A general design method for ultra-long optical path length multipass matrix cells

For the first time, we propose a general design method for ultra-long optical path length (OPL) multipass matrix cells (MMCs) based on multi-cycle mode of two-sided field mirrors. The design idea of the dual circulation mode with two-sided field mirrors is elaborated in detail with the example of MMC based on dual Pickett Bradley White cell (PBWC), and the simple design methods of the other three MMCs based on the dual circulation mode of PBWC and Bernstein Herzberg White cell (BHWC) are given. Further, we propose a general design method for ultra-long OPL MMCs with multi-cycle mode by adding cyclic elements. The OPL of the MMCs designed by this method can reach the order of kilometers or even tens of kilometers. The novel MMCs have the advantages of simple structure, strong spot formation regularity, easy expansion, high mirror utilization ratio, high reuse times of spot spatial position, good stability and extremely high ratio of the optical path length to the volume (RLV). In order to evaluate the performance of the new MMCs, an open-path methane gas sensor with the MMC based on triple PBWC was constructed, which was used to continuously measure the methane in the laboratory, and the feasibility, effectiveness and practicability of the new design method were verified. The design method proposed in this paper provides a new idea for the design of multipass cell (MPC), and the new MMCs designed have great potential application value in the field of high-precision trace gas monitoring.