A new D-shaped tellurite photonic crystal fiber sensor based on the four-wave mixing (FWM) effect with the surface plasmon resonance (SPR) effect is designed and optimized. The substrate of the D-shaped photonic crystal fiber (D-PCF) is tellurite glass, and the polished surface is plated with the gold film and hydrogen gas-sensitive film. An air hole of the inner cladding, which is plated with the gold film and methane gas-sensitive film, is selected as the second sensing channel to simultaneously measure the concentration of hydrogen and methane. Based on the four-wave mixing, the wavelength shifts of the Stokes and anti-Stokes spectra resulting from the variation of the gas concentration can be used to accurately detect the concentrations of methane and hydrogen. Meanwhile, it is found that the SPR effect can increase the wavelength shifts, which means the sensitivity of methane and hydrogen augment. After parameter optimization, the maximum sensitivities of methane and hydrogen are 4.03 nm/% and -14.19 nm/%, respectively. Both the linearities are up to 99.9%. The resolution of methane is 1.25×10-2% and hydrogen is 7.14×10-3%. Moreover, the fiber length of this sensor is only 20 mm, which is conducive to the construction of a compact or ultra-compact embedded FWM fiber sensor.

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/℃. The strain sensitivity for Dip 2 is negligible and its average temperature sensitivity is 60.52 pm/℃ The strain and temperature resolutions are ±3.82 με and ±0.33 ℃.

In this study, we experimentally demonstrate a miniature fiber thermometer based on tip-integrated ZnO-nanowire-nanograting. The sensor has a diameter less than 1 μm and the length of the Bragg grating is sub-10 μm. The ZnO-nanowire-nanograting is sensitive to the environmental temperature change. Thus, the intensity of the light whose wavelength is in the rising or falling region of the nanograting spectrum will vary with the shift in wavelength due to change in temperature. Taking one wavelength (655 nm) in the rise linear region of the nanograting spectrum, a sensitivity of 0.066 nW/℃ in the air is achieved experimentally. The proposed temperature sensor has the superiorities of compactness, stableness, and easy fabrication compared to regular fiber grating sensors, offering great potential for detecting inside minimal volume environments.

Sensing sensitivity is the key performance of optical tweezers. By adjusting the frequency and magnitude of an applied Coulomb force as an input of optical tweezers, we directly measured the sensitivity and signal-to-noise ratio (SNR) of a system and indirectly calculated the actual noise magnitude. Combined with an output filter, the relationship between the SNR and bandwidths was studied. We established the simulation model of a system using Simulink and simulated the relationship between the SNR and magnitude of the input forces and filter bandwidths. In addition, we built an experimental system to determine the relationship between the SNR and the magnitude of the input forces and filter bandwidths. The actual minimum detectable force was measured as 1.827 5×10-17 N at a 1 Hz bandwidth. The experimental results were correlated with the simulation and theoretical results, confirming the effectiveness of the proposed method and demonstrating the high sensitivity of vacuum optical tweezers as mechanical sensors. We proposed a novel method of calibration and measurement of system sensing parameters by applying an actual force that was more direct and precise than the theoretical calculation method that requires accurate fitting parameters, such as the particle radius and density. This method can be employed to analyze the system noise and phase characteristics to confirm and improve the real performance of the system.

Deviation of the H+ concentration from optimum values within the organelles is closely associated with irregular cellular functions that cause the onset of various diseases. Therefore, determining subcellular pH values in live cells and tissues is valuable for diagnostic purposes. In this study, we report a novel ratiometric fluorescence probe 1H-pyrazole-3-carboxylic acid, 4-(benzo[d]thiazol-2-yl)-3-(2,4-dihydroxy-3-methylphenyl)-1H-pyrazole-5-carboxylicacid4-(2-benz othiazolyl)-5-(2,4-dihydroxy-3-methylphenyl), to which we will refer as ThiAKS Green (Thiazole AKyol shifting green), that is pH sensitive. The results presented here show that the probe can penetrate the cell membrane in less than 30 minutes and does not show any detectable toxicity. The measured color shifts up on pH change are linear and most significant around physiological pH (pKa=7.45), thus making this probe suitable for live-cell imaging and intracellular pH measurements. During the long-incubation periods following the application of the probe and the fluorescent microscopy measurements, it shows stable properties and is easy to detect in live cells. In conclusion, the results suggest that ThiAKS Green can be used to obtain precise information on the H+ distribution at various compartments of the live cells.

Metalens are planar lenses composed of the subwavelength arrays, which have unconventional and versatile functionalities to manipulate the light fields compared with the traditional lens. It is noted that the most metalens are designed in a monochromatic mode in the visible or mid-infrared range (mid-IR), however, the broadband range is needed in many practical applications, such as spectroscopy, sensing, and imaging. Here, we design and demonstrate a broadband achromatic dielectric metalens in the mid-IR range of 4 μm - 5 μm for near diffraction-limited (1.0λ) focusing. The broadband achromatic propagation and focusing of the metalens are designed and simulated by constructing and optimizing the phase profile. The Pancharatnam-Berry (P-B) phases of all the elements contribute to the main phase increment of the whole phase profile of the metalens. The additional phase is constructed and optimized by using the random search algorithm to obtain the optimized size of all the elements. The focusing efficiency of the achromatic metalens is also optimized and averaged as the result of phase optimization within a wide band for the building elements, while it is lowered comparing with the regular metalens without broadband achromatic designing. Using this combined designing approach, various flat achromatic devices with the broadband metalens can find a new way for full-color detection and imaging.

The strain-temperature cross-sensitivity problem easily occurs in the engineering strain monitoring of the self-sensing embedded with fiber Bragg grating (FBG) sensors. In this work, a theoretical investigation of the strain-temperature cross-sensitivity has been performed using the temperature reference grating method. To experimentally observe and theoretically verify the problem, the substrate materials, the preloading technique, and the FBG initial central wavelength were taken as main parameters. And a series of sensitivity coefficients calibration tests and temperature compensation tests have been designed and carried out. It was found that when the FBG sensors were embedded on different substrates, their coefficients of the temperature sensitivity were significantly changed. Besides, the larger the coefficients of thermal expansion (CTE) of substrates were, the higher the temperature sensitivity coefficients would be. On the other hand, the effect of the preloading technique and FBG initial wavelength was negligible on both the strain monitoring and temperature compensation. In the case of similar substrates, we did not observe any difference between temperature sensitivity coefficients of the temperature compensation FBG with one free end or two free ends. The curves of the force along with temperature were almost overlapped with minor differences (less than 1%) gained by FBG sensors and pressure sensors, which verified the accuracy of the temperature compensation method. We suggest that this work can provide efficient solutions to the strain-temperature cross-sensitivity for engineering strain monitoring with the self-sensing element embedded with FBG sensors.

Side polished fiber (SPF) has a controllable average roughness and length of the side-polishing region, which becomes a versatile platform for integrating multiple materials to interact with the evanescent field to fabricate all-fiber devices and sensors. It has been widely used in couplers, filters, polarizers, optical attenuators, photodetectors, modulators, and sensors for temperature, humidity, strain, biological molecules, chemical gas, and vector magnetic monitoring. In this article, an overview of the development history, fabrication techniques, fiber types, transmission characteristics, and varied recent applications of SPFs are reviewed. Firstly, the fabrication techniques of SPFs are reviewed, including the V-groove assisted polishing technique and wheel polishing technique. Then, the different types of SPFs and their characteristics are discussed. Finally, various applications of SPFs are discussed and concluded theoretically and experimentally, including their principles and structures. When designing the device, the residual thickness and polishing lengths of the SPF need to be appropriately selected in order to obtain the best performance. Developing all-fiber devices and sensors is aimed at practical usability under harsh environments and allows to avoid the high coupling loss between optical fibers and on-chip integrated devices.