Single-mode fiber, on the other hand, can traverse hundreds or even thousands of kilometers, depending on the light source, wavelength and amount of data to transmit. Visually, telling the difference between multimode and single-mode fiber is nearly impossible, so most manufacturers make it easy on us. Multimode fiber is typically clad in an orange sheath, while single-mode is covered in yellow material.
There are two possible light sources for optical transmission: LED or laser. LEDs are used only in multimode systems, while lasers are the source of choice for single-mode fiber. LEDs cannot operate over single-mode fiber as the light is not coherent enough to travel down such a small passageway--imagine trying to fill an eye-dropper-size opening with water being poured from a bucket.
Data travels down copper lines as a series of on and off pulses to specify whether a bit is a 1 or a 0. Fiber works in the same way, but by pulsing the light source on and off. By pulsing the light faster, more data can be sent down the fiber. There is a limit to how quickly a laser can be turned on and off and still generate enough photons of a required strength to reach the other end and be interpreted. For this reason, high-speed fiber connections rely on multiple wavelengths to transmit data, combining the best of two worlds.