29 research outputs found

    Mode Coupling in Space-division Multiplexed Systems

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    Even though fiber-optic communication systems have been engineered to nearly approach the Shannon capacity limit, they still cannot meet the exponentially-growing bandwidth demand of the Internet. Space-division multiplexing (SDM) has attracted considerable attention in recent years due to its potential to address this capacity crunch. In SDM, the transmission channels support more than one spatial mode, each of which can provide the same capacity as a single-mode fiber. To make SDM practical, crosstalk among modes must be effectively managed. This dissertation presents three techniques for crosstalk management for SDM. In some cases such as intra-datacenter interconnects, even though mode crosstalk cannot be completely avoided, crosstalk among mode groups can be suppressed in properly-designed few-mode fibers to support mode group-multiplexed transmission. However, in most cases, mode coupling is unavoidable. In free-space optical (FSO) communication, mode coupling due to turbulence manifests as wavefront distortions. Since there is almost no modal dispersion in FSO, we demonstrate the use of few-mode pre-amplified receivers to mitigate the effect of turbulence without using adaptive optics. In fiber-optic communication, multi-mode fibers or long-haul few-mode fibers not only suffer from mode crosstalk but also large modal dispersion, which can only be compensated electronically using multiple-input-multiple-output (MIMO) digital signal processing (DSP). In this case, we take the counterintuitive approach of introducing strong mode coupling to reduce modal group delay and DSP complexity

    Mode-Multiplexed Transmission over Conventional Graded-Index Multimode Fibers

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    We present experimental results for combined mode-multiplexed and wavelength multiplexed transmission over conventional graded-index multimode fibers. We use mode-selective photonic lanterns as mode couplers to precisely excite a subset of the modes of the multimode fiber and additionally to compensate for the differential group delay between the excited modes. Spatial mode filters are added to suppress undesired higher order modes. We transmit 30-Gbaud QPSK signals over 60 WDM channels, 3 spatial modes, and 2 polarizations, reaching a distance of 310 km based on a 44.3 km long span. We also report about transmission experiments over 6 spatial modes for a 17-km single-span experiment. The results indicate that multimode fibers support scalable mode-division multiplexing approaches, where modes can be added over time if desired. Also the results indicate that mode-multiplexed transmission distance over 300 km are possible in conventional multimode fibers

    Orbital angular momentum in optical fibers

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    Thesis (Ph.D.)--Boston UniversityInternet data traffic capacity is rapidly reaching limits imposed by nonlinear effects of single mode fibers currently used in optical communications. Having almost exhausted available degrees of freedom to orthogonally multiplex data in optical fibers, researchers are now exploring the possibility of using the spatial dimension of fibers, via multicore and multimode fibers, to address the forthcoming capacity crunch. While multicore fibers require complex manufacturing, conventional multimode fibers suffer from mode coupling, caused by random perturbations in fibers and modal (de)multiplexers. Methods that have been developed to address the problem of mode coupling so far, have been dependent on computationally intensive digital signal processing algorithms using adaptive optics feedback or complex multiple-input multiple-output algorithms. Here we study the possibility of using the orbital angular momentum (OAM), or helicity, of light, as a means of increasing capacity of future optical fiber communication links. We first introduce a class of specialty fibers designed to minimize mode coupling and show their potential for OAM mode generation in fibers using numerical analysis. We then experimentally confirm the existence of OAM states in these fibers using methods based on fiber gratings and spatial light modulators. In order to quantify the purity of created OAM states, we developed two methods based on mode-image analysis, showing purity of OAM states to be 90% after 1km in these fibers. Finally, in order to demonstrate data transmission using OAM states, we developed a 4-mode multiplexing and demultiplexing systems based on free-space optics and spatial light modulators. Using simple coherent detection methods, we successfully transmit data at 400Gbit/s using four OAM modes at a single wavelength, over 1.1 km of fiber. Furthermore, we achieve data transmission at 1.6Tbit/s using 10 wavelengths and two OAM modes. Our study indicates that OAM light can exist, and be long lived, in a special class of fibers and our data transmission demonstrations show that OAM could be considered an additional degree of freedom for data multiplexing in future optical fiber communication links. Our studies open the doors for other applications such as micro-endoscopy and nanoscale imaging which require fiber based remote delivery of OAM light

    Mode Analysis and Applications of Three-Layer Step-Index and Depressed-Core Optical Fibers

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    Optical fibersplay important roles in telecommunication, imaging, sensing, lasers and amplifiers. The rapid development of fiberopticsover the past decades has beenunderpinnedbyvarious designs of the optical fibers and bythe rapid improvement of the understanding of the waveguiding mechanismsand associated models. According to the cross-section refractive index distribution, optical fibers can be generally classified into four basic types: two-layer step-index fiber, three-layer step-index fiber, three-layer depressed-core fiber and hollow-core fiber.Among these four basic fibertypes,the modeproperties and the applications of the three-layer step-index and depressed-core fibers have notbeensufficiently investigated. This thesis presents a detailedmode analysis in three-layer step-index and depressed-core fibers and their applications.A complete dispersion diagram includingthe core and cladding modes in athree-layer step-index optical fiber has beendevelopedfor the first time,using both analyticalmethodandfull-vector finite element method. Modetransition from the cladding-typeto core-type modes as a function of the core radius wasstudiedas a contribution to deepening the knowledge ofconventional step-index optical fibers.Based on the developed complete dispersion diagram for thethree-layer step-index optical fiber, it wasfound that asmall-core fiber with a nano/micro-sized core supportsonly cladding-typemodes. The self-imaging phenomenon ofthe pure cladding modes in thesmall-core fiber has beenstudied, and itscomparison tothe behaviourof the core-typemodes in the conventional multimode fibershas Vbeen carried out. The discrete nature and the exponential growth behaviourof the self-imagingof the cladding-typemodeswas establishedfor the first time.The resultsprovide new insights and design rules fora number of multimode interference devices such as optical couplers, optical modulators, multimode fiber lasers and space-division multiplexing devices. The depressed-core fiber, consisting of a low-index solid coreanda high-index cladding surrounded by air, is in effect a bridge between the conventional step-indexfiber and the tube-type hollow-core fiber from the pointof view of the index profile. In theresearch, a complete dispersion diagram of thedepressed-core fiber has beenobtained for the first time by solving the full-vector eigenvalue equations.The waveguiding in the depressed-core fiber wasanalyzed using thetheory of anti-resonant and the inhibited coupling guiding mechanisms.An asymmetric planar anti-resonant reflecting optical waveguide model (asymmetric planar ARROWmodel)wasproposed for the depressed-core fiber.A high-index polymer-coated no-core fiber as an example of the depressed-core fiberhas beenstudiedtheoretically and experimentally. The appearance of the periodic transmission loss dipsin the spectrum of a long or bent polymer-coated no-core fibersamples reflectsthe anti-resonance nature of the depressed-core fiber. Theexperiments show that the overall change inspectrallossis greater than 31dB at the dip positionaround 1550 nm and the average sensitivity is up to 14.77 dB/m-1, as the bend radius changes from∞ (straight) to47.48 cm.Theresults indicate thatthe polymer-coated no-core fibershave the potential to be used in many devices including curvature sensors and tunable loss filters.VIWhile the superposition of the spectra of multiple modes led to broad dips, it has beenfound that anindividual mode can cause sharp dips in the transmission spectra of apolymer-coated no-core fiber. As a result of this phenomenon and the large thermo-optical and thermal expansion coefficients of the polymer coating, a compact(length \u3c 10 mm), high sensitivity and linear response temperature sensor with the sensitivity as high as -3.784 nm/Chas been demonstrated experimen

    Study, analysis and experimental validation of fiber refractometers based on single-mode, multimode and photonic crystal fibers for refractive index measurements with application for the detection of methane

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    Refractive index measurement has been studied since Ernest Abbé initially designed a refractometer in 1869, which is named the Abbé refractometer. Since then, numerous types of refractometers have been developed by employing either the optical prism-based refractometer or the optical fiber-based refractometer, due to their wide-ranging applications such as for sensingvarious physical, biological and chemical parameters. Recently, a large number of researchers have been developing refractometers based on optical fibers, exploiting mechanisms such as surface plasmon resonance (SPR), multimode interference, fiber Bragg gratings (FBG), long period gratings (LPG), tapered optical fibers, and striped-cladding multimode fibers (MMF), for their advantages in immunity against electromagnetic interference, electrical passivity at the sensing probe, and capability to long term in-situ measurement. This thesis concerns the development of comprehensively functional and accurate models for optical fiber refractometers based on optical intensity modulation, in particular for stripped-cladding MMF refractometry as well as hybrid systems involving a combination of single-mode-multimode fiber refractometery and the all-fiber hybrid refractometer using photonic crystal fibers. A key objective of this work is to characterize the performance of these intensity-based optical fiber refractometers in terms of their power response, sensitivity, resolution, and dynamic range. The simulation results which are corroborated experimentally demonstrate very high sensitivity being obtained in Zone II (i.e. the sensing regime typically employed for measuring a sensing medium index higher than the cladding index but less than or equal to the core index) for all three types of refractometers. However, the sensitivity in Zone III (i.e. the sensing regime for which the sensing medium index is higher than the core index) is very low. A hybrid single-mode fiber - multimode fiber configuration is used to improve the sensitivity in Zone III. On other hand, the sensitivity for Zone I (i.e. the sensing regime typically employed for measuring a sensing medium index lower than the cladding index) has been improved by increasing evanescent wave absorption using the all-fiber hybrid refractometer based on solid-core photonic crystal fibers. As a further potential of the fiber refractometer for applications in biochemical sensing, the proof-of-concept for a methane gas sensor has been demonstrated using supramolecular cryptophane-A which enables to trap the methane molecules. Cryptophane-A incorporated into a functionalized film of StyreneAcrylonitrile (SAN) host is applied to a de-cladded region of the sensor as the sensitive region. The refractive index of this functionalized layer increases proportionally with increasing methane concentration, subsequently inducing variations in the transmitted optical power along the fiber sensor

    Sensing using Specialty Optical Fibers

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    Fiber optic based sensing is a growing field with many applications in civil and aerospace engineering, oil and gas industries, and particularly in harsh environments where electronics are not able to function. Optical fibers can be easily integrated into structures, are immune to electromagnetic interference, can be interrogated from remote distances, and can be multiplexed for distributed measurements. Because of these properties, specialty fiber designs and devices are being explored for sensing temperature, strain, pressure, curvature, refractive index, and more. Here we show a detailed analysis of a multicore fiber (MCF) for sensing, including its design and optimization in simulation, as well as experimental operation when used as sensor. The multicore fiber sensor\u27s performance as a function of temperature, strain, bending, and acoustic waves are all explored. The MCF sensors are shown to be able to withstand temperatures up to 1000°C, making them suitable to be harsh environment sensors. Additionally, a simple method for increasing the sensitivity of the MCF to longitudinal force is shown to multiple the sensitivity of the MCF sensor by a factor of seven. Also, a configuration for decoupling force and temperature will be presented. Finally, a developing all-fiber device, a photonic lantern, will be shown in conjunction with the MCF in order to increase sensitivity, add directional sensitivity, and lower the cost of the sensor interrogation for bending measurements. In addition to the multicore fiber, an analysis of anti-resonant hollow core fiber (ARHCF) is also presented. The fibers\u27 design-dependent propagation losses are explored, as well as their higher order mode content. Also, a potential application of an ARHCF for an in-fiber Raman air sensor is introduced, and the design optimization in simulation is shown

    Design and characterization of few-mode fibers for space division multiplexing on fiber eigenmodes

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    La croissance constante et exponentielle de la demande de trafic de donnĂ©es Internet conduit nos rĂ©seaux de tĂ©lĂ©communications optiques, principalement composĂ©s de liaisons de fibre monomode, Ă  une pĂ©nurie imminente de capacitĂ©. La limite non linĂ©aire de la fibre monomode, prĂ©dite par la thĂ©orie de l'information, ne laisse aucune place Ă  l'amĂ©lioration de la capacitĂ© de communication par fibre optique. Dans ce contexte, la prochaine technologie de rupture dans les transmissions optiques Ă  haute capacitĂ© devrait ĂȘtre le multiplexage par rĂ©partition spatiale (SDM). La base du SDM consiste Ă  utiliser diffĂ©rents canaux spatiaux d'une seule fibre optique pour transmettre des donnĂ©es indĂ©pendantes. Le SDM fournit ainsi une augmentation de la capacitĂ© de transport de donnĂ©es d'un facteur qui dĂ©pend du nombre de chemins spatiaux qui sont Ă©tablis. Une façon de rĂ©aliser le SDM consiste Ă  utiliser des fibres faiblement multimodes (FMF) spĂ©cialisĂ©es, conçues pour prĂ©senter un couplage faible entre les modes guidĂ©s. Un traitement MIMO rĂ©duit peut alors ĂȘtre utilisĂ© pour annuler le couplage rĂ©siduel des modes. Dans cette thĂšse, nous donnons tout d'abord un aperçu des progrĂšs rĂ©cents du multiplexage par rĂ©partition de modes (MDM). Les modes Ă  polarisation linĂ©aire (LP), les modes de moment angulaire orbital (OAM) et les modes vectoriels reprĂ©sentent diffĂ©rentes bases de modes orthogonaux possibles dans la fibre. Nous comparons les travaux utilisant ces modes en termes de conception de fibre proposĂ©e, nombre de modes, complexitĂ© MIMO et rĂ©sultats expĂ©rimentaux de transmission de donnĂ©es. Ensuite, nous introduisons la modĂ©lisation de la fibre optique rĂ©alisĂ©e avec les solveurs numĂ©riques de COMSOL Multiphysics, et nous discutons de quelques travaux utilisant cette modĂ©lisation de fibre. Nous proposons une nouvelle FMF, composĂ©e d'un noyau hautement elliptique et d'une tranchĂ©e adjacente ajoutĂ©e pour rĂ©duire la perte de courbure des modes d'ordre supĂ©rieur. La fibre est conçue et optimisĂ©e pour prendre en charge cinq modes spatiaux avec une dĂ©gĂ©nĂ©rescence de polarisation double, pour un total de dix canaux. La fibre proposĂ©e montre une diffĂ©rence d'indice effectif entre les modes spatiaux supĂ©rieure Ă  1 × 10-3sur la bande C. Ensuite, nous fabriquons la fibre avec un procĂ©dĂ© standard de dĂ©pĂŽt chimique en phase vapeur modifiĂ© (MCVD), et nous caractĂ©risons la fibre en laboratoire. La caractĂ©risation expĂ©rimentale a rĂ©vĂ©lĂ© que la fibre prĂ©sente une propriĂ©tĂ© de maintien de polarisation. Ceci est obtenu grĂące Ă  la combinaison de la structure centrale asymĂ©trique et de la contrainte thermique introduite lors de la fabrication. Nous mesurons la birĂ©fringence avec une technique de rĂ©seau de Bragg inscrit dans la fibre (FBG). En incluant la contrainte thermique dans notre modĂ©lisation de fibre, un bon accord est obtenu entre la birĂ©fringence simulĂ©e et mesurĂ©e. Nous avons rĂ©ussi Ă  effectuer la premiĂšre transmission de donnĂ©es sur la fibre proposĂ©e, en transmettant deux signaux QPSK sur les deux polarisations de chaque mode spatial, sans utiliser de traitement MIMO. Enfin, nous prĂ©sentons une amĂ©lioration d'une technique d'interfĂ©romĂ©trie hyperfrĂ©quence (MICT) prĂ©cĂ©demment proposĂ©e, afin de mesurer expĂ©rimentalement la perte en fonction du mode (MDL) des groupes de modes FMF. En conclusion, nous rĂ©sumons les rĂ©sultats et prĂ©sentons les perspectives d'avenir de cette recherche. En rĂ©sumĂ©, de nouveaux FMF doivent ĂȘtre Ă©tudiĂ©s si nous voulons rĂ©soudre la pĂ©nurie imminente de capacitĂ© de nos technologies systĂšme. Les rĂ©sultats de cette thĂšse indique que le FMF Ă  maintien de polarisation proposĂ©e dans cette recherche reprĂ©sente une amĂ©lioration significative dans le domaine des systĂšmes de transmission MDM sans MIMO pour des liaisons de communication courtes ; c’est-Ă -dire distribuant des donnĂ©es sur une longueur infĂ©rieure Ă  10 km. Nous espĂ©rons que ce travail conduira au dĂ©veloppement de nouveaux composants SD Mutilisant cette fibre, tels que de nouveaux amplificateurs Ă  fibre, ou de nouveaux multiplexeurs/dĂ©multiplexeurs, comme par exemple des coupleurs en mode fibre fusionnĂ©e ou des dispositifs photoniques au silicium.The constant and exponential growth of Internet data traffic demand is driving our optical telecommunication networks, mainly composed of single-mode fiber links, to an imminent capacity shortage. The nonlinear limit of the single-mode fiber, predicted by the information theory, leave no room for optical fiber communication capacity improvements. In this direction, the next disruptive technology in high-capacity communication transmissions is expected to be Space Division Multiplexing (SDM). The basic of SDM consists of using different spatial channels of a single optical fiber to transmit information data. SDM thus provides an increase in the data-carrying capacity by a factor that depends on the number of spatial paths that are established. A way to realize SDM is through the use of specialty few-mode fibers (FMFs), designed to have a weak coupling between the guided modes. A reduced MIMO processing can be used to undo the residual mode coupling. In this thesis, we firstly give an overview of the recent progress in mode division multiplexing (MDM). Linearly polarized (LP) modes, orbital angular momentum (OAM) modes and vector modes represent the possible orthogonal modes guided into the fiber. We compare works, making use of those modes, in terms of proposed fiber design, number of modes, MIMO complexity and data transmission experiments. After that, we introduce the optical fiber modelling performed with the numerical solvers of COMSOL Multiphysics, and we discuss some works making use of this fiber modelling. Next, we propose a novel FMF, composed of a highly elliptical core and a surrounding trench added to reduce the bending loss of the higher order modes. The fiber is designed and optimized to support five spatial modes with twofold polarization degeneracy, for a total of ten channels. The proposed fiber shows an effective index difference between the spatial modes higher than 1×10-3 over the C-band. Afterwards, we fabricate the fiber with standard modified chemical vapor deposition (MCVD) process, and we characterize the fiber in the laboratory. The experimental characterization revealed the polarization maintaining properties of the fiber. This is obtained with the combination of the asymmetric core structure and the thermal stress introduced during the fabrication. We measure the birefringence with a fiber Bragg grating (FBG) technique, and we included the thermal stress in our fiber modelling. A good agreement was found between the simulated and measured birefringence. We successfully demonstrate the first data transmission over the proposed fiber, by transmitting two QPSK signals over the two polarizations of each spatial mode, without the use of any MIMO processing. Lastly, we present an improvement of a previously proposed microwave interferometric technique (MICT), in order to experimentally measure the mode dependent loss (MDL) of FMF mode groups. Finally, we present the conclusions and the future perspectives of this research. To conclude, novel FMFs need to be investigated if we want to solve the imminent capacity shortage of our system technologies. We truly believe that the polarization-maintaining FMF proposed in this research represents a significant improvement to the field of MIMO-free MDM transmission systems for short communication links, distributing data over length less than 10 km. We hope that this work will drive the development of new SDM components making use of this fiber, such as new fiber amplifiers, or new mux/demux, as for example fused fiber mode couplers or silicon photonic devices

    Generation and characterization of cylindrical vector beams in few-mode fiber

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    For the past many decades, the Gaussian laser beam has driven major scientific discoveries that revolutionized the world of optics and photonics. In recent years, there is a burgeoning transformation where significant research has been dedicated in discovering the complex properties of cylindrical vector beams (CVBs). Increasingly, a beam of light with its intensity profile taking the shape of a single doughnut ring has attracted attention of several researchers the world over. Particularly, the so-called CVBs exhibit unique properties when focused owing to their radial and azimuthal distribution of polarization. In comparison to conventional (Gaussian-like) beams inheriting homogeneous polarization, CVBs provide unique light-matter interactions. For example, a radially polarized beam can enhance the imaging resolution of the system significantly with their spatial inhomogeneous polarization by imparting a symmetric and high numerical aperture focus. Moreover, CVBs with their phase and intensity singularities have found broad applications in quantum optics, optical micro/nano-manipulation, surface plasmon polariton, super-resolution imaging, and high-capacity fiber-optic communication. The studies of most widely used CVBs have been explored both in free space optics as well as in guided fiber optics. Further developments will require reliable techniques to generate these CVBs with strong coupling efficiency, high mode purity and high-power handling. For the past few years, the design, fabrication and study of optical fibers that supports CVBs, vortex and orbital angular momentum (OAM) beams have come to the forefront of research in this area. This is true in a sense that mode division multiplexing (MDM) is considered as a preeminent solution to the data capacity limitations faced by the standard single-mode fiber. In addition, vector beams in optical fibers constitute the fundamental basis set of linearly polarized (LP) modes (within the scalar approximation) as well as modes carrying OAM which represent another potential approach for implementing MDM based communications. Therefore, fundamental information and control over the vector beams is key to unravel future fiber communication links and CVB based fiber-optic sensors. For this purpose, it is essential to develop efficient methods to generate these CVBs. Some of the current methods reported for the generation of CVBs employ spiral phase plate, spatial light modulator (SLM), and offset fiber coupling. This thesis elucidates the generation as well as the optical characterization of such propagating cylindrical vector beams in a few-mode fiber. The ultimate purpose would be to develop simple, flexible and cost-effective photonic devices that will allow the efficient generation and stable propagation of the CVB while reducing the overall losses incurred by the system. Most of the methods reported earlier were limited to the measurements of the scalar LP mode groups of a FMF, thus neglecting the underlying vector beams that require delicate spectral and spatial control in order to be detected. In this thesis, three different techniques have been utilized for the generation of CVBs and OAM beams with high output purity. Initially, a tunable mechanical mode converter has been fabricated to demonstrate the generation of cylindrical vector beams supported by FMF in the telecom spectral range. This photonic device is utilized to demonstrate the non-destructive nonlinear characterization of CVB by utilizing the phenomenon of stimulated Brillouin scattering for the first time. We showed how the Brillouin gain spectra of the vector beams in some specialty fibers can be independently identified, measured, and subsequently exploited to probe the corresponding effective refractive indices of the vector beam retrieved from the data. This new characterization method of individual vector beam will have an impact in both light-wave and FMF-based optical sensing applications, which at present, mostly rely on the scalar LP modes. Further, a simple and low-cost technique to generate CVBs via long period fiber grating (LPFG) with very small grating pitch is reported. This work demonstrates that the cost-effective electric arc writing method for the fabrication of LPFGs is open to specialty few-mode fiber that often calls for very small pitch values. Finally, the generation of perfect cylindrical vector beams (PCVB) is demonstrated whose beam profile (i.e. transverse intensity profile) can be easily and precisely controlled. The latter novel method was used in-order to increase the free space coupling efficiency demanded by some specialty FMFs. The tailoring of the beam width and radius is performed via an iris and a diffractive phase mask implemented on a programmable SLM. The technique proposed towards the generation of PCVBs is highly adaptable for its robust nature to generate any arbitrary PCBs as well as perfect vortex beams with any topological order, using the same experimental setup. This experimental analysis is supported and validated via a rigorous theoretical framework that is in concordance with the results obtained
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