Optical head device for reading information stored in a recording medium

Abstract: An optical head device for reading information stored in a recording medium (16) wherein a light beam from a light source is converged on an optical disk and a reflected beam from the optical disk is received by a light receiving element (17) thereby to read out information stored in the optical disk is characterized in comprising a diffraction element (13) including: first diffraction gratings (13e, f) for dividing the light beam from the light source into a 0th order diffracted beam utilized for reading out the information stored in the optical disk and a pair of first order diffracted beams utilized for reading tracking errors; and second diffraction gratings (13a-d) for guiding the reflected beam from the optical disk to the light receiving element. The transmission factors of 0th order diffracted beam of the first and/or second diffraction gratings in the regions located in the vicinity of both ends of the diffraction element are lower than those of the first and/or second diffraction gratings in the regions located in the vicinity of the center of the diffraction element, the ends and center being set in a corresponding direction to the radial direction of the optical disk.

Description: OPTICAL HEAD DEVICE FOR READING INFORMATION STORED IN A RECORDING MEDIUM The present invention relates to an optical head device for reading out various high-density information stored in a track or tracks on a recording medium. In an optical head device for reading out information from an optical disk wherein various high-density information is stored, the reading out of information is performed by projecting a laser beam to produce a beam spot converged on a narrow recording track and detecting a reflected beam therefrom. Fig. 12 shows an example of the optical head device for reading information stored in a recording medium as above mentioned, in which a laser beam emitted from a laser beam source 1 passes through a diffraction grating 2 for producing a beam spot for tracking, and is irradiated onto an optical disk 6 through a collimating lens 4 and an objective lens 5 after being reflected by a beam splitter 3. The reflected beam from the optical disk 6 is transmitted to the beam splitter 3 through the objective lens 5 and the collimating lens 4 and is incident on a photodetector 8 through a plane-concave lens 7. The above reflected beam is converted into an electric signal in the photodetector 8 Fig. 13 shows an example of a prior optical head device wherein a diffraction element 9 comprising a holographic grating (hologram grating) is employed instead of the beam splitter 3. In this prior optical head device, the laser beam source 1, the diffraction grating 2 are aligned with the collimating lens 4, the objective lens 5 and the like, while the photodetector 8 is placed by the side of the laser beam source 1 so that a reflected beam from the optical disk 6 is diffracted by the diffraction element 9 and guided to the photodetector 8. In the prior optical head device as shown in Figs. 12 and 13, the laser beam is converged on the optical disk 6 so as to have substantially the same width as that of a recording track 6a (the recording track 6a is hatched in Fig. 11, for convenience of explanation) as indicated by mark S of Fig. 11 and the recording track 6a is tracked by a tracking servo-mechanism (not shown in the drawings), thereby reading out information recorded on the recording track 6a in accordance with the reflected beam therefrom. Since the recording track 6a formed on the optical disk 6 has a very narrow width of 1 to 2 µm approx., not only is the laser beam to be irradiated onto the recording track 6a required to be converged into a small beam spot by the objective lens 5 having a high numerical aperture (NA), but also the beam spot must have a high luminous intensity. However, there is such a drawback in the above method that an airy ring indicated by mark S₁ is generated in a converged laser beam owing to the secondary maximum elements as well known by one skilled in the art, and if the airy ring extends to the adjacent recording tracks 6a, crosstalk will occur at the time of reading out information stored in the optical disk. An approach to solve the above problem is that as shown in Fig. 14(a), a grating face 2a of the diffraction grating 2 and a grating face 2b opposite thereto, which are shown in Figs. 12 and 13, are respectively provided with a filter 10 mounted thereon, the filter 10 having a light transmitting section 10a the width of which is narrower than that of the laser beam to be transmitted therethrough. As shown in Fig. 14(b), the transmission factors of 0th order diffracted beam in the vicinity of both ends of the diffraction grating, the ends being set in a corresponding direction to the radial direction of the optical disk 6, are controlled with the filter 10 (the portions where the transmission factor of 0th order diffracted beam is controlled, are hatched in Fig. 14), thereby reducing the occurrence of crosstalk at the time of reading information recorded on the recording track 6a. Such a filter 10 is generally produced by a metal vapor deposition and the production of the filter 10 by the vapor deposition brings about complexity in the manufacturing process as well as an increase in the production cost. Since an optical head device usually requires a large number of components and therefore, the addition of the filter 10 further increases the number of components resulting in making the structure of the device complicated all the more. Other prior proposals for using diffraction gratings in optical pickup devices are disclosed in the following documents. EP-A-0 228 620 discloses an optical head device in which a single diffraction grating directs a zeroth order diffracted beam toward an optical recording medium and directs diffracted beams, produced from an optical beam reflected from the medium, to an optical detector assembly. FR-A-2 533 732 discloses an optical recording medium reproducing apparatus employing a diffraction grating to produce the main and sub-beams required for a 3-beam method of tracking control. The reflected beams are deflected by a beam splitter toward respective photodetectors. EP-A-0 195 657 reflecting the subject matter of the preamble of claim 14 discloses an optical pickup device for use with an optical medium, including a compact optical system having two hologram lenses. A light beam from a light source is diffracted toward the medium by a first such lens after being transmitted by the second lens; the reflected beam is transmitted through the first lens and then diffracted by the second lens toward an optical sensor. EP-A-0 305 169, which is relevant only under Article 54(3) EPC, discloses an optical pickup device including a grating assembly between a light source and an information disc, the assembly having a first diffraction grating on its source-side face for dividing the incident beam into a main and two sub-beams for tracking control, and a second diffraction grating on its disc-side face to deflect a part of the light beam reflected from the disc to a photodetector assembly. DE-A-3 522 849 discloses an optical head device using a diffraction grating for diffracting light reflected from an optical recording medium toward a photodetector. EP-A-0 388 840, which also is relevant only under Article 54(3) EPC, discloses an optical pickup device including a diffracting element divided into two regions by a parting line extending in a direction corresponding to a direction transverse to the direction of the track or tracks of a recording medium; one region divides incident light from a light source into sub-spots for performing a 3-spot tracking method and the other region deflects the reflected beams onto a photo-detecting element. One aim of the present invention is to provide an optical head device for reading information stored in a recording medium, capable of preventing the occurrence of crosstalk at the time of reading out information recorded on a recording track without using a filter for controlling the Airy ring of a laser beam. Another aim of the present invention is to provide an optical head device for reading information stored in a recording medium, wherein a diffraction element having first diffraction gratings and second diffraction gratings in the same plane or a diffraction element having the first diffraction grating and the second diffraction grating respectively deposited on the light source side and the optical disk side is employed thereby reducing the number of components thereof. Still another aim of the present invention is to provide an optical head device for reading information stored in a recording medium, wherein in order to prevent the occurrence of crosstalk, a filter function for controlling the transmission factors of the 0th order diffracted beam in the vicinity of both ends of the diffraction element, these ends being set in a corresponding direction to the radial direction of the optical disk, is applied to the first diffraction gratings and/or the second diffraction gratings instead of employing a filter produced by vapor deposition, thereby reducing the number of manufacturing processes as well as the production cost. According to one aspect of the present invention, there is provided an optical head device for reading information stored in a track or tracks on a recording medium wherein a light beam from a light source is converged onto the medium and a reflected beam from the medium is received by a light receiving element thereby to read out information stored in the track or tracks, wherein there is provided diffraction means for forming a 0th order diffracted beam for reading out the information stored in the track or tracks, and including first diffraction grating means for producing from the light beam from the light source a pair of first order diffracted beams utilized for reading tracking errors; and second diffraction grating means for guiding the reflected beam from the recording medium to the light receiving element, the transmission factors for the 0th order diffracted beam of the first and/or second diffraction grating means in regions located at the ends of the diffraction means in a direction transverse to the track or tracks being lower than those of the first and/or second diffraction grating means in regions located generally centrally of the diffraction means in said direction. The first and/or second diffraction grating means can be provided with the functions of dividing a light beam into a 0th order diffracted beam and first order diffracted beams, and of performing a filter function by varying the different diffraction efficiencies for the 0th order diffracted beam and for the first order diffracted beams. This difference in transmission factors may be achieved by arranging that the grating depth of the second diffraction grating means in the regions located at at least one of said ends of the diffraction element is different from the grating depths of the second diffraction grating means in the regions located at the centre of the diffraction element. According to another aspect of the present invention, there is provided an optical head device for reading information stored in a track or tracks on a recording medium by utilizing a light beam from a light source which converges on the recording medium and is reflected from the recording medium to be guided by a diffraction element to a light receiving element thereby to read out information stored on the recording medium, a first diffraction grating being provided for diffracting the light beam travelling from the light source toward the recording medium characterized in that the first diffraction grating is positioned between the light source and the diffraction element, in that said diffraction element includes a second diffraction grating divided into a plurality of regions by at least one parting line which extends in a direction corresponding to the direction transverse to the track or tracks on the recording medium, and parting lines which extend in a direction corresponding to the direction along the track or tracks on the recording medium, and in that said diffraction element is constructed such that the transmission factors for a 0th order diffracted beam at regions of the second diffraction grating located at both ends of the diffraction element are lower than the transmission factors for a 0th order diffracted beam at regions of the second diffraction grating located at the centre of the diffraction element, said ends and centre being set in a direction corresponding to the direction transverse to the track or tracks on the recording medium. According to a further aspect of the present invention, there is provided an optical head device for reading information stored in a track or tracks on a recording medium by utilizing a light beam from a light source which converges onto the recording medium and is reflected from the recording medium to be received by a light receiving element thereby to read out information stored on the recording medium, the device including a diffraction element arranged to be positioned between the light source and the recording medium, said diffraction element including: a first diffraction grating, provided on the light source side of the diffraction element, for dividing the light beam from the light source into a 0th order diffracted beam utilized for reading out the information stored in the recording medium and a pair of first order diffracted beams utilized for reading tracking errors; and a second diffraction grating, provided at the recording medium side of the diffraction element, for guiding the beam reflected from the recording medium to the light receiving element, wherein the transmission factors for the 0th order diffracted beam are lower at both ends of said second diffraction grating than at the centre of said second diffraction grating, said ends and centre being set in a direction corresponding to the direction transverse to the track or tracks on the recording medium. Some embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which:- Figs. 1 to 4 show one embodiment of the present invention. Fig. 1 is an enlarged perspective view of a diffraction element, a photodetector and other components which form the basis of an optical head device for reading information stored in a recording medium. Fig. 2(a) is a plan view of the diffraction element. Fig. 2(b) is a graph showing the distribution of the transmission factors of 0th order diffracted beam in the diffraction element. Fig. 3 is a schematic perspective view showing the whole structure of the optical head device for reading information stored in a recording medium. Fig. 4 is a view illustrating the relationship between an objective lens and an optical disk. Fig. 5 is a plan view showing a diffraction element in another embodiment of the present invention. Figs. 6 to 8 show still another embodiment of the present invention. Fig. 6 is an enlarged perspective view of a diffraction element, a photodetector and other components which form the basis of an optical head device for reading information stored in a recording medium. Fig. 7(a) is a plan view of the diffraction element. Fig. 7(b) is a graph showing the distribution of the transmission factors of 0th order diffracted beam in the diffraction element. Fig. 8 is a schematic perspective view showing the whole structure of the optical head device for reading information stored in a recording medium. Fig. 9 shows a further embodiment of the present invention. Fig. 9(a) is a plan view of a diffraction element and Fig. 9(b) is a graph showing the distribution of the transmission factors of 0th order diffracted beam in the diffraction element. Fig. 10 shows a still further embodiment of the present invention and is a schematic front view showing the whole structure of an optical head device for reading information stored in a recording medium. Fig. 11 is a view illustrating the relationship between a recording track on an optical disk and a beam spot of a laser beam. Fig. 12 is a schematic front view showing the whole structure of a conventional optical head device. Fig. 13 is a schematic front view showing the whole structure of another conventional optical head device. Fig. 14(a) is a view illustrating the diffraction grating with a filter mounted thereon of the optical head device shown in Figs. 12 and 13. Fig. 14(b) is a graph showing the distribution of the transmission factors of 0th order diffracted beam in the diffraction grating shown in Fig. 14(a). DESCRIPTION OF THE EMBODIMENTS With reference to Figs. 1 to 4, an embodiment of the present invention will be hereinafter explained. The optical head device shown in Fig. 3 comprises a laser beam source 12 as a light source, and is designed such that a light beam emitted from a chip 12a of the laser beam source 12 (see Fig. 1) is converged on an optical disk 16 after passing through a diffraction element 13, a collimating lens 14 and an objective lens 15. The reflected beam emitted from the optical disk 16 passing through the objective lens 15 and the collimating lens 14, is incident on the diffraction element 13 and then guided from the diffraction element 13 to a photodetector 17 which has a function of a light receiving element, where the detection of tracking errors and focusing errors as well as the reading of information stored in the optical disk 16 are performed. In Fig. 3, arrows A-A' and B-B' represent the focusing direction and the radial direction respectively and arrow C-C' the tangential direction, i.e., the direction of the alignment of the pits formed on the optical disk 16. As shown in Figs. 1 and 2(a), the photodetector 17 comprises five independent photodetecting sections 17a to 17e, and the diffraction element 13, which is made of a plastic or glass, comprises six regions 13a to 13f which are formed separately from each other in the same plane. Among these six regions, four regions 13a to 13d together constitute a second diffraction grating while other two regions 13e and 13f together constitute a first diffraction grating. These first and second diffraction gratings respectively formed in the regions 13a to 13f are positioned at the surface of the diffraction element 13 opposing to the collimating lens 14. Mark P in Fig. 2(a) indicates the outmost area of the light beam which is to pass through the diffraction element 13. The functions of the first/second diffraction gratings in the regions 13a to 13f in the diffraction element 13 will be hereinafter explained. The light beam that has been emitted from the chip 12a of the laser beam source 12 and passed through the second diffraction grating in the region 13a is divided into a 0th order diffracted beam and first order diffracted beams. This 0th order diffracted beam passes through the collimating lens 14 by which a parallel light beam is produced and this parallel light beam is irradiated onto the pits on the optical disk 16 by the objective lens 15. The 0th order diffracted beam produced at the region 13a has a tilt angle in a specified range with respect to the optical disk 16 after passing through the objective lens 15 as indicated by arrow D of Fig. 4, and is reflected at the same angle as the incident angle thereof as indicated by arrow E in compliance with the light reflection law. Thereafter, the reflected beam passes through the objective lens 15 and the collimating lens 14 and then reaches the surface of the second diffraction grating in the region 13b of the diffraction element 13. The reflected beam from the optical disk 16 is divided by the second diffraction grating in the region 13b into a 0th order diffracted beam to be projected back to the laser beam source 12 as it is and first order diffracted beams to be incident on the photodetector 17. One of the first order diffracted beams produced by the second diffraction grating in the region 13b reaches on a parting line between the photodetecting sections 17b and 17c of the photodetector 17 and then the information stored in the optical disk 16 is detected while the beam spot of the laser beam is moved on the optical disk 16 in compliance with the surface vibration of the optical disk 16 or the like by means of a focus servo-mechanism (not shown in the drawings). The light beam that has been emitted from the laser beam source 12 and passed through the second diffraction grating in the region 13b of the diffraction element 13 is divided into a 0th order diffracted beam and first order diffracted beams. After being reflected by the optical disk 16, this 0th order diffracted beam returns to the region 13a of the diffraction element 13 and then diffracted by the second diffraction grating in the region 13a so as to be guided to the photodetecting section 17a of the photodetector 17, whereby information stored in the optical disk 16 is detected based on the diffracted beam received by the photodetecting section 17a. The light beams that have been emitted from the laser beam source 12 and reached the regions 13e and 13f of the diffraction element 13 are respectively divided into a 0th order diffracted beam and ± first order diffracted beams utilized for detecting tracking errors by the first diffraction gratings in the regions 13e and 13f. After being reflected by the optical disk 16, these first order diffracted beams return to the regions 13c and 13d of the diffraction element 13 and then diffracted by the second diffraction gratings in the regions 13c and 13d so as to be guided to the photodetecting sections 17e and 17d of the photodetector 17. In accordance with the diffracted beams received by the photodetecting sections 17e and 17d, the beam spot of the laser beam is controlled by means of a tracking servo-mechanism (not shown in the drawings) to track a recording track on the optical disk 16. The laser beams that have been emitted from the laser beam source 12 and reached the regions 13c and 13d of the diffraction element 13 are diffracted by the second diffraction gratings in the regions 13c and 13d so that the luminous intensity of each of the 0th order diffracted beams is decreased and these 0th order diffracted beams reach the optical disk 16. Most of the diffracted beams reflected by the optical disk 16 do not diverge to the photodetector 17 but reach the first diffraction gratings in the regions 13e and 13f of the diffraction element 13. The first diffraction gratings and the second diffraction gratings in the regions 13a to 13f are designed such that the transmission factor of the 0th diffracted beam derived from the light beam which has been emitted from the laser beam source 12 and passed through the diffraction element 13 in the foregoing process is high at the second diffraction gratings in the regions 13a and 13b located in the vicinity of the center of the diffraction element 13 as shown in Fig. 2(b), and is low at the second diffraction gratings in the regions 13c and 13d and the first diffraction gratings in the regions 13e and 13f compared with that at the second diffraction gratings in the regions 13a and 13b, the regions 13c to 13f being located in the vicinity of both ends of the diffraction element 13. The above mentioned center and ends of the diffraction element 13 are set in a corresponding direction to the radial direction of the optical disk (B-B' ). As described above, the distribution of the luminous intensity of the light beam at the time when it passes through the diffraction element 13 is equivalent to the distribution of the luminous intensity of the light beam which has passed through the conventional diffraction grating with the filter 10 mounted thereon. Hence, the occurrence of crosstalk can be positively prevented when information stored in the optical disk 16 is detected. In this embodiment, a desired filter function can be applied to the first diffraction gratings in the regions 13e and 13f by changing the diffraction efficiency of 0th order diffracted beam and first order diffracted beam. The diffraction gratings, hence, have both a function of dividing a light beam into a 0th order diffracted beam and first order diffracted beams as well as a filter function. The second diffraction gratings in the regions 13c and 13d respectively have the same function as that of the second diffraction grating in the region 13b, but the diffraction efficiency of 0th order diffracted beam and first diffracted beam can be set to a desired value by simply changing the depths of the above second diffraction gratings. With the above method, it is possible to set the transmission factors of 0th order diffracted beam of the second diffraction gratings in the regions 13c and 13d to be lower than that of the second diffraction grating in the region 13b, and a desired filter function can be accordingly applied to the second diffraction gratings in the regions 13c and 13d. In the foregoing embodiment, the first diffraction gratings and the second diffraction gratings in the regions 13a to 13f of the diffraction element 13 are provided at the surface of the diffraction element 13, opposing to the collimating lens 14, but it is also possible to provide the above first and second diffraction gratings at the opposite surface thereof. Referring now to Fig. 5, another embodiment of the present invention will be explained hereinbelow. A diffraction element 18 of this embodiment comprises five regions 18a to 18e and four regions 18a to 18d among these regions respectively have a second diffraction grating and the other region 18e has a first diffraction grating. Like the first embodiment, the laser beam which has been emitted from the laser beam 12 and passed through the second diffraction grating in a region 18a is reflected at the optical disk 16 and then diffracted by the second diffraction grating in a region 18b so as to be guided to the photodetector 17. On the basis of the diffracted beam received by the photodetector 17, a focus servo-mechanism is controlled. On the other hand, the light beam which has passed through the second diffraction grating in the region 18b is reflected at the optical disk 16 and then diffracted by the second diffraction grating in the region 18a so as to be guided to the photodetector 17, and information stored in the optical disk 16 is detected by the photodetector 17 based on the light beam received by the photodetector 17. The light beam which has been emitted from the laser beam source 12 and passed through the first diffraction grating in a region 18e is reflected at the optical disk 16 and then diffracted by the second diffraction gratings in the regions 18c and 18d of the diffraction element 18 so as to be guided to the photodetector 17. A tracking servo-mechanism is controlled based on the light beam received by the photodetector 17. On the other hand, the light beams which have passed through the second diffraction gratings in the regions 18c and 18d are reflected at the optical disk 16 and then reaches the first diffraction grating in the region 18e. In the optical head device for reading information stored in a recording medium of this embodiment, the transmission factor of the 0th order diffracted beams to pass through the diffraction element 18 is lowered at the regions 18c to 18e located in the vicinity of both ends of the diffraction element 18, compared with the transmission factors of the same at the regions 18a and 18b located in the vicinity of the center of the diffraction element 18, the above ends and center being set in a corresponding direction to the radial direction of the optical disk 16 ( B-B' ). With such an arrangement, a filter function equivalent to that of the conventional diffraction grating with the filter 10 mounted thereon can be achieved by the diffraction element 18. The following description will discuss still another embodiment of the present invention with reference to Figs. 6 to 8. In this embodiment, parts having substantially similar functions to those of the parts which appear in the first embodiment are indicated by the same reference numerals and the description thereof is omitted. In the optical head device for reading information stored in a recording medium of this embodiment, as shown in Fig. 8, a laser beam emitted from the chip 12a of the laser beam source 12 (see Fig. 6) serving as a light source, passes through a first diffraction grating 11, the diffraction element 13 having second diffraction gratings (hologram gratings), the collimating lens 14 and the objective lens 15, and is converged on the optical disk 16. The reflected beam from the optical disk 16 is incident on the diffraction element 13 after passing through the objective lens 15 and the collimating lens 14, and is diffracted by the second diffraction grating of the diffraction element 13 so as to be guided to the photodetector 17 which serves as a light receiving element, by which the detection of tracking errors/focusing errors, and the reading of information stored in the optical disk 16 are performed. As shown in Figs. 6 and 7(a), the photodetector 17 has five independent photodetecting sections 17a to 17e formed separately from each other, while the diffraction element 13 has the second diffraction grating at the collimating lens 14 side thereof. This second diffraction grating is divided into six regions 13a to 13f by a parting line L₁ which extends in a corresponding direction to the radial direction of the optical disk 16 ( B-B' ) and two parting lines L₂ and L₃ which extend in a corresponding direction to the direction of the alignment of the pits formed in the optical disk 16 ( C-C' ). The diffraction element composed of the six regions 13a to 13f which are formed in the same plane is made of a plastic or glass. Mark P in Fig. 7(a) indicates the outmost part of the light beam to pass through the diffraction element 13. As shown in Fig. 7(b), the second diffraction gratings in the regions 13a to 13f are arranged such that the transmission factors of 0th order diffracted beam of the second diffraction gratings in the regions 13c, 13d; 13e, 13f located in the vicinity of both ends of the diffraction element 13 are considerably lower than those of the second diffraction gratings in the regions 13a and 13b located in the vicinity of the center of the diffraction element 13, the above ends and center being set in a corresponding direction to the radial direction of the optical disk 16. The adjustment of the transmission factors can be achieved by changing the depths of the second diffraction gratings. The following description will discuss the functions of the second diffraction gratings in the regions 13a to 13f of the diffraction element 13. A light beam emitted from the laser beam source 12 is diffracted and divided by the first diffraction grating 11 into a 0th order diffracted beam and a pair of first order diffracted beams and these diffracted beams are further divided by the second diffraction gratings in the regions 13a, 13c and 13e. The 0th order diffracted beams among the diffracted beams produced at the above three regions are irradiated on the pits on the optical disk 16 through the collimating lens 14 and the objective lens 15. The reflected beams from the optical disk 16 pass through the objective lens 15 and the collimating lens 14, and then reach the second diffraction gratings in the regions 13b, 13d and 13f in the diffraction element 13 in compliance with the light reflection law. The regions 13a, 13c and 13e constitute one part located at one side of the diffraction element 13 and the regions 13b, 13d and 13f constitute a different part located at the other side thereof, the diffraction element being divided into such two parts by the parting line L₁. The 0th order diffracted beam which has been produced by the first diffraction grating 11 and has reached at the regions 13b, 13d and 13f after being reflected at the optical disc 16 is divided thereat into a 0th order diffracted beam and first order diffracted beams. One of the above first order diffracted beams reaches the parting line ℓ between the photodetecting sections 17b and 17c in the photodetector 17. On the basis of the above first order diffracted beam that has reached the line ℓ, the beam spot of the laser beam is moved on the optical disk 16 in accordance with the surface vibration thereof by means of the focus servo-mechanism (not shown), thereby detecting information stored in the optical disk 16. On the other hand, the pair of first order diffracted beams which have been produced by the first diffraction grating 11 and have reached at the regions 13b, 13d and 13f in the diffraction element 13 after being reflected at the optical disk 16 are diffracted at the above regions to be guided into the photodetecting sections 17d and 17e of the photodetector 17 in order to be utilized for detecting tracking errors as described later, together with the diffracted beams guided into the photodetecting sections 17d and 17e from the regions 13a, 13c and 13e in the diffraction element 13. The light beam which has been emitted from the laser beam source 12 and passed through the second diffraction gratings in the regions 13b, 13d and 13f in the diffraction element 13 after being divided into a 0th order diffracted beam and first order diffracted beams by the first diffraction grating 11 are reflected at the optical disk 16 and then projected back to the regions 13a, 13c and 13e in the diffraction element 13. The 0th order diffracted beam which has been produced by the first diffraction grating 11 and has reached at the regions 13a, 13c and 13e is divided thereat into a 0th order diffracted beam and first order diffracted beams. Then, one of the above first order diffra'cted beams is guided into the photodetecting section 17a of the photodetector 17. The detection of information stored in the optical disk 16 is performed based on the diffracted beam received by the photodetecting section 17a. On the other hand, the pair of first order diffracted beams that have been produced by the first diffraction grating 11 and have reached the regions 13a, 13c and 13e of the diffraction element 13 after being reflected at the optical disk 16 are diffracted in the above regions so as to be guided into the photodetecting sections 17d and 17e of the photodetector 17. On the basis of the diffracted beams guided into the photodetecting sections 17d and 17e of the photodetector 17 from the regions 13a, 13c and 13e and the aforementioned diffracted beams guided into the photodetecting sections 17d and 17e from the regions 13b, 13d and 13f, the beam spot of the laser beam is moved on the recording track on the optical disk 16 by means of the tracking servo-mechanism (not shown). As described earlier, the transmission factors of 0th order diffracted beam of the second diffraction gratings in the regions 13c, 13d; 13e, 13f located in the vicinity of both ends of the diffraction element 13 are considerably lower than those of the second diffraction gratings in the regions 13a and 13b located in the vicinity of the center of the diffraction element 13, the above ends and center being set in a corresponding direction to the radial direction of the optical disk 16. Therefore, the occurrence of crosstalk caused by reading the information recorded on other tracks than a track from which desired information is to be read out, can be positively prevented at the time of reading out information stored in the optical disk 16. The following description will discuss a further embodiment of the present invention with reference to Fig. 9 As shown in Fig. 9(a), a diffraction element 18 in this embodiment comprises a second diffraction grating divided into nine regions 18a to 18i with two parting lines L₁ and L₂ which extend in a corresponding direction to the radial direction of the optical disk 16 and two parting lines L₃ and L₄ which extend in a corresponding direction to the direction of the alignment of the pits on the optical disk 16. As shown in Fig. 9(b), the second diffraction grating having the regions 18a to 18i is designed such that the transmission factors of 0th order diffracted beam in the regions 18d to 18f and 18g to 18i located in the vicinity of both ends of the diffraction element 18 is considerably lower than that in the regions 18a to 18c located in the vicinity of the center of the diffraction element 18, the above ends and center being set in a corresponding direction to the radial direction of the optical disk 16. Fig. 10 shows another embodiment of the present invention. In the optical head device for reading information stored in a recording medium of this embodiment, a diffraction element 20 comprises a first diffraction grating and a second diffraction grating which are formed thereon. More particularly, a second diffraction grating 20a having the aforementioned filter function as well is formed at one surface of the diffraction element 20 opposing to the collimating lens 14, and a first diffraction grating 20b at the other surface of the diffraction element 20 opposing to the laser beam source 12. The second diffraction grating 20a may be divided into a plurality of regions if necessary, and is designed such that the transmission factors of 0th order diffracted beam in the regions located in the vicinity of both ends of the diffraction element 20 are considerably lower than those in the regions located in the vicinity of the center of the diffraction element 20, the ends and center being set in a corresponding direction to the radial direction of the optical disk 16. In this embodiment, parts having substantially similar functions to those of the parts which appear in the first embodiment are indicated by the same reference numerals and the description thereof is omitted. In the foregoing embodiments, the laser beam source 12 and the photodetector 17 are separately disposed, but it is also possible to employ a single element in which the laser beam source 12 and the photodetector 17 are combined thereby realizing the miniaturization of optical head device. With the arrangements described above, not only can the number of components of the optical head device be reduced but also the filter function can be applied to the diffraction element itself resulting in the reduction of the number of manufacturing steps as well as the production cost. The invention being thus described, it will be obvious that the same may be varied in many ways without departing from the scope of the appending claims.

Claims: An optical head device for reading information stored in a track or tracks on a recording medium wherein a light beam from a light source (12) is converged onto the medium (16) and a reflected beam from the medium is received by a light receiving element (17) thereby to read out information stored in the track or tracks, wherein there is provided diffraction means (13;18;11,13;11,18;20) for forming a 0th order diffracted beam for reading out the information stored in the track or tracks, and including first diffraction grating means (13e,13f;18e;11; 20b) for producing from the light beam from the light source a pair of first order diffracted beams utilized for reading tracking errors; and second diffraction grating means (13a to 13d;18a to 18d;13a to 13f;18a to 18i; 20a) for guiding the reflected beam from the recording medium to the light receiving element, the transmission factors for the 0th order diffracted beam of the first and/or second diffraction grating means in regions located at the ends of the diffraction means in a direction transverse to the track or tracks being lower than those of the first and/or second diffraction grating means in regions located generally centrally of the diffraction means in said direction. An optical head device according to claim 1 wherein said diffraction means comprises a diffraction element (13;18) and wherein said first diffraction grating means (13d,13f;18d) and said second diffraction grating means (13a to 13d; 18a to 18d) are positioned at separate regions in the same plane on said diffraction element. An optical head device according to claim 2 wherein a given plurality of said separate regions (13a to 13d, 18a to 18d) have the second diffraction grating means and the remaining region or regions (13e,13f;18e) have the first diffraction grating means. An optical head device according to claim 2 wherein said first and/or second diffraction grating means are provided with the functions of dividing a light beam into a 0th order diffracted beam and first order diffracted beams, and of performing a filter function by varying the diffraction efficiencies for the 0th order diffracted beam and for the first order diffracted beams. An optical head device according to claim 2 wherein said diffraction element (13) is divided into a central section (13a/13b) and two end sections (13c/13f,13d/13e) by two spaced longitudinal parting lines extending in a direction (C-C') corresponding to the direction along the track or tracks of the recording medium, said end and centre sections being set in relation to a direction (B-B') corresponding to the direction transverse to said track or tracks, and being further sub-divided by a transverse parting line extending in said transverse direction, whereby six said regions (13a to 13f) are formed, said second diffraction grating means being formed in four of said six regions, two of said four regions (13a,13b) being located in said central section, the other two of said four regions (13c,13d) being located on one side of said transverse parting line in respective said end sections, said first diffraction grating means being formed in each of the remaining two out of the six regions, said remaining two regions (13e,13f) being located on the other side of said transverse parting line in respective said end portions. An optical head device according to claim 2 wherein said diffraction element (18) is divided into a central section (18a/18b) and two end sections (18c/18d,18e) by two spaced longitudinal parting lines extending in a direction (C-C') corresponding to the direction along the track or tracks of the recording medium, said end and centre sections being set in relation to a direction (B-B') corresponding to the direction transverse to said track or tracks, said centre section and one of said end sections being further sub-divided by a transverse parting line extending in said transverse direction, whereby five said regions (18a to 18e) are formed, said second diffraction grating means being formed in four of said five regions, two of said four regions (18a,18b) being located in said central section on respective opposite sides of said transverse parting line, the other two of said four regions (18c,18d) being both located in the same said one of said end sections on respective opposite sides of said transverse parting line, and said first diffraction grating means being formed in the remaining one of said five regions, said remaining region (18e) comprising the other end section of said diffraction element. An optical head device according to claim 2 wherein said light receiving element (17) comprises a photodetector having a plurality of separate photodetecting sections (17a to 17e) which correspond to a plurality of independent regions of said diffraction element. An optical head device according to claim 7 wherein said light receiving element (17) comprises a photodetector having five separate photodetecting sections (17a to 17e). An optical head device according to claim 2 wherein the grating depth of the second diffraction grating means in the regions (13e 13d;18c,18d) located at at least one of said ends of the diffraction element is different from the grating depths of the second diffraction grating means in the regions (13a,13b;18a,18b) located at the centre of the diffraction element, so that the diffraction element has different transmission factors for said 0th order diffracted beam at said at least one end and at the centre. An optical head device according to claim 2 wherein said diffraction element (13,18) is constructed such that a part of the light beam emitted from the light source (12) passes through a first region (13a;18a) having the second diffraction grating means in the diffraction element to produce a 0th order diffracted beam which is irradiated upon and reflected from the recording medium, and is then further divided into a 0th order diffracted beam and first order diffracted beams at a second region (13b;18b) having the second diffraction grating in the diffraction element, one of said first diffracted beams thereafter reaching a parting line (ℓ) between adjacent photodetecting sections (17b,17c) in the photodetector, to enable a focus servo-mechanism to control a beam spot on the recording medium according to its surface vibration to facilitate proper detection of the information stored in the recording medium. An optical head device according to claim 2 wherein said diffraction element is constructed such that a part of the light beam emitted from the light source passes through a first region (13b;18b) having said second diffraction grating means in the diffraction element to produce a 0th order diffracted beam which is irradiated upon and reflected from the recording medium, and is then further divided into a 0th order diffracted beam and first order diffracted beams at a second region (13a;18a) having the second diffraction grating in a diffraction element, one of said first diffracted beams thereafter reaching a photodetecting section (17a) in the photodetector, for detecting the information stored on the recording medium at said photodetecting section. An optical head device according to claim 2 wherein said diffraction element is constructed such that the light beam emitted from the light source reaching the first diffraction grating means of the diffraction element is divided into a 0th order diffracted beam and ± first order diffracted beams utilized for detecting tracking errors and is then irradiated upon and reflected from the recording medium, and such that the reflected beam from the recording medium is divided by the second diffraction grating means of the diffraction element into a 0th order diffracted beam and first order diffracted beams, one of said first order diffracted beams reaching a photodetecting section (17d,17e) of the photodetector, to enable a tracking servo-mechanism to move a beam spot so as to track a recording track on the recording medium in accordance with said one of said first order diffracted beams received by said photodetecting section. An optical head device according to claim 2 wherein said diffraction element is constructed such that a light beam emitted from the light source reaching the second diffraction grating means located at least one of said ends of the diffraction element is divided so as to produce a 0th order diffracted beam having a decreased luminous intensity, said 0th order diffracted beam being guided onto the optical disk to produce reflected beams most of which reach the first diffraction grating means of the diffraction element and are not directed to the photodetector. An optical head device for reading information stored in a track or tracks on a recording medium by utilizing a light beam from a light source (12) which converges on the recording medium and is reflected from the recording medium to be guided by a diffraction element (13;18) to a light receiving element (17) thereby to read out information stored on the recording medium, a first diffraction grating (11) being provided for diffracting the light beam travelling from the light source toward the recording medium characterized in that the first diffraction grating (11) is positioned between the light source and the diffraction element (13), in that said diffraction element includes a second diffraction grating divided into a plurality of regions by at least one parting line (L₁;L₁,L₂) which extends in a direction corresponding to the direction transverse to the track or tracks on the recording medium, and parting lines (L₂,L₃;L₃,L₄) which extend in a direction corresponding to the direction along the track or tracks on the recording medium, and in that said diffraction element is constructed such that the transmission factors for a 0th order diffracted beam at regions of the second diffraction grating located at both ends of the diffraction element are lower than the transmission factors for a 0th order diffracted beam at regions of the second diffraction grating located at the centre of the diffraction element, said ends and centre being set in a direction corresponding to the direction transverse to the track or tracks on the recording medium. An optical head device according to claim 14 wherein said second diffraction grating formed on the diffraction element (13) is divided into six second diffraction grating regions (13a to 13f) by a parting line (L₁) which extends in a direction corresponding to the direction transverse to the track or tracks on the recording medium and two parting lines (L₂,L₃) which extend in a direction corresponding to the direction along the track or tracks on the recording medium. An optical head device according to claim 14 wherein said second diffraction grating formed on the diffraction element (18) is divided into nine second diffraction grating regions (18a to 18i) by two parting lines (L₁,L₂) which extend in a direction corresponding to the direction transverse to the track or tracks on the recording medium and two parting lines (L₃,L₄) which extend in a direction corresponding to the direction along the track or tracks on the recording medium. An optical head device according to claim 14 wherein said light receiving element (17) comprises a photodetector having a plurality of separate photodetecting sections (17a to 17e) which correspond to a plurality of independent regions of said diffraction element (13;18). An optical head device according to claim 17, wherein said light receiving element (17) comprises a photodetector having five separate photodetecting sections (17a to 17e). An optical head device according to claim 14 wherein the grating depth of the second diffraction grating in the regions (13c to 13e;18d to 18i) located at both said ends of the diffraction element is different from the grating depth of the second diffraction grating in the regions (13a,13b; 18a,18b,18c) located at the centre of the diffraction element so that the diffraction element has different transmission factors for said 0th order diffracted beam at said both ends and at the centre. An optical head device according to claim 17 wherein said diffraction element is constructed such that a portion of the light beam emitted from the light source (12) passes through the second diffraction grating in one part of the diffraction element (13a,13c,13e) after being divided into a 0th order diffracted beam and a pair of first order diffracted beams by the first diffraction grating (11), before being irradiated onto and reflected from the recording medium, and such that the second diffraction grating receives said reflected 0th and first order diffracted beams in a different part (13b,13d,13f) of the diffraction element, said reflected 0th order diffracted beam being thereby diffracted to produce a first order diffracted beam directed onto a parting line (ℓ) between adjacent photodetecting sections (17b,17c) of the photodetector (17), to enable a focus servo-mechanism to control a beam spot on recording medium according to its surface vibration to facilitate proper detection of information stored in the recording medium, while the pair of reflected first order diffracted beams received by the second diffraction grating in said different part (13b,13d,13f) of the diffraction element are diffracted thereby to be guided into two photodetecting sections (17d,17e) of the photodetector for the detection of tracking errors, said one part and said different part of said diffraction element being on opposite sides of a parting line (L₁) which extends in a direction corresponding to the direction transverse to the track or tracks on the recording medium. An optical head device according to claim 17 wherein said diffraction element is constructed such that the light beam emitted from the light source (12) passes through the second diffraction grating in one part (13b,13d,13f) of the diffraction element after being divided into a 0th order diffracted beam and a pair of first order diffracted beams by the first diffraction grating (11) before being irradiated upon and reflected from the recording medium, and such that said 0th order diffracted beam and said pair of first order diffracted beams which have been reflected from the recording medium are received by said second diffraction grating in a different part (13a,13c,13e) of said diffraction element, said reflected 0th order diffracted beam being divided thereat into a 0th order diffracted beam and first order diffracted beams, one of said first order diffracted beams reaching a photodetecting section (17a) in the photodetector for the detection of information stored in the recording medium, said pair of reflected first order diffracted beams being diffracted by said second diffraction grating at said different part of said diffraction element so as to be guided into two photodetecting sections (17e,17d) to enable a tracking servo- mechanism to move a beam spot so as to track a recording track on the recording medium. An optical head device for reading information stored in a track or tracks on a recording medium (16) by utilizing a light beam from a light source (12) which converges onto the recording medium and is reflected from the recording medium to be received by a light receiving element (17) thereby to read out information stored on the recording medium, the device including a diffraction element (20) arranged to be positioned between the light source (12) and the recording medium (16), said diffraction element including: a first diffraction grating (20b), provided on the light source side of the diffraction element, for dividing the light beam from the light source into a 0th order diffracted beam utilized for reading out the information stored in the recording medium and a pair of first order diffracted beams utilized for reading tracking errors; and a second diffraction grating (20a), provided at the recording medium side of the diffraction element, for guiding the beam reflected from the recording medium to the light receiving element (17), wherein the transmission factors for the 0th order diffracted beam are lower at both ends of said second diffraction grating than at the centre of said second diffraction grating, said ends and centre being set in a direction corresponding to the direction transverse to the track or tracks on the recording medium. An optical head device according to claim 22 wherein said second diffraction grating includes a plurality of separate regions.

Inventors: TSUJI SHIGEKI, YOKOTA TAIZO, TSUJI, SHIGEKI, YOKOTA, TAIZO
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1989
Main classification: G11B7
IPC codes: G11B7