Method and apparatus for improving spectral efficiency in wavelength division multiplexed transmission systems

A WDM transmission system comprising:

• a transmitter and a receiver;
• an optical transmission path coupling said transmitter to said receiver;
• wherein said transmitter includes;
• a plurality of 1,2, . . . N optical sources for generating a plurality of N optical channels;
• a first N/2.times.1 multiplexer coupled to predetermined even-numbered optical sources for combining even-numbered optical channels into a first wavelength division multiplexed optical signal;
• a second N/2.times.1 multiplexer coupled to predetermined add-numbered optical sources for combining add-numbered optical channels into a second wavelength division multiplexed optical signal;
• a coupler having first and seconds inputs respectively coupled to said first and second multiplexers such that a wavelength division multiplexed optical signal is produce in which said add-numbered optical channels have a state of polarization that is substantially orthogonal to said even numbered optical channels;
• a data modulator for modulating data from at least one data source, said modulator coupled to at least one of said plurality of optical sources;
• and
• a clock configured to establish a predetermined frequency, said clock being coupled to said at least one data source, said data source coupled to said at least one data modulator, wherein said data is modulated at a rate that is phase locked and substantially equal to said predetermined frequency.