Measurement of chromatic dispersion using two simultaneously modulated wavelengths

In long haul optically amplified submarine communications systems, it is necessary to characterise the system chromatic dispersion correctly. This is important to enable the design of a dispersion compensation map which is used to reduce the system penalties caused, for example, by nonlinear effects. The design of an optically amplified transAtlantic communication link at 5 Gbit/s requires around 140 erbium-doped fibre amplifiers (EDFAs). Each of these has a spectral gain/transmission window (at 1558.65 nm) of approximately 20 nm. Concatenation of the individual amplifiers, each with a slightly offset gain-peak (due to manufacturing tolerances) leads to an overall window that may be as narrow as 3 nm. These parameters set the requirements for a chromatic dispersion measurement system that can perform in the tight spectral window described