Sensitivity is an important index parameter for evaluating the performance of a sensor. To improve the sensitivity of the sensor, we propose and demonstrate a surface plasmon resonance (SPR) fiber sensor based on Ti 3 C 2 T x -Mxene/silver film. A V-shaped photonic crystal fiber (V-PCF) is used as the sensing probe, and a silver film is deposited on its surface by magnetron sputtering to excite the SPR effect. Multimode fibers are used to transmit signals at both ends of the sensor probe. Simulation results show that light can be stably transmitted in the cladding of the V-PCF. Experimental results show that the refractive index (RI) measurement range of the sensor is between 1.333 and 1.420, and the maximum RI sensitivity is 20188 nm/RIU; In order to extend the detection function of the sensor, we coated PDMS film on the Ti 3 C 2 T x -MXene/silver film. When the sensor is used to measure temperature, the maximum temperature sensitivity is 9.33 nm/℃ in the range of 0 ℃-100 ℃. Therefore, the proposed composite film structure sensor in this paper has the advantages of ultra-high detection sensitivity and oxidation resistance, which can have a great competitive advantage in the sensing field.

In order to be compatible with the current optical fiber communication network, we propose a 125 mu m diameter heterogeneous six-core two linear polarization modes fiber with air-hole array and trench (HAT) structure. The coupled power theory and finite element method (FEM) are used to investigate inter-core crosstalk (ICXT), bending loss, and other properties. The HAT structure can effectively suppress ICXT; and the maximum ICXT is less than-38 dB/100 km in C + L band. The effective refractive index difference (Delta neff) between modes is greater than 3 x 10-3, inter-mode crosstalk (IMXT) can be ignored. The maximum intrinsic loss is 0.187 dB/km; relative core multiplicity factor reaches 32.21. The relative spatial efficiency is 18 times that of traditional single-mode fibers. This fiber can be produced by chemical vapor deposition and existing optical fiber manufacturing technology. The designed fiber can be directly connected to the communication network to reduce the cost of equipment renewal, and the communication capacity can be increased by 18 times.

In this paper, a segment-coupled six-mode multiplexer based on multi-core fiber (MCF) is proposed to achieve multiplexing of LP01, LP11, LP21, LP02, LP31 and LP12 modes according to the principle of mode coupling and phase matching. The mode multiplexer consists of a two-part fiber waveguide that supports six modes of transmission at 1.55 mu m wavelength. The transmission performance is analyzed with the finite element method (FEM) and the beam propagation method (BPM). The simulation results show that the insertion loss (IL) of the mode multiplexer is less than 1 dB in the wavelength range of 1.5439-1.5558 mu m, and the lowest IL is 0.1837 dB. The coupling efficiencies (CEs) of the six modes are 99.7 %, 92.0 %, 98.6 %, 98.3 %, 97.4 % and 86.6 %, respectively. The designed mode multiplexer dramatically improves fiber space utilization while achieving more higher-order mode conversion. The mode multiplexer can be applied to the mode division multiplexing (MDM) system to solve the problem of mode multiplexing and demultiplexing.

A surface plasmon resonance (SPR) biosensor based on dual-side polished photonic crystal fiber (PCF) has been presented in this paper. An external gold (Au) coating was used as a plasmonic material to detect changes in an analyte refractive index (RI). Sensor’s structural parameters were optimized for better performance. The proposed sensor’s simulation was conducted using the finite element method (FEM). Analytes within the RI range of 1.30 to 1.40 were used to develop a sensor. The numerical results are expected to have a wavelength sensitivity (WS) of 9400 nm/RIU and amplitude sensitivity (AS) of 27.27 RIU −1 , respectively, and a spectral wavelength resolution of 1.063 × 10 −5 . Furthermore, it is expected to obtain maximum loss of 161,270 dB/m for a Y-polarization (Y-P) odd mode with an analyte RI = 1.37. We obtained an extinction ratio of − 321.54 dB for the polarization filter effect, which is considered to be good. This structure has the advantages of being easy to fabricate, affordable, and expected to show better sensitivity than many other sensors. It assumes to detect a wide range of analytes, making it suitable for organic chemical sensing, pharmaceutical inspection, and biosensor applications.

A novel dual-side polished photonic crystal fiber (PCF) based surface plasmon resonance (SPR) polarization filtering and refractive index (RI) sensor is presented in this paper. The cladding of PCF was composed of eight air holes and the outer surface was dual-sided polished and coated with gold film. Finite element method (FEM) was used for the simulation and design of the proposed sensor. Simulation results showed that sensor performance was better in analytes ranging from 1.33 to 1.42 RI. The wavelength and amplitude sensitivities at resonance wavelengths of 1269 and 995 nm were 27400 nm/RIU and -100.43 RIU-1 for an analyte of 1.42 and 1.41 RI, respectively, along with a spectral resolution of 3.64x10-6 RIU. Maximum loss of 52806 dB/m was obtained for an analyte of 1.41 RI towards Y-Polarization (Y-P). An extinction ratio of -105.374 dB was obtained using a fiber transferring length of 2 mm to achieve better polarization filtering effects. The proposed sensor is made of same size air holes which is easy to fabricate, cost effective, and shows better sensitivity and polarization filtering effects than several other sensors. It is expected that the sensor is suitable for wide RI detection range and capable of providing excellent polarization effect. The proposed sensor can be used for various applications including environmental detection, chemical sensing, biosensors, and pharmaceutical inspection.

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.

This paper presents a dual-core two-parameter optical fiber sensor based on the surface plasmonic resonance (SPR) effect. It is analyzed by the finite element method. The proposed sensor is a dual-channel structure designed with photonic crystal fiber (PCF) as the base material: one channel is coated with a gold film to measure the refractive index (RI) of the solution to be measured, and the other channel is coated with a gold film and polydimethylsiloxane to measure the temperature of the solution to be measured. The exposed microslot structure on both sides reduces the complexity of sensing measurements. The results show that the maximum RI sensitivity of the sensor is 19,900 nm/RIU, and the maximum sensitivity to temperature is 8.7 nm/℃. This work is conducive to realizing a PCF sensor with high sensitivity, large measurement range, real-time monitoring, and easy preparation. As a result, the sensor is expected to be widely used in fields such as biology and chemicals.

Surface plasmon resonance (SPR) sensing methods enable highly sensitive, fast response, and label-free analysis of biomolecular interactions. For SPR sensors, sensitivity and full width at half maximum (FWHM) are two incompatible performance parameters. We propose a refractive index (RI) sensor using a dual-core photonic crystal fiber (PCF) and SPR effects to achieve high sensitivity and a narrow FWHM simultaneously. The air holes of the sensor appear in a hexagonal arrangement, and polishing technology introduces two polishing planes into the cladding. A gold film is deposited on one side of the polished plane to form a highly sensitive RI sensing channel. Five gold nanowires are deposited on the other side of the polished plane to form a RI sensing channel with a narrow FWHM. We analyzed and optimized its structural parameters using the finite element method and determined the optimal structural parameters. The numerical results demonstrate that the maximum sensitivity of the sensor is 21,000 nm/RIU with the narrowest FWHM of 31 nm. Therefore, measuring the refractive index simultaneously with two sensing channels increases the detection accuracy of the measurement. In addition, the findings further indicate that variations in structural parameters do not significantly impact the sensing performance of the sensor, which makes the production of the sensor relatively simple. In conclusion, our work provides a new research method for realizing high sensitivity and a narrow FWHM simultaneously.

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.

In order to meet the actual requirements for sensor miniaturization and integration, we propose a compact dual-parameter surface plasmon resonance (SPR) fiber sensor, which can simultaneously measure the refractive index (RI) and temperature of liquid. The sensor is composed of input–output couplers and two sensing arms. The no-core fibers (NCFs) and microstructured optical fiber (MOF) with deposited films are used as input–output couplers and sensing arms, respectively. One sensing arm is the MOF with silver film for RI measurement, and another is that with silver film and polydimethylsiloxane (PDMS) for temperature measurement. Numerical simulation results show that its cladding mode is more advantageous in stimulating the SPR effect. Experimental results demonstrate that the maximum sensitivity of liquid RI is 4139 nm/RIU in the range of 1.333–1.365, and that of liquid temperature is 4.7 nm/°C in the range of 20 °C–70 °C. The compact structure makes the proposed sensor unique and has a promising application in RI and temperature monitoring.

We propose a homogeneous five-mode twelve-core fiber with a trench-assisted structure, combining a low refractive index circle and a high refractive index ring (LCHR). The 12-core fiber utilizes the triangular lattice arrangement. The properties of the proposed fiber are simulated by the finite element method. The numerical result shows that the worst inter-core crosstalk (ICXT) can achieve at -40.14 dB/100 km, which is lower than the target value (-30 dB/100 km). Since adding the LCHR structure, the effective refractive index difference between LP21 and LP02 mode is 2.8 × 10-3, which illustrates that the LP21 and LP02 modes can be separated. In contrast to without the LCHR, the dispersion of LP01 mode has an apparent dropping, which is 0.16 ps/(nm·km) at 1550 nm. Moreover, the relative core multiplicity factor can reach 62.17, which indicates a large core density. The proposed fiber can be applied to the space division multiplexing system to enhance the fiber transmission channels and capacity.

Temperature greatly influences the activity of biological materials; therefore, it is necessary to simultaneously monitor the temperature and concentration of biological materials. Long-range surface plasmon resonance (LRSPR) sensor has high sensitivity and can detect biomolecules. In this article, we proposed a dual-channel LRSPR sensor that can measure both bovine serum albumin (BSA) concentration and ambient temperature. The proposed sensor used a no-core fiber (NCF) with two channels. One channel was used to deposit magnesium fluoride and silver films on NCF for BSA detection, while the other channel was used to deposit three layers of films [magnesium fluoride, silver, and polydimethylsiloxane (PDMS)] on NCF for temperature detection. The sensitivity of the sensor to detect BSA concentration and temperature is 1.02 nm/(mg/ml) and −2.82 nm/°C, respectively. Cross-sensitivity between the two parameters is solved using a matrix. The sensor offers the advantages of simultaneously monitoring two parameters, having a simple structure and possessing high sensitivity, which makes it promising for a broad range of applications in monitoring BSA concentration and temperature.

Objective As one of the important properties of the light field, polarization plays an important role in the interaction between light and matter. The modulation of polarization plays an indispensable role in optical communication systems, fiber sensors, fiber lasers, and other fields. However, in view of the twist, defects, environment perturbations, and other factors in the process of optical fiber manufacturing, the manufactured optical fiber is not completely uniform, which introduces random birefringence and leads to unpredictable polarization states. Therefore, it is of great practical value to study optical fibers with excellent polarization states. Although the existing single-polarization single- mode negative-curvature hollow-core fiber has the advantages of simple structure, easy preparation, endless single- mode transmission, and low loss, due to the limitation of research habits and optical materials, the current research mainly focuses on common communication bands. But obviously, the mid-infrared band will become the next hot band of the negative-curvature hollow-core fiber. Research shows that a wavelength of 3-5 mu m plays an important role in national defense, medical care, communications, and other fields, especially near the wavelength of 4 mu m, which is an ideal band for quantum cascade detectors to detect low-level light. Single-mode single-polarization light helps to provide a more pure light source for quantum cascade detectors. Therefore, it is of great practical significance to study the single-mode single- polarization negative-curvature hollow-core fiber with a wavelength of 4 mu m. Methods A hollow-core anti-resonant fiber composed of six nested tubes working near 4 mu m is designed, which can transmit single-mode single-polarization with low loss. The influence of structural parameters on fiber performance is calculated by using the control variable method. The capillary wall thickness will lead to an obvious change in the fiber loss with the working band, which is the key factor affecting the characteristics of the negative-curvature hollow-core anti-resonant fiber. Therefore, the capillary wall thickness is analyzed and optimized. Through the scanning study of the capillary wall thickness, the local optimal parameter values of the minimum fundamental mode loss and the maximum high-order mode extinction ratio in the 4 mu m band are determined, and the design goal of the single- mode performance of the fiber is successfully realized. The second step is to optimize the capillary radius. This parameter mainly affects the polarization state of the fiber, and different parameter combinations of the six inner tube radii correspond to different implementation effects. The optimization of capillary radius successfully achieves single-polarization operation in a single-mode state. In the third step, the core diameter of the fiber is optimized. Although the study does not reflect the further optimization effect of the parameters that have been optimized and determined in the previous steps, the parameter design still retains the effective mode area and the maximum transmission power tolerance value of the fiber. The fourth step is to study and characterize the bending resistance of optical fiber. Research shows that this design fully meets the preset requirements for bending resistance and verifies that the natural advantages of negative-curvature hollow-core anti-resonant fibers, such as large effective mode field area and less substrate material coverage, can contribute to the bending resistance of the fiber. Results and Discussions A negative-curvature hollow-core fiber with low-loss single-mode single-polarization transmission is proposed and analyzed by the finite element method. By calculating the influence of fiber parameters on the fiber structure, the high-order mode extinction ratio reaches 163 (Fig. 3), and the fiber successfully realizes single-mode transmission. However, in order to further ensure the single polarization performance of the fiber, the size of the capillary radius is optimized, and the single polarization function is realized based on single-mode transmission (Fig. 4). In order to ensure that the fiber has good bending resistance, the critical bending radius of the fiber is defined, and it is found that the bending loss of the x-polarization fundamental mode of the fiber is always less than 10(-3) dB/m (Fig. 7). In addition, the fiber structure also has a large effective mode field area (Fig. 8), which meets the transmission requirements of high power lasers. The results show that the designed structure achieves both single-polarization performance and single-mode transmission. Conclusions In this paper, a single-mode, single-polarization, low-loss, negative-curvature, hollow-core, and anti-resonant fiber is proposed. The substrate material of the fiber is As40S60, which is specially studied and experimentally prepared by Xi'an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences. Its refractive index is 2. 395 at 4 mu m. It has low intrinsic loss and great chemical stability in the mid-infrared band, which is beneficial to realize the low loss performance of the fiber. The fiber structure adopts a six-nested, capillary-type, negative-curvature, hollow-core, and anti-resonant structure with relatively mature preparation technologies and a simple structure. After optimizing the parameters of the fiber, the single-mode single-polarization effect can be achieved from 3. 99 mu m to 4. 00 mu m. Especially at the wavelength of 4 mu m, the polarization extinction ratio (PER) and high order mode extinction ratio (HOMER) reach 491 and 694, respectively, which meet the conditions of single-polarization single-mode transmission, and the loss is as low as 1. 8x10(-4) dB/m. The fiber also has excellent bending resistance. At the wavelength of 4 mu m, single-mode single-polarization transmission of the fiber can be achieved by selecting the appropriate bending radius at any bending angle. When the bending angle is equal to 0 degrees, and the bending radius is from 1 cm to 10 cm, the confinement loss of the fiber is less than 5. 3x10(-3) dB/m. The negative-curvature, hollow-core, and anti-resonant fiber proposed in this paper has the advantages of simple structure, single-mode single-polarization operation, low loss, and excellent bending resistance. It can not only be applied to the communication industry and medical system but also is expected to provide a more pure light source for quantum cascade detectors operating in the band of 4 mu m.

We present a hill-shaped refractive index (HS-RI) structure in 16-core fiber that can provide 160 channels for the transmission system. The HS-RI mainly settles the issue of the large inter-mode crosstalk (IMXT) between LP21 mode and LP02 mode. We obtain the optimized parameters using the finite element method. Numerical analyses reveal that the inter-core crosstalk (ICXT) of all modes is smaller than −30dB/100km in the C band, and the effective mode refractive index difference (Δneff) is more than 1×10−3. The minimum effective mode field area can achieve 89.80µm2, which greatly reduces the nonlinearity of fiber. The max dispersion is 15.59ps/(nm∗km), which can greatly guarantee signal quality in the transmission process. he relative core multiplexing factor (RCMF) can reach 65.81, which realizes the high space division multiplexing rate. The proposed fiber can be applied to a space division multiplexing system to enhance the fiber transmission channels and capacity.

A negative curvature hollow core fiber (NCHCF) refractive index (RI) sensor based on localized surface plasmon resonance (LSPR) is proposed. The gold nanowires are deposited in four cladding tubes in the y -direction, with the core region serving as the analyte channel. The full vector finite element method (FEM) is used to analyze the influence of various structural parameters on this sensor, and the structural parameters are optimized and selected. Ultimately, an average sensitivity of 9356.59 nm/RIU is accomplished within a spacious refractive index detection scope of RI = 1.28–1.43. The sensor attained a maximum sensitivity of 10,220 nm/RIU at RI = 1.36. In the bargain, there is an excellent linear correlation between the resonance wavelength and the refractive index of the analyte, with a value of 0.99901 and a factor of merit (FOM) range of 119.9563–155.9432 RIU −1 , achieving a resolution of 10 −6 RIU. The sensor has potential applications in various fields such as environmental protection, food safety, and medical diagnostics due to its high sensitivity, spacious detection scope, and positive linear response.

A wide bandwidth and short-length polarization beam splitter (PBS) is proposed based on the tellurite glass dual hollow-core anti-resonant fiber (DHC-ARF). Four circular cladding tubes are introduced in the x-axis direction to form two hollow cores. The modal coupling is realized through the gap between the two hollow cores. The nested tubes are inserted into the cladding tubes between the two hollow cores for realizing ultrawide bandwidth. The bandwidth covers the communication wavelength of 1.31 mu m and 1.55 mu m. The advantage of the proposed PBS is that the beam splitter length is 2.122 cm, The operation bandwidth is 400 nm (1.26-1.66 mu m), where the power extinction ratio is below - 20 dB. The high-order mode extinction ratio is above 100, which can be and the power extinction ratio is - 292.83 dB at 1.55 mu m.

When temperature and refractive index are measured in same spectrum, the measurement range of parameters is generally narrow due to bandwidth limitation. We proposed a sensor for simultaneous measurement of them in a wide wavelength range for this problem. The proposed sensor is a dual-channel structure based on the photonic crystal fiber (PCF) with three air holes and a polished plane. The silver film deposited on the polished plane is responsible for refractive index measurement, and the sodium film and polydimethylsiloxane (PDMS) filled in an air hole for temperature measurement. According to research findings, the two kinds of metal films work in different wavebands, which increases the sensing bandwidth and the measurement range of the parameters to be measured. Numerical results show that when the refractive index of analyte is 1.35 ∼ 1.41 and the temperature is 10 ∼ 60℃, the maximum spectral sensitivity of the sensor is 8400 nm/RIU and 10.2 nm/℃, respectively. The results show that the performance of the sensor is not sensitive to structural parameters. It makes the sensor easy to manufacture. The excellent performances and wide detection range make the proposed sensor have a good application prospect in biochemical detection, environmental monitoring, medical sensing, and other fields.

We propose a homogeneous 13-core 4 linear-polarized modes fiber based on a suspended air-trench V-type index core (SA-VC) structure that can suppress inter-core cross talk (ICXT) and inter-mode cross talk. The SA structure is used to control ICXT, and the VC structure is used to control inter-mode cross talk. Cross talk and other properties of the fiber are calculated by coupled power theory and the finite element method. The numerical results demonstrate that the SA-VC structure has stronger cross talk suppression ability compared with traditional trench structure, the ICXT meets the requirement of lower than − 30 d B / 100 k m , the effective refractive index difference ( Δ n e f f ) between modes is greater than 1.52 × 10 − 3 , the maximum intrinsic loss is 0.189 dB/km, and the relative core multiplicity factor reaches 49.09. Based on the current optical fiber fabrication technology, a manufacturing scheme for this fiber preform is proposed. This design structure aims to be applied to communication systems to increase the transmission capacity of optical fibers.