On-chip Bragg grating waveguides and Fabry-Perot resonators for long-wave infrared operation up to 8.4 µm.

Qiankun Liu,David Bouville,Carlos Alonso-Ramos,Daniel Benedikovic,Enrico Talamas Simola,Vladyslav Vakarin,Laurent Vivien,Jacopo Frigerio,Xavier Le Roux,Andrea Ballabio,Delphine Marris-Morini,Giovanni Isella,Joan Manel Ramirez

doi:10.1364/oe.26.034366

Abstract

Taking advantage of unique molecular absorption lines in the mid-infrared fingerprint region and of the atmosphere transparency window (3-5 µm and 8-14 µm), mid-infrared silicon photonics has attracted more research activities with a great potential for applications in different areas, including spectroscopy, remote sensing, free-space communication and many others. However, the demonstration of resonant structures operating at long-wave infrared wavelengths still remains challenging. Here, we demonstrate Bragg grating-based Fabry-Perot resonators based on Ge-rich SiGe waveguides with broadband operation in the mid-infrared. Bragg grating waveguides are investigated first at different wavelengths from 5.4 µm up to 8.4 µm, showing a rejection band up to 21 dB. Integrated Fabry-Perot resonators are then demonstrated for the first time in the 8 µm-wavelength range, showing Q-factors as high as 2200. This first demonstration of integrated mid-infrared Fabry-Perot resonators paves the way towards resonance-enhanced sensing circuits and non-linear based devices at these wavelengths.

Highlights

In the recent years, mid-infrared (MIR) silicon photonics is attracting a lot of attention [1,2,3]
Taking advantage of the unique molecular absorption lines in the MIR range [4,5], silicon photonics has been proposed as a convincing solution for the development of highperformance and cost-effective MIR integrated sensors
We demonstrated Bragg gratings waveguides and Bragg-grating-based FabryPerot resonators operating in the long-wave MIR region

Summary

Introduction

Mid-infrared (MIR) silicon photonics is attracting a lot of attention [1,2,3]. Germanium (Ge) and Silicon Germanium (SiGe) alloys are strong candidates for extending the operation wavelength of silicon photonics in the mid-IR [10,11,12,13,14,15,16] In this context Ge-rich SiGe graded index waveguides have been recently demonstrated as a promising platform benefiting from the wide transparency window of Ge to achieve deep-MIR operation, beyond 8 μm [17,18,19,20]. The waveguide etching depth (DWG) is 4 μm and the width (WWG) is 5 μm, which provide high optical confinement of the guided mode in the Ge-rich region over a wide range of MIR wavelengths [18]. According to the numerical calculations, an optimized ratio of 70% was obtained to maximize the coupling efficiency, corresponding to WEtch = 3.5 μm Such optimized performance of the device coupling efficiency is attributed to an improved overlap factor between the optical mode and the grating. The minimum transmittance has been calculated as a function of the number of periods and compared to experiments in Fig. 2(d), obtaining comparable trends

Fabry-Perot resonators

Conclusion