Spatially integrated erbium-doped fiber amplifiers enabling space-division multiplexing

L'augmentation exponentielle de la demande de bande passante pour les communications laisse présager une saturation prochaine de la capacité des réseaux de télécommunications qui devrait se matérialiser au cours de la prochaine décennie. En effet, la théorie de l’information prédit que les effets non linéaires dans les fibres monomodes limite la capacité de transmission de celles-ci et peu de gain à ce niveau peut être espéré des techniques traditionnelles de multiplexage développées et utilisées jusqu’à présent dans les systèmes à haut débit. La dimension spatiale du canal optique est proposée comme un nouveau degré de liberté qui peut être utilisé pour augmenter le nombre de canaux de transmission et, par conséquent, résoudre cette menace de «crise de capacité». Ainsi, inspirée par les techniques micro-ondes, la technique émergente appelée multiplexage spatial (SDM) est une technologie prometteuse pour la création de réseaux optiques de prochaine génération. Pour réaliser le SDM dans les liens de fibres optiques, il faut réexaminer tous les dispositifs intégrés, les équipements et les sous-systèmes. Parmi ces éléments, l'amplificateur optique SDM est critique, en particulier pour les systèmes de transmission pour les longues distances. En raison des excellentes caractéristiques de l'amplificateur à fibre dopée à l'erbium (EDFA) utilisé dans les systèmes actuels de pointe, l'EDFA est à nouveau un candidat de choix pour la mise en œuvre des amplificateurs SDM pratiques. Toutefois, étant donné que le SDM introduit une variation spatiale du champ dans le plan transversal de la fibre, les amplificateurs à fibre dopée à l'erbium spatialement intégrés (SIEDFA) nécessitent une conception soignée. Dans cette thèse, nous examinons tout d'abord les progrès récents du SDM, en particulier les amplificateurs optiques SDM. Ensuite, nous identifions et discutons les principaux enjeux des SIEDFA qui exigent un examen scientifique. Suite à cela, la théorie des EDFA est brièvement présentée et une modélisation numérique pouvant être utilisée pour simuler les SIEDFA est proposée. Sur la base d'un outil de simulation fait maison, nous proposons une nouvelle conception des profils de dopage annulaire des fibres à quelques-modes dopées à l'erbium (ED-FMF) et nous évaluons numériquement la performance d’un amplificateur à un étage, avec fibre à dopage annulaire, à ainsi qu’un amplificateur à double étage pour les communications sur des fibres ne comportant que quelques modes. Par la suite, nous concevons des fibres dopées à l'erbium avec une gaine annulaire et multi-cœurs (ED-MCF). Nous avons évalué numériquement le recouvrement de la pompe avec les multiples cœurs de ces amplificateurs. En plus de la conception, nous fabriquons et caractérisons une fibre multi-cœurs à quelques modes dopées à l'erbium. Nous réalisons la première démonstration des amplificateurs à fibre optique spatialement intégrés incorporant de telles fibres dopées. Enfin, nous présentons les conclusions ainsi que les perspectives de cette recherche. La recherche et le développement des SIEDFA offriront d'énormes avantages non seulement pour les systèmes de transmission future SDM, mais aussi pour les systèmes de transmission monomode sur des fibres standards à un cœur car ils permettent de remplacer plusieurs amplificateurs par un amplificateur intégré.The exponential increase of communication bandwidth demand is giving rise to the so-called ‘capacity crunch’ expected to materialize within the next decade. Due to the nonlinear limit of the single mode fiber predicted by the information theory, all the state-of-the-art techniques which have so far been developed and utilized in order to extend the optical fiber communication capacity are exhausted. The spatial domain of the lightwave links is proposed as a new degree of freedom that can be employed to increase the number of transmission paths and, subsequently, overcome the looming ‘capacity crunch’. Therefore, the emerging technique named space-division multiplexing (SDM) is a promising candidate for creating next-generation optical networks. To realize SDM in optical fiber links, one needs to investigate novel spatially integrated devices, equipment, and subsystems. Among these elements, the SDM amplifier is a critical subsystem, in particular for the long-haul transmission system. Due to the excellent features of the erbium-doped fiber amplifier (EDFA) used in current state-of-the-art systems, the EDFA is again a prime candidate for implementing practical SDM amplifiers. However, since the SDM introduces a spatial variation of the field in the transverse plane of the optical fibers, spatially integrated erbium-doped fiber amplifiers (SIEDFA) require a careful design. In this thesis, we firstly review the recent progress in SDM, in particular, the SDM optical amplifiers. Next, we identify and discuss the key issues of SIEDFA that require scientific investigation. After that, the EDFA theory is briefly introduced and a corresponding numerical modeling that can be used for simulating the SIEDFA is proposed. Based on a home-made simulation tool, we propose a novel design of an annular based doping profile of few-mode erbium-doped fibers (FM-EDF) and numerically evaluate the performance of single stage as well as double-stage few-mode erbium-doped fiber amplifiers (FM-EDFA) based on such fibers. Afterward, we design annular-cladding erbium-doped multicore fibers (MC-EDF) and numerically evaluate the cladding pumped multicore erbium-doped fiber amplifier (MC-EDFA) based on these fibers as well. In addition to fiber design, we fabricate and characterize a multicore few-mode erbium-doped fiber (MC-FM-EDF), and perform the first demonstration of the spatially integrated optical fiber amplifiers incorporating such specialty doped fibers. Finally, we present the conclusions as well as the perspectives of this research. In general, the investigation and development of the SIEDFA will bring tremendous benefits not only for future SDM transmission systems but also for current state-of-the-art single-mode single-core transmission systems by replacing plural amplifiers by one integrated amplifier

Development of High-power Single-mode Yb-doped Fiber Amplifiers and Beam Analysis

High-power fiber laser systems enjoy a widespread use in manufacturing, medical, and defense applications as well as scientific research, due to their remarkable power scalability, high electrical to optical efficiency, compactness and ruggedness. However, single-mode fiber power scaling has stagnated in the past years, primarily due to the onset of nonlinear effects such as stimulated Brillouin/Raman scattering and transverse modal instabilities. This thesis addresses the analysis and mitigation of transverse modal instabilities in high-power fiber amplifiers. I describe the high-power fiber amplifier testbed that I set up to test fibers fabricated in house. I will show our results of a Yb-doped fiber amplifier with more than 2.2 kW signal power and beam quality of 1.1 M2. In consequence, I demonstrate mode-selective amplification in a large mode-area Yb-doped fiber using a 3-mode photonic lantern. All three modes were amplified to above 4 W with OSNRs higher than 16 dB. In addition, I show a novel high-speed beam analysis technique to study transverse modal instabilities. To guide fiber designs, I developed a GPU accelerated simulation suite to study the dynamics that occur in high-power fiber amplifiers. A 64 x 64 spatial grid, with 6000 time- and 20000 distance-steps can be solved at 2 min/meter on a GeForce GTX 1080 Ti. Based on these simulations, I will show dynamic transverse modal instability mitigation strategies that rely on mode modulation

Optical Fibers for Space-Division Multiplexed Transmission and Networking

Single-mode fiber transmission can no longer satisfy exponentially growing capacity demand. Space-division multiplexing (SDM) appears to be the only way able to dramatically improve the transmission capacity, for which, novel optical fiber is one of the key technologies. Such fibers must possess the following characteristics: 1) high mode density per cross-sectional area and 2) low crosstalk or low modal differential group delay (DMGD) to reduce complexity of digital signal processing. In this dissertation, we explore the design and characterization of three kinds of fibers for SDM: few-mode fiber (FMF), few-mode multi-core fiber (FM-MCF) and coupled multi-core fiber (CMCF) as well as their applications in transmission and networking. For the ultra-high density need of SDM, we have proposed the FMMCF. It combines advantages of both the FMF and MCF. The challenge is the inter-core crosstalk of the high-order modes. By applying a hole-assisted structure and careful fiber design, the LP11 crosstalk has been suppressed down to -40dB per km. This allows separate transmission on LP01 and LP11 modes without penalty. In fact, a robust SDM transmission up to 200Tb/s has been achieved using this fiber. To overcome distributed modal crosstalk in conjunction with DMGD, supermodes in CMCFs have been proposed. The properties of supermodes were investigated using the coupled-mode theory. The immediate benefits include high mode density and large effective area. In supermode structures, core-to-core coupling is exploited to reduce modal crosstalk or minimize DMGD. In addition, higher-order supermodes have been discovered in CMCFs with few-mode cores. We show that higher-order supermodes in different waveguide array configurations can be strongly affected by angle-dependent couplings, leading to different modal fields. Analytical solutions are provided for linear, rectangular and ring arrays. Higher-order modes have been observed for the first time using S2 imaging method. Finally, we introduce FMF to gigabit-capable passive optical networks (GPON). By replacing the conventional splitter with a photonic lantern, upstream combining loss can be eliminated. Low crosstalk has been achieved by a customized mode-selective photonic lantern carefully coupled to the FMF. We have demonstrated the first few-mode GPON system with error-free performance over 20-km 3-mode transmission using a commercial GPON system carrying live Ethernet traffic. We then scale the 3-mode GPON system to 5-mode, which resulted in a 4dB net gain in power budget in comparison with current commercial single-mode GPON systems

Sensing using Specialty Optical Fibers

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

Mode Coupling in Space-division Multiplexed Systems

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

Cladding waveguide splitters fabricated by femtosecond laser inscription in Ti:Sapphire crystal

Highly-compact devices capable of beam splitting are intriguing for a broad range of photonic applications. In this work, we report on the fabrication of optical waveguide splitters with rectangular cladding geometry in a Ti:Sapphire crystal by femtosecond laser inscription. Y-splitters are fabricated with 30 μm × 15 μm and 50 μm × 25 μm input ends, corresponding to two 15 μm × 15 μm and 25 μm × 25 μm output ends, respectively. The full branching angle θ between the two output arms are changing from 0.5° to 2°. The performances of the splitters are characterized at 632.8 nm and 1064 nm, showing very good properties including symmetrical output ends, single-mode guidance, equalized splitting ratios, all-angle-polarization light transmission and intact luminescence features in the waveguide cores. The realization of these waveguide splitters with good performances demonstrates the potential of such promising devices in complex monolithic photonic circuits and active optical devices such as miniature tunable lasers.This work is supported by the National Natural Science Foundation of China (No. 11404194). Authors acknowledge support from Junta de Castilla y León (Project SA046U16) and MINECO (FIS2015-71933-REDT). Authors would like to thank Prof. Xiaotao Hao from Shandong University for the help on micro-photoluminescence measurement

All-fiber few-mode optical coherence tomography using a modally-specific photonic lantern

ABSTRACT: Optical coherence tomography (OCT) was recently performed using a few-mode (FM) fiber to increase contrast or improve resolution using a sequential time-domain demultiplexing scheme isolating the different interferometric signals of the mode-coupled backscattered light. Here, we present an all-fiber FM-OCT system based on a parallel modal demultiplexing scheme exploiting a novel modally-specific photonic lantern (MSPL). The MSPL allows for maximal fringe visibility for each fiber propagation mode in an all-fiber assembly which provides the robustness required for clinical applications. The custom-built MSPL was designed for OCT at 930 nm and is wavelength-independent over the broad OCT spectrum. We further present a comprehensive coupling model for the interpretation of FM-OCT images using the first two propagation modes of a few-mode fiber, validate its predictions, and demonstrate the technique using in vitro microbead phantoms and ex vivo biological samples

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