Dual determination of strain and temperature using cascaded fiber Fabry–Perot interferometers with wavelength and phase demodulation

Abstract A strain and temperature dual-parameter measurement device using cascaded optical fiber Fabry–Perot interferometers (FPIs) is reported in which both wavelength demodulation and phase demodulation are employed with the the parameters. A single-mode fiber (SMF), a short length of hollow-core fiber (HCF), a short section of SMF, and an HCF are spliced together to form the cascaded FPIs. In this structure, two dominant interference cavities are formed and two FPIs are generated, which are an air-cavity FPI and an air-silica cavity FPI. The sensitivities of the air-cavity FPI and the air-silica cavity FPI to strain or temperature are different, allowing the measurement of both parameters. In terms of wavelength demodulation, the temperature sensitivity of the air-cavity FPI (designated as FPI1) is 1.39 pm/°C and that of the air-silica cavity (designated as FPI2) is 8.56 pm/°C from 25 to 85 °C. In the strain range from 0 to 800 με, the strain sensitivity of FPI1 is 0.5 pm/με and that of FPI2 is 1.13 pm/με. Based on the sensitivity difference of the two FPIs, fixing the sensitivity matrix allows for simultaneous strain and temperature measurements. In addition, the dual parameters may also be measured by tracking the phase variations corresponding to the two FPIs. The reported sensor offers a simple splicing process, easy manufacture, low cost, and stable performance and provides a reference for dual-parameter dynamic measurements.