Fiber amplifiers, directly modulated transmitters and a ring network structure for optical communications

The three technologies that are considered the key elements in building a metropolitan area optical network are studied in this thesis. They are optical amplification, high-speed low cost transmitters and ring network structures. These studies concentrate on cost reduction of these three technologies thus enabling the use of optical networks in small customer base metropolitan areas. The research on optical amplification concentrated first on the solution doping process, at present the most used method for producing erbium doped fiber. It was found that separationing the soot growth and the sintering improved the uniformity of the porous layer. This made the homogeneity of the doping concentration in the fiber core better. The effects of index profile variations that arise from the non-ideal solution doping process were also simulated. In the search for a better doping method a new nanoparticle glass-forming process, the direct nanoparticle deposition, was developed. In this process the doping is done simultaneously with glass formation. Utilizing this new process it was possible to improve the uniformity of the doping resulting in higher usable doping levels and shorter erbium doped fiber lengths in the amplifiers. There were fewer limitations in the amplifier caused by optical non-linearities and polarization mode dispersion since shorter fiber lengths were needed. The double cladding fiber, which avoids the costly coupling of the pump laser into a single mode waveguide, was also studied. This pumping scheme was found to improve the inversion uniformity in the erbium doped fiber core thereby enhancing the power conversion efficiency for the long wavelength band amplifier. In characterizing the erbium doped fiber amplifier the gain and noise figure was measured with a temporal filter setup. It was made of simple, low cost components but yielded accurate measurements since the noise originating from the amplified spontaneous emission was measured at the signal wavelength. In the study of fiber amplifier controlling schemes the input power of the fiber amplifier was successfully used to regulate the pump laser. This feed-forward control scheme provides a simple, low cost control and managment system for the erbium doped fiber amplifier in metropolitan area network applications that require flexible adding and dropping of wavelength channels. The transmitter research focused on the DFB laser due to its simplicity and low cost structure. A solid state Fabry-Perot etalon made from double polished silicon chip was used as a frequency discriminator in the chirp analyser developed for the DFB lasers. This wavelength discriminator did not require repeated calibration or active stabilisation and was controled electrically enabling automatic measurements. The silicon Fabry-Perot etalon was also used for simultaneous spectral filtering and wavelength control of the laser. The usable dispersion limited transmission length was increased when the filter was used in conjunction with the directly modulated distributed feedback laser transmitter. The combination of spatial multiplexing and dense wavelength division multiplexing in ring topology was investigated in the course of the research on the ring network as the feeder part of the metropolitan network. A new way to organize different wavelengths and fibers was developed. This ring network structure was simulated and an experimental ring network built. The results of the studies demonstrated that the same limitations effecting uni-directional ring structures also are the main limitations on the scalability of the spatial and wavelength division multiplexed ring networks based on bi-directional transmission when the node spacing is short. The developed ring network structure demonstrated major cost reductions when compared with the heavy use of wavelength division multiplexing. The node structure was also greatly simplified resulting in less need for different wavelength transmitters in each node. Furthermore the node generated only minor losses for the passing signals thus reducing the need for optical amplification.reviewe

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

Modeling and characterization of cladding-pumped erbium-ytterbium co-doped fibers for amplification in communication systems

Cladding-pumped optical fiber amplifiers are of increased interest in the context of space-division multiplexing but are known to suffer from low power efficiency. In this context, ytterbium (Yb) co-doping can be an attractive solution to improve the performance of erbium (Er) doped fiber amplifiers. We present a detailed direct comparison between Er/Yb-co-doping and Er-doping using numerical simulations validated by experimental results. Two double-cladding fibers, one doped with Er only and the other one co-doped with Er and Yb, were designed, fabricated and characterized. Using the experimentally extracted parameters, we simulate multi-core fiber amplifiers and investigate the interest of Er/Yb-co-doping. We calculate the minimum gain of the amplifiers over a 35-nm spectral window considering various scenarios

Efficient extraction of high pulse energy from partly quenched highly Er3+-doped fiber amplifiers

We demonstrate efficient pulse-energy extraction from a partly quenched erbium-doped aluminosilicate fiber amplifier. This has a high erbium concentration that allows for short devices with reduced nonlinear distortions but also results in partial quenching and thus significant unsaturable absorption, even though the fiber is still able to amplify. Although the quenching degrades the average-power efficiency, the pulse energy remains high, and our results point to an increasingly promising outcome for short pulses. Furthermore, unlike unquenched fibers, the conversion efficiency improves at low repetition rates, which we attribute to smaller relative energy loss to quenched ions at higher pulse energy. A short (2.6 m) cladding-pumped partly quenched Er-doped fiber with 95-dB/m 1530-nm peak absorption and saturation energy estimated to 85 µJ reached 0.8 mJ of output energy when seeded by 0.2-µs, 23-µJ pulses. Thus, according to our results, pulses can be amplified to high energy in short highly Er-doped fibers designed to reduce nonlinear distortions at the expense of average-power efficiency

Fantasies using optical fibers

The activity carried out during the PhD course has concerned special optical fibers with particular refractive index profiles, from Photonic Crystal Fibers (PCFs) to Plastic Optical Fibers (POFs), which is a research topic in continuous evolution and characterized by a great scientific excitement. The aim of the research of the three year PhD course has been to accurately study, and thus to deeply understand the light guiding mechanisms exploited in these kinds of optical fibers. The unusual guiding properties of PCFs with different air-hole arrangements in the fiber cross-section have been investigated both numerically, through a full-vector modal solver based on the Finite Element Method, and experimentally, by considering samples of large mode area PCFs, as well as of nonlinear fibers. Moreover, the properties of Erbium-Doped Fibers (EDFs) with a particular refractive index profile, that is with a depressed-cladding, have been experimentally characterized. By exploiting the bending loss of these active fibers, amplifiers with different configurations have been realized, which cover larger bandwidths with respect to conventional ones, as well as tunable lasers in S, C and L band. Then, in order to design and realize the pre-amplifier stage for a pulsed high power laser useful for industrial applications, single-mode ytterbium-doped fibers with different doping concentrations, with either a single or a double-cladding, have been considered with the aim to optimize the gain performances. Finally, low cost sensors based on the inexpensive plastic fibers have been proposed as an effective solution to the problem of the liquid level measurement. Sensors for both point and continuous measurements of the liquid level, which can be also exploited to distinguish fluids according to their refractive index

Recent advances in radiation-hardened fiber-based technologies for space applications

International audience; In this topical review, the recent progress on radiation-hardened fiber-based technologies is detailed, focusing on examples for space applications. In the first part of the review, we introduce the operational principles of the various fiber-based technologies considered for use in radiation environments: passive optical fibers for data links, diagnostics, active optical fibers for amplifiers and laser sources as well as the different classes of point and distributed fiber sensors: gyroscopes, Bragg gratings, Rayleigh, Raman or Brillouin-based distributed sensors. Second, we describe the state of the art regarding our knowledge of radiation effects on the performance of these devices, from the microscopic effects observed in the amorphous silica glass used to design fiber cores and cladding, to the macroscopic response of fiber-based devices and systems. Third, we present the recent advances regarding the hardening (improvement of the radiation tolerance) of these technologies acting on the material, device or system levels. From the review, the potential of fiber-based technologies for operation in radiation environments is demonstrated and the future challenges to be overcome in the coming years are presented