Abstract

Lithium niobate (LiNbO3) has proven to be one of the most important nonlinear optical materials and is widely used in commercial applications such as electro-optic modulators. Recently, there has been growing interest in the development of periodically-poled lithium niobate (PPLN) and thin-film lithium niobate waveguides for high-power frequency conversion and integrated photonics, respectively [1]. Our own PPLN ridge waveguides have achieved high frequency conversion of 70 % [2] with output power of up to 2.5 W [3]. The ability to fabricate accurate gratings on these waveguides is highly desirable for adding functionality such as grating couplers, filters, and cavities. Kores et al. reported that fifth-harmonic pulsed 213 nm lasers can be used to ablate gratings in lithium niobate on insulator (LNOI) to produce grating couplers [4]. They used two-beam holography to form a 245 µm × 72 µm fixed exposure area achieving up to 130 nm deep ablation of 685 nm period gratings in 20 µm LNOI ridge waveguides. They also fabricated shorter 360 nm period gratings, but these required post-exposure HF etching.

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