Error Analysis of Head-Disk Spacing Measurements Made by Using Optical Interferometry

Abstract

An analytical and simplified method was developed for predicting error propagation encountered when optically measuring head-to-medium spacing in magnetic disk drives. Two kinds of optical interferences observed between the flying head and the glass disk were considered. The first is the homodyne interferometry in which the spacing is obtained from light intensity variation due to interference. The second is polarization interferometry in which the spacing is determined from the phase difference between the parallel and perpendicular components of polarized light. To analytically predict the amount of error propagation, the interference intensity must be presented as a function of its maximum and minimum values which determine the calibration curve. For this purpose, two kinds of accurate and useful approximations were introduced; one is for homodyne interferometry and the other is for polarization interferometry. Using these approximations, the calibration curves were presented in terms of the detected signal and its extremum. Calculation examples were demonstrated for typical head-medium interface conditions and the contribution of error components to the error in spacing measurement was clarified.