Semiconductors like gallium arsenide or indium phosphide convert electrical signals into light (photons) using Laser Diodes or LEDs .
Light pulses travel through Optical Fibers , which use thin glass cores to guide signals over thousands of miles. Photonics - Optical Electronics in Modern Comm.
The convergence of low-loss optical fiber and semiconductor lasers created a medium that could carry data at the speed of light with minimal loss. Photonics - Optical Electronics in Modern Comm.