Patent ID: 6885820

Claim:
A method for controlling a signal to noise ratio of optical add/drop signals of an optical WDM transmission signal in an optical WDM transmission system having a transmitter unit, a receiver unit and at least one optical add/drop multiplexer for transmitting both optical express signals and at least one optical add/drop signal, the method comprising the steps of: determining signal to noise ratios of the express signals in the receiver unit; controlling signal levels of the express signals in the transmitter unit as a function of the determined signal to noise ratios of the express signals; determining a power spectrum of the express signals; adapting a signal level of the at least one optical add/drop signal to the power spectrum determined for the express signals; calculating, prior to the step of adapting, an insertion loss for at least one of the express signals; measuring a signal level of a drop signal; and determining a signal level of an add signal, necessary for the step of adapting, from both the calculated insertion loss and the measured signal level of the drop signal, and setting the signal level of the add signal before injection into the WDM transmission signal.