A tunable optical filter is configured to take point measurements at a few optical frequencies per frequency channel of a DWDM optical network. The measurement frequencies are shifted by pre-determined amounts relative to the optical frequency channel being characterized. Since the spectral shape of the tunable optical filter is known, the center optical frequency, the modulation bandwidth, and the total optical power of the channel can be obtained from as few as three optical measurements. The center optical frequency and the total optical power can be continuously monitored by providing a tunable filter stage coupled to an interleaver stage, and computing a ratio and a weighted sum the optical signals at the two outputs of the interleaver stage.