Mid-IR Optical Frequency Comb Fourier Transform Spectroscopy Using an Antiresonant Hollow-Core Fiber

Optical frequency combs (OFCs) enable high resolution, sensitivity, and speed in spectroscopic measurements. An efficient way of generating an optical frequency comb in the mid-infrared (mid-IR) is the difference frequency generation (DFG) process, which involves the interaction of two input waves in a non-linear crystal, resulting in the generation of a third wave with a frequency that is the difference in frequency of the two input beams [1]. The classical way of extending the path of light-gas interaction in spectroscopic measurements, thus increasing the sensitivity, is to use multi-pass cells (MPC). However, MPCs have disadvantages related to the difficulty of aligning laser light into the cell or optical fringes. An alternative approach is to use the so-called antiresonant hollow-core fibres (ARHCF) [2]. ARHCFs are characterized by a wide low-loss transmission range in the mid-IR, high quality of the delivered beam, and their air core can be filled with the target gas sample, which makes them well suited for laser-based gas sensing.