Integrating cavity based gas cells: a multibeam compensation scheme for pathlength variation

Abstract

We present a four beam ratiometric setup for an integrating sphere based gas cell, which can correct for changes in pathlength due to sphere wall contamination. This allows for the gas absorption coefficient to be determined continuously without needing to recalibrate the setup. We demonstrate the technique experimentally, measuring methane gas at 1651nm. For example, contamination covering 1.2% of the sphere wall resulted in an uncompensated error in gas absorption coefficient of ≈41%. With the ratiometric scheme, this error was reduced to ≈2%. Potential limitations of the technique, due to subsequent deviations from mathematical assumptions are discussed, including severe sphere window contamination.

Investigating the Relation between Absorption and Gas Concentration in Gas Detection Using a Diffuse Integrating Cavity

“…According to Equation 14, the OP is proportional to the optical path length, once the oxygen concentration is constant. Thus, because the oxygen concentration is stable in air, the EOPL of the diffuse cavity was calibrated by comparing the oxygen absorption signal in the cavity and in the air [14][15][16]. Based on this method, each optical path length was measured times, and the error bars are shown in Figure 4.…”

“…Thus, the EOPL should be independent of the detected gas concentration. The experimental methods used to measure the EOPL and its related parameters, such as ρ, L ave , and f have been established and implemented [15,16]. However, detected absorption signals usually show a nonlinear relation with gas concentration at high gas concentration values [17,18] in applications.…”

Ratiometric pathlength calibration of integrating sphere-based absorption cells

“…The four-beam method has been adapted to pathlength calibration of integrating spheres. Details of the technique and underlying theory are provided in [19] and summarised below, as well as in Table 1. Two sources (S1 and S2) are established at ports on the sphere that are at right angles.…”

“…A ratio Q is formed of these four measurements in the absence of absorption [19]; this is termed Q(0). For gas spectroscopy, absorption lines are narrow and therefore this measurement can be made at a neighbouring wavelength where there is no gas absorption.…”

“…A second approach, previously published by the authors of this paper [19] and summarised for comparison in Table 1, draws on the well-known "four-beam" technique for characterisation of water turbidity [20] and absorbance [21]. Despite offering an improvement in pathlength calibration accuracy compared with uncompensated measurements, this method presented a number of disadvantages.…”