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We propose and demonstrate a hybrid few-mode fiber configuration (HFMFC) that enables ultrahigh-channel-count orbital angular momentum (OAM) mode conversion. The HFMFC consists of periodically twisted graded-index few-mode fiber segments and a step-index few-mode fiber segment. Our proposed HFMFC-based multichannel OAM mode converter (OAM-MC) offers an exceptionally high channel count in a wide bandwidth, with customizable channel spacing down to 50 GHz (∼0.4 nm), achieved through optimization of the structural parameters of the HFMFC. By employing this methodology, we have successfully demonstrated 10, 32, 117, and 233 channel OAM mode conversions covering the entire C + L band, representing the highest performance among all reported fiber-based multichannel OAM-MCs to date, for the first time to the best of our knowledge. The suggested ultrahigh-channel-count OAM-MC exhibits promising potential for applications in various fields such as OAM fiber communication, OAM holography, OAM information processing, and OAM metrology.

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.

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., LP 11 ) 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.

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.

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-MnO 2 thin film coating on the fiber surface for the first time, GSH solution would lead to the dissolution of MnO 2 , 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 MnO 2 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.

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 Ge 20 Se 70 Te 10 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.

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 Ti 3 C 2 T x 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 Ti 3 C 2 T x 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.

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 Ti 3 C 2 -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.

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.

In this paper, we fabricate a transmissive fluorescent temperature sensor (TFTS) that based on Er 3+ /Yb 3+ /Mo 6+ tri-doped tellurite fiber, which has the advantages of compactness and simplicity, corrosion resistance, high stability and anti-electromagnetic interference. The doping of Mo 6+ ions will enhance the up-conversion (UC) fluorescence emission efficiency of Er 3+ 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.

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.

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.

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.

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 (Δn eff ) 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 LP 02 mode is less than -60 dB/km. Finally, the effective mode area (A eff ) 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.

A four-hole birefringence tellurite microstructured optical fiber (BTMOF) was designed and fabricated based on 76.5 T e O 2 -6 Z n O -11.5 L i 2 O -6 B i 2 O 3 glass, and its core (slow and fast axes were) measured to be approximately 4.74 µm and 4.29 µm, respectively. The experimentally measured results demonstrated that the maximum supercontinuum (SC) spectra extended from ∼914.1 n m to ∼1885.1 n m when the polarization state of the pump pulse was parallel to the fast axis at 1400 nm with an average power of 460 mW. We performed numerical simulations based on the nonlinear Schrödinger equation, which support the experimentally measured results. The SC generation in birefringent silica microstructured fiber with the same geometric parameters was simulated, and the results showed that the enhanced nonlinear refractive index of the BTMOF yielded a spectrum with a significantly larger bandwidth. Furthermore, the two polarization states along the fast axis and slow axis exhibit different dispersion characteristics, which provide a convenient way of tuning the properties of the generated SC. This work highlights BTMOF as a promising platform for the development of a SC light source, which can be widely used in food quality inspection, early cancer diagnostics, gas sensing, and high-spatial-resolution imaging.

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 Er 3+ /Yb 3+ 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 R 2 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.

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.

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.

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.

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.

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.

We demonstrate the generation of broadband dispersive waves (DWs) and solitons in an 80-cm tellurite microstructured optical fiber (TMOF) designed and fabricated with 78TeO 2 -5ZnO-12LiCO 3 -5Bi 2 O 3 (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.

In this paper, the self-phase modulation (SPM) effect in a double-cladding single-mode tellurite optical fiber (DC-SMTOF) was exploited for temperature sensing. The DC-SMTOF was fabricated based on a TeO 2 -ZnO-Li 2 O-Bi 2 O 3 (TZLB) glass material that has a thermo-optical coefficient as high as -16.4×10 -6 /°C. The temperature sensing performance was evaluated by detecting the 3-dB bandwidth of the SPM spectra with the variation of temperature at different pump wavelength and different average pump power. The temperature sensitivity was obtained to be -2.971 nm/°C with a resolution of 0.0168°C. Both simulation and experiment confirmed that a longer pump wavelength and higher average pump power will result in a higher temperature sensitivity. To the best of our knowledge, this is the first study concerning SPM-based temperature sensing in a tellurite optical fiber. The proposed temperature sensor has a compact structure, and it can realize temperature sensing of high sensitivity without any fiber modification. This work opens the road toward explorations of a novel temperature sensing technology combined with soft glass fibers and nonlinear phenomenon, and is expected to deepen our understanding in the application of these complex nonlinear phenomena.

In this study, the silver mirror reaction was used to coat the silver film on the surface of self-made microstructured fiber (MSF) to stimulate the surface plasmon resonance effect, and Polydimethylsiloxane (PDMS) with a high thermal-optical coefficient was coated on the silver film as temperature-sensitive material. The MSF with silver and PDMS films was coupled with multi-mode fiber on both sides to form the temperature sensor. In this sensor system, the energy is coupled into the cladding of the microstructure fiber by multi-mode fiber, and the surface plasmon resonance can be further excitated in the MSF. When the temperature of the external environment changes, the refractive index of PDMS will also change. At this time, combined with the surface plasmon resonance effect, a resonant absorption peak corresponding to the temperature appears in the transmission spectrum so that the temperature can be measured quickly and accurately. We found that, in the temperature range of 35 °C to 95 °C, the average temperature sensitivity of the sensor during heating and cooling was -0.83 nm/°C and -0.84 nm/°C, respectively. The advantages of this sensor are the simple structure, convenient operation and good reversibility. The relative sensitivity deviation value (RSD = 0.0059) showed that the sensor has high stability. The temperature sensor based on MSF has favorable prospects for use in fields such as medical treatment, biochemical detection and intelligent monitoring.

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 18 M W / c m 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 ∼57 d B . 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.

Titanium nitride (TiN) with outstanding physical and chemical properties has earned considerable attention. However, few studies have been conducted on the ability of TiN in the ultrafast photonics field. In this study, ultrafast fiber lasers operating at 1.5 and 2 μm were realized using TiN nanoparticles (NPs) deposited onto a D-shaped fiber (DF) as the optical modulator. TiN NPs were synthesized via a reduction-nitridation route. As the TiN-DF saturable absorber (SA) was integrated into the erbium-doped fiber laser (EDFL) and thulium-doped fiber laser (TDFL) cavities, femtosecond pulses of 1532 nm and 1960 nm were obtained with high performance. To the best of our knowledge, the mode-locked fiber laser operating around 1530 nm was realized for the first time. Interestingly, TiN-DF SA in the TDFL could be used as a temperature sensitive device, and a novel temperature sensor was proposed with the sensitivity of 88 pm °C -1 . Our results verified that TiN-DF modulators are excellent nonlinear optical devices for ultrafast photonic applications.

Arterial stenosis will reduce the blood flow to various organs or tissues, causing cardiovascular diseases. Although there are mature diagnostic techniques in clinical practice, they are not suitable for early cardiovascular disease prediction and monitoring due to their high cost and complex operation. In this paper, we studied the electromagnetic effect of arterial blood flow and proposed a method based on the deep neural network for arterial blood flow profile reconstruction. The potential difference and weight matrix are used as inputs to the method, and its output is an estimate of the internal blood flow velocity distribution for arterial blood flow profile reconstruction. Firstly, the weight matrix is input into the convolutional auto-encode (CAE) network to extract its features. Then, the weight matrix features and potential difference are combined to obtain the features of the blood velocity distribution. Finally, the velocity features are reconstructed into blood flow velocity distribution by a convolution neural network (CNN). All data sets are obtained from a model of the carotid artery with different rates of stenosis in a uniform magnetic field by COMSOL. The results show that the average root mean square error of the reconstruction results obtained by the proposed method is 0.0333, and the average correlation coefficient is 0.9721, which is better than the corresponding indicators of the Tikhonov, back propagation (BP) and CNN methods. The simulation results show that the proposed method can achieve high accuracy in blood flow profile reconstruction and is of great significance for the early diagnosis of arterial stenosis and other vessel diseases.

We propose a novel scheme of high doped core and stairway-index trench structure to design a manufacturable graded-index 13-core 5-LP mode fiber with low inter-core crosstalk (ICXT) and large mode differential group delay (MDGD). By using the couple power theory and the finite element method (FEM), the change ICXT of with fiber parameters are investigated. The design of core graded profile and trench structure are optimized to achieve better performance and to meet the fabrication conditions. The numerical result demonstrate that this fiber achieves a low ICXT of lower than -30dB/km (Rb≤500 mm). The bending loss values satisfy the ITU-T recommendation G.655 in 195 µm cladding diameter. Furthermore, the dispersion and the MDGD dependences on wavelength are calculated. The relative core multiplicity factor (RCMF) is obtained as 75.17, which realizes the high density multiplexing. The fabrication methods of this fiber are briefly introduced. The designed fiber may be used for Space-division multiplexing (SDM) system to solve the problem of fiber capacity limitation.

We demonstrate broadband mid-infrared cascaded stimulated Raman scattering (SRS) and flat supercontinuum (SC) generation in a chalcogenide optical fiber made from A s 2 S 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 ∼10 d B (from ∼1030 to 3441 nm) was observed. Our results verify that the A s 2 S 5 optical fibers possess promising applications for tunable mid-infrared Raman fiber lasers and SC light sources pumped by 2 µm pulsed lasers.

This paper proposes a highly sensitive surface plasmon resonance (SPR) refractive index sensor based on the photonic crystal fiber (PCF). The optical properties of the PCF are investigated by modulating the refractive index of a liquid analyte. The finite element method (FEM) is used to calculate and analyze the PCF structure. After optimization, the fiber can achieve high linearity of 0.9931 and an average refractive index sensitivity of up to 14,771.4 nm/RIU over a refractive index range from 1.47 to 1.52, with the maximum wavelength sensitivity of 18,000.5 nm/RIU. The proposed structure can be used in various sensing applications, including biological monitoring, environmental monitoring, and chemical production with the modification and analysis of the proposed structure.

In this paper, we proposed a highly sensitive temperature sensor based on self-phase modulation (SPM) in an in-house fabricated microstructured optical fiber (MOF) which had three rings of air holes. The temperature sensing performance was evaluated by detecting the 3 dB bandwidth of SPM spectrum with the variation of temperature at different pump wavelengths and average pump power. At the pump wavelength of 1400 nm with the average pump power of 600 mW, the temperature sensitivity was obtained to be as high as 1.296 nm/°C. Moreover, the theoretical simulation was carried out, the results of which corresponded well with the experiment. To the best of our knowledge, this is the first experimental study concerning SPM-based temperature sensing. This work proves experimentally and theoretically a new temperature sensing mechanism drawing on the SPM effect in optical fibers, which is expected to develop temperature sensors of low cost, simple structure and high sensitivity.

The accuracy of the surface plasmon resonance (SPR) optical fiber sensor is affected by the change of ambient temperature. Therefore, we propose a simple dual channel SPR optical fiber sensor, which can measure both glucose concentration and ambient temperature. The proposed sensor is a two-channel structure based on a no-core optical fiber (NCF): one channel is coated with gold film and polydimethylsiloxane (PDMS) to sense the ambient temperature, and the other is coated with silver film to sense glucose concentration. The experimental results show that the sensor's sensitivity for sensing glucose concentration is 2.882 nm / %, and for sensing temperature is -2.904 nm / °C. By monitoring the real-time temperature, the accuracy of glucose concentration detection was improved. The proposed sensor has a simple and compact structure, and it is suitable for sensing glucose solution or other analyte solutions that need temperature compensation.

This paper proposes a Photonic Crystal Fiber (PCF) refractive index sensor model based on the surface plasmon resonance effect. The proposed PCF model also uses the full vector finite element method to transfer the structure under the anisotropic Perfect Matching Layer (PML) boundary condition. Numerical calculations were carried out on the sensor characteristics. The calculation results show that the elliptical air hole on the left side of the PCF core is coated with a gold-nano film which serves as a Surface Plasmon Resonance (SPR) sensing channel to detect the refractive index of liquid materials. Compared with other structures, the resonant peak generated by the excited SPR effect from the elliptical sensing channel has a high sensitivity to the change of the refractive index of the liquid to be measured. With the help of this attribute, it is relatively easy to adjust the sensitivity. The refractive index range of this structure is within 1.43-1.49 and the sensitivity is up to 12,719.97 nm·RIU -1 . The linearity is good; R 2 = 0.99927, which is very suitable for liquid sensing.

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.

In this paper, we reported a multi-wavelength third-harmonic generation (THG) induced by supercontinuum (SC) in an in-house fabricated suspended-core microstructured optical fiber (MOF). The adjustment of pump wavelength and pump power exerted an influence on SC which simultaneously emitted third harmonic (TH) waves in the visible light range. At the pump wavelength of 1220 nm and the average pump power of 450 mW, a multi-wavelength TH spectrum (373∼589 nm) with over twenty distinct peaks was observed under the phase matching (PM) condition between the fundamental mode and the higher-order modes. To the best of our knowledge, this is the first report on THG in optical fibers with so great a number of wavelengths. The maximal THG conversion efficiency ∼6.791 × 10 -4 was obtained at 1480 nm, 350 mW, which is highly competitive compared with the values reported previously. Furthermore, theoretical simulation has been carried out, which corresponded well with the experimental observation. This multi-wavelength THG in the suspended-core MOF may provide a unique pathway towards tailored multi-wavelength ultrafast light sources for applications in sensing and imaging technologies.

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 -433 p m / ∘ C by numerical simulation, and the experimental result is -160 p m / ∘ 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.

For this study, a temperature sensor utilizing a novel tellurite photonic crystal fiber (PCF) is designed. In order to improve the sensor sensitivity, alcohol is filled in the air holes of the tellurite PCF. Based on the degenerate four-wave mixing theory, temperature sensing in the mid-infrared region (MIR) can be achieved by detecting the wavelength shift of signal waves and idler waves during variations in temperature. Simulation results show that at a pump wavelength of 3550 nm, the temperature sensitivity of this proposed sensor can be as high as 0.70 nm/°C. To the best of our knowledge, this is the first study to propose temperature sensing in the MIR by drawing on four-wave mixing (FWM) in a non-silica PCF., Competing Interests: The authors declare no conflicts of interest.

A step-index tellurite optical fiber with loss ∼0.02  dB/m at ∼1545  nm was fabricated based on TeO 2 -Bi 2 O 3 -ZnO-Na 2 O 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.

A surface plasmon resonance (SPR) liquid refractive index sensor based on photonic crystal fiber (PCF) is proposed. The PCF is made of the exposed core structure, and the gold film is formed by electron beam evaporation within its defects. The sensitivity of the sensor is improved by coating graphene on the surface of the gold film. The experimental results show that the sensitivity of the sensor is increased by 390 nm/RIU after the introduction of graphene, and finally to 2290 nm/RIU. The experiment and simulation have a good consistency. Significantly, the sensor can be reused, and the measurement accuracy can be maintained.

In this paper we propose a gold-plated photonic crystal fiber (PCF) refractive index sensor based on surface plasmon resonance (SPR), in which gold is coated on the external surface of PCF for easy fabrication and practical detection. The finite element method (FEM) is used for the performance analysis, and the numerical results show that the thickness of the gold film, the refractive index of the analyte, the radius of the air hole in the first layer, the second layer, and the central air hole can affect the sensing properties of the sensor. By optimizing the sensor structure, the maximum wavelength sensitivity can reach 11000 nm/RIU and the maximum amplitude sensitivity can reach 641 RIU -1 . Due to its high sensitivity, the proposed sensor can be used for practical biological and chemical sensing.

A photonic crystal fiber (PCF) sensor based on gold nanowires able to detect changes in surface plasmon resonance (SPR) was proposed and numerically investigated through the finite element method. To facilitate real-time detection, the analyte in this sensor was located outside the optical fiber. The effects of diameters of both air hole and gold wires on the sensing characteristics of the sensor were discussed. The sensor was designed to detect liquids with refractive indexes ranging between 1.33 and 1.36. The numerical simulations indicated that sensor structure improved its functionality. The maximum spectral sensitivity reached 9200 nm/RIU over the entire refractive index range. The average spectral sensitivity was estimated to be 5933 nm/RIU, and corresponded to a sensor resolution of 2.81 × 10 -6 RIU. These findings look very promising for future use in detection of liquid.

Mid-infrared cascaded stimulated Raman scattering (SRS) is experimentally investigated in an As-S optical fiber which is fabricated based on As 38 S 62 and As 36 S 64 glasses and whose fiber loss is ∼0.08 dB/m at1545 nm. Using a nanosecond laser operated at ∼1545 nm as the pump source, mid-infrared cascaded SRS up to eight orders is obtained in a 16 m As-S fiber. To the best of our knowledge, this is the first demonstration of SRS of such high order in non-silica optical fibers, and it may contribute to developing tunable mid-infrared Raman fiber lasers using C-band pump sources.

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 2 O-WO 3 -MoO 3 -Nb 2 O 5 (TLWMN, high-index rod) glass, TeO 2 -ZnO-Na 2 O-La 2 O 3 (TZNL, inner cladding) glass, and TeO 2 -ZnO-Li 2 O-K 2 O-Al 2 O 3 -P 2 O 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.

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.

Energy transfer between Tm 3+ and Tb 3+ dependent on the power density of pump laser was investigated in NaYF 4 : Tb 3+ ,Tm 3+ ,Yb 3+ microcrystals. Under the excitation of a 976-nm near-infrared laser at various power densities, Tb 3+ -Tm 3+ -Yb 3+ doped samples exhibited intense visible emissions with tunable color between green and blue. The ratio of blue and green emission were determined by energy transfer between Tm 3+ and Tb 3+ . When the power density of pump laser was low, the energy transfer process from Tm 3+ ( 3 F 4 ) to Tb 3+ ( 7 F 0 ) occurred efficiently. Upconversion processes in Tm 3+ were inhibited, only visible emissions from Tb 3+ with green color were observed. When the power density increased, energy transfer from the 3 F 4 (Tm 3+ ) to 7 F 0 level (Tb 3+ ) was restrained and population on high energy levels of Tm 3+ was increased. Contribution of upconversion emissions from Tm 3+ gradually became dominant. The emission color was tuned from green to blue with increasing the power density. Energy transfer processes between low-lying levels of activators, such as Tm 3+ will greatly reduce the population on certain levels for further high-order upconversion processes. The Tb 3+ -Tm 3+ -Yb 3+ doped phosphors are promising materials for detecting the condition of power density of the invisible near-infrared laser.

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.

The effects of stimulated Raman scattering (SRS) is demonstrated in chalcogenide microstructured optical fiber (MOF) with all-solid AsSe2 core and As2S5 cladding. The first-order Raman Stokes wave is investigated in the MOFs with different core diameters pumped by the picosecond pulses at 1958 nm. The maximum conversion efficiency of -15.0 dB from the pump to first-order Raman Stokes wave is obtained in the MOF with the core diameter of 2.6 μm. The conversion efficiency decreases when the core diameter deviates from 2.6 μm. When the fiber core is larger, the effective nonlinearity is decreased. When the fiber core is smaller, the mode field is difficult to be confined in the core. The walk-off length between the pump and Stokes wave is crucial to the process of SRS according to the analysis of the experimental data. The Raman effects are simulated numerically. The simulated results can agree well with the experiments. It is the first time to demonstrate the Raman effect in AsSe2-As2S5 MOF, to the best of our knowledge.

We report the coherent mid-infrared supercontinuum generation in an all-solid chalcogenide microstructured fiber with all-normal dispersion. The chalcogenide microstructured fiber is a four-hole structure with core material of AsSe2 and air holes that are replaced by As2S5 glass rods. Coherent mid-infrared supercontinuum light extended to 3.3 μm is generated in a 2 cm long chalcogenide microstructured fiber pumped by a 2.7 μm laser.