Bi-directional liquid level fiber optic sensor based on a cascaded hollow core fiber structure

A high sensitivity bi-directional liquid level sensor based on a single mode fiber (SMF) - hollow core fiber (air core diameter of $30 { {\mu m}})$ - hollow core fiber (air core diameter of $20 {\mu}\mathrm{m})$ - SMF (SHHS) structure is reported. The proposed SHHS structure offers two types of radial Fabry-Perot (FP) resonance conditions in silica cladding hence two types of anti-resonant (AR) dips (Dip-20 and Dip-30) are excited in transmission spectra. It is demonstrated that both liquid level and liquid flowing direction can be monitored by simultaneously interrogating the intensity variations of two kinds of AR dips. The maximum liquid level sensitivities of Dip-20 and Dip-30 are estimated to be 0.014 $\text{dB}/{\mu m}$ and 0.005 $\text{dB}/{\mu m}$ , respectively.