A Dual-Parameter Internally Calibrated Fabry-Perot Microcavity Sensor

We propose and experimentally demonstrate multi-parameter sensors based on the antiresonant (AR) reflecting guidance in a single-mode fiber (SMF)-capillary-SMF structure. The sensor is manufactured by simply fusion splicing a certain length of hollow silica capillary between two commercial SMFs. Two prototypes with different lengths of the sandwiched capillary are fabricated and tested. The first sensor works in a reflection mode, based on the combined mechanisms of Fabry-Perot (FP) interference and AR reflecting guidance. As the length of the silica capillary exceeds the critical length for the transition of FP and AR, a typical amplitude modulation (AM) signal can be observed in the reflection spectrum of the device, including the AR envelope and the FP interference signal. Both the FP interference signal and the AR envelope can be used for sensing applications so that multi-parameter measurements can be realized. Measurements of temperature and strain were investigated in the demonstration experiment. The second sensor works in a transmission fashion. Multiple lossy dips exist at the sensor's transmission spectrum as the length of the sandwiched capillary increases due to the AR reflecting guidance in the capillary. The sensor was demonstrated for measuring bending and temperature. Due to macro-bending loss, the intensities of the lossy dips changed with bending; the wavelengths of the lossy dips shifted with temperature due to the temperature-induced changes in the resonance condition. The proposed SMF-capillary-SMF structure-based sensor and the multi-parameter codifying concept could find applications in the multi-parameter sensing field due to its compactness and simplicity.