In optical-fiber technology, that surface of an optical Fiber that is used to Contact the transverse-alignment elements of a component such as a Connector or mechanical splice.

Note: For telecommunications-grade fibers, the reference surface is the outer surface of the cladding. For plastic-clad Silica (PCS) fibers, which have a strippable polymer Cladding (not to be confused with the polymer overcoat of an all-glass fiber), the reference surface may be the core.