Optical head for an optical recording and reproducing apparatus
BACKGROUND OF THE INVENTION 
     This invention relates to an optical head for an optical recording and reproducing apparatus by which information is optically recorded and reproduced on an information carrier surface. 
     FIG. 1 illustrates a conventional optical head disclosed for example in Japanese Patent Laid-Open No. 6076038, in which a light beam emitted from a semiconductor laser 1 is collected by an objective lens 4 and focused in one of guide grooves 3 in an information carrier 2. The reference numeral 5 designates a half prism which allows the passage of the light beam from the semiconductor laser 1 therethrough and which reflects a light beam reflected from the information carrier 2. The reflected light beam from the information carrier 2 is divided into three beams is passed through a cylindrical lens 7 exhibiting a lens function in one direction and the divided beams are introduced into a light receiving element 8 which converts light signals into electrical signals. The light receiving element 8 comprises three light receiving units 9, 10 and 11. 
     With the above construction, the light beam emitted from the semiconductor laser 1 is collected on the information carrier 2 through the half prism 5 and the objective lens 4 and, after being reflected, returns along the same path as the incoming light path. However, the light beam is reflected at the half prism 5 in a direction perpendicular to the light beam directed the semiconductor laser 1 to pass through the prism 6 positioned on the far field region of the reflected light beam. FIG. 2 illustrates the relationship of the prism 6 and the light beam passing through the prism 6, from which it is seen that the light beam 13 is divided into three when passing through the slanted surface 6a and 6c and the top surface 6b of the prism 6, and the divided beams reach the light receiving element 8 after passing through the cylindrical lens 7. The prism 6 has a trapezoidal cross section and has formed thereon side slanted surfaces 6a and 6c and a central flat surface 6b. 
     Description will now be made as to the principle according to which a focus error signal can be obtained. FIGS. 3a, 3b and 3c as well as FIGS. 4a, 4b and 4c are views illustrating relationships between various relative positional relationships of the information carrier 2 to the objective lens 4 and the corresponding shape of the spot on the receiving element 8. In FIGS. 4a, 4b and 4c, the reference numerals 9, 10 and 11 designate the light receiving units on the light receiving element 8, the light receiving unit 10 comprising four light receiving units 10a, 10b, 10c and 10d. As shown in FIGS. 4a, 4b and 4c, the light beam passed through a cylindrical lens 7 provides a light spot (shown by a shaded circle) on the light receiving element 8 which varies in shape as shown in FIGS. 4a, 4b and 4c corresponding with the relative positional relationship between the information carrier 2 and the objective lens 4 as shown in FIGS. 3a, 3b and 3c. Thus, by picking up these changes of the spot shape in the form of the following equation as an electrical signal, a focus error signal can be obtained. 
     
         [light receiving unit 10a output+light receiving unit 10d output]-[light receiving unit 10b output+light receiving unit 10c output . . . (1) 
    
     Next, description will be made as to the principle according to which a tracking error signal can be obtained. FIGS. 5a, 5b and 5c illustrate relative positional relationship between the focused light spot and the guide groove 3 on the information carrier 2, and FIGS. 6a, 6b and 6c illustrate the relationship with respect to the light beams passing through the corresponding prisms. In these figures, the reference numeral 12 designates a light beam focused by the objective lens 4, and 13 designates a light beam passing through the prism 6. As shown in FIGS. 5a and 6a or 5c and 6c, when the light beam 12 shifts from the center of the guide groove 3, the light beam passing through the slanted surface 6a or 6c varies in light amount due to the influence of the diffracted light of the ± first order. Since the light beam which passed through the slanted surfaces 6a and 6c reaches the light receiving units 9 and 11 in FIG. 4, the amount of the light which reaches the light receiving units 9 and 11 varies in accordance with the relative positional relationship between the focused spot and the guide groove 3. By picking up these changes as an electrical signal in the form of the following equation, a tracking error signal can be obtained. 
     
         [light receiving unit 9 output]-[light receiving unit 11 output]. . . (2) 
    
     As the conventional optical heads are constructed in such a manner as above, there was a problem in that cylindrical lenses 7 had to be disposed therein to obtain the focus error signal, increasing the number of optical parts. 
     SUMMARY OF THE INVENTION 
     The present invention was developed to resolve the above mentioned problem, so that the number of optical parts can be reduced, as a result of which the present invention is aimed at providing an optical head that can be miniaturized. 
     The optical head according to the present invention has formed a cylindrical surface in the central portion of its prism. 
     According to the present invention, the central portion of the prism is formed as a cylindrical lens and even when the cylindrical lens is omitted, the light beam which passes therethrough varies in the shape of the light spot on the light receiving unit in accordance with the relative positional relationship between the information carrier and the objective lens, so that the optical head functions in a manner similar to that provided with a cylindrical lens. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more readily apparent from the following detailed description of the preferred embodiment of the present invention taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a diagram showing an optical path of the conventional optical head; 
     FIG. 2 is a view showing the relationship between the light beam and the prism shown in FIG. 1; 
     FIGS. 3a-3d and 4a-4c are diagrams useful for explaining the principle of a means for obtaining the focus error signal in FIG. 1; 
     FIGS. 5a-5c and 6a-6c are diagrams useful for explaining the principle of means for obtaining a tracking error signal in FIG. 1; 
     FIG. 7 is a a diagram showing an optical path of one embodiment of the present invention; 
     FIG. 8 is a perpsective view of a portion of the head shown in FIG. 7; and 
     FIGS. 9a-9c and 10a-10c are diagrams useful for explaining the principle of a means for obtaining the focus error signal in FIG. 7. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIGS. 7 and 8 illustrate one embodiment of the present invention, in which a prism 16 having a substantially trapezoidal cross section has on its central portion between slanted surfaces 16a and 16c a cylindrical surface 16b, and each of the slanted surfaces is disposed so that the direction of dividing line of each of the surfaces optically coincides with the direction of extension of the guide groove 3. 
     The reference numerals which are the same as those used in FIG. 1 indicate identical components. It is to be noted that the cylindrical lens 7 in FIG. 1 is removed 
     The operation will now be described. A diverging light beam emitted from the semiconductor laser 1 passes through the half-prism 5 and the objective lens 4 and focused on the information carrier 2. The light beam is then reflected in the opposite direction along the same light path as before and is reflected by the half-prism 5 toward the prism 16. The reflected light beam passed through the prism 16 is divided into three by the slanted surfaces 16a and 16c and the cylindrical surface 16b. Since the central portion of the prism 16 is a cylindrical lens, the light beam which passes through the central portion varies the spot configuration shown by a shaded circle on the light receiving unit 10 as shown in FIGS. 10a, 10b and 10c in correspondence with the relative positional relationship between the information carrier and the objective lens shown in FIGS. 9a, 9b and 9c. By picking up these changes as an electrical signal in a form similar to the equation (1), the focus error signal can be obtained. 
     Now, how the tracking error signal is obtained will be explained with reference also to FIGS. 5 and 6. As shown in FIGS. 5a and 6a or 5c and 6c, when the light beam 12 is shifted from the center of the guide groove 3 the light quantity of the light beam passing through the slanted surface 16a or 16c of the prism 16 varies due to the influence of the diffracted light of the ± first order. The light beam which passed through the slanted surfaces 16a and 16c reaches the light receiving units 9 and 11 in FIG. 10, so that the quantity of light which reaches the light receiving units 9 and 11 varies in accordance with the relative positional relationship between the focused light spot and the guide groove 3. By picking up this change as an electrical signal in a form identical to the equation (2) the tracking error signal can be obtained. 
     As has been described, according to the present invention, since the central portion of the prism for dividing the light beam into three is formed into a cylindrical surface, the conventional cylindrical lens used for obtaining the focus error signal can be omitted, so that a less expensive and small-sized optical head can be obtained.