Exfo FTB-5600 Distributed PMD Analyzer Manuale Utente Scaricare il pdf (Pagina 122)

Distributed Cumulative PMD Measurement Theory

116 FTB-5600

For very small values of PMD·δν, the ms-difference ΔT

) is proportional

to (PMD·δν)

. Hence, there the measured transmission differences may be

very small, leading to a very poor dynamic range, since the differences

may be of the same magnitude as the coherence noise. In order to use the

largest possible optical-frequency difference δν, the large-step formula

below is used to compute the cumulative PMD:

where ΔT

= 8/45 is a “saturated” value of the ms-difference when

PMD·δν → ∞ and

= 15 is a proportional constant when δν → 0.

When the PMD is obtained from a single measurement at a given time, the

cumulative PMD curve, PMD(z

), will exhibit fluctuations as a function of

distance z, even if the cumulative PMD is constant. Therefore, it can be

advantageous to average over some length Δz, for example 0.5 km to 4 km,

to obtain a clear picture of the cumulative PMD curve. Moreover, the use of

a larger number of random and independent combinations of frequency

and I/O-SOP, for example K = 200 to 500, permits a corresponding

reduction in the measurement uncertainty.

ΔT

() ΔT

2πPMD z

()δν

ΔT

-------------------------------------

⎝⎠

⎛⎞

–

⎝⎠

⎛⎞

exp–=

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Exfo FTB-5600 Distributed PMD Analyzer Manuale Utente Pagina 122