High-speed optical fibre transmission using advanced modulation formats

The rapid growth in interactive bandwidth-hungry services demands ever higher capacity at various stages of the optical network, leading to a potential capacity exhaust, termed the capacity crunch. The main aim of the research work described in this thesis was to help solve the potential capacity crunch by exploring techniques to increase the data rate, spectral efficiency and reach of optical fibre systems. The focus was on the use of advanced signal modulation formats, including optical time-division multiplexing (OTDM), quadrature phase shift keying (QPSK), and 16-state quadrature amplitude modulation (QAM16). QPSK and QAM16 modulations formats were studied in combination with coherent detection and digital signal processing (DSP) for the compensation of transmission impairments. In addition, return-to-zero (RZ) pulses were explored to increase the tolerance towards nonlinearity for coherently detected signals, and nonlinearity compensation (NLC) through the DSP. Initially, to maximise the bit-rate, research was focused on the study of OTDM transmission at 80Gbit/s with the aim to optimise the phase difference between the adjacent OTDM channels. A new technique to achieve bit-wise phase control using a phase-stabilised fibre interferometer was proposed. Faced with a limited fibre capacity, the need to maximise the spectral efficiency became paramount, and thus the need to use phase, amplitude and polarisation domains for signal transmission. In combination with coherent detection the research focused on the performance of optical fibre systems using QPSK and QAM16 modulation formats, including their generation, transmission and detection in single-channel and WDM regimes. This included the study of the impact of pulse shapes, and the mitigation of linear and nonlinear transmission impairments with receiver-based DSP at bit-rates ranging from 42.7 to 224Gbit/s. The technique demonstrated for bit-wise phase control for OTDM was successfully used to demonstrate a new method for QAM16 signal generation. Longest transmission distances (up to 10160km in 112Gbit/s QPSK, 4240km in 112Gbit/s QAM16, and 2000km in 224Gbit/s QAM16) have been achieved with the use of NLC and RZ pulses. The efficiency of these two techniques is explored through a comprehensive set of experiments in both single-channel and WDM transmission experiments. The results can be used in the design of future optical transmission systems

Amplification schemes and multi-channel DBP for unrepeatered transmission

The performance of unrepeatered transmission of a seven Nyquist-spaced 10 GBd PDM-16QAM superchannel using full signal band coherent detection and multi-channel digital back propagation (MC-DBP) to mitigate nonlinear effects is analysed. For the first time in unrepeatered transmission, the performance of two amplification systems is investigated and directly compared in terms of achievable information rates (AIRs): 1) erbium-doped fibre amplifier (EDFA) and 2) second-order bidirectional Raman pumped amplification. The experiment is performed over different span lengths, demonstrating that, for an AIR of 6.8 bit/s/Hz, the Raman system enables an increase of 93 km (36 %) in span length. Further, at these distances, MC-DBP gives an improvement in AIR of 1 bit/s/Hz (to 7.8 bit/s/Hz) for both amplification schemes. The theoretical AIR gains for Raman and MC-DBP are shown to be preserved when considering low-density parity-check codes. Additionally, MC-DBP algorithms for both amplification schemes are compared in terms of performance and computational complexity. It is shown that to achieve the maximum MC-DBP gain, the Raman system requires approximately four times the computational complexity due to the distributed impact of fibre nonlinearity

Polarization multiplexed 16QAM transmission employing modified digital back-propagation

We experimentally demonstrate performance enhancements enabled by weighted digital back propagation method for 28 Gbaud PM-16QAM transmission systems, over a 250 km ultra-large area fibre, using only one back-propagation step for the entire link, enabling up to 3 dB improvement in power tolerance with respect to linear compensation only. We observe that this is roughly the same improvement that can be obtained with the conventional, computationally heavy, non-weighted digital back propagation compensation with one step per span. As a further benchmark, we analyze performance improvement as a function of number of steps, and show that the performance improvement saturates at approximately 20 steps per span, at which a 5 dB improvement in power tolerance is obtained with respect to linear compensation only. Furthermore, we show that coarse-step self-phase modulation compensation is inefficient in wavelength division multiplexed transmission

Nonlinear impairment compensation using expectation maximization for dispersion managed and unmanaged PDM 16-QAM transmission

In this paper, we show numerically and experimentally that expectation maximization (EM) algorithm is a powerful tool in combating system impairments such as fibre nonlinearities, inphase and quadrature (I/Q) modulator imperfections and laser linewidth. The EM algorithm is an iterative algorithm that can be used to compensate for the impairments which have an imprint on a signal constellation, i.e. rotation and distortion of the constellation points. The EM is especially effective for combating non-linear phase noise (NLPN). It is because NLPN severely distorts the signal constellation and this can be tracked by the EM. The gain in the nonlinear system tolerance for the system under consideration is shown to be dependent on the transmission scenario. We show experimentally that for a dispersion managed polarization multiplexed 16-QAM system at 14 Gbaud a gain in the nonlinear system tolerance of up to 3 dB can be obtained. For, a dispersion unmanaged system this gain reduces to 0.5 dB

Optimalization of public transport in the centre of Pilsen in long term period

This Diplomas thesis is focused on the area of public transport in the centre of Pilsen and its optimization in a long term. In the methodology part are defined basic terms of public transport and theoretical outcomes for urban and suburban public transport systems. Next there is a description of means of transport and methods of preference used in public transport. Shortly is introduced the legal framework of public transport in Czech Republic. The processes of transport management especially the process of planning are described in detail. The last subchapter of the methodology part describes integrated transport systems. The analytical part of the thesis begins with the description of the Pilsen as a regional centre and the analysis of its urban and suburban transport system. The deficits of the system are grouped into problem areas; for each problem area is suggested an optimization: the new bus terminal in Šumavská street near the Pilsen main train station, the interconnection of Borská street with the intersection Karlovarská/Lidická and usage of the north-west bypass by public transport

A long-reach ultra-dense 10 Gbit/s WDM-PON using a digital coherent receiver

We investigate the impact of channel spacing and nonlinear transmission over 120 km of standard single mode fiber for a 10 Gbit/s long-reach wavelength division multiplexed passive optical network (WDM-PON). We employed polarization division multiplexed quadrature phase shift keying (PDM-QPSK), which allowed data transmission at 3.125 GBaud, including a 25% overhead for forward error correction. To receive this spectrally efficient modulation format, a digital coherent receiver was employed, allowing for both frequency selectivity and an increased sensitivity of -45 dBm (25 photons/bit).We investigated a channel spacing as low as 5 GHz, for which the loss budget was 48.6 dB, increasing to 54.0 dB for a 50 GHz grid. © 2010 Optical Society of America

Coherent electronic compensation techniques for long-haul optical fibre transmission - Opportunities and challenges

We review recent advances in the use coherent detection and digital signal processing to compensate for dispersion and nonlinearity in long-haul WDM links, and assess the use of these techniques for long-haul and subsea applications. © VDE VERLAG GMBH