The second phase of development in fiber optic communication

Optical loss or attenuation of multimode fiber is higher than that of single mode fiber, usually above 1 dB/km. Current multimode fibers, with a core diameter of 62.5um have typical loss around 3.5 dB/km 850nm and nearly 1.5dB/km at 1300nm. But the multimode fibers specified in ITU-T G.651.1 with a core diameter of 50um have a typical loss of less than 3 dB/km at 850nm and 1 dB/km at 1300nm.

During 1970s, network designers came across a finding that the utilization of the 1300nm wavelength region can substantially increase the repeater spacing since this window provides lower attenuation values. Single mode fibers show almost zero dispersion at near wavelength of 1300. For the current single mode fibers, Zero dispersion wavelength ranges from 1300nm to 1324nm. Researchers decided to take advantage of this phenomenon and focused their efforts to develop InGaAsP semiconductor lasers and detectors capable to operate near 1300nm.

The researchers in all sections of the industry focused their efforts to develop systems that can operate at 1300nm. Their efforts showed positive results by early 1980s. This worldwide effort for the development of InGaAsP semiconductor lasers and detectors operating near 1300 nm has revolutionized the fiber optic telecommunication industry.

The second phase of fiber optic communication systems, based on InGaAsP semiconductor lasers and detectors operating near 1 300 nm, using multimode fibers were used in the early 1980s, but the bit rate of early systems was limited to below 100 Mbit/s because of dispersion in multimode fibers.

The researchers could overcome the above limitation of multimode fibers by the use of single-mode fibers. The history says that a laboratory experiment in 1981 demonstrated transmission of 2 Gigabits per second over 44 kilometers of single mode fiber. The commercial deployment of this new systems took some more time and in 1988, second-generation lightwave systems, operating at bit rates of up to 1.7 Gbit/s with a repeater spacing of about 50 km, were available for commercial use.

ITU-T recommendation G.652 specifies the characteristics of a single-mode optical fiber operating at 1300 nm. Recommendation G. 957 specifies the characteristics of optical systems operating at 1300 nm and suitable for transmitting the bit rates of the synchronous digital hierarchy (SDH) up to STM-16. ITU-T recommendation G.956, which is now Recommendation ITU-T G.955 was extended to include PDH systems as well that operates at 1 300 nm.

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Author: Fiber

Chief Editor of Fiber Optic Mania Magazine

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