Laying and maintaining submarine cables is expensive, and any reduction in capital and operational expenses makes them more attractive than other global data transmission media. The last analogue submarine cable systems were deployed in the mid 1980s and low-loss digital optical fibre cable has come to dominate transoceanic transmission over the last decade. These are placed at regular distances throughout the cable span. Early systems operated at wavelenghts of 1300nm with repeater spacings of 50km. With the advent of 1500 nm technology repeater spans have increased to more than 100km.
Regenerative repeaters are complex opto-electronic devices.
Consequently, a great deal of effort has gone into finding a way of amplifying signals at the optical level wihout converting signals back to electrical signals. It is not widely known beyond submarine transmission specialists that optical amplifiers are already a reality. Optical amplifiers are devices that can be used instead of repeaters in long-haul fibre links. They operate entirely in the optical domain and do not employ any translation of data back to electrical baseband signals. The isolator allows the amplified 1550 nm signal to leave the amplifier but prevents the 1480 nm pump signal from leaving, in effect forcing it to recirculate around the Erbium-doped fibre.