Sensitive temperature measurement with composite multicavity Fabry-Perot interferometers (FPIs) and differential phase demodulation

Abstract

A temperature sensor based on composite multicavity Fabry-Perot interferometers (FPIs) and a sensitivity-enhanced differential phase demodulation method is proposed and demonstrated. The structure was fabricated by partially filled polydimethylsiloxane (PDMS) into a segment of hollow-core fiber (HCF) which is spliced with a single-mode fiber (SMF). In this structure, composite multicavity FPIs are formed, including an air-cavity FPI, a PDMS-cavity FPI and an air-PDMS cavity FPI. The contribution of the different FPIs to the superimposed spectrum is analyzed from the simulated and experimental perspectives. The results demonstrate that the superimposed output spectrum is mainly produced by the air-cavity FPI and the air-PDMS cavity FPI. When the temperature increases, the lengths of the PDMS-cavity and the air-PDMS cavity are enhanced due to the volume expansion of PDMS, and the length of the air-cavity decreases because the air-cavity is compressed by the expanded PDMS. The cavity length variation causes the phase variation in the air-cavity FPI to be the opposite to that in the PDMS-cavity FPI or air-PDMS cavity FPI. Therefore, the phase demodulation sensitivity is enhanced by calculating the differential phase between the air-cavity FPI and the air-PDMS cavity FPI. The experimental results show that the sensitivity amplification factor of differential phase to the single phase of the air-cavity FPI or the air-PDMS cavity FPI is from approximately 1.44–3.25. This phase demodulation method is simpler, faster, more direct, and more convenient than wavelength demodulation from the perspective of demodulating composite interferometers.