Abstract: This invention relates to a technique for inducing frequency selective changes in photo-sensitive materials. It is known to store data in photo-sensitive materials using frequency selective optical data storage (FSDS). In order to improve the storage density, the present invention proposes storing data in the photo-sensitive material using a single side band technique. In one embodiment, a reference pulse is utilised having a frequeny band which encompasses only a single side band of the encoded signal. In another embodiment, a filter is utilised to filter out all frequencies apart from the single side band to be written into the material. As well as being useful for storing data in the photo-sensitive material, the single side band technique can also be used to store filter characteristics.