Multi-wavelength second-harmonic generation in a double-clad high nonlinear fiber induced by multiple phase-matching.

In this study, multi-wavelength second-harmonic generation (SHG) based on self-phase modulation (SPM) broadband supercontinuum (SC) was observed by employing a double-clad high nonlinear optical fiber (HNLF) in conjunction with a femtosecond laser. At a wavelength of 1050 nm and an average pump power of 320 mW, multiple phase-matching conditions were achieved, and SH signals of central wavelengths ∼530.7 nm, ∼525.1 nm, ∼503.5 nm, and ∼478.7 nm were observed, with SHG efficiency reaching ∼1.34 × 10-4. The SHG in this experiment can be attributed to the utilization of a doped optical fiber, where dopants create defect states, facilitating optical-chemical transformation and enhancing second-order polarization susceptibility. Additionally, theoretical simulations were conducted, aligning closely with the experimental findings. To the best of our knowledge, this work marks the first demonstration of multi-wavelength SHG in optical fibers. It offers a distinctive avenue for customizing multi-wavelength ultrafast light sources, exhibiting great application potential in the fields of medical diagnostics and optical sensing.

Generation of optical vortex beams with bandwidth exceeding 550†nm using a helical fiber needle exhibiting strong mode coupling.

A strong-coupling helical fiber needle (HFN) is proposed and demonstrated for the realization of bandwidth-enhanced broadband optical vortex beam (OVB) generation. The HFN is based on a single mode fiber and operates at the dispersion-turning-point (DTP) of the lowest radial order of the cladding mode (i.e., LP11) but with a remarkably high mode coupling efficiency. By utilizing this novel, to the best of our knowledge, HFN, successful generation of the first-order OVB with an impressive bandwidth up to 556 nm at -10 dB and a center wavelength of ∼1570 nm has been achieved. This represents the broadest bandwidth demonstrated among all fiber grating-based OVB generators to date. The proposed HFN-based OVB generator exhibits a relatively compact size, ultra-wide bandwidth, and customizable center wavelength, making it highly promising for applications in optical vortex-based endoscopic imaging as well as particle detection and manipulation.

Er3+/Yb3+/Ho3+ tri-doped no-core fiber temperature sensor based on fluorescence intensity ratio technology: towards high stability and accurate measurements.

In this paper, the green upconversion (UC) fluorescence emission from E r 3+/Y b 3+/H o 3+ tri-doped tellurite glass is investigated for temperature sensing. The doping of H o 3+ ions not only enhances the chance of energy level transition but also avoids the influence of the thermal effect caused by the proximity of 2 H 11/2 and 4 S 3/2 energy levels. The luminescence characteristics at different Y b 3+ and H o 3+ ion concentration doping molar ratios were investigated, and the strongest luminescence characteristics were exhibited when the Y b 3+ ion concentration was at 5 mol% and H o 3+ at 0.2 mol%. Based on this, a tri-doped T e O 2-Z n O-B i 2 O 3 (TZB) no-core fiber was fabricated and connected with multimode fibers (MMFs) to form a temperature sensor. The temperature sensing performance of the tri-doped TZB temperature sensor was evaluated in detail over the temperature range of 255-365 K. The repeatability and stability of the temperature sensor was experimentally verified. The E r 3+/Y b 3+/H o 3+ tri-doped sensor can be used for noninvasive optical temperature sensing in the fields of environmental monitoring, biological sensing, and industrial process temperature control, etc.

MnO2-based dual channel surface plasmon resonance fiber sensor for trace glutathione and refractive index detection.

Glutathione (GSH) plays vital role in human biological systems, so its rapid and sensitive detection is necessary for health condition monitoring. In this work, a simple structure for dual channel GSH and refractive index (RI) detection is proposed. By introducing Au-MnO2 thin film coating on the fiber surface for the first time, GSH solution would lead to the dissolution of MnO2, the change in GSH levels could be monitored over a short period in channel 2. For channel 1, ITO-Ag thin film is applied for RI change detection. After optimization, the GSH detection sensitivity reached about -2.361 nm/mM in the range of 0.005-50 mM, and the RI sensitivity reached 1704.252 nm/RIU in the range of 1.331-1.3895 RIU. Channel 1 could also put into GSH detection in the high concentration scale to enlarge the sensor's range and 0.095 nm/mM of sensitivity is acquired within the range of 50-600 mM. With the presence of MnO2 film, the detection sensitivity increased 25.663 times. Neither channel interferes with the operation of the other. Proposed sensor provides stability, high selectivity and elevation in GSH detection sensitivity, which shows great potential for environmental and biological detection field and their applications.

Multi-octave mid-infrared supercontinuum generation in tapered chalcogenide-glass rods.

We report on the experimental development of short-tapered chalcogenide-glass rods for mid-infrared supercontinuum generation. Multi-octave spectral broadening of femtosecond laser pulses is demonstrated from 1.6 to 15.6 µm in a 5-cm-long tapered Ge20Se70Te10 rod with a waist diameter of 25 µm. Despite the multimode nature of the optical waveguide used, this work clearly shows the potential of such simple post-processed rods for advancing fiber SC sources with infrared glasses, thereby unlocking new possibilities in terms of coupling efficiency, spectral coverage, and output power.

Two fiber-bundle-type FI/FO devices of low insertion loss and cross talk applied for connecting 13-core 5-mode fiber.

Two fiber-bundle-typed fan-in/fan-out (FI/FO) devices, "wavy hexagon-shaped silhouette" type (W-FI/FO) and "tortoise-shaped silhouette" type (T-FI/FO), have been proposed and manufactured based on tapering glass tubes for docking with a self-made 13-core 5-mode fiber. The W-FI/FO device consists of 19 5-mode fibers and has an extended layout based on the 13-core 5-mode fiber structure. It could dock multiple fibers with 19 or 13 cores of the same size standards. When connecting it with 13-core 5-mode, the average losses (ILs) of its five modes are 1.07 dB, 2.95 dB, 3.42 dB, 3.65 dB, and 4.38 dB. The cross talks of the five linearly polarized (LP) modes are -69.1 dB, -64.7 dB, -44.2 dB, -43.9 dB, and -39.1 dB. The T-FI/FO device has a similar core arrangement to the 13-core 5-mode fiber and its average ILs of the five LP modes are 0.23 dB, 1.31 dB, 2.09 dB, 2.66 dB, and 3.03 dB. The cross talks of its five LP modes between adjacent cores are -72.8 dB, -67.8 dB, -43.6 dB, -40.0 dB, and -35.3 dB. The IL and cross talk of the LP01 mode are of satisfactory values, which are 0.23 dB and -72.8 dB, respectively. These two proposed FI/FO devices are expected to be used for high-speed optical interconnection and fiber communication.

Transmissive fluorescent temperature sensor based on Er3+/Yb3+/Mo6+ tri-doped tellurite fiber for real-time thermal monitoring of motors using the FIR technique.

In this paper, we fabricate a transmissive fluorescent temperature sensor (TFTS) that based on Er3+/Yb3+/Mo6+ tri-doped tellurite fiber, which has the advantages of compactness and simplicity, corrosion resistance, high stability and anti-electromagnetic interference. The doping of Mo6+ ions will enhance the up-conversion (UC) fluorescence emission efficiency of Er3+ ions, thus improving the signal-to-noise ratio of TFTS. Using the fluorescence intensity ratio (FIR) technique, the real-time thermal monitoring performance of TFTS is evaluated experimentally. Apart from good stability, its maximum relative sensitivity is 0.01068 K-1 at 274 K in the measured temperature range. In addition, it is successfully used to monitor the temperature variation of the stator core and stator winding of the motor in actual operation. The results show that the maximum error between the FIR-demodulated temperature and the reference temperature is less than 1.2 K, which fully confirms the effectiveness of the TFTS for temperature monitoring. Finally, the FIR-based TFTS in this work is expected to provide a new solution for accurate and real-time thermal monitoring of motors and the like.

Ti3C2Tx MXene Paper-Based Wearable and Degradable Pressure Sensor for Human Motion Detection and Encrypted Information Transmission.

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 Ti3C2Tx MXene 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 Ti3C2Tx MXene 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.

Dual-Layer All-Textile Flexible Pressure Sensor Coupled by Silver Nanowires with Ti3C2-Mxene for Monitoring Athletic Motion during Sports and Transmitting Information.

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.

Highly sensitive surface plasmon resonance temperature sensor based on a hollow core fiber multilayer structure.

A surface plasmon resonance (SPR) temperature sensor based on a hollow core fiber (HCF) is designed in this paper. The sensor is composed of a multi-mode fiber (MMF)-HCF-MMF structure, and the self-made HCF is deposited successively with a thin layer of Au film (50 nm in thickness), gold nanoparticles (10 nm in diameter) and polydimethylsiloxane (PDMS). A series of theoretical and experimental investiagtions are conducted, and the results are as follows: the proposed sensing structure only with Au film can effectively excite the SPR effect, with a sensitivity of (2200 ± 100) nm / RIU in the refractive index (RI) range of 1.3334-1.3811; after adding AuNPS, the sensitivity of the sensor is effectively improved, the sensitivity can be increased to (3100 ± 100) nm / RIU, and after the PDMS coating, temperature sensing can be realized due to its unique temperature-sensitive characteristics, a linear sensitivity of (-2.1 ± 0.1) nm / °C is realized in the temperature range of 25 °C to 100 °C. The sensor has the advantages of simple structure, wide application, large measurement range, high sensitivity, good stability and repeatability. Meanwhile, the internal air hole of HCF leaves a preparation channel for dual-parameter measurement. It has broad application prospect in medical treatment, environmental monitoring and manufacturing industry.

Low crosstalk trapezoid-index thirteen-core single-mode fiber for multi-channel and high-density applications.

Multi-core fiber based on space division multiplexing technology provides a practical solution to achieve multi-channel and high-capacity signal transmission. However, long-distance and error-free transmission remains challenging due to the presence of inter-core crosstalk within the multi-core fiber. Here, we propose and prepare a novel trapezoid-index thirteen-core single-mode fiber to solve the problems that MCF has large inter-core crosstalk and the transmission capacity of single-mode fiber approaches the upper limit. The optical properties of thirteen-core single-mode fiber are measured and characterized by experimental setups. The inter-core crosstalk of the thirteen-core single-mode fiber is less than -62.50 dB/km at 1550 nm. At the same time, each core can transmit signals at a data rate of 10 Gb/s and achieve error-free signal transmission. The prepared optical fiber with a trapezoid-index core provides a new and feasible solution for reducing inter-core crosstalk, which can be loaded into current communication systems and applied in large data centers.

Experimental research of 13-core 5-LP mode fiber with high doped graded-index core and stairway-index trench.

A graded-index 13-core 5-LP mode fiber with high doped core and stairway-index trench structure have been successfully prepared by Hole-drilling method and Plasma vapor deposition. This fiber has 104 spatial channels, realizing large capacity information transmission. By building an experimental platform, the 13-core 5-LP mode fiber have been tested and characterized. The core can stably transmit 5 LP modes. The transmission loss is lower than 0.5 dB/km. Inter-core crosstalk (ICXT) of each layer of core is analyzed in detail. The ICXT can be less than -30 dB/100 km. The test results show that this fiber can stably transmit 5 LP modes, and has the characteristics of low loss and low crosstalk, realizing large capacity transmission. This fiber provides a solution to the issue of limited fiber capacity.

Weakly coupled graded index heterogeneous nineteen-core few-mode fiber.

We propose a novel heterogeneous nineteen-core four-mode fiber. The heterogeneous core arrangement and trench-assisted structure can significantly suppress inter-core crosstalk (XT). In order to control the number of modes in the core, a low refractive index area is introduced in the core. The number of LP modes and the effective refractive index difference (Δneff) of adjacent modes in the core are controlled by changing the refractive index distribution of the core and the parameters of the low refractive index area in the core. And the mode state of low intra-core crosstalk is successfully realized in the graded index core. After the optimization of fiber parameters, each core can stably transmit four LP modes under the optimal fiber parameters, and the inter-core crosstalk of LP02 mode is less than -60 dB/km. Finally, the effective mode area (Aeff) and dispersion (D) of nineteen-core four-mode fiber in C+L band are described. The results show that the nineteen-core four-mode fiber is suitable for terrestrial and undersea communication systems, data centers, optical sensors and other fields.

Multimode interferometric sensor based on the no-core tellurite optical fiber for cryogenic temperature detection.

In this paper, a no-core tellurite optical fiber (NCTOF)-based sensor was proposed for cryogenic temperature detection in refrigeration process. The ultraviolet adhesive (UVA) dual-curing method was operated to stablish a sandwich-like composite structure, in which a section of NCTOF was compactly sandwiched between two segments of silica fiber to form multimode interference. The temperature sensing characteristics in cryogenic range were experimentally investigated by monitoring the transmission spectral movement, where a high sensitivity of 105.6 pm/°C was achieved in the range of -20-0 °C and 51.6 pm/°C in the range of -20-25 °C. The excellent performance was consistent with the simulation analysis. The maximum repeatability standard deviation and stability wavelength error of the sensor are 0.9799 pm/°C and 0.1676 nm, respectively. To the best of our knowledge, this is the first report on using tellurite optical fibers for cryogenic temperature detection, and the UVA dual-curing method provides a reliable solution for the integration and practical application of tellurite optical fiber. The proposed sensor is simple in structure, easy in fabrication, low in cost and excellent in performance. It can be expected to be used in food refrigeration, air-conditioning engineering, medical and health, industrial production, etc.

Preparation and transmission characteristics of seven-core few-mode fiber with low loss and low inter-core crosstalk.

Seven-core five-mode fiber and single-core five-mode fiber with the same core structure by high and low refractive index double rings are prepared based on plasma chemical vapor deposition. The transmission characteristics of the single-core few-mode fiber and the seven-core few-mode fiber are measured and characterized by building an experimental platform. The prepared single-core few-mode fiber can stably transmit five LP modes at 1550 nm, which not only has low loss characteristics, but also has excellent bending resistance. Furthermore, the transmission loss of the prepared seven-core fiber is lower than 0.4 dB/km, and the inter-core crosstalk is lower than -50 dB/km, which realizes the high-density and low-crosstalk transmission of the multi-core fiber. The prepared seven-core few-mode fiber can solve the capacity limitation of single-mode fiber, which will contribute the development of future communication systems.

Suspended-core fiber with embedded GaSe nanosheets for second harmonic generation.

We report an all-fiber scheme for the second harmonic generation (SHG) by embedding gallium selenide (GaSe) nanosheets into a suspended-core fiber (SCF). Based on modes analysis and theoretical calculations, the phase-matching modes from multiple optional modes in the SHG process and the optimal SCF length are determined by calculating the effective refractive index and balancing the SHG growth and transmission loss. Due to the long-distance interaction between pumped fundamental mode and GaSe nanosheets around the suspended core, an SHG signal is observed under a milliwatt-level pump light, and exhibits a quadratic growth with the increased pump power. The SHG process is also realized in a broad wavelength range by varying the pump in the range of 1420∼1700 nm. The SCF with the large air cladding and suspended core as an excellent platform can therefore be employed to integrate low-dimensional nonlinear materials, which holds great promise for the applications of all-fiber structures in new light source generating, signal processing and fiber sensing.

Design of a free-form off-axis three-mirror optical system with a low f-number based on the same substrate.

In this paper, we design a free-form off-axis three-mirror optical system with a low f-number and compact structure, which can be used as an infrared reflection imager. The initial structure is calculated from the near-axis optical transfer matrix based on third-order aberration theory. Particular constraints are designed to install all mirrors on the same substrate for simultaneous milling, which reduces the processing difficulty and effectively avoids errors caused by component assembly. Zernike free-form surfaces are introduced to correct aberrations. This optical system has a field of view of 5∘×5∘ and an f-number of 1.82; the modulation transfer function of the system is higher than 0.6 at 30 lp/mm. The results of the tolerance assignment of the system were verified by the Monte Carlo method, and the machining tolerance is reasonable and easy to achieve. This design not only improves the optical performance of the system but also enhances the feasibility of manufacturing.

Plug-in tip sensor based on upconversion fluorescence in Er3+/Yb3+ co-doped tellurite glass for thermal monitoring of a miniature winding coil.

We demonstrate a plug-in tip sensor with a maximum cross section diameter of only 1 mm for real-time thermal monitoring of a high-density miniature winding coil, which can meet the miniaturization development needs of electromagnetic actuators. Due to the high upconversion luminescence efficiency, tellurite glass with an optimized Er3+/Yb3+ doping ratio is adhered to the end face of silica fiber for a temperature-sensitive tip. Temperature information is demodulated using the fluorescence intensity ratio technique, yielding a nonlinear response with R2 up to 0.9978. Within a wide temperature range of 253.55-442.45 K, the tip sensor exhibits good repeatability, excellent stability, high sensitivity of 52.7 × 10-4 K-1, small absolute error within ±1 K, and fast time response of 2.03 s. It has been successfully proven to be a miniaturized device with strong anti-interference ability for the health management of high-density winding coils.

Flexible Pressure Sensor Decorated with MXene and Reduced Graphene Oxide Composites for Motion Detection, Information Transmission, and Pressure Sensing Performance.

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.

Frequency conversions of nine peaks based on dispersive waves and solitons in a tellurite microstructured optical fiber.

We demonstrate the generation of broadband dispersive waves (DWs) and solitons in an 80-cm tellurite microstructured optical fiber (TMOF) designed and fabricated with 78TeO2-5ZnO-12LiCO3-5Bi2O3 (TZLB) glass. A 1810-nm femtosecond laser is used as the pump source with an average pump power ranging from 33 mW to 175 mW, where the tunable frequency range is 211.1 THz, which corresponds to the tunable wavelength range of 1742.9 nm. At 175 mW, the trapped multiple DWs are located at 923.8 nm, 1039.2 nm, 1121.6 nm, and 1204.6 nm and the multiple solitons are located at 2666.7 nm, 2426.1 nm, 2165.9 nm, 1952.7 nm, and 1842.1 nm. The experimentally obtained maximum DW conversion efficiency is 14%, and the maximum soliton conversion efficiency is 43%. The experimental and theoretical results of pulse evolution in the TMOF agree very well. To the best of our knowledge, this is the first time that nine peaks of frequency conversions have been realized simultaneously in non-silicon fibers. The exceptionally high nonlinearity and broadband-tunable characteristics of the proposed TMOF are promising components for the development of compact and highly efficient tunable mid-infrared fiber lasers, wavelength converters, and time-frequency metrology.