| 摘要 |
PURPOSE:To raise the separation accuracy against a tracking error signal, by detecting a focus error signal from diffracted light of an unequal interval diffraction grating. CONSTITUTION:Laser light 20 is reduced to about 1mum by an objective lens 12 through a beam splitter 13, and is irradiated onto an optical disk 11. Reflected light 21 is reflected by an unequal interval diffraction grating 14, a focus error is detected by a photodetector 16, the light transmits, and a tracking error is detected by a photodetector 18. In case when the diameter of a luminous flux of the reflected light 21 is 4mm., when an interval of the unequal interval grating is selected to 3.9-7.4mum against semiconductor laser light whose wavelength is 830nm, the diffracted light is focused to a position which is 1.5mm. from the center of the unequal interval grating in the vertical direction of the optical axis, and a linear focus is formed. By detecting this light by a 4-split photodetector 16, a focus error signal is obtained. |
