Shining Light on Optical Networking: Page 4 of 6

REDFAs are typically used for submarine cables because they require much less additional equipment. REDFAs also do not add any appreciable delay to the signal, as do OEO repeaters. REDFAs, however, amplify noise in the photonic signal, and over a long distance this noise can hurt the signal.

While the problem with optical transmissions is usually generating enough strength to get to the destination, sometimes the opposite is true. When too many photons get to the receiver, it is blinded--as though it's looking directly into the sun. Adding more photons is not a good thing for receivers. Multimode receivers are immune, for the most part, to being blinded, but single-mode receivers are not. This effect typically happens when the transmitter is mismatched to the distance that needs to be covered.

Single-mode transmitters come in a variety of strengths to transmit over different distances. The maximum distance varies by bandwidth and manufacturer. Although a transmitter and receiver might be designed for the long haul, it would be disastrous to assume that they could operate at shorter distances without the receiver burning out. The receiver design assumes a certain amount of photonic loss over the distance for which the transmitter is designed. When distance or power is mismatched you need a pair of fiber-optic sunglasses, or optical attenuators. A small sliver of shaded glass is placed in the path between two sections of fiber or at the receiver. The attenuator blocks a certain number of photons from going through. Attenuators are rated in decibels according to the amount of photonic loss they elicit.

Once the wavelengths get to the termination device they need to be demultiplexed and sent to the appropriate receiver. The easiest way to do this is by splitting the fiber and shunting the same signals to all the receivers. Then each receiver would look only at photons of a particular wavelength and ignore all the others. This form of narrowband receiver is very expensive and inefficient to manufacture. Instead, most receivers are wideband, allowing wavelengths across most of the laser spectrum to be seen. However, only photons from a given wavelength should be allowed to get to the receiver.