Generalized optical design of two-spherical-mirror multi-pass cells with dense multi-circle spot patterns

We report a set of practical multi-circle spot patterns produced by two-spherical-mirror multi-pass cells (MPCs). Such a set of intricate spot patterns takes full account of the evolution and deformation of spot shapes caused by aberrations on two spherical mirror surfaces by means of a multi-ray calculation model, thus avoiding spot overlaps and reducing an etalon effect. An eight- and nine-multi-circle spot pattern was demonstrated experimentally in order to verify the validity of the calculated results. Furthermore, a 2f spectrum measurement of ambient methane was performed using the eight-multi-circle spot pattern MPC to verify the practicability. An approach to search for multi-circle spot patterns in a two-spherical-mirror MPC is discussed in detail. A set of dense spot patterns results in sensitive, low-cost, compact trace gas sensors based on MPCs, which can be used to implement a large-scale deployment of distributed sensor networks for monitoring pollutants or to realize handheld mobile sensor devices for safety inspection, leakage detection, and medical diagnostics. Published under license by AIP Publishing.