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An overview of the most recent developments and improvements to the low-loss TriPleX Si3N4 waveguide technology is presented in this paper. The TriPleX platform provides a suite of waveguide geometries (box, double stripe, symmetric single stripe, and asymmetric double stripe) that can be combined to design complex functional circuits, but more important are manufactured in a single monolithic process flow to create a compact photonic integrated circuit. All functionalities of the integrated circuit are constructed using standard basic building blocks, namely straight and bent waveguides, splitters/combiners and couplers, spot size converters, and phase tuning elements. The basic functionalities that have been realized are: ring resonators and Mach–Zehnder interferometer filters, tunable delay elements, and waveguide switches. Combination of these basic functionalities evolves into more complex functions such as higher order filters, beamforming networks, and fully programmable architectures. Introduction of the active InP chip platform in a combination with the TriPleX will introduce light generation, modulation, and detection to the low-loss platform. This hybrid integration strategy enables fabrication of tunable lasers, fully integrated filters, and optical beamforming networks.

The thirty-two papers in this special issue focus on the field of programmable photonics, one of the fastest growing fields in photonics with applications ranging from optical signal processing and computing to quantum photonics.

The last few years have seen major progress in harnessing on-chip photon-phonon interactions, leading to a wide range of demonstrations of new functionalities. Utilizing not only the optical response of a nonlinear waveguide - but also acoustic resonances - enables the realization of microwave devices with unprecedented performance, otherwise hard to achieve in all-optical processing schemes or electronically. Here, we overview on-chip stimulated Brillouin scattering (SBS) with special emphasis on microwave sources and microwave signal processing schemes. We review the different material platforms and structures for on-chip SBS, ranging from chalcogenide rib waveguides to hybrid silicon/silicon-nitride structures, high-Q photonic-phononic silica microresonators, and suspended silicon nanowires. We show that the paradigm shift in SBS research - from long length of fibers to chip-scale devices - is now moving toward fully integrated photonic-phononic CMOS chips.

High-resolution signal processing will be a key aspect in current and future communication networks in crowded spectral environments. Signal processing based on the nonlinear optical process known as stimulated Brillouin scattering (SBS) are applicable in high-performance analog signal filtering for radio frequency (RF) communications and in coherent optical communication systems for carrier amplification. To meet the requirements of small size, low weight, and low power, the development of chip-based devices is critical. In this paper, we present an overview of on-chip SBS demonstrations of RF bandpass and bandstop filter functionalities with large frequency tuning and reconfigurability from tens of MHz to almost 0.5 GHz, applicable in RF communications. We also present our recent results on chip-based SBS filtering for optical communications. While traditionally SBS has been deemed to be incompatible with phase-sensitive communications, these latest results highlight the viability of SBS-enhanced high-capacity digital communication networks. We finally discuss the limitations of SBS filtering and also present some strategies to overcome them.