Double-ridged waveguide for efficiently heating ultrafine filament fibers

•A double-ridged waveguide is proposed to facilitate rapid heating of the ultrafine filament fiber.•The proposed design overcomes the difficulty of heating ultrafine filament fibers with conventional equipment.•The effects of the filament fiber size, permittivity, and cavity temperature on heating were investigated.•This design can be used for efficient drying, refining, manufacturing and modification of fibers.