Fully Passive Integrated Optical NOT Gate Using a Phase-Apodized Spiral Bragg Grating

We report the first demonstration of a fully passive on-chip optical NOT gate based on a phase-only waveguide Bragg grating (WBG) implemented on a silicon-on-insulator (SOI) platform. The device consists of a 9-mm-long phase-apodized spiral WBG occupying only <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$0.25 ~\text{mm}^{2}$</tex>, designed to provide a narrowband <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\pi$</tex>-phase shift suitable for high-speed multilevel data inversion. The grating response is synthesized using an inverse layer peeling algorithm and realized through precise modulation of the corrugation spacing. Experimental characterization confirms excellent agreement between the measured and target spectral responses, with a phase peak bandwidth of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\sim 9 \text{GHz}$</tex>. Using this filter, we demonstrate passive inversion of 50-Gbps PAM-2 and 45-Gbaud PAM-3 data streams, representing the first integrated implementation of a WBG-based multilevel NOT gate. The device operates entirely passively, with energy consumption limited by insertion losses, and offers a scalable path toward compact, low-power, ultrafast optical logic circuits.