Identifying channel centre frequency for DWDM power control measurement by using swept filter to detect power variations caused by dithering

In dense wavelength division multiplexing (DWDM) distinguishing between adjacent optical channels in the multiplexed signal can be difficult. The invention modulates the frequency of a channel so that it varies over time, dithering (see Figs. 4A,4B). At the receiver a narrow band wavelength tuneable filter is swept across the multiplexed channel and the power levels of the signal passing through the filter are measured (Fig. 5A). As the filter reaches the shoulders of the frequency modulated channel the power level will vary as the dithering causes the channel signal to move into and out of the filter pass band (a,b). A channel centre frequency (c) approximately midway between the frequencies where oscillating power is measured can then be determined and used for the channel power measurement (Fig. 5B). This power measurement may then be used for power control of the optical signal.