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We report on intra- and inter-modal four-wave-mixing (FWM) in N-fold rotationally symmetric (C_N) single- and multi-core chiral photonic crystal fiber (PCF), created by spinning the preform during fiber drawing. The non-circular modal field is forced to rotate as it propagates along the fiber, resulting in circular birefringence and robust maintenance of circular polarization state. Multi-core chiral C_N PCF supports vortex-carrying helical Bloch modes (HBMs) in which the degeneracy between clockwise and counter-clockwise vortices is lifted. This makes possible new kinds of intermodal polarization modulational instability (PMI). We develop PMI theory for vortex HBMs, and illustrate the results by a series of experiments in which two or more PMI sidebands with different vorticities and polarization states are selectively generated by adjusting the polarization state and topological charge of the pump light. In every case both the topological charge and the spin of the pump light are conserved. We also report generation of a broadband supercontinuum in a single circularly polarized vortex mode.

We combine soliton dynamics in gas-filled hollow-core photonic crystal fibers with a state-of-the-art fiber laser to realize a turnkey system producing few-femtosecond pulses at 8-MHz repetition rate at pump energies as low as 220 nJ. Furthermore, by exploiting the soliton self-frequency shift in a second hydrogen-filled hollow-core fiber, we efficiently generate pulses as short as 22 fs, continuously tunable from 1100 to 1474 nm.

Optical nonreciprocity, which breaks the symmetry between forward and backward propagating optical waves, has become vital in photonic systems and enables many key applications. So far, all the existing nonreciprocal systems are implemented for linearly or randomly polarized fundamental modes. Optical vortex modes, with wavefronts that spiral around the central axis of propagation, have been extensively studied over the past decades and offer an additional degree of freedom useful in many applications. Here, we report a light-driven nonreciprocal isolation system for optical vortex modes based on topology-selective stimulated Brillouin scattering (SBS) in chiral photonic crystal fiber. The device can be reconfigured as an amplifier or an isolator by adjusting the frequency of the control signal. The experimental results show vortex isolation of 22 decibels (dB), which is at the state of the art in fundamental mode isolators using SBS. This device may find applications in optical communications, fiber lasers, quantum information processing, and optical tweezers.

High-brightness sources of coherent and few-cycle-duration light waveforms with spectral coverage from the ultraviolet to the terahertz would offer unprecedented versatility and opportunities for a wide range of applications from bio-chemical sensing to time-resolved and nonlinear spectroscopy, and to attosecond light-wave electronics. Combinations of various sources with frequency conversion and supercontinuum generation can provide relatively large spectral coverage, but many applications require a much broader spectral range and low-jitter synchronization for time-domain measurements. Here, we present a carrier-envelope-phase (CEP)-stable light source, seeded by a mid-infrared frequency comb with simultaneous spectral coverage across seven optical octaves, from the ultraviolet (340¿nm) into the terahertz (40,000¿nm). Combining soliton self-compression and dispersive wave generation in an anti-resonant-reflection photonic-crystal fibre with intra-pulse difference frequency generation in BaGa2GeSe6, the spectral brightness is two to five orders of magnitude above that of synchrotron sources. This will enable high-dynamic-range spectroscopies and provide numerous opportunities in attosecond physics and material sciences. J.B. and his group acknowledge financial support from the European Research Council via ERC Advanced Grant ‘TRANSFORMER’ (788218) and ERC Proof of Concept Grant ‘mini-X’ (840010), the European Union’s Horizon 2020 for FET-OPEN ‘PETACom’ (829153), FET-OPEN ‘OPTOlogic’ (899794), Laserlab-Europe (EU-H2020 654148), Marie Skłodowska-Curie grant no. 860553 (‘Smart-X’), MINECO for Plan Nacional FIS2017-89536-P, AGAUR for 2017 SGR 1639, MINECO for ‘Severo Ochoa’ (SEV- 2015-0522), Fundació Cellex Barcelona, CERCA Programme/Generalitat de Catalunya and the Alexander von Humboldt Foundation for the Friedrich Wilhelm Bessel Prize. We thank I. Tyulnev and M. Enders for their assistance.

We report a nonreciprocal reconfigurable light-driven isolator for vortex modes based on topology-selective Brillouin scattering in chiral PCF. The isolation rates are higher than 22 dB and 23 dB for 1st- and 2nd-order optical vortices.

We report a turn-key system producing few-fs pulses at 8 MHz repetition rate for pump energies as low as 220 nJ, and shifting their central wavelength continuously between 1100 nm and 1400 nm.

We report supercontinuum generation in chiral, thus circularly birefringent, all-normal dispersion fibers, where low noise can be achieved for pumping along the slow as well as the fast axis due to suppression of coherent coupling.

We will discuss a carrier-to-envelope phase-controlled high brightness source of light covering 7 optical octaves. The spectral brightness up to 5 orders of magnitude higher than the brightest synchrotron and ranges from 340 nm to 40,000 nm.

—A general method is presented for exciting discrete modes in waveguides of arbitrary geometry. Guided modes supported by the waveguide are first calculated using a finite difference frequency domain model. High efficiency holograms to excite these discrete modes are then generated using the Direct Search hologram generation algorithm. The Direct Search algorithm is optimised such that the inherent properties of waveguide modes are exploited to give faster execution times. A nodeless antiresonant photonic crystal fibre is considered as a test geometry, in which high-purity modes are experimentally excited and in-coupling efficiencies of up to 32.8% are obtained.

We present an ultra-high brightness source of carrier-to-envelope phase-controlled light waveforms providing simultaneous spectral coverage across 7 optical octaves. The spectral brightness 2–5 orders of magnitude above synchrotron sources ranging from 340 nm to 40,000 nm

Optical fiber-mediated optogenetic activation and neuronal Ca2+ recording in combination with fMRI provide a multi-modal fMRI platform. Here, we developed an MRI-guided robotic arm (MgRA) as a flexible positioning system with high precision to real-time assist optical fiber brain intervention for multi-modal animal fMRI. Besides the ex vivo precision evaluation, we present the highly reliable brain activity patterns in the projected basal forebrain regions upon MgRA-driven optogenetic stimulation in the lateral hypothalamus. Also, we show the step-wise optical fiber targeting thalamic nuclei and map the region-specific functional connectivity with whole-brain fMRI accompanied by simultaneous calcium recordings to specify its circuit-specificity. The MgRA also guides the real-time microinjection to specific deep brain nuclei, which is demonstrated by an Mn-enhanced MRI method. The MgRA represents a clear advantage over the standard stereotaxic-based fiber implantation and opens a broad avenue to investigate the circuit-specific functional brain mapping with the multi-modal fMRI platform., Fiber optic implantation in deep areas of the rodent’s brain for MRI combined with optogenetics is challenging. Here the authors use an MRI-guided robotic arm as the navigation method for accurate fiber optic placement and precise microinjection during multi-modal fMRI, optogenetics and calcium recordings.

Soliton dynamics can be used to temporally compress laser pulses to few fs durations in many different spectral regions. Here we study analytically, numerically and experimentally the scaling of soliton dynamics in noble gas-filled hollow-core fibers. We identify an optimal parameter region, taking account of higher-order dispersion, photoionization, self-focusing, and modulational instability. Although for single-shots the effects of photoionization can be reduced by using lighter noble gases, they become increasingly important as the repetition rate rises. For the same optical nonlinearity, the higher pressure and longer diffusion times of the lighter gases can considerably enhance the long-term effects of ionization, as a result of pulse-by-pulse buildup of refractive index changes. To illustrate the counter-intuitive nature of these predictions, we compressed 250 fs pulses at 1030 nm in an 80-cm-long hollow-core photonic crystal fiber (core radius 15 µm) to ∼5 fs duration in argon and neon, and found that, although neon performed better at a repetition rate of 1 MHz, stable compression in argon was still possible up to 10 MHz.

Circular-symmetric optical fibres are ideal for studying nonlinear optical processes involving optical vortices [1] . Chiral photonic crystal fibres (PCFs), fabricated either by post-processing techniques or by spinning the preform during fibre drawing, have been shown to support circularly polarized helical Bloch modes (HBMs), which are superpositions of multiple vortices with topological charges l ( m ) = l (0) + mN , where N is the number of azimuthal periods and m the order of the m -th azimuthal Bloch harmonic [2] . Here we report on the importance of the preservation of topological charge in polarisation modulational instability (PMI) between circularly polarised modes in a circularly birefringent chiral PCF with a 3-fold rotationally symmetric core.

Knowledge of the phase and amplitude of an optical field is of critical importance in many applications, for example in measuring the carrier envelope phase of an ultrashort pulse. While the intensity spectrum can easily be obtained using a spectrometer, there is no direct method for measuring the phase. Retrieving phase information requires use of an interferometer to compare the signal phase against the phase of a known reference source. To resolve ambiguity in the sign of the phase, the field quadrature can be measured at the output of a polarising beam splitter [1] . Interferometry requires, however, that the reference source is stable and has a spectrum at least as broad as the one under study, which can in some cases be multi-octave in extent. Here we report an experiment in which the reference is provided by a supercontinuum generated in a twisted all-normal dispersion (t-ANDi) fibre.

Stimulated Brillouin scattering (SBS) in optical fibres, in which guided light is parametrically reflected by coherent acoustic phonons, provides a powerful and flexible mechanism for controlling light, and has recently been demonstrated with orbital angular momentum (OAM) modes in a special annular-core fibre [1] . Helically twisted photonic crystal fibre (PCF) robustly supports circularly-polarised helical Bloch modes (HBMs) and displays OAM birefringence, i.e., modes with equal and opposite OAM orders have different propagation constants [2] . Here we report for the first time SBS of circularly-polarised HBMs in a chiral 4-core PCF with 3-fold symmetry.

New methods are urgently required to identify degradation and failure mechanisms in high energy density energy storage materials such as Ni-rich LiNi0.8Mn0.1Co0.1O2 cathodes (NMC811) for Li-ion batteries. Understanding and ultimately avoiding these mechanisms requires in-situ tracking of the complex electrochemical processes that occur in different parts of battery cells. Here we demonstrate a new operando spectroscopy method that enables the tracking of electrolyte chemistry, applied here for high energy density Li-ion batteries with a NMC811 cathode, during electrochemical cycling. This is achieved by embedding a novel hollow-core optical fibre probe inside the battery to monitor the evolution of electrolyte species by background-free Raman spectroscopy. Our data reveals changes in the ratio of carbonate solvents and electrolyte additives as a function of the cell voltage, as well as changes in the lithium-ion solvation dynamics. This advanced operando methodology delivers a new way to study battery degradation mechanisms, and the understanding it develops should contribute to extending the lifetime of next-generation batteries.

Improved analytical tools are urgently required to identify degradation and failure mechanisms in Li-ion batteries. However, understanding and ultimately avoiding these detrimental mechanisms requires continuous tracking of complex electrochemical processes in different battery components. Here, we report an operando spectroscopy method that enables monitoring the chemistry of a carbonate-based liquid electrolyte during electrochemical cycling in Li-ion batteries with a graphite anode and a LiNi

We report the first, to the best of our knowledge, observation of cross-phase modulational instability (XPMI) of circularly polarized helical Bloch modes carrying optical vortices in a twisted photonic crystal fiber with a three-fold symmetric core, formed by spinning the fiber preform during the draw. When the fiber is pumped by a superposition of left-circular polarization (LCP) and right-circular polarization (RCP) modes, a pair of orthogonal circularly polarized sidebands of opposite topological charge is generated. When, on the other hand, a pure LCP (or RCP) mode is launched, the XPMI gain is zero, and no sidebands are seen. This observation has not been seen before in any system and is unique to chiral structures with -fold rotational symmetry. The polarization state and topological charge of the generated sidebands are measured. By decomposing the helical Bloch modes into their azimuthal harmonics, we are able to deduce the selection rules for the appearance of modulational instability sidebands. We showed that the four waves in the nonlinear mixing process must exhibit the same set of azimuthal harmonic orders.

Stimulated Brillouin scattering (SBS) has many applications; for example, in sensing, microwave photonics, and signal processing. Here, we report the first experimental study of SBS in chiral photonic crystal fiber (PCF), which displays optical activity and robustly maintains circular polarization states against external perturbations. As a result, circularly polarized pump light is cleanly backscattered into a Stokes signal with the orthogonal circular polarization state, as is required by angular momentum conservation. By comparison, untwisted PCF generates a Stokes signal with an unpredictable polarization state, owing to its high sensitivity to external perturbations. We use chiral PCF to realize a circularly polarized continuous-wave Brillouin laser. The results pave the way for a new generation of stable circularly polarized SBS systems with applications in quantum manipulation, optical tweezers, optical gyroscopes, and fiber sensors.

Performing quantitative in situ spectroscopic analysis on minuscule sample volumes is a common difficulty in photochemistry. To address this challenge, we use a hollow-core photonic crystal fiber (HC-PCF) that guides light at the center of a microscale liquid channel and acts as an optofluidic microreactor with a reaction volume of less than 35 nL. The system was used to demonstrate in situ optical detection of photoreduction processes that are key components of many photocatalytic reaction schemes. The photoreduction of viologens (XV2+) to the radical XV•+ in a homogeneous mixture with carbon nanodot (CND) light absorbers is studied for a range of different carbon dots and viologens. Time-resolved absorption spectra, measured over several UV irradiation cycles, are interpreted with a quantitative kinetic model to determine photoreduction and photobleaching rate constants. The powerful combination of time-resolved, low-volume absorption spectroscopy and kinetic modeling highlights the potential of optofluidic microreactors as a highly sensitive, quantitative, and rapid screening platform for novel photocatalysts and flow chemistry in general.

We demonstrate a fibre-optic sensing method capable of monitoring chemical changes within Li:ion cells under real working conditions. Our technique is based on optofluidic single-ring hollow-core fibres, which uniquely allow light to be guided at the centre of a microfluidic channel. We integrate the fibres into working Li:ion cells, use them to take sub-microlitre samples of the electrolyte liquid, and analyse these by background-free Raman spectroscopy to identify early signs of battery degradation. Our approach complements existing battery monitoring systems and will enable us to identify degradation mechanisms that currently limit the lifetime and capacity of state-of-the-art energy storage systems.

We designed a head-mounted three-photon microscope for imaging deep cortical layer neuronal activity in a freely moving rat. Delivery of high-energy excitation pulses at 1,320 nm required both a hollow-core fiber whose transmission properties did not change with fiber movement and dispersion compensation. These developments enabled imaging at >1.1 mm below the cortical surface and stable imaging of layer 5 neuronal activity for >1 h in freely moving rats performing a range of behaviors. A head-mounted three-photon microscope based on a custom-designed optical fiber and dispersion compensation enables imaging of activity from neuronal populations deep in the cortex of freely moving rats.

Optical fibers have been considered an optimal platform for third-order parametric down-conversion since they can potentially overcome the weak third-order nonlinearity by their long interaction length. Here we present, in the first part, a theoretical derivation for the conversion rate both in the case of spontaneous generation and in the presence of a seed beam. Then we review three types of optical fibers and we examine their properties in terms of conversion efficiency and practical feasibility.

We report efficient Raman conversion of vortex modes in twisted hydrogen-filled hollow-core photonic crystal fibers. Circular birefringence provides topological protection and broadband guidance of helically-phased modes over long distances.

We report generation of circularly polarized supercontinuum light (850-1650 nm) in a single vortex mode using a chiral three-core PCF. We show chiral multicore PCFs enable robust maintenance of circularly polarized vortex modes. © 2020 The Author(s)

We report the first experimental demonstration of cross-phase modulational instability of circularly polarized vortex modes in a twisted PCF with a threefold symmetric core. The polarization and topological charge of the generated sidebands are measured. © 2020 The Author(s)

We designed a head-mounted three-photon microscope for imaging deep cortical layer neuronal activity in a freely moving rat. Delivery of high-energy excitation pulses at 1,320 nm required both a hollow-core fiber whose transmission properties did not change with fiber movement and dispersion compensation. These developments enabled imaging at1.1 mm below the cortical surface and stable imaging of layer 5 neuronal activity for1 h in freely moving rats performing a range of behaviors.

We compare the properties of the broadband supercontinuum (SC) generated in twisted and untwisted solid-core photonic crystal fibers when pumped by circularly polarized 40 picosecond laser pulses at 1064 nm. In the helically twisted fiber, fabricated by spinning the preform during the draw, the SC is robustly circularly polarized across its entire spectrum whereas, in the straight fiber, axial fluctuations in linear birefringence and polarization-dependent nonlinear effects cause the polarization state to vary randomly with the wavelength. Theoretical modelling confirms the experimental results. Helically twisted photonic crystal fibers permit the generation of pure circularly polarized SC light with excellent polarization stability against fluctuations in input power and environmental perturbations.

We introduce the concept of Doppler-assisted tomography (DAT) and show that it can be applied successfully to non-invasive imaging of the internal microstructure of a photonic crystal fiber. The fiber is spun at ~10 Hz around its axis and laterally illuminated with a laser beam. Monitoring the time-dependent Doppler shift of the light scattered by the hollow channels permits the azimuthal angle and radial position of individual channels to be measured. An inverse Radon transform is used to construct an image of the microstructure from the frequency-modulated scattered signal. We also show that DAT can image sub-wavelength features and monitor the structure along a tapered fiber, which is not possible using other techniques without cutting up the taper into several short pieces or filling it with index-matching oil. The non-destructive nature of DAT means that it could potentially be applied to image the fiber microstructure as it emerges from the drawing tower, or indeed to carry out tomography on any transparent microstructured cylindrical object.

© 2019 IEEE Optofluidic hollow-core photonic crystal fiber (HC-PCF) uniquely allows light to be guided at the centre of a microfluidic channel. The system maximizes the interaction of light with infiltrated chemicals and (nano)particles, offering unique opportunities for in-situ optical monitoring of a range of photochemical and catalytic reactions [1,2]. Our current goal is to extend this work to hybrid colloidal systems comprising a particulate light absorber and a molecular catalyst for photocatalytic fuel production [3]. Here we use HC-PCF microreactors to study novel light-absorbing particles for such systems: graphitic, N-doped, and amorphous carbon-nanodots (CNDs) that offer a unique combination of scalability, biocompatibility, water solubility, and stable optical properties [4].

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.

Single-ring hollow-core photonic crystal fibre (SR-PCF), consisting of a ring of thin-walled glass capillaries surrounding a central hollow core, can offer remarkably low transmission loss [1], and is finding applications in, e.g., wavelength conversion and pulse compression in gases, high-power beam delivery and circular dichroism [2]. As with all microstructured fibres, it is highly desirable to continuously measure the internal structural parameters (e.g. the capillary diameter) during fibre drawing. This would improve the yield of useful fibre lengths, as well as offering better control of structural uniformity along the fibre. Successful tapering of hollow-core fibres also requires a non-destructive method of verifying structural integrity along the taper.

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].

Thermal tapering of step-index and photonic crystal fibres (PCF) has recently emerged as an effective tool for controlling nonlinear optical effects through offering axially varying dispersion and high effective nonlinearity [1,2]. At the same time, rapid developments in hollow-core PCF have opened up new possibilities for gas-based nonlinear optics. In particular, single-ring PCF (SR-PCF), consisting of a circular ring of thin-walled capillaries surrounding a central hollow core, can offer low-loss, easily predictable windows of transparency, effectively single-mode guidance and is relatively simple to fabricate [3]. Here we show, for the first time to our knowledge, the fabrication and non-invasive characterization of tapered SR-PCF.

This is an introduction to the feature issue of Optical Materials Express on Optical Fibers: Materials and Applications.

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 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 circular polarization state, typically exhibiting weak linear birefringence. Here we report a revolutionary approach based on helically-twisted hollow-core photonic crystal fiber, which displays circular birefringence, thus robustly maintaining 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 fully-controllable polarization state., Comment: 5 pages, 4 figures

© 2018 SPIE. Higher-order modes are controllably excited in water-filled kagomè-, bandgap-style, and simplified hollow-core photonic crystal fibers (HC-PCF). A spatial light modulator is used to create amplitude and phase distributions that closely match those of the fiber modes, resulting in typical launch efficiencies of 10-20% into the liquid-filled core. Modes, excited across the visible wavelength range, closely resemble those observed in air-filled kagomè HC-PCF and match numerical simulations. These results provide a framework for spatially-resolved sensing in HC-PCF microreactors and fiber-based optical manipulation.

Direct observation of third-order spontaneous down conversion, where three low energy photons are generated from a single one, remains a challenging task. We report here on different strategies based on engineered optical fibers. These include gas-filled hollow-core photonic crystal fibers, hybrid- and tapered fibers.

We report CARS in hollow-core gas-filled PCF. The results pave the way to single-shot broadband (>4000 cm−1) CARS under ambient conditions. A detection limit of 300 ppm was reached with 200 mW overall laser power.

We present a technique for measuring the phase and amplitude distribution of the light guided in a twisted coreless photonic crystal fiber (or indeed any fiber), enabling full characterization of the orbital angular momentum states.

We demonstrate efficient generation of 1.35-optical-cycle (14.5 fs) and 60 µJ mid-IR pulses at 160 kHz repetition rate. The CEP-stable, 21 W mid-IR waveforms are self-compressed inside a gas-filled antiresonant-reflection photonic crystal fiber (ARR-PCF).

A hollow-core single-ring photonic crystal fiber (SR-PCF) consists of a ring of capillaries arranged around a central hollow core. Spinning the preform during drawing introduces a continuous helical twist, offering a novel means of controlling the modal properties of hollow-core SR-PCF. For example, twisting geometrically increases the effective axial propagation constant of the LP01-like modes of the capillaries, providing a means of optimizing the suppression of HOMs, which occurs when the LP11-like core mode phase-matches to the LP01-like modes of the surrounding capillaries. (In a straight fiber, optimum suppression occurs for a capillary-to-core diameter ratio d/D=0.682.) Twisting also introduces circular birefringence (to be studied in a future Letter) and has a remarkable effect on the transverse intensity profiles of the higher-order core modes, forcing the two-lobed LP11-like mode in the untwisted fiber to become three-fold symmetric in the twisted case. These phenomena are explored by means of extensive numerical modeling, an analytical model, and a series of experiments. Prism-assisted side-coupling is used to measure the losses, refractive indices, and near-field patterns of individual fiber modes in both the straight and twisted cases.

Two novel kinds of anti-resonant polygonal-core fibers are introduced, including a triangular-core fiber-design with surrounding silica-glass webs [1], and a negatively-curved square-core fiber-design.

Hollow-core fibres with a single ring of glass capillaries surrounding the core are receiving increased attention due to their relative simplicity, low loss, and broad transmission windows [1, 2]. Recently it was reported that such fibres are effectively single-mode at all guided wavelengths if the inner diameter d of the cladding capillaries and the core diameter D satisfy the relationship d/D = 0.682 [3]. Endlessly single-mode behaviour is highly desirable in applications such as beam delivery and fibre gyroscopes. Here we address another important issue: the dependence of the bend-sensitivity on fibre structure and wavelength. We present a simple analytical expression for the minimum radius to which the fibre can be bent before the losses increase dramatically. The analysis can also be used to calculate the optimum bend-radius for suppression of a given higher-order mode.

Recent developments in single-ring hollow-core photonic crystal fibre (SR-PCF) have made possible robust single-mode guidance over a broad bandwidth [1]—an important attribute in many applications, for example high power delivery of laser light and ultrafast gas-based nonlinear optics. Here we report for the first time that these fibres can be designed to transmit just one circular polarisation state if they are twisted continuously during fibre drawing [2], forming a helical structure.