Nonlinearities introduced by power amplifiers in communication systems have been a subject of interest for industry and academia for a long time. On one hand the search for increased range intrinsically incites operators to boost input power. On the other hand this leads to stronger nonlinear phenomena in amplifiers which in turn increase the bit error rate. The situation led to the emergence of digital predistortion algorithms that compensate the power-amplifier's nonlinearities and improve the overall performance of the link. A rich corpus of literature exists on this topic but it mainly addresses wireless microwave networks. The case of digital predistortion specifically tailored for optical communications is much less explored. This paper focuses on predistortion via filter-lookup-tables and tackles the issue of non-uniform spacing. The work is mainly aimed at CO-OFDM systems including low-cost semiconductor optical amplifiers.