Showing papers by "P. St. J. Russell published in 2019"

Formation of optical supramolecular structures in a fibre laser by tailoring long-range soliton interactions

Abstract: Self-assembly of fundamental elements through weak, long-range interactions plays a central role in both supramolecular DNA assembly and bottom-up synthesis of nanostructures. Optical solitons, analogous in many ways to particles, arise from the balance between nonlinearity and dispersion and have been studied in numerous optical systems. Although both short- and long-range interactions between optical solitons have attracted extensive interest for decades, stable soliton supramolecules, with multiple aspects of complexity and flexibility, have thus far escaped experimental observation due to the absence of techniques for enhancing and controlling the long-range inter-soliton forces. Here we report that long-range soliton interactions originating from optoacoustic effects and dispersive-wave radiations can be precisely tailored in a fibre laser cavity, enabling self-assembly of large numbers of optical solitons into highly-ordered supramolecular structures. We demonstrate several features of such optical structures, highlighting their potential applications in optical information storage and ultrafast laser-field manipulation. Optical solitons have been studied in a variety of systems for their unique properties. Here, the authors report on optically observed solitonic supramolecules, made up of large-scale structures of many solitons interacting within a fiber cavity, and study their fundamental characteristics.

Polarization-Tailored Raman Frequency Conversion in Chiral Gas-Filled Hollow-Core Photonic Crystal Fibers

Abstract: Broadband-tunable sources of circularly polarized light are crucial in fields such as laser science, biomedicine, and spectroscopy. Conventional sources rely on nonlinear wavelength conversion and polarization control using standard optical components and are limited by the availability of suitably transparent crystals and glasses. Although a gas-filled hollow-core photonic crystal fiber provides pressure-tunable dispersion, long well-controlled optical path lengths, and high Raman conversion efficiency, it is unable to preserve a circular polarization state, typically exhibiting weak linear birefringence. Here we report a revolutionary approach based on a helically twisted hollow-core photonic crystal fiber, which displays circular birefringence, thus robustly maintaining a circular polarization state against external perturbations. This makes it possible to generate pure circularly polarized Stokes and anti-Stokes signals by rotational Raman scattering in hydrogen. The polarization state of the frequency-shifted Raman bands can be continuously varied by tuning the gas pressure in the vicinity of the gain-suppression point. The results pave the way to a new generation of compact and efficient fiber-based sources of broadband light with a fully controllable polarization state.

Route from single-pulse to multi-pulse states in a mid-infrared soliton fiber laser

Abstract: State-of-the-art ultrafast mid-IR fiber lasers deliver optical solitons with durations of several hundred femtoseconds. The Er- or Ho-doped fluoride gain fibers generally used in these lasers have strong anomalous dispersion at ∼3 µm, which generally forces them to operate in the soliton regime. Here we report that a pulse-energy clamping effect, caused by the buildup of intracavity nonlinearities, limits the shortest obtainable pulse durations in these mid-infrared soliton fiber lasers. Excessive intra-cavity energy results in soliton instability, collapse and fragmentation into a variety of stable multi-pulse states, including phase-locked soliton molecules and harmonically mode-locked states. We report that the spectral evolution of the mid-IR laser pulses can be recorded between roundtrips through stretching their second-harmonic signal in a 25-km-length of single-mode fiber. Using a modified dispersive Fourier transform set-up, we were able to perform for the first time spectro-temporal measurements of mid-IR laser pulses both in the pulsed state and during pulse collapse and fragmentation. The results provide insight into the complex nonlinear dynamics of mid-IR soliton fiber lasers and open up new opportunities for obtaining a variety of stable multi-pulse mode-locked states at mid-IR wavelengths.

Spatio-temporal measurement of ionization-induced modal index changes in gas-filled PCF by prism-assisted side-coupling.

Abstract: We report the use of prism-assisted side-coupling to investigate the spatio-temporal dynamics of photoionization in an Ar-filled hollow-core photonic crystal fiber. By launching four different LP core modes we are able to probe temporal and spatial changes in the modal refractive index on timescales from a few hundred picoseconds to several hundred microseconds after the ionization event. We experimentally analyze the underlying gas density waves and find good agreement with quantitative and qualitative hydrodynamic predictions. Moreover, we observe periodic modulations in the MHz-range lasting for a few microseconds, indicating nanometer-scale vibrations of the fiber structure, driven by gas density waves.

Full-field characterization of helical Bloch modes guided in twisted coreless photonic crystal fiber.

Abstract: It was recently reported that a photonic crystal fiber (PCF) with no structural core guides light if a permanent chiral twist is introduced by spinning the fiber preform during the draw. The intriguing guidance mechanism behind this novel effect has many remarkable features; for example, it intrinsically supports circularly polarized helical Bloch modes (HBMs) that carry multiple optical vortices, making twisted PCFs of interest in fields such as optical micro-manipulation, imaging, quantum optics, and optical communications. Here we report for the first time, to the best of our knowledge, that a twisted coreless PCF supports not just one but a family of guided HBMs, each member of which has a unique transverse field distribution and harmonic spectrum. By making detailed interferometric measurements of the near-field phase and amplitude distributions of HBMs, and expanding them as a series of Bessel beams, we are able to extract the amplitude of each azimuthal and radial HBM harmonic. Good agreement is found with the numerical solutions of Maxwell’s equations. The results shed light on the properties of this curious new optical phenomenon.

Supercontinuum Generation in a As 2 S 3 -Silica Nanospike Waveguide Pumped by Tm-Doped Fiber Laser

Abstract: The development of compact bright supercontinuum (SC) sources is of particular interest, especially in the mid-IR where is a lack of suitable sources [1,2]. Here we report first results on SC generation in a As 2 S 3 -silica dual nanospike waveguide [3, 4] pumped in different multi-pulse regimes by a high-power femtosecond Tm-doped fiber laser mode-locked by nonlinear polarization evolution [5–7].

Optical traps and anti-traps for glass nanoplates in hollow waveguides.

Abstract: We study theoretically the optical forces acting on glass nanoplates introduced into hollow waveguides, and show that, depending on the sign of the laser detuning relative to the nanoplate resonance, optomechanical back-action between nanoplate and hollow waveguide can create both traps and anti-traps at intensity nodes and anti-nodes in the supermode field profile, behaving similarly to those experienced by cold atoms when the laser frequency is red or blue detuned of an atomic resonance. This arises from dramatic distortions to the mode profile in the hollow waveguide when the nanoplate is off-resonant, producing gradient forces that vary strongly with nanoplate position. In a planar system, we show that when the nanoplate is constrained by an imaginary mechanical spring, its position exhibits strong bistability as the base position is varied. We then treat a two-dimensional system consisting of an anti-resonant nanoplate in the hollow core of a photonic crystal fiber, and predict the stable dark trapping of nanoplate at core center against both translational and rotational motion. The results show that spatial and angular position of nano-scale objects in hollow waveguides can be optically controlled by launching beams with appropriately synthesized transverse field profiles.

Generation of Broad-Band Circularly Polarised Supercontinuum in Chiral Photonic Crystal Fibre

Abstract: Supercontinuum (SC) generation in solid-core photonic crystal fibres (PCFs) is well established and has diverse applications in science and technology [1,2]. In a conventional SC, the polarization state is not maintained over the whole spectrum due to cross-phase modulation and may in addition vary with power. Recently, it has been reported that continuously twisted PCFs exhibit circular birefringence [3,4]. Here we report the first experimental demonstration of a twisted PCF that robustly maintains circular polarization state across the whole spectrum, independent of power.

Dispersion Engineering of Schott-SF6 Photonic Crystal Fibres for Nonlinear Applications in the Infrared

Abstract: An empirical model for estimating the dispersion of solid-core Schott-SF6 photonic crystal fibre is presented. Finite element calculations and experimentally measured power-dependent spectra confirm the accuracy of the model. © 2019 The Author(s)

Reconstruction of Guided Helical Bloch Modes in Twisted Coreless Photonic Crystal Fibre using Bessel Beams

Abstract: Light beams carrying orbital angular momentum (OAM) are of fundamental interest in many fields of research [1]. Recently it has been reported that helical Bloch modes (HBMs) carrying multiple values of OAM are guided in chirally twisted photonic crystal fibre (PCF), even when there is no core [2]. HBMs are not supported by conventional waveguides and would be very difficult to realise using multi-beam interference. Here we report the first complete experimental characterisation of the phase and amplitude of the complex HBM fields in twisted coreless PCF. We compare the measurements with finite element simulations, based on precise scanning electron micrographs of the fibre structure, obtaining excellent agreement. We then expand the numerically calculated HBM fields as a series of Bessel beams, allowing us to calculate the strengths and azimuthal orders of the HBM harmonics with a fidelity greater than 95%.

Pump-Probe Study of Plasma Dynamics in Gas-Filled Photonic Crystal Fiber Using Counterpropagating Solitons

Abstract: Transient changes in polarizability alter the phase-matching conditions in nonlinear optical processes in a gas-filled hollow-core photonic crystal fiber. Understanding these polarizability changes is crucial for describing light-matter interactions in $e.g.$ high-field laser science. Although techniques to detect these changes exist, they are complex and have not been proven to work in confined geometries. In this study, the wavelength shift of the dispersive wave emitted by a soliton in a fiber is used to probe the plasma at the temporal focus of a counterpropagating pump soliton. By varying the delay between pump and probe, the buildup of plasma is monitored and its spatial profile reconstructed.

Highly Efficient Thresholdless Ultraviolet Frequency Conversion in H 2 -Filled Photonic Crystal Fibers

Abstract: Thresholdless ultraviolet frequency conversion with high efficiency is demonstrated in a hydrogen-filled kagome-PCF. Raman coherence waves, prepared by a visible pump, up- or downshift the frequency of an ultraviolet signal upon fulfillment of phase-matching conditions. © 2019 The Author(s)

Dual-Colour-Pump Broadband CARS in Single-Ring Gas-Filled Photonic Crystal Fibre

Abstract: Selective detection and chemical analysis of trace gases at concentration levels of parts per million (ppm) and below is of critical importance in environmental monitoring [1] and medicine [2]. Spectroscopic techniques offer high gas-type selectivity and are widely used for measuring the concentration of specific molecular species. Raman spectroscopy provides two key advantages. First, the pump wavelength can be freely chosen, independently of the absorption lines of the gas. Second, the highest Raman frequency shift of any gas is the vibrational transition of H 2 at 125 THz, which means that all known Raman active molecules can be detected with one spectrometer [3].

The Curious Properties of Twisted Photonic Crystal Fibers

Abstract: Helically twisted hollow and solid core photonic crystal fibers display many curious properties: perfect circular birefringence, circular dichroism, birefringence between modes of equal and opposite azimuthal order, and guidance even when there is no core. © 2019 The Author(s)