  As a fundamental optical approach to interferometry, Sagnac interference has
been widely used for reflection manipulation, precision measurements, and
spectral engineering in optical systems. Compared to other interferometry
configurations, it offers attractive advantages by yielding a reduced system
complexity without the need for phase control between different pathways, thus
offering a high degree of stability against external disturbance and a low
wavelength dependence. The advance of integration fabrication techniques has
enabled chip-scale Sagnac interferometers with greatly reduced footprint and
improved scalability compared to more conventional approaches implemented by
spatial light or optical fiber devices. This facilitates a variety of
integrated photonic devices with bidirectional light propagation, showing new
features and capabilities compared to unidirectional-light-propagation devices
such as Mach-Zehnder interferometers (MZIs) and ring resonators (RRs). Here, we
present our latest results for functional integrated photonic devices based on
Sagnac interference. We outline the theory of integrated Sagnac interference
devices with comparisons to other integrated photonic building blocks such as
MZIs, RRs, photonic crystal cavities, and Bragg gratings. We present our latest
results for Sagnac interference devices realized in integrated photonic chips,
including reflection mirrors, optical gyroscopes, basic filters, wavelength
(de)interleavers, and optical analogues of quantum physics.
