On-Chip Optical Power Limiter for Quantum Communications

This article presents an on-chip optical power limiter that utilizes the thermo-optical defocusing effect. A pair of input and output waveguides is designed to mimic emitting and receiving antennas. The waveguides are separated by a free-space region filled with poly-methyl-meth-acrylate (PMMA) material, which has a negative thermal-optic coefficient that causes a decrease in refractive index with an increase in temperature. As the power in the input waveguide increases, the refractive index of the free-space region decreases, which in turn increases the radiated beam's divergence angle with respect to input power. The empirical findings demonstrate that the non-linear divergence angle can be written as theta 0+kP${\theta }_0 + kP$, where theta 0 represents the divergence angle of the equivalent Gaussian beam, k is a waveguide-specific constant, and P is the input power. The edge of the receiving waveguide is tapered to adjust the coupling of the divergent beam to the output waveguide. The taper width is optimized to minimize insertion loss. The devices are two orders lengthwise smaller compared to the bulk demonstration, and they exhibit low loss ranging from 0.2 to 10 dB. Photonic integrated circuits (PICs), valued for miniaturization, cost-effectiveness, and energy efficiency, are explored for secure communication. This article focuses on on-chip optical power limiters (OPLs) utilizing a thermo-optic defocusing effect in PMMA. Crucial for thwarting Trojan-Horse Attacks, these OPLs boast low losses and compact dimensions, holding promise to enhance security and efficiency in quantum communication systems.image