Compatible optical pickup

Abstract: A compatible optical pickup capable of recording or reproducing information on or from compact disc (CD) family media such as CD rewritable (CD-RW) by adopting a element (135) having a plurality of gratings. The optical diffraction element (135) has a first region (135a) which directly transmits the first and second light incident thereto, and a second region (135b) bounding the first region, which directly transmits the incident first light and diffractingly transmits the second light toward an optical axis. Preferably a separate optical diffraction element (135) is used in the form of a stepped planar lens in which one or more stepped pattern periods each having a plurality of annular gratings are arranged in the second region, wherein the depths of the gratings become smaller moving away from the optical axis. Alternatively, the stepped pattern may be formed on one face of an objective lens (137').

Description: The present invention relates to optical pickups capable of adopting disc type recording media having different formats, and more particularly to an optical pickup adopting a planar lens having a stepped grating, capable of recording or reproducing information on or from a compact disc (CD) family media including a CD rewritable (CD-RW) and a digital versatile disc (DVD). The need has been identified for an optical recording and/or reproducing apparatus capable of recording and/or reproducing information on and/or from a DVD at a high density be compatible with CD family media such as CD, recordable CD (CD-R), CD-RW, CD interactive (CD-I) and CD plus graphics (CD+G). The standard thickness of existing CD family media is 1.2mm, whereas the thickness of DVDs has been standardized to 0.6mm in consideration of the allowable error in the tilt angle of a disc and the numerical aperture (NA) of an objective lens. Accordingly, when recording or reproducing information on or from a CD using an optical pickup for DVDs, spherical aberrations occur due to a difference in the thicknesses therebetween. Such spherical aberration cannot provide a light intensity sufficient for recording an information (radio frequency) signal or may deteriorate the signal reproduced from the CD. Also, DVDs and CD family media utilize different wavelengths of light for reproduction: CDs use light having a wavelength of about 780nm as a light source for reproduction, whereas DVDs use light having a wavelength of about 650nm as a light source therefor. Thus, an optical pickup is required compatible with CDs, having an optical source capable of emitting different wavelengths of light, and a structure in which optical spots can be formed at different focal positions. Referring to Figure 1, a conventional compatible optical pickup comprises a first optical source 21 for emitting light having a wavelength of about 650nm and a second optical source 31 for emitting light having a wavelength of about 780nm. The first optical source 21 is appropriate for a relatively thin disc 10a, such as DVDs, and the second optical source 31 is appropriate for a relatively thick disc 10b such as CDs. Light emitted from the first optical source 21 is condensed by a first collimator lens 23, parallelly incident onto a first polarization beam splitter (PBS) 25, and then reflected by the first PBS 25 toward the thin disc 10A. After reflected by the thin disc 10a, the reflected light is transmitted through the first PBS 25 and is then received by a first photodetector 27. Here, an interference filter 41 for changing the paths of light emitted from the first and second optical sources 21 and 31, a 1/4-wavelength plate 43, a variable diaphragm 45 and an objective lens 47 for condensing light incident thereto are situated on an optical path between the first PBS 25 and the disc 10. Light emitted from the second optical source 31 is condensed by a second collimator lens 33, parallelly incident onto a second PBS 35, transmitted through a condenser lens 37, and then incident onto the interference filter 41. The light is reflected by the interference filter 41 and passes through the 1/4-wavelength plate 43, the variable diaphragm 45 and the objective lens 47 in sequence to form an optical spot on the thick disc 10b. Light reflected by the relatively thick disc 10b is incident onto the interference filter 41 through the objective lens 47, the variable diaphragm 45 and the 1/4-wavelength plate 43, and then reflected by the interference filter 41 heading toward the second PBS 35. The light is reflected by the second PBS 35 and received by a second photodetector 39. The interference filter 41, an optical element for transmitting or reflecting incident light depending on the wavelength of the incident light, transmits the light emitted from the first optical source 21 and reflects the light emitted from the second optical source 31. The 1/4-wavelength plate 42 is an optical member for changing the polarization direction of the incident light. Light emitted from the first and second optical sources 21 and 31 pass through the 1/4-wavelength plate 43 twice, heading toward the first and second PBSs 25 and 35 while the phase of the polarized light is delayed 90°. Referring to Figure 2, the variable diaphragm 45 comprises a first region 45a the size of which is variable, and a second region 45b bounding the first region 45a, and selectively transmits the incident light. The variable diaphragm 45 corresponds to a region of the objective lens 27 having a NA of 0.6 or less. The first region 45a, which corresponds to a region of the objective lens 47 having a NA of 0.45 or less, completely transmits the light emitted from the first and second optical sources 21 and 31. The second region 45b, which is coated with multiple dielectric thin films to a thickness on a micrometer scale, corresponds to a region of the objective lens having a NA of 0.45-0.6. That is, the second region 45b completely transmits light having a wavelength of 650nm and completely reflects light having a wavelength of 780nm. For the purpose of eliminating optical aberration occuring at the second region 45b, the first region 45a is formed of a SiO 2 thin film. The objective lens 47 condenses light emitted from the first and second optical sources 21 and 31 to form optical spots on the recording surface of the discs 10a and 10b, respectively. The conventional compatible optical pickup described above, and as disclosed in EP-A-0747893, which is used for the two-part form delimitation, can be adopted to a CD-R by using two optical sources having different wavelengths. However, because the conventional compatible optical pickup requires a variable diaphragm having first and second regions, which is manufactured through sophisticated and expensive processes, assembling of such optical pickup becomes complicated and costly. In addition, the variable diaphragm completely reflects the light for recording on a CD-R, which is emitted from the second optical source and incident onto a region having a NA of 0.45 or more, and thus it cannot be applied to an optical pickup for CD-RWs, which requires a NA of 0.5 or more and high optical efficiency for recording. An aim of at least preferred embodiments of the present invention is to provide a compatible optical pickup capable of recording or reproducing information on or from digital versatile discs (DVDs) and compact disc (CD) family media including CDs, recordable CDs (CD-Rs) and CD rewritables (CD-RWs). According to the present invention there is provided an optical pickup as set forth in claim 1 appended hereto. Preferred features of the invention will be apparent from the dependent claims and the description which follows. According to the present invention there is provided an optical pickup comprising: a first optical source for emitting a first light having a first predetermined wavelength; a second optical source for emitting a second light having a longer wavelength relative to the first light; and an objective lens for condensing the first and second lights onto optical discs having different thicknesses; characterised by: an optical diffraction means having a first inner region which transmits the first and second light incident thereto without diffraction, and a second outer region which transmits the incident first light without diffraction and diffractingly transmits the second light toward an optical axis. Preferably, the optical diffraction means is a stepped planar lens in which one or more stepped pattern periods each having a plurality of annular gratings are arranged in the second region. Preferably, the depths of the gratings become smaller moving away from the optical axis. Preferably, the stepped pattern comprises a first stepped pattern formed within a first annular radial range of the stepped planar lens, and a second stepped pattern formed within a second annular radial range. Preferably, the first stepped pattern is formed within a radial range of about 1000-1150µm; and the second stepped pattern is formed within a radial range of about of 1150-1200µm. Preferably, the second outer region is immediately adjacent the first inner region. Preferably, the compatible optical pickup comprises: an optical path changing means for changing the traveling path of an incident light. Preferably, the stepped pattern is formed in one side of the stepped planar lens, facing the optical path changing means. Preferably, the optical diffraction means is built-in at one side of the objective lens, facing the optical path changing means. Preferably, one or more units of a stepped pattern having a plurality of annular gratings are arranged in the second outer region, wherein the depths of the gratings in each unit become smaller moving away from the optical axis, and the maximum depth thereof is d . Preferably, the maximum depth of the or each grating d is about 6.4µm. Preferably, the optical pickup comprises a photodetector for detecting an information signal and an error signal from the first and second light which have been reflected by the optical discs and passed through the optical path changing means. Preferably, the optical pickup comprises: a first collimator lens disposed on the optical path between the second optical source and the optical path changing means, for pre-condensing and transmitting the incident light; and a second collimator lens disposed on the optical path between the optical path changing means and the objective lens, for condensing and transmitting the incident light, to set the optical focal length between the second optical source and the optical discs, thereby raising the optical efficiency. Preferably, the optical pickup comprises: a first optical unit including a first optical source for emitting a first light having a wavelength of 650nm, and a first photodetector for receiving the first light; a second optical unit including a second light source for emitting a second light having a wavelength of 780nm, and a second photodetector for receiving the second light; an optical path changing means for changing the paths of the first and second light; and an objective lens for condensing the first and second lights from the optical path changing means to form optical spots onto optical discs having different thicknesses. 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 is a schematic view showing the optical arrangement of a conventional compatible optical pickup; Figure 2 is a schematic view of the variable diaphragm of Figure 1; Figure 3 is a schematic view showing the optical arrangement of a compatible optical pickup according to a preferred embodiment of the present invention; Figure 4 is a perspective view of the stepped planar lens adopted as an embodiment of an optical diffraction means according to the present invention; Figure 5 shows a grating pattern formed in the second region of the stepped planar lens according to the present invention; Figure 6 is a diagram illustrating the diffraction efficiency according to the depth and interval of the grating pattern formed in the second region of the stepped planar lens according to the present invention; Figure 7 is a graph showing the change in diffraction efficiency of the first and second lights of Figure 6, according to the depth of the grating pattern; Figure 8 is a partial view of a compatible optical pickup, for illustrating the operation of the stepped planar lens according to the present invention; Figure 9 is a graph illustrating a variation of optical efficiency with respect to the focal length of collimator lense; Figure 10 is a schematic view of a stepped planar lens built-in objective lens as another optical diffraction means according to the present invention; and Figure 11 is a schematic view showing the optical arrangement of a compatible optical pickup according to another embodiment of the present invention. Referring to Figure 3, a compatible optical pickup according to a preferred embodiment of the present invention includes first and second optical sources 111 and 121 for emitting light having different wavelengths, an optical path changing means for changing the path of an incident light, an objective lens 137 for condensing the incidnet light so as to form an optical spot on an optical disc 100, an optical diffraction means, a photodetector 141 for detecting an information (radio frequency) signal and an error signal from the incident light which is reflected by the optical disc 100 and passes through the optical path changing means. The first optical source 111 emits a first light I having a wavelength of about 650nm. The emitted first light I is used to record or reproduce information on or from a relatively thin disc 100a such as a digital versatile disc (DVD). Also, the second optical source 121 emits a second light II having a wavelength of about 780nm, and the emitted second light II is used to record or reproduce information on or from a relatively thick optical disc 100b such as a compact disc (CD) or a CD rewritable (CD-RW). The optical path changing means includes a first beam splitter 113 for changing the traveling path of the first light I emitted from the first optical source 111, and a second beam splitter 131 disposed between the first beam splitter 113 and the objective lens 137, for changing the traveling path of the light incident thereto. The first beam splitter 113 makes the first light I incident from the first optical source 111 toward the optical disc 100, and the first and second lights I and II, which have been reflected by the optical disc 100, toward the photodetector 141. The first beam splitter 113 may have a cubic structure instead of the planar structure shown in Figure 3. The second beam splitter 131 changes the traveling paths of the light incident thereto by partially transmitting or reflecting the first and second lights I and II incident thereto. In the case of having the optical arrangement shown in Figure 3, the second beam splitter 131 reflects a part of the second light II emitted from the second optical source 121 such that it heads toward the optical disc 100, and transmits the rest of the second light II. Also, the second beam splitter 131 transmits a part of the first light I emitted from the first optical source 111 such that it heads toward the optical disc 100, and reflects the rest of the first light I. The second beam splitter 131 transmits a part of the light, which has been reflected by the optical disc 100, such that it heads toward the photodetector 141. The compatible optical pickup according to the present embodiment may further comprise a monitoring detector 143 capable of detecting the output of light emitted from the first and second optical sources 111 and 121. The monitoring detector 143 detects the output of light emitted from the first and second optical sources 111 and 121, by receiving the first light I reflected by the second beam splitter 131, which has been emitted from the first optical source 111, and the second light II transmitted through the second beam splitter 131, which has been emitted from the second light source 121. Here, because the first and second optical sources 111 and 121 are selectively driven according to the type of adopted optical disc, the first and second light I and II are not received by the monitoring photodetector 143 at the same time. The optical diffraction means may be a stepped planar lens 135 disposed on an optical path between the optical path changing means and the objective lens 137. The stepped planar lens 135 directly transmits the first light I and diffractingly transmits the second light II toward an optical axis. Referring to Figures 3 and 4, the stepped planar lens 135 has a first region 135a, which corresponds to a region of the objective lens 137 having a numerical aperture (NA) of 0.3 or less, and a second region 135b bounding the first region 135a, which corresponds to a region having an NA of 0.3-0.5. Also, the diameter of a region A indicated by dashed lines in Figure 4 corresponds to the effective diameter of the objective lens 137 having an NA of 0.6. The first region 135a is capable of directly transmitting the incident first and second light I and II, wherein it has no pattern. Meanwhile, the second region 135b has an annular grating pattern having a stepped section. The first region 135 has a 0th-order diffraction efficiency of about 100%, thereby directly transmitting both the first and second lights I and II. Here, the 0th-order diffraction efficiency is expressed as the percentage of the transmitted light with respect to the amount of an incident light. The second region 135b has a 0th-order diffraction efficiency of about 100% with respect to the first light I incident thereto, and a 0th-order diffraction efficiency of about 0% with respect to the second light II incident thereto. Also, a 1st-order diffraction efficiency of the second region 135b with respect to the second light II is about 70%. Here, the 1st-order diffraction efficiency is expressed as the percentage of the 1st-order diffracted light with respect to the amount of incident light. Figure 5 shows an example of a stepped pattern of the stepped planar lens 135. The graph of Figure 5 shows the section of patterns formed on the quadrant I of the X-Y coordinate of Figure 4, and the sectional view of Figure 5 shows the stepped pattern of the second region 135b. Referring to Figure 5, the stepped pattern is formed of a plurality of lattices whose depths become smaller as they move farther away from the optical axis, wherein a maximum depth d of the stepped pattern is ideally approximately 6.4µm. Here, the stepped pattern may be repeated. That is, one stepped pattern may be formed within a first radial range, preferably of about 1000-1150µm of the stepped planar lens 135, and another stepped pattern may be further formed within a second radial range, preferably annularly disposed about the first range, and preferably in the range of about 1150-1200µm. Such stepped pattern of the stepped planar lens 135 is formed facing the optical path changing means. The maximum depth d of the pattern is determined by a method which will be explained later. The diffraction efficiency of the stepped planar lens 135 is determined according to the interval and depth of the stepped pattern. Referring to Figure 6, the vertical axis represents the optical axis of the stepped planar lens 135 and the horizontal axis represents the distance in a radial direction. Also, T represents one stepped pattern period, and α, β, γ, as coefficients which are equal to or greater than 0 and less than 1, satisfy the relationship of α < β < γ. Also, no represents the refractive index of air, which is usually equal to 1, and n 1 represents a refractive index of the stepped planar lens 135. Here, assuming that x represents an arbitrary position on the radial direction of the stepped planar lens 135, the stepped planar lens 135 has a transmission coefficient T m of the stepped planar lens 135 at each position in the radial direction, which is expressed by where m represents the order of diffraction, λ represents the wavelength, and d represents the maximum depth of the stepped pattern. The optical efficiency is equal to the absolute square of the transmission coefficient T m . Also, from the equation (1) , the transmission of the stepped planar lens 135 according to the section thereof is expressed by the following function (2) Also, the stepped planar lens 135 include gratings in stepped patterns, and the number of gratings in one stepped pattern period is expressed by N ≒ λ 1 (λ 2 -λ 1 ) where N is an integer which represents the number of gratings in one stepped pattern period, λ 1 represents the wavelength of the first light and λ 2 represents the wavelength of the second light. Also, the step difference of each grating of the stepped planar lens 135 is equal to each other, and such step difference causes a phase difference in the second light II, which satisfies the following equation (4), such that the phase difference of the second light II incident onto the first region 135a having an NA of 0.3 or less equals to that of the second light II incident onto the second region 135b, thereby eliminating spherical aberration. δ i = 2πi(n-n 0 )d i λ where δ i represents the optical phase difference by the i-th step difference from the optical center of the stepped planar lens and d i represents the depth of the i-th step difference. Figure 7 is a graph showing the diffraction efficiency of light emitted from different optical sources according to the depth of the pattern, which is considered in designing a stepped planar lens. In Figure 7, the X-axis represents the depth of the pattern, and the Y-axis represents the diffraction efficiency. A curve indicated by dotted lines represents the 0th-order diffraction efficiency with respect to the first light I, and a curve indicated by solid lines and with blank circles represents the -1st-order diffraction efficiency with respect to the second light II, and a curve indicated by solid lines represents the 0th-order diffraction efficiency with respect to the second light II.. Here, the minus (-) means a diffraction toward the optical axis of the objective lens 137. In the graph of Figure 7, when the depth of the pattern is at about 6400nm (6.4µm), the 0th-order diffraction efficiency of the first light I is about 1 and that of the second light II is about 0. Also, the -1st- order diffraction efficiency of the second light II is about 0.75. Thus, preferably, the maximum depth d of the pattern of the stepped planar lens 135 according to the present invention is set to about 6.4µm such that the 0th-order diffraction efficiency of the first light I in the second region 135b is about 100% and that of the second light II therein is close to 0%. Referring to Figure 8, in the operation of the stepped planar lens 135, the stepped planar lens 135 directly transmits the first light I in both the first and second regions 135a and 135b. Also, the stepped planar lens 135 directly transmits the second light II in the first region 135a while diffractingly transmitting the second light II to the -1st-order in the second region 135b. Thus, a part of the first light I transmitted through the stepped planar lens 135 is incident onto a region of the objective lens 137, having a NA of 0.3-0.5, and condensed onto the thin optical disc 100a, whereas the second light II transmitted through the stepped planar lens 135 is incident onto a region of the objective lens 137, having an NA of 0.3 or less, and condensed onto the thick optical disc 100b. As described above, in the optical pickup of the present invention, the first and second lights I and II can be focused at different positions by disposing the stepped planar lens 135 on the optical path. Referring to Figure 3, the objective lens 137 has a NA of, for example, 0.6, to be appropriate for the formation of the relatively thin optical disc 100a, and condenses the incident first and second lights I and II to be focused onto the optical discs 100a and 100b, respectively. The photodetector 141 receives the lights respectively reflected by the optical discs 100a and 100b, and passed through the objective lens 137, the stepped planar lens 135 and the optical path changing means, to detect an error signal and an information (RF) signal from the received lights. Preferably, the optical pickup according to the present invention includes a grating 123, first and second collimator lenses 125 and 133, and a light receiving lens 139. The grating 123, which diffractingly transmits an incident light to detect a tracking error signal by a three-beam method, is disposed on the optical path between the second optical source 121 and the second beam splitter 131. The grating 123 diffractingly transmits light emitted from the second optical source 121 to be at least the 0-th and ±1-st order light. The first and second collimator lenses 125 and 133 cause the incident light to converge to set the optical focal lengths between the first and second optical sources 111 and 121, and the optical discs 100. The first collimator lens 125 is disposed on the optical path between the second optical source 121 and the second beam splitter 131, and pre-condenses the emanated light emitted from the second optical source 121. The first collimator lens 125 makes the optical focal length short such that light emitted from the second optical source 121 is suitable for a CD-RW that requires a high optical efficiency. The second collimator lens 133 is disposed on the optical path between the objective lens 137, and the first and second optical sources 111 and 121, and collimates the light heading toward the optical disc 100. Figure 9 illustrates the optical efficiency with respect to the focal lengths of the first and second collimator lenses 125 and 133. As shown in Figure 9, when the focal length of each of the first and second collimator lenses 125 and 133 are about 25 mm, the optical efficiency is 15.7% relative to the incident light, and 50.2% at a focal distance of 12 mm. Thus, in the case where a CD-RW, which requires an optical power of 8-21 mW depending on the writing speed, is adopted as the thick optical disc 100b, the optical focal length can be shortened to about 12 mm by using the first collimator lens 125, thereby raising the optical efficiency. As a result, a desired optical power can be obtained, which is not absolutely dependent on the optical output from the second optical source 121. The light receiving lens 139 is disposed between the first beam splitter 113 and the photodetector 141 and causes astigmatism of the light passing through the same to detect a focus error signal from the light. The optical diffraction means may be built-in at one side 137'a of an objective lens 137' as shown in Figure 10, rather than being in the form of a separated stepped planar lens. As in the above-mentioned stepped planar lens 135, the optical diffraction means shown in Figure 10 may have a first region 135'a for directly transmitting the incident light, and a second region 135'b bounding the first region 135'a, for directly transmitting the first light I and diffractingly transmitting the second light II toward the optical axis. Also, one or more stepped patterns each having a plurality of annular gratings are formed in the second region 135'b, wherein the depths of the gratings become smaller moving away from the optical axis. The stepped pattern formed in the second region 135'b is substantially the same as that mentioned above, and thus a detailed explanation will not be repeated here. Referring to Figure 11, a compatible optical pickup according to another embodiment of the present invention comprises a first optical unit 210 for emitting and receiving a first light I' having a wavelength of about 650nm, a second optical unit 220 for emitting and receiving a second light II' having a wavelength of about 780nm, an optical path changing means for changing the paths of the first and second light I' and II', an objective lens 237 for condensing the incident light to form an optical spot on an optical disc 200, and an optical diffraction means. The compatible optical pickup shown in Figure 11 is substantially the same as the compatible optical pickup illustrated with reference to Figure 3 in that the optical diffraction means such as a stepped planar lens 235 is disposed on the optical path. However, unlike the compatible optical pick according to the first embodiment of the present invention, the compatible optical pickup of Figure 11 comprises separate photodetectors, for example, first and second photodetectors 219 and 227, for receiving the light emitted from first and second optical sources 211 and 221, respectively. The first optical unit 210 comprises the first optical source 211 for emitting the first light I', a beam splitter 213 for changing the traveling path of the incident light, a monitoring photodetector 215 for receiving light, which has been emitted from the first optical source 211 and then diverged by the beam splitter 213, to detect the optical output of the first optical source 211, and the first photodetector 219 for receiving light that has been reflected by a relatively thin optical disc 200a and then passed through the beam splitter 213. The first optical unit 210 may further comprise a first collimator lens 214 for changing the radiating incident light into parallel light, and a light receiving lens 217 disposed between the beam splitter 213 and the first photodetector 219. The light receiving lens 217 may be a planar hologram lens as shown in Figure 11. The second optical unit 220 comprises a second optical source 211 for emitting the second light II', a hologram optical element 223 for changing the preceding path of the incident light, and a second photodetector 227 for receiving light that has been reflected by a relatively thick optical disc 200b and diffracted by the hologram optical element 223. The second optical unit 220 may further comprise a second collimator lens 225 for condensing the incident emanated light into parallel light. The optical path changing means disposed on the optical path between the objective lens 237, and the first and second optical units 210 and 220, which is for changing the traveling path of the light, comprises a PBS 231 for transmitting/reflecting the incident light according to polarization direction, and a phase retardation plate 233 for causing lag in-phase to the incident light. The optical diffraction means is comprised of the stepped planar lens 235 as shown in Figure 11. The optical diffraction means is substantially the same as the optical diffraction means illustrated with reference to Figures 4 through 9, and thus a detailed explanation thereof is omitted here. In the case where the compatible optical pickup according to the present invention comprises a reflection mirror M on the optical path and parallel light is incident onto the reflection mirror M, the arrangement of the first and second optical units 210 and 220 and the PBS 231 may be fixed, and the objective lens 237, the stepped planar lens 235, the phase retardation plate 233 and the reflection mirror M may be moved in the radial direction of the optical disc 200 to follow tracks. The compatible optical pickup according to the present invention comprises the optical diffraction means such as the stepped planar lens, on the optical path, which has a simple structure and does not need separate driving thereof, can be applied to optical discs having different formats, for example, CD family media including CD-RW, and DVD. Also, the stepped planar lens can be mass-produced at low cost using a mother substrate having a pattern corresponding to a desired grating pattern. Also, as for the second light for use in recording or reproducting information on or from the CD-RW, most of the light passed through the second region having a NA of 0.3-0.5 is used by being diffracted to the -1st order toward the optical axis, thereby raising the optical efficiency in use relative to the conventional optical pickup. Also, the optical efficiency can be raised by shortening the optical focal length with the first and second collimator lenses, rather than by the optical power of the second optical source alone. While this invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention as defined by the appended claims.

Claims: An optical pickup comprising: a first optical source (111) for emitting a first light having a first predetermined wavelength; a second optical source (121) for emitting a second light having a longer wavelength relative to the first light; and an objective lens (137) for condensing the first and second lights onto optical discs having different thicknesses; characterised by : an optical diffraction means (135) having a first inner region (135a) which transmits the first and second light incident thereto without diffraction, and a second outer region (135b) which transmits the incident first light without diffraction and diffractingly transmits the second light toward an optical axis. The optical pickup of claim 1, wherein the optical diffraction means (135) is a stepped planar lens in which one or more stepped pattern periods each having a plurality of annular gratings are arranged in the second region (135b). The optical pickup of claim 2, wherein the depths of the gratings become smaller moving away from the optical axis. The optical pickup of claim 2 or 3, wherein the stepped pattern comprises a first stepped pattern formed within a first annular radial range of the stepped planar lens, and a second stepped pattern formed within a second annular radial range. The optical pickup of claim 4, wherein the first stepped pattern is formed within a radial range of about 1000-1150µm; and the second stepped pattern is formed within a radial range of about of 1150-1200µm. The optical pickup of any of claims 1 to 5, wherein the second outer region is immediately adjacent the first inner region. The optical pickup of any of claims 1 to 6, further comprising: an optical path changing means (131) for changing the traveling path of an incident light. The optical pickup of any of claims 2 to 7, wherein the stepped pattern is formed in one side of the stepped planar lens, facing the optical path changing means (131). The optical pickup of claims 8, wherein the optical, diffraction means (135b) is built-in at one side of the objective lens (137'), facing the optical path changing means. The optical pickup of any of claims 2 to 9, wherein one or more units of a stepped pattern having a plurality of annular gratings are arranged in the second outer region, wherein the depths of the gratings in each unit become smaller moving away from the optical axis, and the maximum depth thereof is d . The optical pickup of any of claims 2 to 10, wherein the maximum depth of the grating d is about 6.4µm. The optical pickup of any of claims 1 to 11, comprising a photodetector for detecting an information signal and an error signal from the first and second light which have been reflected by the optical discs and passed through the optical path changing means. The optical pickup of claim 1, further comprising: a first collimator lens (125) disposed on the optical path between the second optical source and the optical path changing means, for pre-condensing and transmitting the incident light; and a second collimator lens (133) disposed on the optical path between the optical path changing means and the objective lens, for condensing and transmitting the incident light, to set the optical focal length between the second optical source and the optical discs, thereby raising the optical efficiency. The optical pickup of any of claims 1 to 13, comprising: a first optical unit (210) including a first optical source (211) for emitting a first light having a wavelength of 650nm, and a first photodetector (219) for receiving the first light; a second optical unit (220) including a second light source (221) for emitting a second light having a wavelength of 780nm, and a second photodetector (227) for receiving the second light; an optical path changing means (231) for changing the paths of the first and second light; and an objective lens (237) for condensing the first and second lights from the optical path changing means (231) to form optical spots onto optical discs having different thicknesses.

Inventors: CHO KUN-HO, CHOI MYOUNG-SOO, KIM JONG-RYULL, KIM KUN-SOO, LEE CHUL-WOO, OH HYUNG-TAEK, SEONG PYONG-YONG, YOO JANG-HOON, YU JONG-HWA, CHO, KUN-HO, CHOI, MYOUNG-SOO, KIM, JONG-RYULL, KIM, KUN-SOO, LEE, CHUL-WOO, OH, HYUNG-TAEK, SEONG, PYONG-YONG, YOO, JANG-HOON, YU, JONG-HWA, Cho, Kun-ho 103-106 Doosan Donga Apt., Choi, Myoung-soo, c/o Samsung Electronics Co Ltd, Kim, Jong-ryull, 201 Hyundai Villa,, Kim, Kun-soo, c/o Samsung Electronics Co Ltd, Oh, Hyung-taek, c/o Samsung Electronics Co Ltd, Yoo, Jang-hoon, 102-307 Daerim Hyundai 1-cha Apt., Yu, Jong-hwa, c/o Samsung Electronics Co Ltd
Applicants: SAMSUNG ELECTRONICS CO LTD, SAMSUNG ELECTRONICS CO., LTD.
Application year: 1999
Main classification: G11B7
IPC codes: G11B7, G02B5