A type of optical fiber with a relatively thick inner core that allows light rays to propagate along multiple modes, or physical paths, through the fiber.The number of modes is sensitive to the core diameter, the numerical aperture (NA), and the wavelength. A core diameter that is large in relationship to the wavelength supports a large number of modes. MMF typically has a core diameter of 62.5 microns or 50 microns as illustrated in Figure M-3. The light source used in an MMF transmission system generally is either a light-emitting diode (LED) or a vertical cavity surface-emitting laser (VCSEL). LEDs operate in the 850 nm range, emit poorly focused light beams, and couple effectively to 62.5 micron MMF.VCSELs typically operate in the 1300 nm and 1310 nm ranges, emit well-focused light beams, and couple effectively to both 62.5 micron and 50.0 micron fiber.The numerical aperture mathematically describes the angle of acceptance, which defines the light-gathering ability of the fiber. These interrelated terms all establish the size of the angle at which the light source can effectively inject a signal into the fiber.The wider the angle from axial, the higher the mode, the more modes supported, the more light reaches the far end. So, a fiber optic transmission system (FOTS) employing an LED connected to a fiber with a 62.5 micron core and a relatively large numerical aperture supports a great many modes. A FOTS employing a VSCEL connected to a fiber with a 50.0 micron core and a relatively small numerical 0V +V -V 0 1 0 0 1 1 1 0 0 1 0 0 1 1 1 0 Baseband Digital Signal Modulated MLT-3 Signal aperture supports many fewer modes of light.The latter example is preferable.There are two general categories of modes: