Efficient Modeling and Simulation of Wavelength Division Multiplexing Dual Polarization QPSK Optical Fiber Transmission

Due to enormous growth in communications, wavelength division multiplexing (WDM) systems are popular because these systems allow us to expand the capacity of the networks without laying more optical fiber cables. In this thesis, we have systematically derived the coupled nonlinear Schrödinger (CNLS) equations, including a consistent definition of the complex envelope, Fourier transform, the state of polarization, and derivation under the engineering notation. After a discussion of coarse step based second order symmetrized split-step Fourier (SSSF) simulation method, which is applicable to the numerical solution of the CNLS equations, an analytical step-size selection based local error method is applied to the WDM optical fiber communication systems. With systematical simulation study of both standard single mode fiber (SSMF) fiber links and true-wave reduced slope (TWRS) fiber links. It is found that similar to the single channel systems, the global simulation accuracy for the vector propagation can be satisfied using the local error bound (LEB) obtained from a scalar propagation model for the same global error over a large range of simulation accuracy and differential group delay (DGD). Furthermore, carefully designed numerical simulations are used to show that the proposed local error method leads to higher computational efficiency compared to other prevalent step-size selection schemes in vector WDM simulations. The scaling of the global simulation error with respect to the number of optical fiber spans is demonstrated, and global error control for multi-span WDM simulations is proposed

Efficient Modeling and Simulation of Wavelength Division Multiplexing Dual Polarization QPSK Optical Fiber Transmission

Due to enormous growth in communications, wavelength division multiplexing (WDM) systems are popular because these systems allow us to expand the capacity of the networks without laying more optical fiber cables. In this thesis, we have systematically derived the coupled nonlinear Schrödinger (CNLS) equations, including a consistent definition of the complex envelope, Fourier transform, the state of polarization, and derivation under the engineering notation. After a discussion of coarse step based second order symmetrized split-step Fourier (SSSF) simulation method, which is applicable to the numerical solution of the CNLS equations, an analytical step-size selection based local error method is applied to the WDM optical fiber communication systems. With systematical simulation study of both standard single mode fiber (SSMF) fiber links and true-wave reduced slope (TWRS) fiber links. It is found that similar to the single channel systems, the global simulation accuracy for the vector propagation can be satisfied using the local error bound (LEB) obtained from a scalar propagation model for the same global error over a large range of simulation accuracy and differential group delay (DGD). Furthermore, carefully designed numerical simulations are used to show that the proposed local error method leads to higher computational efficiency compared to other prevalent step-size selection schemes in vector WDM simulations. The scaling of the global simulation error with respect to the number of optical fiber spans is demonstrated, and global error control for multi-span WDM simulations is proposed

Optical time domain add-drop multiplexing employing fiber nonlinearities

Het in dit proefschrift beschreven onderzoek richt zich op het ontrafelen van in het tijdsdomein gestapelde optische signalen, ook wel optical time division multiplexing (OTDM) genoemd, en de bijbehorende technologische uitdagingen. Dit werk richt zich in het bijzonder op het toevoegen en extraheren van een specifieke datastroom uit een OTDM signaal. De component die deze functie uitvoert kan worden aangeduid als een add-drop multiplexer (ADM). Deze ADMs kunnen worden onderverdeeld in twee categorieën. De eerste categorie is gebaseerd op oplossingen die gebruik maken van halfgeleider materiaal en de tweede categorie benut de niet-lineariteit van een glasvezel. Een onderzochte halfgeleider materiaal ADM techniek is gebaseerd op het crossabsorption modulation (XAM) effect in een electro-absorptie modulator (EAM). Een model, gebaseerd op propagatie-vergelijkingen in halfgeleider materiaal, is ontwikkeld om de invloed van het XAM effect te kunnen simuleren. Resultaten verkregen met dit model komen goed overeen met experimenteel verkregen resultaten. Foutvrij extraheren (demultiplexen) van een 10 Gb/s datakanaal uit een 80 Gb/s OTDM signaal, met behulp van XAM in een EAM is experimenteel aangetoond. Een nieuw concept genaamd cross-polarisatie rotatie (XPR) is geïntroduceerd om het contrast ratio van de EAM demultiplexer te verbeteren. Ondanks verbetering van het contrast ratio van de demultiplexer is er geen significante verbetering van de prestatie waarneembaar. Mogelijkheden om de EAM in een 160 Gb/s demultiplexer configuratie te gebruiken zijn onderzocht. De kwaliteit van de EAM als optische schakelaar is sterk afhankelijk van het maximaal toegestane ingangsvermogen. Een hoger vermogen van het optische kloksignaal leidt tot een sterker absorptie verzadigingseffect. De snelheid van de EAM als optische schakelaar is begrensd door de hersteltijd van de vrije elektronen en gaten in de halfgeleider, gezamenlijk de carriers genoemd. Een verhoging van de negatieve biasspanning leidt tot een verkorting van de carrier hersteltijd. Een nadeel van het gebruik van een hogere biasspanning is de bijkomende hogere absorptie wat resulteert in een hoger vereist ingangsvermogen om de absorptie te verzadigen, omdat anders een verslechtering van de signaal-ruis verhouding onvermijdelijk is. Een belangrijk deel van het proefschrift richt zich op ADMs die de niet-lineariteit van een glasvezel benutten. Een van de meest veelbelovende oplossingen is gebaseerd op de nonlinear optical loop mirror (NOLM). Een geheel optische tijdsdomein ADM gebaseerd op een NOLM structuur is voor het eerst gedemonstreerd op datasnelheden boven de 80 Gb/s. Simulaties en experimenteel onderzoek zijn uitgevoerd op 160 Gb/s en 320 Gb/s. De prestatie limiterende factoren in de NOLM gebaseerde ADM zijn overspraak van naburige kanalen voor het extraheren van een kanaal en incomplete verwijdering van het geëxtraheerde kanaal voor het toevoegen van een nieuw kanaal. De jitter op het controle- en datasignaal en een niet geoptimaliseerde NOLM ingangskoppelaar verslechteren de kwaliteit van de ADM. De behaalde resultaten openen mogelijkheden om in de toekomst het systeem op te waarderen naar 640 Gb/s. De conversie van twee 10 Gb/s non-return to zero (NRZ) golflengte gestapelde kanalen (WDM) naar één 20 Gbs return-to-zero (RZ) OTDM signaal is experimenteel gekarakteriseerd. Het conversie principe is gebaseerd op four-wave mixing (FWM) in een sterk niet-lineare vezel (HNLF). Een voordeel van deze conversie techniek is dat er geen extra NRZ naar RZ conversiestap vereist is. Een tweede voordeel is de transparantie van FWM ten opzichte van de gebruikte modulatie techniek. Zo is deze techniek bijvoorbeeld ook geschikt voor fasegemoduleerde datasignalen. De beperkingen van deze conversie techniek zijn onderzocht. Conversie van 2x10 Gb/s WDM naar 20 Gb/s OTDM is experimenteel aangetoond, maar simulaties wijzen uit dat deze techniek niet geschikt is voor conversie van 4x40 Gb/s WDM naar 160 Gb/s OTDM, omdat het optische vermogen van het geconverteerde signaal erg laag is als gevolg van de lage efficiëntie van het FWM proces. Een alternatieve ADM techniek die ook bestudeerd is, is gebaseerd op cross-phase modulatie (XPM) spectrale verbreding in combinatie met filtering. Het voordeel van deze techniek is het geringere aantal benodigde componenten voor de constructie van een complete ADM in vergelijking met een ADM gebaseerd op een NOLM of een Kerr shutter. Simulaties en experimenteel werk demonstreren de mogelijkheden van deze techniek. Een geheel optische tijddomein ADM voor fasegemoduleerde signalen is voor de eerste maal aangetoond. Add-drop multiplexing van een 80 Gb/s RZ-DPSK OTDM signaal gebaseerd op de Kerr shutter met 375 meter HNLF is experimenteel gedemonstreerd. De fase-informatie in het signaal is behouden in de complete ADM. Praktische beperkingen in de experimentele set-up begrensden de datasnelheid tot 80 Gb/s. Een ADM experiment op 320 Gb/s met amplitude gemoduleerde signalen geeft een indicatie van de mogelijkheden van de Kerr shutter als ultrasnelle schakelaar

Fiber Laser Based Nonlinear Spectroscopy

To date, nonlinear spectroscopy has been considered an expensive technique and confined mostly to experimental laboratory settings. Over recent years, optical-fiber lasers that are highly reliable, simple to operate and relatively inexpensive have become commercially available, removing one of the major obstacles to widespread utilization of nonlinear optical measurement in biochemistry. However, fiber lasers generally offer relatively low output power compared to lasers traditionally used for nonlinear spectroscopy, and much more careful design is necessary to meet the excitation power thresholds for nonlinear signal generation. On the other hand, reducing the excitation intensity provides a much more suitable level of user-safety, minimizes damage to biological samples and reduces interference with intrinsic chemical processes. Compared to traditional spectroscopy systems, the complexity of nonlinear spectroscopy and imaging instruments must be drastically reduced for them to become practical. A nonlinear spectroscopy tool based on a single fiber laser, with electrically controlled wavelength-tuning and spectral resolution enhanced by a pulse shaping technique, will efficiently produce optical excitation that allows quantitative measurement of important nonlinear optical properties of materials. The work represented here encompasses the theory and design of a nonlinear spectroscopy and imaging system of the simplest architecture possible, while solving the difficult underlying design challenges. With this goal, the following report introduces the theories of nonlinear optical propagation relevant to the design of a wavelength tunable system for nonlinear spectroscopy applications, specifically Coherent Anti-Stokes Spectroscopy (CARS) and Förster Resonance Energy Transfer (FRET). It includes a detailed study of nonlinear propagation of optical solitons using various analysis techniques. A solution of the generalized nonlinear Schrödinger equation using the split-step Fourier method is demonstrated and investigation of optical soliton propagation in fibers is carried out. Other numerical methods, such as the finite difference time domain approach and spectral-split step Fourier methods are also described and compared. Numerical results are contrasted with various measurements of wavelength shifted solitons. Both CARS and FRET test-bed designs and experiments are presented, representing two valuable biochemical measurement applications. Two-photon excitation experiments with a simplified calibration process for quantitative FRET measurement were conducted on calmodulin proteins modified with fluorescent dyes, as well as modified enhanced green fluorescent protein. The resulting new FRET efficiency measurements showed agreement with those of alternative techniques which are slower and can involve destruction of the sample. In the second major application of the nonlinear spectroscopy system, CARS measurement with enhanced spectral resolution was conducted on cyclohexane as well as on samples of mouse brain tissue containing lipids with Raman resonances. The measurements of cyclohexane verified the ability of the system to precisely determine its Raman resonances, thus providing a benchmark within a similar spectral range for biological materials which have weaker Raman signal responses. The improvement of spectral resolution (resonance frequency selectivity), was also demonstrated by measuring the closely-spaced resonances of cyclohexane. Finally, CARS measurements were also made on samples of mouse brain tissue which has a lipids-based Raman signature. The CARS spectrum of the lipid resonances matched well with other cited studies. The imaging of mouse brain tissue with Raman resonance contrast was also partially achieved, but it was hindered by low signal to noise ratio and limitations of the control hardware that led to some dropout of the CARS signal due to power coupling fluctuations. Nevertheless, these difficulties can be straightforwardly addressed by refinement of the wavelength tuning electronics. In conclusion, it is hoped that these efforts will lead to greater accessibility and use of CARS, FRET and other nonlinear spectral measurement instruments, in line with the promising advances in optics and laser technology

Overcoming degradation in spatial multiplexing systems with stochastic nonlinear impairments

Single-mode optical fibres now underpin telecommunication systems and have allowed continuous increases in traffic volume and bandwidth demand whilst simultaneously reducing cost- and energy-per-bit over the last 40 years. However, it is now recognised that such systems are rapidly approaching the limits imposed by the nonlinear Kerr effect. To address this, recent research has been carried out into mitigating Kerr nonlinearities to increase the nonlinear threshold and into spatial multiplexing to offer additional spatial pathways. However, given the complexity associated with nonlinear transmission in spatial multiplexed systems subject to random inter-spatial-path nonlinearities it is widely believed that these technologies are mutually exclusive. By investigating the linear and nonlinear crosstalk in few-mode fibres based optical communications, we numerically demonstrate, for the first time, that even in the presence of significant random mixing of signals, substantial performance benefits are possible. To achieve this, the impact of linear mixing on the Kerr nonlinearities should be taken into account using different compensation strategies for different linear mixing regimes. For the optical communication systems studied, we demonstrate that the performance may be more than doubled with the appropriate selection of compensation method for fibre characteristics which match those presented in the literature

MULTI-WAVELENGTH SPECKLE REDUCTION FOR LASER PICO-PROJECTORS USING DIFFRACTIVE OPTICS

Personal electronic devices, such as cell phones and tablets, continue to decrease in size while the number of features and add-ons keep increasing. One particular feature of great interest is an integrated projector system. Laser pico-projectors have been considered, but the technology has not been developed enough to warrant integration. With new advancements in diode technology and MEMS devices, laser-based projection is currently being advanced for pico-projectors. A primary problem encountered when using a pico-projector is coherent interference known as speckle. Laser speckle can lead to eye irritation and headaches after prolonged viewing. Diffractive optical elements known as diffusers have been examined as a means to lower speckle contrast. Diffusers are often rotated to achieve temporal averaging of the spatial phase pattern provided by diffuser surface. While diffusers are unable to completely eliminate speckle, they can be utilized to decrease the resultant contrast to provide a more visually acceptable image. This dissertation measures the reduction in speckle contrast achievable through the use of diffractive diffusers. A theoretical Fourier optics model is used to provide the diffuser’s stationary and in-motion performance in terms of the resultant contrast level. Contrast measurements of two diffractive diffusers are calculated theoretically and compared with experimental results. In addition, a novel binary diffuser design based on Hadamard matrices will be presented. Using two static in-line Hadamard diffusers eliminates the need for rotation or vibration of the diffuser for temporal averaging. Two Hadamard diffusers were fabricated and contrast values were subsequently measured, showing good agreement with theory and simulated values. Monochromatic speckle contrast values of 0.40 were achieved using the Hadamard diffusers. Finally, color laser projection devices require the use of red, green, and blue laser sources; therefore, using a monochromatic diffractive diffuser may not optimal for color speckle contrast reduction. A simulation of the Hadamard diffusers is conducted to determine the optimum spacing between the two diffusers for polychromatic speckle reduction. Experimental measured results are presented using the optimal spacing of Hadamard diffusers for RGB color speckle reduction, showing 60% reduction in contrast

Optical guiding in meter-scale plasma waveguides

We demonstrate a new highly tunable technique for generating meter-scale low density plasma waveguides. Such guides can enable electron acceleration to tens of GeV in a single stage. Plasma waveguides are imprinted in hydrogen gas by optical field ionization induced by two time-separated Bessel beam pulses: The first pulse, a J_0 beam, generates the core of the waveguide, while the delayed second pulse, here a J_8 or J_16 beam, generates the waveguide cladding. We demonstrate guiding of intense laser pulses over hundreds of Rayleigh lengths with on axis plasma densities as low as N_e0=5x10^16 cm^-3