Tracking Servo Signal Simulation from STM Surface Profiles

Certain optical data systems rely upon the ability of the laser to accurately track along grooves in the substrate. The topography and physical properties of the substrate, laser beam characteristics (wavelength, spot size), and physical optics of the optical head all play a role in determining whether tracking is possible on a given optical disk. The effects of each of these variables can be found by modelling the disk as a phase surface which modulates the phase of the incident laser light. Scalar diffraction thcory[l,2] can be used to predict the reflected light intensity as a function of position across the disk for a given system. It is then possible to predict the servo signals which are used for tracking. The primary substrate parameters which affect these tracking signals are the groove depth, the relative widths of the groove and land (duty cycle), the track pitch, the refractive index, and the groove topography. All of these disk properties, excluding the refractive index, are found using Scanning Tunneling Microscopy (STM)[3]. This gives a detailed three dimensional surface profile for the disk which is then used in the theoretical calculation of the tracking servo signals. These calculated values are compared to actual experimental data obtained using an optical disk tester. Disks were measured which have varying depth and varying track pitch at fixed depth.