  The ability to amplify optical signals is of pivotal importance across
science and technology. The development of optical amplifiers has
revolutionized optical communications, which are today pervasively used in
virtually all sensing and communication applications of coherent laser sources.
In the telecommunication bands, optical amplifiers typically utilize gain media
based on III-V semiconductors or rare-earth-doped fibers. Another way to
amplify optical signals is to utilize the Kerr nonlinearity of optical fibers
or waveguides via parametric processes. Such parametric amplifiers of
travelling continuous wave have been originally developed in the microwave
domain, and enable quantum-limited signal amplification with high peak gain,
broadband gain spectrum tailored via dispersion control, and ability to enable
phase sensitive amplification. Despite these advantages, optical amplifiers
based on parametric gain have proven impractical in silica fibers due to the
low Kerr nonlinearity. Recent advances in photonic integrated circuits have
revived interest in parametric amplifiers due to the significantly increased
nonlinearity in various integrated platforms. Yet, despite major progress,
continuous-wave-pumped parametric amplifiers built on photonic chips have to
date remained out of reach. Here we demonstrate a chip-based travelling-wave
optical parametric amplifier with net signal gain in the continuous-wave
regime. Using ultralow-loss, dispersion-engineered, meter-long, silicon nitride
photonic integrated circuits that are tightly coiled on a photonic chip, we
achieve a continuous parametric gain of 12 dB that exceeds both the on-chip
optical propagation loss and fiber-chip-fiber coupling losses in the optical
C-band.
