Optical pickup for an optical disc player

Abstract: An optical pickup for an optical disc player has a laser for emitting a laser beam. The pickup is suitable for optical discs which are different in recording density. The laser beam from the laser is divided into two beams by a beam splitter. A first objective provided for focusing one of the laser beams on one of the optical discs. A beam expander is provided for expanding an intensity distribution in the optical axis. A second objective provided for focusing the expanded laser beam on the other disc with a spot having a larger diameter.

Description: BACKGROUND OF THE INVENTION The present invention relates to an optical pickup for an optical disc player, and more particularly to a pickup having a single laser for recording and reproducing information on a disc with different recording density. Optical CD discs are known such as a read only CD and a writable CD-R disc which is used as a ROM. Further, as a writable optical disc having a high recording density, a write once disc and an erasable disc are available. Information is recorded on the disc and reproduced with a laser beam. These discs are different from the CD in the material of the recording surface. For example, the write once disc has a tellurium or bismuth recording surface on which the lasers burn pits for recording. In another type of the write once disks, the lasers are focused on a recording surface coated with a selenium antimony (Sb 2 Se 3 ) thin film, or an oxide tellurium (TeOx) thin film, or a thin film of organic pigment, which changes the reflectivity of the light. The erasable disc uses as the recording surface, an amorphous alloy made of rare earth metals such as gallium, terbium, and others. In a magneto-optical recording method, the recording surface of the disc is initially magnetized to form a magnetic field in a direction perpendicular to the surface. The laser heats a predetermined area of the disc to elevate the temperature above Curie temperature, which is about 150°C, thereby reversing the direction of the magnetic field. To read the recorded information, the laser is irradiated onto the recording surface so that polarized wave front slightly rotates as a result of the Kerr effect. Thus only the polarized wave deflected by the rotation is read by a photodetector, thereby enabling the information to be read. Japanese Patent Application Laid-Open 61-220173 which reflects the subject-matter of the preamble of the independent claims discloses a system for reproducing a CD having two areas, the recording densities of which are different from each other. Fig. 8 shows the system. A laser beam emitted from a laser 1 is divided by a half mirror 2 into different directions. The beam in one of the directions is focused on a recording surface of an optical disc 7 through a beam splitter 3, a quarter-wave plate 4, a reflecting mirror 5, and an objective 6. The reflected beam from the disc 7 passes in return and is reflected by the beam splitter 3 and focused on a photodetector 9 through a cylindrical lens 8. The beam reflected on the half mirror 2 in the other direction is focused on the disc 7 through a reflecting mirror 10, a beam splitter 11, a quarter-wave plate 12, a reflecting mirror 13, and an objective 14. The reflected beam from the disc 7 is reflected on the beam splitter 11 and focused on the photodetector 9 through a cylindrical lens 15. The NA (numerical aperture) of the objective 14 is different from that of the objective 6 in accordance with the difference between the recording densities. Another example of such a system is shown in Fig. 9. This system which comprises two lasers 16 and 17 each of which produces a laser beam having a different wavelength. The laser beam emitted from the laser 16 is focused on a disc (not shown) through a beam splitter 18, a half-wave plate 19, a beam splitter 20, a quarter-wave plate 21, and an objective (not shown). The reflected beam from the disc is reflected on the beam splitter 20 and focused on photodetector 22. The laser beam emitted from the laser 17 is reflected on the beam splitter 18 and focused on the disc in the same manner as the beam from the laser 16. The reflected beam from the disc is focused on the photodetector 22 in the same manner as described above. In the former system, it is necessary to provide the optical parts, such as the beam splitter and the wave plate, separately for the respective beams. Thus, the system is complicated in construction and the manufacturing cost increases. On the other hand, in the latter system, since two beams of the different wavelengths are focused on the disc, chromatic aberration is produced, which is difficult to correct. Accordingly, it is impossible to reproduce two kinds of signals such as audio signal and video signal recorded on the disc at the same time. Furthermore, it is necessary to control a spot diameter of the laser beam in accordance with the recording density of the optical disc. If the disc has a high recording density, the beam must be focused on the disc at a small spot diameter. If the disc has a low recording density, the beam is focused on the disc at a large diameter. Therefore, if the beam having the large spot diameter is used for reproducing the information in the high recording density, crosstalk is produced between adjacent tracks and pits of short pitch can not be reproduced because of the large spot. If the beam having the small spot diameter is used on the disc having the low recording density, intermodulation is generated, so that it is impossible to properly reproduce the information on the disc. SUMMARY OF THE INVENTION An object of the present invention is to provide an optical pickup which is simple in construction, thereby reducing the size of the pickup and the manufacturing cost. Another object of the present invention is to provide a pickup which may record and reproduce information on optical discs having different recording densities with respect to each other. The present invention provides an optical pick-up according to the appended independent claims 1 and 4. In an optical pickup of one embodiment of the present invention, a laser beam is emitted from a single laser for recording and reproducing information on an optical disc having different recording densities. The laser beam is splitted into different directions. The beam in one of the directions is focused on the optical disc through a first objective. The beam in the other direction is expanded along the optical axis by a beam expander to reduce the diameter of the beam and the beam is focused on the disc through a second objective. The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings. BRIEF DESCRIPTION OF DRAWINGS Fig. 1 is a perspective view schematically showing an optical pickup for an optical disc player according to the present invention; Fig. 2 is a schematic side view explaining an operation of the pickup; Fig. 3 is a diagram showing truncation of a laser beam; Fig. 4 is a diagram explaining an operation for the separation of a beam; Fig. 5 is a diagram explaining an operation of a modification of the pickup; Figs. 6a and 6b schematically show another embodiment of the present invention; Fig. 7 schematically show a further embodiment; Fig. 8 is a diagram showing a conventional optical pickup; and Fig. 9 is a diagram showing a part of another conventional pickup. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Fig. 1 showing an optical pickup according to the present invention, a semiconductor laser 30 emits a laser beam which is reflected on a half mirror 31 as a beam splitter, and hence divided into two directions. The laser beam in one of the directions is focused on a first optical disc (not shown) through an objective 32. The reflected beam from the disc is focused on a photodetector 37 through the objective 32, the half mirror 31, and a cylindrical lens 29. The beam passing through the half mirror 31 enters a beam expander 25 comprising a large lens 33 and a small lens 34. The beam expander 25 has a function to expand the intensity (power) distribution of the light having a Gaussian distribution in the direction of the optical axis thereby narrowing the distribution as shown in Fig. 2. The beam is reflected on a prism 35 in a vertical direction and focused on a second disc through an objective 36. The second disc has a lower recording density than the first disc. The pickup may be used for a single disc having different areas, the recording densities of which are different from each other. The reflected beam from the disc passes through the objective 36, prism 35, expander 25, and half mirror 31, and is focused on the photodetector 37 through the lens 29. The objectives 32 and 36 have the same numerical aperture (NA). The operation of the pickup is described with reference to Fig. 2. The laser beam emitted from the semiconductor laser 30 is reflected on the half mirror 31 and divided into the different directions, namely to the objective 32 and to the expander 25. The laser beam directed to the objective 32 has a wide width Gaussian distribution A in intensity distribution. The skirt of the Gaussian distribution has a substantially larger width than the diameter of the objective 32. Consequently, the side skirt of the distribution is largely truncated by the objective so that a central portion approximate to a plane wave shown by the hatching is irradiated onto the disc. On the other hand, the laser beam directed to the objective 36 is expanded in the optical axis as shown by the distribution B. As a result, the skirt of the distribution is slightly truncated. Referring to Fig. 3 showing a Gaussian distribution of the laser beam, a spot diameter 2ω0 of the beam focused on disc (not shown) through a lens is determined by the NA of the lens and the wavelength of the beam and expressed by the equation as follows, 2ω0 = k · λ/NA where k is constant. Since the NAs of both lenses 32 and 36 are equal, the spot diameter is determined by the constant k. In the Gaussian distribution of the light intensity, the constant k changes with the diameter of the beam profile incident on the lens. If the position of the profile on the distribution is at a position a where the light intensity is one-half of the central portion, the constant k is 0.5. The constant k at a position (position b) is 0.8, and 1.2 at a zero position c. Namely, the constant k decreases as the amount of the truncation increases. Therefore, the spot diameter reduces as the truncation increases. Thus, the spot of the beam focused on a disc 38 through the objective 36 has a large diameter because of the small truncation, and the spot of the beam passing through the objective 32 has a small diameter because of a large truncation. Furthermore, the powers of the beams focused on the discs through the objectives 32 and 36 are equalized as described hereinafter. If the Gaussian beam having the spot diameter 2ω0 (Fig. 3) is transmitted through a circular opening having a radius R, the transmittance T is represented as follows. T = 1 - exp (-2m 2 ) where m = a/ω0 (constant). Thus, if the transmittance of the half mirror 31 is t, the reflectance thereof is r, the truncation of the beam profile on the objective 32 reflected on the half mirror 31 is m1, and the truncation of the beam profile on the objective 36 compressed by the expander is m2, transmittances T1 and T2 of the objectives are represented as follows. T1 = 1 - exp (-2m1 2 ) T2 = 1 - exp (-2m2 2 ) t : r = T1 : T2 t X T2 = r X T1 t = (T1 / T2) r It will be seen from the equation, that if the transmittance t is t = r x T1/T2 , the beam powers on the disc are equalized. The separation of the reflected beam for the photodetector 37 is performed by polarizing the beam. As shown in Fig. 4, a collimator lens 40 is provided in front of the semiconductor laser 30, and a λ/4-wave plate 31B is disposed at the incident side of the objective 36. The reflected beam from the disc 38 is polarized by the λ/4-wave plate 31B in a direction perpendicular to the paper of the figure as shown by marks +. The polarized reflected beam passes the beam expander and is reflected on the half mirror 31. The beam is applied to a photodetector 37a passing through a polarizing beam splitter 41. On the other hand, the reflected beam passing the objective 32 passes the half mirror 31 as shown by dotted line arrows and is reflected by the splitter 41 to a photodetector 37b. If either of the objectives 32 and 36 is not used, the unused objective is defocused, thereby preventing the reflected beam from returning to the photodetector 37. Alternatively, as shown in Fig. 5, a λ/2-wave plate 31A is disposed in front of the semiconductor laser 30 to rotate the polarized light of the beam at 90 degrees so that the beam reflected only in one direction from the half mirror 31 is transmitted or reflected. In accordance with the present invention, the optical pickup is provided with a single laser and means for focusing the laser beam from the laser on the disc with different spot diameters. Therefore, the pickup is simplified in construction, and the size and the manufacturing cost thereof are reduced. Figs. 6a and 6b show another embodiment of the pickup. A lens 33A is variably disposed at the incident side of the large lens 33 of the expander 25 so as to form a zoom lens. As shown in Fig. 6a, when the lens 33A is disposed near the lens 33, the truncation of the beam becomes large, so that a plane wave of the Gaussian distribution is used and the spot diameter is reduced. As shown in Fig. 6b, when the lens 33A is disposed away from the lens 33, the spot diameter becomes large since the truncation is reduced. In the embodiment, since the half mirror 31 and the objective 32 are omitted, the construction is further simplified. Fig. 7 shows a further embodiment. In the embodiment, actuators 39 are provided on the small lens 34 of the expander 25 to move the lens 34 in the focus direction and the tracking direction. In the embodiment, since an actuator (not shown) of the objective 36 can be omitted, the weight of the optical pickup is reduced, so that the pickup can be moved at a high speed when searching the recorded information. While the presently preferred embodiments of the present invention have been shown and described, it is to be understood that these disclosures are for the purpose of illustration and that various changes and modifications may be made.

Claims: An optical pickup for an optical disc player having a laser (30) for emitting a laser beam, two objectives (32, 36) for focusing the laser beam on an optical disc (38) having a high recording density portion and a low recording density portion or a high recording density disc or on a low recording density disc, and a beam splitter (31) for dividing the laser beam into a first beam directed to said high recording density portion or said high recording density disc through one objective (32) and a second beam directed to said low recording density portion or said low recording density disc through the other objective (36); characterized in that a beam expander/reducer (25) is disposed in the path of the second beam for narrowing the intensity distribution of the second beam in the optical axis, so that the one objective (32) effects a large truncation on the first beam whereby a smaller diameter spot is formed on said high recording density portion or said high recording density disc and that the other objective (36) effects a small truncation on the second beam whereby a larger diameter spot is formed on said low recording density portion or said low recording density disc. The optical pickup according to Claim 1, wherein if the transmittance of the beam splitter (31) is t, the reflectance thereof is r, the truncation of the beam profile on the objective (32) is m1, and the truncation of the beam profile on the objective (36) is m2, transmittance T1 and T2 of the objectives are represented as follows: T1 = 1 - exp (-2m1 2 ) T2 = 1 - exp (-2m2 2 ) t : r = T1 : T2 The optical pickup according to Claim 1, wherein the beam expander/reducer (25) comprises a large diameter lens (33) and a small diameter lens (34). An optical pickup for an optical disc player having a laser (30) for emitting a laser beam, and an objective (36) for focusing the laser beam on an optical disc having a high recording density portion and a low recording density portion or a high recording density disc or a low recording density disc characterized in that a beam expander/reducer (25) is disposed in the path of the laser beam for narrowing the intensity distribution of the laser beam in the optical axis, a zoom lens (33A) is provided in front of the beam expander/reducer (25) on the laser side and is movable along the optical axis of the beam expander/reducer (25), so as to change the degree of truncation of the beam by the objective (36) so that by a large truncation, a small diameter spot is formed on said high recording density portion, or said high recording density disc, and by a small truncation, a large diameter spot is formed on said low recording density portion or said low recording density disc. The optical pickup according to Claim 4, wherein the beam expander comprises a large diameter lens (33) and a small diameter lens (34). The optical pickup according to Claim 5, wherein the small diameter lens (34) is provided with actuators (39) to be moved in a focus direction and in a tracking direction.

Inventors: YANAGAWA NAOHARU, YANAGAWA, NAOHARU, Yanagawa, Naoharu, c/o Tokorozawa Koujou
Applicants: PIONEER ELECTRONIC CORP, PIONEER ELECTRONIC CORPORATION
Application year: 1993
Main classification: G11B7
IPC codes: G11B7, G11B11, G11B13