High fiber-to-fiber net gain in erbium-doped thin film lithium niobate waveguide amplifier as an external gain chip

Miniaturized erbium-doped waveguide amplifiers attracted great interests in recent decades due to their high gain-effi-ciency and function-scalability in the telecom C-band. In this work, an erbium-doped thin film lithium niobate waveguide amplifier achieving >10 dB off-chip (fiber-to-fiber) net gain and >20 mW fiber-output amplified power is demonstrated, thanks to the low-propagation-loss waveguides and robust waveguide edge-couplers prepared by the photolithography assisted chemomechanical etching technique. Systematic investigation on the fabricated waveguide amplifiers reveals remarkable optical gain around the peak wavelength of 1532 nm as well as the low fiber-coupling loss of −1.2 dB/facet. A fiber Bragg-grating based waveguide laser is further demonstrated using the fabricated waveguide amplifier as the external gain chip, which generates >2 mW off-chip power continuous-wave lasing around the gain peak at 1532 nm. The un-ambiguous demonstration of fiber-to-fiber net gain of the erbium-doped thinfilm lithium niobate (TFLN) waveguide amplifier as well as its external gain chip application will benefit diverse fields demanding scalable gain elements with high-speed tunability.