  Microring resonators made from silicon are becoming a popular microscale
device format for generating photon pairs at telecommunications wavelengths at
room temperature. In compact devices with a footprint less than $5\times
10^{-4}$ mm$^2$, we demonstrate pair generation using only a few microwatts of
average pump power. We discuss the role played by important parameters such as
the loss, group-velocity dispersion and the ring-waveguide coupling coefficient
in finding the optimum operating point for silicon microring pair generation.
Silicon photonics can be fabricated using deep ultraviolet lithography
wafer-scale fabrication processes, which is scalable and cost-effective. Such
small devices and low pump power requirements, and the side-coupled waveguide
geometry which uses an integrated waveguide, could be beneficial for future
scaled-up architectures where many pair-generation devices are required on the
same chip.
