Patent ID: 7206520

Claim:
An optical communication device, comprising: an electrical modulation control unit to produce a first modulation control signal comprising a plurality of first channel signals and a second modulation control signal comprising a plurality of second channel signals that are respectively at the different channel frequencies of the first channel signals and respectively carry the same channel information as the first channel signals, wherein two adjacent channel signals in each of the first and the second modulation control signals have a relative phase shift of 90 degrees, and wherein each channel signal in the first modulation control signal has a relative phase shift of 90 degrees with respect to a corresponding channel signal at the same channel frequency in the second modulation control signal; and a Mach-Zehnder optical modulator comprising an input port to receive an optical carrier at an optical carrier frequency, a first optical path and a second optical path which receive a first portion of the optical carrier an a first optical carrier and a second portion of the optical carrier as a second optical carrier, respectively, and an output port to combine light from the first and second optical paths to produce an optical output signal which carries output channel signals having the same channel information of the first and second channel signals and a frequency spacing between two adjacent output channel signals being twice a frequency spacing between two adjacent channel signals in the first and second modulation control signals, wherein the first optical path receives and responds to the first modulation control signal to modulate the first optical carrier to carry the first channel signals on both sides of the optical carrier frequency, and the second optical path receives and responds to the second modulation control signal to modulate the second optical carrier to carry the second channel signals on both sides of the optical carrier frequency and to produce a phase shift of 90 degrees in light in the second optical path relative to light in the first optical path.