Overcoming optical amplifier intermodulation distortion using coding in multichannel communications systems

In future optical networks on frequency division multiple access (FDMA), e.g. TV distribution, optical amplifiers will be required to assure adequate reception. A key figure of merit is η, the efficiency with which channels can be packed into a given bandwidth relative to the ideal packing of 100% corresponding to abutting channels (channel spacing=channel bandwidth). Packing can be severely limited by intermodulation distortion due to inherent amplifier nonlinearity. Assuming constant envelope signaling, the authors use a recently developed amplifier model to show that, by employing practical coding methods, η can be greatly increased. A simple, general algebraic formula for η is derived in terms of amplifier, receiver, and code parameters. For illustrative purposes the authors consider the packing of 50-Mb/s channels into a 1-THz optical bandwidth, assuming typical amplifier and receiver parameters