Patent CA-2018314-A1 (CA)
IPC: G11B7/08
Filed: 1990-06-05
Abstract: The  invention  discloses  an  optical  head  device having:-a diffraction grating element divided into a first diffraction grating and a second diffraction grating by a plane including an optical  axis that connects a light emitting point on a semiconductor laser to a focal point on a recording medium such as an optical disk, so that the first and second diffraction gratings have a substantially equal diffraction angle,  a substantially equal optical utilization efficiency, and different focal distances of first order diffracted lights, each of the first order diffracted lights being produced from a reflected light from the recording medium, at the respective diffraction gratings; and a photodetector positioned at a mid-position between two focal points produced when there occurs no focusing error,  the photo-detecting means having a plurality of photo-detecting   parts   juxtaposed   in   a   direction substantially  orthogonal  to  an  average  diffracting direction of the diffraction grating element.    In this optical head, the first diffraction grating and second diffraction grating  are designed  such  that lights diffracted at the first diffraction grating and second   diffraction   grating   are   guided   into   the photo-detecting means to form light spots in the vicinity of centers of the corresponding photo-detecting parts, and the light spots are aligned in a direction substantially orthogonal to the average diffracting direction of the diffraction  grating  element.   The  above  arrangement enables to produce a small size optical head device and improve  the  detection of  focusing errors  and tracking errors.     The invention also discloses an optical head device having:-a diffraction grating element divided into first to forth diffraction gratings by two planes including the optical axis which connects the light emitting point of the  semiconductor  laser  to  the  focal  point  on  the recording medium.   In this diffraction grating element, the first to forth diffraction gratings are paired off so that each pair is symmetrical.  When focal points of first order diffracted lights in the first to forth diffraction gratings  are  plotted  in  three  dimensional  orthogonal coordinates on condition that a direction Z parallel with the optical axis is plotted as the Z-coordinate, at least the Z-coordinates of the focal points of first diffracted lights  in  diffraction  gratings   in  each  pair  are substantially identical, and the Z-coordinate of the focal point for first order diffracted lights in a first pair of diffraction gratings differs from the Z-coordinate of the focal point for first order diffracted lights in a second pair of diffraction gratings.   The optical head device further has a photodetector disposed at a mid-position between the focal points of the first order diffracted lights produced in the first pair of diffraction gratings and the focal points of the first order diffracted lights produced in the second pair of diffraction gratings, the photodetector having a plurality of photo-detecting parts which  correspond  to  the  first  to  forth  diffraction gratings respectively.  In this optical head device, the diffracted lights produced in the pairs of diffraction gratings  are  guided  to  the  photodetector  so  as  to respectively form light spots in proximity to a center of the  corresponding  photo-detecting  part.    The  above arrangement enables to produce a small size optical head device and improve the detection of focusing errors and tracking errors.