Compressed channel spacing for optical heterodyne communication systems

A method for operating an optical heterodyne communication system wherein an optical signal including a plurality of optical channels is transmitted on a single optical waveguide, comprising the steps of:

• selecting the optical channels;
• grouping the optical channels into blocks, each block including at least two optical channels having a predetermined channel spacing in the frequency spectrum;
• selecting a block spacing .DELTA.f.sub.B between blocks in the frequency spectrum such that when an optical signal, including a selected optical channel centered at frequency f.sub.s, is heterodyned with a local oscillator lightwave at a frequency f.sub.LO to produce an intermediate frequency signal centered at frequency f.sub.IF, interference in the selected channel resulting from adjacent blocks is less than a predetermined value;
• combining the said optical channels into said optical signal;
• sending the optical signal from an optical transmitter to an optical heterodyne receiver;
• and
• extracting an information signal from the selected optical channel.

An optical heterodyne communication system comprising:

• an optical transmitter for transmitting an optical signal including a plurality of optical channels, said optical channels being grouped into blocks wherein each block includes at least two optical channels having a predetermined channel spacing in the frequency spectrum and adjacent blocks are spaced in the frequency spectrum by a block spacing .DELTA.f.sub.B such that when an optical signal, including a selected optical channel centered at a frequency f.sub.S, is heterodyned with a local oscillator lightwave at a frequency f.sub.LO to produce an intermediate frequency signal centered at frequency f.sub.IF, interference in the selected channel from adjacent blocks is less than a predetermined value;
• an optical receiver for receiving said optical signal including means for heterodyning said optical signal with a local oscillator lightwave to produce an intermediate frequency signal and means responsive to the intermediate frequency signal for extracting an information signal from the selected optical channel;
• and
• optical waveguide means for carrying said optical signal from said optical transmitter to said optical receiver.

A method of transmitting an optical signal to reduce average channel spacing in a coherent optical communication system having a transmitter and a receiver including a local oscillator, comprising the steps of:

• a) grouping optical channels of the transmitter into blocks wherein spacing between optical channels in a block is selected so that optical interference between channels in a block does not exceed a predetermined level at the receiver when the optical signal is heterodyned with a lightwave from the local oscillator;
• b) selecting spacing between blocks so that optical interference of optical channels of adjacent blocks with a selected optical channel to be received at the receiver does not exceed the predetermined level when the optical signal is heterodyned with the lightwave from the local oscillator;
• and
• c) combining the optical channels and blocks into the said optical signal and transmitting the optical signal with said optical channel spacing and said block spacing.

In an optical heterodyne system, having a transmitter and a receiver including a local oscillator, a method for transmitting an optical signal including a plurality of optical channels, comprising the steps of:

• a) grouping pairs of optical channels together to form blocks, wherein the optical channels are spaced within said blocks such that optical interference does not exceed an predetermined at the receiver after heterodyning the optical signal with a lightwave from the local oscillator, and wherein a spacing between blocks is such that interference from adjacent optical channels with the selected optical channel when heterodyned with the local oscillator lightwave does not exceed a predetermined level;
• b) transmitting the optical signal to a receiver;
• c) where a lower frequency optical channel in a block is a selected channel to be received by the receiver, negatively displacing the selected channel to a lower frequency relative to the lower frequency channel by the frequency of the local oscilator;
• and
• d) where a higher frequency optical channel in the block is a selected channel to be received by the receiver, positively displacing the selected channel to a higher frequency relative to the higher frequency channel by the frequency of the local oscilator.