Design and optimization of hollow-optical-fiber gas cell for infrared spectroscopy

We optimized the configuration of a hollow-fiber gas cell used in a Fourier-transform infrared spectroscopy system for quantitative analyses of low-concentration gas components with small sample volume. Numerical calculation shows that the signal-to-noise ratio (SNR) of this measurement system is maximized in a condition dependent on a fiber's bore size and length, transmission loss of the fiber, and absorption coefficient of the objective gas component. From the design calculation, a hollow optical fiber with a large diameter of 2'mm was chosen, and a measurement system was constructed with the fiber. In analyses of measured absorption spectra, evaluation of the total area of multiple peaks with different rotation levels enables precise evaluation of gas components with high SNR. Experimental results with sample gases containing nitric oxide showed that a minimum detection limit is obtained owing to gas-cell optimization and numerical procedures.