The first introduced single mode fibers belonged to G652.A category. G.652.B category is an improved version of G.652.A category. When introduced first, the G.652.A category offered low attenuation at 1310nm, the wavelength around at which optical transmitter and receiver systems were developed. Multimode fibers were used to transmission until then. Multimode fibers offered low attenuation at 1300nm compared to the attenuation at 850nm. Still the attenuation at 1300nm were at a range of 0.7 to 1.0 dB/km.
G,652.A was a breakthrough in optical communications in 1970s. With the use of G.652.A fiber, long distance optical transmission became a reality. Repeater and regenerator distance was reduced drastically. The dispersion at 1310 nm region was also zero. G.652.A fiber ruled the fiber optic telecommunication world for some time until the users realized the effect of polarization mode dispersion. Single mode optical fibers that have improved attenuation and Polarization mode dispersion characteristics were developed by fiber manufacturers. ITU-T recognized that fiber as ITU-T G.652.B category.
The below table of comparison gives an idea about the differences in characteristics of both G.652.A and G.652.B fibers.
|1||Attenuation at 1310 nm||dB/km||Max. 0.5||Max. 0.4|
|2||Attenuation at 1550 nm||dB/km||Max. 0.4||Max. 0.35|
|3||PMD Link Design Value||ps/√km||Max. 0.5||Max. 0.2|
The major difference is in attenuation and polarization mode dispersion. As can be seen in the table above, attenuation coefficient at 1310 nm and 1550 nm are high for category A fibers compared to category B fibers. The reduction is attenuation gave more margin for system operators and allowed longer span of repeaters and regenerators.
Drastic reduction in polarization mode dispersion link design value allowed high bit rate datat transmission at lower bit error rate. The earlier deployed fiber optic cables with category A fibers might have all replaced by this time. From the year 2000 onwards ITU started listing the dispersion unshifted single mode optical fibers in tables. That was when the low water peak fibers started getting popularity in the market.
Optical fibers during 1987-88 had high attenuation limits such as 1.0 dB/km at 1300 nm and 0.5 dB/km at 1550 nm. It is high now, when we are familiar with the ultra loss fibers introduced by fiber manufacturers. During that period attenuation value of 1.0 dB was quite low.