| 摘要 |
PURPOSE:To reduce the beam diameter without lowering the detection sensitivity to improve the precision of following-up of an object lens, by constituting the device so that the luminous flux reflected from a recording face is converged on a light receiving face of a differential photodetector with a prescribed beam diameter. CONSTITUTION:A luminous flux 10A emitted from a semiconductor 6 becomes a convergent luminous flux 10B by a coupling lens 7 and is irradiated onto a recording face 1A through a polarizing beam splitter (PS) 5 and an object lens 3. The reflected light is reduced to a luminous flux 10C and is reflected by the PS5 and is condensed onto a differential photodetector 8. Since the luminous flux made convergent by the lens 7 is extended to luminous fluxes 10B, 10C, and 10A and is converged onto the light receiving face, the beam diameter is made shorter, and the length between the object lens 3 and the light receiving face is made longer, and therefore, the precision of following-up of the object lens 3 is improved without lowering the detection sensitivity. |
