The invention includes a method for optically encoding data for transmission over a wavelength division multiplexed optical communications system comprising the steps of: generating a periodic series of optical pulses defining a series of time slots, wherein one pulse appears in each time slot; filtering the pulses to produce carrier pulses extending over more than one time slot; and modulating the pulses with data for transmission. Preferably, the filter gives rise to the pulses having a temporal profile with a minimum in the centre of each of the time slots adjacent to the time slot for that pulse. The resulting data format allows for greater spectral efficiency in a WDM optical transmission system as compared with conventional RZ and NRZ data formats.