Negative Curvature Hollow Core Research Articles

Raman amplifier based on stimulated Raman scattering in a methane-filled hollow core fiber

This article reports on a single pass amplifier based on stimulated Raman scattering in a methane-filled negative curvature hollow core fiber (HCF) to transition 1.06 μm power to 1.54 μm. The researchers measured the highest average Raman power at a single frequency in a methane filled HCF to date of 4.92 W (246 μJ/pulse), with a high average quantum efficiency of 95.9%. A numerical model for the system was developed and shows good agreement with measured thresholds and efficiencies. Model results from a trade space study indicate configuration regimes necessary to maximize 1.54 μm power while avoiding power loss from the secondary shift.

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

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.

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.

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.

Low loss negative curvature hollow core fiber in visible red-light and near-infrared bands

Considering the low loss transmission and commercialization of hollow core fiber, negative curvature hollow core fiber (NC-HCF) has become a research hotspot in recent years. We report an innovative NC-HCF with an antiresonant layer inside the cladding tubes based on semi-ellipse and semi-circle structures. Based on the COMSOL simulation, it exhibits an ultralow loss from 0.6 to 1.6μm transmission band. On the one hand, as for semi-ellipse and semi-circle with antiresonant layer structure, the fiber lowest confinement loss (CL) of LP01 mode reaches a lowest value of 7.608 × 10 − 4 dB / km at 0.78 μm, and the high-order mode suppression ratio (HOMER) value can reach 4069. On the other hand, the semi-circle and semi-ellipse with antiresonant layer fiber structure shows the lowest CL of 1.675 × 10 − 4 dB / km at 0.79 μm, and the value of HOMER is 1776. Moreover, based on the fabrication tolerance analysis, the hollow core fibers proposed indicate better performance.

Analysis of Negative Curvature Hollow Core Fiber Coupled with Single Mode and Graded-Index Fibers

Analysis of Negative Curvature Hollow Core Fiber Coupled with Single Mode and Graded-Index Fibers

Negative curvature hollow core fiber sensor for the measurement of strain and temperature.

Three different types of strain and temperature sensors based on negative curvature hollow core fiber (NCHCF) are proposed. Each sensor is produced by splicing a small section of the NCHCF between two sections of single mode fiber. Different types of interferometers are obtained simply by changing the splicing conditions. The first sensor consists on a single Fabry-Perot interferometer (FPI). The remaining two configurations are attained with the same sensing structure, depending on its position in relation to the interrogation setup. Thus, a double FPI or a hybrid sensor, the latter being composed by an FPI and a Michelson interferometer, are formed. The inline sensors are of submillimeter size, thus enabling nearly punctual measurements.

Mid-infrared hollow core fiber drawn from a 3D printed chalcogenide glass preform

We report the fabrication of a microstructured optical fiber drawn from a soft glass 3D printed preform. For this proof of concept, a chalcogenide glass that is well known for its capability to be shaped at low temperature and its mid-infrared transmission was selected: Te20As30Se50. The obtained negative curvature hollow core fiber shows several transmission bands in the 2–12 µm range that are reproduced numerically using finite element-based simulations and coupled mode theory.

Negative Curvature Hollow Core Fiber Based All-Fiber Interferometer and Its Sensing Applications to Temperature and Strain.

Negative curvature hollow core fiber (NCHCF) is a promising candidate for sensing applications; however, research on NCHCF based fiber sensors starts only in the recent two years. In this work, an all-fiber interferometer based on an NCHCF structure is proposed for the first time. The interferometer was fabricated by simple fusion splicing of a short section of an NCHCF between two singlemode fibers (SMFs). Both simulation and experimental results show that multiple modes and modal interferences are excited within the NCHCF structure. Periodic transmission dips with high spectral extinction ratio (up to 30 dB) and wide free spectral range (FSR) are produced, which is mainly introduced by the modes coupling between HE11 and HE12. A small portion of light guiding by means of Anti-resonant reflecting optical waveguide (ARROW) mechanism is also observed. The transmission dips, resulting from multimode interferences (MMI) and ARROW effect have a big difference in sensitivities to strain and temperature, thus making it possible to monitor these two parameters with a single sensor head by using a characteristic matrix approach. In addition, the proposed sensor structure is experimentally proven to have a good reproducibility.

Light transport and vortex-supported wave-guiding in micro-structured optical fibres.

In hydrodynamics, vortex generation upon the transition from smooth laminar flows to turbulence is generally accompanied by increased dissipation. However, vortices in the plane can provide transport barriers and decrease losses, as it happens in numerous geophysical, astrophysical flows and in tokamaks. Photon interactions with matter can affect light transport in ways resembling fluid dynamics. Here, we demonstrate significant impact of light vortex formation in micro-structured optical fibres on the energy dissipation. We show possibility of vortex formation in both solid core and hollow core fibres on the zero energy flow lines in the cladding. Through intensive numerical modelling using different independent approaches, we discovered a correlation between appearance of vortices and reduction of light leakage by three orders of magnitude, effectively improving wave guiding. This new effect potentially might have strong impact on numerous practical applications of micro-structured fibres. For instance, a strong light localization based on the same principle can also be achieved in the negative curvature hollow core fibres.

Design of Negative Curvature Hollow Core Fiber Based on Reinforcement Learning

In negative curvature hollow core fibers (NCHCFs), light guidance is based on the capillary structure in the cladding. To achieve desirable fiber propagation properties, various designs of the capillary structure have been proposed in literature. However, the design process so far depends more or less on experience. In this article, we propose a reinforcement learning (RL) based method of systematically optimizing the capillary structure to achieve low average confinement loss (CL) for a given operating wavelength range and core radius. We use a recurrent neural network (RNN) to interactively study the properties of different capillary structures. The wavelength averaged CLs of the resulting designs are more than one order of magnitude lower than the lowest average CL of prior designs in literature. The same approach can be applied to search for optimum capillary structures in terms of other fiber propagation properties such as bending loss (BL), higher order modes extinction ratio (HOMER), overlap of the optical mode with the capillary structure, or a trade-off among these properties.

基于负曲率空芯光纤的光泵太赫兹光纤激光器的理论研究

Based on the compact and efficient optically pumped terahertz laser(OPTL) technology, an optically pumped terahertz fiber laser(OPTFL) based on a negative curvature hollow core fiber was designed. This OPTFL used a hollow-core fiber with polymethylpentene(PMP) material as operation gas chamber and was filled with methanol gas and pumped by 9P(36) continuous-wave(CW) CO2 laser. Based on rate equations and the transmission theory in hollow core fiber, factors affecting the output characteristics of OPTFL were analyzed. By investigating inner microstructure of hollow-core fiber, a negative curvature hollow-core fiber for efficiently transmitting terahertz waves was proposed. Considering the designed negative curvature hollow-core fiber, the feasibility of long cavity OPTFL was analyzed. Theoretical calculations showed that by appropriately increasing the cavity length of the proposed OPTFL, the terahertz output power was expected to reach the order of 100 milliwatts with optimal operating conditions. The results provide a new method for the OPTFL with high power and high performance.

负曲率空芯光纤限制损耗的影响因素

负曲率空芯光纤限制损耗的影响因素

Multiphoton fluorescence excitation and detection with a single negative curvature hollow core fibre

In this letter, we propose a single fiber-based sensor setup allowing for simultaneous excitation and detection of multiphoton fluorescence. Presented sensor’s key element is the negative curvature hollow core fiber (NCHCF) with three transmission bands in the visible spectral range (414–423 nm, 510–552 nm and 680–784 nm), allowing for nearly dispersion-free guidance of 160 fs-long laser pulses at 730 nm photon wavelength. Total temporal broadening of a laser pulse propagating in the proposed sensor setup is only (7 ± 1) fs. The NCHCF output beam was additionally focused with a microlensed, multimode fiber tip, increasing the efficiency of multiphoton absorption. The usefulness of the sensor for the multiphoton spectroscopy experiments is tested on the solutions of fluorescein and flavin adenine dinucleotide. This optical fiber sensor combines simplicity, minimal size, and good optical properties, and can be found an interesing solution for the non-linear optical methods used in chemistry, biology and medicine.

Features of light leakage from the negative curvature hollow core fibers (Erratum)

Features of light leakage from the negative curvature hollow core fibers (Erratum)

Features of light leakage from the negative curvature hollow core fibers

We would like to continue the discussion of specific features of light leakage in the negative curvature hollow core fibers (NCHCFs). In our previous work, it was shown that consideration of the transverse Poynting vector component of air core modes in the vicinity of core–cladding boundary wall and its streamlines is of great importance for understanding light localization mechanism in NCHCFs. Using a simple model of a capillary with the azimuthally periodic distribution of the refractive index in the capillary wall, we demonstrate that the transverse Poynting vector component behavior is similar to that of NCHCFs. We also consider the impact of a supporting tube on light leakage in NCHCFs.

Strong Light Localization and a Peculiar Feature of Light Leakage in the Negative Curvature Hollow Core Fibers

In this paper we would like to continue a discussion started in our previous work and devoted to the mechanism of light localization in hollow core microstructured fibers with a noncircular core-cladding boundary. It has been shown in many works that, for waveguide microstructures with different types of core-cladding boundary shape, the positions of the transmission bands’ edges can be predicted by applying the well-known anti–resonant reflecting optical waveguide (ARROW) model. At the same time, the ARROW model cannot explain the strong light localization and guiding at high material loss inside the transmission bands which are observed in negative curvature hollow core fibers, for example. In this paper we want to clarify our previous findings and consider the light localization process from another point of view, namely, by comparing the light leakage process in waveguide microstructures with different shapes of the core-cladding boundary. The results are discussed based on the ARROW model and a new approach associated with the consideration of spatial dispersion occurring under the interaction of the air core mode with the core-cladding boundary.

Application of Negative Curvature Hollow-Core Fiber in an Optical Fiber Sensor Setup for Multiphoton Spectroscopy

In this paper, an application of negative curvature hollow core fiber (NCHCF) in an all-fiber, multiphoton fluorescence sensor setup is presented. The dispersion parameter (D) of this fiber does not exceed the value of 5 ps/nm × km across the optical spectrum of (680–750) nm, making it well suited for the purpose of multiphoton excitation of biological fluorophores. Employing 1.5 m of this fiber in a simple, all-fiber sensor setup allows us to perform multiphoton experiments without any dispersion compensation methods. Multiphoton excitation of nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FAD) with this fiber shows a 6- and 9-fold increase, respectively, in the total fluorescence signal collected when compared with the commercial solution in the form of a hollow-core photonic band gap fiber (HCPBF). To the author’s best knowledge, this is the first time an NCHCF was used in an optical-fiber sensor setup for multiphoton fluorescence experiments.

Dispersion measurement of microstructured negative curvature hollow core fiber

We present measurements of the dispersion of two negative curvature fibers at and around 1 μm. These fibers are suitable for beam delivery of both high power and ultrashort pulse lasers with one designed to operate at 1030 nm and the other designed for dual wavelength delivery at 515 and 1030 nm. Both fibers were found to exhibit anomalous dispersion measured to be 2.17±0.25 ps nm−1 km−1 for the fundamental mode rising to 4.69±0.17 ps nm−1 km−1 for the first-order mode in the 1030-nm fiber. Dispersion in the double wavelength fiber was found to be higher at 3.81±0.03 ps nm−1 km−1 for the fundamental mode guiding at 1030 nm.