| 摘要 |
PROBLEM TO BE SOLVED: To highly accurately write servo track signals by correctly positioning a magnetic head arm based on a stable shift information. SOLUTION: A luminous flux emitted from a semiconductor laser light source 52 is turned to a parallel luminous flux at a collimator lens 51, pin-pointed through a lens 50 onto a linear diffraction grating 35 set on a magnetic head arm 32, then reflected and diffracted. As a result, the±diffraction light of a first order are projected. The diffraction light after passing the lens 50 becomes two parallel luminous fluxes of parallel optical axes. The luminous fluxes are deflected by brazed diffraction gratings 55a, 55b and synthesized at four grating areas of the diffraction grating 35 with wavefronts agreed. Accordingly, sine wave signals which show quarter-cycle phase difference in accordance with the movement of the diffraction grating 35 are obtained at a four division sensor 54. The magnetic head arm 32 is highly accurately positioned by the phase shift signals to write servo track signals.
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