OPTICAL HEAD

PURPOSE:To obtain good focus error signals by separately controlling luminous fluxes in such a manner that the ordinary luminous flux increases an astigmatism by suppressing coma aberrations and that the extraordinary luminous flux suppresses both of the coma aberration and the astigmatism. CONSTITUTION:This optical head has a semiconductor laser 1, a composite prism 2, a collimator lens 6 constituting a condenser device, an objective lens 7, an aberration controller 9, a first photodetector 11, a second detector 12 and a sub-mount 15. The photodetectors 11, 12 are formed on the same semiconductor substrate 10 and the semiconductor laser 1 is fixed via the sub-mount 15 to this substrate 10. A composite prism is fixed via the aberration controller to the substrate 7. The aberration controller 9 separately controls the luminous fluxes in such a manner that the ordinary luminous flux increases the astigmatism by suppressing the coma aberrations and that the extraordinary luminous flux suppresses both of the coma aberration and the astigmatism. As a result, the application of the method for detecting the focus error signals respectively adequate for the ordinary luminous flux and the extraordinary luminous flux is made possible.