In the previous article, I briefly explained and compared two types of optical sources used in transmitters: LED and LD. Today, I am going to discuss what happens at the other end of a fiber link -- detectors. Optical detectors, as the name implied, can detect the amount of light received. Our very own eyes are a pair of detectors as they can receive light information with the retina and transmit that light data to our brain. In the visible light spectrum, our eyes are great detectors to inspect fiber break or light leakage. However, most fiber works in the invisible wavelength spectrum where human eyes won't be able to see. That is the where the optical detectors come in .