Optical pickup for high-density recording/reproducing

Abstract: An optical pickup for high-density recording/reproducing, which can read closely packed recorded information which exceeds the limits normally associated with a given cut-off frequency, the pickup including an objective lens (18, 18') having divided portions (20-22, 20'-22') for respectively transmitting a zero-order optical component and ±1st-order diffracted components. To detect a signal for reproduction, a focusing lens (28) is used for focusing the zero-order component and ±1st-order diffracted components passing through the objective lens (18, 18') so as to be mutually interfered. Stable high-density optical recording/reproduction can be realized regardless of the numerical aperture of the objective lens (18, 18') and optical wavelength.

Description: The present invention relates to an optical pickup for high-density recording/reproducing, and more particularly, to an optical pickup for the high-density recording/reproducing which can close information recording interval along a track on a light-recording medium. To increase the recording density of an optical recording medium, the size of a beam spot formed on the medium should be reduced as much as possible, which is generally accomplished through an objective lens of an optical pickup. Even assuming a lens with absolutely no aberration as the objective lens, there is a limit to how small the spot can become. The diameter of the beam spot is directly proportional to optical wavelength, but is inversely proportional to the numerical aperture of a lens. Thus, high-density recording and reproducing is generally realized by reducing the spot size formed on the light-recording medium, using a light source of a short wavelength (e.g., using a second harmonic generator) and an objective lens having a large numerical aperture. However, short-wavelength light sources are expensive, and even if an objective lens with large numerical aperture is used, the spot stability decreases according to the tilt of the recording medium (e.g., an optical disk). Accordingly, different methods are required to achieve high stability at relatively low cost. Figure 1 illustrates a conventional optical pickup to explain the principle of the recording and reproduction by the optical pickup. In Figure 1, a reference numeral 1 denotes a light source; a reference numeral 2 denotes a collimating lens for collimating a light beam generated from the light source; a reference numeral 3 denotes a beam splitter for separating light beams incident to an optical disk and light beams reflected therefrom; a reference numeral 4 denotes an objective lens for focusing an incident light beam on the optical disk within a diffraction limit, after having passed through the beam splitter; a reference numeral 5 denotes an optical signal detector for detecting signals by receiving the reflected light beams; and a reference numeral 6 denotes an optical disk. That is, the light beam generated from the light source 1 is incident upon the objective lens 4 via the collimating lens 2 and beam splitter 3 and thus focused on the recording surface of the optical disk 6, to be reflected and travel back to the optical signal detecting device 5 via the objective lens 4 and beam splitter 3 for detection. The reflected beams are detected as a reproduced information signal and a servo tracking signal for controlling the objective lens 4. As shown in Figure 2, recording marks 7 (e.g., pits) for recording an information unit are spaced along tracks of the optical disk 6 with a predetermined pitch p. A beam spot 8 of the incident light is focused via the objective lens 4 and moves (in the direction of the arrow) according to the rotation of the optical disk 6 along the line (track) of the recording marks 7. Though, theoretically, the recording density will increase with a narrower pitch or track spacing of the recording marks 7, there are practical limits in doing so conventionally, which will be described substantially as follows. Referring to Figure 3, the light beams focused on the optical disk are reflected and diffracted from the boundary of the recording marks 7 on the recording surface of the optical disk, to produce a zero-order light component 9 and ±1st-order diffracted light components 10 and 11. The diffracted light components 10 and 11 partially overlap the zero-order light component 9, causing mutual interference. Here, the zero-order light beam is reincident upon the objective lens with the same diameter as that of the objective lens. An alteration of light amount occurs on the portions whereon zero-order light component 9 and ±1st-order diffracted light ones 10 and 11 interfere with each other due to a relative phase difference therebetween. This change of the light amount constitutes the signal for reproduction detected by the optical signal detecting device 5. In other words, the information recorded on the optical disk can be reproduced from the change of the light amount (i.e., if such diffraction and interference do not happen, the reproduction signal cannot be acquired), which can be briefly expressed in a following formula: where λ is the light wavelength, NA is the numerical aperture of an objective lens, a is half of the diameter of the objective lens, p is the pitch of the recording marks 7 (Figure 2), and h is the distance between the optical axes of the zero-order light component 9 and the ±1st-order diffracted light components 10 and 11 (Figure 3). For reference, 1/p designates a cut-off frequency. Here, if the pitch p drops below λ/2NA (i.e., h≥2a), interference between the zero-order light and ±1st-order diffracted light components does not occur. Under the conventional environment of the optical pickup, the data on the optical disk, when recorded at close interval to exceed the limit of a given cut-off frequency, cannot be read. In other words, there is a limit to increasing the recording density of an optical recording medium. EP-A-0 190698 (Matsushita) reflecting the preamble of claims 1 and 12 describes a method and apparatus for reproduction of optically recorded signals. In reproduction of signals recorded on an optical disk, a laser beam is irradiated onto the disk and interference between reflected defracted light of +1th order or -lth order and reflected defracted light of 0 th order is coersively induced, and by detecting the interference light, the high frequency signal can be reproduced. JP-A-4047528 (Olympus) details apparatus by which data from an optical disk is reproduced by detecting first order and 0 th order defracted light corresponding to a pit interval. It is an aim of preferred embodiments of the present invention to provide an optical pickup for high-density recording/reproducing to increase the recording density regardless of optical wavelength or the numerical aperture of an objective lens and to enable for optical stability. According to a first aspect of the invention there is provided an optical pickup for high-density recording/reproducing, comprising: a light source for generating a light beam; an objective lens for focusing the light beam on a light-recording medium; and an optical signal detecting means for detecting an optical signal by receiving reflected light beams diffracted and reflected by the light-recording medium, wherein said objective lens includes a first portion for transmitting a zero-order optical component of the reflected light beams and second and third portions for respectively transmitting at least some of each ±1st-order diffracted optical components of the reflected light beams, the pick-up being characterised in that said objective lens is coated opaquely except for the first, second and third portions thereof. Also, it is preferable that the present invention further comprises means for controlling the diameter of a light beam generated by the light source so that the light beam is incident to the first portion of the objective lens. Preferably, the aperture of said slit is substantially the same size as the first portion of said objective lens. The pick-up preferably further comprises a collimating lens, installed between said light source and slit, for collimating the light beam generated from said light source. The pickup may also comprise means for making the zero-order optical component and ±1st-order diffracted optical components of the reflected light beams passing said objective lens mutually interfere. Preferably, means are provided for selecting the zero-order optical component, among the reflected light beams passing said objective lens. Said optical signal detecting means preferably comprises: a focusing lens for focusing the zero-order optical and ±1st-order diffracted optical components of the reflective light beams transmitted respectively by the first, second and third portions of said objective lens, to make them interfere with each other; and an optical detector for detecting a signal for reproducing information recorded on said light-recording medium according to the change of light amount due to the mutual interference between the zero-order optical and list-order diffracted optical components of the reflective light beams focused on said focusing lens. Preferably, said optical signal detecting means includes means for detecting signals for informing the track and focus locations of said objective lens from the zero-order optical component of the reflected light beams transmitted by the first portion of said objective lens, and this means for detecting may comprise: a slit having an aperture for passing the zero-order optical component of the reflected light beams coming via said objective lens, for blocking the ±1st-order diffracted optical components; a focusing lens for focusing the zero-order optical component of the reflected light beams passing through the aperture of said slit; an edge prism for reflecting some of the zero-order optical component of the reflected light beams focused by said focusing lens, and passing the remainder thereof, centered on the optical axis; an optical detector for detecting a signal for informing the track location of said objective lens from the zero-order optical component of the reflected light beams reflected by said edge prism; and another optical detector for detecting a signal for informing the focus location of said objective lens from the zero-order optical component of the reflected light beams passing through said edge prism. Preferably, the optical pickup is further provided with a collimating lens for collimating the generated light beam, a first slit with an aperture for passing the collimated light beam therethrough, to restrict the diameter of the collimated light beam to a predetermined diameter, and a first beam splitter for reflecting the light beams passing through the aperture of said slit toward said objective lens, and transmitting the reflected light beams passing through said objective lens, and wherein said objective lens is arranged for focusing the light beam passing through the aperture of said first slit, the first portion transmitting the zero-order light of the reflected beams diffracted and reflected by the light-recording medium in parallel, and the second and third portions transmitting at least a portion of each list-order diffracted light diffracted in the jitter direction of the reflected light beams, wherein said first, second and third portions are not overlapped and, wherein, said detecting means comprises: a second beam splitter for reflecting some of the reflected light beams while passing the remainder thereof, in order to divide the light amount of the reflective light beams coming straight through said first beam splitter; a first focusing lens for focusing the zero-order optical and ±1st-order diffracted optical components of the reflected light beams reflected by said second beam splitter so as to be interfered mutually; a first optical detector for receiving the zero-order optical and ±1st-order diffracted optical components of the reflected light beams focused by said focusing lens and detecting a signal for the reproduction of the information recorded on said light-recording medium; a second slit having an aperture, for passing the zero-order optical component of the reflected light beams coming straight through said second beam splitter and intercepting the ±1st-order diffractive optical components thereof; a second focusing lens for focusing the zero-order optical component of the reflected light beams passing through the aperture of said second slit; an edge prism for reflecting some of the zero-order optical component of the reflected light beams focused by said second focusing lens, and passing the remainder thereof; a second optical detector for receiving the zero-order optical component of the reflective beams reflected by said edge prism and detecting a signal for informing the track location of said objective lens; and a third optical detector for receiving the zero-order optical component of the reflected light beams passing through said edge prism and detecting a signal for informing the focus location of said objective lens. According to a second aspect of the invention, there is provided an objective lens for use in an optical pick for high-density recording/reproducing, wherein the objective lens is provided with first, second and third portions, the first portion being arranged to transmit light from an optical source to an optical recording medium and to transmit zero order reflected light from the optical recording medium towards an optical detector and the second and third portions being arranged to transmit ± 1st order diffracted reflected light from the optical recording medium towards the optical detector, the objective lens being chracterised in that said objective lens is coated opaquely except for the first, second and third portions thereof. For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example, to the accompanying diagrammatic drawings, in which: Figure 1 shows a conventional optical pickup; Figure 2 shows recording marks of an optical disk and a beam spot following the recording marks; Figure 3 shows beam spots diffracted and reflected from the optical disk; Figure 4 shows a pickup for high-density recording/reproducing according to an embodiment of the present invention; Figure 5 is a plan view showing an incident surface of an objective lens used in an optical pickup for high-density recording/reproducing in accordance with the present invention; Figure 6 is a schematic profile showing the paths of the incident light beams via the optical pickup for high-density recording/reproducing; Figure 7 is a schematic profile showing the paths of the reflected light beams via the optical pickup for high-density recording/reproducing; and Figure 8 is a plan view showing another embodiment with respect to the objective lens used in the optical pickup for high-density recording/reproducing according to the present invention. In Figure 4, for example, a laser diode is used as a light source 12 for emitting a laser light beam. A collimating lens 13 collimates the light beams emitted from the laser diode 12. A first slit 14 in front of the collimating lens 13 has an aperture 15 of a predetermined diameter for restricting the light beam from the collimating lens 13. Light beam 16 passing through the aperture 15 of the first slit 14 is reflected by a first beam splitter 17 toward an objective lens 18. Here, in lieu of the first slit, a transparent medium material which is completely coated opaquely, save the portion corresponding to the aperture, can be used or a collimating lens (similarly coated) serves as the first slit. Referring to Figure 5, the objective lens 18 is composed of a first portion 20 formed on the center of its light-receiving (or light-emitting) surface, second and third portions 21 and 22 formed in contact with the outer sides of first portion 20, and a hatched portion 23. The portions 20, 21 and 22 are aligned in the direction perpendicular to a radial direction of the optical disk, i.e., in the jitter direction. Referring to Figure 6, the first portion 20 of the objective lens 18 receives light beam 16 whose diameter is substantially adjusted to that of the first portion 20 by the aperture of the first slit as described above. The light beam 16 is focused up to its maximum diffractive limitation through objective lens 18, to be formed in a spot on the surface of the optical disk 19. The spot on the optical disk 19 follows the recording marks by the rotation of the optical disk 19 by a spindle motor (not shown), to thereby be diffracted and reflected on the boundary of the recording marks. Referring to Figure 7, a zero-order optical component 24 among the reflected light beam by the optical disk 19 passes through the first portion 20 of the objective lens 18 by the same course as the incident light beam. Among the reflected light beams, the ±1st-order diffracted optical components 25 and 26 diffracted in the jitter direction pass through the second and third portions 21 and 22 of the objective lens 18, respectively, to thereby continue on in parallel with the zero-order optical component 24. Referring back to Figure 4, the reflected light beams comprising the zero-order optical component 24 and ±1st-order diffracted optical component 25 and 26 go straight through the first beam splitter 17, with some of the light being reflected by the second beam splitter 27, and the remainder continuing on forward. A first focusing lens 28 focuses the zero-order optical component 24 and the ±1st-order diffracted optical components 25 and 26 reflected by the second splitter 27 on a first optical detector 29 in a spot, whereby the different components of the reflected light beams interfere mutually. Accordingly, first optical detector 29 detects signals for reproduction according to the change of the light amounts due to the mutual interference between the zero-order optical component 24 and the ±1 order diffracted optical components 25 and 26 from the spot of received light beams thereon. Meanwhile, the zero-order optical component 24 of the reflected light beams travelling straight the second beam splitter 27 passes through an aperture 31 of a second slit 30, but the ±1st-order diffracted optical components 25 and 26 are intercepted by the second slit 30. The zero-order optical component 24 passing through the aperture 31 of the second slit 30 is focused by the second focusing lens 32, some of which is reflected by an edge prism 33 and the remainder passes without obstruction. Some of the zero-order optical component 24 reflected by the edge prism 33 is received by the second optical detector 34, to be detected as a signal for indicating a track location of the objective lens. The remainder of the zero-order optical component passing through the edge prism 33 is received by a third optical detector 35, to be detected as a signal for indicating a focus location of the objective lens. According to such an embodiment, the distance between the optical axes, in the jitter direction, of zero-order optical and ±1st-order diffracted optical components of the reflected light beams is greater than or equal to the diameter of the first portion on the objective lens used in the present invention (h≥2a). In other words, since the signal for the reproduction can be detected even though the pitch of the recording marks on the optical disk is less than or equal to λ/2NA, reproduction at high density (above cut-off frequency) is possible. In addition, high-density recording can be realized by reducing the interval of the recording marks on the optical disk regardless of the numerical aperture of the objective lens or the optical wavelength. Next, Figure 8 shows another embodiment of an objective lens. An objective lens 18' includes a first portion 20' formed for receiving the zero-order optical component adjacent to an optical axis and second and third portions 21' and 22' formed on both sides thereof, for partially receiving the ±1st-order diffracted optical components. Compared with the case of forming the aperture portion of an objective lens in a perfect circle in order to completely receive the ±1st-order diffracted optical components as in the prior embodiment, the embodiment in Figure 8 has an advantage in that the diameter of the objective lens can be made smaller. In addition, instead of dividing the objective lens into the portions for receiving the components of the reflected light beams respectively as in the above embodiments, a lens having a much larger diameter than the conventional objective lens can be provided for the receiving operation. As described above, in embodiments of the present invention, the recording and reproduction of data at high density (above cut-off frequency) can be realized, without deteriorating optical stability and regardless of optical wavelength or the numerical aperture of the objective lens. Also, embodiments of the present invention enable the application to an optical pickup for recording/reproduction at super-high density, through the simultaneous use of an objective lens having a large numerical aperture and a short-wavelength light source.

Claims: An optical pickup for high-density recording/reproducing, comprising: a light source (12) for generating a light beam; an objective lens (18, 18') for focusing the light beam on a light-recording medium (19); and an optical signal detecting means for detecting an optical signal by receiving reflected light beams diffracted and reflected by the light-recording medium, wherein said objective lens (18, 18') includes a first portion (20, 20') for transmitting a zero-order optical component of the reflected light beams and second and third portions (21, 21', 22, 22') for respectively transmitting at least some of each ±1st-order diffracted optical components of the reflected light beams, the pick-up being characterised in that said objective lens (18, 18') is coated opaquely except for the first, second and third portions (20-22, 20'-22') thereof. An optical pickup for high-density recording/reproducing as claimed in claim 1, further comprising means (14, 15) for controlling the diameter of a light beam generated by said light source (12) so that the light beam is incident to the first portion of said objective lens. An optical pickup for high-density recording/reproducing as claimed in claim 2, wherein said controlling means comprises: a slit (14) having an aperture (15) pierced with a predetermined diameter, said slit being installed between said light source (12) and said objective lens (18, 18') in order to restrictively transmit the light beam from said light source to the first portion (20, 20') of said objective lens through the aperture (15). An optical pickup for high-density recording/reproducing as claimed in claim 3, wherein the aperture (15) of said slit (14) is substantially the same size as the first portion (20, 20') of said objective lens (18, 18'). An optical pickup for high-density recording/reproducing as claimed in claim 3 or 4, further comprising a collimating lens (13), installed between said light source (12) and slit (14), for collimating the light beam generated from said light source (12). An optical pickup for high-density recording/reproducing as claimed in any of the preceding claims, further comprising means (28) for making the zero-order optical component and ±1st-order diffracted optical components of the reflected light beams passing said objective lens (18, 18') mutually interfere. An optical pickup for high-density recording/reproducing as claimed in any of the preceding claims, further comprising means (30) for selecting the zero-order optical component, among the reflected light beams passing said objective lens (18, 18'). An optical pickup for high-density recording/reproducing as claimed in any of the preceding claims, wherein said optical signal detecting means comprises: a focusing lens (28) for focusing the zero-order optical and ±1st-order diffracted optical components of the reflective light beams transmitted respectively by the first, second and third portions (20-22, 20'-22') of said objective lens (18, 18'), to make them interfere with each other; and an optical detector (29) for detecting a signal for reproducing information recorded on said light-recording medium according to the change of light amount due to the mutual interference between the zero-order optical and ±1st-order diffracted optical components of the reflective light beams focused on said focusing lens (28). An optical pickup for high-density recording/reproducing as claimed in any of the preceding claims, wherein said optical signal detecting means (30-35) includes means for detecting signals for informing the track and focus locations of said objective lens (18, 18') from the zero-order optical component of the reflected light beams transmitted by the first portion of said objective lens (18, 18'). An optical pickup for high-density recording/reproducing as claimed in claim 9, wherein said detecting means comprises: a slit (30) having an aperture (31) for passing the zero-order optical component of the reflected light beams coming via said objective lens (18, 18'), for blocking the ±1st-order diffracted optical components; a focusing lens (32) for focusing the zero-order optical component of the reflected light beams passing through the aperture (31) of said slit (30); an edge prism (33) for reflecting some of the zero-order optical component of the reflected light beams focused by said focusing lens (32), and passing the remainder thereof, centered on the optical axis; an optical detector (34) for detecting a signal for informing the track location of said objective lens from the zero-order optical component of the reflected light beams reflected by said edge prism (33); and another optical detector (35) for detecting a signal for informing the focus location of said objective lens from the zero-order optical component of the reflected light beams passing through said edge prism (33). An optical pickup according to claim 1, wherein there is further provided with a collimating lens (13) for collimating the generated light beam, a first slit (14) with an aperture (15) for passing the collimated light beam therethrough, to restrict the diameter of the collimated light beam to a predetermined diameter, and a first beam splitter (17) for reflecting the light beams passing through the aperture (15) of said slit (14) toward said objective lens (18, 18'), and transmitting the reflected light beams passing through said objective lens (18, 18'), and wherein said objective lens (18, 18') is arranged for focusing the light beam passing through the aperture (15) of said first slit (14), the first portion (20, 20') transmitting the zero-order light of the reflected beams diffracted and reflected by the light-recording medium (19) in parallel, and the second and third portions (21, 21', 22, 22') transmitting at least a portion of each ±1st-order diffracted light diffracted in the jitter direction of the reflected light beams, wherein said first, second and third portions (20-22, 20'-22') are not overlapped and, wherein, said detecting means comprises: a second beam splitter (27) for reflecting some of the reflected light beams while passing the remainder thereof, in order to divide the light amount of the reflective light beams coming straight through said first beam splitter (17); a first focusing lens (28) for focusing the zero-order optical and ±1st-order diffracted optical components of the reflected light beams reflected by said second beam splitter (27) so as to be interfered mutually; a first optical detector (29) for receiving the zero-order optical and ±1st-order diffracted optical components of the reflected light beams focused by said focusing lens (28) and detecting a signal for the reproduction of the information recorded on said light-recording medium; a second slit (30) having an aperture (31), for passing the zero-order optical component of the reflected light beams coming straight through said second beam spitter (27) and intercepting the ±1st-order diffractive optical components thereof; a second focusing lens (32) for focusing the zero-order optical component of the reflected light beams passing through the aperture (31) of said second slit (30); an edge prism (33) for reflecting some of the zero-order optical component of the reflected light beams focused by said second focusing lens (32), and passing the remainder thereof; a second optical detector (34) for receiving the zero-order optical component of the reflective beams reflected by said edge prism and detecting a signal for informing the track location of said objective lens (18, 18'); and a third optical detector (35) for receiving the zero-order optical component of the reflected light beams passing through said edge prism (33) and detecting a signal for informing the focus location of said objective lens (18, 18'). An objective lens for use in an optical pick for high-density recording/reproducing, wherein the objective lens (18, 18') is provided with first, second and third portions (20-22, 20'-22'), the first portion (20, 20') being arranged to transmit light from an optical source (12) to an optical recording medium (19) and to transmit zero order reflected light from the optical recording medium (19) towards an optical detector and the second and third portions (21, 21', 22, 22') being arranged to transmit + 1st order diffracted reflected light from the optical recording medium (19) towards the optical detector, the objective lens being chracterised in that said objective lens (18, 18') is coated opaquely except for the first, second and third portions (20-22, 20'-22') thereof.

Inventors: LEE YONG-JAE, LEE, YONG-JAE
Applicants: SAMSUNG ELECTRONICS CO LTD, SAMSUNG ELECTRONICS CO., LTD.
Application year: 1996
Main classification: G11B7
IPC codes: G11B7