Analytic model for light guidance in single-wall hollow-core anti-resonant fibers

Confinement loss prediction in diverse anti-resonant fibers through neural networks

In this work, genetic algorithm (GA) is employed to optimize convolutional neural networks (CNNs) for predicting the confinement loss (CL) in anti-resonant fibers (ARFs), achieving a prediction accuracy of CL magnitude reached 90.6%, which, to the best of our knowledge, represents the highest accuracy to date and marks the first instance of using a single model to predict CL across diverse ARF structures. Different from the previous definition of ARF structures with parameter groups, we use anchor points to describe these structures, thus eliminating the differences in expression among them. This improvement allows the model to gain insight into the specific structural characteristics, thereby enhancing its generalization capabilities. Furthermore, we demonstrate a particle swarm optimization algorithm (PSO), driven by our model, for the design of ARFs, validating the model's robust predictive accuracy and versatility. Compared with the calculation of CL by finite element method (FEM), this model significantly reduces the cost time, and provides a speed-up method in fiber design driven by numerical calculation.

Polarization sensitive optical side leakage radiometry for distributed characterization of anti-resonant hollow-core fibers

A novel technique referred to as optical side leakage radiometry is proposed and experimentally demonstrated for non-destructive and distributed characterization of anti-resonant hollow-core optical fibers with high spatial resolution. Through in-depth analysis of the leakage light collection, we discover a unique polarization dependence, which is validated by our experiment. By leveraging this effect and employing Fourier filtering, this method enables accurate quantification of propagation attenuations for fundamental and higher order modes (with the uncertainty of <1 dB/km), identification of localized defects (with the resolution of ∼5 cm), and measurement of ultra-low spectral phase birefringence (at the level of 10-7) in two in-house-fabricated nested antiresonant nodeless hollow-core fibers. Such a fiber characterization approach, boasting unprecedently high accuracy and a potentially wide dynamic range, holds the potential to become an indispensable diagnosis tool for monitoring and assisting the manufacture of high-quality anti-resonant hollow-core fiber.

Antiresonant fiber structures based on swarm intelligence design

In this work, we obtained a new, to the best of our knowledge, structure of anti-resonant fiber (ARF) by an adaptive particle swarm optimization (PSO) algorithm. Different from the prior method of stacking elemental parts and optimizing parameters through experience or algorithm, we decompose some classic structures into points and optimize the positions of these points through swarm intelligence. The fiber structure is reconstructed by interpolation, and some new structures with low confinement loss (CL) and high higher order mode extinction ratio (HOMER) are obtained. These novel ARFs exhibit similar structural characteristics, and are named as "the bulb-shaped ARFs". Among these structures, the minimum achieved CL is 2.21 × 10-5dB/m at 1300 nm and the maximum achieved HOMER exceeds 14,000. This work provides a method with high degree of freedom in the design of non-uniform cross-section waveguides and helps to discover new fiber structures.

Laser-induced damage of an anti-resonant hollow-core fiber for high-power laser delivery at 1 µm

We demonstrate high-power laser delivery exceeding 1 kilowatt through a 5-meter homemade anti-resonant hollow-core fiber (AR-HCF) at 1-µm wavelength. Laser-induced damage to the fiber coating and jacket glass is experimentally observed respectively for different incident laser powers from a few hundred watts up to nearly 1.5 kilowatts. The cladding microstructure of the AR-HCF is free of damage at the incident end when 80% of the 1.5-kW incident power is coupled in. The deviation of an incident laser beam from the core to the cladding causes no damage but only deterioration of the coupling efficiency. The potential of the AR-HCF for higher-power laser delivery is discussed.

Interpreting light guidance in antiresonant and photonic bandgap waveguides and fibers by light scattering: analytical model and ultra-low guidance

Here, we introduce a quasi-analytic model that allows studying mode formation in low refractive index core waveguides through solely focusing on the cladding properties. The model isolates the reflection properties of the cladding from the modes via correlating the complex amplitude reflection coefficient of the cladding to the complex effective index of the fundamental core mode. The relevance and validity of the model are demonstrated by considering a single-ring anti-resonant fiber, revealing unexpected situations of exceptionally low loss. Our model explains mode formation by light scattering, which conceptually provides deep insights into the relevant physics.

Confinement loss of anti-resonant capillaries with curved boundaries

A systematic analysis of the dependence of the confinement loss of an anti-resonant capillary on the curvature of the core surround is presented. The core boundary is described by circular arcs and the construction allows for a wide range of core shapes to be considered. It is found that both negative and positive curvatures substantially reduce the confinement loss relative to that of a circular anti-resonant capillary and that this effect is insensitive to the size of the core relative to the wavelength and to the properties of the glass capillary wall. In contrast, for a solid core surround there is a small increase in the confinement loss with curvature. Results of scalar and vector calculations are shown to be similar. A qualitative explanation of the results is proposed based on azimuthal confinement of the wave fields generated by the curved boundaries.

Quantitative analysis of anti-resonance in single-ring, hollow-core fibres

The dependence of the confinement loss of unjacketed and jacketed single-ring fibres on structural parameters and the wavelength is analysed with reference to an anti-resonant model for which an analytic expression for the loss is available. Provided leakage through the gaps between the cladding capillaries is suppressed, the loss of unjacketed structures follows the prediction of the analytic model closely in terms of the scaling with respect to the radius and glass thickness of the capillaries, and the ratio of the wavelength to the core radius. The absolute value of the confinement loss and its dependence on the dielectric constant differ significantly from the analytic model; these differences are discussed in terms of the negative curvature of the core-cladding boundary. The loss of jacketed structures does not follow the anti-resonant model as closely, but there is sufficient similarity to conclude that anti-resonance in the glass and air regions of the cladding is key to understanding the guidance mechanism.

Approximate model for analyzing band structures of single-ring hollow-core anti-resonant fibers

Precise knowledge of modal behavior is of essential importance for understanding light guidance, particularly in hollow-core fibers. Here we present a semi-analytical model that allows determination of bands formed in revolver-type anti-resonant hollow-core fibers. The approach is independent of the actual arrangement of the anti-resonant elements, does not enforce artificial lattice arrangements and allows determination of the effective indices of modes of preselected order. The simulations show two classes of modes: (i) low-order modes exhibiting effective indices with moderate slopes and (ii) a high number of high-order modes with very strong effective index dispersion, forming a quasi-continuum of modes. It is shown that the mode density scales with the square of the normalized frequency, being to some extent similar to the behavior of multimode fibers.

Hollow-core conjoined-tube negative-curvature fibre with ultralow loss

Countering the optical network ‘capacity crunch’ calls for a radical development in optical fibres that could simultaneously minimize nonlinearity penalties, chromatic dispersion and maximize signal launch power. Hollow-core fibres (HCF) can break the nonlinear Shannon limit of solid-core fibre and fulfil all above requirements, but its optical performance need to be significantly upgraded before they can be considered for high-capacity telecommunication systems. Here, we report a new HCF with conjoined-tubes in the cladding and a negative-curvature core shape. It exhibits a minimum transmission loss of 2 dB km⁻¹ at 1512 nm and a <16 dB km⁻¹ bandwidth spanning across the O, E, S, C, L telecom bands (1302–1637 nm). The debut of this conjoined-tube HCF, with combined merits of ultralow loss, broad bandwidth, low bending loss, high mode quality and simple structure heralds a new opportunity to fully unleash the potential of HCF in telecommunication applications.

Countering the optical network ‘capacity crunch’ requires developments in optical fibres. Here, the authors report a hollow-core fibre with conjoined tubes in the cladding and a negative-curvature core shape. It exhibits a transmission loss of 2 dB/km at 1512 nm and less than 16 dB/km bandwidth in the 1302–1637 nm range.

Nonlinear dynamic of picosecond pulse propagation in atmospheric air-filled hollow core fibers

Atmospheric air-filled hollow core (HC) fibers, representing the simplest yet reliable form of gas-filled hollow core fiber, show remarkable nonlinear properties and have several interesting applications such as pulse compression, frequency conversion and supercontinuum generation. Although the propagation of sub-picosecond and few hundred picosecond pulses are well-studied in air-filled fibers, the nonlinear response of air to pulses with a duration of a few picoseconds has interesting features that have not yet been explored fully. Here, we experimentally and theoretically study the nonlinear propagation of ~6 ps pulses in three different types of atmospheric air-filled HC fiber. With this pulse length, we were able to explore different nonlinear characteristics of air at different power levels. Using in-house-fabricated, state-of-the-art HC photonic bandgap, HC tubular and HC Kagomé fibers, we were able to associate the origin of the initial pulse broadening process in these fibers to rotational Raman scattering (RRS) at low power levels. Due to the broadband and low loss transmission window of the HC Kagomé fiber we used, we observed the transition from initial pulse broadening (by RRS) at lower powers, through long-range frequency conversion (2330 cm^-1) with the help of vibrational Raman scattering, to broadband (~700 nm) supercontinuum generation at high power levels. To model such a wide range of nonlinear processes in a unified approach, we have implemented a semi-quantum model for air into the generalized nonlinear Schrodinger equation, which surpasses the limits of the common single damping oscillator model in this pulse length regime. The model has been validated by comparison with experimental results and provides a powerful tool for the design, modeling and optimization of nonlinear processes in air-filled HC fibers.

Analytic model for the complex effective index of the leaky modes of tube-type anti-resonant hollow core fibers

Due to their promising applications, hollow-core fibers, in particular, their anti-resonant versions, have recently attracted the attention of the photonics community. Here, we introduce a model that approximates, using the reflection of a wave on a single planar film, modal guidance in tube-type anti-resonant waveguides whose core diameters are large compared to the wavelength. The model yields analytic expressions for the real and imaginary parts of the complex effective index of the leaky modes supported, and is valid in all practically relevant situations, excellently matching all the important dispersion and loss parameters. Essential principles such as the fourth power dependence of the modal loss on the core radius at all wavelengths and the geometry-independent transition refractive index, below which modal discrimination favors the fundamental mode are discussed. As application examples, we use our model for understanding higher-order mode suppression in revolver-type fibers and for uncovering the tuning capabilities associated with nonlinear pulse propagation.

Characterization of a liquid-filled nodeless anti-resonant fiber for biochemical sensing

We theoretically and experimentally characterize a liquid-filled nodeless anti-resonant fiber (LARF) that could find versatile applications in biochemical sensing. When a hollow-core nodeless anti-resonant fiber (HARF) is filled with a low refractive index liquid such as water or aqueous solutions in the whole hollow area, it preserves its anti-resonant reflection waveguiding mechanism with attributes encompassing the broad transmission bandwidth in UV, visible, and near IR; the neglectable confinement loss; and the acceptable single-mode quality. In comparison with other forms of hollow fiber, the moderate core size of our ARF allows both a large analyte-light overlap integral and a fast liquid flow rate. Such a LARF platform offers a promising route for creating compact, integrable and biocompatible all-fiber multifunctional optofluidic devices for in-situ applications. A proof-of-concept experiment of Raman spectroscopy using ethanol is presented, and applications in fluorescence spectroscopy, resonant Raman spectroscopy, noninvasive biochemical analysis, and interferometric sensing are in prospect.

Bending loss characterization in nodeless hollow-core anti-resonant fiber

We report high performance nodeless hollow-core anti-resonant fibers (HARFs) with broadband guidance from 850 nm to >1700 nm and transmission attenuation of ~100 dB/km. We systematically investigate their bending loss behaviors using both theoretical and experimental approaches. While a low bending loss value of 0.2 dB/m at 5 cm bending radius is attained in the long wavelength side (LWS) of the spectrum, in this paper, we pursue light guidance in the short wavelength side (SWS) under tight bending, which is yet to be explored. We analytically predict and experimentally verify a sub transmission band in the SWS with a broad bandwidth of 110 THz and an acceptable loss of 4.5 dB/m at 2 cm bending radius, indicating that light can be simultaneously guided in LWS and SWS even under tight bending condition. This provides an unprecedented degree of freedom to tailor the transmission spectrum under a tight bending state and opens new opportunities for HARFs to march into practical applications where broadband guidance under small bending radius is a prerequisite.

Bending-induced mode non-degeneracy and coupling in chalcogenide negative curvature fibers

We study bend loss in chalcogenide negative curvature fibers with different polarizations, different tube wall thicknesses, and different bend directions relative to the mode polarization. The coupling between the core mode and tube modes induces bend loss peaks in the two non-degenerate modes at the same bend radius. There is as much as a factor of 28 difference between the losses of the two polarization modes. The fiber with a larger tube wall thickness, corresponding to a smaller inner tube diameter, can sustain a smaller bend radius. The bend loss is sensitive to the bend direction when coupling occurs between the core mode and tube modes. A bend loss of 0.2 dB/m at a bend radius of 16 cm, corresponding to 0.2 dB/turn, can be achieved in a chalcogenide negative curvature fiber.

Single-ring hollow core optical fibers made by glass billet extrusion for Raman sensing

We report the fabrication of the first extruded hollow core optical fiber with a single ring of cladding holes, and its use in a chemical sensing application. These single suspended ring structures show antiresonance reflection optical waveguiding (ARROW) features in the visible part of the spectrum. The impact of preform pressurization on the geometry of these fibers is determined by the size of the different hole types in the preform. The fibers are used to perform Raman sensing of methanol, demonstrating their potential for future fiber sensing applications.

Hybrid transmission bands and large birefringence in hollow-core anti-resonant fibers

We identify, for the first time to our best knowledge, a new type of transmission band having hybrid resonance nature in hollow-core anti-resonant fibers (ARF). We elucidate its unique phase-locking feature of the electric field at the outermost boundary. Exploiting this hybrid band, large birefringence in the order of 10(-4) is obtained. Our analyses based on Kramer-Kronig relation and transverse field confinement interpret the link between the hybrid transmission band and the large birefringence. Guided by these analyses, an experimentally realizable polarization-maintaining ARF design is proposed by introducing multi-layered dielectric structure into a negative curvature core-surround. This multi-layered ARF possesses characteristics of low loss, broad transmission band and large birefringence simultaneously.

Higher-order mode suppression in chalcogenide negative curvature fibers

We find conditions for suppression of higher-order core modes in chalcogenide negative curvature fibers with an air core. An avoided crossing between the higher-order core modes and the fundamental modes in the tubes surrounding the core can be used to resonantly couple these modes, so that the higher-order core modes become lossy. In the parameter range of the avoided crossing, the higher-order core modes become hybrid modes that reside partly in the core and partly in the tubes. The loss ratio of the higher-order core modes to the fundamental core mode can be more than 50, while the leakage loss of the fundamental core mode is under 0.4 dB/m. We show that this loss ratio is almost unchanged when the core diameter changes and so will remain large in the presence of fluctuations that are due to the fiber drawing process.