Dual-Comb Spectroscopy using a Single Frequency-Shifting Loop

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Dual-comb spectroscopy (DCS) is a multi-heterodyne interferometrie technique that employs two optical frequency combs (OFCs) with different line spacing. When DCS is carried out with a pair of mode-locked OfCs, the need of a tight phase locking between them leads to sophisticated hardware and/or software implementations [1]. A strategy to simplify the DCS architecture, demonstrated with several comb modalities, is the use of a bidirectional laser cavity to produce two counter-propagating combs, each one having different line spacing [2,3]. Alternatively, but offering much more flexibility on the mode separation, electro-optic (EO) frequency combs can also be used for carrying out DCS. In that DCS scheme, both EO combs are fed by a single cw laser, which ensures by default a high degree of mutual coherence between them [4]. In the same vein, but comparatively much less exploited, frequency-shifting loops (FSLs) are also an easily implementable comb platform. Based on the recirculation of light in a loop containing an acousto-optic frequency shifter (AOFS), FSLs are capable of producing spectra with hundreds or even thousands of lines, without resorting to non-linear broadening or optimized fast driving electronics [5]. Recently, we have demonstrated DCS using two different FSLs [6]. However, this system requires the use of a low-bandwidth stabilization system to compensate for the uncorrelated fluctuations between the two combs, a requirement also found in other DCS schemes, and necessary for increasing the acquisition time (and, hence, the measurement signal-to-noise ratio) beyond the ms time scale.