[Image:Singlemode fibre structure.svg|thumb|right|The structure of a typical
single-mode fiber.
1. Core 8 µm diameter
2. Cladding 125 µm dia.
3. Buffer 250 µm dia.
4. Jacket 400 µm dia.]] In
fiber-optic communication, a
single-mode optical fiber (
SMF) is an
optical fiber designed to carry light only directly down the fiber - the transverse mode. Modes are the possible solutions of the
Helmholtz equation for waves, which is obtained by combining
Maxwell's equations and the boundary conditions. These modes define the way the wave travels through space, i.e. how the wave is distributed in space. Waves can have the same mode but have different frequencies. This is the case in single-mode fibers, where we can have waves with different frequencies, but of the same mode, which means that they are distributed in space in the same way, and that gives us a single ray of light. Although the ray travels parallel to the length of the fiber, it is often called
transverse mode since its
electromagnetic vibrations occur perpendicular (transverse) to the length of the fiber. The 2009
Nobel Prize in Physics was awarded to
Charles K. Kao for his theoretical work on the single-mode optical fiber.