Preforms are presented which yield, upon drawing, new, improved, speckle-free output optical fibers, providing speckle-free, smooth output with flat top transmission of light from gaussian or few mode sources. The production of these preforms is also presented. The preforms, and the fibers produced in varying core dimensions from about 100 m to above 1000 m, are based on a structured silica section of mode mixing area adjacent to the inner core, or in the case of non-circular core, within the core. Modified Plasma Vapor Deposition process achieves the structured sections. The structured sections include pairs of layers, where a thin down-doped layer is alternated with a much thicker core material layer. The ratio of the thickness of the core layer to the thickness of the down-doped layer is about 3 to 25. The paired layers are between about 8 to 30-layer pairs. Both circular inner core and non-circular core examples are discussed.

A multicore fiber includes: a plurality of core portions made of glass; and a cladding portion made of glass and configured to surround outer periphery of the plurality of core portions. The cladding portion has tensile stress of 20 MPa or lower in a region on outer periphery side than a core portion that is closest to outer periphery of the cladding portion in the plurality of core portions.