Estimating System OSNR With a Digital Coherent Transceiver

We demonstrate a technique of evaluating optical signal to noise ratio (OSNR) associated with an optical carrier in a fiber-optic system using a commercial coherent optical transceiver equipped with digital signal processing capability. The procedure of measuring noises caused by the transmitter and the receiver is outlined, and these transceiver-related noises need to be taken into account in order to accurately evaluate the system OSNR. Through digital polarization-demultiplexing and polarization nulling in the receiver, it is possible to extract the noise underneath the optical signal, and to measure the tilt of the ASE noise spectrum within the optical signal bandwidth.