Information recording and/or reproducing apparatus

Abstract: An information recording/reproducing apparatus for recording and/or reproducing information on and/or from an optical recording medium comprises a light source (50), at least two objective lenses (4,5) corresponding to at least two kinds of optical recording media (1,2) having different thicknesses of protective layer, respectively, a holder (6) for supporting these objective lenses (4,5), selection means for selecting one of these objective lenses in accordance with that one of the optical recording media (1,2) to be read, the light of the light source (50) being directed onto that optical recording medium (1,2) through the objective lens (4,5) selected by the selection means, thereby performing recording and/or reproducing of the information.

Description: The present invention relates to an apparatus for recording and/or reproducing information on an optical recording medium, for example a magneto-optical disk drive, a direct read after write drive (DRAW), a phase changing disk drive, CD-ROM or the like. The disks for use in such an apparatus are provided with a protective layer (hereinafter, referred to as a cover glass) such as glass, resin (PC, PMMA or the like) formed on its side surface on which the reading light and the writing light are incident, in order to protect or guard the recording layer of the recording medium. This cover glass has different thicknesses in accordance with the types of the disk, even in the same DRAW disk. For example, the thicknesses of the disks are 1.2 mm for the magneto-optical disk, 1.2 mm and 0.6 mm for the phase changing disk, and 1.2 mm and 0.09 mm for the standard DRAW disk of 356 mm. An objective lens for use in an optical pick-up of the information recording and/or reproducing apparatus has generally a numeral aperture NA of 0.45 - 0.6, and is designed by taking aberration caused by the above cover glass into consideration. On considering allowable level at the reading/writing property of the signal, in the case of the cover glass having 1.2 mm thickness, the limit of the allowable level is in the order of ±0.05. If the allowable level exceeds this limit value, the reading/writing property of the signal becomes considerably deteriorated. If the reading/writing operation of the signal is performed by exchanging disks having cover glass of different thickness, for example a disk having 1.2 mm thickness of cover glass and a disk having 0.6 mm thickness of cover glass, therefore, it is difficult to perform this reading/writing operation of the signal with one information recording and/or reproducing apparatus. That is, two information recording and/or reproducing apparatus must be provided. In order to remove the above defect, for example, Japanese Patent Application Opened No. 241,095/93 provides the feature of inserting parallel plates between a light source and a collimator to compensate for a spherical aberration caused by the difference in thickness of the cover glass of the disk. However, the optical pick-up disclosed in the Japanese Patent Application Opened No. 241,095/93 has the following problem. For example, a spherical aberration component ω 40 of a coefficient of a wave front aberration and a value ν of the wave front aberration, which are caused, for example, in case of passing the light through the cover glass having thickness t can be obtained from "Optics 14" (1985), pp. 219 - 221 and expressed as follows:- ω40 = (t/8) · {(n 2 -1)/n 3 } · NA 4 ν = 0.0745 ω 40 wherein the numeral aperture of the projective lens is NA, and the refractive index is n. Therefore, in a pick-up having NA = 0.55 and n = 1.57 and using light of wavelength λ = 780 nm, if a disk having a cover glass of t = 1.2 mm is exchanged with a disk having a cover glass of t = 0.6 mm, a spherical aberration of ω40 = 0.00260, ν = 0.248 λrms is generated. While in case of compensating the above spherical aberration by arranging the parallel plates for compensation between the light source and the collimator lens, if the numerical aperture at the light source side is NA' = 0.25 and the refractive index is n = 1.57, the thickness t' of the parallel plates is expressed as follows:- t' = {n 3 /(n 2 -1)} · (8/NA' 4 ) · ω 40 = 14.07 mm Moreover, in order to insert such thick parallel plates between the light source and the collimator lens, the light source must be moved by a following distance 1: 1 = t'(1 - 1/n) = 5.11 mm. Thus, the optical pick-up disclosed in the Japanese Patent Application Opened No. 241,095/93 requires a moving mechanism for inserting the compensating parallel plates and a mechanism for moving the light source in accordance with the insertion of the parallel plates, so that the construction of the apparatus becomes complicated and the optical system becomes large. In the reading/writing operation, moreover, the objective lens is moved in dependence upon the eccentricity of the disk by a tracking servo, thereby causing incoincidence between the light axis of the objective lens and the light axis of the collimator lens, resulting in a generation of comma aberration or astigmatism. Accordingly, it is desirable to eliminate the above described disadvantages of the conventional information recording and/or reproducing apparatus. In particular, it is desirable to provide an information recording and/or reproducing apparatus capable of performing information reading/writing operations for plural kinds of optical recording media having cover glasses of different thicknesses by means of the same apparatus. Information recording and/or reproducing apparatus embodying a first aspect of the present invention comprises: two objective lenses corresponding to two kinds of optical recording media having different thicknesses of a protective layer, respectively; a holder capable of being rotated around an axis parallel to an optical axis of the objective lens and capable of being moved along said axis and for holding said two objective lenses; and a drive means for driving the holder in order to displace a corresponding objective lens in accordance with kinds of the optical recording media in an optical path ; characterised in that the two objective lenses are closely juxtaposed on the holder in a tracking direction. JP-06-236570 discloses an information recording/reproducing apparatus comprising: two objective lenses corresponding to two kinds of optical recording media having different thicknesses of a protective layer, respectively; a holder capable of being rotated around an axis parallel to an optical axis of the objective lens and capable of being moved along said axis and for holding said two objective lenses; and a drive means for driving the holder in order to displace a corresponding objective lens in accordance with kinds of the optical recording media in an optical path. However, the lenses are separated by 180° and therefore the holder must be rotated a large distance in order to change lenses. Information recording and/or reproducing apparatus embodying a first aspect of the present invention may further comprise a detecting means for detecting kinds of the optical recording media. The drive means may be a tracking actuator for moving a light spot on the optical recording medium in the tracking direction. The holder preferably has a bearing means capable of being rotated about a supporting axis provided parallel to the optical axis of the objective lens and capable of being moved along the supporting axis, and said two objective lenses are integrally formed and arranged on the same radius with respect to the bearing means of the holder. Information recording and/or reproducing apparatus embodying a first aspect of the present invention may further comprise an objective lens detection means for detecting which objective lens of said two objective lenses is displaced in the optical path. Information recording and/or reproducing apparatus embodying a first aspect of the present invention may further comprise a holding means for holding the position of the objective lens displaced by the drive means. In this case, the construction becomes simple and the apparatus can be downsized and inexpensive. According to the first aspect of the present invention, the reading/writing operation can be performed for the plurality kinds of the optical recording medium in the single apparatus without causing the deterioration of the optical property due to the difference in thickness of the protective layer of the optical recording medium, by selecting the suitable objective lens in accordance with kinds of the optical recording medium due to the thickness of the protective layer. The plural of objective lenses are exchanged and selected in accordance with the kinds of the optical recording medium, so that the distance between the optical recording medium and the objective lens can be optionally selected as compared with the case of inserting the correction optical element between the objective lens and the optical recording medium, the objective lens having small operating distance may be used, and whole construction of the apparatus can be made thin. Reference will now be made, by way of example, to the accompanying drawing, in which: Fig. 1 is a schematic view of a first information recording and/or reproducing apparatus embodying the present invention; Fig. 2 shows a perspective view of the actuator shown in Fig. 1; Fig. 3 shows a plan view of the actuator shown in Fig. 2; Fig. 4 shows a sensor for discriminating the kind of disks for use in the apparatus shown in Fig. 1; Figs. 5 and 6 show a sensor for use in discriminating which lens is in use in the apparatus shown in Fig. 1; and Fig. 7 is an explanatory view showing an output of the lens detecting sensor. In the drawings, like parts are shown by corresponding reference characters throughout. Figs. 1 to 3 show the construction of part of a first information recording and/or reproducing apparatus embodying the present invention. As shown in Fig. 1, this embodiment is adapted to two kinds of disks, that is, a disk 1 with a cover glass (protective layer) having a thickness of t1 (=0.6 mm), and a disk 2 with a cover glass (protective layer) having a thickness of t2 (=1.2 mm). The light from a light source 50 illuminates the disk 1 or 2 after being passed through a collimator 51 to form a parallel luminous flux and reflected by a mirror 18 and then through an actuator 17 comprising an objective lens 3 and its drive means. The return light reflected by the disk 1 or 2 is separated from the return path and is incident on alight detector (not shown) by means of a well known method. In this case, the information can be read/written by performing a positional control such as focusing control and tracking control of the objective lens 3 to the disk and a detection process of the read signal based on the return light. Figs. 2 and 3 are a perspective view and a plan view respectively showing the detailed construction of the actuator 17 shown in Fig. 1. In this embodiment, the actuator 17 is formed by a, so called, shaft sliding method. A base 10 of a magnetizable substance has a shaft 12 studded thereon, and a holder 6 is provided on the shaft 12 rotatably and slidably in the axial direction (in the focusing direction Fo). That is, the shaft 12 is made a rotation center of the holder 6. The holder 6 is also provided with projections 30a, 30b arranged at substantially symmetrical positions to the shaft 12, respectively, the one projection 30a at the side of the mirror 18 being provided with an opening, in which the objective lens 3 is mounted and the other projection 30b being provided with a balancer 19. In this embodiment, the objective lens 3 is formed by two lenses; that is a lens 4 (N.A.=0.6) corresponding to the disk 1 and a lens 5 (N.A.=0.4) corresponding to the disk 2, which are made of plastics integrally. This lens 3 is mounted on the holder 6 in such a manner that the two lenses 4, 5 are juxtaposed in the tracking direction Tr, i.e. these objective lenses 4, 5 lie side by side on the same radius about the rotation center of the holder 6. In this case, the position of the holder 6 is substantially the same in the perpendicular direction to the surface of the optical recording medium 1 or 2, in such a manner that the light spot is in respective focusing conditions for the corresponding disks 1, 2. The holder 6 is provided with two sectorial openings 31a, 31b positioned opposite to each other in the tracking direction about the shaft 12, and inner yokes 32a, 32b provided on the base 10 are arranged in the sectorial openings 31a, 31b. The base 10 is provided with outer yokes 33a, 33b so as to make them opposite to the inner yokes 32a, 32b through the holder 6, and permanent magnets 11a, 11b are mounted respectively inside the outer yokes 33a, 33b in such a manner that the same poles are positioned opposite to each other. The holder 6 is also provided with a focusing coil 8 wound thereon at its lower side and at an outer periphery thereof, and two plane tracking coils 9a, 9b; 9c, 9d are mounted on the focusing coil 8 at portions opposite to the tracking direction in such a manner that one vertical arm of respective tracking coils is opposite to the permanent magnet 11a or 11b. In this way, the magnetic circuit is constructed to effect the magnetic flux on the focusing coil 8 and two corresponding tracking coils 9a, 9b ; 9c, 9d by respective permanent magnets 11a, 11b. Moreover, the holder 6 is provided, on the projection 30a having the objective lens 3 mounted thereon, with stoppers 34a, 34b for limiting the excessive rotational movement of the holder 6 by abutting them against the outer yokes 33a, 33b, and with a piece of magnetizable substance 7 on the projection 30b, on which the balancer 19 is provided. Moreover, the base 10 is provided with a permanent magnet 13 magnetized in the tracking direction Tr so as to oppose the magnetizable substance 7 provided on the holder 6. This permanent magnet 13 is provided with yokes 14,15 at its respective poles. In the above construction, when the reading/writing operation is performed for the disk 1, pulse-shaped DC currents having a predetermined magnitude in the first direction are applied to the tracking coils 9a, 9b ; 9c, 9d to rotate the holder 6 in the direction of A in Fig. 3, and as the holder 6 rotates, the magnetizable piece 7 provided on the holder 6 is magnetically drawn by the permanent magnet 13 through the yoke 15, thereby placing the lens 4 over the mirror 18. In this case, the stopper 34b of the holder 6 is abutted to the outer yoke 33b to limit excessive rotating movement of the holder 6. Under such conditions, the tracking error signals are supplied to the tracking coils 9a, 9b ; 9c, 9d, thereby performing the tracking control by slightly rotating the holder 6 about the shaft 12, and the focusing error signals are supplied to the focusing coil 8, thereby performing the focusing control by sliding the holder 6 along the shaft 12, resulting in a reading/writing operation of the disk 1. In case of performing a reading/writing operation for the disk 2 from the conditions shown in Fig. 3, pulse-shaped DC currents having a predetermined magnitude in the second direction opposite to the first direction are applied to the tracking coils 9a, 9b ; 9c, 9d to rotate the holder 6 in the direction of B in Fig. 3 against the magnetic attraction force of the magnetizable piece 7 due to the yoke 15, and as the holder 6 rotates, the magnetizable piece 7 is magnetically sucked by the permanent magnet 13 through the yoke 14, thereby placing the lens 5 over the mirror 18. In this case, the stopper 34a of the holder 6 is abutted to the outer yoke 33a to limit excessive rotating movement of the holder 6. Under such conditions, as in the case with the above described case, the tracking error signals are supplied to the tracking coils 9a, 9b ; 9c, 9d, thereby performing the tracking control by slightly rotating the holder 6 about the shaft 12, and the focusing error signals are supplied to the focusing coil 8, thereby performing the focusing control by sliding the holder 6 along the shaft 12, resulting in a reading/writing operation of the disk 2. The disks 1, 2 are discriminated by providing discriminating sensor holes to a cartridge for accommodating respective disks, and by detecting the presence of the sensor hole with a switch or the like, thereby selecting the corresponding lens as described above in accordance with the discriminated result, resulting in a reading/writing operation of the disk. In the case of changing the holder 6 from the reading/writing condition for the disk 2 to the reading/writing condition for the disk 1, the holder 6 is rotated in the direction of A in Fig. 3 against the magnetic attraction force of the magnetizable piece 7 due to the yoke 14 by means of the pulse-shaped DC currents having a predetermined magnitude in the first direction applied to the tracking coils 9a, 9b ; 9c, 9d, thereby placing the lens 4 on the mirror 18, and then the rotating movement of the holder 6 is held by the magnetic attraction force of the magnetizable piece 7 due to the yoke 15. In addition to the discrimination of the disk by the cartridge explained above, a sensor shown in Fig. 4 may be used for such a discrimination. This sensor is secured to a carriage (not shown) for holding the actuator 17. This sensor comprises a light emitting diode LED placed at its center portion and two photodetectors provided on both sides of the light emitting diode LED in the tracking direction. The light receiving surface of each photodetector is divided into two, so that one photodetector has two photodetector portions PD1 and PD2 and the other photodetector has two photodetector portions PD3 and PD4. In the case of the disk 1, on which the cover glass of the disk is thin and a recording and/or reproducing surface is near the sensor, a large amount of the reflected light of the light emitting diode LED is incident on the photodetectors PD2 and PD3 which are positioned near the light emitting diode LED. On the contrary, in the case of the disk 2, on which the cover glass of the disk is thick and a recording and/or reproducing surface is far the sensor, a large amount of the reflected light of the light emitting diode LED is incident on the photodetectors PD1 and PD4 which are positioned far the light emitting diode LED. Therefore, signals for discriminating the thickness of the cover glass and the kind of the disk can be obtained by operating the output signals of the photodetectors arithmetically, such as [PD2 + PD3] - [PD4 + PD1]. Moreover, this sensor is used to perform the following arithmetic operation: {PD2 + PD4} - {PD1 + PD3} so that the slant signal of the disk can be obtained. This signal adjusts the slant of the actuator for the disk, so that a preferred signal having no cross talk and jitter can be obtained. In this way, the disk discrimination sensor and the disk slant sensor are commonly constituted by the same sensor, so that the number of the sensors can be decreased. In order to discriminate the kinds of disk, firstly, either one of the lenses is utilized to read out the information recorded on the control track of the disk, or the bar code shaped information provided on the innermost or outermost periphery portion of the disk, thereby discriminating the kinds of disk with the read result. Therefore, the lenses may be selected by the use of this discriminated result. In this case, it is preferable that firstly, the lens 5 having small NA (numeral aperture) is used to read out the information, since if the lens having small NA is used, the deterioration of the spherical aberration becomes small in the case of varying the thickness of the cover glass, so that the signal on the disk can easily be read out. In this way, in the case of the information recording and/or reproducing apparatus selecting the lens by reading out the information of the disk, under the condition of non-loaded disk, and the switched-on condition, it is preferable to select one of the lens, preferably, the lens having small NA. A sensor 101 for determining which lens is over the mirror 18 is shown in Figs. 5, 6 and 7. In this embodiment, the projection 30a of the holder 6 is provided, at its tip portion, with reflection members 100a, 100b at the places corresponding to respective lenses 4, 5. The surface of the reflection member is made as a mirror finished surface. The holder 6 is formed by a plastics material such as polycarbonate (PC), liquid crystal polymer (LCP) or the like and colored white so as to have a high reflection factor. The surface of the holder 6 around the reflecting members 100a, 100b is made as a coarse surface to prevent unreasonable reflection of light from these areas. The sensor 101 for discriminating the lens comprises a light emitting diode (LED) 102 positioned at the center of the sensor and first photodetector (PD1) 103a and second photodetector (PD2) 103b, which are positioned on either side of the LED. In operation, the light emanating from the light emitting diode 102 is reflected by the reflection member 100a and is incident on the photodetectors 103a, 103b, and then the outputs of the photodetectors 103a, 103b are operated arithmetically by a differential amplifier 104 to obtain its differential output [PD1 - PD2]. In the condition that the lens 4 is selected, as shown in Fig. 5, the light emanating from the light emitting diode 102 is reflected on the reflecting member 100a and incident on the photodetectors 103a, 103b. In this case, the reflecting member 100a is slanted towards the sensor 101 as shown in Fig. 5, and then the light incident on the photodiode 103b is shifted outside thereof, so that the amount of light incident on the photodetector 103b is less than that of the photo-detector 103a. Therefore, as shown in Fig. 7, the output of the differential amplifier 104 becomes positive {[PD1 - PD2] > 0}. On the contrary, as shown in Fig. 6, under the condition that the lens 5 is selected the reflecting member 100b is slanted towards the sensor 101 in the direction opposite to that shown in Fig. 5, the output of the differential amplifier 104 becomes negative {[PD1 - PD2] < 0}, (as shown in Fig. 7). That is, it can be discriminated whether or not any one of the lenses 4, 5 is selected and used in accordance with positive or negative of DC component of the output from the differential amplifier [PD1 - PD2]. Therefore, the possibility of recording and/or reproducing information onto and/or from the disk by the erroneous or undesirable lens can be avoided. Moreover, the positional signal in the tracking direction signal of the lenses 4, 5 can also be obtained by the signal caused by decreasing the offset components A or B shown in Fig. 7 from the output of the sensor 101, [PD1-PD2]. Alternatively, the speed signal in the tracking direction of the lenses 4, 5 can be obtained by the signal caused by differentiating the output of the sensor 101, [PD1-PD2]. Therefore, these signals can be used as a driving current of the tracking coil and as a coarse access drive of the actuator 17 (not shown). That is, the positional or speed sensor for the lens can be co-used with the detection or discriminating sensor for the lens, so that the number of the sensors can be decreased. In this way, according to the present embodiment, the appropriate one of the two lenses 4, 5 is chosen in accordance with the disks 1, 2, that is, the thickness of the cover glass on the disks, so that the optical performance, particularly, the spherical aberration due to the thickness of the cover glass as in the case of inserting the aberration compensating optical element, does not deteriorate, and then the optical element for aberration correction need not be inserted between the lenses 4,5 and the disks 1,2, respectively, so that the differential distance of the lenses 4,5 can be made small and thus the apparatus as a whole can be reduced in size. Moreover, the exchange of the lenses 4,5 is performed by the actuator 17 for performing the tracking control without providing a separate exchanging means, so that the construction can be made simple, small and inexpensive. In the conditions, such as the reading/writing condition for the disk 1 and the reading/writing condition for the disk 2, also, an operation neutral position of the holder 6 in the tracking direction Tr thereof is magnetically held by the magnetizable piece 7, the permanent magnet 13 and the yokes 14, 15, so that the tracking control can be made stably. As the lens 3, moreover, juxtaposed lenses 4, 5 are integrally formed by a plastic molding, so that the distance between the two lenses 4, 5 can be made very small. The rotating angle of the holder 6 in case of exchanging the lenses can also be made small, so that the exchanging control can be made small. The lenses 4, 5 are mounted on the holder 6 in such a manner that light spots for the corresponding disks 1, 2 are positioned in focusing conditions, respectively, at the same positions of the focusing direction of the holder 6, so that the moving amount of the holder 6 in the focusing direction can be made most small. In the first embodiment, moreover, the actuator 17 is of a shaft sliding system, but other systems, for example, a system of supporting the holder for holding the objective lens by four wires can effectively be applied to the present invention. The operation neutral position of the holder 6 in the tracking direction Tr thereof can be selected by flowing the bias current through only a tracking coil in the desired direction, without using the magnetizable piece 7 and the permanent magnet 13. Alternatively, when the holder 6 is provided with a reflecting member or the like, thereby forming a reflective sensor, it is also possible to detect which of the lenses 4, 5 is selected.

Claims: An information recording/reproducing apparatus comprising: two objective lenses (4, 5) corresponding to two kinds of optical recording media (1, 2) having different thicknesses of a protective layer, respectively; a holder (6) capable of being rotated around an axis parallel to an optical axis of the objective lens (4, 5) and capable of being moved along said axis and for holding said two objective lenses (4, 5); and a drive means (17) for driving the holder (6) in order to displace a corresponding objective lens (4, 5) in accordance with kinds of the optical recording media in an optical path; characterised in that the said two objective lenses (4, 5) are closely juxtaposed on the holder (6) in a tracking direction An information recording/reproducing apparatus as claimed in claim 1, further comprising a detecting means for detecting kinds of the optical recording media (1, 2). An information recording/reproducing apparatus as claimed in claim 1 or 2, wherein the drive means (17) is a tracking actuator for moving a light spot on the optical recording medium (1, 2) in the tracking direction. An information recording/reproducing apparatus as claimed in claim 1, 2 or 3, wherein the holder (6) has a bearing means (12) capable of being rotated about a supporting axis provided parallel to the optical axis of the objective lens (4, 5) and capable of being moved along the supporting axis, and said two objective lenses (4, 5) are integrally formed and are arranged on the same radius with respect to the bearing means (12) of the holder (6). An information recording/reproducing apparatus as claimed in any preceding claim, further comprising an objective lens detection means (101) for detecting which objective lens (4, 5) of said two objective lenses (4, 5) is displaced in the optical path. An information recording/reproducing apparatus as claimed in any preceding claim, further comprising a holding means (7, 13) for holding the position of the objective lens (4, 5) displaced by the drive means (17).

Inventors: IKEGAME TETSUO, NAKANO OSAMU, IKEGAME, TETSUO, NAKANO, OSAMU
Applicants: OLYMPUS OPTICAL CO, OLYMPUS OPTICAL CO., LTD.
Application year: 1995
Main classification: G11B7
IPC codes: G11B7, G11B19