Investigation of performance issues affecting optical circuit and packet switched WDM networks

Optical switching represents the next step in the evolution of optical networks. This thesis describes work that was carried out to examine performance issues which can occur in two distinct varieties of optical switching networks. Slow optical switching in which lightpaths are requested, provisioned and torn down when no longer required is known as optical circuit switching (OCS). Services enabled by OCS include wavelength routing, dynamic bandwidth allocation and protection switching. With network elements such as reconfigurable optical add/drop multiplexers (ROADMs) and optical cross connects (OXCs) now being deployed along with the generalized multiprotocol label switching (GMPLS) control plane this represents the current state of the art in commercial networks. These networks often employ erbium doped fiber amplifiers (EDFAs) to boost the optical signal to noise ratio of the WDM channels and as channel configurations change, wavelength dependent gain variations in the EDFAs can lead to channel power divergence that can result in significant performance degradation. This issue is examined in detail using a reconfigurable wavelength division multiplexed (WDM) network testbed and results show the severe impact that channel reconfiguration can have on transmission performance. Following the slow switching work the focus shifts to one of the key enabling technologies for fast optical switching, namely the tunable laser. Tunable lasers which can switch on the nanosecond timescale will be required in the transmitters and wavelength converters of optical packet switching networks. The switching times and frequency drifts, both of commercially available lasers, and of novel devices are investigated and performance issues which can arise due to this frequency drift are examined. An optical packet switching transmitter based on a novel label switching technique and employing one of the fast tunable lasers is designed and employed in a dual channel WDM packet switching system. In depth performance evaluations of this labelling scheme and packet switching system show the detrimental impact that wavelength drift can have on such systems

Novel Fibers and Components for Space Division Multiplexing Technologies

Passive devices and amplifiers for space division multiplexing are key components for future deployment of this technology and for the development of new applications exploring the spatial diversity of light. Some important devices include photonic lantern (PL) mode multiplexers supporting several modes, fan-in/fan-out (FIFO) devices for multicore fibers (MCFs), and multimode amplifiers capable of amplifying several modes with low differential modal gain penalty. All these components are required to overcome the capacity limit of single mode fiber (SMF) communication systems, driven by the growing data capacity demand. In this dissertation I propose and develop different passive components and amplifiers for space division multiplexing technologies, including PL mode multiplexers with low insertion loss and low mode dependent loss to excite different number of modes into few mode fibers (FMFs). I demonstrate a PL with a graded index core that better matches the mode profiles of a graded index FMF supporting six spatial modes with mode dependent loss (MDL) ranging from 2- to 3-dB over the entire C-band. Multicore fibers can alleviate the capacity limit of single mode fibers by placing multiple single mode cores within the same fiber cladding. However, interfacing single mode fibers to MCFs can be challenging due to physical limitations, in this dissertation I develop and fabricate different types of FIFO devices to couple light into MCFs with high efficiency and having up to 19 cores. I demonstrate high coupling efficiency with insertion loss below 0.5 dB per FIFO into a 4-core MCF and below 1 dB for a 19-core MCF. Multimode erbium doped fiber (EDF) amplifiers are required to amplify each mode within the few mode transmission fiber, the main challenge is to provide an amplifier with low differential modal gain, in this dissertation I present the first coupled-core amplifier concept compatible with FMFs. A 6-core coupled-core EDF can be spliced with low insertion and low MDL to a FMF supporting 6 spatial modes via a slight taper transition. The amplifier introduces 1.8 MDL with gain variation over the entire C-band below 1-dB

Advances in Optical Amplifiers

Optical amplifiers play a central role in all categories of fibre communications systems and networks. By compensating for the losses exerted by the transmission medium and the components through which the signals pass, they reduce the need for expensive and slow optical-electrical-optical conversion. The photonic gain media, which are normally based on glass- or semiconductor-based waveguides, can amplify many high speed wavelength division multiplexed channels simultaneously. Recent research has also concentrated on wavelength conversion, switching, demultiplexing in the time domain and other enhanced functions. Advances in Optical Amplifiers presents up to date results on amplifier performance, along with explanations of their relevance, from leading researchers in the field. Its chapters cover amplifiers based on rare earth doped fibres and waveguides, stimulated Raman scattering, nonlinear parametric processes and semiconductor media. Wavelength conversion and other enhanced signal processing functions are also considered in depth. This book is targeted at research, development and design engineers from teams in manufacturing industry, academia and telecommunications service operators

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

Crosstalk mitigation techniques in multi-wavelength networks comprising photonic integrated circuits

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