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Title: Wavelength-flattened directional coupler based mid-infrared chemical sensor using bragg wavelength in subwavelength grating structure
Authors: Dong, Bowei
Hu, Ting
Luo, Xianshu
Chang, Yuhua
Guo, Xin
Wang, Hong
Kwong, Dim-Lee
Lo, Guo-Qiang
Lee, Chengkuo
Keywords: Photonics Sensors
DRNTU::Engineering::Electrical and electronic engineering
Issue Date: 2018
Source: Dong, B., Hu, T., Luo, X., Chang, Y., Guo, X., Wang, H., Kwong, D.-L., et al. (2018). Wavelength-Flattened Directional Coupler Based Mid-Infrared Chemical Sensor Using Bragg Wavelength in Subwavelength Grating Structure. Nanomaterials, 8(11), 893-. doi:10.3390/nano8110893
Series/Report no.: Nanomaterials
Abstract: In this paper, we report a compact wavelength-flattened directional coupler (WFDC) based chemical sensor featuring an incorporated subwavelength grating (SWG) structure for the mid-infrared (MIR). By incorporating a SWG structure into directional coupler (DC), the dispersion in DC can be engineered to allow broadband operation which is advantageous to extract spectroscopic information for MIR sensing analysis. Meanwhile, the Bragg reflection introduced by the SWG structure produces a sharp trough at the Bragg wavelength. This sharp trough is sensitive to the surrounding refractive index (RI) change caused by the existence of analytes. Therefore, high sensitivity can be achieved in a small footprint. Around fivefold enhancement in the operation bandwidth compared to conventional DC is achieved for 100% coupling efficiency in a 40 µm long WFDC experimentally. Detection of dichloromethane (CH2Cl2) in ethanol (C2H5OH) is investigated in a SWG-based WFDC sensor 136.8 µm long. Sensing performance is studied by 3D finite-difference time domain (FDTD) simulation while sensitivity is derived by computation. Both RI sensing and absorption sensing are examined. RI sensing reveals a sensitivity of −0.47% self-normalized transmitted power change per percentage of CH2Cl2 concentration while 0.12% change in the normalized total integrated output power is realized in the absorption sensing. As the first demonstration of the DC based sensor in the MIR, our device has the potential for tertiary mixture sensing by utilizing both changes in the real and imaginary part of RI. It can also be used as a broadband building block for MIR application such as spectroscopic sensing system.
ISSN: 2079-4991
DOI: 10.3390/nano8110893
Schools: School of Electrical and Electronic Engineering 
Rights: © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (
Fulltext Permission: open
Fulltext Availability: With Fulltext
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