The invention has for its object to provide a monitoring unit, particularly for optical burst mode signals in asynchronous optical networks based on, e.g., Internet or ATM (ATM=asynchronous transfer mode). The monitoring unit for burst mode signals comprises a detector for performing a Q-factor measurement in dependence on at least one variable and at least one fixed amplitude threshold value and on at lest one variable and at least one fixed phase value. This detector contains a burst mode amplitude detector for generating a fixed amplitude threshold value for each burst, a burst mode clock recovery circuit for generating a fixed phase value for each burst, a burst detector for detecting the beginning of each burst, and a control logic for generating a variable amplitude threshold value and a variable phase value for each burst.