The performances of fiber-optic distributed acoustic sensing (DAS) systems are fundamentally limited by the trade-off between the sensing distance and response bandwidth (RB), constraining the effectiveness of the DAS for long-haul sensing applications greatly. To break such a limitation, this paper proposes a novel Zadoff-Chu (ZC) based nonlinear frequency modulation (ZC-NLFM) scheme that combines the zero-correlation property of the ZC method with the chirp diversity of NLFM. The generated ZC-NLFM pulses exhibit excellent sidelobe and inter-pulse interference suppression, enabling high-sensitivity DAS demodulation even under low signal-to-noise ratio conditions. Furthermore, by employing the large-effective-area fiber (LEAF) with the lower attenuation, higher stimulated Brillouin scattering threshold, and Raman amplification, a wide-frequency-range, long-distance, and low-noise DAS (WLL-DAS) with high strain sensitivity of 94.34<inline-formula id="M1"> <math id="mathml_M1" display="inline" overflow="scroll"> <mrow class="MJX-TeXAtom-ORD"> <mrow class="MJX-TeXAtom-ORD"> <mrow class="MJX-TeXAtom-ORD"> <mtext>p</mtext> </mrow> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mrow class="MJX-TeXAtom-ORD"> <mrow class="MJX-TeXAtom-ORD"> <mrow class="MJX-TeXAtom-ORD"> <mo mathvariant="sans-serif">ε</mo> </mrow> </mrow> </mrow> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mrow> <mo fence="true" stretchy="true" symmetric="true">/</mo> <mrow class="MJX-TeXAtom-ORD"> <mrow class="MJX-TeXAtom-ORD"> <mpadded width="0"> <mphantom> <mrow class="MJX-TeXAtom-ORD"> <mrow class="MJX-TeXAtom-ORD"> <mtext>p</mtext> </mrow> </mrow> <mrow class="MJX-TeXAtom-ORD"> <mrow class="MJX-TeXAtom-ORD"> <mrow class="MJX-TeXAtom-ORD"> <mrow class="MJX-TeXAtom-ORD"> <mo mathvariant="sans-serif">ε</mo> </mrow> </mrow> </mrow> </mrow> <mrow class="MJX-TeXAtom-ORD"> <msqrt> <mrow class="MJX-TeXAtom-ORD"> <mtext>Hz</mtext> </mrow> </msqrt> </mrow> </mphantom> </mpadded> </mrow> </mrow> <mo fence="true" stretchy="true" symmetric="true"></mo> </mrow> </mrow> <mrow class="MJX-TeXAtom-ORD"> <msqrt> <mrow class="MJX-TeXAtom-ORD"> <mtext>Hz</mtext> </mrow> </msqrt> </mrow> </mrow></math></inline-formula> over a wide RB of up to 7 kHz and an ultralong sensing distance of 148 km at the spatial resolution of 10 m is achieved simultaneously. Compared to the conventional DAS, the proposed WLL-DAS achieves a 22-fold increase in the RB over an ultralong distance of &gt;140 km, significantly extending the performance boundary and application range of the DAS.

Abstract The optical fiber Fabry-Perot interferometer (FPI) has been widely investigated as a potential temperature sensor. To function as a temperature sensor, the cavity of the FPI is typically constructed from either silica fibers or polymers. The silica cavity FPIs can function at temperatures exceeding 1 000 °C. However, their temperature sensitivity is constrained by their relatively low thermal optical coefficients and thermal expansion of silica materials. Although the polymer cavity FPI exhibits high temperature sensitivity, its cavity is susceptible to deterioration in high-temperature environments. Here, to overcome this challenge and achieve high-sensitivity temperature sensing in a high-temperature environment, we propose a new type of temperature FPI sensor by inserting and sealing a section of Cr20Ni80 metal microwire inside a section of a silica hollow core fiber (HCF) spliced to a standard single-mode fiber (SMF). The FPIs exhibit a high degree of temperature sensitivity due to the high thermal expansion of the Cr20Ni80 metal microwire. Since the Cr20Ni80 metal has a high melting temperature of 1 400 °C, such FPIs can function in high-temperature environments. Moreover, the temperature sensitivity of this FPI can be modified without affecting its reflection spectrum by changing the length of the metallic microwire within the hollow core fiber. The experimental results indicate that the proposed FPIs exhibit temperature sensitivity of greater than −0.35 nm/°C within the temperature range of 50 °C to 440 °C. Our proposed metal microwire-based FPIs are economical, robust, simple to fabricate, and capable of functioning in high-temperature environments, rendering them appealing options for practical applications.

Abstract A single-layer graphene structure is put forward to generate quadruple plasmon-induced transparency (PIT) at the terahertz frequency by coupling the bright-dark mode and bright-bright mode originated from five graphene strips. Based on the research on the electric field intensity of the PIT transparent window, it is suggested that intense fatal interference occurs among the bright and dark modes. The PIT reaction of the structure is analyzed and simulated with the coupled mode theory (CMT) and the finite difference time domain (FDTD) approach. Tunable multi-frequency switching is achieved under this quadruple transparency effect, since the maximum modulation depth (MD) is as high as 95% and the minimum insertion loss (IL) is 0.17 dB. Besides, the time delay and the group refractive index within the PIT windows can be up to 0.744 ps and 722, respectively. The proposed structure shows another fascinating ability to be sensitive to the nearby refractive index with the sensitivity of up to 0.91 THz/RIU. Therefore, this structure offers another novel thinking to design the multichannel switches, slow light instruments, and sensors in the terahertz band.

Abstract In recent years, the design and synthesis of fluorescent sensors have drawn considerable attention for the specific and sensitive detection of biologically and environmentally important metal ions in living cells and aqueous solutions. Herein, we reported the photophysical properties of a Schiff base fluorescent sensor N′-(2-hydroxy-3-methoxybenzylidene)-3-methoxybenzohydrazide (RN5) that possesses a high affinity for the Ni2+ at a nanomolar concentration. Through fluorescence titration analysis at 600 nm, we confirmed that the sensor RN5 exhibited a colorimetric (from colorless to yellow) and fluorometric turn-on response (fluorescence enhancement) in CH3CN:H2O (2/8, v/v) medium in the presence of Ni2+, whereas no noticeable emission was observed in the case of other examined metals ion. The complexation reaction of Ni2+ with RN5 has been investigated by the colorimetric, absorption spectroscopy, fluorescence spectroscopy, and Fourier transform infrared spectroscopy (FTIR). The 1:1 binding mode was confirmed between the sensor RN5 and Ni2+ by job plot evaluation and theoretically confirmed by DFT studies. The detection limit for Ni2+ was found to be 12.8 nM estimated by the titration method, which was lower than the guideline for drinking water, 1.2 μM by the Environmental Protection Agency (EPA), USA. The sensor RN5 exhibited the excellent fluorescence sensing behavior in a wide pH range of 5–12. The turn-on fluorescence behavior of the Ni2+ with the sensor RN5 was fast, so that it could be employed for quantitative and qualitative determination. Test kits, containing RN5 were fabricated which could perform as the suitable and complete assay “in-the-field” detection of Ni2+. Finally, the sensor RN5 was applied successfully for the detection of Ni2+ in the real water samples.

Heavy metal ions have drawn enough attention due to their harm to both humans and environments, and there has been an urgent need for stable and quick detection of them. Besides, the manganese ion is one of the most abundant ions in urban water systems, which needs to be monitored carefully. Hereby, a carbon dots (CDs) (the microwave method) based surface plasmon resonance (SPR) Mn2+ sensor has been proposed and discussed, which could achieve high detection sensitivity of 6.383 nm/lg(ppb) in the range of 0 ppb–200 ppb on manganese ions, with the detection limit of 0.3462 ppb. The proposed sensor also possesses high stability upon time and temperature, and it also has great ion selectivity against 9 other ions. Practical usage scenarios have been tested on the human serum, tap water, lake water, and river water, which confirm that the proposed sensor holds great potential for both blood tests and environmental monitoring.

Optical fiber sensors have gained significant attention in recent years owing to their remarkable advantages of remote operation and rapid response. The integration of optical fiber sensing with the microfluidics technology has paved the way for the establishment of optical fiber optofluidic sensing. Optical fiber optofluidic systems possess the advantages of the low invasiveness, compact structure, excellent biocompatibility, and the ability to handle small analyte volumes, rendering them particularly suitable for serving as chemical sensors and biosensors. In this paper, we present an in-depth overview of optical fiber optofluidic chemical sensors and biosensors. Firstly, we provide a comprehensive summary of the types of optical fibers commonly employed in optofluidic chemical and biosensing, elucidating their distinct attributes and performance characteristics. Subsequently, we introduce and thoroughly analyze several representative sensing mechanisms employed in optical fiber optofluidic systems and main performance parameters. Furthermore, this review delves into the modification and functionalization of optical fibers. Additionally, we showcase typical biosensing and chemical sensing applications to demonstrate the practicality and versatility of optical fiber optofluidic sensing. Finally, the conclusion and outlook are given.

The practical development of compact modern nanophotonic devices relies on the availability of fast and low-cost fabrication techniques applicable to a wide variety of materials and designs. We have engraved a split grating geometry on stainless steel using femtosecond laser processing. This structure serves as a template to fabricate efficient plasmonic sensors, where a thick gold layer is grown conformally on it. The scanning electron microscope (SEM) images confirm the generation of the split laser-induced periodic spatial structures. The optical reflectance of our sensors shows two dips corresponding to the excitation of surface plasmon resonances (SPRs) at two different wavelengths. Furthermore, the asymmetric shape of these spectral responses reveals a strong and narrow Fano resonance. Our computational electromagnetism models accurately reproduce the reflectivity of the fabricated structure. The spectral responses of both the simulated and fabricated structures are fitted to the Fano model that coherently combines the narrow SPRs with the broad continuum background caused by diffraction. The parameters extracted from the fitting, such as the resonance wavelengths and line widths, are used to evaluate the performance of our device as a refractometric sensor for liquids. The maximum sensitivity and figure of merit are 880 nm/RIU and 80 RIU−1, respectively. Besides the compact design of our sensing device, its performance exceeds the theoretical maximum sensitivity of a classical Kretschmann setup.

In this paper, a dual Mach-Zehnder interferometer for measuring both temperature and strain is proposed and verified by experiments. The sensor configuration involves cascading a four-core fiber and a double-clad fiber between two single-mode fibers. By exploiting the different responses of the two Mach-Zehnder interferometers to temperature and strain, we construct a matrix using two selected resonance dips from the transmission spectra, so that both temperature and strain can be measured simultaneously. The experimental results show the sensor’s remarkable performance, with the maximum temperature sensitivity of −94.2 pm/°C and the maximum strain sensitivity of 2.68 pm/µε. The maximum temperature error and strain error are found to be ±0.35 °C and ±4.8 µε, respectively. Compared with other optical fiber sensors, the sensor has high sensitivity, a simple structure, and ease to manufacture and implement, making it a structure choice for applications in quality inspection of materials.

For expanding the amplitude-frequency response range of the differential cross-phase multiply (DCM) algorithm in the φ-OTDR system, a temporal spline interpolation (TSI) method is proposed to pre-process Rayleigh backscattering (RBS) signals. Through the TSI method, the discrete temporal signals characterizing RBS traces are subjected to interpolation, facilitating a reduction in differential approximation errors. This, in turn, establishes a heightened level of precision in phase demodulation, especially relevant across extensive sensing distances. By comparing the recovered time-domain waveforms and the corresponding power spectral densities without and with the TSI, the above improvement effect has been experimentally validated by utilizing the TSI. The results show that, with the TSI, the amplitude-frequency response range of the DCM algorithm is enlarged by 2.78 times, and the new relationship among fpulse, f, and D under the root mean square error (RMSE) tolerance less than 0.1 can be expressed as 1.9(D+1)f ≤ fpulse. This contribution underscores a substantial advancement in the capabilities of the DCM algorithm, holding promise for refined performance in optical fiber sensing applications.

In this paper, a theoretical analysis of how the excitation conditions affect the sapphire fiber Fabry-Perot interferometer (SFPI) visibility was performed. The conditions were considered, in which an SFPI was excited by a single-mode fiber (SMF), a multimode fiber (MMF), and a fiber collimator. The finite difference method (FDM) was used to realize the numerical solution of the modal electric fields, and then, the modal excited distributions in the sapphire fiber and the SFPI visibility were calculated. The results showed that different numbers of modes were excited in sapphire fibers under different excitation conditions and finally affected the fringe visibility of the SFPI. The fiber collimator excited the fewest modes and the visibility remained at the highest level. Finally, an experiment was performed, and the experimental results agreed well with the theoretical results.