Coupling control in a two-step nano-pixel polarization rotator for enhanced breath sensing

As the global population ages more rapidly due to improved life expectancy, early disease detection is becoming more important to maintain health. We focus on a compact breath sensor that uses infrared photonic circuits and cavity ring-down spectroscopy to identify disease markers. To measure gas concentrations at ppm-order levels, the sensor relies on an amplifier-assisted and loop-based optical system, but unwanted wavelength peaks can interfere with detection. A polarization rotator is needed to filter out these signals, yet most existing devices cannot be easily integrated into small-scale photonic circuits. We designed a nano-pixel waveguide polarization rotator that converts the light polarization in two steps: first, converting TE00 to TE10, then transforming TE10 to TM00. Our previous work showed that although high quality of intermediate mode TE1 is achieved, the PR does not satisfy our 10 dB requirement for efficient breath sensing. In this study, we improved this ratio by refining the coupling efficiency between the two conversion sections. Employing 3D-FDTD simulations, we achieved a compact 4.56×0.96 μm polarization rotator, improving the polarization extinction ratio (PER) value from 8 dB to a potential 35 dB PER at the working wavelength 1550 nm.