Engineers working in the telecommunication networks handling the second generation fiber optic communication systems felt the need to find an operating wavelength which has lower attenuation. The singlemode fibers used for the second generations offered a good balance of dispersion and attenuation, but the relatively higher value of attenuation at 1310nm limited the repeater spacing.
The loss at 1300nm/1310nm offered by the optical fbers used in the second generation was in the range of 0.5 dB/km. Optical fibers with 0.2 dB/km loss at 1550nm was available in 1979 itself. The problem was the higher dipsersion at 1550nm, which was more than 18 ps/nm.km. Due to this higher dispersion values, operational wavelength of 1550nm could not be used though the attenuation was lower. The introduction of third phase lightwave systems operating at 1 550nm over singlemode fiber was considerably delayed by the higher fiber dispersion near 1 550nm.
Conventional InGaAsP semiconductor lasers could not be used because of pulse spreading occurring as a result of simultaneous oscillation of several longitudinal modes. The dispersion problems were handled either by using dispersion-shifted fibers designed to have minimum dispersion near 1 550 nm or by limiting the laser spectrum to a single longitudinal mode. Dispersion shifted fibers were categorized under recommendation G.653 by ITU-T.
The laboratory experiments indicated the possibility of transmitting information at bit rates of up to 4 Gbit/s over distances in excess of 100 km in 1985. Third generation lightwave systems operating at 2.5 Gbit/s became available commercially in 1992.
Third generation systems were capable of operating at a bit rate of up to 10 Gbit/s. The best performance is achieved using dispersion-shifted fibres in combination with lasers oscillating in a single longitudinal mode. Third generation systems were marked by the use of 1550nm wavelength for transmitting optical signals. Singlemode optical fibers recommended in ITU-T G.653 are not used these days for telecommunication networks due to the non-linear issues in optical communication. Though the initial thought of utilizing the 1550nm region of lower attenuation by shifting the zero dispersion from 1310n to 1550nm was proven practically not suitable for high bit-rate transmissions.
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