Performance optimization of hollow-core fiber photothermal gas sensors

We report performance optimization of hollow-core photonic bandgap fiber (HC-PBF) photothermal (PT) gas sensors. The PT phase modulation efficiency of a C₂H₂ filled HC-PBF (HC-1550-02) is found independent of the pump modulation frequency for up to ∼330  kHz, but starts to drop quickly to 10% of the maximum value at a couple of megahertz. With a 1.1 m long HC-PBF gas cell with angle-cleaved single-mode fiber/HC-PBF joints to reduce reflection and a modified 3×3 Sagnac interferometer with balanced detection for phase demodulation, a noise equivalent concentration of ∼67  ppbC₂H₂ is achieved with a 1 s time constant, and it goes down to ∼18  ppb with 145 s integration time. The system has good long-term stability and exhibits signal fluctuations of <1% over a ∼5  h period.