| 摘要 |
A double focal-point magneto-optic pick-up head is provided. This magneto-optic pick-up head reads data from an optical disc by using a diffraction grating to diffract a laser beam into two diffracted beams of equal intensity which are focused on the optical disc at two focal points. One focal point is pinpointed on one groove portion of the optical disc, and the other focal point is pinpointed on one neighboring land portion of the same. This allows the two reflected beams to have a difference in propagation length equal to twice the depth of the land portion. The reflected beams are subsequently diffracted again by the diffraction grating into four beams, two of which are directed to interfere with each other to thereby produce an interference beam representative of a read signal from the optical disc. Since the depth is fixed by manufacture, the difference in propagation length between the two reflected beams from the two focal points on the optical disc can always be maintained at a constant. The interference between the two beams for the read signal is thus precise, allowing the output data from the magneto-optic pick-up head to be reliable.
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