Patent ID: 7873284

Claim:
An apparatus, comprising: a Mach-Zehnder interferometer for encoding three or more data bits onto an optical carrier during each coding interval for a symbol, comprising: an optical splitter having an input port and first and second output ports, an optical combiner having first and second input ports and an output port, a first optical path coupling the first output port of the optical splitter to the first input port of the optical combiner; and a second optical path coupling the second output port of the optical splitter to the second input port of the optical combiner; and a digital signal generator; and wherein at least one of the first and second optical paths in the Mach-Zehnder interferometer includes an electro-optical phase shifter and an electro-optical attenuator; and wherein the digital signal generator is configured to apply a first set of binary-valued voltage signals to a control input of the phase shifter and a second set of binary-valued voltage signals to a control input of the attenuator.