The present invention relates to a surface interaction type multimode optical fiber coupler. A representative embodiment of the present invention comprises a plurality of optical fibers with at least one having an expanded core section. Sections of the optical fibers including at least a portion of the expanded core sections are fused together forming a fused section. At least one of the optical fibers is suitable for multimode operations. According to an embodiment of the present invention, a method of fabrication an optical fiber coupler comprises: providing a plurality of optical fibers with at least one having an expanded core section; and maintaining sections of the optical fibers including at least a portion of the expanded core sections in contact and simultaneously heating at least a portion of the sections that are in contact to form a fused section until a predetermined end condition is reached.