Visible Spectral-Domain Optical Coherence Tomography for Photonic Integrated Circuits Characterization

TL;DR

This paper introduces a high-resolution, nondestructive spectral-domain optical coherence tomography method for characterizing visible photonic integrated circuits, enabling detailed diagnostics with single-port access.

Contribution

It adapts spectral-domain OCT for visible PICs, providing a practical, high-resolution diagnostic tool that surpasses previous infrared-based methods.

Findings

Achieved 8 um axial resolution in silicon nitride

Resolved multiple echoes in waveguide ring resonators

Demonstrated imaging of PIC-QMC transition regions

Abstract

Visible photonic integrated circuits underpin applications ranging from AR/VR to quantum control, yet lack a high-resolution, nondestructive diagnostic comparable to the optical frequency-domain reflectometry used in infrared silicon photonics. Here we adapt spectral-domain optical coherence tomography to measure guided-mode back-reflections in visible PICs. Broadband visible light injected into a circuit generates back-reflections that interfere with a depth-referencing local oscillator, and the resulting spectral fringes are recorded on a spectrometer. We validate the approach by resolving multiple round-trip echoes in a waveguide-coupled ring resonator using only single-port access. We then extend it to circuits integrated with diamond quantum micro-chiplets, clearly resolving input and output facets as well as PIC--QMC transition regions. The system achieves shot-noise-limited sensitivity, 50 dB dynamic range, 8 um axial resolution in silicon nitride, and a 2 mm imaging depth at 6 dB roll-off. SD-OCT therefore provides a practical, high-resolution diagnostic for visible PICs that uses a broadband probe source and requires only single-port optical access, enabling rapid characterization of propagation loss, backscattering, and dispersion.