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. Effective optical path length (EOPL) is an important index to evaluate the ability of increasing optical path length for an integrating cavity. The EOPL has been studied to be related to inner surface reflectivity, cavity shape and dimension, port fraction. For different spectroscopic techniques, whether the EOPL variation tendency is universally valid is investigated in this work. The EOPLs of a cubic integrating cavity in different port fractions have been measured using time-resolved spectroscopy and tunable diode laser absorption spectroscopy (TDLAS) respectively. With the increasing of the port fraction, the EOPL reduces gradually. However, measured EOPL results shows a deviation between the two technique methods. By analyzing, the deviation is caused by a reflectivity difference of the inner coating at different laser wavelength for the two spectral methods. By introduction of the reflectivity correction, the deviation could be eliminated. This demonstrates that EOPL extending law of integrating cavity was universally valid in different spectroscopy application.