| 摘要 |
PURPOSE:To prevent a loss of light quantity and to make the device low in cost. CONSTITUTION:In order to transmit one of the light beams with two wavelengths emitted from a light source 1 and to diffract another light beam, a phase diffraction grating 11 is interposed between the light source 1 and a photodetector 8. When the relation of the wavelengths is lambda1<lambda2, the phase difference of the phase diffraction grating 11 to the first light beam with the wavelength lambda1 is 2mpi (m is integer). The detection of a tracking error signal is performed as the difference between the (+ or -) first order beams of the second light beam with wavelength lambda2 and a reproduced RF signal is obtained from zero-order beam of the first light beam. Since cutting a part of the emitted light by a filter 2 is unnecessitated, the loss of light quantity is not occurred. Since the adjustment of the position of a single photodetector 8 is easily carried out, the assembling work of the photodetector 8 is simplified. The photodetector 8 is simplified and a comparatively expensive and conventional dichroic mirror 9 is unnecessitated. |
