Fabrication-Tolerant CWDM Filter Based on Cascaded Mach–Zehnder Interferometers With Low Phase Error Waveguide Bend

We demonstrate a low phase error waveguide bend designed through the use of adjoint shape optimization. This bend features low loss and crosstalk, and its 1.8 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula>m width and 4 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula>m radius reduce phase error caused by width deviations to about one-sixth of that in conventional arc bend. Utilizing this bend, we developed a cascaded MZI-based coarse wavelength division multiplexing (CWDM) filter with high tolerance to random fabrication variation of waveguide width. The spectral shift of the filter is about five times smaller than that of conventional filter under both global and local width deviations. Additionally, due to the small bend radius, we achieved a compact total device footprint of 0.01 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\text{mm}^{2}$</tex-math></inline-formula>. The filter was then fabricated using a 193-nm dry lithography process. Experimental results show that the filter exhibits a loss of 2.34 dB and crosstalk of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mathbf{-}$</tex-math></inline-formula>25.85 dB. Measured spectra from different die across the wafer are highly consistent, with spectral shifts of only 3.67 nm due to deviations in waveguide thickness. Our design strategy offers a solution for the application of phase-sensitive devices.