Field Programmable Photonic Gate Arrays

The field programmable photonic gate array (FPPGA) is an integrated photonic device/subsystem that operates similarly to a field programmable gate array in electronics. It is a set of programmable photonics analogue blocks (PPABs) and of reconfigurable photonic interconnects (RPIs) implemented over a photonic chip. The PPABs provide the building blocks for implementing basic optical analogue operations (reconfigurable/independent power splitting and phase shifting). Broadly they enable reconfigurable processing just like configurable logic elements (CLE) or programmable logic blocks (PLBs) carry digital operations in electronic FPGAs or configurable analogue blocks (CABs) carry analogue operations in electronic field programmable analogue arrays (FPAAs). Reconfigurable interconnections between PPABs are provided by the RPIs. This chapter presents basic principles of integrated FPPGAs. It describes their main building blocks and discusses alternatives for their high-level layouts, design flow, technology mapping and physical implementation. Finally, it shows that waveguide meshes lead naturally to a compact solution.

Programmable Integrated Photonics for Classical Applications

Programmable photonics can also be applied to classical systems. While these systems rely mainly on pure fixed application-specific photonic circuits, the advantages of programmability are decisive when systems grow in complexity. This chapter reviews application areas where programmable photonics can enable technology: optical switching interconnection and routing to provide myriad powerful solutions that encompass broadcasting and wavelength selective operation; artificial intelligence (AI) and neurocomputing to enables the implementation of reconfigurable analogue cores required for the operation of neural networks and neurophotonic systems. Microwave and analogue photonics are particularly appealing areas which require substantial cost reductions and broadband operation flexibility simultaneous with most of the required functionalities in a single chip, which span 5G communications, satellite payloads, electromagnetic and photonic radar, Internet of Things and autonomous driving. Finally, it covers optical signal processing, including examples in signal filtering and sensing, controllable Talbot effect and more speculative futuristic applications, e.g. synthetic domains.

Practical Implementation of Programmable Photonic Circuits

In covering the fundamentals and ideal implementation of integrated multi-port interferometers and waveguide meshes, we saw that solutions with larger integration density of programmable unit cells enables the synthesis of more complex circuits. However, photonic integrated circuits (PICs) generally suffer from design and fabrication errors and other non-ideal working conditions, which compromises performance and scalability. In addition, PICs require the development of two additional tiers (electronic hardware and software) to allow their programmability, optimisation and (re)configuration. This chapter introduces basic practical considerations of programmable PIC design and reviews experimental demonstrations of both multi-port interferometers and waveguide mesh arrangements. It analyses the main error sources and their impact on circuit performance and investigates the most challenging evolution obstacles for very large-scale programmable PICs. It introduces an analytical method for arbitrary waveguide mesh analysis. Finally, it presents a general architecture for the control subsystem and introduces the software framework and main algorithms.

Integrated Multi-port Interferometers

Integrated multi-port interferometers are the second step in programmable photonics. Initially conceived and designed to implement fixed linear optics unitary transformations between N input and N output ports to support mode transformations in quantum optics, the rapid development of this field and the possibility of applying these structures to other areas stimulated development of devices capable of supporting arbitrary linear transformations by suitable programming and surpassing use in square matrix transformations and unitary operators. This chapter reviews some basic concepts on unitary N×N matrices and then describes in detail the basic design principles and integrated optics implementations of triangular and rectangular designs for multi-port interferometers. It then considers more specific interferometer designs, which do not provide possible implementation of general linear transformations but are optimised for performing important operations, e.g. the Fourier transform. Lastly, it covers the linear universal component, which integrates both universal beam couplers and integrated multi-port interferometers.

Introduction to Programmable Integrated Photonics

Programmable integrated photonics (PIP) aims at designing common integrated optical hardware configurations, which—by suitable programming—can implement a variety of functionalities that can be elaborated for basic or more complex operations in many application fields. It follows a different approach to that of application specific photonic integrated circuits (ASPICs), which have dominated during the last few decades. The interest in PIP is driven by the surge of a considerable number of emerging applications in the fields of telecommunications, quantum information processing, sensing and neurophotonics that will require flexible, reconfigurable, low-cost, compact and low-power-consuming devices, much as field programmable gate array (FPGA) devices operate in electronics. This chapter serves as a general introduction to the book and reviews the main basic principles and recent advances in PIP, including fabrication platforms, design principles, architecture choices, challenges and limitations. and provides a brief introduction to the applications of this new field.

Integrated Waveguide Meshes

Integrated waveguide meshes are the third evolutionary step in programmable photonics, and rely on the large-scale repetition of interferometric waveguide elements and phase actuators conceived and designed to implement a common hardware platform that enables the programming of arbitrary photonic circuit topologies and design parameters by suitable setting of its control signals. In contrast to linear feedforward-only programmable circuits, these circuits enable the synthesis of optical cavities, optical loops and feedbackward paths. Their improved versatility entails a paradigm shift regarding the development of application-specific photonic integrated circuits. Here, the long and costly test cycles can be potentially substituted by off-the-shelf ready-to-use circuits. This allows them to be applied to myriad systems requiring optical linear processing and dynamic configuration. This chapter introduces the basic design principles and the implementations of the most popular designs. It then provides a comparative analysis of alternative waveguide mesh arrangements and their performance evaluation.

Reconfigurable Application-Specific Photonic Integrated Circuits

Reconfigurable photonic integrated circuits are the first evolutionary step in programmable photonics. These are fundamentally application-specific circuits designed to carry a given functionality and that feature some degree of flexibility enabled by the possibility of tuning some of its internal parameters by means of appropriate control signals. Another distinctive feature of these circuits resides in the fixed nature of their physical topology and designs. There is a vast amount of reconfigurable integrated circuits reported in the literature. This chapter provides some representative examples for other functionalities, including filtering, RF phase shifting and time delay lines, beam formers, wave generators and other multi-functional application specific circuits. It prepares the reader to understand how these can be emulated with the more generic programmable photonic circuits discussed later in the book.

Basic Building Blocks and Techniques

Programmable integrated photonics (PIP) relies on designing suitable basic building blocks (BBBs) able to carry elementary signal processing operations and interconnection hardware architectures that offer very high spatial regularity. The most popular BBBs proposed so far are based on elementary 2×2 tunable photonics coupling components capable of providing independent setting of the power coupling ratio. Additionally, they are based on the phase shift experienced by the incoming signals from two input waveguides in their transition to the two output waveguide ports of the device. This chapter deals primarily with these components. First, it considers the basic matrix methods required to describe their operation, either standalone or in combination with others to form more complex structures. Next, it describes the main technology approaches for the implementation of BBBs, including 3dB Mach–Zehnder tunable couplers, directional couplers, and beamsplitters, followed by how these BBBs are employed to build up a tunable basic unit (TBU), which is the elementary constituent of waveguide mesh circuits. It concludes by describing the devices and techniques relevant in multiport devices, and discusses the equivalence between mode conversion and linear optics matrix transformations and the universal linear combiner.

Programmable Integrated Photonics for Quantum Systems

Programmable photonics can find applications in myriad areas including the quantum information field, which encompasses communications, computing, sensing and tomography. Large-scale bulk optics setups previously prevented the development of more complex and scalable quantum optics configurations. Linear optic systems with the required fidelity require a strict control of interference through demanding phase stability mechanisms. Integrating a considerable number of photonic elements on a chip in order to implement multi-port interferometers has become the only viable technological path towards quantum information systems. This chapter introduces the applications of programmable photonics to quantum information systems. After introducing the general framework of a programmable quantum photonic system integrated on a chip and briefly describing the role of more external components such as sources and detectors, it covers the relationship between reconfigurable integrated optic circuits and linear optical quantum gates, quantum transport simulation, boson sampling and complex Hadamard and quantum Fourier transforms.