Roadmap on all-optical processing

The ability to process optical signals without passing into the electrical domain has always attracted the attention of the research community. Processing photons by photons unfolds new scenarios, in principle allowing for unseen signal processing and computing capabilities. Optical computation can be seen as a large scientific field in which researchers operate, trying to find solutions to their specific needs by different approaches; although the challenges can be substantially different, they are typically addressed using knowledge and technological platforms that are shared across the whole field. This significant know-how can also benefit other scientific communities, providing lateral solutions to their problems, as well as leading to novel applications. The aim of this Roadmap is to provide a broad view of the state-of-the-art in this lively scientific research field and to discuss the advances required to tackle emerging challenges, thanks to contributions authored by experts affiliated to both academic institutions and high-tech industries. The Roadmap is organized so as to put side by side contributions on different aspects of optical processing, aiming to enhance the cross-contamination of ideas between scientists working in three different fields of photonics: optical gates and logical units, high bit-rate signal processing and optical quantum computing. The ultimate intent of this paper is to provide guidance for young scientists as well as providing research-funding institutions and stake holders with a comprehensive overview of perspectives and opportunities offered by this research field

Monolithically integrated coherent comb de-multiplexer using facetless semiconductor ring lasers

Due to the growing demand for faster Internet connections and the growth of services requiring high bandwidths, the migration of the networks to higher and higher bit rates is inevitable. The surge in data usage due to real time application in the entertainment, business and medical sector has placed tremendous pressure on the current infrastructure and its ability to keep up with the constant traffic increase. Wavelength division multiplexing (WDM) systems, which currently dominate long-haul optical transmission links employ guard bands between channels to avoid interference. This leads to inefficient use of the system bandwidth, which is limited by the erbium doped fibre amplifier (EDFA). A solution to this problem is to use coherent WDM (Co-WDM), or called optical orthogonal frequency division multiplexing (OFDM) to increase the bandwidth efficiency. In Co-WDM or optical OFDM, the interference from adjacent orthogonal channels can be eliminated at the receiver end. Therefore, the spacing between adjacent channels can be reduced without using guard bands. Moreover, the series of individual lasers used in WDM networks can be replaced with a single coherent comb source. However, Co-WDM still requires the use of discrete bulky components, such as narrow band filters to operate as a comb demultiplexer, modulators and multiplexer. To justify the potential move from WDM to Co-WDM a significant reduction in cost, size and production time is necessary. The photonic integrated circuit (PIC) offers an attractive solution to integrate all components on a single chip. The vision of the larger research project was to design and fabricate a fully integrated Co-WDM transceiver. The initial demonstration PIC will comprise an integrated coherent comb generator, a comb filter and an integrated electro-absorption modulator (EAM). However, this work mainly focused on the filter aspect of the PIC to demultiplex the narrowly spaced comb lines (<25 GHz). The filter has two functions: to demultiplex the comb lines, and to selectively amplify the individual comb line through injection locking. The filter is designed to be facet-less so that it can be suitable for integration with other devices. The fabrication process of the PIC employs Standard UV lithography and a regrowth-free process to further reduce the time, cost and complexity making it suitable for mass production. The process of optimizing the facet-less lasers in term of fabrication yield, consistency and filtering performance will be presented

Optical comb injection for optical demultiplexing and harmonic frequency locking

With the continued growth of internet traffic, new optical communication infrastructures capable of dramatically increasing network bandwidth are being considered. Optical superchannels consisting of densely packed channels will be required for future networks, which could potentially be implemented using optical frequency combs - optical sources which consists of a series of discrete, equally spaced frequency lines. Optical combs can increase the spectral efficiency of these superchannels by allowing the channels to be more densely packed, while simultaneously reducing the number of components required (decreasing the energy consumption), and simplifying the digital signal processing required. Despite these advantages, the trade-off between cost and performance must be favourable in order for optical combs to become feasible for use in future communication networks. Photonic integrated circuits integrate several components together on a single semiconductor chip. These photonic circuits reduce both the cost and power consumption of devices, and hence recent research has been focused on creating suitable on-chip coherent optical comb transmitters. This thesis investigates an approach which is being used to demultiplex narrowly spaced optical combs on a photonic integrated circuit. By injection locking a laser to one of the lines in the optical comb (i.e, forcing a laser to lase with the frequency of that comb line), the comb line can be amplified and demultiplexed. This work investigates the physics of these active demultiplexers, both experimentally and numerically. It is found that the optimal side mode suppression ratio is obtained when the ratio of the comb's power to the injected laser's power is small, which also indicates optimal performance occurs when the locking range of the injected laser is at its smallest. The relaxation oscillations of the injected laser affect how well the comb can be demultiplexed, and as a result better side mode suppression ratios can be achieved at larger comb spacings. Further, it is shown that the relaxation oscillations within the injected laser can become undamped due to the comb injection, and frequency lock to fractions of the optical comb spacing. The injected laser can even become locked at detunings between the comb lines, creating a new output optical comb through nonlinear processes. The above phenomena are investigated numerically using two dimensional maps, and it is found that Arnol'd tongues appear in the injected laser's locking map

Systems Engineering for Silicon Photonic Devices

The increasing integration of digital information with our daily lives has led to the rise of big data, cloud computing, and the internet of things. The growth in these categories will lead to an exponential increase in the required capacity for data centers and high performance computation. Meanwhile, due to bottlenecks in data access caused by the limited energy and bandwidth scalability of electrical interconnects, computational speedup can no longer scale with demand. A better solution is necessary in order to increase computational performance and reduce the carbon footprint of our digital future. People have long thought of photonic interconnects, which can offer higher bandwidth, greater energy efficiency, and orders-of-magnitude distance scalability compared to electrical interconnects, as a solution to the data access bottleneck in chip, board, and datacenter scale networks. Over the past three decades we have seen impressive growth of photonic technology from theoretical predictions to high-performance commercially available devices. However, the dream of an all-optical interconnection network for use in CPU, Memory, and rack-to-rack datacenter interconnects is not yet realized. Many challenges and obstacles still have to be addressed. This work investigates these challenges and describe some of the ways to overcome them. First we will first examine the pattern sensitivity of microring modulators, which are likely to be found as the first element in an optical interconnect. My work will illustrate the advantage of using depletion mode modulators compared to injection mode modulators as the number of consecutive symbols in the data pattern increases. Next we will look at the problem of thermal initialization for microring demultiplexers near the output of the optical interconnect. My work demonstrates the fastest achieved initialization speed to-date for a microring based demultiplexer. I will also explore an thermal initialization and control method for microrings based on temperature measurement using a pn-junction. Finally, we will look at how to control and initialize microring and MZI based optical switch fabrics, which is the second element found in a optical interconnect. Work here will show the possibility of switching high-speed WDM datastreams through microring based switches, as well as methods to deal with the complexities inherent in control and initialization of high-radix switch topologies. Through these demonstrations I hope to show that the challenges facing optical interconnects, although very real, are surmountable using reasonable engineering efforts

Optical Communication

Optical communication is very much useful in telecommunication systems, data processing and networking. It consists of a transmitter that encodes a message into an optical signal, a channel that carries the signal to its desired destination, and a receiver that reproduces the message from the received optical signal. It presents up to date results on communication systems, along with the explanations of their relevance, from leading researchers in this field. The chapters cover general concepts of optical communication, components, systems, networks, signal processing and MIMO systems. In recent years, optical components and other enhanced signal processing functions are also considered in depth for optical communications systems. The researcher has also concentrated on optical devices, networking, signal processing, and MIMO systems and other enhanced functions for optical communication. This book is targeted at research, development and design engineers from the teams in manufacturing industry, academia and telecommunication industries

Development of germanium/silicon integration for near infrared detection

Silicon (Si) is the base material for electronic technologies and is emerging as a very attractive platform for photonic integrated circuits (PICs). PICs allow optical systems to be made more compact with higher performance than discrete optical components. Applications for PICs are in the area of fibre-optic communication, biomedical devices, photovoltaics and imaging. Germanium (Ge), due to its suitable bandgap for telecommunications and its compatibility with Si technology is preferred over III-V compounds as an integrated on-chip detector at near infrared wavelengths. There are two main approaches for Ge/Si integration: through epitaxial growth and through direct wafer bonding. The lattice mismatch of ~4.2% between Ge and Si is the main problem of the former technique which leads to a high density of dislocations while the bond strength and conductivity of the interface are the main challenges of the latter. Both result in trap states which are expected to play a critical role. Understanding the physics of the interface is a key contribution of this thesis. This thesis investigates Ge/Si diodes using these two methods. The effects of interface traps on the static and dynamic performance of Ge/Si avalanche photodetectors have been modelled for the first time. The thesis outlines the original process development and characterization of mesa diodes which were fabricated by transferring a ~700 nm thick layer of p-type Ge onto n-type Si using direct wafer bonding and layer exfoliation. The effects of low temperature annealing on the device performance and on the conductivity of the interface have been investigated. It is shown that the diode ideality factor and the series resistance of the device are reduced after annealing. The carrier transport mechanism is shown to be dominated by generation–recombination before annealing and by direct tunnelling in forward bias and band-to-band tunnelling in reverse bias after annealing. The thesis presents a novel technique to realise photodetectors where one of the substrates is thinned by chemical mechanical polishing (CMP) after bonding the Si-Ge wafers. Based on this technique, Ge/Si detectors with remarkably high responsivities, in excess of 3.5 A/W at 1.55 μm at −2 V, under surface normal illumination have been measured. By performing electrical and optical measurements at various temperatures, the carrier transport through the hetero-interface is analysed by monitoring the Ge band bending from which a detailed band structure of the Ge/Si interface is proposed for the first time. The above unity responsivity of the detectors was explained by light induced potential barrier lowering at the interface. To our knowledge this is the first report of light-gated responsivity for vertically illuminated Ge/Si photodiodes. The wafer bonding approach followed by layer exfoliation or by CMP is a low temperature wafer scale process. In principle, the technique could be extended to other materials such as Ge on GaAs, or Ge on SOI. The unique results reported here are compatible with surface normal illumination and are capable of being integrated with CMOS electronics and readout units in the form of 2D arrays of detectors. One potential future application is a low-cost Si process-compatible near infrared camera