A fiber optic mode scrambler includes a multi-mode optical fiber formed with a core and a cladding around the core and a non-adiabatic cross-sectional shape change zone in the optical fiber. The fiber further has a bending region extending over a length of the optical fiber. The optical fiber has a non-zero curvature at the bending region and a device for maintaining the curvature of the optical fiber at the bending region. Mode scramblers can be provided that are accurately adjusted to match with different desired optical characteristics. A well-controlled manufacturing method for the mode scrambler is also described.