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

Fibres optiques amplificatrices pompées par la gaine pour les réseaux de communication

Avec la demande croissante en matière de consommation Internet, une nouvelle génération de systèmes de communication est présentement en cours de développement. L'une des caractéristiques de cette nouvelle génération est qu'elle exploite le multiplexage spatial afin d'augmenter la capacité et le niveau d'intégration des réseaux. Dans ce contexte, il est judicieux de s'intéresser à la place que pourrait occuper le multiplexage spatial dans la technologie de fibres amplificatrices. À court terme, le multiplexage spatial pourrait être utilisé conjointement avec le pompage par la gaine afin de diminuer la complexité et le nombre de composants dans les nœuds des réseaux de communication ayant de nombreux ports d'entrée et de sortie à amplifier à un même endroit, en permettant l'amplification simultanée des signaux présents dans plusieurs cœurs d'une même fibre optique et à partir d'une seule source de pompage. Cependant, avec le pompage par la gaine, la puissance pompe est répartie sur l'entièreté de la gaine, menant ainsi à une faible intensité lumineuse de la pompe, ce qui a pour effet d'accentuer les effets de saturation de l'amplification, les rendant ainsi moins compatibles avec les réseaux reconfigurables. Une application alternative pour le pompage par la gaine est l'amplification de signaux dans la bande L. En effet, il s'agit d'une application où les niveaux d'inversion de population d'ions d'erbium requis sont plus bas, ce qui est compatible avec la forte saturation propre au pompage par la gaine. Dans ce contexte, cette thèse vise à explorer l'intérêt du développement de fibres amplificatrices pompées par la gaine pour les réseaux de communication et à proposer des améliorations concrètes relatives à cette technologie. D'abord, le chapitre 1 vise à déterminer si, et sous quelles conditions, le co-dopage à l'erbium-ytterbium est préférable au dopage à l'erbium seul dans les fibres amplificatrices pompées par la gaine qui amplifient la bande C dans les réseaux de communication. Ce chapitre permet de conclure que le co-dopage à l'erbium-ytterbium est généralement uniquement préférable à l'erbium seul lorsque l'amplificateur doit être opéré en régime de forte saturation ou lorsque la région spectrale couverte par l'amplificateur ne s'étend pas en bas de 1535 nm. Ensuite, le chapitre 2 étudie l'impact de la géométrie de distribution des ions d'erbium sur le gain et la compression du gain dans une fibre amplificatrice pompée par la gaine. Plus spécifiquement, le dopage en anneau autour du cœur est comparé au dopage uniforme dans le cœur et à un profil de dopant qui couvrirait à la fois le cœur et une région périphérique du cœur afin de couvrir presque entièrement le mode fondamental. Ce chapitre permet de conclure que, parmi ces géométries, le dopage en anneau autour du cœur est celui qui permet de minimiser les effets de saturation. Puis, le chapitre 3 présente une nouvelle méthode de couplage latéral de la pompe dans la gaine, sans fusion et sans altération du signal, qui permet d'augmenter l'efficacité de couplage de la pompe significativement par rapport aux méthodes alternatives sans fusion. Les chapitres 4 à 6 présentent un processus d'optimisation ayant pour but d'utiliser les analyses et développements des chapitres 1 à 3 pour concevoir un amplificateur à fibre à cœurs multiples répondant de façon optimale aux spécifications requises pour un amplificateur s'insérant dans un nœud de réseau reconfigurable et couvrant la bande C. Le chapitre 4 présente la fabrication et la caractérisation de la première fibre amplificatrice à cœurs multiples utilisant une géométrie de distribution des ions actifs en anneau autour de chacun des cœurs. Ses performances s'avèrent être décevantes dû au taux élevé d'agrégats d'ions d'erbium et à la présence d'ASE dans la gaine. Dans le chapitre 5, un nouveau design de cœur est testé, en fabriquant une fibre amplificatrice à cœur unique, afin de corriger les deux principales lacunes de la fibre précédente. Dans le chapitre 6, le design présenté au chapitre 5 est réutilisé dans une seconde itération de fibre amplificatrice à cœurs multiples. Cette fibre est caractérisée en injectant de la puissance pompe dans la gaine et de la puissance signal simultanément dans tous les cœurs à l'aide d'un fan-in/fan-out pour mesurer le gain, le facteur de bruit ainsi que les variations de gain entre les cœurs. Le chapitre 7 explore une application alternative du pompage par la gaine en investiguant l'utilisation de couches concentriques hétérogènes de dopage à l'erbium seul et de co-dopage à l'erbium-ytterbium afin de maximiser l'efficacité de conversion de puissance dans les fibres amplificatrices pompées par la gaine et couvrant la bande L. Le chapitre 8 démontre qu'il est possible de recycler la puissance pompe résiduelle à la sortie d'une fibre amplificatrice multicœur pompée par la gaine en y inscrivant un réseau de Bragg intra-gaine. Cette méthode a l'avantage de ne pas nécessiter l'ajout de composants additionnels, à l'entrée et à la sortie, qui peuvent induire des pertes d'insertion. Finalement, le chapitre 9 est une discussion générale sur les progrès effectués dans le cadre de cette thèse ainsi que les défis qui restent à surmonter par rapport à l'utilisation des fibres amplificatrices pompées par la gaine dans les réseaux de communication

Advanced fiber components for optical networks

Due to the tremendous growth in data traffic and the rapid development in optical transmission technologies, the limits of the transmission capacity available with the conventional erbium-doped amplifiers (EDFA), optical filters and modulation techniques have nearly been reached. The objective of this thesis is to introduce new fiber-optic components to optical networks to cope with the future growth in traffic and also to bring down the size and cost of the transmission equipment. Improvements in performance and in scalability of the optical networks are studied through simulations and experimental network set-ups. High-power single-mode laser sources operating at 980 nm are important in pumping EDFAs and Raman amplifiers. In this thesis, two new practical, fiber-coupled configurations of stable high-power cladding-pumped Yb-doped fiber sources operating at 977 nm are presented: a fiber laser and an ASE (amplified spontaneous emission) or superfluorescent source. Sources are based on high numerical aperture Yb-doped jacketed air-clad fiber and high brightness pump diodes. L-band EDFAs are used to expand amplification bandwidth beyond the C-band wavelengths. Traditional L-band EDFAs are costly devices, which are core-pumped with expensive high-power single-mode diodes. Cladding-pumping technology brings down the cost of the pump diodes in L-band EDFAs, since high-power but low-cost multimode pump diodes can then be used. Additionally, the flexibility in designing erbium-doped fiber is improved. In this thesis, a new design for L-band EDFA based on GTWave cladding-pumping technology is introduced. Simultaneous noise reduction and transient suppression in the amplifier is achieved by using a gain-clamping seed-signal. To increase the spectral efficiency of the optical transmission systems optical filters having square spectral response and linear phase, leading to zero dispersion both in-band and out-of-band, are required. The application of inverse scattering technique in conjunction with advanced fiber Bragg grating writing technique significantly reduces in-band dispersion and greatly improves grating characteristics. In this thesis, the in-band and out-of-band dispersion penalty of a cascade of linear-phase fiber Bragg grating (FBG) filters is experimentally measured and compared to the results with conventional apodized FBG filters. Fiber Bragg grating based distributed feedback fiber lasers (DFB FL) are attractive alternatives to semiconductor lasers. Output power and efficiency of DFB FLs can be significantly increased by using a master-oscillator-and-power-amplifier (MOPA) configuration, consequently degrading optical signal to noise ratio (OSNR) and RIN of the master source. These trade-offs are studied in several MOPA configurations using core-pumped and cladding-pumped EDFAs as power amplifiers and compared to the results with a high-power stand-alone DFB-FLs, i.e. DFB FLs pumped with a high-power pump source. Finally, the performance and scalability of a bidirectional and a high-density metropolitan WDM ring networks is analyzed. Results show that the scalability limitation imposed by the amplified RIN arising from the Rayleigh backscattering in bidirectional WDM ring networks can be avoided by using low gain shared-pump EDFAs and directly modulated transmitters. In high-density metropolitan WDM networks based on non-zero dispersion shifted fibers the main limiting nonlinearity is four-wave mixing. In metropolitan areas distributed Raman amplification (DRA) is the most effective means reduce the effect of four-wave mixing.reviewe

Single-Frequency EYDFA with polarization-maintaining fibers for gravitational wave detection

In 2015, the space-time distortion caused by GW150914 was found - a pivotal event that inaugurated the era of interferometric gravitational wave astronomy. As of today, gravitational wave observations are routinely made with proper sky localization by the world-wide operating detector network of the second generation. The implementation of cryogenic cooling can reduce the coating thermal noise in the next detector generation. In this case, optics made of fused silica are not suitable because of fused silica's large mechanical loss at low temperatures. Crystalline silicon is an alternative material but not transparent at 1064nm; therefore, other laser wavelengths, e.g. 1.5um, must be used. Single-frequency EYDFAs based on LMA fibers can deliver the required output power at 1.5um. A PM setup, however, has not been demonstrated on the desired ~100W power level so far; also, there has been no demonstration of any successful longterm operation (> hours) even of a non-PM setup. In this work, a prototype amplifier with PM fibers is presented on a laboratory- and advanced engineering-level. A numerical FEM analysis of the pump wavelength dependence of the Yb3+ ASE and non-linear SBS has been performed; off-peak pumping was found to suppress the unwanted Yb3+ ASE considerably. The achievable output power at 1.5um was limited by the Yb3+ ASE if the simulated amplifier was pumped from 880nm to 990nm; the onset of the Yb3+ ASE was linked to a deterioration of the Yb3+-to-Er3+ energy transfer. The simulated amplifier was limited by SBS if pumped at wavelengths shorter than 880nm or longer than 990nm. The power threshold was approximatable by adapting a well-known threshold approximation for passive fibers. Uncontrolled gain, e.g. resulting from a seed laser failure, must be prevented by interlocking the pumping process. In this work, the required reaction time has been studied with single-mode fibers by a combined experimental and numerical approach. It was found that a potential emergency-off system must switch-off the pumping process well below ~100us and/or ~300us to prevent catastrophic gain for the Yb3+ ASE and/or Er3+ ASE, respectively. An electronic circuit was designed; the board in PCB format was found capable to meet this requirement. The PCB prototype was installed as part of the engineering-level amplifier. A high-power single-frequency EYDFA made from 25/300 PM fibers is presented; the amplifier was implemented with low seed input power to match available GWD-compatible seed laser sources. A pump wavelength of 940nm was used. The pre-amplifier delivered 1.07W output power with low ASE power levels and operated free of SBS. The maximum output power of the high-power amplifier was 110W with 44.4+-0.3% optical-to-optical efficiency. The Er3+ ASE extinction ratio was 48.34dB at maximum output power; the Yb3+ ASE was negligible. SBS-free operation was confirmed by monitoring the amplifier noise at MHz frequencies. The PER ranged from 9.8dB to 12.6dB, probably owed to the used gain fiber. Further power scaling was limited by thermal fiber damage assumed to originate from photodarkening. Moreover, an advanced prototype with a revised cooling approach is presented. The performance of two suitable 25/xxx gain fibers was compared at the ~50W level over a 2-week period. The Nufern fiber showed a growing attenuation, i.e. 14.7+-2.2% per 13 days, that was tentatively attributed to the formation of P1 type color centers from POHCs; further research needs to be undertaken to confirm. The iXblue fiber seemed more heat resilient under operation. Furthermore, the PER from the iXblue fiber was in the range of 15.2dB to 20.7dB; the fundamental mode power was 95.7%. It was concluded that the iXblue fiber is suited to be used in GWD-compatible laser sources

Surpassing the nonlinear conversion efficiency of soliton microcombs

Laser frequency combs are enabling some of the most exciting scientific endeavours in the twenty-first century, ranging from the development of optical clocks to the calibration of the astronomical spectrographs used for discovering Earth-like exoplanets. Dissipative Kerr solitons generated in microresonators currently offer the prospect of attaining frequency combs in miniaturized systems by capitalizing on advances in photonic integration. Most of the applications based on soliton microcombs rely on tuning a continuous-wave laser into a longitudinal mode of a microresonator engineered to display anomalous dispersion. In this configuration, however, nonlinear physics precludes one from attaining dissipative Kerr solitons with high power conversion efficiency, with typical comb powers amounting to ~1% of the available laser power. Here we demonstrate that this fundamental limitation can be overcome by inducing a controllable frequency shift to a selected cavity resonance. Experimentally, we realize this shift using two linearly coupled anomalous-dispersion microresonators, resulting in a coherent dissipative Kerr soliton with a conversion efficiency exceeding 50% and excellent line spacing stability. We describe the soliton dynamics in this configuration and find vastly modified characteristics. By optimizing the microcomb power available on-chip, these results facilitate the practical implementation of a scalable integrated photonic architecture for energy-efficient applications