Pulsed photothermal interferometry for high sensitivity gas detection with hollow-core photonic bandgap fibre

Pulsed photothermal interferometry (PTI) gas sensor with hollow-core photonic bandgap fibre (HC-PBF) is demonstrated with a Sagnac interferometer-based phase detection system. Under the condition of constant peak pump power, the optimal pulse duration is found to be >1.2 μs for detecting low-concentration of trace gases in nitrogen, limited by thermal conduction of gases within the hollow-core. Preliminary experiments with a 0.62-m-long HC-PBF gas cell, low peak power (~20.2 mW) and a boxcar averager with 10k average times demonstrated a detection limit of 3.3 p.p.m acetylene. Detection limit down to ppb or lower is expected with high peak power pump pulses.