Non-invasive on-chip light observation by contactless waveguide conductivity monitoring

Photonic technologies lack non-invasive monitoring tools to inspect the light inside optical waveguides. This is one of the main barriers to large scale integration, even though photonic platforms are potentially ready to host thousands of elements on a single chip. Here,we demonstrate non-invasive light observation in silicon photonics devices by exploiting photon interaction with intra-gap energy states localized at the waveguide surface. Light intensity is monitored by measuring the electric conductance of the silicon core through a capacitive access to the waveguide. The electric contacts are located at suitable distance from the waveguide core, thus introducing no measurable extra-photon absorption and a phase perturbation as low as 0.2 mrad, comparable to thermal fluctuations below 3 mK. Light monitoring with a sensitivity of -30 dBm and a dynamic range of 40 dB is demonstrated in waveguides and high-Q resonators, and for the tuning of coupled-resonator optical filters. This approach realizes a ContactLess Integrated Photonic Probe (CLIPP), that is simple, inherently CMOS compatible, noninvasive and scalable to hundreds of probing points per chip. The CLIPP concept provides a viable route to real-time conditioning and feedback control of densely-integrated photonic systems.