Dual Mode Mid-Infrared Chemical Sensor Using Bragg Wavelength in Subwavelength Grating Incorporated Broadband Directional Coupler

In this paper, we present a broadband directional coupler (DC) based dual mode chemical sensor for the mid-infrared (MIR). For the first time, we propose chemical sensing based on refractive index (RI) change in a compact DC by incorporating a subwavelength grating (SWG) structure into conventional DC that introduces a sharp Bragg wavelength. Meanwhile, the SWG structure can also engineer the dispersion in the DC to achieve broadband operation so that the device can operate in a wide wavelength range that benefits spectrum analysis in the MIR. Fivefold enhancement in the operation bandwidth compared to conventional DCs has been achieved for 100% coupling efficiency experimentally. The sensing capability of the device for dichloromethane (CH
₂
Cl
₂
) detection in ethanol (C
₂
H
₅
OH) is investigated by simulation. A sensitivity of -0.47% self-normalized transmitted power change per percentage of CH
₂
Cl
₂
concentration is revealed in the RI sensing mode while 0.12% change in the total integrated output power is realized in the absorption sensing mode.