Influence of Lamé-type cladding geometry on the connector-compatible diameter in 4-core MCF

Lamé-shaped cladding in multicore fibers causes a systematic difference between ellipse-based and connector-relevant maximum diameters. Measurements of 47 manufactured 4-core fibers showed that the ellipse approximation underestimated the maximum diameter by 0.287 µm on average and up to 0.871 µm. Bootstrap analysis of the observed distribution gave a maximum false-positive insertion probability of 38.3%. A conservative Marcuse coupling analysis further showed that, at 0.654-µm diametral clearance, the fractions satisfying both insertion and 0.35-dB loss criteria were 89.4% for ellipse-based screening and 70.2% for Lamé-aware screening. Thus, cladding shape affects both ferrule insertability and the optical alignment margin in multifiber interfaces.