Have you ever wondered by seeing the Polarization Mode Dispersion values mentioned in ITU-T recommendations for dispersion unshifted single mode fibers? The current recommendation in force specifies 0.2 ps/sqrt.km for G.652.B category and 0.5 ps/sqrt.km for G.652.C fibers. Interestingly the earlier version of single mode fiber has low PMD values compared to its predecessor.
Attenuation at the water peak region is quite high for earlier silica glass fibers. That is the wavelength region where the silica fibers deviated from theoretical relation established by Rayleigh for wavelength versus loss relationship. Rayleigh proved mathematically that the light drops its power that is indirectly proportional to the fouth power of wavelength. Rayleigh’s equation suggests that the optical power loss is wavelength dependent.
Attenuation is lower at longer wavelengths. We know that attenuation at 1310 nm is higher than that at 1550 nm. Dispersion unshifted single mode fiber generally shows an attenuation of less than 0.35 dB/km at 1310 nm and 0.2 dB/km at 1550 nm. This may vary depending on fiber manufacturer. Attenuation G.652.C fiber at 1550 nm is specified low compared to the one specified for G.652.B (0.35 dB/km for B category and 0.3 dB/km for C category). Interestingly the PMD is higher for G.652.C category fibers.
C category fibers are intended for operation from 1260 nm to 1625 nm. The low attenuation at water peak wavelength made it suitable for coarse wavelength division multiplexing systems. As the requirements moved towards dense wavelength division multiplexing at wavelength regions of 1530 to 1565 nm and also an option to use water peak wave lengths for some applications, polarization mode dispersion came in to picture. For single mode fibers to support DWDM systems, PMD must be lower. The high PMD of C category fibers were a hindrance to use these fibers in DWDM systems.
Fiber manufacturers developed low water peak fiber that has low PMD values. The B category and C category fibers mentioned in earlier versions of ITU recommendations had a PMD value of 0.5 ps/sqrt.km. Year 2000 issue of ITU recommendations had this value. B category fibers were predominant at that time. Low water peak fibers were showing their entry. Naturally, fiber manufacturers, whose preform making capacities were tuned for B category fibers pushed for improved PMD values for B category. Thus the B category got an advantage to be used in DWDM systems by offering PMD values less than or equal to 0.2 ps/sqrt.km.
Manufacturers developed low water peak fibers having less than or equal to 0.2 ps/sqrt.km PMD that is categorized under D category, leaving the C category with a higher value for PMD.