Negative Curvature Fiber Research Articles

High-performance dual-core conjoined tube negative curvature fiber polarization beam splitter

High-performance dual-core conjoined tube negative curvature fiber polarization beam splitter

Negative curvature fiber (NCF) polarization filter with large polarization loss ratio and ultralow loss in the terahertz range

A high-performance polarization filter based on the negative curvature fiber (NCF) with an asymmetric cladding structure is proposed and analyzed. The resonant effects of the x-polarization fundamental mode (XPFM) of the NCF are enhanced by the single-layer semi-elliptical tubes with a gradient wall thickness in the x-axis direction, while the anti-resonant effects of the y-polarization fundamental mode (YPFM) are strengthened by the nested double-layer elliptical tubes in the y-axis direction. The optimized structure shows a polarization loss ratio of greater than 100 in the range of 0.98–1.02 THz in addition to propagation losses of 0.94 dB/m and 7.43 × 10−4 dB/m at 1 THz for XPFM and YPFM, respectively. A polarization loss ratio of 1,276 is achieved and the polarization loss ratio is 6,321 at 0.98 THz. Our results show that the NCF polarization filter delivers better performance than recently reported filters in the terahertz band and the results reveal a new strategy to design high-performance polarization filters.

Influence of cladding tube structure on transmission characteristics of negative curvature terahertz fiber

A novel type of polarization negative curvature terahertz fiber has been designed, which has a cladding region composed of two long semi-circular cladding tubes in the horizontal direction and four long semi-elliptical cladding tubes in the oblique direction. The optimal fiber dimensions are determined to investigate the internal relationship between polarization characteristics and fiber cladding structure. The transmission performances of the negative curvature fiber are investigated by varying the thickness of fiber cladding tubes and adjusting the structural characteristics of cladding tubes in different directions. Relatively good transmission performances can be obtained with a cladding tube thickness of 80 μm. The birefringence can be stably maintained at the order of 10−4, and the confinement loss of the x-polarized mode at 2.55 THz is 0.036 dB cm−1. Moreover, the dispersion coefficient of the two polarized modes is stable between ±0.6 ps (THz·cm)−1 in the 2.3–2.6 THz band. The bent performance of the designed fiber at 2.52 THz is also discussed. The results show that the fiber has low bending sensitivity, and can still maintain excellent birefringence and low confinement loss in the bending state. For the y polarization, the birefringence can be basically stabilized at the order of 10−4 within a bending radius of 10–60 cm.

Frequency selective low-loss transmission in negative curvature hollow-core fibers

A new low-loss negative curvature fiber design, named as the pole-anchored nested tubular negative curvature fiber, which exhibits frequency selective capabilities in the near-infrared region is proposed. This unique approach involves the addition of nested tubular elements anchored by vertical poles, which serve as an in-fiber resonator. The resulting frequency selective resonant loss dips exhibit high finesse and relative loss modulation depth. The concept of frequency selectivity in negative curvature fibers introduces unique possibilities for future applications of hollow-core structures.

A metasurface structure-assisted terahertz negative curvature fiber

Negative curvature fibers have the benefits of high bandwidth, low loss, and provide higher transmission performances. Birefringence of the fiber is particularly important, with potential application value in communication, imaging and other fields. In this paper, a metasurface structure-assisted terahertz negative curvature fiber is designed to improve the birefringence characteristic. This work breaks through the conditional design ideas, and theoretically analyzes the effect of adding gold microstructures in the cladding structures on the transmission characteristics of negative curvature fibers. The investigation results show that the birefringence of the designed fiber exhibits a significant increase in most frequency range between 1.75–2.4 THz compared to the results before adding gold microstructures. The maximum birefringence reaches as high as 0.00216 when the frequency is 2.175 THz and θ is 90°. Compared to the original frequency, the birefringence has increased by nearly an order of magnitude. The highest birefringence has been increased to 3.5 times. In addition, the influences of the gold microstructures on the confinement loss, waveguide dispersion, and effective mode field area are also investigated. This fiber provides a reference for the design of high birefringence terahertz fibers in the future.

Low-loss weak-coupling 6-mode hollow-core negative curvature fiber based on symmetric double-ring nested tube

Few-mode optical fibers have played an increasingly important role in breaking through the transmission capacity limitations of single-mode optical fiber and alleviating the bandwidth crisis in optic fiber communication systems in recent years. Nevertheless, traditional solid core few-mode optical fibers usually suffer optical fiber nonlinearity and mode coupling, leading to mode crosstalk between channels. Hollow core negative curvature fibers (HC-NCF) have attracted widespread attention due to their advantages, such as low latency, low nonlinearity, low dispersion, low transmission loss, and large operating bandwidth. In this work, a novel low-loss few-mode HC-NCF with symmetrically double ring nested tube structure is designed, which supports six core modes including LP<sub>01</sub>, LP<sub>11</sub>, LP<sub>21</sub>, LP<sub>02</sub>, LP<sub>31<i>a</i></sub>, and LP<sub>31<i>b</i></sub>. The designed optical fiber is based on silica dioxide substrate and adopts a unique symmetrical double ring nested cladding structure, which can effectively suppress the coupling between the core mode and the cladding mode. The finite element method (FDE) is used to numerically analyze the properties of the proposed few-mode HC-NCF and optimize the structural parameters of the few-mode HC-NCF. Moreover, the confinement loss and bending loss of all core modes are investigated. The simulation results show that the proposed few-mode HC-NCF can support the independent transmission of six weakly coupled core modes (with the effective refractive index difference greater than 1×10<sup>–4</sup> between the adjacent core modes, which greatly avoids the coupling between the adjacent modes in the fiber core). In the 400 nm bandwidth (1.23–1.63 μm, covering the O, E, S, C, and L bands), all six modes in the fiber core maintain low loss transmission. Moreover, in the range of 1.3–1.63 μm, the confinement loss (CL) of LP<sub>01</sub>, LP<sub>11</sub> and LP<sub>21</sub> mode are all less than 1×10<sup>–3</sup> dB/m, and the CL of LP<sub>02</sub> and LP<sub>31<i>b</i></sub> mode are both less than 3×10<sup>–3</sup> dB/m. The CL of each mode reaches the lowest value at 1.4 μm, and the LP<sub>01</sub> mode has the lowest CL of 4.3×10<sup>–7</sup> dB/m. In addition, for a bending radius of 7 cm, each mode maintains the low bending loss characteristic in a certain operating wavelength range. In the range of 1.23–1.61 μm, the BL of LP<sub>01</sub> is less than 4.5×10<sup>–4</sup> dB/m, and the BL of LP<sub>11</sub> is less than 1.3×10<sup>–3</sup> dB/m. The tolerance analysis shows that even with the deviation of structural parameters of ±1%, the few-mode HC-NCF can still maintain the characteristic of low-loss and weak coupling. The designed few-mode HC-NCF has ultra-low CL and bending-insensitive characteristics while supporting independent transmission of six modes, which will find huge potential applications in future high performance mode division multiplexing systems.

Nested multibar cladding elements in negative curvature fibers for CO laser guidance

A numerical study on the multi-bar nested cladding design of chalcogenide glass-based negative curvature hollow-core fiber was carried out to achieve a low-loss light guidance in the mid-infrared spectrum centered at 5.4 μm. Fiber design parameters were systematically optimized, and the effect of the nested bars on the confinement and total loss performance of a five-tubular cladding structure was investigated. An ultra-low transmission loss of 0.112 dB km−1 at 5.4 μm was achieved with As2Se3 triple-bar negative curvature fiber while maintaining low bending sensitivity. The design is also suitable for high transmission performance with alternative infrared glasses and can be potentially used for low-loss light guidance in a wide mid-infrared spectrum.

Double nested dual-core negative curvature fiber polarization beam splitter

In this paper, a polarization beam splitter（PBS） based on double nested dual-core negative-curvature fiber (DNHC-NCF) is designed. Two hollow cores are constructed by eight double nested cladding tubes arranged at certain angles. The influences of structural parameters on the performances of the DNHC-NCF PBS have been studied by using the finite element method (FEM). According to the simulation results, the structure of the proposed DNHC-NCF PBS is optimized. The final results indicate that the length of the proposed DNHC-NCF PBS is 1.38 cm and the extinction ratio (ER) is less than −20 dB in the wavelength range of 1.48–1.80 µm (320 nm, covers S + C + L + U bands). Besides, the higher order mode extinction ratio (HOMER) in the wavelength range of 1.3–1.71 µm is greater than 100, showing good single-mode characteristic as well. It is believed that the proposed double nested dual-core negative curvature fiber polarization beam splitter will have important applications in the fields of optical communication system.

Hybrid nested negative curvature fiber with ultra-low-loss in the terahertz band

A novel hybrid nested negative curvature fiber (NCF) based on the three-layer anti-resonant effect is described. The elliptical tubes cladding structure exhibits the advantageofflexibledesign and can effectively reduce propagation losses. The influences of the nested methods and structural parameters on the propagation characteristic are analyzed numerically. Since high-resistance silicon (HRS) has an ultra-low absorption coefficient in the THz frequency range, the effective material loss is reduced. Meanwhile, the three-layer nested structure suppresses the confinement loss, the total loss is as low as 6.62 × 10−5 dB/m at 1.1 THz. The effects of the number of cladding tubes and dielectric layers on the propagation loss are compared and analyzed. The optimized NCF shows a loss of less than 10−4 dB/m between 1 and 1.06 THz and 1.1–1.24 THz. The result reveals a new method to design broadband ultra-low-loss hollow-core NCFs.

Negative curvature fiber for suppressing high-order radial OAM modes transmission

The combination of terahertz (THz) wave and the orbital angular momentum (OAM) multiplexing technology can further improve the communication capacity. Compared to other outer cladding structures, the negative curvature structure has been proven to provide stronger confinement effect on electromagnetic waves. Here, we propose a novel polymer (COC TOPAS) fiber which consists a central hollow-core, annular region and single layer negative curvature circle tubes as outer cladding for terahertz OAM modes transmission. The mode map is established by mathematical analysis of cut-off conditions for the vector modes, and the excitation of high-order radial modes in the fiber is successfully suppressed. In addition, the effective refractive index, confinement loss, effective mode area, mode purity and dispersion characteristics of the fiber are investigated under the condition of ‘single mode’ operation in the frequency range of 2.0–3.0 THz.

Hollow Core Double-Clad Fiber Coupler for Nonlinear Micro-Endoscopy

We report the design, fabrication and characterization of the first hollow core double-clad fiber coupler for nonlinear micro-endoscopy. It is made of a pure silica negative curvature hollow core double-clad fiber and a highly multimode fiber fused along each other. The fabrication of such a coupler was made possible thanks to the use of a soft (borosilicate) glass multimode fiber which can be fused along the negative curvature fiber without altering its geometry, thanks to its much lower fusion temperature. A 60% power transfer from the double-clad fiber to the multimode one was measured at 500 nm. Demonstration of two-photon fluorescence and second harmonic generation imaging was performed on unlabeled biological tissues with an endoscopic head mounted on the hollow core double-clad fiber coupler to demonstrate its applicability. This device greatly simplifies the integration of nonlinear micro-endoscopes based on hollow core double clad fibers.

Surface Plasmon Resonance-Based Gold-Coated Hollow-Core Negative Curvature Optical Fiber Sensor.

The hollow-core fiber-based sensor has garnered high interest due to its simple structure and low transmission loss. A new hollow-core negative-curvature fiber (HC-NCF) sensor based on the surface plasmon resonance (SPR) technique is proposed in this work. The cladding region is composed of six circular silica tubes and two elliptical silica tubes to reduce fabrication complexity. Chemically stable gold is used as a plasmonic material on the inner wall of the sensor structure to induce the SPR effect. The proposed sensor detects a minor variation in the refractive indices (RIs) of the analyte placed in the hollow core. Numerical investigations are carried out using the finite element method (FEM). Through the optimization of structural parameters, the maximum wavelength sensitivity of 6000 nm/RIU and the highest resolution of 2.5 × 10-5 RIU are achieved in the RI range of 1.31 to 1.36. In addition, an improved figure of merit (FOM) of 2000 RIU-1 for Y-polarization and 857.1 RIU-1 for X-polarization is obtained. Because of its simple structure, high sensitivity, high FOM, and low transmission loss, the proposed sensor can be used as a temperature sensor, a chemical sensor, and a biosensor.

Hollow-core negative curvature fiber with high birefringence for temperature sensing based on surface plasmon resonance effect

A hollow-core negative curvature fiber (HC-NCF) with high birefringence for temperature sensing based on surface plasmon resonance effect is proposed. For the HC-NCF, the cladding layer is composed of six silica tubes, and the thickness and radius of the two silica tubes in the y-axis direction are different from those of other ones. Moreover, the two silica tubes in the y-axis direction are filled with the gold wires, and the ethanol to be tested is filled in other silica tubes and air area. As the temperature changes, the coupling state between the x-pol core mode and surface plasmon polarization mode will change. The temperature sensitivity can be up to 3.1 nm / ° C in the range of 20°C to 40°C. Moreover, the proposed HC-NCF temperature sensor simultaneously achieves the maximum R of 3.333 × 10 − 2 ° C and maximum FOM of 269 ° C − 1, which indicates that it has high sensitivity, good linearity, high R, and large FOM. The proposed HC-NCF temperature sensor is expected to be useful in the field of the environmental monitoring.

A novel plasmonic sensor using solid core D-shaped negative curvature optical fiber with AU–TiO2 layer

Recently, unique structure fiber enacts a significant role in sensing applications. A novel D-shaped negative curvature fiber based surface plasmon resonance (SPR) sensor is proposed. The design consists of negative curvature solid core fiber surrounded by a cladding composed of double-layer adjoined airholes. An outside layer of the cladding contains semi-circular airholes, and the interior layer has small circular air holes to overwhelm the higher-order modes. The external sensing approach detects changes in the refractive index between 1.33 and 1.37. Employing gold as a plasmonic material, SPR is developed on top of the photonic crystal fiber (PCF). Between the gold layer and the PCF, a small layer of titanium dioxide is placed for binding the plasmonic material to the fiber. And it acts as an adhesive layer. The finite element method is used to perform numerical analyses. After improving the structural parameters, an enhanced sensitivity of 26 000 nm RIU−1 is achieved. Because of its enhanced sensitivity, this sensor is used in a bio-medical application to detect and prevent diseases in the early stage.

Bending hollow-core negative-curvature fiber for refractive index sensing based on surface plasmon resonance effect

We numerically investigated a novel refractive index (RI) sensor based on a liquid-filled and Au-coated hollow-core negative-curvature fiber (HC-NCF). In the design, the cladding area of the HC-NCF was composed of only one ring of six silica tubes, all the air holes in the fiber were filled with analytic liquids. The fiber bent in either the x or y-axis direction and Au-coated inside the single cladding tube in the corresponding bending direction. The influences of Au-coated HC-NCF structure parameters on the propagation characteristics under different bending directions were investigated using the finite element method (FEM). Moreover, the sensing performances in the RI range of 1.30–1.35 were evaluated through analyzing the position of the confinement loss peak. The simulation results showed that the HC-NCF RI sensor in the y-axis bending direction showed higher sensitivity, and the maximum sensitivity was 6.974 µm/RIU. The sensor shows high sensitivity and provides an effective method for measuring the RI of liquids.

Dual hollow-core negative curvature fiber polarization beam splitter covering the O + E + S + C + L communication band

In this paper, a dual hollow-core negative curvature fiber is proposed for the polarization beam splitter. The effects of the structure parameters of the dual hollow-core negative curvature fiber on the coupling length and coupling length ratio of the x -polarized and y -polarized core modes and the higher-order mode extinction ratio are analyzed by the finite element method. Moreover, the normalized output powers of the x -polarized and y -polarized modes in the cores A and B and the corresponding extinction ratio are also investigated by the mode coupling theory. The simulation results show that a dual hollow-core negative curvature fiber polarization beam splitter with the length of 6.45 cm can achieve a broad bandwidth of 400 nm (1.23–1.63 µm), covering the O + E + S + C + L communication band. In addition, the higher-order mode extinction ratio is greater than 100 in the considered wavelength range, which means that it has good single-mode characteristics. It is believed that the proposed dual hollow-core negative curvature fiber polarization beam splitter will have significant applications in the optical communication system.

Numerical simulation of a truncated cladding negative curvature fiber sensor based on the surface plasmon resonance effect

A refractive index (RI) sensor based on the surface plasmon resonance effect is proposed using a truncated cladding negative curvature fiber (TC-NCF). The influences of the TC-NCF structure parameters on the sensing performances are investigated and compared with the traditional NCF. The simulation results show that the proposed TC-NCF RI sensor has an ultra-wide detection range from 1.16 to 1.43. The maximum wavelength sensitivity reaches 12400 nm/RIU, and the corresponding R 2 of the polynomial fitting equation is 0.9999. The maximum and minimum resolutions are 2.56 × 10−5 and 8.06 × 10−6, respectively. In addition, the maximum amplitude sensitivity can reach –379.1 RIU−1 when the RI is chosen as 1.43. The proposed TC-NCF RI sensor could be useful in biochemical medicine, environmental monitoring, and food safety.

Real-time azimuth determination for the post-processing of negative curvature fibers

In this paper, with the laser side irradiation of the NCFs, the forward scattering patterns can be obtained through the scattered spectra. Some characteristic values of the scattering patterns are introduced, and they correspond to the integral intensity of the electric field and are closely related to the azimuth. We proposed two methods to determine the azimuth of monolayer and bilayer NCFs, and the corresponding errors are less than 2° and 1°, respectively. In addition, we discussed the influence of the structural parameters of the NCFs and the relative position of the screen, and the results show that the methods have universal applicability and screen position insensitivity. The methods can be applied to the online azimuth determination of the NCFs without damage and pollution to the NCFs.

Ultralow loss hollow-core negative curvature fibers with nested elliptical antiresonance tubes.

Hollow-core negative curvature fibers can confine light within air core and have small nonlinearity and dispersion and high damage threshold, thereby attracting a great deal of interest in the field of hollow core fibers. However, reducing the loss of hollow-core negative curvature fibers is a serious problem. On this basis, three new types of fibers with different nested tube structures are proposed in the near-infrared spectral regions and compared in detail with a previously proposed hollow-core negative curvature fiber. We used finite-element method for numerical simulation studies of their transmission loss, bending loss, and single-mode performance, and then the transmission performance of various structural fibers is compared. We found that the nested elliptical antiresonant fiber 1 has better transmission performance than that of the three other types of fibers in the spectral range of 0.72-1.6 µm. Results show that the confinement loss of the LP01 mode is as low as 6.45×10-6 dB/km at λ = 1.06 µm. To the best of our knowledge, the record low level of confinement loss of hollow-core antiresonant fibers with nested tube structures was created. In addition, the nested elliptical antiresonant fiber 1 has better bending resistance, and its bending loss was below 2.99×10-2 dB/km at 5 cm bending radius.

A self-verification temperature sensor based on surface plasmon resonance in a hollow core negative curvature fiber

We proposed and simulated a surface plasmon resonance (SPR) temperature sensor with two loss peaks in a hollow core negative curvature fiber (HC-NCF). Inner walls of the anti-resonant tubes in HC-NCF were plated with gold films to stimulate SPR, while the thermo-optic mixture of toluene and chloroform was filled in the air holes in HC-NCF to modulate the coupling between core modes and surface plasmon polaron modes (SPPMs). Simulation results showed that two SPPMs with opposite thermo-optic respond effects were excited at two separate wavelength bands due to their different dispersion characteristics. Temperature measurement sensitivities of −3.976 nm °C−1 and 1.071 nm °C−1 were obtained for the two SPPMs, while the sensitivity reached −5.047 nm °C−1 when detected the wavelength interval between the two SPPMs loss peaks. The two separate loss peaks could also be utilized in self-verification. The designed temperature sensor based on HC-NCF and SPR depicts high sensitivity and self-verification, which could be utilized for high precision and stable temperature monitoring.