Celebrating the 20 anniversary of Optics Express, this paper reviews the evolution of optical fiber communication systems, and through a look at the previous 20 years attempts to extrapolate fiber-optic technology needs and potential solution paths over the coming 20 years. Well aware that 20-year extrapolations are inherently associated with great uncertainties, we still hope that taking a significantly longer-term view than most texts in this field will provide the reader with a broader perspective and will encourage the much needed out-of-the-box thinking to solve the very significant technology scaling problems ahead of us. Focusing on the optical transport and switching layer, we cover aspects of large-scale spatial multiplexing, massive opto-electronic arrays and holistic optics-electronics-DSP integration, as well as optical node architectures for switching and multiplexing of spatial and spectral superchannels.

Exploiting a particular wave property for a particular application necessitates components capable of discriminating in the basis of that property. While spectral or polarisation decomposition can be straightforward, spatial decomposition is inherently more difficult and few options exist regardless of wave type. Fourier decomposition by a lens is a rare simple example of a spatial decomposition of great practical importance and practical simplicity; a two-dimensional decomposition of a beam into its linear momentum components. Yet this is often not the most appropriate spatial basis. Previously, no device existed capable of a two-dimensional decomposition into orbital angular momentum components, or indeed any discrete basis, despite it being a fundamental property in many wave phenomena. We demonstrate an optical device capable of decomposing a beam into a Cartesian grid of identical Gaussian spots each containing a single Laguerre-Gaussian component, using just a spatial light modulator and mirror.

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Silicon photonic devices and integrated circuits have undergone rapid and significant progresses during the last decade, transitioning from research topics in universities to product development in corporations. Silicon photonics is anticipated to be a disruptive optical technology for data communications, with applications such as intra-chip interconnects, short-reach communications in datacenters and supercomputers, and long-haul optical transmissions. Bell Labs, as the research organization of Alcatel-Lucent, a network system vendor, has an optimal position to identify the full potential of silicon photonics both in the applications and in its technical merits. Additionally it has demonstrated novel and improved high-performance optical devices, and implemented multi-function photonic integrated circuits to fulfill various communication applications. In this paper, we review our silicon photonic programs and main achievements during recent years. For devices, we review highperformance single-drive push-pull silicon Mach-Zehnder modulators, hybrid silicon/III-V lasers and silicon nitrideassisted polarization rotators. For photonic circuits, we review silicon/silicon nitride integration platforms to implement wavelength-division multiplexing receivers and transmitters. In addition, we show silicon photonic circuits are well suited for dual-polarization optical coherent transmitters and receivers, geared for advanced modulation formats. We also discuss various applications in the field of communication which may benefit from implementation in silicon photonics.

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