Optical Amplifiers

Naturally, fiberoptic cables cannot completely transmit a signal indefinitely without some loss in the power.  Due to impurities in the glass, some of the signal can be absorbed or scattered so that the signal’s amplitude decreases significantly.  To combat this loss of signal, the light must pass through a repeater.  A repeater is just an amplifier that takes the original depleted signal and retransmits an amplified version.  The major drawback in this system is that these repeaters can significantly slow the rate of data transfer (being one more obstacle the signal must pass through).  However, there are two ways to minimize the impact of repeaters on the speed of the optical signal.

1)         Improve the quality of the cable

If the glass has fewer impurities, the signal will be able to travel longer distances without having to be amplified.  In the mid-1960s, commercial optical glass lost so much power traveling through the glass that optical fibers would need repeaters approximately every 50 meters just to maintain the signal.  By comparison, today’s optical fibers can transmit light nearly 80 kilometers without needing reamplification.

2)         Improve the quality of the repeater

Until the 1990s, optical systems used repeaters electro-optical hybrid systems to amplify the signal.  These amplifiers would receive the incoming light signal, convert it into an electrical signal, amplify the signal electronically, then retransmit the signal using diode lasers.  As should be evident, this component slows the transfer of data significantly by requiring the signal to be converted from optical to electrical then back to optical.

In the pursuit of higher speeds, all-optical repeaters have been developed so that the signal needs no conversion to an electrical signal to be amplified.  Such repeaters are known as erbium-doped fiber amplifiers (EDFAs).  Essentially, glass in an optical fiber is doped with erbium.  The erbium is kept at a state of population inversion by diode lasers that excite the atoms.  When a photon interacts with an excited atom of erbium, the excited atom undergoes stimulated emission (as in a laser) which reradiates a photon that is coherent with the incident photon and travels in the same direction.  In this way, the signal can be amplified without any conversion to an electric signal.