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

Unified and vector theory of Raman scattering in gas-filled hollow-core fiber across temporal regimes

Raman scattering has found renewed interest owing to the development of gas-filled hollow-core fibers, which constitute a unique platform for exploration of novel ultrafast nonlinear phenomena beyond conventional solid-core-fiber and free-space systems. Much progress has been made through models for particular interaction regimes, which are delineated by the relation of the excitation pulse duration to the time scales of the Raman response. However, current experimental settings are not limited to one regime, prompting the need for tools spanning multiple regimes. Here, we present a theoretical framework that accomplishes this goal. The theory allows us to review recent progress with a fresh perspective, makes new connections between distinct temporal regimes of Raman scattering, and reveals new degrees of freedom for controlling Raman physics. Specific topics that are addressed include transient Raman gain, the interplay of electronic and Raman nonlinearities in short-pulse propagation, and interactions of short pulses mediated by phonon waves. The theoretical model also accommodates vector effects, which have been largely neglected in prior works on Raman scattering in gases. The polarization dependence of transient Raman gain and vector effects on pulse interactions via phonon waves is investigated with the model. Throughout this Perspective, theoretical results are compared to the results of realistic numerical simulations. The numerical code that implements the new theory is freely available. We hope that the unified theoretical framework and numerical tool described here will accelerate the exploration of new Raman-scattering phenomena and enable new applications.

Effect of rotational Raman response on ultra-flat supercontinuum generation in gas-filled hollow-core photonic crystal fibers

We experimentally and numerically investigate flat supercontinuum generation in gas-filled anti-resonant guiding hollow-core photonic crystal fiber. By comparing results obtained with either argon or nitrogen we determine the role of the rotational Raman response in the supercontinuum formation. When using argon, a supercontinuum extending from 350 nm to 2 µm is generated through modulational instability. Although argon and nitrogen exhibit similar Kerr nonlinearity and dispersion, we find that the energy density of the continuum in the normal dispersion region is significantly lower when using nitrogen. Using numerical simulations, we find that due to the closely spaced rotational lines in nitrogen, gain suppression in the fundamental mode causes part of the pump pulse to be coupled into higher-order modes which reduces the energy transfer to wavelengths shorter than the pump.

Mitigating nonlinear penalty induced from a low-speed optical supervisory channel on coherent signals

With the increasing signal rates of a long-haul backbone dense-wavelength-division-multiplexing (DWDM) transmission system, e.g., from 100 Gb/s to 400 Gb/s and even to 800 Gb/s, optical path impairments simultaneously become more severe. Harmful factors being formerly insignificant become noticeable, e.g., nonlinear phase noise (NPN) on main DWDM channels induced by the cross-phase modulation (XPM) from the low-speed optical supervisory channel (OSC). Field trials show that a greater than 5.13-dB penalty can be observed on the shortest channel of 400G DP-16QAM-PCS over G.654.E links, which greatly degrades the overall transmission performance and limits the maximum reach. In this paper, we propose a dual-OSC structure with opposite signals to compensate for performance degradation caused by OSC-induced NPN. This method involves no extra digital signal processing (DSP), which is not only simple but also applicable for universal signal rates. By experimental demonstration, a 1.32-dB gain in Q (dB) for 200G DP-16QAM transmission over 1618-km G.652.D can be done, almost achieving the same performance as the no OSC case.

Effect of nonlinear lensing on the coupling of ultrafast laser pulses to hollow-core waveguides

Gas-filled hollow-core fibers are a flexible platform for the manipulation of ultrafast laser pulses through a variety of nonlinear optical effects. Efficient high-fidelity coupling of the initial pulses is very important for system performance. Here we study the effect of self-focusing in gas-cell windows on the coupling of ultrafast laser pulses into hollow-core fibers using (2+1)-dimensional numerical simulations. As expected, we find that the coupling efficiency is degraded and the duration of the coupled pulses changed when the entrance window is too close to the fiber entrance. The interplay of nonlinear spatio-temporal reshaping and the linear dispersion of the window create different results depending on the window material, pulse duration, and pulse wavelength, with longer-wavelength beams more tolerant of high intensity in the window. While shifting the nominal focus to compensate can restore some of the lost coupling efficiency, it improves the pulse duration only marginally. From our simulations we derive a simple expression for the minimum distance between the window and the HCF entrance facet. Our results have implications for the often space-constrained design of hollow-core-fiber systems, especially where the input energy is not constant.

Designing multi-mode anti-resonant hollow-core fibers for industrial laser power delivery

We investigate the design of hollow-core fibers for the delivery of 10s of kilowatt average power from multi-mode laser sources. For such lasers, delivery through solid-core fibers is typically limited by nonlinear optical effects to 10s of meters of distance. Techniques are presented here for the design of multi-mode anti-resonant fibers that can efficiently couple and transmit light from these lasers. By numerical simulation we analyze the performance of two anti-resonant fibers targeting continuous-wave lasers with M² up to 13 and find they are capable of delivering MW-level power over several kilometers with low leakage loss, and at bend radii as small as 35 cm. Pulsed lasers are also investigated and numerical simulations indicate that optimized fibers could in principle deliver nanosecond pulses with greater than 100 mJ pulse energy over distances up to 1 km. This would be orders of magnitude higher power and longer distances than in typical machining applications using the best available solid core fibers.

Cd7SiAs6, a Nonchalcopyrite Arsenide with a Strong Nonlinear-Optical Response

A new arsenide, Cd₇SiAs₆, has been successfully synthesized and characterized. It is the first arsenide that adopts a nonchalcopyrite structure and possesses a strong nonlinear-optical (NLO) response. In the structure, the CdAs₃ trigonal planar unit, a kind of π-conjugated planar NLO-active group, was identified for the first time. Furthermore, theoretical calculations reveal that the CdAs₃ planar unit contributes more to the NLO effect than the CdAs₄ tetrahedron does. The result may provide valuable insights for the future exploration of IR NLO materials, especially for application above 10 μm.

Efficient self-compression of ultrashort near-UV pulses in air-filled hollow-core photonic crystal fibers

We report generation of ultrashort near-UV pulses by soliton self-compression in kagomé-style hollow-core photonic crystal fibers filled with ambient air. Pump pulses with the energy of 2.6 µJ and duration of 54 fs at 400 nm were compressed temporally by a factor of 5, to a duration of ∼11 fs. The experimental results are supported by numerical simulations, showing that both Raman and Kerr effects play a role in the compression dynamics. The convenience of using ambient air and the absence of glass windows that would distort the compressed pulses makes the setup highly attractive as the basis of an efficient table-top UV pulse compressor.

Noise-related polarization dynamics for femto and picosecond pulses in normal dispersion fibers

We report how the complex intra-pulse polarization dynamics of coherent optical wavebreaking and incoherent Raman amplification processes in all-normal dispersion (ANDi) fibers vary for femto and picosecond pump pulses. Using high temporal resolution vector supercontinuum simulations, we identify deterministic polarization dynamics caused by wavebreaking and self-phase modulation for femtosecond pulses and quasi-chaotic polarization evolution driven by Raman amplification of quantum noise for picosecond pulses. In contrast to cross-phase modulation instability, the Raman-based polarization noise has no power threshold and is reduced by aligning the higher energy polarization component with the lower index axis of the fiber. The degree of polarization stability is quantified using new time domain parameters that build on the spectrally averaged degree of coherence used in supercontinuum research to quantify the output spectral stability. We show that the spectral coherence is intrinsically linked to polarization noise, and that the noise will occur in both polarization maintaining (PM) and non-PM fibers, spanning a broad range of pulse energies, durations, and fiber birefringence values. This analysis provides an in-depth understanding of the nonlinear polarization dynamics associated with coherent and incoherent propagation in ANDi fibers.

Impact of nonlinear effects on transmission losses of hollow-core antiresonant negative curvature optical fiber

We investigate the impact of input pulse duration and peak power of a femtosecond laser on pulse broadening and propagation losses in selected hollow-core antiresonant fiber (HC-ARF). The mixed effects of strong self-phase modulation and relatively weak Raman scattering broaden the spectral width, which in turn causes a portion of the output spectrum to exceed the transmission band of the fiber, resulting in transmission losses. By designing and setting up a gas flow control system and a vacuum system, the nonlinear behavior of the fiber filled with different pressurized gases is investigated. The experimental results show that replacing the air molecules in the fiber core with argon can weaken pulse broadening and increase the transmittable peak power by 14 MW for a given 122 MW input, while a vacuum system can reduce the nonlinearity to a larger extent, therefore enhancing the transmission of HC-ARF by at least 26 MW.

Fabrication of tubular anti-resonant hollow core fibers: modelling, draw dynamics and process optimization

The fabrication of hollow core microstructured fibers is significantly more complex than solid fibers due to the necessity to control the hollow microstructure with high precision during the draw. We present the first model that can recreate tubular anti-resonant hollow core fiber draws, and accurately predict the draw parameters and geometry of the fiber. The model was validated against two different experimental fiber draws and very good agreement was found. We identify a dynamic within the draw process that can lead to a premature and irreversible contact between neighboring capillaries inside the hot zone, and describe mitigating strategies. We then use the model to explore the tolerance of the draw process to unavoidable structural variations within the preform, and to study feasibility and limiting phenomena of increasing the produced yield. We discover that the aspect ratio of the capillaries used in the preform has a direct effect on the uniformity of drawn fibers. Starting from high precision preforms the model predicts that it could be possible to draw 100 km of fiber from a single meter of preform.

Hollow Core Optical Fibers for Industrial Ultra Short Pulse Laser Beam Delivery Applications

Hollow core fibers were introduced many years ago but are now starting to be used regularly in more demanding applications. While first experiments mainly focused on the characterization and analysis of the fibers themselves, they are now implemented as a tool in the laser beam delivery. Owing to their different designs and implementations, different tasks can be achieved, such as flexible beam delivery, wide spectral broadening up to supercontinuum generation or intense gas-laser interaction over long distances. To achieve a constant result in these applications under varying conditions, many parameters of these fibers have to be controlled precisely during fabrication and implementation. A wide variety of hollow core fiber designs have been analyzed and implemented into a high-power industrial beam delivery and their performance has been measured.