Design of flexible hollow core fiber based photoacoustic gas sensor with high cell constant and compact size

Here we designed, optimized, and proposed a flexible low frequency resonant photoacoustic (PA) gas sensor by using a large core leaky hollow core fiber (L-HCF). The influences from the dimensions, the transmission loss and the bending loss on the performance of the flexible PA gas sensor were systematically investigated. In this work, the optimized inner diameter and length of the L-HCF were 1.7 mm and 300 mm, respectively. The L-HCF based PA cell constant was calculated to be 12115 Pa/(W·cm^-1). The minimum detectable limit (MDL) for trace C₂H₂ detection achieved 23.0 ppb when the lock-in integration time was 200 s by using a near-infrared distributed feedback (DFB) laser source and a low-cost electrical micro-electro-mechanical system (MEMS) microphone. Besides, the amplitude decay ratio of the of the PA signal was only 11.3% when the bending radius of the L-HCF was 100 mm. The normalized noise equivalent absorption (NNEA) coefficient is calculated to be 6.6 × 10^-9 W•cm^-1•Hz^-1/2. The L-HCF based PA cell was proved to own merits of compact size, high cell constant, small gas volume and low cost.