Ultrasensitive detection of methane, isotopic carbon dioxide, carbon monoxide, formaldehyde, acetylene, and ethylene is performed in the spectral range 2.5-5 μm using intracavity spectroscopy in broadband optical parametric oscillators (OPOs). The OPOs were operated near degeneracy and synchronously pumped either by a mode-locked erbium (1560 nm) or thulium (2050 nm) fiber laser. A large instantaneous bandwidth of up to 800 cm~(-1) allows for simultaneous detection of several gases. We observe an effective path-length enhancement due to coherent interaction inside the OPO cavity and achieve part-per-billion sensitivity levels. The measured spectral shapes are in good agreement with a model that takes into account group delay dispersion across the broad OPO frequency band.
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