Nonlinearity compensation and information rates in fully-loaded C-band optical fibre transmission systems

Nonlinearity compensation and achievable information rates were investigated in fully-loaded C-band communication systems considering transceiver limitations. It is found that the efficacy of nonlinearity compensation in enhancing the achievable information rates depends on the modulation formats and transmission distances

Joint Superchannel Digital Signal Processing for Effective Inter-Channel Interference Cancellation

Modern optical communication systems transmit multiple frequency channels, each operating very close to its theoretical limit. The total bandwidth can reach 10 THz limited by the optical amplifiers. Maximizing spectral efficiency, the throughput per bandwidth is thus crucial. Replacing independent lasers with an optical frequency comb can enable very dense packing by overcoming relative drifts. However, to date, interference from non-ideal spectral shaping prevents exploiting the full potential of frequency combs. Here, we demonstrate comb-enabled multi-channel digital signal processing, which overcomes these limitations. Each channel is detected using an independent coherent receiver and processed at two samples-per-symbol. By accounting for the unique comb stability and exploiting aliasing in the design of the dynamic equalizer, we show that the optimal spectral shape changes, resulting in a higher signal-to-noise ratio that pushes the optimal symbol rate towards and even above the channel spacing, resulting in the first example of frequency-domain super-Nyquist transmission with multi-channel detection for optical systems. The scheme is verified both in back-to-back configuration and in single span transmission of a 21 channel superchannel originating from a 25 GHz-spaced frequency comb. By jointly processing three wavelength channels at a time, we achieve spectral efficiency beyond what is possible with independent channels. At the same time, one significantly relaxes the hardware requirements on digital-to-analog resolution and bandwidth, as well as filter tap numbers. Our results show that comb-enabled multi-channel processing can overcome the limitations of classical dense wavelength division multiplexing systems, enabling tighter spacing to make better use of the available spectrum in optical communications

Analytical optimization of wideband nonlinear optical fiber communication systems

In the design of fiber links for both continental and transoceanic optical communication systems, the optimization of span length is of high importance from both performance and cost perspectives. In this work, the maximization of signal-to-noise ratio (SNR) is investigated by optimizing the span length in wideband (up to 4.5-THz) Nyquist-spaced optical fiber communication systems. A simple and accurate closed-form expression of the optimal span length is provided, and a quick estimation of SNR is also described for practically feasible and cost-effective span length values

Information rates in Kerr nonlinearity limited optical fiber communication systems

Achievable information rates of optical communication systems are inherently limited by nonlinear distortions due to the Kerr effect occurred in optical fibres. These nonlinear impairments become more significant for communication systems with larger transmission bandwidths, closer channel spacing and higher-order modulation formats. In this paper, the efficacy of nonlinearity compensation techniques, including both digital back-propagation and optical phase conjugation, for enhancing achievable information rates in lumped EDFA- and distributed Raman-amplified fully-loaded C −band systems is investigated considering practical transceiver limitations. The performance of multiple modulation formats, such as dual-polarisation quadrature phase shift keying (DP-QPSK), dual-polarisation 16 −ary quadrature amplitude modulation (DP-16QAM), DP-64QAM and DP-256QAM, has been studied in C −band systems with different transmission distances. It is found that the capabilities of both nonlinearity compensation techniques for enhancing achievable information rates strongly depend on signal modulation formats as well as target transmission distances

Volterra-Assisted Optical Phase Conjugation: A Hybrid Optical-Digital Scheme for Fiber Nonlinearity Compensation

Digital nonlinearity compensation (NLC) schemes such as digital backpropagation and Volterra equalization are well known to be effective techniques in mitigating optical fiber nonlinearity, thus offering improved transmission performance. Alternatively, optical NLC, and specifically optical phase conjugation (OPC), has been proposed to relax the digital signal processing complexity. In this paper, a novel hybrid optical-digital NLC scheme combining OPC and a Volterra equalizer is proposed, termed Volterra-Assisted OPC (VAO). It has a twofold advantage: it overcomes the OPC limitation in asymmetric links and substantially enhances the performance of Volterra equalizers. When NLC is operated over the entire transmitted optical bandwidth, the proposed scheme is shown to outperform both OPC and Volterra equalization alone by up to 4.2 dB in a five-channel, 32 GBaud PM-16QAM transmission over a 1000 km EDFA-amplified fiber link. Moreover, VAO is also demonstrated to be very robust when applied to long-transmission distances, with a 2.5-dB gain over OPC-only systems at 3000 km. VAO combines the advantages of both optical and digital NLC offering a promising tradeoff between performance and complexity for future high-speed optical communication systems

Analytical optimization of wideband nonlinear optical fiber communication systems

In the design of fiber links for both continental and transoceanic optical communication systems, the optimization of span length is of high importance from both performance and cost perspectives. In this work, the maximization of signal-to-noise ratio (SNR) is investigated by optimizing the span length in wideband (up to 4.5-THz) Nyquist-spaced optical fiber communication systems. A simple and accurate closed-form expression of the optimal span length is provided, and a quick estimation of SNR is also described for practically feasible and cost-effective span length values

Information rates in Kerr nonlinearity limited optical fibre communication systems

Achievable information rates of optical communication systems are inherently limited by nonlinear distortions due to the Kerr effect occurred in optical fibres. These nonlinear impairments become more significant for communication systems with larger transmission bandwidths, closer channel spacing and higher-order modulation formats. In this paper, the efficacy of nonlinearity compensation techniques, including both digital back-propagation and optical phase conjugation, for enhancing achievable information rates in lumped EDFA- and distributed Raman-amplified fully-loaded C −band systems is investigated considering practical transceiver limitations. The performance of multiple modulation formats, such as dual-polarisation quadrature phase shift keying (DP-QPSK), dual-polarisation 16 −ary quadrature amplitude modulation (DP-16QAM), DP-64QAM and DP-256QAM, has been studied in C −band systems with different transmission distances. It is found that the capabilities of both nonlinearity compensation techniques for enhancing achievable information rates strongly depend on signal modulation formats as well as target transmission distances

Millimetre-Resolution Photonics-Assisted Radar

Radar is essential in applications such as anti-collision systems for driving, airport security screening, and contactless vital sign detection. The demand for high-resolution and real-time recognition in radar applications is growing, driving the development of electronic radars with increased bandwidth, higher frequency, and improved reconfigurability. However, conventional electronic approaches are challenging due to limitations in synthesising radar signals, limiting performance. In contrast, microwave photonics-enabled radars have gained interest because they offer numerous benefits compared to traditional electronic methods. Photonics-assisted techniques provide a broad fractional bandwidth at the optical carrier frequency and enable spectrum manipulation, producing wideband and high-resolution radar signals in various formats. However, photonic-based methods face limitations like low time-frequency linearity due to the inherent nonlinearity of lasers, restricted RF bandwidth, limited stability of the photonic frequency multipliers, and difficulties in achieving extended sensing with dispersion-based techniques. In response to these challenges, this thesis presents approaches for generating broadband radar signals with high time-frequency linearity using recirculated unidirectional optical frequency-shifted modulation. The photonics-assisted system allows flexible bandwidth tuning from sub-GHz to over 30 GHz and requires only MHz-level electronics. Such a system offers millimetre-level range resolution and a high imaging refresh rate, detecting fast-moving objects using the ISAR technique. With millimetre-level resolution and micrometre accuracy, this system supports contactless vital sign detection, capturing precise respiratory patterns from simulators and a living body using a cane toad. In the end, we highlight the promise of merging radar and LiDAR, foreshadowing future advancements in sensor fusion for enhanced sensing performance and resilience