A method of monitoring a WDM optical signal, comprising: • receiving a WDM optical signal having a plurality of channels; • detecting the optical signal after filtering the WDM optical signal with a tunable filter to determine a channel plan of the WDM optical signal; • based on the channel plan that has been determined by detecting the WDM optical signal, reconfiguring at least a center wavelength and bandwidth of the tunable filter optical transfer function to determine a signal performance parameter of the WDM optical signal, the method, further comprising: • filtering the optical signal with a tunable filter configured to have at least a first optical transfer function, wherein the first optical transfer function has a width narrower than a bandwidth of each of the optical channels; • detecting the filtered optical signal to determine a second optical transfer function optimized to measure a parameter of a channel in the WDM optical signal; • filtering the optical signal with the tunable filter configured to have the second optical transfer function; and • while the optical signal is being filtered by the tunable filter configured with the second optical transfer function, detecting the optical signal to determine a signal performance parameter of the WDM optical signal. • receiving a WDM optical signal having a plurality of channels; • detecting the optical signal after filtering the WDM optical signal with a tunable filter to determine a channel plan of the WDM optical signal; • based on the channel plan that has been determined by detecting the WDM optical signal, reconfiguring at least a center wavelength and bandwidth of the tunable filter optical transfer function to determine a signal performance parameter of the WDM optical signal, the method, further comprising: • filtering the optical signal with a tunable filter configured to have at least a first optical transfer function, wherein the first optical transfer function has a width narrower than a bandwidth of each of the optical channels; • detecting the filtered optical signal to determine a second optical transfer function optimized to measure a parameter of a channel in the WDM optical signal; • filtering the optical signal with the tunable filter configured to have the second optical transfer function; and • while the optical signal is being filtered by the tunable filter configured with the second optical transfer function, detecting the optical signal to determine a signal performance parameter of the WDM optical signal. • filtering the optical signal with a tunable filter configured to have at least a first optical transfer function, wherein the first optical transfer function has a width narrower than a bandwidth of each of the optical channels; • detecting the filtered optical signal to determine a second optical transfer function optimized to measure a parameter of a channel in the WDM optical signal; • filtering the optical signal with the tunable filter configured to have the second optical transfer function; and • while the optical signal is being filtered by the tunable filter configured with the second optical transfer function, detecting the optical signal to determine a signal performance parameter of the WDM optical signal.