Measurement of high pressure and high temperature using a dual-cavity Fabry-Perot interferometer created in cascade hollow-core fibers.

A compact dual-cavity Fabry-Perot interferometer (DC-FPI) sensor is proposed and demonstrated based on a hollow-core photonic bandgap fiber (HC-PBF) spliced with a hollow-core fiber (HCF). The HC-PBF, which has low transmission loss, was used as the first FPI cavity and also acted as a bridge between the lead-in single-mode fiber and the HCF. The HCF was used as the second FPI cavity and also acted as a micro gas inlet into the first FPI cavity. A DC-FPI sensor with different cavity lengths of 226 and 634 μm in the first FPI and the second FPI was created. Both gas pressures ranging from 0-10 MPa and temperatures ranging from 100-800°C were measured using the DC-FPI sensors together with a fast Fourier transform and phase-demodulation algorithm. Experimental results showed that the first FPI cavity was gas pressure sensitive but temperature insensitive, while the second FPI cavity was temperature sensitive but gas pressure insensitive. A high gas pressure sensitivity of 1.336 μm/MPa and a temperature sensitivity of 17 nm/°C were achieved in the DC-FPI sensor. Moreover, the cross sensitivity between the gas pressure and temperature was calculated to be ∼-15  Pa/°C and ∼0.3°C/MPa. The proposed DC-FPI sensors provide a promising candidate for the simultaneous measurement of high pressures and high temperatures at some precise locations.