Holographically fabricated 2D grating coupler for integrated large beam delivery

Abstract

In integrated optics, etched grating couplers are commonly used to generate free space beams and interact locally with optical systems [1]. Etched grating and waveguide couplers possess small-scale inconsistencies [2], often intensified by the high index contrast of these devices, which can lead to effective index variations and result in phase errors across the fabricated devices. These typically restrict the size of successful devices to a few-millimeter length. Here, we employ a holographic UV writing approach to fabricate a 2D grating coupler in doped planar silica-on-silicon to provide a large beam into free space. We show the potential to fabricate fiber-coupled compact devices to deliver IR beams for atoms/ion trap applications [3].
A direct UV writing technique using a pulsed 213 nm laser [4] was employed to fabricate a grating coupler in a hydrogen-loaded planar device, consisting of: silicon substrate, SiO₂ thermal oxide underclad, Ge doped photosensitive silica core, and phosphorous/boron doped overclad. The device was fabricated in two writing phases. First, we fabricated a channel waveguide containing in-plane 60° blazed gratings (6-mm long) to create an expanded beam within the planar layer. Details of the fabrication technique are available in [5]. Later, a modified interferometer (shown in Fig. 1(a and b)) was employed to inscribe out-of-plane 45° blazed gratings [6] using a prism coupling approach. Fig1. (c) illustrates the schematic of a 2D grating coupler which consists of in-plane 60° blazed gratings and 45° out-of-plane blazed gratings. Fig. 1(d) shows a photograph of the fabricated 2D grating coupler mounted on the characterization setup. A fiber V-groove assembly was used to launch 780 nm light into the waveguide containing the 60° in-plane blazed gratings. These produced an expanded beam within the core silica layer. This wider beam was then reflected perpendicular to the core layer by the 45° out-of-plane blazed gratings (4.4 × 3.9 mm) coupling to free space. We will present details of the fabrication approach and results from our latest devices, including the efficiency of the fabricated device.

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Identifiers

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

ORCID for Peter Horak: orcid.org/0000-0002-8710-8764

ORCID for Christopher Holmes: orcid.org/0000-0001-9021-3760

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

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

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

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

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