| 摘要 |
PURPOSE: To facilitate exact alignment of two beams in the detection of focussing error in the differential beam size method by making two beams incident on an optical detector in a manner that the beams are deflected from the optical axis by a deflecting means so as to have specific deflection angles which are equal to each other but opposite in sign. CONSTITUTION: In a beam splitter(BS) 1, its end faces from which two split detection beams outgo are slanted with minute, equal angles θ, θ' symmetrically to each other. Hence, the two beams outgoing from the BS 1 also are slanted with equal angles. In this case, the BS 1 is moved along the optical axis to change the distance between the beams on the surface of the detector 3. When, for example, the detector 3 is placed at positions of A, A', A", the distance between the beams is narrowed in the order shown as a, a', a" on the detector. For equalizing the beam sizes of the two beams on the two detectors, the converging lens 2 alone is moved along the optical axis for adjustment independent of the BS 1. By adjusting the positions of the detector 3 together with its package in a plane vertical to the optical axis, the positional relations between two beams and two corresponding detectors are made to satisfy desired conditions. |
