What is the source to transmit information in a optical fiber?

Optical fiber is used to carry light ... either visible or else infra-red. So the source you want to use is a stable, reliable source of light, that's small enough to efficiently couple into the tiny window at the end of the fiber, and whose brightness can be changed easily and fast in order to put information onto the light (modulate it). It also helps if the light consists entirely of a single color (wavelength), because different wavelengths travel down the fiber at different speeds, so multiple wavelengths could get spread out and arrive at the far end in a real mess. The best, most widely used source available today is the laser diode.