their limited photonic
integrability. Here we identify a path to broadband gas-phase spectroscopy on a
chip. We design a low-noise III-V-on-silicon comb generator on a photonic chip,
that emits a flat-top spectrum of 1400 lines at a repetition frequency of 1.0
GHz, a feature never approached by other ultra-miniaturized comb synthesizers.
With dual-comb spectroscopy, our near-infrared electrically-pumped laser
records high-resolution (1 GHz) sensitive multiplexed spectra with resolved
comb lines, in times as short as 5 microseconds. Isotope-resolved 12C/13C
detection in carbon monoxide is performed within 1 millisecond. With further
developments, entire high-resolution spectroscopy laboratories-on-a-chip may be
manufactured at the wafer scale. In environmental sensing, broad networks of
spectrometers could be densely field-deployed to simultaneously monitor, in
real time, sources and sinks of greenhouse gases, industrial pollution or
noxious emissions of motor vehicles.
