  Coherent laser beams in the 3 to 20 {\mu}m region of the spectrum are most
applicable for chemical sensing by addressing the strongest vibrational
absorption resonances of the media. Broadband frequency combs in this spectral
range are of special interest since they can be used as a powerful tool for
molecular spectroscopy offering dramatic gains in speed, sensitivity,
precision, and massive parallelism of data collection. Here we show that a
frequency comb realized through subharmonic generation in an optical parametric
oscillator (OPO) based on orientation-patterned gallium phosphide (OP-GaP)
pumped by a Kerr-lens mode-locked 2.35-{\mu}m laser can reach a continuous
wavelength span of 3-12 {\mu}m, thus covering most of the molecular 'signature'
region. The key to achieving such a broad spectrum is to use a low-dispersion
cavity entailing all gold-coated mirrors, minimally dispersive and optically
thin intracavity elements, and a specially designed pump injector. The system
features a smooth ultra-broadband spectral output that is phase coherent to the
pump laser comb, 245-mW output power with high (>20%) optical conversion
efficiency, and a possibility to reach close to unity conversion from a
mode-locked drive, thanks to the non-dissipative downconversion processes and
photon recycling.
