Laser direct writing: Enabling monolithic and hybrid integrated solutions on the lithium niobate platform

Applications in life sciences and information technology require all-optical solutions. In the inevitable race towards miniaturized optical circuits, all-integrated solutions will prevail against bulk setups. Because of its outstanding nonlinear properties, lithium niobate (LiNbO3) emerged as the key platform for integrated optics. In this paper, we discuss the direct femtosecond (fs) laser inscription technique whose flexibility enables the realization of two- and three-dimensional embedded optical waveguides in various optical materials. Linear and nonlinear components for monolithic and hybrid waveguide devices are characterized and their perspectives are reviewed, e.g. couplers, Bragg reflectors, frequency converters, amplitude modulators and gain modules. Finally, we demonstrate a monolithic LiNbO3 waveguide chip that combines a frequency doubling and a modulating unit. Thumbnail image of Schematic of a hybrid fs laser written chip that comprises a rare-earth-doped laser section (a), a frequency doubling unit (b), Bragg reflectors (c), waveguide splitters (d) and an amplitude modulator (e).