A Wavelength-Insensitive All-Silicon In-Line Optical Power Monitor for Multimode Silicon Photonics

We demonstrate an in-line all-silicon optical power monitor at near-infrared wavelengths which is able to identify the optical powers of different modes multiplexed in a silicon multimode waveguide. The device is a 12-μm-wide slab waveguide embedded with three PN junctions which can produce photocurrents through various sub-bandgap absorption mechanisms. The positions of these PN junctions are well engineered so that their photocurrents in response to the hybrid mode are distinctly different. A theoretical model is built to describe the nonlinear relationship between the optical powers of multiplexed modes and the photocurrents of different PN junctions. Based on the model, the fabricated device can simultaneously extract the powers of multiplexed TE₀, TE₁ and TE₂ modes. Specifically, the minimum detectable powers for TE₀/TE₁/TE₂ modes are 2.22/1.83/1.17μW, respectively, while the absolute percentage measurement errors are less than 8.9%/5.6%/6.1%. As the wavelength sweeps from 1500 nm to 1570 nm, the variation ranges of the photocurrents are less than 5%. Furthermore, we study two scenarios in which the linear approximation can be used to accelerate the optical powers calculation speed, i.e., the low-power mode in which the nonlinear photocurrent component is neglectable, and the small-signal mode in which the fluctuation amplitudes of optical powers are very weak. The absolute percentage measurement errors for TE₀/TE₁/TE₂ modes are less than 12.8%/11.5%/21.9% and 4.8%/4.5%/2.5% in these two scenarios.