INFLUENCE ON THE READ-OUT SIGNAL OF THE HEIGHT PROFILE OF THE PITS (OR BUMPS) ON OPTICAL DISKS

When the diffraction pattern of the focused laser beam of the read-out system falls on the plane surface between the pits (or bumps), substantially all the light is reflected back to pass through the focusing objective. When it falls on a bump (or pit), phase differences are imparted to the light in the diffraction pattern, and some of the light returning from the disc is diffracted to pass outside the focusing objective; it is this reduction in the light returning through the objective that constitutes the read-out signal. The strength of this signal depends on the height profile of the bump or pit which (because of the photoresist process) will show more or less 'rounding' of the edges. The diffraction theory of the read-out signal is described, and numerical results are given. It is shown that the 'ideal' height profile is the rectangular form, with no rounding of the edges. This form is not achievable in practice, but the results show that very satisfactory values of the read-out signal can nevertheless be achieved in the presence of moderate amounts of rounding.