histroy of telecommunication

The need for reliable long-distance communication systems has existed since antiquity.
Over time, the sophistication of these systems has gradually improved, from smoke signals to telegraphs and finally to the first coaxial cable, put into service in 1940.
As these communication systems improved, certain fundamental limitations presented themselves.
Electrical systems were limited by their small repeater spacing (the distance a signal can propagate before attenuation requires the signal to be amplified), and the bit rate of microwave systems was limited by their carrier frequency.
In the second half of the twentieth century, it was realized that an optical carrier of information would have a significant advantage over the existing electrical and microwave carrier signals.
However, no coherent light source or suitable transmission medium was available.
After the development of lasers in the 1960s solved the first problem, development of high-quality optical fiber was proposed as a solution to the second.
Optical fiber was finally developed in 1970 by Corning Glass Works with attenuation low enough for communication purposes (about 20dB/km).
At the same time GaAs semiconductor lasers were developed that were compact and therefore suitable for fiber-optic communication systems.
After a period of intensive research from 1975 to 1980, the first commercial fiber-optic communication system was developed, which operated at a wavelength around 0.8 µm and used GaAs semiconductor lasers.
This first generation system operated at a bit rate of 45 Mbit/s with repeater spacing of up to 10 km.
The second generation of fiber-optic communication was developed for commercial use in the early 1980s, operated at 1.3 µm, and used InGaAsP semiconductor lasers.
These systems were initially limited by dispersion.
In 1981 the single-mode fiber was revealed to greatly improve system performance.
By 1987, these systems were operating at bit rates of up to 1.7 Gb/s with repeater spacing up to 50 km.
Third-generation fiber-optic systems operated at 1.55 µm and had loss of about 0.2 dB/km.
They achieved this despite earlier difficulties with pulse-spreading at that wavelength using conventional InGaAsP semiconductor lasers.
Scientists overcame this difficulty by using dispersion-shifted fibers designed to have minimal dispersion at 1.55 µm or by limiting the laser spectrum to a single longitudinal mode.
These developments eventually allowed 3rd generation systems to operate commercially at 2.5 Gbit/s with repeater spacing in excess of 100 km.
The fourth generation of fiber-optic communication systems used optical amplification to reduce the need for repeaters to increase fiber capacity.
This improvement caused a revolution that resulted in the doubling of system capacity every 6 months starting in 1992 until a bit rate of 10 Tb/s was reached by 2001.
Recently, bit-rates of up to 14 Tbit/s have been reached over a single 160 km line using optical amplifiers.