Evolution of Lightwave Systems

The extensive research to find a suitable transmission medium in the form of optical fibers started in 1975 and can be divided into various generations spreading over a time period of 25 years from 1975 to 2000.More researches and advancements in each generation led to the increase in BL product and decrease in  the losses associated with optical fibers.

Generations of Optical Fibers:-
Developments in the field of optical fiber communication can be broadly classified into five generations as given below.

1) First Generation:-

a) The first generation light wave system became commercially available in the early 1980s.

b)Their operating wavelength was nearly 0.8 mm and they used Ga As semiconductor lasers.

They operated at a bit rate of 45Mb/s and allowed repeater spacings of upto 10km.

2)Second Generation:-

a) The second generation light wave  systems became commercially available in 1987.

b) Their operating wavelength was 1.3mm and they used In GaAsp lasers.

c) They operated at bit rate upto 1.7 GB/s and allowed repeater spacings of upto 50 kms.

3) Third Generation:-

a) The third generation  light wave systems became commercially available in 1990.

b) Their operating wavelength was 1.55mm and they used lasers oscillating in a single longitudinal mode. In GaAsp lasers could not to used due to simultaneous oscillations by several longitudinal modes resulting in pulse spreading commonly known as dispersion.

c) They can operate at a bita rate uoto 10GB/s and allow repeater spacings of 60-70 km The repeater spacing can be increased by increasing the sensitivity of the receiver by using homodyne or heterodyne detection scheme . The light wave systems employing such scheme are known as coherent light wave systems.

4) Fourth Generation:-

a) The fourth generation light wave systems became commercially available in 1992.

b)Their operating wave length is in the range 1.45 um - 1.62um.

Rhey used wave length - division multiplexing (WDM) for increasing the bit rate and optical amplifications is provided by erbium doped fiber amplifiers spaced 60-80 km apart and compensate the fiber losses.

5)Fifth Generation:-

a) The light wave systems which evolved after the year 2000 comes under fifth generation light wave systems. These light wave systems or fiber optic communication systems are concerned with extending the operating wavelength range over which a WDM system can operate simultaneously.

b) The conventional wavelength window , known as C band (1.53-1.57 um)

is being extended on both the long and short wavelength sides resulting in L and S bands respectively.

c) Besides this to cover the entire range of wavelength region from 1.30mm to 1.65 mm , a new kind of fiber known as dry fiber has also been developed.

d) They can operate at a bit rate of 2.56 Tb/s (64 WDM channels at 19 Gb/s over 4 fiber pairs) and attempt to increase the bit rate of each channel