Methane detection system based on a new type of dense optical multi-pass cell

Abstract

ObjectiveTo increase the optical path of gas absorption in a multi-pass cell (MPC), a dense spot pattern has been obtained on MPC to improve the accuracy of trace gas detection. In this study, an algorithm was proposed to calculate the spot patterns of a new type of dense optical MPC. MethodIn this novel algorithm, the reflected ray equation and spherical equation were combined to obtain the position coordinates of light spots on a mirror surface. The calculation was repeated by changing the reflected optical path parameters until a new dense light spot pattern was obtained. To verify the accuracy of the proposed algorithm, a series of spot numerical simulations and methane (CH4) concentration detection experiments were conducted. ResultsResults showed that in an MPC with a double spherical mirror, a series of dense spot patterns located on the mirror surface was obtained by calculating off-axis incident rays using the new algorithm, effectively increasing optical path length, and obtaining an MPC with a long optical path of 12.92 m. CH4 concentration was detected by combining the long optical path MPC with a tunable diode laser absorption spectroscopy. A good linear relationship was established between the calibrated CH4 concentration and the peak of the second harmonic signal detected by the system. The minimum detection limit of the system for CH4 was 2.33 ppb at an integration time of 20 s obtained via Allan deviation analysis. The long optical path MPC system achieved high detection sensitivity. Hence, a new method for calculating the dense spot pattern of an MPC is provided in this work.