Opportunities and Challenges for Long-Distance Transmission in Hollow-Core Fibres

Anti-resonant hollow-core fiber of the Nested Antiresonant Nodeless type (NANF) has been showing a steady decrease in loss over the last few years, gradually approaching that of standard Single-Mode Fiber (SMF). It already by far outperforms SMF as to non-linear effects, which are three to four orders of magnitude lower in NANF than in SMF. Theoretical predictions and experimental evidence also hint at a much wider usable bandwidth than SMF, potentially amounting to several tens of THz. Propagation speed is 50% faster, a key feature in certain contexts. In this paper we investigate the potential impact of possible future high-performance NANF on long-haul optical communication systems, assuming NANF continues on its current steady path towards better performance. We look at the system throughput in different long-haul scenarios, addressing links of various length, from 100~km to 4,000~km, and different NANF optical bandwidths, loss and total launch power. We compare such throughput with a benchmark state-of-the-art SMF Raman-amplified C+L system. We found that NANF might enable relative throughput gains vs.~the benchmark on the order of 1.5x to 5x, at reasonable NANF and system parameter values. We also study the problem of the impact of NANF Inter-Modal-Interference (IMI) on system performance and show that a value of -60~dB/km, close to the currently best reported values, is low enough to have no substantial harmful effect. We finally look at a more long-term scenario in which NANF loss gets below that of SMF and we show that in this context repeterless or even completely amplifierless systems might be possible, delivering 300-400 Tb/s per NANF, over 200 to 300~km distances. The system simplification and ease of wideband exploitation implied by these systems might prove quite attractive especially in densely populated regions where inter-node distances are modest. While several technological hurdles remain towards NANF systems becoming practical contenders, in our opinion NANF appears to have the potential to become an attractive and possibly disruptive alternative to conventional solid-core silica fibers

Nonlinearity-free Coherent Transmission in Hollow-Core Antiresonant Fiber

We demonstrate the first multi-terabit/s WDM data transmission through hollow-core antiresonant fiber (HC-ARF). 16 channels of 32-GBd dual-polarization (DP) Nyquist-shaped 256QAM signal channels were transmitted through a 270-m long fiber without observing any power penalty. In a single-channel high power transmission experiment, no nonlinearity penalty was observed for up to 1 W of received power, despite the very low chromatic dispersion of the fiber (<2 ps/nm/km). Our simulations show that such a low level of nonlinearity should enable transmission at 6.4 Tb/s over 1200 km of HC-ARF, even when the fiber attenuation is significantly greater than that of SMF-28. As signals propagate through hollow-core fibers at close to the speed of light in vacuum such a link would be of interest in latency-sensitive data transmission applications

Devices and Fibers for Ultrawideband Optical Communications

Wavelength-division multiplexing (WDM) has historically enabled the increase in the capacity of optical systems by progressively populating the existing optical bandwidth of erbium-doped fiber amplifiers (EDFAs) in the C-band. Nowadays, the number of channels—needed in optical systems—is approaching the maximum capacity of standard C-band EDFAs. As a result, the industry worked on novel approaches, such as the use of multicore fibers, the extension of the available spectrum of the C-band EDFAs, and the development of transmission systems covering C- and L-bands and beyond. In the context of continuous traffic growth, ultrawideband (UWB) WDM transmission systems appear as a promising technology to leverage the bandwidth of already deployed optical fiber infrastructure and sustain the traffic demand for the years to come. Since the pioneering demonstrations of UWB transmission a few years ago, long strides have been taken toward UWB technologies. In this review article, we discuss how the most recent advances in the design and fabrication of enabling devices, such as lasers, amplifiers, optical switches, and modulators, have improved the performance of UWB systems, paving the way to turn research demonstrations into future products. In addition, we also report on the advances in UWB optical fibers, such as the recently introduced nested antiresonant nodeless fibers (NANFs), whose future implementations could potentially provide up to 300-nm-wide bandwidth at less than 0.2 dB/km loss

Terahertz Hollow Core Antiresonant Fibre

Research on fibres operating in the terahertz frequency range is rapidly growing with numerous potential applications such as in spectroscopy, imaging, security, and transmission. However, designing a terahertz fibre with controllable and desirable transmission characteristics is challenging due to the complex cladding structure. In this thesis, we study hollow core antiresonant photonic crystal fibre (HC-ARPCF) for electromagnetic transmission and refractometric sensing in the terahertz regime. The HC-ARPCF consists of an air-core surrounded by a structured polymer cladding, which confines most of the power within the air-core region. The idea behind hollow-core antiresonant fibres is that light is guided in the hollow air core, thus drastically reducing the transmission loss. Guidance of light is achieved via reflection provided by thin membranes of the antiresonant tubes that surround the core, behaving effectively as a Fabry-P´erot cavity. At antiresonant frequencies, the thin membranes reflect the light towards the core because of the higher refractive index of the membranes. The guidance mechanism of the HC-ARPCF can also be explained due to the inhibited coupling mechanism (coupling between core and cladding mode is forbidden in guidance), where the cladding mode maintains a lower density of states (ηeff) than the fundamental core mode. Inhibited coupling guidance in HC-ARPCF offers broad bandwidth. At resonance frequencies, the light couples to the thin membranes and the core mode becomes more lossy, which can assist in gas sensing. The idea for the terahertz HC-ARPCF is inspired by those in the well-developed infrared and mid-infrared range. The effect of cladding pattern, cladding material, and cladding sector angle are analysed to investigate and tune the transmission loss, bending loss, and modal properties. The detailed simulations of several designs give a new understanding of the effect of the cladding elements on the leakage loss. The HCARPCFs are considered as a suitable candidate for low loss and broadband terahertz transmission. In addition, we model and simulate a simple hollow-core antiresonant terahertz waveguide, show the linear properties and explore the mechanism of achieving nonlinearity. First, the linear properties of HC-ARPCF are discussed, and then the nonlinear properties of the same structure are demonstrated, considering a gas-filled core in the terahertz regime. Furthermore, this thesis describes two different fabrication techniques for terahertz HC-ARPCF, using Zeonex and UV-resin as the bulk materials via a 3D printing process. The Zeonex filaments are made by using a Filabot EX2 Filament Extruder designed for filament production. To measure the effective material loss of the Zeonex, a circular disc with an uneven thickness of 0.65±0.05mmand a diameter of 24mmis printed.We demonstrate the first successful fabrication of Zeonex and UV resin fibre using Fused Decomposition modelling (FDM) and Steriolithography Apparatus (SLA) methods, respectively, to investigate the surface quality and thickness variations of the printed structure. These printing approaches have potential to replace conventional costly terahertz fibre drawing process. The fabricated fibres are then experimentally investigated for terahertz transmission. Fibres fabricated using the FDM and SLA methods are also investigated numerically and the results are compared against the experimental results. The detailed simulations suggest their attenuation can be improved by orders of magnitude with improvements in the quality of the fabrication process. We also discuss the possible post-processing techniques that can be useful for improving fibre quality and consistency in future work.Thesis (Ph.D.) -- University of Adelaide, School of Electrical & Electronic Engineering, 202

Novel Specialty Optical Fibers and Applications

Novel Specialty Optical Fibers and Applications focuses on the latest developments in specialty fiber technology and its applications. The aim of this reprint is to provide an overview of specialty optical fibers in terms of their technological developments and applications. Contributions include:1. Specialty fibers composed of special materials for new functionalities and applications in new spectral windows.2. Hollow-core fiber-based applications.3. Functionalized fibers.4. Structurally engineered fibers.5. Specialty fibers for distributed fiber sensors.6. Specialty fibers for communications

Design and Analysis of Advanced Photonic Devices for Electromagnetic Transmission and Sensing

In this thesis, we report the investigation of advanced photonic devices for electromagnetic transmission and biochemical sensing in the terahertz and optical regimes. The choice of material for designing a terahertz device is deemed to be one of the most crucial factors. First, we consider materials that are frequently used in making terahertz devices. We experimentally demonstrate the optical, thermal, and chemical properties of various chosen glasses, polymers, and resin to select the optimal material for terahertz. Second, we perform a broad review on terahertz optical fibres—this includes various fibre categories, their guiding mechanisms, fabrication methodologies, possible experimental methodologies, and applications. Third, we analyse and demonstrate the design of various fibre structures for terahertz transmission and sensing, and then perform experiments on a hollow core antiresonant fibre. We demonstrate successful fabrication of an asymmetrical Zeonex fibre using a novel fabrication method. This is carried out by using a tabletop horizontal extruder designed for producing polymer filaments. The fabricated fibre is then experimentally investigated for terahertz transmission and gas sensing. Fourth, we study optical fibre based surface plasmon resonance biosensors for operation in the optical regime. Theoretical studies are undertaken to obtain the best possible sensor in consideration of performance, experimental feasibility, and fabrication. One of the optimized sensors is then fabricated as a possible candidate for possible realworld sensing applications. Finally, we study metasurface planar devices for achieving high sensitivity and quality factor in the terahertz regime. We first demonstrate a tunable graphene metasurface that can achieve multi-band absorption and high refractometric sensing. Later, we demonstrate on an all-dielectric metasurface that reports highest Q-factor in the terahertz regime. We fabricate and experiment on the dielectric metasurface and find good agreement with the simulation.Thesis (Ph.D.) -- University of Adelaide, School of Electrical & Electronic Engineering, 202

Optimisation des interféromètres fibrés pour la stabilisation d'oscillateurs laser

La grande cohérence temporelle du signal émis par un oscillateur laser, en comparaison aux autres sources lumineuses, constitue une des propriétés les plus remarquables du laser. Aujourd'hui, un grand nombre d'applications allant de la spectroscopie à la télédétection reposent sur un laser stabilisé, c'est-à-dire d'un laser mono fréquence dont la cohérence temporelle est améliorée par asservissement à une référence externe. Le développement des horloges atomiques optiques et des détecteurs d'ondes gravitationnelles, deux efforts de recherche de portée mondiale qui repoussent continuellement les limites technologiques, est par ailleurs étroitement lié aux progrès dans le domaine de la stabilisation laser. À ce jour, les signaux laser les plus cohérents ont été produits par asservissement à des cavités en espace libre de type Fabry-Perot hautement découplées d'un environnement de laboratoire contrôlé. Dans les cinquante dernières années, ces cavités ont été le sujet d'un intense effort de recherche par les principaux laboratoires de métrologie nationaux ; leur performance est présentement limitée par des fluctuations thermodynamiques de la taille des miroirs délimitant la cavité, une limite qui semble pour l'instant difficile à surmonter. Il y a un intérêt à étudier d'autres types de références externes, non seulement pour établir s'il est possible de vaincre la limite de stabilité actuelle en poussant le progrès dans une direction alternative, mais aussi parce que la complexité des meilleures cavités en espace libre les rend peu attrayantes dans toutes les applications pour lesquelles une stabilité laser record n'est pas requise. Les travaux de recherche présentés dans cette thèse sont unis sous le thème de l'optimisation des interféromètres à fibre optique. Ces interféromètres, tout comme les cavités Fabry-Perot qui ont fait l'objet d'une recherche beaucoup plus poussée, peuvent agir à titre de référence externe pour la stabilisation laser ; ils affichent une performance typiquement déterminée soit par le bruit de photodétection, soit par les fluctuations de longueur et d'indice de la fibre constituant le bras de délai dans une configuration Michelson ou Mach-Zehnder. Fondamentalement, la stabilité ultime d'un laser verrouillé à un tel interféromètre est limitée par les fluctuations d'origine thermodynamique du parcours optique dans la fibre, une limite qui est plus élevée que la limite analogue dans une cavité en espace libre dans la mesure où une fibre de silice conventionnelle est considérée. En comparaison, ces interféromètres fibrés sont toutefois plus simples à construire et à opérer. Les trois premiers articles présentés dans cette thèse s'intéressent au cas d'un interféromètre fibré limité par le bruit de photodétection. Cette condition est normalement atteinte lorsqu'une relativement courte fibre optique est utilisée. On y montre qu'une limite de stabilité fondamentale associée au bruit de grenaille peut être calculée. Surtout, on établit pour la première fois que cette limite peut être asymptotiquement atteinte peu importe la méthode de détection et le point d'opération utilisés, pour autant que deux conditions soient respectées : que tous les signaux interférométriques disponibles soient indépendamment enregistrés et combinés de façon optimale et que le caractère cyclostationnaire du bruit de grenaille soit soigneusement considéré pour la construction de la forme d'onde de démodulation (si le signal laser est modulé). On présente aussi une nouvelle méthode, similaire à la détection auto-hétérodyne, permettant de minimiser l'impact des bruits de photodétection d'origine technique et faciliter l'opération à la limite du bruit de grenaille. Les deux derniers articles de cette thèse s'intéressent plutôt au cas d'un interféromètre dont la performance est limitée par le bruit de longueur optique fondamental de la fibre, ce qui est pertinent pour une fibre de longueur intermédiaire. On y montre que des fibres à coeur creux récemment développées, les fibres antirésonantes, exhibent des propriétés les rendant utilisables aux fins de la stabilisation laser et potentiellement compétitives vis-à-vis des fibres conventionnelles et peut-être même des cavités en espace libre. En particulier, on démontre que ces fibres à coeur creux, lorsque évacuées, affichent un coefficient de rétrodiffusion Rayleigh qui est de quatre ordres de grandeur inférieur à celui d'une fibre conventionnelle ; leur bruit thermoconductif est aussi plus faible par 5 dB à 20 kHz et peut vraisemblablement être diminué davantage. De futurs travaux pourront confirmer si le bruit fondamental de ces fibres est aussi faible qu'envisagé pour les basses fréquences, ce qui permettrait de considérer leur utilisation pour faire véritable compétition aux cavités ultra-stables.The great temporal coherence of the signal emitted by a laser oscillator, compared to other light sources, constitutes one of the most remarkable properties of the laser. Today, a large number of applications ranging from spectroscopy to remote sensing rely on a stabilized laser, that is, a single-frequency laser whose temporal coherence is improved by locking it to an external reference. The development of optical atomic clocks and gravitational wave detectors, two world-class research efforts that are constantly pushing technological boundaries, is also closely related to advances in laser stabilization. To this day, the most coherent laser signals have been produced by locking lasers to free-space Fabry-Perot cavities highly decoupled from a controlled laboratory environment. In the last fifty years, such cavities have been the subject of an intense research effort by the main national metrology laboratories; their performance is currently limited by thermodynamic fluctuations in the dimensions of the mirrors delimiting the cavity, a limit which currently appears difficult to overcome. There is an interest in studying other types of external references, not only to establish whether it is possible to beat the current stability limit by pushing progress in an alternative direction, but also because the complexity of the best free-space cavities makes them unattractive in all applications for which record stability is not required. The research work presented in this thesis is united under the theme of the optimization of fiber interferometers. Such interferometers, just like the Fabry-Perot cavities which have been the subject of much more extensive research, can act as an external reference for laser stabilization; they display a performance typically determined either by the photodetection noise or by the length and index fluctuations of the optical fiber constituting the delay arm in a Michelson or Mach-Zehnder configuration. Fundamentally, the ultimate stability of a laser locked to such an interferometer is limited by thermodynamic fluctuations of the optical path through the fiber, a limit which is higher than the analogous limit in a free-space cavity when a conventional silica fiber is considered. In comparison, however, these fiber interferometers are simpler to build and operate. The three first articles presented in this thesis focus on the case of a fiber interferometer limited by photodetection noise. This condition is normally reached when a relatively short optical fiber is used. It is shown that a fundamental stability limit associated with shot noise can be calculated. Most importantly, it is established for the first time that this limit can be asymptotically attained regardless of the detection method and the operating point that are used, provided that two conditions are met: that all available interferometric signals are independently recorded and combined in a optimal manner and that the cyclostationary nature of shot noise is carefully considered when constructing the demodulation waveform (if the laser signal is modulated). We also introduce a new method, similar to self-heterodyne detection, which allows the minimization of technical photodetection noise and facilitates shot-noise-limited operation. The last two articles of this thesis rather concern the case of an interferometer whose performance is limited by the fiber's fundamental optical path noise, which is relevant when the delay fiber is of intermediate length. It is shown that recently developed antiresonant hollow-core fibers exhibit properties that make them suitable for the purposes of laser stabilization and potentially competitive with conventional silica fibers and perhaps even free-space cavities. In particular, it is demonstrated that these hollow-core fibers, when evacuated, display a Rayleigh backscattering coefficient which is four orders of magnitude lower than that of a conventional fiber; their thermoconductive noise is also lower by 5 dB at 20 kHz and can likely be reduced further. Future work will confirm whether the fundamental noise of these fibers at low frequencies is as low as currently anticipated, which would allow their consideration to truly compete with ultra-stable free-space cavities