Effective optical path length measurement of integrating cavity using time-resolved spectroscopy and tunable diode laser absorption spectroscopy

Abstract

Integrating cavities are commonly used in trace gas detection and weak absorption measurement. The effective path length (L) is an important index for evaluating the ability to increase the optical path length of an integrating cavity. Studies have demonstrated that the effective path length (L) is related to the inner surface reflectivity, cavity shape and dimension, and port fraction. However, the measured effective path length (L) of an integrating cavity generally varies with the spectroscopic technique used in practical applications. In this study, the effective path lengths (L) of a cubic integrating cavity with different port fractions were measured using time-resolved spectroscopy and tunable diode laser absorption spectroscopy (TDLAS). The value of L gradually decreased with an increase in the port fraction. Further, the measured L results showed a deviation between the two measurement techniques. The reason for the different effective optical paths obtained by the two spectroscopic techniques was investigated. An analysis showed that the difference in the effective optical paths was due to the reflectivity difference at the different laser wavelengths used for the two spectral methods. Correcting the reflectivity eliminates the difference in the effective optical paths.