Modulated ringdown comb interferometry for sensing of highly complex gases.

Gas samples relevant to health and the environment typically contain many molecular species that span a huge concentration dynamic range. Mid-infrared frequency comb spectroscopy with high-finesse cavity enhancement has allowed the most sensitive multispecies trace-gas detections so far. However, the robust performance of this technique depends critically on ensuring absorption-path-length enhancement over a broad spectral coverage, which is severely limited by comb-cavity frequency mismatch if strongly absorbing compounds are present.

Experimental confirmation of phase profile of Hermite-Gauss beams.

Phase distribution of Hermite-Gauss (HG) beams generated by a gas laser is investigated experimentally by studying their interference with a plane wave and diffraction by a single slit by selecting pairs of bright lobes with different phases. Experimentally recorded interference and diffraction profiles support HG mode phase profiles expounded on in this paper. We find that the phase difference between one bright lobe and another is not simply zero or but increases (or decreases) uniformly in steps of as the number of zeros between them increases, in agreement with analytic function theory.