A novel D-shaped fiber structure combined with a fiber Bragg grating for refractive index (RI) and temperature sensing is proposed and experimentally investigated. Thanks to the etched cladding of D-shaped fiber, the resulting interference is strongly influenced by the surrounding refractive index. By monitoring the dip wavelength of D-shaped fiber structure, the refractive index can be measured. Theoretical analysis and simulations were conducted to investigate the cladding mode propagating behavior influenced by the surrounding RI. Combined with the temperature sensing property of fiber Bragg grating (FBG), the possibility of simultaneously measuring the RI and temperature relies on monitoring the resonance dip of the D-shaped fiber modal interferometer and the Bragg wavelength of the FBG. Experimental results showed that RI sensitivity of -31.79 nm/RIU within the range of 1.333 ~ 1.428, and temperature sensitivity of 28.7 pm/°C can be obtained. The proposed sensor with simple configuration, low cost and easy fabrication is suitable for refractive index and temperature sensing applications.

Recently, optoelectronic oscillators (OEOs) have shown good behavior in microwave signal generation. In this letter, we propose a rational harmonic mode-locking OEO (RHML-OEO) to generate microwave frequency combs (MFCs) with tunable comb intervals. By embedding an extra Mach–Zehnder modulator (MZM) into the OEO cavity, the intensity of the oscillating mode can be modulated by the signal applied on the MZM, and sidebands having fixed phase differences are produced in addition to the oscillating mode. A main feature of the RHML-OEO is that there is a frequency detuning between the modulation signal and longitudinal mode spacing of the OEO. Consequently, MFCs with large comb intervals can be generated even with a low-frequency modulation signal. In our experiment, third-order, fifth-order, and tenth-order RHML-OEOs are realized to generate MFCs with different frequency intervals of 1.128, 1.88, and 3.76 MHz when the modulation frequencies are 564, 940 kHz, and 1.253 MHz, respectively.

An in-fiber Mach-Zehnder interferometer is proposed for the discrimination of strain and temperature. The sensor is based on two cascaded standard single mode fibers using three peanut tapers fabricated by simple splicing. The cascaded structure excites more frequency components, which induce four sets of interference dips in the transmission spectrum. One set of the spectrum dips have different sensitivities to temperature and strain from those of the other three. The sensor can discriminate strain and temperature by monitoring the wavelength shifts of two spectrum dips. Repeated experiments are taken both for strain and temperature increasing and decreasing scenarios. Experimental results show that Dip 1 has an average strain sensitivity of −0.911 pm/µε and an average temperature sensitivity of 49.98 pm/°C. The strain sensitivity for Dip 2 is negligible and its average temperature sensitivity is 60.52 pm/°C The strain and temperature resolutions are ±3.82 µε and ±0.33 °C.

A voltage sensor with high resolution and large measurement range based on an optoelectronic oscillator (OEO) is proposed and experimentally demonstrated. The key component in the cavity to select the oscillating signal is a finite impulse response (FIR)-microwave photonic filter (MPF) which consists of a sinusoidal broadband optical signal, an unbalanced Mach–Zehnder interferometer (MZI), a section of dispersion compensating fiber, and a photodetector. The center frequency of the FIR-MPF is mainly determined by the free spectral range (FSR) of the FIR-MPF. In the lower arm of the MZI, a cylindrical piezoelectric ceramic (PZT) wrapped with a section of optical fiber acts as voltage sensing head. Due to the inverse piezoelectric effect of PZT, the variation of the voltage will cause radial deformation of the cylindrical PZT and then lead to the change of the FSR of the MZI, determining the shift of center frequency of FIR-MPF as well as the frequency of the oscillating signal of the OEO. Thus, by monitoring the shift of the oscillation frequency of the OEO using an electric spectrum analyzer or a digital signal processor, a high-speed interrogation and high-resolution voltage measurement can be realized. Additionally, in the proposed scheme, an infinite impulse response (IIR)-MPF consisting of a fiber ring resonator is cascaded with the FIR-MPF to ensure the single-mode oscillation of the OEO. The experimental results show that a total range of 1700 V voltage sensing from – 200 V to 1500 V is accomplished with the voltage sensitivity of 0.25 GHz/100 V and the resolution of 0.3 V. By adjusting the proportion of the length of single mode fiber between two branches of MZI, the impact of temperature can be greatly reduced. The proposed sensor offers advantages such as a large measurement range, high resolution, high-speed interrogation, and stability to temperature disturbances, making it highly suitable for sensing applications in smart grids.

We proposed and successfully validated improved the sensitivity of magnetic field sensing using the Vernier effect in a dual-loop optoelectronic oscillator (OEO) incorporating cascaded L-shaped Terfenol-D-FBGs. Thanks to the time delay introduced by the dispersion compensating fiber (DCF) in the OEO cavity, precise conversion between the sensing FBG wavelength changes and the OEO oscillation frequency can be achieved. The central wavelengths of the sensing Terfenol-D-FBG change along with the magnetic field. Therefore, the magnetic field can be monitored by measuring the oscillating frequency of the OEO. In the proposed scheme, two reflection signals from the cascaded FBGs pass through two fiber paths with slightly different lengths, inducing the Vernier effect in the OEO, which significantly enhances the sensitivity of the sensor. Additionally, a bias magnetic field is applied to the sensing Terfenol-D alloy to further improve the sensitivity. Experimental results demonstrate that by applying a bias magnetic field on both sides of the sensing Terfenol-D, the sensitivity of the single-loop OEO is increased from 421 Hz/mT to 560 Hz/mT. Utilizing the Vernier effect, the magnetic field sensitivity of the dual-loop OEO reaches up to −16.54 kHz/mT, with a 29-times improvement over the single-loop OEO's 560 Hz/mT. By finely adjusting the path length difference, we further enhance the performance of the sensor, achieving sensitivities of −24.58 kHz/mT, −28.49 kHz/mT and −35.94 kHz/mT, respectively, which are 44 times, 51 times and 64 times higher than the single-loop OEO structure. Besides, to overcome the influence of temperature, a temperature compensation sensor is designed using a pair of L-shaped Terfenol-D alloys to host the cascaded sensing FBG and reference FBG. The experimental results show that the proposed L-shaped Terfenol-D with cascaded FBG can reduce the magnetic field error caused by temperature crosstalk to 0.03 mT.