PDF of this tutorialIn the previous sections, we covered how PMD, and specifically DGD, can disperse the transmitted optical bit and cause bit errors at the receiver. If there are relatively few bit errors at the receiver, then usually other mechanisms of the transmission system can satisfactorily recover the lost transmitted information. However, if the bit errors are too numerous, then the transmitted information is too corrupt to recover and the transmission link should be considered out of service.
The quantity of bit errors encountered at the receiver are directly influenced by the amount of PMD in a fiber optic transmission span. In Topic 1, we covered how the DGD component of PMD was measured in the time separation of the polarized pulses of the optical bit. This separation is measured in picoseconds (ps, equal to 10-12 seconds). The average separation can be calculated from the PMD coefficient of the fiber. PMD coefficients are in units of ps per √ km, or ps/(√ of km). Some fibers installed before the mid 1990’s have coefficients of about 1–2ps/(√ of km). We can look at some example calculations to be expected with fibers of this quality for a fiber route about the distance from San Francisco to Los Angeles:
DGD = (PMD coefficient) x (√ of distance) DGD = (1–2ps/(√ of km))((√ of 500 km)) DGD = 22–44 ps
DGD of this magnitude, in an OC–192/STM–64 transmission system, can be expected to result in a bit-error rate that is severe enough to cause service problems. Some general rules on limitations of distances caused by PMD are given in Figure 7.
Figure 7. Distance Chart for PMD
