A high sensitivity refractive index sensor based on a single mode-small diameter no core fiber structure is proposed. In this structure, a small diameter no core fiber (SDNCF) used as a sensor probe, was fusion spliced to the end face of a traditional single mode fiber (SMF) and the end face of the SDNCF was coated with a thin film of gold to provide reflective light. The influence of SDNCF diameter and length on the refractive index sensitivity of the sensor has been investigated by both simulations and experiments, where results show that the diameter of SDNCF has significant influence. However, SDNCF length has limited influence on the sensitivity. Experimental results show that a sensitivity of 327 nm/RIU (refractive index unit) has been achieved for refractive indices ranging from 1.33 to 1.38, which agrees well with the simulated results with a sensitivity of 349.5 nm/RIU at refractive indices ranging from 1.33 to 1.38.

Based on a linear piezoelectric constitutive relation and a nonlinear magnetostrictive constitutive relation, a nonlinear magnetoelectric (ME) effect model for lead zirconate titanate (PZT)/Terfenol-D nanobilayer on a substrate has been developed. In this study, the nonlinear ME coefficients at bending mode for two cases (without surface stress and with surface stress) are calculated by using Gurtin-Murdoch theory. The difference between two cases and the influence of residual surface tension are discussed. At the same time, the clamping effect of the substrate on ME effect is studied by altering the thickness ratio of the substrate and selecting different substrate materials. The influences of frequency of the magnetic field, PZT volume fraction on the ME effect are investigated, respectively. Finally, the dependence of ME effect on pre-stress is presented. The results show that for the nanobilayer, both the residual surface tension and surface stress have non-ignored effects on the ME effect. Besides, the resonant frequency of the nanobilayer is very low at the bending mode, which can be enhanced by increasing the thickness ratio of the substrate. Also, the substrate can weaken the ME effect due to the clamping effect, and a more soft substrate material should be selected for large ME effect. In addition, pre-stress plays an important role in the nonlinear ME coupling effect of the model developed.

Stretchable optical fiber sensors (SOFSs), which are promising and ultra-sensitive next-generation sensors, have achieved prominent success in applications including health monitoring, robotics, and biological–electronic interfaces. Here, we report an ultra-sensitive multi-functional optical micro/nanofiber embedded with a flexible polydimethylsiloxane (PDMS) membrane, which is compatible with wearable optical sensors. Based on the effect of a strong evanescent field, the as-fabricated SOFS is highly sensitive to strain, achieving high sensitivity with a peak gauge factor of 450. In addition, considering the large negative thermo-optic coefficient of PDMS, temperature measurements in the range of 30 to 60 °C were realized, resulting in a 0.02 dBm/°C response. In addition, wide-range detection of humidity was demonstrated by a peak sensitivity of 0.5 dB/% RH, with less than 10% variation at each humidity stage. The robust sensing performance, together with the flexibility, enables the real-time monitoring of pulse, body temperature, and respiration. This as-fabricated SOFS provides significant potential for the practical application of wearable healthcare sensors.

Magnetoelectric (ME) effect in a Ni/PZT/Terfenol-D composite cantilever was tested under three different magnetic loading modes. The frequency-dependent ME effect and dual-peak phenomenon were observed in the experiment. The influence of orientations of magnetic fields on the dual-peak phenomenon of ME coupling was investigated. Magnetic field distribution inside the ME composite structure was simulated, which agrees well with experimental data. The experiment results indicate that ME coefficient versus bias magnetic field curve presents a novel dual-peak phenomenon near the resonant frequency, and the ME coefficient which depends upon the amplitude and orientation of magnetic field presents a nonlinear shift whether at the resonant frequency or not. In addition, the optimal angle corresponding to the largest ME coefficient for different bias fields were obtained. The proposed ME composites-based sensors can be used for detecting or harvesting magnetic signals of uncertain orientations and amplitudes in complex environments.

A polarization-dependent all-fiber comb filter based on a combination effect of multimode interference and Mach-Zehnder interferometer was proposed and demonstrated. The comb filter was composed with a short section of multimode fiber (MMF) fusion spliced with a conventional single mode fiber on the one side and a short section of a different type of optical fiber on the other side. The second type of optical fiber is spliced to the MMF with a properly designed misalignment. Different types and lengths of fibers were used to investigate the influence of fiber types and lengths on the performance of the comb filter. Experimentally, several comb filters with free spectral range (FSR) values ranging from 0.236 to 1.524 nm were achieved. The extinction ratio of the comb filter can be adjusted from 6 to 11.1 dB by varying polarization states of the input light, while maintaining the FSR unchanged. The proposed comb filter has the potential to be used in optical dense wavelength division multiplexing communication systems.