| 摘要 |
PURPOSE:To obtain a focus error signal having no error even if mechanical and positional shift of an optical system are generated, by correcting the positional shift to four photodetecting elements of a beam spot on a photodetector due to the mechanical and positional shifts, etc., by electrical and circuital processings. CONSTITUTION:Outputs of four photodetecting elemens 4A, 4B, 4C and 4D of a photodetector 4 are fetched as outputs A, B, C and D through pre-amplifiers 5A-5D, and supplied to a correcting circuit 10. The correcting circuit 10 amplifies separately the outputs of 4C and 4D so that the sum of outputs of the elements 4A, 4B is equalized to the sum of outputs of 4C and 4D. That is to say, the gain of a gain cotrol amplifier 11 and 12 is controlled by a voltage being proportional to S=(A+B)/(C+D), so that the sum (C'+D') of its outputs C', D' is equalized to (A+B). A, B, C' and D' and supplied to an operating circuit 20, and focus error signal F'=(A+C')-(B+D') is obtained. However, when the gain of the amplifier 11 and 12 is varied, the gain of a signal F' is also varied, therefore, in reality, a signal F'=F'/R' is fetched as a focus error signal. |
