To provide small-sized focus detecting device capable of detecting with resistance to disturbance noise by a method wherein functions of detection of variation of a beam spot shape having variation of face run-out of an optical disk and of conversion of a light-electricity signal are executed individually by a plurality of optical fibers and a non-dividing type photodetector.
A light beam 1a from a laser light source 1 is divided into a penetration light 2a and a reflection light 2b by a beam splitter BS2. The reflection light 2b is converted into a parallel right by a collimate lens 3 to be collected on an optical disk 5 by an objective lens 4. A reflection light 5a by the optical disk 5 is again divided into penetration lights 2c, 2d through the lenses 4, 3 and BS2. The penetration light 2c is divided into a penetration light 6a and a branch light 6b by a light passage branch circuit 6 to be incident on optical fibers 7a, 7b. Diameters of beam spots on the optical fibers 7a, 7b are varied along with variation of face run-out of the optical disk 5. Electric signals in relation to optical intensities of the optical fibers 7a, 7b are outputted from photodetectors, then a focus error signal is obtained from a comparator.
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UJIIE MASAHIKO
KAYANO KAZUNORI
TSUCHIYA RIKIO
SUGIURA HIROTOSHI
