  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.
