Figure 1 shows a diagram of single mode fiber.  The core is 9 microns.  The cladding is 125 microns in diameter.  The laser creates a straight beam of light. Figure 2 shows a diagram of multi mode fiber.  The core is between 50 and 62.5 microns.  The cladding is 125 microns in diameter.  The LED’s make beams of light that reflect off the inside of the core.