Hollow-core Negative Curvature Fiber Research Articles

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.

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.

Enhancing polarization maintenance and spectral filtering in negative curvature hollow-core fibers

A new design of polarization-maintaining and spectral filtering negative curvature hollow-core fiber tailored for the telecommunication bands in the near-infrared region is presented. The optical fiber, consisting of a six-tube silica structure, incorporates vertically nested tubes anchored radially by a pole structure. By contrast, standard nested tubes in the horizontal direction form the asymmetric fiber structure, which encounters birefringence. This unique fiber design not only preserves the polarization states of light but also exhibits frequency selective transmission exclusively in the vertical direction due to the pole structure. Through fiber design optimization, a transmission loss below 0.1 dB/km for spectrally filtered wavelengths is achieved, with birefringence on the order of 10−5 within the wavelength range of 1.45 µm to 1.60 µm. These results demonstrate significant improvements in terms of birefringence, distinct loss separation between horizontally and vertically polarized states, and a reduced number of spectrally filtered wavelengths compared to previously reported findings. The proposed fiber design holds untapped potential for applications requiring selective transmissions with specific polarization.

Highly sensitive temperature sensor using nested hollow-core anti-resonant fibers

A highly sensitive temperature sensor based on composite nested negative curvature hollow-core fiber is proposed. Three resonant tubes are nested inside the cladding tubes to induce a resonant coupling between the fundamental core mode and the dielectric mode of the nested resonant tube. Three glass-sheet conjoined nested tubes are introduced to suppress the comparatively low loss higher-order mode LP02. All the air-holes are filled with temperature-sensitive liquid of ethanol. As the temperature changes, the resonant dip in the transmission spectrum corresponding to the coupling will shift. Numerical simulation results indicate that the temperature sensitivity is 3.36 nm °C−1 in the range of 20 °C–40 °C. Since there is no need of any special post-processing, such as selective infiltration or material coating for the sensors proposed, the sensor provides a promising mechanism for the optical fiber temperature sensing.

High Resolution and Sensitivity Negative Curvature Hollow Core Fiber Refractive Index Sensor Based on LSPR

High Resolution and Sensitivity Negative Curvature Hollow Core Fiber Refractive Index Sensor Based on LSPR

Design of a negative curvature hollow-core fiber for transmitting a single-mode dark hollow beam

We propose a nested negative curvature hollow-core fiber that can stably transmit a single-mode (LP11 mode) dark hollow beam with low loss. A pair of resonant tubes is nested inside the cladding tubes in the horizontal direction to enhance the confinement loss of the fundamental mode LP01. Two pairs of glass sheets at different positions within the four cladding tubes in the non-horizontal direction are introduced to suppress the comparatively low loss higher-order modes LP21 and LP02. The numerical simulation results indicate that the loss ratio between the fundamental mode and LP11 mode can reach 28703 when the transmission wavelength is 1.54 µm, and the LP11 mode has the lowest confinement loss of 0.0049 dB/m. Meanwhile, the loss ratios of higher-order modes such as LP21, LP02, and LP31 with the target LP11 mode are above 100. The effective bandwidth of single-mode dark hollow beam transmission can reach 5 nm. This provides an optical fiber solution for single-mode dark hollow beam transmission, which has potential applications in atomic optics, quantum information, and nonlinear optics.

Orbital Angular Momentum in Fibers

Orbital angular momentum (OAM) of light beams with helical phase front, as an alternative spatial dimension resource of photons, has attracted extensive attention over the past decades. Very recently, OAM in fibers has facilitated rapid development of various fiber-based applications, such as optical communications, optical manipulation, optical sensing, optical imaging, nonlinear optics, and quantum science. Comprehensive and in-depth understanding of OAM in fibers is of great interest to researchers in this area. Here, we give a tutorial overview of OAM in fibers. We first introduce the basic theories, including the eigenmode analysis method, mode coupling theory, and spin-orbit mapping of light in fibers. We then give a detailed classification of OAM fibers and discuss versatile spatial mode bases in fibers. After that, we review various OAM fiber designs, including conventional single-mode fibers, conventional multi-mode fibers, ring-core fibers, air-core fibers, photonic crystal fibers, negative-curvature hollow-core fibers, multi-core fibers, active fibers (e.g., erbium-doped fibers), etc. Besides, we describe the state-of-the-art commercial fiber manufacturing processes as well as testing and characterization methods for OAM fibers. We also review various active and passive OAM fiber devices. Moreover, we introduce several typical advanced applications based on OAM fibers, such as capacity scaling fiber-optic communications, remote vectorial Doppler velocimetry, super-resolution optical imaging, and high-dimensional quantum cryptography. Finally, we briefly discuss future challenges and prospects of OAM in fibers.

Orbital Angular Momentum Resonances Arising from Mode Coupling in Hollow-Core Fibers

It is known that the growth of the orbital part of the angular momentum of the fundamental air core mode of the negative curvature hollow-core fibers (NCHCFs) with circular polarization occurs at the edge of the transmission bands due to the spin–orbit interaction. In this paper, we consider the resonant behavior of orbital angular momentum (OAM) for a linearly polarized fundamental air core mode in straight and bent NCHCF. The resonant growth of OAM is associated with coupling between the linear polarized fundamental air core mode and the cladding capillary wall modes. The coupling between the modes arises due to crossing and anti-crossing. This OAM growth can be explained by a complex interaction of energy fluxes of the coupled modes under crossing. This phenomenon may be useful for a deeper understanding of the processes arising from mode coupling in fiber optics and nanophotonics, as well as for generating and transmitting OAM modes in micro-structured optical fibers.

High-Sensitive Curvature Sensor Based on Negative Curvature Hollow Core Fiber

A high-sensitive curvature sensor based on the negative curvature hollow core fiber (NCHCF) is proposed and demonstrated. The graded index fiber (GIF) is introduced between single-mode fiber (SMF) and NCHCF for the cascaded structure of SMF-GIF-NCHCF-GIF-SMF. Compared to the structure without GIF, the coupling efficiency is improved by 16.5%. For the cascaded structure, multi-mode interference (MMI) and anti-resonance (AR) mechanisms coexist in the transmission spectra. The experimental results show that MMI-based dips have a blue shift as curvature increases while AR is not sensitive to its variation. As such, the self-referencing approach is employed for improving the performance of the curvature sensor with a sensitivity of up to −5.27 nm/m−1. Furthermore, the temperature sensitivity of the sensor is −0.003 nm/°C, indicating negligible temperature crosstalk. With the advantages of compactness, efficient coupling, high sensitivity, and negligible temperature crosstalk, the curvature sensor shows prospects in structural security monitoring, the aviation industry, energy exploration, and other fields.

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.

Highly sensitive methane detection using a mid-infrared interband cascade laser and an anti-resonant hollow-core fiber.

For over a decade hollow-core fibers have been used in optical gas sensors in the role of gas cells. However, very few examples of actual real-life applications of those sensors have been demonstrated so far. In this paper, we present a highly-sensitive hollow-core fiber based methane sensor. Mid-infrared distributed feedback interband cascade laser operating near 3.27 µm is used to detect gas inside anti-resonant hollow-core fiber. R(3) line near 3057.71 cm-1 located in ν3 band of methane is targeted. Compact, lens-free optical setup with an all-silica negative curvature hollow-core fiber as the gas cell is demonstrated. Using wavelength modulation spectroscopy and 7.5-m-long fiber the detection limit as low as 1.54 ppbv (at 20 s) is obtained. The demonstrated system is applied for a week-long continuous monitoring of ambient methane and water vapor in atmospheric air at ground level. Diurnal cycles in methane concentrations are observed, what proves the sensor's usability in environmental monitoring.

Parametric Optimization for Low Loss Negative Curvature Hollow Core Fiber With Elliptical Tube

Negative curvature hollow core fiber (NC-HCF) with elliptical tubes is beneficial for low confinement loss (CL) due to the rich degree of freedom in tube curvature. In this paper, an empirical formula for optimizing the structural parameters of NC-HCF with elliptical tubes is presented, which is aimed at the minimum CL. It is obtained by analyzing the trade-off between the mode coupling induced loss and the gap induced leakage loss, according to the theory of mode coupling between core fundamental mode and tube ring mode, and the geometric arrangement. Consequently, a relationship between gap and elliptical tube diameter in the radial direction has been found to be 1/2, with corresponding formula validated by the numerical investigation and simulation for a wide wavelength range and different elliptical tube numbers. The parametric optimization formula provides a design guideline for low loss NC-HCF, which can be an effective tool for further exploration and application.

Mid-infrared negative curvature hollow-core fiber with elliptically nested tubular structure

Arsenic triselenide chalcogenide negative curvature hollow-core fiber with ellipse-nested tubular cladding elements was numerically presented for the low-loss light guidance in the mid-infrared region. Confinement and total losses were calculated for the fiber with a five-tube design as low as 0.34 dB/km and 4.55 dB/km at 10.6μm, respectively. The single-mode light guidance characteristic of the proposed design was investigated, and a strong suppression on the higher-order modes shows that the fiber design favors the light guidance with the fundamental mode. A dispersion control study was carried out to optimize the primary design parameters to shift the zero-dispersion wavelength close to 10.6μm. The bending loss performance of the proposed design was analyzed in the targeted spectrum. The proposed negative curvature hollow-core fiber could be potentially used in low-loss guidance for high-power lasers and could also be utilized for several other applications, such as surgery and drug delivery in the mid-IR spectrum.

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.

Optimization method for low-loss single-mode bending negative curvature anti-resonant hollow-core fiber designed by mode modification.

A method of designing negative curvature anti-resonant hollow-core fibers (NC-AR-HCFs) with bending resistance is proposed, by which the fundamental mode (FM) and higher-order mode (HOM) can be adjusted. An asymmetric double-ring negative curvature hollow-core fiber (ADR-NC-HCF) is proposed to verify the method. The ADR-NC-HCF achieves the FM loss of 0.8dB/km at 1550nm under the bending radius of 20mm. The coupling relation between the modes in ADR-NC-HCFs is analyzed revealing the physical principle of the design method. Based on the principle, the fiber can be directionally optimized to achieve a lower loss of the FM or higher-order mode extinction ratio.

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 Modeling of Guided Mode for Negative Curvature Hollow-core Fibers

Numerical Modeling of Guided Mode for Negative Curvature Hollow-core Fibers

Enhancing fiber atom interferometer by in-fiber laser cooling

We demonstrate an inertia sensitive atom interferometer optically guided inside a 22-cm-long negative curvature hollow-core photonic crystal fiber with an interferometer time of 20 ms. The result prolongs the previous fiber guided atom interferometer time by three orders of magnitude. The improvement arises from the realization of in-fiber {\Lambda}-enhanced gray molasses and delta-kick cooling to cool atoms from 32 {\mu}K to below 1 {\mu}K in 4 ms. The in-fiber cooling overcomes the inevitable heating during the atom loading process and allows a shallow guiding optical potential to minimize decoherence. Our results permit bringing atoms close to source fields for sensing and could lead to compact inertial quantum sensors with a sub-millimeter resolution.