Delivering light through a long fiber to perform mid-infrared (IR) spectroscopy offers a great potential for remote sensing in medical applications and in hazardous or inaccessible environments. Recently developed silica hollow-core fibers (HCFs) provide low-loss waveguiding, as well as low nonlinearity and dispersion, thus emerging as excellent candidates for this kind of spectroscopy. Besides, compared to conventional mid-IR fibers with solid glass cores, the length scale of HCFs can be increased by one order of magnitude to tens of meters.

In the paper by Castro-Marin et al., fiber-delivered spectroscopy is performed by introducing a HCF in one arm of an unbalanced Michelson interferometer. The operation of this scheme relies on the use of a mid-IR optical frequency comb, which produces long sequences of optical pulses with high mutual coherence. This allows the pulses of the local oscillator arm to coherently gate the reflected pulses coming from a sample located at the distal end of the HCF. A mirror scanning results in the generation of interferograms, from which the spectral information of the sample (and also of the delivery fiber) can be retrieved through Fourier analysis, similar to the coherent heterodyne detection employed in dual-comb spectroscopy.

Compared to previous schemes, the above approach benefits from a set of advantages, such as the suppression of interfering features along the delivery path and the localization of all the detection devices at the proximal end of the fiber. The authors validate their system by performing reflectance spectroscopy of a plastic sample through a fiber of 40 m, showing that their novel scheme holds the promise of enabling multi-species gas monitoring in remote environments.

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