Specialty Optical Fibers Handbook

splices and connectors results in a calculated span length of 72 km. For many

years, almost a third of the optical transmission window between 1260 and

1625 nm was unusable because of the presence of a large ‘‘water peak’’ at

1383 nm with loss as high as 2 dB/km. Today, zero water peak (ZWP) fibers,

described in detail later, open previously unusable spectrum for low-cost coarse

wavelength division multiplexing (CWDM).

The more interesting case is that of multiple spans, in which noise accumu-

lates in an amplifier chain. An expression that approximates the OSNR for a link

comprising N

span

spans of loss L

span

in which each amplifier has noise figure, NF ,

is given in [1] as

OSNR ¼ 58 þ P

ch

 NF  L

span

 10 log 10(N

span

), (5:1)

where all units are in dB ...