  Silicon photonics has developed into a mainstream technology driven by
advances in optical communications. The current generation has led to a
proliferation of integrated photonic devices from thousands to millions -
mainly in the form of communication transceivers for data centers. Products in
many exciting applications, such as sensing and computing, are around the
corner. What will it take to increase the proliferation of silicon photonics
from millions to billions of units shipped? What will the next generation of
silicon photonics look like? What are the common threads in the integration and
fabrication bottlenecks that silicon photonic applications face, and which
emerging technologies can solve them? This perspective article is an attempt to
answer such questions. We chart the generational trends in silicon photonics
technology, drawing parallels from the generational definitions of CMOS
technology. We identify the crucial challenges that must be solved to make
giant strides in CMOS-foundry-compatible devices, circuits, integration, and
packaging. We identify challenges critical to the next generation of systems
and applications - in communication, signal processing, and sensing. By
identifying and summarizing such challenges and opportunities, we aim to
stimulate further research on devices, circuits, and systems for the silicon
photonics ecosystem.
