Patent ID: 7787778

Claim:
A method of controlling a polar optical transmitter comprising a signal path comprising a multi-bit digital stage cascaded with an analog stage having a pair of branches for supplying a respective drive signal to each branch of a dual branch Mach-Zehnder (MZ) modulator, the method comprising steps of: independently dithering a set of two or more selected components of a signal traversing the signal path using a respective digital dither signal for each selected component; detecting at least one artefact of the dither signals in an optical signal at an output of the MZ modulator; and adjusting at least one parameter of the transmitter based on the detection result; wherein the step of adjusting at least one parameter of the transmitter comprises steps of: calculating a cost function value based on the detected artefacts; and adjusting the at least one parameter of the transmitter based on the calculated cost function value wherein each cost function is a selected function of one or more of the detected artefacts, which is dependent upon a respective parameter; wherein the cost function defines a control surface which has either a maximum or a minimum corresponding to a desired feature of the output optical E-field, and wherein the step of adjusting at least one parameter comprises adjusting the respective parameter to drive a value of the cost function to the corresponding maximum or minimum; and wherein: the dither comprises a common dither signal applied to each drive signal; the cost function is a modulation depth of the output optical signal at a frequency corresponding to that of the dither signal; and the parameter is a system balance of the signal path.