Fiber Temperature Sensor Based on Cascaded Fabry–Perot Interferometers and Harmonic Vernier Effect

Novel optical fiber temperature sensors, comprising two cascaded Fabry–Perot interferometer (FPI) cavities, have been proposed to realize ultrahigh temperature sensitivity based on the mechanism of harmonic Vernier effect (HVE). Three reference cavities, FPIr1, FPIr2, and FPIr3, were cascaded to the sensing cavity (FPIs) to fabricate sensor1, sensor2, and sensor3, respectively. Sensor1 is designed for traditional Vernier effect (TVE)-based temperature experiments with almost the same detuning as sensor2 is; sensor2 and sensor3 are designed for HVE-based temperature experiments with the harmonic order ${i}$ = 1 and three times the difference in detuning. The internal intersection points in the spectra of HVE-based sensors can provide accurate spectral position and shift. The single FPIs shows linear response to temperature with a sensitivity of ~1.064 nm/°C; The TVE-based sensor1 shows temperature sensitivity of $\sim -8.430$ nm/°C, indicating a magnification factor of $\sim -7.92$ ; the HVE-based sensor2 shows temperature sensitivity of ~15.341 nm/°C, indicating a harmonic magnification factor of ~14.42, which is similar to (1 + 1) times 7.92; the sensor3, based on HVE ${i}$ = 1 too, shows temperature sensitivity of ~50.351 nm/°C, which is three times larger than that of sensor2 for the three times smaller detuning. These results indicate that HVE-based sensors can achieve high sensitivity with a large detuning, which means easier fabrication, and can get ultrahigh sensitivity with a small detuning. These polydimethylsiloxane (PDMS)-based temperature sensors, due to their small size and resistance to electromagnetic interference and excellent performance of PDMS, can meet different requirements for temperature.