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

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

Experimental comparison of nonlinear compensation in long-haul PDM-QPSK transmission at 42.7 and 85.4 Gb/s

We compare the nonlinear limits of 42.7 and 85.4 Gb/s PDM-QPSK transmission. Intra-channel nonlinearity compensation is demonstrated, with increase of optimum launch power by 1dB in each case and corresponding increase in maximum reach to 11635km and 8100km, respectively. © VDE VERLAG GMBH

Experimental characterisation of QAM16 at symbol rates up to 42Gbaud

We experimentally investigate generation and detection of QAM16 at symbol rates up to 42Gbaud. The error floor due to the finite receiver bandwidth and resolution was characterised as a function of baud rate. Transmission over 320km at 35Gbaud is demonstrated. ©2010 IEEE

Pulse-shaping versus digital backpropagation in 224Gbit/s PDM-16QAM transmission

We investigate the transmission performance of 224Gbit/s polarization-division-multiplexed 16-state quadrature amplitude modulation (PDM-16QAM) for systems employing standard single mode fiber (SSMF) and erbium doped fiber amplifiers (EDFAs). We consider the effectiveness of return-to-zero (RZ) data pulses with varying duty cycles and digital backpropagation (DBP) in reducing nonlinear distortion in wavelength-division- multiplexed (WDM) links with 3, 5, 7 and 9 channels. Similar improvement in transmission reach of 18-25% was achieved either by pulse-carving at the transmitter or by DBP, yielding maximum transmission distances of up to 1760km for RZ-pulse-shapes and 1280km for NRZ. © 2011 Optical Society of America

Mitigation of fiber nonlinearity using a digital coherent receiver

Coherent detection with receiver-based DSP has recently enabled the mitigation of fiber nonlinear effects. We investigate the performance benefits available from the backpropagation algorithm for polarization division multiplexed quadrature amplitude phase-shift keying (PDM-QPSK) and 16-state quadrature amplitude modulation (PDM-QAM16). The performance of the receiver using a digital backpropagation algorithm with varying nonlinear step size is characterized to determine an upper bound on the suppression of intrachannel nonlinearities in a single-channel system. The results show that for the system under investigation PDM-QPSK and PDM-QAM16 have maximum step sizes for optimal performance of 160 and 80 km, respectively. Whilst the optimal launch power is increased by 2 and 2.5 dB for PDM-QPSK and PDM-QAM16, respectively, the Q-factor is correspondingly increased by 1.6 and 1 dB, highlighting the importance of studying nonlinear compensation for higher level modulation formats. © 2010 IEEE