Compact and Ultra-Low-Loss Inline Optical Power Monitor Based on Multimode Interference for Silicon Photonic Integrated Circuits

Abstract

We propose and experimentally demonstrate an ultrasmall, low-loss inline photodetector based on multimode interference in a silicon multimode waveguide. Periodic self-focusing enables heavily doped electrodes to be placed close to the waveguide with negligible loss. The detector provides <0.1 dB insertion loss over a wide wavelength range with a total length of 4.7 μm. Owing to photoconductive operation, short electrode spacing, and a photoconductive gain of approximately 340, a sensitivity of 75 mV/mW is achieved with a few microwatts of electrical power consumption. Integration into a silicon ring resonator confirms accurate monitoring of resonant transmission, with enhanced sensitivity of 234 mV/mW at resonance. These results establish a compact, low-power monitor suitable for dense photonic integrated circuits.