Beam shaping for optical data storage

The basic instrument used for optical data storage is a scanning laser microscope. Each device contains a beam from a laser diode that is collimated, shaped and focused with an objective lens to produce a microscopic spot on the recording media. The reflected light is collected by the objective lens and directed to data and servo detectors with a beam splitter. Data density on the storage medium is primarily defined by the size and shape of the focused laser beam used to scan the data. Several interesting techniques have been used to shape the focused spot in a way that decreases the primary feature of the spot, thus increasing density. For example, both amplitude and phase filters have been used to decrease the central lobe, at the expense of increased sidelobe levels. Effects of the sidelobes can be minimized with special electronic circuits. The configuration of the readout optics also can influence density. That is, optical filters can be placed in the collection pupil to improve the system transfer function. When combined with electronic shaping circuits, the optical filters significantly improve device performance. This paper reviews the techniques used for beam shaping in optical data storage with an explanation of each technique and its success or failure.