  Light source is indispensable component in on-chip system. Compared with
hybrid or heterogeneous integrated laser, monolithically integrated laser is
more suitable for high density photonic integrated circuit (PIC) since the
capability of large-scale manufacturing, lower active-passive coupling loss and
less test complexity. Recent years have seen the spark of researches on
rare-earth ion doped thin film lithium niobate (REI:TFLN), demonstrations have
been made both in classical and quantum chips. However, low output power and
limited quantum emitting efficiency hinder the application of the chip-scale
laser source based on REI:TFLN. Here a highly efficient integrated laser
assisted by cascaded amplifiers is proposed and experimentally prepared on
Erbium-doped TFLN. A slope efficiency of 0.43% and a linewidth of 47.86 kHz are
obtained. The maximum integrated laser power is 7.989 {\mu}W. Our results show
a viable solution to improve efficiency by self-amplification without changing
the intrinsic quantum emitting efficiency of the material, and our design has
potential application in incorporating with functional devices such as optical
communications, integrated quantum memory and quantum emission.
