Extended Data Fig. 3: Cross-polarized back-focal-plane (BFP) imaging techniques for a grating-coupled L3 cavity.

Two orthogonally polarized far-field profiles are imaged by orienting the input polarization E_in at a + 45^∘ (a) or − 45^∘ (b) angle from the dominant cavity polarization axis (dashed line in inset). The complete cavity emission profile \(S(\overrightarrow{k})\) can be reconstructed by summing both images (c) or approximated from a single polarized image (d), yielding near-identical images with quantitative agreement between the extracted η₀.