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On today's SM ad-hoc call, issues regarding measurements with worst-case dispersion were raised. It is often difficult to obtain worst-case dispersion fiber using the fiber types specified in the 802.3 standard. Other fiber types can
potentially be used for this purpose, such as dispersion-shifted fibers that move the zero-dispersion wavelength (ZDW) near 1550 nm. When operated near 1310 nm these fibers produce substantial dispersion, but the sign of the dispersion will likely be either
only positive or only negative. (At this point, I have not bothered to calculate which one, and it may depend on the manufacturer and fiber category.) But a glass half full is better than an empty one.
Some dispersion-shifted fibers have cut-off wavelengths longer than 1300 nm. These are not assured to be single-mode at the wavelengths of interest. As a work-around, one can apply mode suppression loops to rid the fiber of the higher
order modes after each connection and splice in the test bed. Another way is to use IEC B2 fibers, a.k.a. ITU G.953 fibers. These are dispersion shifted, but also offer cabled cut-off wavelength maxima of 1270 nm, so remain single-mode at the wavelengths
of interest. There are two sub-categories, IEC B2_a and B2_b (ITU G.953.A and 953.B), each having different dispersion specs. The mode field diameter of these fibers is smaller than others, but should still couple adequately to the source.
Obtain the ZDW and ZDS of the fiber sample from the manufacturer. From these the dispersion at the operating wavelength can be calculated as a function of fiber length. Select the fiber length that produces the worst-case dispersion
for that operating wavelength. Hope this helps. Paul Kolesar |