Abstract: Multipass optical absorption cell is often used in the field of absorption spectroscopy measurements, and is one of the most effective means to enhance the detection performance of the system. Based on the structural principle of Herriott cell, a high-temperature multi-path optical absorption cell suitable for the measurement of molecular absorption spectra of trace gases in the middle infrared band was designed in this work. The design has the characteristics of high temperature resistance, long optical path and easy adjustment of optical path. The temperature of the multipass optical absorption cell is up to 300 ℃, which reduces the influence of the high adsorbability of some gases in the environments of normal or low temperatures. The effective optical path of the cell is 14.28 m, which improves the detection sensitivity of the system. The optical absorption cell also avoids the pollution of the mirror surface caused by the contact between the gas to be measured and the mirror surface, and can be disassembled without the need to readjust. To verify the feasibility of the design, a mid-infrared quantum cascade laser (QCL) with a central wavelength of 7.16 μm was combined with the Herriott-type high-temperature multipass optical absorption cell. The second harmonic (2f) signals of SO2 and SO3 gas in the multipass optical absorption cell were measured using wavelength modulation spectroscopy and the volume fractions of SO2 and SO3 were calibrated. The SO2 volume fraction data were measured continuously for 1000 s, and the theoretical minimum detection limit of SO2 of the system was found to be 0.04 cm3/m3 through Allan variance analysis. Compared to the high temperature SO2 detection system used before, the detection sensitivity of the newly designed system has increased by 49 times. In addtion, the results show that the design of high temperature multi-path cell not only meets the requirements of SO3 gas measurement environment, but also improves the detection sensitivity of the system. This work provides an effective method for concentration measurement of trace gas molecules in the middle infrared band at high temperatures.

Key words: mid-infrared absorption spectrum, high-temperature multipass optical absorption cell, sulfur trioxide;sulfur dioxide, wavelength modulation