Performance of Direct-Detection Mode-Group-Division Multiplexing using Fused Fiber Couplers

© 2015 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works[EN] We present an end-to-end performance evaluation of a mode-group-division multiplexing system that uses direct detection instead of coherent detection, avoiding complex digital signal processing. The system transmits four data channels through a step-index fiber supporting six spatial modes comprising four mode groups, considering the two-fold degeneracy of the LPlm modes for l ≠ 0. Multiplexing and demultiplexing is performed using two- and three-core fused fiber couplers, each one phase-matched to a group of degenerate modes. These devices are analyzed through a field-based model that describes, for the first time to our knowledge, crosstalk between all the fiber modes. Propagation through the few-mode fiber is modeled considering differential modal attenuation, intermodal dispersion, chromatic dispersion, and both intergroup and intragroup modal coupling. The end-to-end link is described by a concatenation of matrix operators describing the optical field transfer functions for the multiplexer, fiber and demultiplexer. Error-free transmission of four 32-Gb/s OOK modulated data channels through a 1-km link proves the feasibility of the proposed direct-detection mode-group-division multiplexing approach.The work of I. Gasulla was supported by the Fulbright Commission and the Spanish Ministerio de Educacion through the Programa Nacional de Movilidad de Recursos Humanos del Plan Nacional de I-D + i2008-2011. The work of J. M. Kahn was supported by a Google Faculty Research Award.Gasulla Mestre, I.; Kahn, JM. (2015). Performance of Direct-Detection Mode-Group-Division Multiplexing using Fused Fiber Couplers. Journal of Lightwave Technology. 33(9):1748-1760. doi:10.1109/JLT.2015.2392255S1748176033

Multiplexage par division modale pour les applications à courte distance

Le multiplexage par division de mode (MDM) a reçu une attention considérable de la part des chercheurs au cours des dernières années. La principale motivation derrière l'utilisation de différents modes de fibre optique est d'augmenter la capacité des réseaux de transport. Les expériences initiales ont montré une grande complexité dans le traitement de signal (DSP) du récepteur. Dans cette thèse, nous étudions la viabilité et les défis de la transmission de données sur des fibres à quelques modes (FMF) pour des systèmes MDM à complexité de DSP réduite. Nos études comprennent à la fois une transmission de données cohérente et non cohérente. Dans notre première contribution, nous démontrons, pour la première fois, la transmission de données sur 4 canaux dans une nouvelle fibre OAM sans démultiplexage de polarisation optique. Nous utilisons une complexité de DSP réduite: deux jeux d'égaliseurs MIMO (multiple-input multiple-output) 2 × 2 au lieu d'un bloc égaliseur MIMO 4 × 4 complet. Nous proposons un nouveau démultiplexeur de mode permettant de recevoir simultanément deux polarisations d'un mode et de réaliser électriquement un démultiplexage de polarisation dans le récepteur DSP. Nous étudions également la pénalité OSNR due aux imperfections dans le démultiplexeur de mode et nous examinons la vitesse de transmission maximum accessible pour notre système. Dans notre deuxième contribution, nous étudions les dégradations modales dans les systèmes OAM-MDM, en nous concentrant sur leur effet sur la performance et la complexité du récepteur. Dans notre étude expérimentale, nous discutons pour la première fois de l'impact de deux modes non porteurs de données sur les canaux de données véhiculés par les modes OAM. Deux types différents de fibres OAM sont étudiés. Nous caractérisons notre liaison MDM en utilisant les techniques de mesure du temps de vol et de réponse impulsionnelle. Nous discutons des conclusions des résultats de caractérisation en étudiant l'impact des interactions modales sur la complexité de l'égaliseur du récepteur pour différents scénarios de transmission de données. Dans le troisième chapitre, nous étudions un nouveau FMF à maintien de polarisation et conduisons deux séries d'expériences de transmission de données cohérentes et de radio sur fibre (RoF). Nous démontrons pour la première fois, la transmission de données sans MIMO sur six et quatre canaux dans les systèmes cohérents et RoF, respectivement. Nous démontrons également, pour la première fois, la transmission de données RoF sur deux polarisations d'un mode dans une FMF. Nous discutons de la dégradation des performances due à la diaphonie dans de tels systèmes. Nous étudions également l'impact de la courbure sur cette fibre dans un contexte de RoF. La propriété de maintien de polarisation de cette fibre sous courbure est étudiée à la fois par des expériences de caractérisation et de transmission de données.Mode division multiplexing (MDM) has received extensive attention by researchers in the last few years. The main motivation behind using different modes of optical fiber is to increase the capacity of transport networks. Initial experiments showed high complexity in DSP of the receiver. In this thesis, we investigate the viability and challenges for data transmission over specially designed few mode fibers (FMF) for MDM systems with reduced DSP. Our studies include both coherent and non-coherent data transmission. In our first contribution, we demonstrate, for the first time, data transmission over 4 channels in a novel OAM fiber without optical polarization demultiplexing. We use reduced DSP complexity: two sets of 2×2 multiple-input multiple-output (MIMO) equalizers instead of a full 4×4 MIMO equalizer block. We propose a novel mode demultiplexer enabling us to receive two polarizations of a mode simultaneously and conducting polarization demultiplexing electrically in receiver DSP. We also investigate the OSNR penalty due to imperfections in the mode demultiplexer and we examine the maximum reachable baud rate for our system. In our second contribution, we study the modal impairments in OAM-MDM systems, focusing on their effect on receiver performance and complexity. In our experimental study, for the first time, we discuss the impact of two non-data carrying modes on data channels carried by OAM modes. Two different types of OAM fibers are studied. We characterize our MDM link using time-of-flight and impulse response measurement techniques. We discuss conclusions from characterization results with studies of the impact of modal interactions on receiver equalizer complexity for different data transmission scenarios . In the third contribution, we study a novel polarization-maintaining FMF and conduct two sets of coherent data transmission and non-coherent radio over fiber (RoF) experiments. We demonstrate for the first time, MIMO –Free data transmission over six and four channels in coherent and RoF systems, respectively. We also demonstrate, for the first time, RoF data transmission over two polarizations of a mode in a FMF. We discuss the performance degradation due to crosstalk in such systems. We also study the impact of bending on this fiber in RoF context. The polarization maintaining property of this fiber under bending is studied both via characterization and data transmission experiments

Few-Mode Transmission Technology for Ultra-High Capacity Optical Networks

Tesis por compendio[ES] En esta Tesis Doctoral, se propone diferentes técnicas de acoplo y conversión modal destinadas a aumentar la capacidad de transporte en sistemas de telecomunicaciones sobre fibra óptica. En particular, el objetivo principal es el desarrollo de la tecnología necesaria para conseguir una multiplexación modal utilizando un número limitado de modos, de manera controlada. Para ello, se estudian dos escenarios MDM con dos longitudes de onda distinta. Por un lado, usando la longitud de onda de 850 nm sobre SSMF favoreciendo la utilización de componentes ópticos y electro-ópticos de coste mucho menor que sus equivalentes en la banda C+L. Esta novedosa tecnología de transmisión permitirá una nueva generación de interconexiones ópticas de muy alta capacidad aplicable a enlaces chip-a-chip, a backplanes ópticos y también a clústeres de computación de altas prestaciones y centros de conmutación de red. Por otro lado, usando la longitud de onda de 1550 nm sobre guías ópticas basadas en SOI, es decir, Si (silicio) sobre sustrato de SiO2 (óxido de silicio) favoreciendo la utilización de dispositivos basados en tecnología integrada que ofrecen un menor tamaño, mejor repetibilidad y robustez que los dispositivos basados en fibra óptica. Para ello, se propone el uso de acopladores ópticos fusionados siendo un elemento indispensable a la hora de multiplexar y demultiplexar los distintos modos ópticos en un enlace MDM a 850 nm. Esta técnica permite multiplexar/demultiplexar los modos ópticos cuando el tipo de acoplador óptico utilizado es simétrico (DC, del inglés directional coupler), siendo necesario la utilización de un conversor de modos. También se estudia la posibilidad de convertir el modo óptico mediante la utilización de un acoplador óptico asimétrico (ADC, del inglés asymmetrical directional coupler), no siendo necesario utilizar un conversor de modos y simplificando el esquema MDM. Además, en esta tesis doctoral también se propone y evalúa el diseño de un conversor de modos mecánico basado en SSMF. Esta técnica permite obtener el primer modo de orden superior con una alta calidad y sin la necesidad de utilizar un ADC. Después de esto, se propone y evalúa la posibilidad de utilizar acopladores comerciales (diseñados a 1550 nm) a la longitud de onda de 850 nm permitiendo de esta forma reducir la necesidad de utilizar acopladores ópticos y conversores modales específicamente diseñados en dicha longitud de onda. Esta técnica reduciría los costes del sistema al necesitar un menor número de dispositivos y aprovechar los dispositivos diseñados a 1550 nm, siendo más económicos que los diseñados a 850 nm. En esta Tesis también se propone el uso de ADCs en guías strip basadas en SOI para la conversión y multiplexación de los modos ópticos desde la guia fundamental a la guia de dos modos, a la longitud de onda de 1550 nm. Para ello se estudia y demuestra experimentalmente diferentes diseños con el fin de obtener el diseño más robusto frente a las tolerancias de fabricación consiguiendo un rendimiento óptimo. Además, el uso de DCs sobre guías ridge es comúnmente utilizado y ofrece mejores prestaciones que el basado en guías strip, por ese motivo esta Tesis estudia y evalúa el uso de ADCs sobre guías ridge mediante el método de análisis de los índices efectivos de los supermodos par e impar. De esta forma se realiza una comparación entre los diseños óptimos de ambas estructuras (strip y ridge) con el objetivo de averiguar qué diseño ofrece mejores prestaciones. Por último, se propone y estudia el diseño de un acoplador grating capaz de multiplexar y demultiplexar los modos ópticos del modo fundamental y del primer orden superior desde la guia óptica a la fibra óptica y viceversa. Para ello se proponen diferentes diseños con el objetivo de conseguir un diseño más tolerante y eficiente frente a los errores por desalineamiento obteniendo un acoplo óptimo.[CA] En aquesta Tesi Doctoral, es proposen diferents tècniques d'acoblament i conversió modal destinades a augmentar la capacitat de transport en sistemes de telecomunicacions sobre fibra òptica. En particular, l'objectiu principal és el desenrotllament de la tecnologia necessària per a aconseguir una multiplexació modal utilitzant un número limitat de modes, de manera controlada. Per a això, s'estudien dos escenaris MDM amb dos longituds d'onda distinta. D'una banda, usant la longitud d'ona de 850 nm sobre SSMF afavorint la utilització de components òptics i electro-òptics de cost molt menor que els seus equivalents en la banda C+L. Aquesta nova tecnologia de transmissió permetrà una nova generació d'interconnexions òptiques de molt alta capacitat aplicable a enllaços chip-a-chip, a backplanes òptics i també a clústers de computació d'altes prestacions i centres de commutació de xarxa. D'altra banda, usant la longitud d'ona de 1550 nm sobre guies òptiques basades en SOI, és a dir, Si (silici) sobre substrat de SiO2 (òxid de silici) afavorint la utilització de dispositius basats en tecnologia integrada que ofereixen una menor grandària, millor repetibilitat i robustesa que els dispositius basats en fibra òptica. Per a això, es proposa l'ús d'acobladors òptics fusionats sent un element indispensable a l'hora de multiplexar i demultiplexar els distints modes òptics en un enllaç MDM. Aquesta tècnica permet multiplexar/demultiplexar els modes òptics quan el tipus d'acoblador òptic utilitzat és simètric (DC, de l'anglès directional coupler), sent necessari la utilització d'un convertidor de modes. També s'estudia la possibilitat de convertir el mode òptic per mitjà de la utilització d'un acoblador òptic asimètric (ADC, de l'anglès asymmetrical directional coupler), no sent necessari utilitzar un convertidor de modes i simplificant l'esquema MDM. Es mes, en aquesta tesi doctoral també es proposa i avalua el disseny d'un convertidor de modes mecànic basat en SSMF. Aquesta tècnica permet obtindre el primer mode d'orde superior amb una alta qualitat sense la necessitat d'utilitzar un ADC. Després d'açò, es proposa i avalua la possibilitat d'utilitzar acobladors comercials (dissenyats a 1550 nm) a la longitud d'ona de 850 nm permetent d'esta manera reduir la necessitat d'utilitzar acobladors òptics i convertidors modals específicament dissenyats en la dita longitud d'ona. Aquesta tècnica reduiria els costos del sistema al necessitar un menor nombre de dispositius i aprofitant els dispositius dissenyats a 1550 nm, sent més econòmics que els dissenyats a 850 nm. En aquesta Tesi també es proposa l'ús de ADCs en guies strip basades en SOI per a la conversió i multiplexació dels modes òptics des de la guia fonamental a la guia de dos modes, a la longitud d'ona de 1550 nm. Per a això s'estudia i demostra experimentalment diferents dissenys a fi de obtindré el disseny més robust enfront les toleràncies de fabricació aconseguint un rendiment òptim. A més, l'ús de DCs sobre guies ridge és comunament utilitzat i ofereix millors prestacions que el basat en guies strip, per eixe motiu aquesta Tesi estudia i avalua l'ús de ADCs sobre guies ridge per mitjà del mètode d'anàlisi dels índexs efectius dels supermodes parell i imparell. D'aquesta manera es realitza una comparació entre els dissenys òptims de les dos estructures (strip i ridge) amb l'objectiu d'esbrinar quin disseny ofereix millors prestacions. Finalment, es proposa i estudia el disseny d'un acoblador grating capaç de multiplexar i demultiplexar els modes òptics del mode fonamental i del primer orde superior des de la guia òptica a la fibra òptica i viceversa. Per a això es proposen diferents dissenys amb l'objectiu d'aconseguir un disseny més tolerant i eficient enfront dels errors per desalineament obtenint un acoblament òptim.[EN] In this Ph.D. thesis, different mode coupling and mode conversion techniques with the aim to increase the transport capacity in telecommunications systems over optical fiber are proposed. Concretely, the main aim is the development of the technology to achieve MDM using a limited controlled number of modes. Two different MDM scenarios based on two distinct wavelengths have been considered. On one hand, using the 850 nm wavelength over SSMF favors the use of optical and electro-optical devices with costs much lower than their equivalent in the C+L band. This novel transmission technology enables a new generation of very high capacity optical interconnections applicable to chip-to-chip links, to optical backplanes, and also to high-performance computing clusters and network switching centre interconnections. On the other hand, using the 1550 nm wavelength over optical waveguides based on SOI, i.e., Si (Silicon) above SiO2 substrate (silicon oxide), allows the use of integrated devices offering a less size, better repeatability and robustness in comparison with the optical fiber devices. Fused fiber couplers are proposed as key elements to (de)multiplex different fiber modes in a MDM link at 850 nm. The use of a symmetric directional coupler (DC) as a (de)multiplexer requires the use of an additional mode converter. The use of an asymmetrical directional coupler (ADC) as optical (de)multiplexer and mode converter is proposed, avoiding the necessity of an additional mode converter and simplifying the MDM scheme. Furthermore, in this Ph.D. thesis it is also proposed and evaluated the design of a mechanical mode converter at 850 nm using a SSMF. This technique permits to obtain the first high order mode with high quality and without the necessity of using an ADC. After that, it is analyzed and investigated the employment of commercial optical couplers (designed at 1550 nm) at 850 nm wavelength operation, thus avoiding the use of optical couplers and mode converters specifically designed at 850 nm wavelength. The MDM system costs are reduced as fewer devices are required and commercial components designed at 1550 nm are cheaper than the counterparts at 850 nm. In this Ph.D. thesis it is also considered the employment of ADCs over strip waveguides based on SOI technology for the conversion and multiplexing of the optical modes, from single-mode waveguide to high order mode waveguide at the 1550 nm wavelength. Thus, it has been studied and experimentally investigated different designs aimed to achieve the most robust configuration, in which the yield is less affected by the fabrication tolerances. Furthermore, the use of DCs over ridge waveguides is commonly employed and it offers better performance than strip waveguides. For this reason, the Ph.D. thesis studies and evaluates the use of ADCs with ridge waveguides by considering the effective refractive indexes of the even and odd supermodes analysis. In this way, a comparison between strip and ridge structures is done in order to find the optimum design that offer the best features. Finally, it is analyzed the design of a grating coupler capable of multiplexing and demultiplexing the fundamental and the high order mode from the waveguide to the optical fiber and vice versa. Thus, different designs are evaluated in order to achieve a design more robust and efficient to the coupling misalignments.García Rodríguez, D. (2018). Few-Mode Transmission Technology for Ultra-High Capacity Optical Networks [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/115938TESISCompendi

Design of LP01 to LPlm Mode Converters for Mode Division Multiplexing

Mode division multiplexing (MDM) over few mode fiber (FMF) has been proposed as an alternative solution to tackle the capacity limitations of optical networks based on standard single mode fiber (SMF). These limitations are caused by the fiber nonlinear effects. MDM is realized through excitation of different fiber spatial modes, each mode being an independent transmission channel. Therefore, MDM over FMF requires mode conversion (basically from fundamental mode to higher order modes and vice versa) as well as mode multiplexing and demultiplexing. Mode conversion, multiplexing and demultiplexing can be realized through different techniques. It can be achieved using free-space optics based on matching the profile of an input mode to the profile of an output mode using phase mask or spatial light modulator. Mode conversion and (de)multiplexing can also be achieved using waveguide structures. These mode converters and (de)multiplexers are mainly based on optical fiber and planar waveguide, which include fiber grating, tapering, lanterns, planar lightwave circuit (PLC), photonic crystal fiber (PCF), mode selective coupler (MSC) and Y-junction. It is worth mentioning that more than one technique may be applied to realize a specific converter/ (de)multiplexer for a specific mode. In general, Mode converters and (de)multiplexers based on free space optics are polarization insensitive and wavelength independent, but they result in high insertion loss and are bulky. On the other hand, all-waveguide mode converters and (de)multiplexers have high mode conversion efficiency (less insertion loss and high extinction ratio) and are compact, but they are wavelength dependent. Recently, many research works demonstrate the design, analysis and fabrication of several types of mode converters and (de)multiplexers. However, almost all the proposed devices are specific to a certain number of modes, therefore, they result in mode-specific designs. The explosive growth of traffic over telecommunication networks, especially in the access networks mandates that more and more modes would be (de)multiplexed to respond to the high traffic demands. As a result, proposing a universal mode converter and (de)multiplexer, that can convert and (de)multiplex any required number of modes is needed. In this thesis, mode converters and (de)multiplexers are thoroughly investigated. A universal LP01 to LPlm mode converter and (de)multiplexer is proposed. The mode converter is based on tapered circular waveguides and the (de)multiplexer is based on symmetric directional couplers. An LP01 to LP02 is first introduced. It consists of a tapered circular waveguide followed by a non-tapered circular waveguide. Inside the second waveguide, a circular tapered element is inserted. The initial tapered waveguide allows excitation of LP02 mode as well as other LP0m modes (m > 2). The second waveguide (comprising the circular section and the inner tapered element) is used to make conversion to be mainly from LP01 to LP02. Simulation shows that conversion efficiency of almost 100% at the central wavelength of O- S- and C-band, and above 98% over the S- and C-band is achieved. Moreover, suppression of non-desired higher order modes is more than 10 dB over the whole O-, S- and C-band. In particular, suppression is more than 19 dB over the entire C-band. The analysis also shows that the performance of the mode converter is not sensitive to slight variations of the converter’s parameters. In addition, the same converter can be used for converting LP02 back to LP01. Further, a (de)multiplexer for an LP02 and an LP01 mode is designed using the mode converter combined with a symmetric directional coupler. The multiplexer is broadband and has insertion loss less than 0.5 dB over the C-band. The proposed design is fabricated by inscribing it in the bulk of a borosilicate glass using a femtosecond laser. The converter has an insertion loss of less than 1 dB for the entire C-band and a total length of 2.22mm. this fabricated prototype validates the proposed mode converter design. The LP01 to LP02 mode converter structure can also be used to convert to other LP0m mode by proper tuning its parameters. After extensive simulations and optimizations, an LP01 to LP0m mode converter is proposed. The proposed converter structures are designed not only to provide high performances (low insertion losses and high extinction ratios), but also to be able to be fabricated by respecting the fabrication requirements (in terms of lengths and refractive indices). As a case study, six mode converters, converting LP01 to LP0m, with m = 2 to 7 are reported. The structures have insertion losses ranging from 0.1 dB to 2.5 dB. These performance results outperform all reported similar mode converters. To (de)multiplex the resulting LP0m modes, a (de)multiplexer based on symmetric directional couplers is proposed. This kind of devices are easy to design and fabricate and provide low insertion loss and cross talk. As an example, the first five modes (LP01 to LP05) are (de)multiplexed with an insertion loss less than 2.5 dB and cross talk less than -15 dB at the design wavelength. These results outperform the reported results for similar devices. The LP01 to LP0m mode converter structure is modified by inserting more inner elements to be able to convert to any LPlm mode. Therefore, a universal LP mode converter structure is proposed. The number and parameters of these inner elements depend on the desired LPlm mode. For instance, structures to convert LP01 to LP11, LP21 and LP31 are provided. These modes require between 5 to 6 inner elements with different radii and lengths. The simulation results for these three structures shows that an insertion loss less than 1.9 dB and an extinction ratio higher than 10 dB are achieved for the three modes at the design wavelength of 1550nm. Furthermore, the three modes (LP11, LP21 and LP31) are (d)multiplexed using a symmetric directional coupler with an insertion loss less than 0.9 dB and a cross talk below -17 dB for the three modes at the design wavelength. All the parameters of the presented mode converters and (de)multiplexers are designed to allow them to be fabricated using 3D femtosecond laser inscription technique

Test of mode-division multiplexing and demultiplexing in free-space with diffractive transformation optics

open5openGIANLUCA RUFFATO, 1; FILIPPO ROMANATO, ; 1Department of Physics and Astronomy ‘G. Galilei’, University of Padova; 2Laboratory for Nanofabrication of Nanodevices, c.so Stati Uniti 4GIANLUCA RUFFATO, 1; Romanato, Filippo; Ruffato, Gianluca; Astronomy ‘. G. Galilei’, University of Padova; 2Laboratory for Nanofabrication of Nanodevices, c. so Stati Uniti

Space Division Multiplexing in Optical Fibres

Optical communications technology has made enormous and steady progress for several decades, providing the key resource in our increasingly information-driven society and economy. Much of this progress has been in finding innovative ways to increase the data carrying capacity of a single optical fibre. In this search, researchers have explored (and close to maximally exploited) every available degree of freedom, and even commercial systems now utilize multiplexing in time, wavelength, polarization, and phase to speed more information through the fibre infrastructure. Conspicuously, one potentially enormous source of improvement has however been left untapped in these systems: fibres can easily support hundreds of spatial modes, but today's commercial systems (single-mode or multi-mode) make no attempt to use these as parallel channels for independent signals.Comment: to appear in Nature Photonic

All-fiber mode-group-selective photonic lantern using graded-index multimode fibers

We demonstrate the first all-fiber mode-group-selective photonic lantern using multimode graded-index fibers. Mode selectivity for mode groups LP01, LP11 and LP21 + LP02 is 20-dB, 10-dB and 7-dB respectively. The insertion loss when butt coupled to multimode graded-index fiber is below 0.6-dB. The use of the multimode graded-index fibers in the taper can significantly reduce the adiabaticity requirement

Mode-division multiplexed transmission with inline few-mode fiber amplifier

We demonstrate mode-division multiplexed WDM transmission over 50-km of few-mode fiber using the fiber\u27s LP01 and two degenerate LP11 modes. A few-mode EDFA is used to boost the power of the output signal before a few-mode coherent receiver. A 6x6 time-domain MIMO equalizer is used to recover the transmitted data. We also experimentally characterize the 50-km few-mode fiber and the few-mode EDFA

Orbital angular momentum vector modes (de)multiplexer based on multimode micro-ring

Orbital angular momentum (OAM) multiplexing has emerged as an important method to increase the communication capacities in future optical information technologies. In this work, we demonstrate a silicon integrated OAM (de)multiplexer with a very simple structure. By simply tapping the evanescent wave of two different whispering gallery modes rotating inside a multimodal micro-ring resonator, four in-plane waveguide modes are converted to four free-space vector OAM beams with high mode purity. We further demonstrate chip-to-chip OAM multiplexing transmission using a pair of silicon devices, which shows low-level mode crosstalk and favorable link performance