Patent Publication Number: US-RE40895-E

Title: Optical head and apparatus for and method of storing and reproducing optical information

Description:
This is a reissue application of U.S. Pat. No.  6 , 680 , 894 , issued Jan.  20 ,  2004 . 
     This application is based on the patent application No. 2000-313673 filed in Japan, the contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical head, and an apparatus for and a method of storing and reproducing optical information. 
     2. Description of the Related Art 
     An optical storage technique using an optical disk with a pattern of a pit shape, which is one of information storage media of high densities and mass storage volumes, is increasing its applications such as a digital audio disk, a video disk, a document file disk and further a data file. In the optical storage technique, the information is stored (recorded) and reproduced with high accuracy and reliability by applying a minutely condensed light beam onto the optical disk. The storing or reproducing action solely depends on the optical system. Fundamental functions of an optical head, which is a main portion of the optical system, are roughly divided into convergence for forming a minute spot of diffraction limit, focus control and tracking control of the optical system, and detection of pit signals. These functions are achieved by combining any one of various optical systems with any one of various photoelectric transferring and detecting methods in accordance with the object and use of the optical system. 
     On the other hand, in recent years, an optical disk of high density and mass storage volume, which is a called DVD, has been put to practical use and highlighted as an information storage medium which can treat a very large volume of information such as moving images. In the DVD optical disk, the pit size on the surface for storing information is reduced in order to increase its storing density in comparison with that of a compact disk (shortly referred to “CD” hereinafter) which is one of conventional optical disks. Accordingly, in the optical head for storing and reproducing information on the DVD optical disk, its wave length of the light source defining the spot diameter or its numerical aperture (shortly referred to “NA” hereinafter) of the convergent lens is different from that of the CD. In this connection, as for the CD, the wave length of the light source is set to about 0.78 μm while the NA is set to about 0.45. On the other hand, as for the DVD optical disk, the wave length of the light source is set to about 0.63-0.65 μm while the NA is set to about 0.6. Therefore, in order to store or reproduce the information for the two kinds of optical disks of the CD and the DVD using one optical disk drive, the optical head is required to have two optical systems. On the other hand, because the optical head is required to be small, thin and inexpensive, there is such a tendency that parts of the optical systems of the CD and the DVD are used in common to the best of their ability. For example, a technique may be used such that only one light source for the DVD is used while two kinds of convergent lenses for the DVD optical disk and the CD are used. Further, it may be also used such a technique that only one convergent lens is used in common, and then the NA is mechanically or optically changed such that the NA is set to a larger value when the DVD optical disk is used while the NA is set to a smaller value when the CD is used. 
     Meanwhile, in order to reproduce a CD-R which is rapidly and increasingly prevailing in recent years, there has been widely produced an optical head with light sources having wave lengths which are optimized for the DVD and the CD (CD-R), respectively. In  FIG. 10 , there is shown an example of an interchangeable optical head for reproducing the DVD and CD-R. In the optical head, the interchangeability between the DVD and the CD is achieved by using a convergent lens including a portion optimized for the DVD and another ring-shaped portion optimized for the thickness of the CD, and by using a light source for the CD and another light source for the DVD. 
     Hereinafter, the optical head will be described with reference to drawings. As shown in  FIG. 10 , in the optical head, on an optical path between a disk  8  (i.e., DVD  8 a or CD  8 b) and a photo detector  54 , there are disposed an objective lens  7 , a collimator lens  3 , a rising mirror  9  and beam splitters  5 ,  6 . A light source  51  for reproducing the DVD is disposed on an optical path which is diverged from the beam splitter  5  while another light source  52  for reproducing the CD is disposed on an optical path which is diverged from the beam splitter  6 . In the optical head, the objective lens  7  has a shape to be able to form a spot having a size which is optimized for both of the DVD  8 a and the CD  8 b, the thickness of the base material of the DVD being smaller, and the thickness of the base material of the CD being larger (see FIGS.  5 A and  5 B). Meanwhile, the beam splitter  5  is composed of a plate having surfaces parallel to each other (parallel plate). As for both of a light beam  62 a for reproducing the DVD  8 a and a light beam  62 b for reproducing the CD  8 b, the reflected light beams of those generate astigmatism in the beam splitter  5 . The DVD  8 a is reproduced while composing the photo an detector  54  of a quartered detector and using the so-called astigmatism method as the focus error detecting method. Hereupon, as the tracking detecting method, the so-called phase difference method is used if the DVD  8 a is a DVD-ROM while the push-pull method is used if the DVD  8 a is DVD-RAM. In this connection, as the tracking method for the CD  8 b, the so-called three beam method is generally used using diffracted light which is generated by a grating (diffraction grating)  60  disposed between the light source  52  for the CD and the beam splitter  6 . Moreover, in order to stabilize the push-pull signals of the DVD-RAM, it is also suggested to use the so-called differential push-pull (DPP) method using diffracted light which is generated by a grating  59  disposed between the light source  51  for the DVD and the beam splitter  5 , as same as the case of reproducing the CD  8 b. 
     However, in the conventional interchangeable optical head using the respective light sources for the DVD and the CD, it is required to provide at least two beam splitters. Meanwhile, if different tracking error detecting methods are used for the DVD and the CD, respectively, namely if the DPP method is used for reproducing the DVD while the three beam method is used for reproducing the CD, it is required that respective optical elements with diffraction gratings are provided for both of the DVD and the CD. Therefore, the above-mentioned matters prevent the optical head from being made smaller or more inexpensive. 
     SUMMARY OF THE INVENTION 
     The present invention has been developed to solve the conventional problems described above and has an object to provide a small and inexpensive optical element of an optical head. 
     An optical head according to the present invention includes (i) a light emitting element in which a plurality of light sources are integrally formed, the light sources being able to emit beams having different wave lengths to one another, (ii) an optical system for converging a beam emitted by any one of the light sources of the light emitting element onto an optical information storage medium, (iii) an optical separator for separating a reflected beam coming from the optical information storage medium from the beam coming from the light source, and (iv) a light receiving element for detecting light quantities of the reflected beam separated by the optical separator. 
     In the above-mentioned optical head, the optical separator separates the reflected beam coming from the optical information storage medium from the beam coming from the light source by means of reflection and transmission of the beam. 
     Further, in the above-mentioned optical head, an aligning direction of the light sources preferably inclines by about 45 degrees in a rotational direction around an axis of the beam coming from the light source on the basis of a reflection axis of the optical separator. 
     Moreover, in the above-mentioned optical head, the light receiving element for receiving a zero-order-diffracted light, which is located at a central position, is preferably composed of regions of at least a number equal to four times of a number of the light sources, the regions being composed of region sets each of which includes four regions, each of the sets receiving the reflected beam which has been emitted by the light source and then reflected by the optical information storage medium. 
     Further, in the above-mentioned optical head, the optical separator is preferably composed of a parallel plate, that is a plate having surfaces parallel to each other. 
     In addition, the above-mentioned optical head further includes an optical element disposed between the light sources and the optical system, which includes two kinds of diffraction gratings disposed on front and back portions thereof. Hereupon, depths, pitches and angles for the axis of the beams of the two kinds of diffraction gratings are different from each other, respectively. 
     An apparatus for storing and reproducing optical information according to the present invention includes (i) a light emitting element in which a plurality of light sources are integrally formed, the light sources being able to emit beams having different wave lengths to one another, (ii) an optical system for converging a beam emitted by any one of the light sources of the light emitting element onto an optical information storage medium, (iii) an optical separator for separating a reflected beam coming from the optical information storage medium from the beam coining from the light source, (iv) a light receiving element for detecting light quantities of the reflected beam separated by the optical separator, and (v) an electric circuit for transforming optical signals detected by the light receiving element to electric signals so as to output signals stored in the optical information storage medium as the electric signals. 
     A method of storing and reproducing optical information according to the present invention, for an apparatus for storing and reproducing the optical information with an optical head, the optical head including (i) a light emitting element in which a plurality of light sources are integrally formed, the light sources being able to emit beams having different wave lengths to one another, (ii) an optical system for converging a beam emitted by any one of the light sources of the light emitting element onto an optical information storage medium, (iii) an optical separator for separating a reflected beam coming from the optical information storage medium from the beam coming from the light sources and (iv) a light receiving element for detecting light quantities of the reflected beam separated by the optical separator, includes the steps of (a) identifying the kind of the optical information storage medium, (b) making the light emitting element emit the beam having wave length corresponding to the identified kind of the optical information storage medium and then covering the beam onto the optical information storage medium, and (c) detecting the reflected beam coming from the optical information storage medium and then outputting signals stored in the optical information storage medium as electric signals. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Various characteristics and advantages of the present invention will become clear from the following description taken in conjunction with the preferred embodiments with reference to the accompanying drawings throughout which like parts are designated by like reference numerals, in which: 
         FIG. 1  is a schematic section view of an optical head when a CD is reproduced; 
         FIG. 2  is a schematic perspective view of the optical head when the CD is reproduced; 
         FIG. 3  is a schematic section view of the optical head when a DVD is reproduced; 
         FIG. 4  is a schematic perspective view of the optical head when the DVD is reproduced; 
         FIGS. 5A and 5B  are views showing functions of objective lenses; 
         FIG. 6  is a view showing positions of spots on the DVD; 
         FIG. 7  is a view showing positions of spots on the CD; 
         FIG. 8  is a perspective view showing a detailed construction of a grating element; 
         FIG. 9  is a schematic view showing an apparatus for reproducing optical information; and 
         FIG. 10  is a schematic section view of an optical system in a conventional optical head. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings. In the drawings, like parts are designated by like reference numerals. 
     An optical head according to the embodiment of the present invention is an interchangeable optical head for a DVD and a CD. In the optical head, there are used one light emitting element in which a plurality of light sources for the DVD and the CD are integrally formed therewith, and a convergent lens in which a portion thereof is optimized for the DVD while another ring-shaped portion thereof is optimized for the CD (or the thickness of the base material of the CD) so that interchangeability between the DVD and the CD is achieved. Hereinafter, the optical head according to the present embodiment will be described with reference to  FIGS. 1  to  4 , each of which shows the constriction of an optical system of the optical head. In those figures, the members common with those in  FIG. 10  are given the same reference numerals as those in FIG.  10 . 
     In the optical head, on an optical path between an optical disk  8  (i.e., DVD  8 a or CD  8 b) and a photo detector  4 , there are provided an objective lens  7 , a collimator lens  3  and a beam splitter  5 . Further, a dual channel semiconductor laser  1 , in which a light source  2 a for reproducing the DVD and a light source  2 b for reproducing the CD are integrally formed therewith, is disposed on an optical path which is diverged from the beam splitter  5 . In the optical head also, the respective light sources  2 a,  2 b for the DVD and the CD are used as same as the conventional case. However, in the optical head according to the present embodiment, the dual channel semiconductor laser  1  in which the light sources  2 a,  2 b having respective wave lengths are integrated, is used as different from the conventional case. Corresponding to that, only the one beam splitter  5  is used as the optical separator. 
     The objective lens  7  is a convergent lens, which is fundamentally optimized for the DVD. However, a portion of the objective lens  7  (convergent lens), which is formed of a ring-shaped zone, is optimized for the thickness of the base material of the CD. This feature is as same as that of the conventional case. Referring to  FIGS. 5A and 5B , a specific portion is provided at least on the input plane or output plane of the objective lens  7  (position different from that of the ordinary lens). In the specific portion, there is provided a doughnut-shaped or ring-shaped intermediate region A 2  having an outer diameter which is smaller than the overall effective diameter of the lens for the region through which the light passes. Further, a center region A 1  is provided at the inner side of the intermediate region A 2  while a peripheral region A 3  is provided at the outer side of the intermediate region A 2 . The curvature of each of the center region A 1  and the peripheral region A 3  is optimized for the DVD  8 a in which the thickness of the base material thereof is smaller. The curvature of the intermediate region A 2  is optimized for the CD  8 b in which the thickness of the base material thereof is larger. The intermediate region A 2  may be divided into a plurality of sub-regions in accordance with circumstances. It is preferable that the photo detector  4  is designed such that light in the far axis region does not reach the photo detector  4  when the information in the CD  8 b having a larger thickness is reproduced, namely the light reaches only the center region A 1  and the intermediate region A 2  of the objective lens  7 . 
     Accordingly, as shown by broken lines in  FIG. 5B , when the information is stored or reproduced to or from the CD  8 b having the thick base material, the light beam having the wave length of 780 nm for reproducing the CD, coming from the light source  52  for reproducing the CD passes through the center region A 1  and the intermediate region A 2  and then converged onto the CD  8 b. On that occasion, even if the curvature of the center region A 1  corresponding to the near axis region is optimized for the thin DVD  8 a, occurrence of spherical aberration is reduced because the near axis light passes near the center axis of the lens. Thus, when the information is stored or reproduced to or from the DVD  8 a, the beam having the wave length of 650 nm for reproducing the DVD, coming from the light source  51  for reproducing the DVD passes through the center region A 1  and the peripheral region A 3  each of which has the optimized curvature for the thin disk, and then forms a focus on the surface with the information of the thin disk  8 a. If the numerical aperture (NA) of the region corresponding to the near axis region and the far axis region of the objective lens  7  is set to about 0.45, smaller spots can be formed on the thick disk so that there can be formed spots having sizes which are optimized for the CD. 
     Hereupon, the beam splitter is composed of at optical member (i.e., parallel plate) of a plate shape having planes parallel to each other. The light passing through the beam splitter  5  causes an astigmatism. 
       FIGS. 1 and 2  show such a state that the CD  8 b, in which the thickness of the base material thereof is larger, is reproduced. A beam  12 b coming from the light source  2 b in the semiconductor laser  1 , which emits the beam with the wave length of 780 nm for the CD, is reflected by the beam splitter  5  and the rising mirror  9  in turn, transformed to a parallel beam by the collimator lens  3 , and then converged onto the CD  8 b by the objective lens  7 . The principle that the beam is converged onto the CD  8 b, is the same as that of the conventional case. Therefore, it is possible to form spots having sizes which are optimized for the CD  8 b. 
     When the CD  8 b is reproduced also, the reflected beam coming from the CD  8 b reaches the beam splitter  5  through the objective lens  7 , collimator lens  3  and the rising mirror  9 . On that occasion, the astigmatism is caused in the beam splitter  5 . The reflected beam passes through the beam splitter  5  and then reaches the photo detector  4 . The light receiving section of the photo detector  4 , which receives the light for reproducing the CD is composed of a quartered detector of rectangular shape so that the signal for detecting the focus error is generated by means of the so-called astigmatism method. 
     Meanwhile, the so-called three-beam method is used as the procedure for detecting the tracking error when the CD  8 b is reproduced. Each of three beams is a diffracted beam which is generated by a grating  11 b formed on a primary surface side of a grating element  10  disposed between the semiconductor laser  1  and the beam splitter  5 . 
     A detailed construction of the grating element  10  is shown in FIG.  8 . 
       FIGS. 3 and 4  show such a state that the DVD  8 a, in which the thickness of the base material thereof is smaller, is recorded or reproduced. A beam  12 a coming from the light source  2 a in the semiconductor laser  1 , which emits the beam with the wave length of 650 nm for the DVD, is converged onto the DVD  8 a through the beam splitter  5 , the rising mirror  9 , collimator lens  3  and the objective lens  7  in turn, the same as the case of reproducing the CD. On that occasion, the principle that the beam is converged onto the DVD  8 a, is the same as that of the conventional case. Therefore, it is possible to form spots having sizes which are optimized for the DVD  8 a. Because the beam splitter  5  is composed of the parallel plate, the reflected beam coming from the DVD  8 a also causes the astigmatism when it passes through the beam splitter  5 , the same as the case of reproducing the CD  8 b. The light receiving section of the photo detector  4 , which receives the light for reproducing the DVD is composed of a quartered detector of rectangular shape so that the signal for detecting the focus en or is generated by means of the so-called astigmatism method. Hereupon, as the tracking detecting method, the so-called phase difference method is used if the DVD  8 a is a DVD-ROM while the push-pull method is used if the DVD  8 a is DVD-RAM. 
     Further, in order to stabilize the push-pull signal of the DVD-RAM, the so-called differential push-pull (DPP) method is used. In the DPP method, there is used a diffracted beam which is generated by a grating  11 a formed on a secondary surface side of the grating element  10  disposed between the semiconductor laser  1  and the beam splitter  5 . 
     In the photo detector  4 , the light receiving element for receiving a zero-order diffracted light, which is located at a central position, is composed of light receiving regions of at least a number equal to four times of a number of the light sources. Hereupon, on both sides of the light receiving element for receiving the zero-order diffracted light, the light receiving element for receiving the positive first-order diffracted light and the light receiving element for receiving the negative first-order diffracted light are disposed, respectively (i.e., one by one). In the light receiving element, the light receiving regions are composed of region sets each of which includes four light receiving regions. Thus, the light receiving regions are arranged so as to receive the reflected beam which has been emitted by the light source and then reflected by the optical disk. The photo detector  4  may be, for example, a quartered detector. As shown in  FIGS. 2 and 4 , in the photo detector  4 , the light receiving section  4 b for receiving the CD reproducing beam and the light receiving section  4 a for receiving the DVD reproducing beam are arranged in such a manner that the six photo detecting elements make a matrix composed of two columns and three rows. Hereupon, these are arranged in such a manner that the parting lines for forming the quartered detectors of rectangular shapes are continuously connected to one another. By arranging the light receiving sections as described above, the sizes of the quartered detectors can be increased in relation to the interval between the light spot of the CD reproducing beam and the light spot of the DVD reproducing beam so that the extent which can detect the focus signal is increased. 
     If the detectors are arranged as described above, the aligning direction of the light spot of the CD reproducing beam and the light spot of the DVD reproducing beam becomes parallel to the above-mentioned parting lines of the detectors. Accordingly, it is preferable that the aligning direction inclines by 45 degrees in the rotational direction around the optical axis of the reproducing beam, in relation to the parallel plate  5  which causes the astigmatism. That is, it is necessary that in the semiconductor laser  1 , the aligning direction of the light source  2 a having the wave length for the DVD and the light source  2 b having the wave length for the CD inclines by 45 degrees in the rotational direction around the axis of the emitted beam in relation to the parallel plate  5 . 
     Meanwhile, because the so-called phase difference method is used as the procedure for detecting the tracking error during the reproducing process of the DVD-ROM while the push-pull method is used as the procedure for detecting the tracking error during the reproducing process of the DVD-RAM, it is necessary that the transferred track image on the photo detector  4 a for reproducing the DVD (the image of the pit arrays in the DVD  8 a which is transferred onto the photo detector  4 a) is parallel to the parting lines for forming the quartered detectors of the photo detector  4 a. Therefore, the optical axis extending from the rising mirror  9  to the photo detector  4 a through beam splitter  5  is disposed in the direction inclined by 45 degrees in relation to the track of the DVD  8 a (pit array). 
     As described above, when the DVD-RAM is reproduced, the so-called DPP method is used in order to improve the stability of the tracking error signal. Hereupon, the pitch, direction and depth of the grating  11 a formed on the front (primary) surface of the grating element  10  are different from those of the grating  11 b formed on the back (secondary) surface of the grating element  10 , respectively. In  FIGS. 6 and 7 , there are shown the spots located on the DVD-RAM and the spots located on the CD, respectively. As shown in  FIG. 6 , when the DPP method is used as the procedure for detecting the tracking error during the DVD-RAM reproducing process, it is preferable that the so-called sub-beam generated by the diffraction is separated from the reading spot in the radial direction of the disk by the distance equal to ½ (half) of the track pitch (in this case, pitch of land to land). On the other hand, as shown in  FIG. 7 , because the so-called three beam method is used as the procedure for detecting the tracking error during the CD reproducing process, it is preferable that the so-called sub-beam generated by the diffraction is separated from the reading spot in the radial direction of the disk by the distance equal to ¼ (quarter) of the track pitch (in this case, pitch of pit array to pit array). Accordingly, if the spot intervals (interval between the sub-beam and the reading beam) of the DVD  8 a and CD  8 b are identical to each other, the track based angle of the grating  11 a must be different from that of the grating  11 b. Further, because their optical distances from the semiconductor laser  1  are different from each other, the pitches of the gratings  11 a,  11 b are also different from each other. Regarding to the depths of the gratings, in view of the efficiency for using light, it is advantageous to design the depth of the grating  11 a for the DVD in such a manner that the beam coming from the light source  2 b for the CD is not diffracted to the best of its ability. Meanwhile, it is advantageous to design the depth of the grating  11 b for the CD in such a manner that the beam coming from the light source  2 a for the DVD is not diffracted to the best of its ability. Accordingly, it is preferable that the depths of the gratings  11 a and  11 b are also different from each other. Therefore, in the present embodiment, the optical depth of the grating  11 a is set to the value identical to the wave length of 780 nm of the light emitted by the light source  2 b for reproducing the CD. In consequence, although the beam having the wave length for reproducing the DVD is diffracted, the beam having the wave length for reproducing the CD is not diffracted. On the other hand, the optical depth of the grating  11 b is set to the value identical to the wave length of 650 nm of the light emitted by the light source  2 a for reproducing the DVD. In consequence, although the beam having the wave length for reproducing the CD is diffracted, the beam having the wave length for reproducing the DVD is not diffracted. 
       FIG. 9  shows an apparatus for reproducing optical information (optical disk drive), which is provided with the above-mentioned optical head. The optical disk drive can reproduce the information of the CD and the DVD. The control circuit  20  can make the light source  2 a or  2 b of the semiconductor laser  1  in the optical head emit the beam. In the optical head, the beam emitted by the light source  2 a or  2 b is reflected by the beam splitter  5  and the mirror  9 , and then converged by the convergent lens  3  to become a parallel beam. Thus, a minute light spot is formed onto the optical disk  8  from the parallel beam by the objective lens  7 . The reflected beam coming from the optical disk  8  passes through the objective lens  7  and the convergent lens  3 , and then is reflected by the mirror  9 . Further, the reflected beam passes through the beam splitter  5  and then is detected by the photo detector  4 . The control circuit  20  drives a spindle motor  42  to rotate the optical disk  8  on the basis of the electrical signal coming from the quartered detector of the photo detector  4 . Further, the control circuit  20  actuates the actuator to perform the tracking control on the optical disk  8 . In addition, the control circuit  20  outputs the signals stored in the optical disk  8  to the external device as electrical signals. Because the construction and action of the control system described above is as same as that of the conventional optical disk drive, detailed descriptions as to the control system are omitted. 
     Although the above-mentioned optical information processor is one for only reproducing use, the above-mentioned optical head can be of course used also for an optical information processor having a storage function. 
     When the optical information processor is used, it is required that the control circuit  20  identifies the kind of the optical information storage medium (CD or DVD). Various methods of identifying the storage media have been suggested and used. The control circuit  20  is provided with an identification section  21  for identifying whether the storage medium is a CD or DVD using any one of the above-mentioned methods. When an optical information storage medium is set in the processor, the identification section  21  of the control circuit  20  judges the kind of the optical information storage medium. Then, the control circuit  20  makes the light source  2 a or  2 b in the semiconductor laser  1  emit a light in accordance with the judgement. Thus, the reproducing/storing of the optical information storage is performed. 
     In the optical head according to the present invention which stores or reproduces the optical information using beams whose wave lengths are different from each other, the light emitting element, in which a plurality of light sources which emit beams with different wave lengths are integrated, is used. In consequence, it may be possible to reduce the number of optical parts such as the beam splitter, etc. so that the optical head may be small-sized while lowering its cost. Accordingly, the apparatus for storing or reproducing the optical information may be also small-sized while lowering its cost. 
     Although the present invention has been fully described in connection with the preferred embodiments thereof with reference to the accompanying drawings, it is to be noted that various changes and modifications are apparent to those skilled in the at. Such changes and modifications are to be understood as included within the scope of the present invention as defined by the appended claims unless they depart therefrom.