Broadband and Low Crosstalk Polarization- Insensitive Waveguide Crossing Using Subwavelength Structure

A novel polarization-insensitive waveguide crossing with broad bandwidth and low crosstalk has been demonstrated using two-dimensional periodic subwavelength structure arrays on silicon-on-insulator (SOI) platform. The duty cycle of the sub-wavelength structures in the taper and crossing regions has been optimized to create a higher refractive index in the center, with lower indices on either side along the propagation direction. This tailored refractive index profile effectively confines both the TE<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> and TM<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> modes to the center of the taper and crossing regions, facilitating broadband transmission with low crosstalk (CT) for both modes. The proposed crossing features a compact footprint of 10.4 × 10.4 μm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup>. Simulation results indicate an insertion loss (IL) of 0.19 dB for the TE<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> mode and 0.27 dB for the TM<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> mode at the wavelength of 1550 nm. Both modes maintain a broad bandwidth from 1400 nm to 1700 nm, with ILs below 1.01 dB (0.53 dB) and CT levels below <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ - $</tex-math></inline-formula>38 dB (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ - $</tex-math></inline-formula>40 dB) for the TE<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> (TM<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub>) mode. The crossing structure is straightforward and can be fabricated in a single etching step. The device was fabricated, and measurement were conducted over a wavelength range of 1500 nm to 1600 nm. However, due to the limitations in the grating coupler's bandwidth and measurement equipment, other bands could not be assessed. The measured results show ILs below 0.78 dB (0.43 dB) and CT below <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ - $</tex-math></inline-formula>31 dB (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ - $</tex-math></inline-formula>33 dB) for the TE<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> and TM<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> modes over the measured wavelength range. This outstanding performance makes it well-suited for on-chip optical interconnection applications.