In fiber-optic communication, a single-mode optical fiber (SMF) (monomode optical fiber, single-mode optical waveguide, or unimode fiber) is an optical fiber designed to carry only a single ray of light (mode). This ray of light often contains a variety of different wavelengths. Although the ray travels parallel to the length of the fiber, it is often called the transverse mode since its electromagnetic vibrations occur perpendicular (transverse) to the length of the fiber. The 2009 Nobel Prize for Physics was given to Charles K. Kao for his theoretical work on the single-mode optical fiber.