Optical pickup apparatus with reduced astigmatic aberration

Abstract: The present invention provides an optical pickup apparatus capable of preventing astigmatic aberration. The optical pickup in accordance with the present invention comprises one or more light sources and beam splitting means which is placed in the light-incident path beginning from the light sources and selectively reflects or transmits the light beams emitted by the light sources, the light-incident side of the beam splitting means not being parallel with the light-exiting side of the beam splitting means. The beam splitting means is a wedge-shaped plate, the light-incident side being more inclined than the light-exiting side with respect to the optical axis. The optical pickup employing the wedge-shaped beam splitting means creates an optical spot with little astigmatism on the optical disk. Moreover, it remarkably reduces the manufacturing cost since it adopts plate-type beam splitters instead of prism-type beam splitters.

Description: The present invention relates to an optical pickup apparatus for preventing astigmatic aberration. Optical pickup apparatuses for retrieving/recording data from/on optical disks such as CDs or DVDs employ beam splitters to separate the laser beam emitted by a laser diode from the beam reflected by the optical disk or to separate the optical path for CDs from the optical path for DVDs. Most commonly used beam splitters are prism-type beam splitters and plate-type beam splitters. FIG. 1A and 1B depict a prism-type beam splitter comprising two identically-shaped prisms attached to each other. The attached surfaces of the prisms are arranged to be inclined at a predetermined angle, for example 45 degrees, with respect to the optical axis and selectively transmit or reflect impinged beams depending upon the polarization thereof. Two differently-polarized beams B1 and B2 incident upon the surface from different angles are combined into a beam B3 in FIG. 1A, whereas an incident beam B1 is divided into two differently-polarized beams B2 and B3 in FIG 1B. FIG. 2A and 2B depict a plate-type beam splitter placed to be inclined at a predetermined angle with respect to the optical axis. Like the prism-type beam splitter, the surfaces of the plate-type beam splitter selectively transmit or reflect impinged beams depending upon the polarization thereof. Therefore, the plate-type beam splitter functions in much the same was as the prism-type beam splitter does, as shown in FIGS. 2A and 2B. The plate-type beam splitter, however, causes astigmatic aberration since the plate-type beam splitter may be disaligned with the incident light beam as shown in FIG. 3. To retrieve/record data from/on an optical disk, the laser beam emitted by a laser diode should be focused to a single point on the optical disk. The plate-type beam splitters, therefore, are not appropriate for this purpose and accordingly most optical pickups employ prism-type beam splitters in the light-incident path from the laser diode to the optical disk. The plate-type beam splitters are sometimes used to deliberately induce astigmatic aberration in the optical path from the optical disk to optical detectors. FIG. 4 shows the optical characteristics of an optical pickup employing the plate-type beam splitter in the light-incident path from the laser diode to the optical disk. FIG. 4A depicts the wave front error (WFE) characteristic versus optical axis tilt and FIG. 4B depicts the interferogram. For normal operation of the optical pickup, the WFE should remain within 0.07λ as long as the tilt angle remains within ± 2°. It is easily seen in FIGS. 4A and 4B that the WFE exceeds the required limit and the interferogram is remarkably distorted, which demonstrates the reason the plate-type beam splitters are not acceptable in the light-incident path of the optical pickup. Meanwhile, FIG. 5A and 5B show the optical characteristics of an optical pickup employing the prism-type beam splitter in the light-incident path. Unlike the previous example, it is seen that the WFE curves for tangential and radial are almost identical each other, the WFE satisfies the required condition, and the interferogram shows little distortion. The prism-type beam splitters, however, increase manufacturing cost since it is much more expensive than the plate-type beam splitters. Moreover, prism-type beam splitters are constructed by attaching two prisms together, however, this joining work of two prisms is difficult in a manufacturing process, and such attached two sides sometimes degrade the performance of the prism-type beam splitters. EP-A-0 881 635 JP-A-10 269 607 and JP-A-63 052 341 show some examples of optical pickup devices in which a wedge-shaped beam splitter is used in order to avoid or decrease astigmatic aberration. It is a primary object of the invention to provide an optical pick apparatus which does not cause astigmatic aberration by employing wedge-shaped beam splitters in the light incident path. An optical pickup apparatus in accordance with the present invention is defined in appended independent claim 1. Further embodiments are defined in dependent claims 2 and 3. BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are included to provide a further understanding of the invention, illustrate the preferred embodiments of the invention, and together with the description, serve to explain the principles of the present invention. In the drawings: FIG. 1A and 1B show the principles of a prism-type beam splitter; FIG. 2A and 2B show the principles of a plate-type beam splitter; FIG. 3 shows the optical path modified by a plate-type beam splitter; FIG. 4A and 4B show the optical characteristics obtained by a plate-type beam splitter; FIGS. 5A and 5B show the optical characteristics obtained by a prism-type beam splitter; FIGS. 6A and 6B show the wedge-shaped plate in accordance with an example and simulation results; FIGS. 7A and 7B show the optical characteristics obtained by the wedge-shaped plate in accordance with an example; FIG. 8 shows another example of the optical pickup for preventing astigmatic aberration; and FIG. 9 shows an embodiment of the optical pickup for preventing astigmatic aberration in accordance with the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In order that the invention may be fully understood, preferred embodiments thereof will now be described with reference to the accompanying drawings. FIG. 6A and 6B show the shape of the wedge-shaped beam splitter in accordance with an example and simulation results for finding an optimal angle of the wedge-shaped beam splitter. The wedge-shaped beam splitter is configured so that the surface on which the laser beam emitted by the laser diode is incident is more inclined than the opposite surface by an angle of  with respect to the optical axis, as shown in FIG. 6A. The angle  optimal for the optical pickup is determined by simulations considering the plate width (t) and incident angle (). FIGS. 7A and 7B show the wave front error and interferogram obtained when a wedge-shaped beam splitter whose angle  is found by the simulations is placed in the light-incident path between the laser diode and the optical disk. It is seen that the optical characteristics are very similar to those shown in FIG. 5A. FIGS. 8 and 9 show examples of the optical pickup employing the wedge-shaped beam splitter. The pickup apparatus shown in FIG. 8 comprises a hologram pickup module (HPM) 10 containing both laser diode and photo diode for CDs, another hologram pickup module (HPM) 11 for DVDs placed at a right angle to the HPM 10 with respect to a beam splitter, a wedge-shaped beam splitter 12 for dividing or combining laser beams emitted from the HPM 10 and HPM 11, a collimating lens 13 for converting divergent beams received from the wedge-shaped beam splitter 12 into parallel beams, and an objective lens 14 for focusing the parallel beams to a single point on the recording layer of an optical disk 1. Suppose that the wedge-shaped beam splitter has an optimal angle  of 1.3° obtained by simulations considering the plate width (t) and incident angle (). The laser beam emitted from the HPM 10 for CDs passes through the wedge-shaped beam splitter 12 and then enters the collimating lens 13. In this case, the astigmatic aberration of the laser beam is effectively suppressed by the wedge-shaped beam splitter 12 with the angle . The collimated laser beam is focused on the recording layer of the optical disk 1 by the objective lens 14. The laser beam is reflected by the optical disk 1 and moves toward the wedge-shaped beam splitter 12. After passing through the wedge-shaped beam splitter 12, the reflected laser beam is collected by the photo diode in the HPM 10. The photo diode converts the collected laser beam into electrical signals. On the other hand, the laser beam emitted from the HPM 11 for DVDs is reflected by the wedge-shaped beam splitter by 90° and enters the collimating lens 13. The collimated laser beam is focused on the recording layer of the optical disk 1 by the objective lens 14. The laser beam is reflected by the optical disk 1 and moves toward the wedge-shaped beam splitter 12. The laser beam reflected by the optical disk 1 is reflected again by the wedge-shaped beam splitter by 90° and reaches the HPM 11. The photo diode of the HPM 11 converts the collected laser beam into electrical signals. In summary, the optimal angle  which minimizes the astigmatic aberration can be determined by simulations considering the plate width (t) and incident angle () and the wedge-shaped beam splitter with the angle  can be effectively utilized in the optical pickup for recording/retrieving data on/from the optical disk. In the above-explained embodiment, the angle  other than 1.3° can be selected from the range of 1.2°∼1.5°, or more reduced range of 1.25°∼1.45° which is preferable to the former range. An embodiment of the optical pickup employing two wedge-shaped beam splitters is shown in FIG. 9, wherein the HPM 10 for CDs is replaced with a laser diode, photo diode for CDs, and a wedge-shaped beam splitter placed at the intersection of the light path. Similarly, several different configurations are obtainable with the wedge-shaped beam splitter in accordance with the present invention. The optical pickup employing the wedge-shaped beam splitter in accordance with the present invention creates an optical spot with little astigmatism on the optical disk. Moreover, it remarkably reduces the manufacturing cost since it adopts plate-type beam splitters instead of prism-type beam splitters. The wedge-shaped beam splitter in accordance with the present invention can be usefully employed in optical pickups for DVDs which currently employs more than one prism-type beam splitters.

Claims: An optical pickup apparatus for preventing astigmatic aberration, comprising: a first light source (CD LD) and a light receiver (CD PD) placed separately; a first wedge-shaped beam splitter (12), said first wedge-shaped beam splitter being placed in a light path of divergent or convergent light and having a first side opposing a second side; a second light source (11) placed apart from said first light source (CD LD), a second wedge-shaped beam splitter (12'), which is placed in a light path of divergent or convergent light and in an intersection of light beams emitted by said second light source (11) and light beams passing through said first wedge-shaped beam splitter (12), said second wedge-shaped beam splitter (12') having a first side opposing a second side; a beam focuser (14) to focus light beams passing through said second wedge-shaped beam splitter (12') toward an optical disk (1); and wherein said first and second sides of each of said first (12) and second (12') wedge-shaped beam splitters are not parallel to each other. The apparatus set forth in claim 1, wherein said first (12) and second (12') wedge-shaped beam splitters have a angle difference of 1.2 °~1.5 ° between their light-incident side and light-exiting side with respect to an axis of a incident beam. The apparatus set forth in claim 2, wherein said first (12) and second (12') wedge-shaped beam splitters have the angle difference of 1.25 °~1.45 °.

Inventors: LEE HYOUNG JAE, LEE, HYOUNG JAE
Applicants: LG ELECTRONICS INC, LG ELECTRONICS INC.
Application year: 2000
Main classification: G11B7
IPC codes: G11B7, G02B13