摘要： Numerous practical applications, including health and environmental protection, need compact and high sensitivity gas sensors. This work reports an experimental investigation on a compact multipass cell (MPC) designed and optimized with a genetic algorithm (GA) with long, adjustable optical path length (OPL) using only inexpensive spherical mirrors. A four-mirror based MPC with the GA developed here, offers great design flexibility in comparison to a two-mirror solution. As an example, a stable 24 m OPL was reached within only 80 cc volume. Moreover, by changing the mirror positions various stable OPLs can be achieved in a controllable way. The presented MPC consists of four mirrors in a configuration similar to a bow-tie. As a result, the symmetry between the mirrors is broken (mirrors are not parallel to each other) and an astigmatic spot pattern with a high fill factor is obtained. Additionally, the use of the folded optical path geometry causes higher compactness in contrast to an astigmatic mirror-based MPC. Compared to other dense-pattern or folded spherical mirrors MPCs reported previously, longer OPL in the same volume can be obtained. In order to accurately calculate the line-sphere intersection points and reflection angles based on algorithms reported in custom ray tracing software was developed. It also allows to determine the optimal MPC configuration with specified design constraints (mirror diameters, their focal lengths and desirable OPL) by using a GA. To verify the simulation results, we assembled a MPC with four 1” in diameter, 25 mm focal length mirrors mounted in kinematic holders and fixed them to an aluminum base. By changing the angle and distance between the mirrors, different MPC configurations were tested. Several OPLs ranging from 4.5 m to 28 m were achieved with the GA. The longest OPL, with sufficient output beam quality for such mirrors, was 24 m. In order to prove the agreement between the simulation and experiment, a 16 m and 24 m OPL configurations were prepared and the time-of-flight inside the MPC was measured by injecting a 10 ns pulse laser into the cavity. The first pulse (registered at 0 ns delay in Fig. 1c) corresponds to light partially reflected from the optical plate situated near the MPC input, whereas the second one arrives from the output of MPC. By measuring the time delay between both pulses, the actual OPL was calculated. The experimentally obtained OPLs were 16.11 m and 23.88 m, which is in good agreement with the simulated values of 16.1 m and 23.82 m respectively. In conclusion, we present a compact, four-mirror MPC, designed and optimized with a GA in several OPL variants. Then two of them were experimentally verified through time-of-flight measurement inside the MPC.