Optical pickup having catadioptric objective lens

[0002] This application claims the benefit of Korean Application No. 98-13317 filed Apr. 14, 1998, in the Korea Industrial Property Office, the disclosure of which is hereby incorporated by reference.

[0003] The present invention relates to an optical pickup having a catadioptric objective lens.

[0004] There are various methods under study for increasing recording capacity of an optical recording and reproducing apparatus through reducing the size of a focused spot by decreasing the wavelength of light and increasing a numerical aperture (NA) of an objective lens.

[0005] FIG. 1 is a conceptual diagram of a conventional optical focusing system which is used as an objective lens for an optical disk 4 and generates a near field to reduce the size of a focused spot on the disk 4. The conventional optical focusing system includes a conventional aspherical lens 1 and a spherical lens 2. The spherical lens 2 is typically called a solid immersion lens. A slider 3 is located between the spherical lens 2 and the disk 4 to load and move the objective lens thereon. The slider 3 moves the spherical lens 2 with respect to the surface of the disk 4 while maintaining a set distance between the spherical lens 2 and the disk 4 of less than 100 nm.

[0006] The aspherical lens 1 refracts the laser light emitted from a light source (not shown). The spherical lens 2 focuses the laser light refracted by the aspherical lens 1 on an inner surface (that surface located toward the disk 4) of the spherical lens 2. The inner surface of the spherical lens 2, on which the laser light is focused, forms a near field. As a result, information may be recorded on the disk 4 or read from the disk 4, via the near field.

[0007] When the spherical lens 2 is formed of a material having a refractive index “n,” an angle at which the laser light is focused on the inner surface of the spherical lens 2 becomes large, and the momentum of the laser light is reduced. As a result, an effect of reducing the wavelength of the laser light to λ/n is generated. Thus, the NA is increased to NA/λ. Therefore, the size of the focused spot which is finally formed in the surface of the spherical lens 2 is proportional to 1/n². Accordingly, the size of the spot can be reduced by fabricating the spherical lens 2 using a material having a refractive index different from the refractive index “n.”

[0008] However, as the aspherical lens 1 and the spherical lens 2 are separately manufactured, it is difficult to assemble and adjust an optical focusing system having the desired optical features. Since the optical focusing system needs an incident laser light having a beam diameter greater than or equal to 3 mm, the size of all optical components, including a light receptor, is quite large. When an incident laser beam slants off a normal angle with respect to the optical disk, due to vibration of a moving optical pickup or a rotating optical disk, it is difficult to accurately record or reproduce signals. The shortest wavelength of the light from currently available laser diode light sources is about 600 nm, while the NA of the objective lenses is approximately 0.6. Thus, when an NA greater than or equal to 0.6 is needed, the performance of the optical pickup is very sensitive to the incident beam slant. As a result, it is difficult to use conventional optical focusing systems in commercial optical recording and reproducing apparatus.

[0009] Accordingly, it is an object of the present invention to provide an optical pickup having a catadioptric objective lens having an excellent performance with respect to an incident beam slant.

[0010] It is also an object of the present invention to provide an optical pickup having a catadioptric objective lens enabling a reduction of the size of a focused spot.

[0011] It is yet a further object of the present invention to provide an optical pickup with lightweight compact optical components

[0012] Additional objects and advantages of the invention will be set forth in part in the description which follows, and, in part, will be obvious from the description, or may be learned by practice of the invention.

[0013] To accomplish the above objects of the present invention, there is provided an optical pickup for recording and reproducing information on a recording medium using a near field, the optical pickup having a unit for emitting laser light to a recording medium and detecting the light reflected from the recording medium, and a catadioptric objective lens including a first reflective area for reflecting the light incident from the unit, a focusing area on which the light incident from the unit is focused as a focused spot, and a second reflective area for reflecting the light reflected from the first reflective area toward the focusing area, in which the first reflective area and the focusing area forms a first surface of the catadioptric objective lens and the second reflective area becomes a part of a second surface of the catadioptric objective lens located in the vicinity of the unit, wherein the unit and the catadioptric objective lens are integrated and the light from the unit is incident to the catadioptric objective lens via the portions of the catadioptric lens other than the second reflective area of the second surface.

[0014] These and other objects and advantages of the invention will become apparent and more readily appreciated from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings of which:

[0015] FIG. 1 is a diagram of a conventional optical focusing system;

[0016] FIG. 2 is a diagram of an optical pickup system according to a first embodiment of the present invention; and

[0017] FIG. 3 is a diagram of an optical pickup system according to a second embodiment of the present invention.

[0018] Reference will now be made in detail to the present preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout.

[0019] FIG. 2 is a diagram of an optical system according to a first preferred embodiment of the present invention. The optical pickup system shown in FIG. 2, includes a unit 20 integrating a laser diode 21 (acting as a light source) and a photodiode 23 (acting as a light receptor) and a beam splitter 22 (preferably a prism). Accordingly, the unit 20 may also be referred to as an “integrated unit”. A catadioptric objective lens 30 is located between the unit 20 and a disk 40. The catadioptric objective lens 30 focuses the light emitted from the laser diode 21 of the unit 20 on the disk 40 and transmits the light reflected from the disk 40 to the unit 20. In the first embodiment, the catadioptric lens 30 serves as a slider.

[0020] The objective lens 30 has a recess, or other suitable structure, on the opposite side of the catadioptric lens from the disk 40, for receiving and supporting the unit 20. This recess may be referred to as a “connection portion,” as the unit 20 and the catadioptric objective lens 30 are connected. Note that being connected only connotes a fixed relationship there between, and one of ordinary skill in the art will recognize that the actual relationship will vary depending upon the construction of the unit 20 and the catadioptric objective lens 30. The beam splitter 22 and the catadioptric objective lens 30 are preferably fabricated using the same material having a desired refractive index.

[0021] Referring to FIG. 2, the laser light emitted from the laser diode 21 of the unit 20 is transmitted via the beam splitter 22 and is incident on the catadioptric objective lens 30 in a divergent form. As the beam splitter 22 and the catadioptric objective lens 30 are fabricated using the same material, the light from the laser diode 21 is incident to the catadioptric objective lens 30, without having refracted. For convenience of explanation, the surface of the catadioptric objective lens 30 facing the disk 40 is referred to as a first surface while the surface facing the laser light source is referred to as a second surface.

[0022] The catadioptric objective lens 30 includes a first reflective area 30-1 on the first surface, a focusing area 30-3 on which a focused spot is formed and a second reflective area 30-2 on the second surface. The second reflective area 30-2 comprises the entire second surface except that portion of the second surface contacting (or mated with) the unit 20. The second reflective area 30-2 is provided with an aspherical shape. The first surface of the catadioptric objective lens 30 transmits the light only at the focusing area 30-3 in the center portion of the first surface and reflects light toward the second reflective area 30-2 at all other areas.

[0023] The laser light reflected from the first reflective area 30-1 of the first surface and the second reflective area 30-2 of the second surface is focused on the focusing area 30-3 of the first surface in the form of a focused spot. The focused spot forms a near field. Therefore, information can be read from, and recorded on, the portion of the disk 40 located within a distance of one wavelength of the laser light from the focusing area 30-3 of the first surface. It is preferable that the distance between the first surface and the disk 40 is less than 100 nm.

[0024] During a reading operation, the near field formed by the catadioptric objective lens 30 is altered by an information recording layer on the disk 40. The reflected light representative of such alteration is reflected by the beam splitter 22 and is focused onto the photodiode 23.

[0025] Although the first embodiment of the present invention has been described with respect to a specific configuration, it will be recognized that the first embodiment is not limited to the specific configuration. For example, the laser diode 21 and the photodiode 23 in the unit 20 may be attached to the beam splitter 22 (as opposed to integrated) or even mounted as separate units spaced apart a predetermined distance.

[0026] FIG. 3 is a diagram of an optical pickup system according to a second preferred embodiment of the present invention. The optical system shown in FIG. 3 is obtained by modifying the optical system shown in FIG. 2 to create a unit 20-1 for use with a magneto-optical disk. Generally, a Wollaston prism 24 is placed between the beam splitter 22 and the photodiode unit 25. The Wollaston prism 24 separates reflected light from the magneto-optical disk 40-1 according to a polarization state of the reflected light. Further detailed description of the Wollaston prism 24 will be omitted in the interest of brevity as those of ordinary skill in the art are familiar with the properties of a Wallaston prism.

[0027] The photodiode unit 25 in which two photodiodes are packaged is used instead of the photodiode 23 so that two light beams separated by the wollaston prism 24 can be detected.

[0028] Otherwise, the elements of FIG. 3 having the same reference numerals as those of FIG. 2 perform the same functions as those of FIG. 2, such that a detailed description thereof will be omitted. The operations of the additional elements will be described below.

[0029] The light emitted from the laser diode 21 is focused on the magneto-optical disk 40-1 and the light reflected from the magneto-optical disk 40-1 is incident toward the beam splitter 22 along the same optical path as the focusing path. The beam splitter 22 reflects the incident light to the Wollaston prism 24. The Wollaston prism 24 splits the incident light and outputs two light beams to the photodiode 25. The photodiode 25 focuses the two light beams from the Wollaston prism 24, one each, onto pair of packaged photodiodes in the photodiode unit 25.

[0030] Although the second embodiment of the present invention has been described with respect to a specific configuration, it will be recognized that the second embodiment is not limited to the specific configuration. For example, the laser diode 21 and the photodiode unit 25 in the unit 20 may be attached to the beam splitter 22 (as opposed to integrated) or even mounted in separate units spaced apart a predetermined distance.

[0031] Although a few preferred embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.

[0032] The preferred embodiments have been described where the unit 20 (or unit 20-1) and the objective lens 30 have the same refractive index. However, the unit 20 (or unit 20-1) and the objective lens 30 can be fabricated using different materials having different refractive indexes. The above embodiments teach that the objective lens 30 as functioning as a slider, similar to the slider shown in FIG. 1. However, a separate slider can be located between the objective lens 30 and the disk 40 or disk 40-1.

[0033] As described above, the present invention enables components of an optical pickup system to be lightweight and compact. The present invention also provides a thin optical pickup system which can be used in slim devices such as notebook computers.