A refractive index sensor based on a multi-core micro/nano fiber is proposed for low refractive index solutions. At first, the mode field distribution of the tapered multi-core fiber is analyzed with the finite element model (FEM). After that, the relationship between the refractive index sensitivity and the diameter of the multi-core micro/nano fiber is calculated. At last, four sensors with different sizes are explored, and when the taper length is 16.20 mm, the refractive index sensitivity of the sensor can reach 5815.50 nm/RIU, which agrees with the theoretical analysis. The refractive index measurement error is less than 0.5 ‰, which has a high practical application value. The longer the taper length is, the smaller the fiber diameter is. According to the theoretical analysis, when the fiber diameter is less than 4.864 μm, the structure sensor’s refractive index sensitivity is higher than 10000 nm/RIU. At the same time, when the sensor’s taper length is 15.99 mm, its temperature sensitivity is -0.1084 nm/℃. Compared with single-mode fiber, the sensor proposed here has the advantages of stability, compact structure, and high sensitivity, which has a potential in the field of seawater salinity measurement.

In this paper, we propose a compact plasmonic sensor structure comprised of a metal-dielectric-metal (MDM) waveguide, and a baffle plate in waveguide core and two side-coupled rectangular cavities. In this structure, two Fano resonances are achieved and can be tuned independently by changing the structural parameters of the cavities. Especially, when the resonant wavelengths of the two Fano resonances are the same, the sensing sensitivity can be enhanced by coupling between two Fano resonances. By investigating the transmission spectrum, the effect of structural parameters on Fano resonances and the refractive index sensitivity of the sensor structure are analyzed in detail. The numerical simulations demonstrate a sensitivity as high as 1295nm/RIU and a figure of merit of 1647.

A highly sensitive and temperature-compensated methane sensor based on a liquid-infiltrated photonic crystal fiber (PCF) is proposed. Two bigger holes near the core area are coated with a methane-sensitive compound film, and specific cladding air holes are infiltrated into the liquid material to form new defective channels. The proposed sensor can achieve accurate measurement of methane concentration through temperature compensation. The sensitivity can reach to 20.07 nm/% with a high linearity as the methane concentration is within the range of 0%–3.5% by volume. The proposed methane sensor can not only improve the measurement accuracy, but also reduce the metrical difficulty and simplify the process.

We have demonstrated a distributed vibration sensor based on phase-sensitive optical time-domain reflectometer (φ-OTDR) system exhibiting immunity to the laser phase noise. Two laser sources with different linewidth and phase noise levels are used in the φ-OTDR system, respectively. Based on the phase noise power spectrum density of both lasers, the laser phase is almost unchanged during an extremely short period of time, hence, the impact of phase noise can be suppressed effectively through phase difference between the Rayleigh scattered light from two adjacent sections of the fiber which define the gauge length. Based on the phase difference method, the external vibration can be located accurately at 41.01 km by the φ-OTDR system incorporating these two lasers. Meanwhile, the average signal-to-noise ratio (SNR) of the retrieved vibration signal by using Laser I is found to be ~37.7 dB, which is comparable to that of ~37.5 dB by using Laser II although the linewidth and the phase noise level of the two lasers are distinct. The obtained results indicate that the phase difference method can enhance the performance of φ-OTDR system with laser phase-noise immunity for distributed vibration sensing, showing potential application in oil-gas pipeline monitoring, perimeter security, and other fields.

We have proposed and demonstrated a double-cladding fiber (DCF) with cladding-mode resonance property for broadband acoustic vibration sensing. Since the fundamental mode in the core waveguide is able to be coupled to LP05 mode in the tube waveguide once the phase-matching condition is fulfilled, the transmission spectrum can exhibit a dip with a large extinction ratio. an acoustic vibration could induce the wavelength shift of such transmission spectrum, so that the intensity variation at a wavelength near the dip is coded with the information of the acoustic vibration signal. By demodulating the response of intensity variation, the frequency of the applied acoustic vibration signal can be recovered. Such a DCF-based sensor with an intensity modulation could measure the acoustic vibration with a broadband frequency range from 1 Hz to 400 kHz and exhibits the maximum signal-to-noise ratio (SNR) of ~80.79 dB when the vibration frequency is 20 kHz. The obtained results show that the proposed DCF-based acoustic vibration sensor has a potential application in environmental assessment, structural damage detection, and health monitoring.

We have reported high-energy pulse generation from a Q-switched Er-doped fiber laser based on black phosphorus (BP) with a drop-casting method. Poly dimethyl diallyl ammonium (PDDA) is employed to protect BP. A passive Q-switching operation is achieved by using the BP/polyvinyl alcohol (PVA) film as the saturable absorbers (SAs). When the pump power increases from 130.4 mW to 378 mW, the repetition rate increases from 13.33 kHz to 26.6 kHz, and the pulse duration decreases from 10.67 μs to 7.11 μs. The maximum pulse energy is 468.03 nJ, to the best of our knowledge, which is the highest pulse energy produced from Q-switched fiber laser based on BP-SA at 1.5 μm. The obtained high-energy pulse demonstrates that BP/PVA film fabricated by such a drop-casting method can provide an ideal SA for high pulse energy generation from fiber lasers, and it has a potential application of remote sensing.

In this paper, the localized surface plasmon resonance (LSPR) peak of Ag-Fe and Au-Fe alloy nanoparticles for the triangular prism is calculated by using the discrete dipole approximation (DDA) method. We investigate the variation of the resonance wavelength, refractive index sensitivity, and figure of merit with the particles size and alloy compositions. We perform a comparative study on the refractive index sensitivity and figure of merit of alloys in order to find the considered (Ag-Fe and Au-Fe) alloys with high sensitivity. The refractive index sensitivity of the Au-Fe alloy is found higher than that of the Ag-Fe alloy. Therefore, to optimize the size of alloy nanoparticles (NPs) for the triangular prism, the figure of merit is calculated and observed that the optimized size is 50 nm and 20 nm for Ag-Fe and Au-Fe alloys, respectively. A comparison of Ag-Fe shows that the Au-Fe alloy NPs have greater figure of merit (FOM) and thus may be more suitable for applications in biosensing.

Sub-nanometer displacement measurement is still a challenge in the current sensor field. In this study, a new type of displacement sensor is designed which is based on the coupling effect of two balanced gain and loss resonators. The optical properties of the sensor have been studied through the coupled mode theory and scatter matrix. The pole effect in the coupling system can be used to measure the sub-nanometer displacement. The resolution of the sensor can reach 0.001 nm over a dynamic range of 20 nm. The sensor has the highest sensitivity within the range of one nanometer. The environmental disturbance and structure parameter perturbation have been demonstrated to make trivial effect on the sensor performance.

Standing on the potential for high-speed modulation and switching in the terahertz (THz) regime, all-optical approaches whose response speeds mainly depend on the lifetime of nonequilibrium free carriers have attracted a tremendous attention. Here, we establish a novel bi-direction THz modulation experiment controlled by femtosecond laser for new functional devices. Specifically, time-resolved transmission measurements are conducted on a series of thin layers Bi2Se3 films fabricated straightforwardly on Al2O3 substrates, with the pump fluence range from 25 μJ/cm2 to 200 μJ/cm2 per pulse. After photoexcitation, an ultrafast switching of THz wave with a full recovery time of ~10 ps is observed. For a longer timescale, a photoinduced increase in the transmitted THz amplitude is found in the 8 and 10 quintuple layers (QL) Bi2Se3, which shows a thickness-dependent topological phase transition. Additionally, the broadband modulation effect of the 8 QL Bi2Se3 film is presented at the time delays of 2.2 ps and 12.5 ps which have a maximum modulation depth of 6.4% and 1.3% under the pump fluence of 200 μJ/cm2, respectively. Furthermore, the absorption of α optical phonon at 1.9 THz shows a time-dependent evolution which is consistent with the cooling of lattice temperature.

A novel Young’s modulus measurement scheme based on fiber Bragg gratings (FBG) is proposed and demonstrated experimentally. In our method, a universal formula relating the Bragg wavelength shift to Young’s modulus is derived and metal wires are loaded strain by using the static stretching method. The Young’s modulus of copper wires, aluminum wires, nickel wires, and tungsten wires are separately measured. Experimental results show that the FBG sensor exhibits high measurement accuracy, and the measurement errors relative to the nominal value is less than 1.0%. The feasibility of the FBG test method is confirmed by comparing it with the traditional charge coupled device (CCD) imaging method. The proposed method could find the potential application in the material selection, especially in the field that the size of metal wires is very small and the strain gauges cannot be qualified.

Surface plasmon resonance (SPR) sensor based on the blue phosphorene/MoS2 hetero-structure is presented to enhance the performance parameters, i.e., sensitivity, detection accuracy, and quality factor. The blue phosphorene/MoS2 hetero-structure works as an interacting layer with the analyte for the enhancement of the sensitivity of the sensor. It is observed that the sensitivity of blue phosphorene/MoS2 based sensor (i.e., structure-II) is improved by 5.75%, from the conventional sensor (i.e., structure-III). Further, an additional silicon nanolayer is introduced between the metal layer and blue phosphorene/MoS2 hetero-structure. The sensitivity of the proposed blue phosphorene/MoS2 hetero-structure with a silicon layer SPR sensor, i.e., structure-I, is enhanced by 44.76% from structure-II and 55.75% from structure-III due to an enhancement in the evanescent field near the metal-analyte interface. Finally, it is observed that at the optimum thickness of silicon between the gold layer and blue phosphorene/MoS2, performance parameters of the sensor are enhanced.