  Dual-comb spectroscopy allows for the rapid, multiplexed acquisition of
high-resolution spectra without the need for moving parts or low-resolution
dispersive optics. This method of broadband spectroscopy is most often
accomplished via tight phase locking of two mode-locked lasers or via
sophisticated signal processing algorithms, and therefore, long integration
times of phase coherent signals are difficult to achieve. Here we demonstrate
an alternative approach to dual-comb spectroscopy using two phase modulator
combs originating from a single continuous-wave laser capable of > 2 hours of
coherent real-time averaging. The dual combs were generated by driving the
phase modulators with step-recovery diodes where each comb consisted of > 250
teeth with 203 MHz spacing and spanned > 50 GHz region in the near-infrared.
The step-recovery diodes are passive devices that provide low-phase-noise
harmonics for efficient coupling into an enhancement cavity at picowatt optical
powers. With this approach, we demonstrate the sensitivity to simultaneously
monitor ambient levels of CO2, CO, HDO, and H2O in a single spectral region at
a maximum acquisition rate of 150 kHz. Robust, compact, low-cost and widely
tunable dual-comb systems could enable a network of distributed multiplexed
optical sensors.
