  Lithium niobate on insulator (LNOI) is an emerging photonic platform with
great promises for future optical communications, nonlinear optics and
microwave photonics. An important integrated photonic building block, active
waveguide amplifiers, however, is still missing in the LNOI platform. Here we
report an efficient and compact waveguide amplifier based on erbium-doped LNOI
waveguides, realized by a sequence of erbium-doped crystal growth, ion slicing
and lithography-based waveguide fabrication. Using a compact 5-mm-long
waveguide, we demonstrate on-chip net gain of > 5 dB for 1530-nm signal light
with a relatively low pump power of 21 mW at 980 nm. The efficient LNOI
waveguide amplifiers could become an important fundamental element in future
lithium niobate photonic integrated circuits.
