Coherent wavelength division multiplexing: a novel transmission format for high spectral density optical communication networks

In this thesis a novel transmission format, named Coherent Wavelength Division Multiplexing (CoWDM) for use in high information spectral density optical communication networks is proposed and studied. In chapter I a historical view of fibre optic communication systems as well as an overview of state of the art technology is presented to provide an introduction to the subject area. We see that, in general the aim of modern optical communication system designers is to provide high bandwidth services while reducing the overall cost per transmitted bit of information. In the remainder of the thesis a range of investigations, both of a theoretical and experimental nature are carried out using the CoWDM transmission format. These investigations are designed to consider features of CoWDM such as its dispersion tolerance, compatibility with forward error correction and suitability for use in currently installed long haul networks amongst others. A high bit rate optical test bed constructed at the Tyndall National Institute facilitated most of the experimental work outlined in this thesis and a collaboration with France Telecom enabled long haul transmission experiments using the CoWDM format to be carried out. An amount of research was also carried out on ancillary topics such as optical comb generation, forward error correction and phase stabilisation techniques. The aim of these investigations is to verify the suitability of CoWDM as a cost effective solution for use in both current and future high bit rate optical communication networks

Modal correction for fiber-coupling efficiency in free-space optical communication systems through atmospheric turbulence

High-speed free-space optical communication systems have recently used fiber-optical components. The coupling efficiency with which the received laser beam can be coupled into a single-mode fiber is noticeably limited by atmospheric turbulence due to the degradation of its spatial coherence. Fortunately, adaptive optics (AO) can alleviate this limitation by partially correcting the turbulence-distorted wavefront. The coupling efficiency improvement provided by Zernike modal AO correction is numerically evaluated. It is found that the first 3-20 corrected polynomials can considerably improve the fiber-coupling efficiency. The improvement brought by AO is compared with that brought by a coherent fiber array. Finally, a hybrid technique that integrates AO and a coherent fiber array is proposed. Results show that the hybrid technique outperforms each of the two above-mentioned techniques. (C) 2009 Elsevier GmbH. All rights reserved.

High speed optical communication systems: From modulation formats to radically new fibres

High volumes of data traffic along with bandwidth hungry applications, such as cloud computing and video on demand, is driving the core optical communication links closer and closer to their maximum capacity. The research community has clearly identifying the coming approach of the nonlinear Shannon limit for standard single mode fibre [1,2]. It is in this context that the work on modulation formats, contained in Chapter 3 of this thesis, was undertaken. The work investigates the proposed energy-efficient four-dimensional modulation formats. The work begins by studying a new visualisation technique for four dimensional modulation formats, akin to constellation diagrams. The work then carries out one of the first implementations of one such modulation format, polarisation-switched quadrature phase-shift keying (PS-QPSK). This thesis also studies two potential next-generation fibres, few-mode and hollow-core photonic band-gap fibre. Chapter 4 studies ways to experimentally quantify the nonlinearities in few-mode fibre and assess the potential benefits and limitations of such fibres. It carries out detailed experiments to measure the effects of stimulated Brillouin scattering, self-phase modulation and four-wave mixing and compares the results to numerical models, along with capacity limit calculations. Chapter 5 investigates hollow-core photonic band-gap fibre, where such fibres are predicted to have a low-loss minima at a wavelength of 2μm. To benefit from this potential low loss window requires the development of telecoms grade subsystems and components. The chapter will outline some of the development and characterisation of these components. The world's first wavelength division multiplexed (WDM) subsystem directly implemented at 2μm is presented along with WDM transmission over hollow-core photonic band-gap fibre at 2μm. References: [1]P. P. Mitra, J. B. Stark, Nature, 411, 1027-1030, 2001 [2] A. D. Ellis et al., JLT, 28, 423-433, 2010.

Spatial-spectral flexible optical networking:enabling switching solutions for a simplified and efficient SDM network platform

The traffic carried by core optical networks grows at a steady but remarkable pace of 30-40% year-over-year. Optical transmissions and networking advancements continue to satisfy the traffic requirements by delivering the content over the network infrastructure in a cost and energy efficient manner. Such core optical networks serve the information traffic demands in a dynamic way, in response to requirements for shifting of traffics demands, both temporally (day/night) and spatially (business district/residential). However as we are approaching fundamental spectral efficiency limits of singlemode fibers, the scientific community is pursuing recently the development of an innovative, all-optical network architecture introducing the spatial degree of freedom when designing/operating future transport networks. Spacedivision- multiplexing through the use of bundled single mode fibers, and/or multi-core fibers and/or few-mode fibers can offer up to 100-fold capacity increase in future optical networks. The EU INSPACE project is working on the development of a complete spatial-spectral flexible optical networking solution, offering the network ultra-high capacity, flexibility and energy efficiency required to meet the challenges of delivering exponentially growing traffic demands in the internet over the next twenty years. In this paper we will present the motivation and main research activities of the INSPACE consortium towards the realization of the overall project solution. © 2014 Copyright SPIE.

Fibre impairment compensation using artificial neural network equalizer for high-capacity coherent optical OFDM signals

We propose an artificial neural network (ANN) equalizer for transmission performance enhancement of coherent optical OFDM (C-OOFDM) signals. The ANN equalizer showed more efficiency in combating both chromatic dispersion (CD) and single-mode fibre (SMF)-induced non-linearities compared to the least mean square (LMS). The equalizer can offer a 1.5 dB improvement in optical signal-to-noise ratio (OSNR) compared to LMS algorithm for 40 Gbit/s C-OOFDM signals when considering only CD. It is also revealed that ANN can double the transmission distance up to 320 km of SMF compared to the case of LMS, providing a nonlinearity tolerance improvement of ∼0.7 dB OSNR.

SDN enabled dynamically reconfigurable high capacity optical access architecture for converged services

Dynamically reconfigurable time-division multiplexing (TDM) dense wavelength division multiplexing (DWDM) long-reach passive optical networks (PONs) can support the reduction of nodes and network interfaces by enabling a fully meshed flat optical core. In this paper we demonstrate the flexibility of the TDM-DWDM PON architecture, which can enable the convergence of multiple service types on a single physical layer. Heterogeneous services and modulation formats, i.e. residential 10G PON channels, business 100G dedicated channel and wireless fronthaul, are demonstrated co-existing on the same long reach TDM-DWDM PON system, with up to 100km reach, 512 users and emulated system load of 40 channels, employing amplifier nodes with either erbium doped fiber amplifiers (EDFAs) or semiconductor optical amplifiers (SOAs). For the first time end-to-end software defined networking (SDN) management of the access and core network elements is also implemented and integrated with the PON physical layer in order to demonstrate two service use cases: a fast protection mechanism with end-to-end service restoration in the case of a primary link failure; and dynamic wavelength allocation (DWA) in response to an increased traffic demand.

Simulation of an apodizer's effect for high-density optical storage

The effect of an apodizer with two parallel taper refractive surfaces is theoretically investigated for high-density optical storage. The apodizer may modulate an incident Gaussian beam into an annular beam. Simulation shows that with the increasing inner radius of the modulated beam, the focal spot shrinks obviously. The depolarization effect gets strong simultaneously, which induces the circular symmetry loss of the focal spot. In this process, pattern density of the orthogonal and longitudinal diffractive fields increases remarkably.

Rare earth doped beta-diketone complexes as promising high-density optical recording materials for blue optoelectronics

Some kinds of rare earth beta-diketone complexes with blue-violet light absorption edge were synthesized using the ligands of thenoyltrifluoroacctone (HTTA), 2, 2'-dipyridyl (BIPY) and different metal ions (Gd3+, Sm3+ and La3+). Their contents, structures and optoelectronic parameters were monitored by elemental analysis, MS, IR and UV spectra. The solubility of rare earth beta-diketone complexes in 2, 2, 3, 3-tetrafluoro-1-propanol (TFP) and absorption properties of their films in the region 300-800 nm were measured. The influence on the difference of absorption maximum from rare earth beta-diketone complexes to beta-diketone ligand by different metal ions was studied. In addition, the thermal stability of rare earth beta-diketone complexes was also reported. (C) 2005 Elsevier B.V. All rights reserved.