| 摘要 |
PURPOSE:To read a high density data by detecting a beat wave made by modulation of a diffraction grating and recorded information of an information medium with a photodetector after returning to an objective lens. CONSTITUTION:Detection light from a semiconductor laser 1 is converged by a lens L0 and modulated by the modulating diffraction grating 13 in its focus position and then converged on the information medium 7 by a lens L1. Its return light is transmitted via a lens L2 through a demodulating diffraction grating 16 in its focus position to be demodulated, and is converged by a lens L and then received and detected by a pin photodiode 10. A laser beam emitted from the semiconductor laser 1 is modulated by the modulating diffraction grating 13, and the beat wave is made by this modulated laser beam and an information pit P of the information medium 7. When a frequency of this beat wave is lower than a cut-off frequency of the objective lens, the beat wave in the return light can be transmitted by the lens L2. Since no information can be read out under such a circumstance, the beat wave is converged and demodulated by the demodulating diffraction grating 16 and then received and detected by the pin photodiode 10. |
