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• Optical fiber sensors possess advantages including small size, quick response, resistance to electromagnetic interference and long-range monitoring capabilities. • Multiple optical principles have been discussed for each of their merits and drawbacks in heavy metal ion sensing. • Different types of materials that could be utilized for trace heavy metal ion detection have also been discussed for further comparison. • The analysis on the current state and future development direction were numerated in different aspects. The ecological environment suffers from multiple pollution from many aspects, and the heavy metal ions (HMI) pose a significant threat to human health. Detecting these ions is challenging due to their trace amounts and diverse types. Optical fiber sensors offer numerous advantages over conventional techniques in HMI sensing, including small size, quick response, resistance to electromagnetic interference, and long-range monitoring capabilities. In this paper, several optical methods for HMI detection have been described, namely fluorescence, fiber grating, fiber modal interference, surface plasmon resonance, and optical absorbance. The principles, structures, and sensitive materials of these methods are discussed in detail. The advantages and disadvantages of each approach are illustrated to facilitate a better comparison. Furthermore, the current status and future directions for the development of HMI optical fiber sensors are also discussed, which could enlighten the future HMI sensor design. [ABSTRACT FROM AUTHOR]

Optical magnetic field sensors have gained prominence due to their compact size, high sensitivity, and broad dynamic range. Photonic crystal fiber (PCF) sensors have emerged as a significant role in optical sensing. Traditional numerical design methods of PCF such as finite element method (FEM) are computationally demanding and often restrict the design space due to iterative trial-and-error processes. In this work, a bidirectional design approach using a deep learning model based on deep neural networks (DNNs) and augmented gray wolf optimizer (AGWO) is proposed for surface plasmon resonance (SPR)-based PCF magnetic field sensor. This bidirectional design method contains forward modeling and inverse design. The forward modeling predicts the optical responses of sensor structures accurately in 55.99 ms with an ${R}$ -squared value of 0.9942. The inverse design approach, with a tandem network configuration for addressing the nonuniqueness challenge in inverse design, identifies the sensor design aligning with desired optical responses in less than 0.5 s. In addition, AGWO is utilized to further enhance the accuracy and convergence ability of DNN. The key results indicate that our proposed approach reduces computational effort and enhances design effectiveness in PCF sensor design, which can also offer an alternative method for the design of various optical devices.

• This is the first time that cascaded FWM in HNLF–GO is investigated. • GO is a good nonlinear optical material for nonlinear wavelength conversion. • FWM conversion efficiency increases to a maximum of −14.5 dB. • The maximum spacing between signal and pump is 21 nm. The all-optical wavelength conversion using cascaded four-wave mixing (FWM) phenomena with graphene oxide (GO) and a highly nonlinear fiber (HNLF) device is demonstrated experimentally. GO has a strong third-order nonlinear effect and is an excellent nonlinear optical material for nonlinear optical wavelength conversion. The group velocity matching of pump and signal, HNLF, and GO amplification promote the cascaded nonlinear frequency mixing process. From the experimental and analytical results, the maximum spacing between signal and pump is 21 nm, and specifically, the order of cascaded FWM light increases from order 1 to order 2 with increasing GO, and the first-order FWM conversion efficiency increases to a maximum of −14.5 dB. To the best of our knowledge, this is the first time that cascaded FWM-based all-optical wavelength conversion in HNLF–GO with wide wavelength selectivity is investigated with combined pump and signal light. Our findings not only provide an effective method for achieving all-optical wavelength conversion in cascaded FWM but also offer the possibility of fabricating high-performance nonlinear optical devices. [Display omitted] [ABSTRACT FROM AUTHOR]

The rapid development of wearable electronic devices and the diversification of social needs urgently demand high-performance wearable pressure sensors capable of stable operation in special environments. In this article, a flexible pressure sensor composed of biphasic multidimensional hybrid conductive nanocomposites (CNCs) and surface microstructures was proposed. Changes of conductive path happened both at microscopic (CNC-induced) and macroscopic (microneedle-induced) levels, synergistically improving the sensor’s performance. The influence of conductive filler and surface microstructure on the sensitivity and sensing range of the sensor was systematically investigated, revealing the intrinsic connection between material property, geometric structure, and sensing performance, and yielding the optimal fabrication parameters. In addition to high sensitivity over large sensing range, the proposed sensor also provided a fast response time, was unaffected to temperature fluctuations, and exhibited good repeatability and environmental stability. To mimic human fingertip skin, a sandpaper template was used to create external microstructures on the lower electrode of the pressure sensor, enabling the recognition of object roughness and further expanding the sensor’s application range. Additionally, the sensor’s good waterproof property made it suitable for underwater applications. As a proof of concept, the sensor was employed for underwater activity detection and information transmission.

Microwave ablation is a minimally invasive technique using microwave energy to heat and destroy abnormal cells in tumors, in which temperature monitoring is critical to achieve the desired thermal therapy effect while protecting surrounding healthy tissue. This article proposes for the first time using a surface plasmon resonance (SPR)-based temperature sensor for the real-time monitoring of tissue temperature changes during microwave ablation. The SPR sensor has the advantages of simple structure, low cost, large measurement range, and satisfactory sensitivity. The SPR sensor adopted a multimode fiber (MMF)–thin-core fiber (TCF)–MMF structure, yielding a sensitivity of $-1.781 \pm 0.031$ nm/°C in the range of $22~^{\circ }$ C– $94~^{\circ }$ C. Furthermore, a commercially available fiber Bragg grating (FBG) array sensor was used as a reference, confirming the effectiveness of the proposed SPR sensor in the application.

A widely tunable third-harmonic generation (THG) in a mid-infrared step-index tellurite optical fiber was investigated in this work. By pumping a 3 cm-long tellurite optical fiber using an optical parametric oscillator with the pump wavelength of 1180~3500 nm, a tunable THG from 393 to 1167 nm was achieved. To the best of our knowledge, this is the broadest THG in the optical fiber. The THG conversion efficiency with different fiber lengths was investigated, which showed that the THG conversion efficiency decreased with the fiber length increasing. In addition, the third-harmonic (TH) signal pattern showed that it did not arise from phase matching between the fundamental mode of the pump and the higher-order mode of the TH signal, but rather resulted from the tellurite optical fiber’s high nonlinearity. It provides a unique pathway for the development of new light sources.

In this paper, we propose and demonstrate a ratiometric fluorescence temperature sensor based on an innovative silica-tellurite composite, which is capable of sensing dynamic human thermal information in real time. The temperature-sensitive upconversion fluorescence emission is generated by a miniaturized tellurite glass tip upon near-infrared excitation and collected by silica optical fiber channels. The ambient temperature information is demodulated via a fluorescence intensity ratio technique. This is the first time that Er3+/Yb3+ co-doped tellurite glass, which has fragile mechanical strength and a distinctive softening temperature, has been successfully extracted into a thin-walled silica tube using a negative pressure method. This provides a robust packaging for tellurite glass, allowing it for reliable thermometry in various complex scenarios. When applied to a human body, the sensitive tip located in the armpit and tongue fossa can continuously and accurately acquire the thermal information with a fast response time within 1 s. With its miniature size, robust packaging, intrinsic safety, fast response, and high reliability, the proposed ratiometric fluorescence sensor holds great potential for real-time human (in vivo/ex vivo) health assessment in clinical medicine, personalized healthcare, human-machine interfaces, and other intricate scenarios.

The development of flexible pressure sensors with ultraviolet (UV) shielding property is of great significance for outdoor activity monitoring devices. In this work, we propose a flexible pressure sensor based on polyacrylonitrile (PAN)/ TiO2/MXene nanofiber membrane by combining the electrospinning and dip-coating methods, and the addition of TiO2 nanoparticles (NPs) and MXene flakes greatly improves the UV-protective performance of the sensor. The high ultraviolet protection factor (UPF) of the proposed sensor is ~24716, and it has a relatively stable transmittance for UV light of different power. This UV-protective sensor not only shows excellent pressure sensitivity (0.15148 kPa−1) in a wide range of 350 kPa but also has excellent breathability as well as good cycle stability. The results show that the PAN/TiO2/MXene sensor has excellent UV protection performance and pressure sensing ability, which can be used for outdoor activity monitoring. It can monitor various sports conditions when people are outdoors, demonstrating broad application prospects in fields such as intelligent sports and wearable electronics.

The dependence of fluorescence intensity on temperature change has made fluorescent microsphere temperature measurement a widely available technology. In this work, blue and orange polystyrene microspheres were introduced into sodium carboxymethyl cellulose (CMC-Na) hydrogel to successfully construct a dual-emission fluorescent material. This dual-phase material inherits the excellent optical properties of microspheres and the gel property of CMC-Na and shows long-term thermal and structural stability. Single-wavelength ultraviolet excitation (365 nm) shows that the emission peaks at 429 and 580 nm are different from the fluorescence quenching caused by temperature change, providing a ratio measurement method for temperature measurement. This material is then packaged with the tip of the quartz optical fiber to produce the fluorescence intensity ratio (FIR) optical fiber temperature sensor. The experimental results show that FIR has a linear relationship with temperature from 10 °C to 60 °C, and the sensitivity of temperature is 0.00466 $^{\circ }\text{C}^{-1}$ . In addition, the satisfactory reliability, stability, and timely response characteristics of the sensor prove that the proposed fluorescent material is an ideal choice for temperature sensing and has great application potential in this field.

In this article, we propose and experimentally demonstrate a simple and low-cost fiber-optic sensor for temperature-independent ultraviolet (UV) detection. The sensor utilizes a photosensitive material, i.e., acrylic and hydrogenated epoxy resin, which is tightly wrapped around one end of a multimode optical fiber (MMF) and processed by light curing and heat curing. Upon absorbing UV radiation, the phosphor in the resin compound undergoes population inversion, resulting in the emission band around 480 nm, and the peak intensity of the emission band is positively correlated with the UV excitation intensity. The UV sensing characteristics of the sensor are experimentally investigated by tracking the photoluminescence peak intensity of the output spectrum, demonstrating a fast response of 0.1–0.2 s, high sensitivity of 315.78 (mW/cm2)−1, small absolute error within −0.28 to 0.42 mW/cm2, and good repeatability ( $R > $ 95%) and stability (RSD < 1.3%). In addition, the peak intensity is proved to be insensitive to temperature variation: as temperature varies from −20.5 °C to 56.3 °C, it shows only a small fluctuation, relative standard deviation (RSD) being less than 4.7%, indicating that the photothermal effects have negligible effect on the UV measurement accuracy. With the advantages of simple structure, easy fabrication, low cost, and excellent performance, the proposed sensor is expected to be used for reliable UV detection in practical applications.

Er3+/Yb3+ co-doped tellurite glass (TeO2-ZnO-WO3-Li $_{{2}}\text{O}$ -BaF2-Nb2O3-Er2O3 + Yb2O3, TZWLBN) was investigated in the range of 253–443 K, and by varying the molar ratio of Yb3+ ions, the ion concentration of the strongest luminescent emission was found. Based on it, by combining mold casting and fiber drawing techniques, a no-core tellurite fiber (NTF) was fabricated, and then connected with two multimode fibers (MMFs) to form a temperature sensor. Under a pumping power of 2.20 mW, a comprehensive evaluation was conducted to investigate the temperature sensing characteristics of this sensor, employing the fluorescence intensity ratio (FIR) technique. The repeatability and stability of the experimental protocol were established through numerous heating-cooling cycles and constant temperature test. The highest possible absolute and relative sensitivities were determined as 0.00264 and 0.0114 $\text{K}^{-{1}}$ . The temperature sensor exhibited a maximum temperature error of 0.88 K when employed for monitoring the temperature of the metal–oxide–semiconductor field effect transistor (MOSFET) in complex circuits, thereby demonstrating its practical applicability in thermal detection scenarios.

In this article, we propose a magnetic field sensor based on a multimode-D-type-multimode (MMF-D-type-MMF) fiber structure employing surface plasmon resonance (SPR). The sensor’s surface is coated with a layer composed of indium tin oxide (ITO), silver (Ag), and a 2-D material known as Bi2O2Se. Bi2O2Se exhibits varying magnetic permeability within a specific magnetic field strength range, a characteristic confirmed in practical tests. To enhance the sensor’s sensitivity to magnetic fields, we implemented several optimization measures. These measures encompassed adjustments to: 1) the concentration of the magnetic fluid (MF); 2) the thickness of the Bi2O2Se layer; and 3) the selection of fiber type. We conducted simulations and experimental tests to refine these parameters. Consequently, the proposed D-type sensor demonstrates a sensitivity of 0.4325 nm/GS within the 40–150 GS magnetic field strength range, representing a significant 75.31% sensitivity improvement achieved through the aforementioned enhancement measures. When compared to other fiber-optic magnetic field sensors, our proposed sensor stands out for its exceptional sensitivity, straightforward design, robust repeatability, strong anti-interference capabilities, and high stability. As a result, it holds significant potential for a wide range of applications in aerospace, aviation, environmental monitoring, and related fields. The sensor has a high sensitivity in the range of 40–150 GS, which can monitor the magnetic field strength in life in real time and determine whether it meets the human body safety magnetic field standard.

In this article, Er $^{{3}+}$ /Yb $^{{3}+}$ co- doped TeO2-Li $_{{2}}\text{O}$ -ZnF2-WO3-Nb2O5 (TZWLN) glasses were prepared using the melt annealing methods, and the rare Earth (RE) ion concentrations were optimized to achieve the strongest green up-conversion (UC) luminescence. Based on it, a temperature sensor drawing on the fluorescence intensity ratio (FIR) technique was developed via a dual solidification process for real-time temperature monitoring of automotive 3-D LiDAR (A3DL). The sensing performance of the sensor was evaluated in the temperature range of 252 to 453 K. The maximum absolute sensitivity ( ${S}_{a}{)}$ was $3.98\times 10^{-{3}}\,\,\text{K}^{-{1}}$ at 453 K, and the maximum relative sensitivity ( ${S}_{r}{)}$ was 0.0146 $\text{K}^{-{1}}$ at 252 K. Global rapid thermal cycling repetition experiments and local temperature jump stability experiments were conducted, and temperature information was demodulated using the FIR mathematical model. In the repetition experiments, the maximum residual sum of squares (RSSs) was $9.98\times 10^{-{3}}$ , and in the temperature jump experiments, the standard deviation of demodulated temperatures was 0.064 and 0.063. It is worth noting that the maximum measurement error of the sensor in the real-time temperature detection of the onboard LiDAR was only 0.77 K. To the best of our knowledge, this was the first report on using fluorescent temperature sensors for real-time temperature detection of A3DL. The proposed sensor has the advantages of excellent stability and repeatability, resistance to electromagnetic interference and corrosion, as well as the capacity to avoid cross-sensitivity. It offers an effective and reliable measurement scheme for real-time temperature detection in A3DL.

In this article, a Mach Zehnder interference temperature sensor is proposed by tightly joining a double-cladding tellurite optical fiber (DTF) between two sections of silica single-mode fiber (SMF) using the fusion splicing method. In the real-time temperature measurement of glycerin solution, this Mach-Zehnder Interferometric (MZI) sensor exhibits a temperature sensitivity as high as 313.8 pm/°C in the range of $10 ^{\circ} \text{C}\sim 60 ^{\circ} \text{C}$ . The stability-induced temperature error and the maximum repeatability standard deviation are $\pm 0.5 ^{\circ} \text{C}$ and $\pm 0.4 ^{\circ} \text{C}$ , respectively. Attractively, the sensor is virtually unresponsive to changes in refractive index of the external environment, making it suitable for temperature measurement of a wide range of high-refractive-index organics. Therefore, the sensor is expected to be further applied in the manufacture, storage and application of chemical reagents.

Paper-based flexible sensors are of great significance for promoting the development of green wearable electronic devices due to their good degradability and low cost. In this work, a paper-based wearable pressure sensor with a sandwich structure is proposed, which is assembled from a sensing layer printed with Ti3C2TxMXene ink, an interdigitated electrode printed in the same simple and economical way, and two polyethylene terephthalate films. The demonstrated paper-based pressure sensor exhibits excellent sensitivity in a wide pressure sensing range, as well as cyclic stability at a certain pressure. The sensor can be attached to the human body’s surface to monitor various pressure-related physical activities. Using a self-designed mobile phone APP, the special pressure signals collected from the sensor can be transmitted and translated, and an intelligent and encrypted information transmission system can be established. Since only ordinary printing paper and Ti3C2TxMXene ink are used, the pressure sensor is easy to prepare, economical, and environmentally friendly, and it can be degraded by stirring in water without generating electronic waste. It can be foreseen that the proposed sensor shows bright application potential in the sustainable development of healthcare and human–computer interaction fields.

At present, wearable flexible pressure sensors have broad application prospects in fields such as motion monitoring and information transmission. However, it is still a challenge to design flexible pressure sensors with high sensitivity over a large sensing range and simple fabrication. Here, we use a simple “dipping-drying” method to fabricate a fabric-based flexible pressure sensor by coupling silver nanowires (AgNWs) with Ti3C2-MXene. The interaction between MXene and AgNWs helps realize a dual-layer sensing network, achieving good synergistic effects between pressure sensitivity and sensing range. The effects of the material combination and dip-coating sequence on the sensor’s performance are systematically studied. The results show that the sensor was impregnated sequentially with AgNWs solution, and the MXene solution has the highest sensitivity (0.168 kPa–1) over a wide range (190 kPa). Meanwhile, it has the advantages of low response hysteresis and detection limit, as well as good linearity and durability. We further demonstrate the application of this sensor in human physiological signal monitoring and motion pattern recognition. It can also encrypt and transmit information according to different pressing states. In addition, the proposed pressure sensor array exhibits spatial resolution detection capabilities, laying the foundation for applications in the fields of motion monitoring and human–computer interaction.

In this paper, we propose and experimentally validate passively mode-locked erbium-doped fiber lasers utilizing CNFs as saturable absorbers (SAs). A mode-locked pulse was observed, with a central wavelength of 1562 nm and a bandwidth of 3.56 nm after inserting the CNF film into the ring cavity. The pulse duration and repetition rates are 1.28 ps and 54 MHz, respectively. The maximum output power can reach 1.93 mW. To our knowledge, this is the first report of CNF as SA in erbium-doped fiber lasers.

It is a challenging and valuable work to design and manufacture temperature-compensated all-fiber ultraviolet (UV) sensor. In this article, we creatively use a simple and compact optical fiber sensor based on surface plasmon resonance (SPR) for simultaneous detection of UV and ambient temperature. Different coatings [Ag/polydimethylsiloxane (PDMS) and Ag/ZnO/PDMS] are used to split a length of no-core optical fiber (NCF) into two sensing regions, which generates a transmission spectrum with two distinct SPR dips. The proposed sensor achieves a temperature sensitivity of −2.542 nm/°C in the 15 °C–35 °C range and a UV sensitivity of −0.605 nm/(mW/cm2) in the range of 0–17.20 mW/cm2. The matrix established by the wavelength shift of two dips can demodulate UV and temperature changes, and provide temperature compensation for UV detection. In addition, even in surroundings of complex and changeable UV intensity and ambient temperature, the sensor shows excellent stability and repeatability, as well as great potential in UV sensing, providing a reference for the development of multiparameter optical fiber sensors.

A novel combination of structural design and sensing theory is proposed for an optical fiber magnetic field and temperature two-parameter sensor based on the Mach–Zehnder interferometer (MZI) and Faraday rotation effect. The sensor structure includes a polydimethylsiloxane (PDMS) microcavity and a small Section of offset spliced fiber. Numerical analysis shows that the sensor exhibits high-temperature sensitivity, which has been experimentally verified. The sensor has a temperature sensitivity of −846 pm/°C in the 20 °C to 35 °C and a magnetic field intensity sensitivity of 190 pm/Gs in the range of 0–150 Gs. The temperature and magnetic field intensity responses of the sensor are linear and have been optimized for sensitivity demodulation. The sensor is well-suited for standardized monitoring of temperature and magnetic field in applications requiring accurate measurements.

A type of surface plasmon resonance (SPR) magnetic field sensor based on Au/gold nanoparticles (AuNPS)/magnetic fluid (MF)-processed hollow core fiber is proposed in this work, which fully utilizes SPR’s high sensitivity to refractive index (RI) and the magnetron RI characteristic of MF. The metal material Au excites SPR, and AuNPS excite the localized surface plasmon resonance (LSPR). The coupling of surface plasmon polariton (SPP) with localized surface plasmon (LSP) increases the local electric field strength, thus improving the sensor’s sensitivity. Both the theoretical analysis on the sensing mechanism and the experimental findings demonstrate that this novel SPR magnetic field sensor has a wide measurement range and high sensitivity. In the range of magnetic field strength from 0 to 603 Gs, the sensitivity can reach up to 156 pm/Gs, and the linearity is good (the coefficient of determination is 0.996). Compared with other optical fiber magnetic field sensors, the sensor proposed in this article has the benefits of easy fabrication, excellent sensitivity, simple structure, strong anti-interference ability, and wide measurement range, thus exhibiting significant application potentials in the fields of military drills, aerospace, etc.

A label-free glucose biosensor has been proposed and demonstrated in this article based on a tapered two-mode fiber (TTMF) sandwiched between two single-mode fibers (SMFs). The tapering technique helps excite the ${\textit {LP}}_{{11}}$ mode in TTMF, forming uniform interference fringe containing the ${\textit {LP}}_{{01}}$ and ${\textit {LP}}_{{11}}$ modes. The experimental results show that the sensor’s sensitivity increases, as the TTMF waist diameter decreases, and at the TTMF diameter of $13.12 \mu \text{m}$ , the maximum glucose sensitivity can reach 0.112 nm/mg/mL, while the temperature sensitivity is as low as 16.43 pm/°C. With compact structure, label-free sensing capacity, and ease fabrication, this proposed biosensor has bright application prospect in the fields of food safety, disease diagnosis, and clinical analysis.

An antiinterference temperature sensor based on the Mach-Zehnder interferometer (MZI) is proposed, which is formed by successively fusing a homemade Kagome hollow-core photonic crystal fiber (HC-PCF) with two no-core fibers (NCFs) and two single-mode fibers (SMFs). Because of the antiresonant reflection optical waveguide properties, the interference between hollow-core mode and glass mode is achieved in Kagome HC-PCF, which significantly simplifies the overall sensing architecture and maintains better stability of the sensing signal. Using a Kagome HC-PCF length of $497 \mu \text{m}$ , the maximum temperature sensitivity of the proposed sensor is obtained as 38.39 pm/°C. Meanwhile, it exhibits a low sensitivity of 0.5041 pm/ $\mu \varepsilon $ to ambient stress variation, −0.28 nm/ $\text{m}^{-{1}}$ to curvature change, and 9.35 nm/RIU to external refractive index (RI) change. This MZI temperature sensor has the advantages of compact structure, easy fabrication, low cost, small size, high sensitivity, and antiinterference capability, which can be expected to be applied to temperature sensing in complex environments.

In this article, a Section of ultrafine no-core fiber (UNCF) is fused between two sections of single-mode fiber (SMF) to form a multimode interference (MMI)-based sensor, which enables dual-parameter measurement of liquid level and temperature. The effect of UNCF diameter on the liquid-level sensing performance is analyzed theoretically and experimentally, and the results show that the sensitivity can be improved by reducing the UNCF diameter. When the UNCF diameter is reduced to $30 \mu \text{m}$ , the sensitivity of the sensor is 1008.2 pm/mm for water with a refractive index (RI) of 1.333 in the liquid-level range of 0–30 mm. Its temperature sensitivity is −221.02 pm/°C over a detection range of 5 °C–60 °C. Attractively, the sensor shows almost no response to pressure variation. When applied to the liquid-level measurement of a water cooler, the sensitivity is 1898 pm/mm. The sensor has the advantages of high sensitivity, good repeatability and stability, and satisfactory anti-interference ability. It has great potential for applications in the health management of lasers, chips, transformers, and other instruments.

Biosensors have important implications for medical research and disease prevention. High-quality optical microcavities with whispering-gallery modes (WGMs) can enhance light-matter interactions in small volumes, making them ideal for making biosensors. However, the sensitivity of the optical microcavity biosensors can further improve. The sensing performance of the biosensor based on a microtube or microbubble can be further improved by exceptional points (EPs) in non-Hermitian physics. Non-Hermitian systems are open systems, exchanging energy with the external environment. EPs as one representative feature of non-Hermitian systems are a branch point singularity, where eigenvalues and their corresponding eigenvectors coalesce in the parameter space of the system. In the vicinity of EPs, the eigenvalues exhibit non-trivial topology, which can be exploited for highly sensitive detections. Based on this principle, a novel configuration with multiple layers is proposed, where the coupling of modes can be tailored for EPs by choosing the thickness of layers. Simulated results demonstrated that the design has strong biosensing performance. The sensitivity can be enhanced by 1000 times more than that of conventional methods. The ability to prevent environmental interference has also been improved. This is significant for highly sensitive low-concentration biomolecule detection in microtubes in the future.

In this article, the temperature sensing characteristics of two fundamental solitons in a glycerin-filled six-hole microstructured optical fiber (MOF) were systematically exploited through experiment and simulation. The six-hole MOF has a large duty cycle, which can increase the space to fill the temperature-sensitive material glycerin for improving the temperature sensing sensitivity. By detecting the center wavelength shift of 3 dB bandwidth of two fundamental solitons at 1050 nm, 390 mW, a maximum sensitivity of −0.601 nm/°C was obtained based on the first fundamental soliton and −0.598 nm/°C was obtained based on the second. Simulation results combined with the experimental results showed that the higher the average pump power, the higher the temperature sensitivity, and the temperature sensitivity of the first was slightly higher than that of the second. The difference in sensing characteristics between the two fundamental solitons discovered in this study could provide a novel solution to solve the cross-sensitivity of dual-parameter sensing. To the best of our knowledge, this is the first research about utilizing two fundamental solitons for temperature sensing, which broadens the boundary of the nonlinear-based sensors, and has promising prospects in the fields of national defense security, disease monitoring, agriculture production, etc.

In this work, Q-switched pulses were achieved by using silver sulfide (Ag2S) films as saturable absorbers. Ag2S nanoparticles (NPs) were synthesized by a hydrothermal method. By inserting the Ag2S film into an erbium-doped fiber laser cavity, Q-switching was realized with a central wavelength of 1558 nm for the first time. Interestingly, by integrating the single mode-multimode-single mode fiber sensing structure into the Q-switched fiber laser, the all-fiber refractive index sensing device was realized based on the principle of multi-mode interference. For alcohols with different refractive indices, the red-shift of the central wavelength could be observed with a sensitivity of 251 nm/RIU. These results confirmed that Ag2S NPs possess great potential in nonlinear optics and optical sensing.

In this paper, a reliable anti-interference probe based on fluorescence intensity ratio technique is proposed for cryogenic temperature detection in refrigeration process, the sensing unit of which is based on a self-made coreless tellurite optical fiber (TOF) co-doped with Er3+/Yb3+ ions. The temperature information was demodulated by analyzing the fluorescence spectral changes. In order to meet the needs of practical applications, the dual-curing method was applied to realize the TOF splicing, and the thin-walled silica tube was used to provide robust packaging. For the first time, the anti-cross-sensitive properties of the fluorescent probe under vibration and liquid interference were quantified, and the repeated response deviation was on the order of 10−5. The actual working capacity of the probe was experimentally investigated by applying it to measuring the circulating exhale and cryogenic equipment, in which a fast time response of about 2.11 s and a small absolute error of 0.80 K were obtained. The probe has good stability and repeatability, and can work stably even in complex environments with vibrations and liquids, showing strong anti-interference ability. In addition, the robust package and easy method make the probe suitable for mass fabrication.

Although fiber-based flexible piezoresistive pressure sensors have received extensive attention because of their simple fabrication and easy integration, the common practice of using a single material as the sensing layer often leads to unsatisfactory sensitivity and a limited sensing range. Herein, we exploit the combination of reduced graphene oxide (rGO) and two-dimensional transition-metal carbides and nitrides (MXene), use a polyester filament (PET) as the fiber matrix, and fabricate an MX/rGO PET-based flexible pressure sensor using the “dipping-drying” method. A systematic study is conducted concerning the effect of the dip-coating sequence and material combination on the sensor’s resistance and sensitivity, which reveals that MX/rGO PET has the smallest resistance and the highest sensitivity (1.24 kPa–1). A series of tests are conducted to evaluate the pressure sensing characteristics of the MX/rGO PET-based pressure sensor, confirming its good linearity, fast response speed, low detection limit, and stable performance. In addition, the sensor has been successfully used to monitor various human joint activities and physiological signals such as breathing, demonstrating great application potential in the field of personal health care. To further enhance the practical utility, an APP has been designed to analyze and display the collected signals, and the constructed sensor network also provides an ingenious method for information encryption and transmission via pressure sensing. In all, the MX/rGO PET-based pressure sensor proposed in this work is expected to provide a competitive scheme for wearable flexible electronic devices in information transmission and human–computer interaction in the future.

In this paper, a compact fluorescent probe based on fluorescence intensity ratio (FIR) technique is proposed for real-time thermal monitoring of chips, the sensing unit of which is based on a self-made tellurite optical fiber (TOF) co-doped with Er3+/Yb3+ ions. Temperature information was demodulated by analyzing the up-conversion fluorescence emission spectra generated using a 980 nm laser pump. Experimentally characterized the relationship between the FIR related to two thermally coupled energy levels and the ambient temperature, and on-line investigated the temperature change of Central Processing Unit (CPU) and North Bridge Chip for about 80 min during the cycle startup/shutdown process. The proposed probe exhibits a fast time response of about 1.1 s in a step temperature-varying environment and a small absolute error of &lt;0.85 K in the real-time thermal monitoring of the CPU. To the best of our knowledge, this is the first report on fluorescent probes for thermal detection of chips. The proposed probe has the characteristics of anti-electromagnetic interference, explosion-proof, corrosion resistance, and can effectively avoid cross-sensitivity problems caused by environmental parameters. It provides a novel and reliable method for real-time thermal monitoring of chips.

This paper proposes a dual-core photonic crystal fiber (PCF) refractive index sensor. And we use the full vector finite element method (FEM) under the anisotropic perfect matching layer (PML) boundary condition to study the effects of various parameters of the sensor on performances. By two airholes which filled with the liquid to be analyzed, the two cores coupled with each other, the refractive index range is 1.36–1.44. Numerical calculations were carried out on the sensor characteristics. The simulation results show that: when the refractive index range is 1.36–1.42, the average sensitivity can be up to 10,315 nm/RIU (refractive index unit), the highest sensitivity can be up to 13,502 nm/RIU when the liquid refractive index is 1.42, the quadratic fitting coefficient R12= 0.99969. And when the refractive index range is 1.42–1.44, the average sensitivity can be up to 17,980 nm/RIU, the highest sensitivity can be up to 22,380 nm/RIU when the liquid refractive index is 1.44, the quadratic fitting coefficient R22= 0.99999. The sensor we proposed is suitable for biomolecules and chemistry, as its conventional wide detection range with high sensitivity. Most importantly, a new idea of broadening the detection range is put forward for reference.

In this paper, an annular winding structure made of single-mode optical fiber is proposed as the core of a cost-efficient and reliable annular optical fiber temperature sensor (AOFTS). The sensor is mainly due to the multi-mode interference effect to achieve real-time response to external temperature changes. The experimental results show that the average temperature sensitivity of the sensor is about 255.5 pm/°C in the temperature range of −20^∘C−110^∘C; it has higher sensitivity at low temperatures. At −20^∘C, the sensitivity of the AOFTS reaches 450 pm/°C. The sensor has the advantages of simple fabrication, low fabrication cost, strong stability, and good reproducibility and repeatability. It has great application prospects in the field of low-temperature detection.

In order to improve the performance and simplify the structure of an optical antenna for a space laser communication system, we design a free-form off-axis three-mirror optical antenna with an integrated primary/tertiary-mirror structure. The adoption of the integrated primary/tertiary-mirror structure improves efficiency of light energy utilization and reduces the complexity of optical processing and assembly. The introduction of free-form optical elements and optical structure constraints helps to correct the off-axis aberration and realize a large field of view. The obtained optical antenna has the magnification of five times and field of view of 2.4^∘×2.4^∘. The image quality obtained here reaches the diffraction-limited level. At the communication wavelength of 808 nm, the wavefront error is better than λ/22, and the system has a high energy concentration. The proposed optical antenna could not only improve tracking accuracy of the optical antenna in space but also greatly reduce the complexity of the laser communication system. It has reference significance and application value for free-space laser communication.

As a new type of carbon-based material, carbon nanofibers (CNFs) have attracted much attention due to their unique physical structure and optical properties. In this paper, we propose the application of CNFs as the saturable absorber (SA) and established a passively mode-locked thulium-doped fiber laser (TDFL) for verification. By mixing sodium carboxymethyl cellulose solution with CNFs, CNF SA was prepared, the nonlinearity of which was tested as follows: the modulation depth was ∼1.3%, and the saturation intensity was 18MW/cm^2. By inserting the CNF SA into the TDFL ring cavity, mode-locked laser pulses of a central wavelength of 1954.47 nm and a 3 dB bandwidth of 5.93 nm were obtained. The spectral pulse width was 1.31 ps; the repetition frequency was 32.68 MHz; and the signal-to-noise ratio (SNR) was calculated to be ∼57dB. To our knowledge, this is the first time that CNFs have been reported as SAs for mode-locked lasers in the 2 µm wavelength region. Our work provides a new reference for using carbon-based materials in the realization of ultrafast lasers, and the proposed CNFs are highly advantageous in the development of ultrahigh-speed optical modulators and next-generation high-performance nonlinear photonic devices.

We demonstrate broadband mid-infrared cascaded stimulated Raman scattering (SRS) and flat supercontinuum (SC) generation in a chalcogenide optical fiber made from As_2S_5 glass. By using a 2 µm nanosecond laser as the pump source, mid-infrared cascaded SRS up to six orders ranging from 2149 to 3425 nm was experimentally observed, and this all-fiber Raman laser operating at 3.43 µm was realized for the first time to our knowledge. By introducing a 2 µm femtosecond laser as the excited source, the broadband flat mid-infrared SC with the spectral range of ∼10dB (from ∼1030 to 3441 nm) was observed. Our results verify that the As_2S_5 optical fibers possess promising applications for tunable mid-infrared Raman fiber lasers and SC light sources pumped by 2 µm pulsed lasers.

Semiconducting polymer nanoparticles (SPNs) possess many special photophysical and chemical properties. However, little research has been done on the potential of SPNs in laser technology. In this work, we present the ultrafast pulses generation at 1.5 and 2 μm through proposing the deposition of SPNs onto a side-polished fiber (SPF) platform as the nonlinear optical modulator. SPNs are designed and prepared through a combination of density functional theory and nanoreprecipitation method. The prepared SPNs not only exhibit strong linear absorption but also has nonlinear saturable absorption characteristics. Once SPF-SPNs saturable absorber (SA) is integrated into the laser cavity, ultrafast lasers of 1.5 and 2 μm can be achieved with high performance. Our results show that SAs based on the SPNs and SPF are promising nonlinear optical modulators for broadband ultrafast pulse generation.

A surface plasmon resonance (SPR) temperature sensor based on a photonic crystal fiber (PCF) filled with silver nanowires is proposed in this paper. We inject ethanol solution filled with silver nanowires into the grapefruit PCF to realize temperature sensing. The sensitivity of the sensor can reach −433pm/^∘C by numerical simulation, and the experimental result is −160pm/^∘C. Simulations and experiments show that the wavelength of the resonance peak will blueshift with the invalidity of silver nanowires, and the resonance effect of the sensor will weaken. It can provide reference for the realization and application of other SPR sensors based on PCF.

A high-loss broadband photonic crystal fiber polarization filter based on surface plasmon resonance is designed. The designed filter has two symmetrical large holes, and the holes are coated with a metal film, which greatly improves the loss. The characteristics of the polarization filter are calculated by the finite element method. The thickness of the metal layer and the arrangement and size of air holes near the core are discussed. The optimum structure of a metal-filled polarization filter in photonic crystal fibers is obtained. By modulating the parameters, the loss of the filter in the $y$y direction at 1.31 µm is 3261.7 dB/cm, while the loss in the $x$x direction is small. If the length of the optical fiber is set to 200 µm, the extinction ratio is greater than 20 dB, and the width is 1 µm, respectively. The filter has a simple structure and excellent performance.

A step-index tellurite optical fiber with loss ∼0.02  dB/m at ∼1545  nm was fabricated based on TeO_2−Bi_2O_3−ZnO−Na_2O glass. With a nanosecond laser operated at ∼1545  nmas the pump source, second-harmonic generation (SHG) was observed in the 2.5 m tellurite fiber with conversion efficiency up to ∼1.15% at 20 mW, which, to the best of our knowledge, is the highest for non-silica optical fibers. The experimental phenomenon can be explained by the χ^(2)-induced model of nonlinear polarization at the core–cladding interface. Furthermore, it can also be explained by a χ^(3)-induced model via the four-wave-mixing effect, where a quasi-SH signal and a millimeter wave were generated simultaneously. This new model may help shed light on the physical mechanism of SHG in optical fibers.

In this study, we design a refractive index (RI) sensor using a novel cadmium telluride photonic crystal fiber (TPCF). Based on four-wave mixing (FWM), the changes in RI can be accurately detected, and RI sensing in the mid-infrared region (MIR) can be achieved by detecting wavelength shifts in the Stokes and anti-Stokes spectra caused by the changes in RI of the liquid to be measured. When the pump wavelength of FWM lies in the normal and abnormal dispersion regions of the TPCF, the RI response of the idler frequency wave and the signal wave are analyzed by numerical simulation methods. The simulation results show that the RI sensitivity of the sensor can be as high as 7692 nm/RIU with a linearity is up to 99.9% at the pump wavelength of 3380 nm. To our knowledge, the RI sensing sensitivity of the MIR is presented for the first time in this study by using FWM in the non-silicon PCF.

All-optical dynamic photonic bandgap (PBG) control by an optical Kerr effect (OKE) is investigated in an all-solid double-clad tellurite photonic bandgap fiber (PBGF) which is fabricated based on TeO_2−Li_2O−WO_3−MoO_3−Nb_2O_5 (TLWMN, high-index rod) glass, TeO_2−ZnO−Na_2O−La_2O_3 (TZNL, inner cladding) glass, and TeO_2−ZnO−Li_2O−K_2O−Al_2O_3−P_2O_5 (TZLKAP, outer cladding) glass. To the best of our knowledge, this is the first demonstration of all-optical dynamic PBG control in optical fibers. This PBGF has a high nonlinear refractive index which can lead to a significant OKE and induce the generation of all-optical dynamic PBG control. The transmission spectrum is simulated with the pump peak power increasing from 0 to 300 kW, which shows an obvious PBG shift. Dynamic PBG control is demonstrated both numerically and experimentally at the pump peak power of 200 kW (ON or OFF) at the signal of 1570 nm.

In this paper, a section of tellurite no-core optical fiber (NCF) is compactly sandwiched between two segments of tellurite single-mode optical fibers (SMF) to form multimode interference (MMI) sensor for refractive index (RI) measurement. Theoretical investigation reveals that the sensitivity of this MMI-based RI sensor can be enhanced by reducing the diameter of the NCF. As the NCF diameter is lowered to 20 µm, the sensor presents good RI sensing performance in the mid-infrared range of 2-5 µm and the sensitivity rises with the increase of the wavelength. At 4-5 µm and within the detectable RI range of 1.36-1.63, the maximum sensitivity is 3069.1 nm/RIU. The proposed sensor combines the material superiority of tellurite glass and the structural advantage of the SMF-NCF-SMF sensing unit, exhibiting high sensitivity and wide detection range in terms of RI sensing. It provides a new method for detecting the concentration of toxic organic compounds with high RI.

We investigate the supercontinuum (SC) generation in an 1 cm long silica photonic crystal fiber (PCF) pumped by the pulse sources with single, dual, and triple wavelengths, respectively. The silica PCF has two zero-dispersion wavelengths at 900 and 2620 nm, respectively. When pumped by a single wavelength, the SC spectral range covers about 1000 nm. When pumped by dual and triple wavelengths, the SC spectral range covers wider than 2000 nm. Both the SC spectral range and the flatness are improved obviously when pumped by triple wavelengths. The maximum SC spectral range is obtained when the silica PCF is pumped by the triple wavelengths at 800, 1450, and 1785 nm. The SC spectral range covers 2810 nm from 350 to 3160 nm wider than three octaves. The 10 dB bandwidth covers 2280 nm from 450 to 2730 nm wider than two octaves. This is the first investigation on comparison of the SCs generated by different pump wavelengths up to three experimentally. The generated SC spectra have covered the full transmission window of silica fiber.

We experimentally demonstrate mid-infrared (MIR) supercontinuum (SC) generation spanning ∼2.0 to 15.1 μm in a 3 cm-long chalcogenide step-index fiber. The pump source is generated by the difference frequency generation with a pulse width of ∼170  fs, a repetition rate of ∼1000  Hz, and a wavelength range tunable from 2.4 to 11 μm. To the best of our knowledge, this is the broadest MIR SC generation observed so far in optical fibers. It facilitates fiber-based applications in sensing, medical, and biological imaging areas.