Use of high frequency test signals for monitoring of transparent WDM optical data networks

The article describes testing of the optical paths in transparent WDM (wavelength diversity multiplexing) data networks. It shows how additional high frequency modulation superposed on the optical carrier can be used to establish a test channel without affecting useful signals. Bit error rate (BER) measurements are described for 10 Gigabit/sec useful signals transmitted, using 50 Mbit/s data rate. This test method worked satisfactorily for ASK-NRZ as well as for DPSK modulation, to provide early warning of degradation of the useful signal. The article also deals with monitoring of individual channels in optical networks and the test setup is presented, showing superposed digital test signal modulation.