Telecommunication engineers and the researchers found a drawback of using electronic repeaters in the third generation fiber optic communication systems. Electronic repeaters need continuous monitoring and maintenance. The optical signals in third generation fiber optic communication systems operating at the wavelength of 1550nm were regenerated by using electronic repeaters that were spaced at every 70 to 80 kilometers.
The advent of optical fiber amplifiers changed the scenario. Optical fiber amplifiers were introduced in 1989 and marks the fourth generation of fiber optic communication systems along with the introduction of wavelength division multiplexing (WDM). Impact of fiber amplifiers have been significant that it offered long distance communication by eliminating expensive and difficult-to-maintain electronic amplifiers.
The fourth phase of lightwave systems took advantages of using the optical amplification for increasing the repeater spacing and of wavelength division multiplexing (WDM) for increasing the aggregate bit rate. The introduction of the WDM technique started a revolution in optical communication that resulted in doubling the system capacity every 6 months. In most wavelength division multiplexing systems optical fiber attenuation was compensated using erbium-doped fiber amplifiers installed repeated at every 70 to 80 kilometers.
Erbium-doped fiber amplifiers were available in the market in 1990, though the development of optical fibers as amplifiers were finished in 1985. Fiber amplifiers operate in the 1530 to 1565nm, which is the DWDM operating wavelength. This window is called C-band. An experiment in 1991 revealed the possibility of 2.5 Gigabits per second for 21,000 kilometers and 5 Gigabits per second for 14,300 kilometers by using a recirculating-loop configuration, adding hopes to the efforts for building intercontinental fiber optic network under the sea. Transatlantic and Transpacific submarine fiber optic cable systems were introduced by 1996. Many submarine fiber optic cables have been installed worldwide, since then.