| Optical orthogonal frequency division multiplexing (OOFDM) is a novel fiber transmissiontechnology proposed recently with a high spectral efficiency. Quickly it has become one of theresearch topics in the optical communication society. The optical channel is divided into severalorthogonal sub-channels in the OOFDM system by employing popular digital signal processingtechnology. The data are transmitted through these sub-channels by being carried on a set oforthogonal optical subcarriers. This kind of optical multicarrier modulation is inspired by thewireless communication. The high tolerance of the fiber channel distortion such as chromaticdispersion, polarization mode dispersion and the flexibilities of OOFDM due to its intrinsicsoftware definition make it one of the most competitive candidates in the next generation ofoptical networks.However, the OOFDM signal is sensitive to the nonlinear distortion. The intermodulationdistortion will cause the OOFDM system severe performance degradation. The bottleneck of theOOFDM transmitters with higher modulation formats will be conventional optical modulators,such as Mach-Zehnder modulators, electro-absorption modulated lasers, which have nonlineartransfer functions. To reduce the nonlinear effect, the optical modulation depth must have aback-off, but this will lower the effective optical signal-to-noise ratio as well. Therefore, thenonlinear compensation technique of the optical modulators is very important to the OOFDMsystems and it can be broadened to all the optical transmission systems which have poortolerance to the nonlinearities. Digital pre-distortion (DPD) algorithm is one kind of effectivemethods to compensate for the nonlinearities induced by the optical modulators by estimating thenonlinear transfer function of the modulators with a math model and pre-distorting the inputsignal by the inverse function of the math model. Besides, the DPD scheme needs a fewcomponents only so that it has a huge advantage. |
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