Frequency-divide-and-conquer approach to creating frequency combs in the infrared*

The technique of optical Frequency Combs - a broadband series of some million sharp equidistant phase-locked frequency spikes from mode-locked femtosecond lasers - has revolutionized optical frequency metrology and led to creation of optical atomic clocks. Also, frequency combs opened new capabilities in molecular spectroscopy since they provide a unique combination of large wavelength coverage, high spectral resolution, and the ability of parallel spectral data acquisition. Practical comb sources based on Ti:sapphire, Yb- and Er- fiber lasers, are limited to the near-IR spectral region, $\lambda <$ 2 $\mu $m, while to take the full advantage of frequency combs for molecular spectroscopy, one would need operation at longer ( $>$ 2.5 $\mu $m) wavelengths, that is in the region of fundamental rotational-vibrational absorption bands. Here we implement a new approach for creating broadband mid-infrared frequency combs based on a subharmonic optical parametric oscillator (OPO). The source conveniently down converts the frequency comb of an erbium fiber laser with a center wavelength of 1560nm and produces a 2/3-octaves-wide frequency comb centered at $\lambda $ = 3.1 $\mu $m with 40{\%} quantum efficiency. The frequency comb is phase-locked to the pump laser and has other remarkable coherence properties which we studied by interfering the outputs of two identical OPOs pumped by the same laser.