J. Eur. Opt. Society-Rapid Publ. 2025, 21, 34 Ó The Author( s), published by EDP Sciences, 2025 https:// doi. org / 10.1051 / jeos / 2025030 Available online at: https:// jeos. edpsciences. org
Journal of the European Optical Society-Rapid Publications
RESEARCH ARTICLE High-efficiency broadband grating out-coupler for thick silicon waveguides with a tuned SiGe layer
Sidra Tul Muntaha 1, 2,*, Isaac Doughan 1, Matteo Cherchi 2, 3, a,
, Matthieu Roussey 1, and Timo Aalto 2
1 |
Center for Photonics Sciences, University of Eastern Finland, Yliopistokatu 7, PO Box 111, 80101, Joensuu, Finland |
2 |
VTT Technical Research Center of Finland, Tietotie 3, 02150 Espoo, Finland |
3 |
Xanadu Quantum Technologies, 24th fl, 777 Bay Street, Toronto, ON M5B 2H7, Canada |
Received 25 April 2025 / Accepted 12 June 2025
Abstract. We design a novel grating out-coupler while maintaining a Gaussian-shaped far-field output. The approach is based on the patterning of a grating in a SiGe layer on the top surface of a 3-lm-thick Si waveguide. The grating aims at coupling the guided signal into a beam vertically out from the surface of the chip, i. e., outcoupling. The use of a high refractive index alloy, i. e., silicon-germanium, allows a tunability of the effective index of the guided mode and to modify the field distribution enabling a higher efficiency of the grating. It yields a theoretical efficiency of 75 % at the central wavelength of 1583 nm. The overall length of this grating coupler is 500 lm, and the output light is detected at 1 lm above the grating. This type of long grating coupler is intended to be eventually used as sensor in LiDAR( Light detection and ranging) applications.
Keywords: Silicon germanium, Grating out-coupler, Long device, Broad-band, LiDAR applications.
1 Introduction
Silicon photonics has been in the spotlight for many years owing to the revolution it has undergone. This platform enabled to perform, on-a-chip, nonlinear optics [ 1 ], high speed optical communications [ 2 ], mid-infrared devices [ 3 ], photonics crystals [ 4 ], optoelectronic integrated circuits [ 5 ], and more recently, LiDAR( light detection and ranging) sensors [ 6 ]. Silicon-on-insulator platforms are proven to play a significant role in next-generation optical interconnects as it already shows [ 7 – 9 ]. Beam steering mechanism constitutes an important part of LiDAR systems. In our previous publication [ 10 ], optical phased arrays have been demonstrated successfully to carry out non-mechanical one-dimensional( 1D) beam steering. However, twodimensional( 2D) beam steering and, ultimately, angle scanning, demand extensive device geometry development and more advanced steering mechanisms.
Despite the numerous high-performance optical components available on silicon-on-insulator( SOI) platform, in and out light coupling to and from silicon chips remain a challenge. The issue of the large modal mismatch between the waveguide and single-mode fibre, mostly due to the
a The affiliation of this author has changed during the publication process. He was affiliated to VTT Technical Research Center of Finland while working on this paper. * Corresponding author: sidra. muntaha @ photonics. fi refractive index difference, is met with mainly two solutions, i. e., edge coupling and vertical grating couplers [ 11 – 13 ].
Considering a common edge out-coupling in silicon waveguide, the output beam signal remains relatively well collimated in the horizontal direction, but it tends to quickly diverge vertically, because the waveguide is thin. It yields additional optical component, e. g., lensed, to collimate the output beam, which is one requirement in LiDARs. An alternative approach is to use a grating outcoupler, i. e., patterning a grating on the surface of the waveguide. It allows the beam to propagate out of the waveguide, eventually perpendicularly to the surface of the chip. Such a vertical grating out-coupler offers a flexibility in terms of measurement techniques and require less post-processing as compared to the edge couplers [ 14 ], which is an advantage for the development of portable devices. Researchers have been working on the grating on micron-scale waveguides for single-mode fibre coupling [ 15 ] and have demonstrated experimentally a grating coupler in 1.5 lm thick SOI rib waveguides with a 3 dB bandwidth. Apart from the fundamental mode, higher number of modes can also couple to these thicker waveguides. A diffraction grating operating in reflection has also been investigated [ 3 ] to allow beam steering in the vertical direction by tuning the wavelength of the light source.
In addition to the aforementioned difficulties, the size of the beam is one of the biggest challenges for LiDAR applications. Creating a large beam(> 100 lm size) is more
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