Roadmap of optical communications

© 2016 IOP Publishing Ltd. Lightwave communications is a necessity for the information age. Optical links provide enormous bandwidth, and the optical fiber is the only medium that can meet the modern society's needs for transporting massive amounts of data over long distances. Applications range from global high-capacity networks, which constitute the backbone of the internet, to the massively parallel interconnects that provide data connectivity inside datacenters and supercomputers. Optical communications is a diverse and rapidly changing field, where experts in photonics, communications, electronics, and signal processing work side by side to meet the ever-increasing demands for higher capacity, lower cost, and lower energy consumption, while adapting the system design to novel services and technologies. Due to the interdisciplinary nature of this rich research field, Journal of Optics has invited 16 researchers, each a world-leading expert in their respective subfields, to contribute a section to this invited review article, summarizing their views on state-of-the-art and future developments in optical communications

Generation and optimization of picosecond optical pulses for use in hybrid WDM/OTDM networks

The burgeoning demand for broadband services such as database queries, home shopping, video-on-demand, remote education, telemedicine and videoconferencing will push the existing networks to their limits. This demand was mainly fueled by the brisk proliferation of Personal Computers (PC) together with the exceptional increases in their storage capacity and processing capabilities and the widespread availability of the internet. Hence the necessity, to develop high-speed optical technologies in order to construct large capacity networks, arises. Two of the most popular multiplexing techniques available in the optical domain that are used in the building of such high capacity networks, are Wavelength Division Multiplexing (WDM) and Optical Time Division Multiplexing (OTDM). However merging these two techniques to form very high-speed hybrid WDM/OTDM networks brings about the merits of both multiplexing technologies. This thesis examines the development of one of the key components (picosecond optical pulses) associated to such high-speed systems. Recent analysis has shown that RZ format is superior to conventional NRZ systems as it is easier to compensate for dispersion and nonlinear effects in the fibre by employing soliton-like propagation. In addition to this development, the use of wavelength tunability for dynamic provisioning is another area that is actively researched on. Self-seeding of a gain switched Fabry Perot laser is shown to one of the simplest and cost effective methods of generating, transform limited optical pulses that are wavelength tunable over very wide ranges. One of the vital characteristics of the above mentioned pulse sources, is their Side Mode Suppression Ratio (SMSR). This thesis examines in detail how the pulse SMSR affects the performance of high-speed WDM/OTDM systems that employ self-seeded gain-switched pulse sources

Label-controlled optical switching nodes

Optical networks are evolving from initially static optical circuits and subsequently optical circuit switching towards optical packet switching in order to take advan- tage of the high transport capacity made available by WDM systems in a more °exible and e±cient way. Optically labeling of packets and routing the packets's payload optically under control of its label allows the network nodes to route and forward IP data without having to process the payload, thus keeping it in the optical domain; this is a promising solution to avoid electronic bottlenecks in routers. All-optical label switching can therefore be used to route and forward packets independent of their length and payload bitrate. Several optical signal labeling techniques have been proposed in previous re- search reported in literature; orthogonal labeling and time-serial labeling have been studied in this thesis. This thesis studies two orthogonal modulation label- ing techniques: one based on FSK labels with an IM payload, and another one on SCM labeling for a DPSK modulated payload. A time-serial labeling method based on IM labels with IM or DPSK payload is also presented and studied. The ¯rst two techniques assume electronic processing of the labels in the node, and hence assume that labels can be transmitted at a much lower bitrate than the payload data rate. The third technique assumes all-optical signal processing in the nodes, capable of handling a label at the same bitrate or slightly lower than the payload data. Labels at low bitrate in comparison with the payload bitrate are desirable in systems where the label processing will be conducted in the electrical domain, while labels at the same bitrate as the payload can be used in systems where the processing is conducted in the optical domain, exploiting all-optical processing techniques. These three techniques have been chosen because they are compatible with the existing networks, since the modulation format, bitrates, transmission properties, and other features of the signals are similar to the ones used for commercially available applications. Thus, they can be considered important candidates for migration scenarios from optical circuit switching towards optical burst switching networking. Orthogonal labeling based on FSK/IM is a promising scheme for implementing the labeling of optical signals, and it is the technology of choice in the STOLAS project. This technique o®ers advantageous features such as a relaxed timing de- lineation between payload and label, and ease of label erasure and re-writing of new labels. By using wavelength-agile tunable laser sources with FSK modula- tion capability, wavelength converters, and passive wavelength routing elements, a scalable modular label-controlled router featuring high reliability can be built. In this thesis, several aspects of the physical parameters of an FSK/IM labeling scheme within a routing node have been studied and presented. Optical ¯ltering requires special care, since the combined FSK/IM scheme has a broader spectrum than that of pure intensity modulated signals. The requirements on the limited extinction ratio for the IM signal can be relaxed at low bitrates of the label signal or, alternatively, by introducing data encoding. Optical labeling by using FSK/IM represents a simple and attractive way of implementing hybrid optical circuit and burst switching in optical networks. Architecturally, similar advantages can be mentioned for the second orthogo- nal labeling technique studied in this thesis, based on SCM labels and a DPSK payload. In-band subcarriers carrying low bitrate labels located at a frequency equal to half the bitrate of the payload signal can be inserted introducing only low power penalties. Wavelength conversion can be implemented by using passive highly nonlinear ¯bers and exploiting the four-wave mixing e®ect. This thesis also studies the design of two functional blocks of an all-optical core node proposed in the LASAGNE project, namely the all-optical label and payload separator and the wavelength converter unit for a time-serial labeling scheme. The label and payload processor can be realized exploiting nonlinear e®ects in SOAs. An implementation using polarization division multiplexing to transport the external control light for an IM/IM time-serial scheme was demon- strated. Label and payload processors with self-contained control signals were also demonstrated, either using a DPSK signal to simultaneously transport the payload data and the control signal or inserting a CW dummy in between the label and the payload, which were based on IM-RZ format. A study on single- and multi- wavelength conversion based on FWM in a HNLF was presented. This approach allows transparent wavelength conversion (independent of the data format used) at high bitrates (the nonlinear e®ects in a ¯ber are obtained at ultrafast speeds). The labeling techniques explored have indicated a viable way of migration towards optical burst packet switched networks while signi¯cantly improving the throughput of the routing nodes

Fully-photonic digital radio over fibre for future super-broadband access network applications

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel UniversityIn this thesis a Fully-Photonic DRoF (FP-DRoF) system is proposed for deploying of future super-broadband access networks. Digital Radio over Fibre (DRoF) is more independent of the fibre network impairments and the length of fibre than the ARoF link. In order for fully optical deployment of the signal conversion techniques in the FP-DRoF architecture, two key components an Analogue-to-Digital Converter (ADC) and a Digital-to-Analogue Converter (DAC)) for data conversion are designed and their performance are investigated whereas the physical functionality is evaluated. The system simulation results of the proposed pipelined Photonic ADC (PADC) show that the PADC has 10 GHz bandwidth around 60 GHz of sampling rate. Furthermore, by changing the bandwidth of the optical bandpass filter, switching to another band of sampling frequency provides optimised performance condition of the PADC. The PADC has low changes on the Effective Number of Bit (ENOB) response versus analogue RF input from 1 GHz up to 22 GHz for 60 GHz sampling frequency. The proposed 8-Bit pipelined PADC performance in terms of ENOB is evaluated at 60 Gigasample/s which is about 4.1. Recently, different methods have been reported by researchers to implement Photonic DACs (PDACs), but their aim was to convert digital electrical signals to the corresponding analogue signal by assisting the optical techniques. In this thesis, a Binary Weighted PDAC (BW-PDAC) is proposed. In this BW-PDAC, optical digital signals are fully optically converted to an analogue signal. The spurious free dynamic range at the output of the PDAC in a back-to-back deployment of the PADC and the PDAC was 26.6 dBc. For further improvement in the system performance, a 3R (Retiming, Reshaping and Reamplifying) regeneration system is proposed in this thesis. Simulation results show that for an ultrashort RZ pulse with a 5% duty cycle at 65 Gbit/s using the proposed 3R regeneration system on a link reduces rms timing jitter by 90% while the regenerated pulse eye opening height is improved by 65%. Finally, in this thesis the proposed FP-DRoF functionality is evaluated whereas its performance is investigated through a dedicated and shared fibre links. The simulation results show (in the case of low level signal to noise ratio, in comparison with ARoF through a dedicated fibre link) that the FP-DRoF has better BER performance than the ARoF in the order of 10-20. Furthermore, in order to realize a BER about 10-25 for the ARoF, the power penalty is about 4 dBm higher than the FP-DRoF link. The simulation results demonstrate that by considering 0.2 dB/km attenuation of a standard single mode fibre, the dedicated fibre length for the FP-DRoF link can be increased to about 20 km more than the ARoF link. Moreover, for performance assessment of the proposed FP-DRoF in a shared fibre link, the BER of the FP-DRoF link is about 10-10 magnitude less than the ARoF link for -19 dBm launched power into the fibre and the power penalty of the ARoF system is 10 dBm more than the FP-DRoF link. It is significant to increase the fibre link’s length of the FP-DRoF access network using common infrastructure. In addition, the simulation results are demonstrated that the FP-DRoF with non-uniform Wavelength Division Multiplexing (WDM) is more robust against four wave mixing impairment than the conventional WDM technique with uniform wavelength allocation and has better performance in terms of BER. It is clearly verified that the lunched power penalty at CS for DRoF link with uniform WDM techniques is about 2 dB higher than non-uniform WDM technique. Furthermore, uniform WDM method requires more bandwidth than non-uniform scheme which depends on the total number of channels and channels spacing