Fusion splicing
Joining two fibers permanently by aligning their ends and melting them together with an electric arc. The lowest-loss, lowest-reflection fiber joint, and the default for any connection that never needs to be undone.
A fusion splice welds two cleaved fiber ends into a continuous strand: the splicer aligns the fibers, fires a calibrated arc that softens the glass, feeds the ends together, and the joint solidifies as effectively one fiber. Done properly, the splice point has no interface. That is precisely its advantage over any connector.
Typical performance for single-mode fiber: 0.02–0.05 dB loss (0.1 dB is a rejection threshold in most practice) and no measurable back-reflection (return loss beyond 60–70 dB, since there is no index step to reflect from). This reflection-free property matters as much as the low loss: links dense with connectors accumulate both loss and feedback risk, while spliced plant is optically quiet.
Loss, when it appears, comes from geometry: lateral core offset (the dominant term: a 0.5 µm offset against a ~10 µm mode field already costs ~0.05 dB), angular misalignment from poor cleaves, core deformation from wrong arc parameters, and, between dissimilar fibers, mode-field-diameter mismatch, which sets a floor no splicer can align away (SMF to high-NA specialty fiber routinely costs a few tenths of a dB; thermal core expansion during a long arc can partially bridge it).
Two splicer families: core-alignment machines image the cores and align them actively (the standard for low-loss and specialty work); cladding-alignment machines rely on fixed V-grooves and fiber geometry (faster, cheaper, standard in mass FTTH deployment, slightly higher typical loss). Ribbon splicers fuse twelve at a stroke.
Process discipline decides outcomes: strip, clean, cleave (the cleave angle, under 0.5–1°, is the single biggest operator-controlled variable), splice, then protect. The bare joint is fragile and gets a heat-shrink sleeve with a strength member, or a recoat for space-constrained assemblies. Splicers estimate loss optically after the arc, but the estimate is geometric; trust OTDR or power-meter verification for anything critical. Proof-tension after splicing catches latent cracks before deployment does.