Intensity, polarization, and phase information in optical disk systems

Digital information in optical data storage systems can be encoded in the intensity, in the polarization state, or in the phase of a carrier laser beam. Intensity modulation is achieved at the surface of the storage medium either through destructive interference from surface–relief features (e.g., CD or DVD pits) or through reflectivity variations (e.g., alteration of optical constants of phase–change media). Magneto–optical materials make use of the polar magneto–optical Kerr effect to produce polarization modulations of the focused beam reflected from the storage medium. Both surface–relief structures and material–property variations can create, at the exit pupil of the objective lens of the optical pickup, a phase modulation (this, in addition to any intensity or polarization modulation or both). Current optical data storage systems do not make use of this phase information, whose recovery could potentially increase the strength of the readout signal. We show how all three mechanisms can be exploited in a scanning optical microscope to reconstruct the recorded (or embedded) data patterns on various types of optical disk.