Multi-nested antiresonant hollow-core fiber with ultralow loss and single-mode guidance.

We propose an antiresonant hollow-core fiber design that exhibits ultralow loss and exceptional single modedness at 1.55 µm. In this design, the confinement loss of less than 10-6 dB m-1 can be obtained with excellent bending performance even at a tight bending radius of 3 cm. At the same time, a record-high higher-order mode extinction ratio of 8 × 105 can be achieved in the geometry by inducing strong coupling between the higher-order core modes and cladding hole modes. These guiding properties make it an excellent candidate for applications in hollow-core fiber-enabled low-latency telecommunication systems.

Antiresonant hollow-core fiber Bragg grating design.

Fiber Bragg gratings (FBGs) inscribed in hollow-core fibers hold a potential to revolutionize the field of gas photonics by enhancing the performance and versatility of hollow-core fiber-based matter cells. By effectively transforming these cells into cavities, FBGs can significantly extend the effective length of light-matter interactions. Traditional FBG inscription methods cannot be extended to hollow-core fibers, because light in the fundamental mode is predominantly confined to the hollow region where an index change cannot be induced. In this Letter, we propose a bi-thickness dual-ring hollow-core antiresonant fiber (DRHCF) design that achieves substantial overlap between the fundamental mode and cladding glass in a well-controlled manner, ensuring a strong FBG response with a minimal insertion loss. Through detailed numerical investigations, we demonstrate the feasibility of creating a high reflection FBG in the DRHCF using standard FBG inscription techniques. The proposed device is expected to have a length of <1 cm and the insertion loss of <0.3 dB, including splice loss.

Integration of an anti-resonant hollow-core fiber with a multimode Yb-doped fiber for high power near-diffraction-limited laser operation.

We proposed and demonstrated mode cleaning in a high-power fiber laser by integrating an anti-resonant hollow-core fiber (AR-HCF) into a multimode laser cavity of an ytterbium (Yb)-doped fiber (YDF). An in-house mode-matched AR-HCF was fusion-spliced to a commercial multimode LMA-YDF, ensuring efficient fundamental mode coupling. The AR-HCF inflicts a high propagation loss selectively on higher-order modes, facilitating fundamental mode operation. Thus, the AR-HCF works as an efficient spatial mode filter embedded in the multimode fiber laser cavity and reinforces preferential amplification of the fundamental mode. Beam quality factor enhancement was achieved from M2 = 2.09 to 1.39 at an output power of 57.7 W (pump-power limited). The beam quality can be further improved by refining the AR-HCF fabrication. The proposed technique has a great potential to be exploited in other multimode fiber laser cavities involving erbium- or thulium-doped fibers and obviates the need for complicated specialty active fiber designs. Compared with the commonly used fiber bending technique, our method can achieve an efficient higher-order mode suppression without inducing mode-field deterioration.

Understanding bending-induced loss and bending-enhanced higher-order mode suppression in negative curvature fibers.

We present a numerical analysis on bending-induced loss and bending-enhanced higher-order mode suppression in negative curvature fibers. We provide underlying mechanisms on how geometrical parameters affect the bending properties. We find that fiber parameters influence the bending performance by altering the resonant coupling conditions, as well as light leakage through inter-tube gaps. We identify regions in the parameter space that exhibit excellent bending properties and offer general guidelines for designing negative curvature fibers that are less sensitive to bending. Moreover, we explore the possibility of enhancing higher-order core mode suppression through mechanical bending. We find that up to nine-fold increase in the higher-order mode extinction ratio can be achieved by bending the fiber.

Effect of decreasing pressure on soliton self-compression in higher-order modes of a gas-filled capillary.

We numerically investigate soliton self-compression in the higher-order modes of a gas-filled capillary with decreasing pressure. We demonstrate four times enhancement in the compression with the decreasing pressure compared to the equivalent constant pressure case in the HE12 mode, reaching sub-cycle duration of 1.85 fs at its output. Moreover, the negative pressure gradient effectively suppresses the intermodal coupling in the later stage of the compressor, which helps to maintain high output mode purity. These findings are of direct benefit for applications that require ultrashort light pulses in unconventional spatial beam profiles, including in nonlinear frequency conversion, microscopy, micromachining, and particle manipulation.

In-line hollow-core fiber-optic bandpass filter.

We present an antiresonant hollow-core fiber-based bandpass optical filter. The device is realized by tapering down a section of tubular hollow-core fiber to a ratio of less than 0.5. Sweeping of the tube wall thickness-induced resonant bands in the down- and up-transition sections of the taper suppresses the blue side of the spectrum, while the red side filtering exploits the increased confinement loss at the taper waist that depends sharply on the wavelength-to-core-diameter ratio. These working principles of the filter make it possible to customize the location and width of the passband by tailoring the fiber design and taper profile. We achieve a 350-nm-wide bandpass filter with the minimum insertion loss of 1.3 dB in the passband and up to 40 dB suppression in the lossbands. We anticipate the filter to become one of the essential components in all-hollow-core fiberized optical systems.

Anti-resonant hollow-core fiber fusion spliced to laser gain fiber for high-power beam delivery.

We present the selective excitation of the fundamental mode in an anti-resonant hollow-core fiber (ARHCF) fusion-spliced with a commercial large mode area (LMA) fiber. By designing and fabricating a single-ring ARHCF that is mode-matched to a LMA fiber and by splicing the two using a CO2 laser-based splicer, we achieve a coupling efficiency of 91.2% into the fundamental mode. We also demonstrate an all-fiber integration of an ARHCF with a commercial ytterbium-doped fiber in a laser cavity for beam delivery application. Coupling of the single-mode laser output beam into the fundamental mode of the ARHCF is demonstrated with 90.4% efficiency (<0.45dB loss) for up to 50 W continuous wave beam in a stable and alignment-free all-fiber laser setup.

Analyzing mode index mismatch and field overlap for light guidance in negative-curvature fibers.

We numerically investigate the role of cladding geometries in two widely used anti-resonant hollow-core fiber designs with negative curvatures, the tubular negative-curvature fiber and ice-cream-cone negative-curvature fiber. The confinement loss governed by the inhibited coupling between the modes in the core and cladding is thoroughly examined systematically against the core-cladding curvature for both types. We show that, in addition to the mode-index mismatch, the mode-field overlap also plays a key role in determining the loss. Simultaneously, we find the ice-cream-cone negative-curvature fiber can exhibit better loss performance than the tubular design within a specific range of the curvature. This enhancement is achieved without sacrificing the transmission bandwidth and is relatively robust against the fabrication error.

Effect of the second ring of antiresonant tubes in negative-curvature fibers.

We present a numerical investigation on the effect of introducing the second ring of antiresonant tubes on the guiding properties of the negative-curvature fiber. We determine the range of structural parameters for achieving the optimum light guidance in the double-ring geometry. Our study shows that the double-ring negative-curvature fiber can improve the confinement loss by up to four orders of magnitude with considerably better bending and single-mode performance when compared to its single-ring counterpart.

Band-edge mediated frequency down-conversion in a gas-filled anti-resonant hollow-core fiber.

We demonstrate frequency down-conversions of femtosecond pulses through dispersive wave generation and degenerate four-wave mixing in a gas-filled anti-resonant hollow-core fiber. These are achieved by exploiting the rapid variation of the dispersion in the fiber's transmission band edge. In this approach, the wavelength of the down-shifted radiation is governed solely by the thickness of the dielectric wall at the core-cladding interface, while other system parameters are accountable only for inducing sufficient nonlinear phase shifts. With the right choice of cladding wall thickness, the concept can be applied directly for generating high-power mid-infrared femtosecond pulses.

Dissipative solitons with extreme spikes in the normal and anomalous dispersion regimes.

Prigogine's ideas of systems far from equilibrium and self-organization (Prigogine & Lefever. 1968 J. Chem. Phys.48, 1695-1700 (doi:10.1063/1.1668896); Glansdorff & Prigogine. 1971 Thermodynamic theory of structures, stability and fluctuations New York, NY/London, UK: Wiley) deeply influenced physics, and soliton science in particular. These ideas allowed the notion of solitons to be extended from purely integrable cases to the concept of dissipative solitons. The latter are qualitatively different from the solitons in integrable and Hamiltonian systems. The variety in their forms is huge. In this paper, one recent example is considered-dissipative solitons with extreme spikes (DSESs). It was found that DSESs exist in large regions of the parameter space of the complex cubic-quintic Ginzburg-Landau equation. A continuous variation in any of its parameters results in a rich structure of bifurcations.This article is part of the theme issue 'Dissipative structures in matter out of equilibrium: from chemistry, photonics and biology (part 1)'.

Modulation instability in higher-order nonlinear Schrödinger equations.

We investigate the dynamics of modulation instability (MI) and the corresponding breather solutions to the extended nonlinear Schrödinger equation that describes the full scale growth-decay cycle of MI. As an example, we study modulation instability in connection with the fourth-order equation in detail. The higher-order equations have free parameters that can be used to control the growth-decay cycle of the MI; that is, the growth rate curves, the time of evolution, the maximal amplitude, and the spectral content of the Akhmediev Breather strongly depend on these coefficients.

Positive and negative curvatures nested in an antiresonant hollow-core fiber.

We propose a negative curvature hollow-core fiber that has a nested elliptical element in the antiresonant tubes. The additional elliptical element effectively adds two curvatures, namely, a positive and a negative curvature. Our numerical study shows that it enhances the confinement of the light in the core. Moreover, the nested elements provided an extra degree of freedom that can be exploited to suppress higher-order modes through the change of the ellipticity. The resulting low confinement loss and single-mode guidance properties of the proposed fiber make it a suitable candidate for applications in ultrashort pulse delivery and gas-based nonlinear systems.

Mid-infrared supercontinuum generation in supercritical xenon-filled hollow-core negative curvature fibers.

We present an investigation on the generation of supercontinuum in the mid-infrared (mid-IR) spectral region. Namely, we study a silica-based anti-resonant hollow-core fiber which has good guidance properties in the mid-IR filled with supercritical xenon providing the necessary high nonlinearity. Our numerical study shows that by launching a 200 nJ pump of 100 fs centered at 3.70 µm, a supercontinuum that spans from 1.85 to 5.20 µm can be generated. Such sources are potentially useful for applications, such as the remote sensing of various molecules, medical imaging diagnosis, and surgery.

Extreme amplitude spikes in a laser model described by the complex Ginzburg-Landau equation.

We have found new dissipative soliton in the laser model described by the complex cubic-quintic Ginzburg-Landau equation. The soliton periodically generates spikes with extreme amplitude and short duration. At certain range of the equation parameters, these extreme spikes appear in pairs of slightly unequal amplitude. The bifurcation diagram of spike amplitude versus dispersion parameter reveals the regions of both regular and chaotic evolution of the maximal amplitudes.

Observation of Coexisting Dissipative Solitons in a Mode-Locked Fiber Laser.

We show, experimentally and numerically, that a mode-locked fiber laser can operate in a regime where two dissipative soliton solutions coexist and the laser will periodically switch between the solutions. The two dissipative solitons differ in their pulse energy and spectrum. The switching can be controlled by an external perturbation and triggered even when switching does not occur spontaneously. Numerical simulations unveil the importance of the double-minima loss spectrum and nonlinear gain to the switching dynamics.

From rogue wave solution to solitons.

Using a generalized nonlinear Schrödinger equation, we investigate the transformation of a fundamental rogue wave solution to a collection of solitons. Taking the third-order dispersion, self-steepening, and Raman-induced self-frequency shift as the generalizing effects, we systematically observe how a fundamental rogue wave has an impact on its surrounding continuous wave background and reshapes its own characteristics while a group of solitons are created. Applying a local inverse scattering technique based on the periodization of an isolated structure, we show that the third-order dispersion and Raman-induced self-frequency shift generates a group of solitons in the neighborhood where the rogue wave solution emerges. Using a volume interpretation, we show that the self-steepening effect stretches the rogue wave solution by reducing its volume. Also, we find that with the Raman-induced self-frequency shift, a decelerating rogue wave generates a red-shifted Raman radiation while the rogue wave itself turns into a slow-moving soliton. We show that when third-order dispersion, self-steepening, and Raman-induced self-frequency shift act together on the rogue wave solution, each of these effects favor the rogue wave to generate a group of solitons near where it first emerges while the rogue wave itself also becomes one of these solitons.

Plasma-induced asymmetric self-phase modulation and modulational instability in gas-filled hollow-core photonic crystal fibers.

We study theoretically the propagation of relatively long pulses with ionizing intensities in a hollow-core photonic crystal fiber filled with a Raman-inactive noble gas. Because of photoionization, an extremely asymmetric self-phase modulation and a new kind of "universal" plasma-induced modulational instability appear in both normal and anomalous dispersion regions. We also show that it is possible to spontaneously generate a plasma-induced continuum of blueshifting solitons, opening up new possibilities for pushing supercontinuum generation towards shorter and shorter wavelengths.

Theory of photoionization-induced blueshift of ultrashort solitons in gas-filled hollow-core photonic crystal fibers.

We show theoretically that the photoionization process in a hollow-core photonic crystal fiber filled with a Raman-inactive noble gas leads to a constant acceleration of solitons in the time domain with a continuous shift to higher frequencies, limited only by ionization loss. This phenomenon is opposite to the well-known Raman self-frequency redshift of solitons in solid-core glass fibers. We also predict the existence of unconventional long-range nonlocal soliton interactions leading to spectral and temporal soliton clustering. Furthermore, if the core is filled with a Raman-active molecular gas, spectral transformations between redshifted, blueshifted, and stabilized solitons can take place in the same fiber.

Creeping solitons in dissipative systems and their bifurcations.

We present a detailed numerical study of creeping solitons in dissipative systems. A bifurcation diagram has been constructed for the region of transition between solitons and fronts. It shows a rich variety of transitions between various types of localized solutions. For the first time, we have found a sequence of period-doubling bifurcations of creeping solitons, and also a symmetry-breaking instability of creeping solitons. Creeping solitons may involve many frequencies in their dynamics, and this can result, in particular, in a multiplicity of zig-zag motions.