Vol. 56, Issue 1, pp. 121-131 (2026)

Abstract

The multi-pass cell (MPC) is an effective approach to extend the interaction path between laser radiation and gas molecules, thereby improving the detection sensitivity of laser-based gas sensors. However, an inaccurate determination of the MPC optical path length can introduce significant errors in direct absorption spectroscopy. In this work, we proposed a fast and precise technique—optical frequency domain interferometry (OFDI)—for measuring the MPC length, particularly suitable for configurations where the laser entrance and exit share the same window. Both the conventional absorption-based method and the proposed OFDI method were implemented and compared. Experimental results show that for a 295.4 m-long MPC, the absorption method yielded an uncertainty of approximately 1 m, while the OFDI approach reduced this to about 5 cm. Using the corrected MPC length, a carbon dioxide sensor was developed, achieving a detection sensitivity of 1.2 ppm with an integration time of 60 s. The proposed OFDI method provides a simple and highly effective solution for accurately measuring the optical path length of multi-pass cells and other optical systems.