  Optical isolators are indispensible components in nearly all photonic systems
as they help ensure unidirectionality and provide crucial protection from
undesirable reflections. While commercial isolators are exclusively built on
magneto-optic (MO) principles they are not readily implemented within photonic
integrated circuits due to the need for specialized materials. Importantly, the
MO effect is generally weak, especially at shorter wavelengths. These
challenges as a whole have motivated extensive research on non-MO alternatives.
To date, however, no alternative technology has managed to simultaneously
combine linearity (i.e. no frequency shift), linear response (i.e. input-output
scaling), ultralow insertion loss, and large directional contrast on-chip. Here
we demonstrate an optical isolator design that leverages the unbeatable
transparency of a short, high quality dielectric waveguide, with the
near-perfect attenuation from a critically-coupled absorber. Our design concept
is implemented using a lithium niobate racetrack resonator in which phonon
mediated Autler-Townes splitting (ATS) breaks the chiral symmetry of the
resonant modes. We demonstrate on-chip optical isolators at wavelengths one
octave apart near 1550 nm and 780 nm, fabricated from the same lithium
niobate-on-insulator wafer. Linear optical isolation is demonstrated with
simultaneously <1 dB insertion loss, >39 dB contrast, and bandwidth as wide as
the optical mode that is used. Our results outperform the current best-in-class
MO isolator on-chip on both insertion loss and isolator figures-of-merit, and
demonstrate a lithographically defined wavelength adaptability that cannot yet
be achieved with any MO isolator.
