The form of discs considered has λ/4-deep pits along the track, the spacings and lengths of which are modulated. A laser beam is focused on the track by means of a high-aperture objective, and the light which is scattered back through the objective is detected. The diffraction theory of this system is treated, both for simple periodic and for more complicated forms of disc. Formulas are obtained for the fundamental and harmonics of the read-out signal. The influences of focus error and aberrations are studied, for certain cases of which numerical results are given. Also treated are cross-talk between adjacent tracks and intermodulation between the pit-spacing frequencies and those of the length modulation of the pits.
© 1979 Optical Society of AmericaPDF Article