The growing maturity of integrated photonic technology makes it possible to build increasingly large and complex photonic circuits on the surface of a chip. Today, most of these circuits are designed for a specific application. However, the increase in complexity creates an opportunity for a generation of photonic circuits that can be programmed in software for a wide variety of functions through a mesh of on-chip waveguides, tunable beam couplers and optical phase shifters. Here we discuss the state of this emerging technology, not just the recent developments in photonic building blocks and circuit architectures, but also the higher levels in the technology stack for the electronic control and programming strategies. We also cover the various possible applications in linear matrix operations, quantum information processing and microwave photonics and examine how these generic chips can accelerate the development of future photonic circuits by providing a higher-level platform for prototyping novel optical functionalities without the need for custom chip fabrication.

Abstract:We use the coupling matrix formalism to investigate continuouswave and pulse propagation through microring coupled-resonator optical waveguides (CROWs). The dispersion relation agrees with that derived using the tight-binding model in the limit of weak inter-resonator coupling. We obtain an analytical expression for pulse propagation through a semi-infinite CROW in the case of weak coupling which fully accounts for the nonlinear dispersive characteristics. We also show that intensity of a pulse in a CROW is enhanced by a factor inversely proportional to the inter-resonator coupling. In finite CROWs, anomalous dispersions allows for a pulse to propagate with a negative group velocity such that the output pulse appears to emerge before the input as in "superluminal" propagation. The matrix formalism is a powerful approach for microring CROWs since it can be applied to structures and geometries for which analyses with the commonly used tight-binding approach are not applicable.

We address the trade-offs among delay, loss, and bandwidth in the design of coupled-resonator optical waveguide (CROW) delay lines. We begin by showing the convergence of the transfer matrix, tight-binding, and time domain formalisms in the theoretical analysis of CROWs. From the analytical formalisms we obtain simple, analytical expressions for the achievable delay, loss, bandwidth, and a figure of merit to be used to compare delay line performance. We compare CROW delay lines composed of ring resonators, toroid resonators, Fabry-Perot resonators, and photonic crystal defect cavities based on recent experimental results reported in the literature.

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We review and present additional results from our work on multilayer silicon nitride (SiN) on silicon-on-insulator (SOI) integrated photonic platforms over the telecommunication wavelength bands near 1550 and 1310 nm. SiN-on-SOI platforms open the possibility for passive optical functionalities implemented in the SiN layer to be combined with active functionalities in the SOI. SiN layers can be integrated onto SOI using a front-end or back-end of line integration process flow. These photonic platforms support low-loss SiN waveguides, low-loss and low-crosstalk waveguide crossings, and low-loss interlayer transitions using adiabatic tapers. Novel ultra-broadband and efficient grating couplers as well as polarization management devices are enabled by the close coupling between the silicon and SiN layers.

Ultra-compact waveguide electroabsorption optical switches and photodetectors with micron- and sub-micron lengths and compatible with silicon (Si) waveguides are demonstrated using the insulator-metal phase transition of vanadium dioxide (VO(2)). A 1 μm long hybrid Si-VO(2) device is shown to achieve a high extinction ratio of 12 dB and a competitive insertion loss of 5 dB over a broad bandwidth of 100 nm near λ = 1550 nm. The device, operated as a photodetector, can measure optical powers less than 1 μW with a responsivity in excess of 10 A/W. With volumes that are about 100 to 1000 times smaller than today's active Si photonic components, the hybrid Si-VO(2) devices show the feasibility of integrating transition metal oxides on Si photonic platforms for nanoscale electro-optic elements.

This document provides supplementary information to "Silicon photonic transmitter for polarization-encoded quantum key distribution," http://dx.doi.org/10.1364/optica.3.001274. Details on the refractive index and absorption change as well as the electroluminescence in the forward-biased silicon (Si) PIN diodes are described. © 2016 Optical Society of America http://dx.doi.org/10.1364/optica.3.001274.s001 REAL AND IMAGINARY REFRACTIVE INDEX CHANGE IN SI WAVEGUIDE PIN DIODESThe modulation of the refractive index in silicon (Si) is typically through the plasma dispersion effect. The carrier density changes both the real and imaginary parts of the refractive index, which can cause, for example, a reduced extinction ratio in an optical attenuator or modulator, and polarization dependent loss in a polarization controller. According to [1], the changes in the refractive index Δn and absorption Δα of Si near a wavelength of 1550 nm arewhere ΔN e and ΔN h are changes in the free electron density and free hole density measured in cm −3 . By incorporating both Δn and Δα into a mode-solver, the coupled changes in the real and imaginary parts of the effective index as a function of carrier density can be modelled. Experimentally, for the Si waveguide PIN diode with the cross-section illustrated in Fig. S1(a), which is similar to the one used in the present work, the measured phase-shift and attenuation as function of the applied forward bias voltage are shown in Fig. S1(b) and Fig. S1(c), respectively [2]. The length of the Si PIN diode used for these measurements was 500 μm. The measured differential phase-shift was about −7.3π/(mm · V) and the corresponding differential absorption change was about 20 dB /(mm · V). These figures have been reproduced from [2]. The Si PIN diodes we have used in the current work had P++ and N++ regions that were 700 nm away from the waveguide core compared to 800 nm in Fig. S1. The reduced separation would lead to a slightly lower series resistance. ELECTROLUMINESCENCE FROM SI PIN DIODESWe observed that the Si waveguide PIN diodes in forward bias could generate weak electroluminescence. Fig. S2(a) shows the electroluminescence spectrum of a 1000 μm-long PIN diode at several forward bias voltages. The electroluminescence is broadband and centered near a wavelength of 1150 nm, close to the bandgap energy of Si (1.1 eV = 1130 nm). This electroluminescence is not power efficient due to the indirect bandgap of Si. Fig. S2(b) shows the current vs. voltage relationship of the diode. Fig. S2(c) shows the total optical power collected

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