Abstract
A double common-path differential heterodyne interferometer is proposed to measure the flying height modulation (FHM) of a slider in a hard disk drive. A transverse Zeeman He–Ne laser is employed as the light source and high-speed phase measurement is applied for signal processing. The resolution and the sampling frequency of FH measurement are 0.1 nm and 500 kHz, respectively. According to the symmetrical optical path design, the environmental noise and Abbe error from the disk can be adaptively compensated. The experimental results show that the stability of the system is sub-nanometer in a normal laboratory without air conditioning, and the FHM of ±3 nm is achieved when the pitch of the rotating disk is around 1.2 µm.
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