| 摘要 |
PURPOSE:To detect a focus error signal with high accuracy by securing coincidence between the passing diameter and the opening diameter when the focused light passes through the opening of the 1st photodetector as well as between the passing diameter and the opening diameter when the scattered light passes through the opening of the 2nd photodetector in a focused mode. CONSTITUTION:In a focused mode the focused light received from a spherical lens 6 is coincident with an opening 7a of the 1st photodetector 7 and also with an opening 8a of the 2nd photodetector 8 respectively. Thus A=0 and B=0 are satisfied and the differential output (A-B) is equal to zero. The scattered light passed through the opening 7a and then a focusing point F' has a smaller diameter than the opening 8a and therefore passes through the opening 8a. Thus B=0 is satisfied and the output (A-B) is plus. While the scattered light passed through the opening 7a and then a focusing point F'' has a larger diameter than the opening 8a and is therefore made incident on a fringe 8b of the opening 8a. Thus the output (A-B) is minus. As a result, the focusing detection is attained by obtaining the differential output (A-B) between both photodetectors 7 and 8.
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