US Patent 7751062 Optical flying height measurement and calibration with dual sliders

A method for optical flying height measurement, and a flying height tester. The method the method comprises the steps of aligning a first slider and a second slider adjacent a transparent disk; varying a rotational speed of the disk relative to the first slider whereby a spacing between the first slider and the disk changes; measuring changes in a first reflected light intensity caused by interference of a light signal portion reflected from the first slider and a light signal portion reflected from a surface of the disk, wherein the rotational speed is varied over a range such that one maximum and minimum intensities pair is measured in the reflected light intensity; measuring a second reflected light intensity caused by interference of a light signal portion reflected from the second slider and a light signal portion reflected from a surface of the disk at a fixed rotational speed between the disk and the second slider; and calculate a flying height of the second slider using the measured maximum and minimum intensities pair and the measured second reflected light intensity.