| 摘要 |
A laser diode (L) source beam is received by a beam splitter prism (U) that directs the beam through collimating (K) and focussing (O) lenses and a semi-transmissive mirror (S) behind which the disc is located. The focussing lens (O) can be subjected to an axial vibration. Reflected signals from the disc are directed by a lens (LS) onto a photodetector (PH) having square of four sensor segmetns. Difference values based on the sensor signals are derived that describe the adjustments to be made to the focussing lens position to provide accurate adjustment of the beam on the disc.
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