An optical data transmission system and method employs positive only data modulation, with an offset applied to the positive modulated values before generating modulated drive current signals for a light emitting component. The offset is such that the minimum drive current falls in a range where the electron-to-photon efficiency of the light emitting component is substantially constant. The drive current thus falls in a more linear part of its current vs. intensity characteristic. This reduces distortion and hence enables increased data rate.