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Ammonia sensor based on QEPAS with HC-PBF as reference cell

Quartz-enhanced photoacoustic spectroscopy (QEPAS) sensors involves with many desirable features, such as being small and portable, with fast continuous in situ measurements possible. In QEPAS systems, reference cells filled with gas mixtures for wavelength locking and calibration are used to improve the precision and stability of the trace gas concentration measurement. For this study, a 5 m length hollow core photonic bandgap fiber (HC-PBF) splicing with single mode fibers at two ends was manufactured as reference cell, which has long absorption path, low transmission loss and easy connectivity. Hollow cores under high pressure (3.0 × 105 Pa) were filled with a certified mixture of ammonia and nitrogen gas to reach equilibrium rapidly. The experiment results indicated that absorption spectra of reference cell with a low loss <3.5dB maintained stable after 150 days at atmosphere. For trace gas detection, 1.531 μm DFB laser with wavelength modulation technique was demonstrated based on QEPAS. A normalized noise equivalent absorption coefficient (NNEA) of 1.18×10-7 cm-1W/√Hz was obtained at room temperature and pressure of 760 Torr. This results in a minimum detection limit of 3.6ppm for noise equivalent concentration within a 1s lock in integration time.