  Chip-based optical amplifiers can significantly expand the functionalities of
photonic devices. In particular, optical-parametric amplifiers (OPAs), with
engineerable gain-spectra, are well-suited for nonlinear-photonic applications.
Chip-based OPAs typically require long waveguides that occupy a large
footprint, and high pump powers that cannot be easily produced with chip-scale
lasers. We theoretically and experimentally demonstrate a
microresonator-assisted regenerative OPA that benefits from the large
nonlinearity enhancement of microresonators and yields a high gain in a small
footprint. We achieve 30-dB parametric gain with only 9 mW of cw-pump power and
show that the gain spectrum can be engineered to cover telecom channels
inaccessible with Er-based amplifiers. We further demonstrate the amplification
of Kerr-soliton comb lines and the preservation of their phase properties.
Additionally, we demonstrate amplification by injection locking of
optical-parametric oscillators, which corresponds to a regenerative amplifier
pumped above the oscillation threshold. Novel dispersion engineering techniques
such as coupled cavities and higher-order-dispersion phase matching can further
extend the tunability and spectral coverage of our amplification schemes. The
combination of high gain, small footprint, low pump power, and flexible
gain-spectra engineering of our regenerative OPA is ideal for amplifying
signals from the nanowatt to microwatt regimes for portable or space-based
devices where ultralow electrical power levels are required and can lead to
important applications in on-chip optical- and microwave-frequency synthesis
and precise timekeeping.
