Modal decomposition in helical side-core fibers through controlled excitation

Modal decomposition in helical side-core fibers (HSCFs) is challenging due to the helical geometry and coupling between the central and helical wound side cores, which creates a rich spectrum of supermodes beyond simple individual core modes. Here, we introduce a controlled excitation methodology utilizing spatially and spectrally resolved (S²) imaging enhanced with principal component analysis (PCA) to separate and characterize competing modal interference mechanisms. Our experiments reveal two distinct, launch-dependent mode pair interferences: single-mode fiber splicing excites intra-core guided-mode interference, while free-space coupling excites inter-core supermode interference, characterized by a significantly larger group delay. We further show that this supermode excitation is critically dependent on the inter-core phase-matching condition, a finding confirmed by control experiments on a phase-mismatched fiber where the effect is suppressed. Leveraging this validated mode identification, we report mode-resolved propagation losses for left-circularly polarized (LCP) modes: 0.079 ± 0.005 dB/m for the fundamental mode (HE₁₁⁺) and 0.267 ± 0.015 dB/m for the first higher-order mode (HE₂₁⁺). This work establishes a systematic framework for characterizing complex helical fibers, providing essential design parameters for high-power fiber lasers and sensing applications.