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

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.

Figures (3)

• Figure 1: Overview of OCT for PICs inspection. (a) CAD rendering of the optical setup: a fibre-based Michelson interferometer delivers the combined sample--reference field to a commercial spectrometer. Interference with the reference arm amplifies weak backscattering from PICs---often treated as a nuisance---thereby revealing circuit loss, dispersion and coupling losses across architectures that are otherwise difficult to diagnose. (b) Representative inspection contexts and length scales for visible-PIC OCT: a 4-inch wafer containing many millimetre- to centimetre-scale devices (top), and a heterogeneously integrated QMC-on-PIC socket (bottom). In practice, the OCT probe in (a) can be scanned across intact wafers or diced chips before or after dicing. The shown diamond QMC on PIC is about $\sim 100\,\mu\mathrm{m}\times 30\,\mu\mathrm{m}$. (c) Calibration of axial resolution and dynamic range. (d) Imaging depth and spectral roll-off. (e) Detection sensitivity with shot-noise-limited performance per CCD pixel.
• Figure 2: (a) OCT reconstruction reveals that a short input pulse yields through-port echoes spaced by the round-trip time $T_{\mathrm{rt}}$; a grating-coupler back-reflection re-excites the ring on the return path, producing a delayed echo train. The detected amplitude decay captures the total loss in the ring resonator. (b) FDTD simulations of two simple waveguides with different widths. (c) The red curve shows echo broadening due to group-velocity dispersion. The cyan curve shows restoration of the broadened pulse to its intrinsic pulse width, $\tau = 53\,\mathrm{fs}$ (equivalent to $16\,\mu\mathrm{m}$ axial resolution), using a two-step numerical dispersion-compensation procedure. The model derivation is provided in the SI.
• Figure 3: (a) Top-view scattering image of the PIC, with five B-scan profiles on the right corresponding to the labeled depth locations within the circuit. (b) Ratio of back-reflection strengths from the input and output facets for a QMC channel compared with a reference loop. (c) The averaged roll-off corrected A-scan profiles of the QMC channel and the reference loop.