DUV laser written gratings on zinc doped lithium niobate waveguides

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.
We report initial results on using our 213 nm small spot writing technique to fabricate gratings on PPLN waveguides. Small spot writing is a two-beam holographic method in which each beam is focused to a single 14 μm spot with an EOM in one beam to control the phase of the interference pattern at the focus, as shown in Fig. 1b. This enables the writing of structures with near-arbitrary gratings [5]. The PPLN used is a magnesium-doped wafer with a layer of zinc diffused into the surface to form planar waveguides; this is then ductile diced to form single-mode ridge waveguides with a mode that is well-matched to standard fiber. Using 4 mW average power at 12.5 kHz, the 533 nm period gratings, shown in Fig. 1b, were fabricated over a range of total fluences from 30 mJ cm⁻² to 50 kJ cm⁻². Surface damage was observed at fluences over 6 kJ cm⁻². Using a fluence of 0.5 kJ cm⁻², 40 mm gratings were written along the length of the 10 μm wide PPLN waveguides with no damage to the waveguide found.
We will report on further metrology and optical characterisation of our gratings, as well as gratings at different periods, and more complex apodised gratings.

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Identifiers

ORCID for Qazi Ahmed: orcid.org/0000-0003-0731-5390

ORCID for Paul C. Gow: orcid.org/0000-0002-3247-9082

ORCID for James C. Gates: orcid.org/0000-0001-8671-5987

ORCID for Peter G.R. Smith: orcid.org/0000-0003-0319-718X

ORCID for Corin Gawith: orcid.org/0000-0002-3502-3558

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