  Mid-infrared spectroscopy, an important and widespread technique for sensing
molecules, has encountered barriers stemming from sources either limited in
tuning range or excessively bulky for practical field use. We present a
compact, efficient, and broadly tunable optical parametric oscillator (OPO)
device surmounting these challenges. Leveraging a dispersion-engineered
singly-resonant OPO implemented in thin-film lithium niobate-on-sapphire, we
achieve broad and controlled tuning over an octave, from 1.5 to 3.3 microns by
combining laser and temperature tuning. The device generates > 25 mW of
mid-infrared light at 3.2 microns, offering a power conversion efficiency of
15% (45% quantum efficiency). We demonstrate the tuning and performance of the
device by successfully measuring the spectra of methane and ammonia, verifying
our approach's relevance for gas sensing. Our device signifies an important
advance in nonlinear photonics miniaturization and brings practical field
applications of high-speed and broadband mid-infrared spectroscopy closer to
reality.
