OPTICAL PICKUP AND OPTICAL RECORDING APPARATUS

Abstract: An optical pickup is capable of projecting laser beams having different polarization directions or different divergence angles from laser sources formed in a surface emitting laser array and varying the effective numerical aperture of an optical system provided with an objective lens and the like. The optical pickup has a high-degree compatibility with both a CD-standards optical disk having a low recording density and a large thickness and a DVD-standards optical disk having a high recording density and a small thickness. Furthermore, a plurality of laser sources can be formed in a compact space by adopting the surface emitting laser array, and the numerical aperture of the optical system can be controlled by making a selection of laser beams. Accordingly, it is possible to realize an optical pickup having compatibility by means of an extremely simple construction, and it is also possible to inexpensively provide a small-size high-performance optical pickup.

Description: The present invention relates to an optical pick-up and an optical recording system for performing a process such as a recording process to or a reproducing process from an optical recording medium such as an optical disk. As an optical disk which is recently available, a compact disk (CD) based on the compact disk standards (CD standards) is known. The substrate of the compact disk has a thickness of 1.2 mm, and a track pitch formed on the surface of the substrate to record information is 1.6 µm. In order to reproduce the CD, an optical pick-up comprising a laser diode for emitting a laser beam having a wavelength of 0.78 µm and an objective lens having a numerical aperture of 0.45 is used. In recent years, various developments have been made to increase the recording density of the optical disk,. and it has been examined in the developments to increase the numerical aperture of the objective lens. When the numerical aperture of the objective lens is increased, the optical resolution is improved. For this reason, the linear recording density on the optical disk can be increased, and the pitch of recording tracks can be decreased. Therefore, the recording density can be considerably increased. However, when the numerical aperture of the objective lens is increased, a phenomenon that the aberration of a light spot focused by the objective lens increases occurs. More specifically, due to axial runout of the optical disk and axial runout of a turntable which has the optical disk attached thereto and is rotated, inclination (so-called tilt) of the optical disk and the objective lens occurs, and coma aberration occurs in this light spot due to this tilt. For this reason, by only increasing the numerical aperture, focusing performance is not improved, and an optical disk having a high recording density cannot be realized. For this reason, it is considered that decreasing the substrate of the optical disk in thickness, allows the numerical aperture of the objective lens to be increased while preventing degradation of aberration (coma aberration) caused by tilt. On the basis of this consideration, standardization of the high-density optical disk advances. One of the standards is the standard of a digital video disk (DVD). In this DVD standard, the thickness of the optical disk is set to 0.6 mm and the track pitch is set to 0.74 µm. With this DVD, a recording or reproducing operation is performed by using an optical pick-up for irradiating a laser beam having a wavelength of 0.65 µm or 0.635 µm through an objective lens having a numerical aperture of 0.6. When the optical disk having a thin substrate based on this standard is used, the recording density can be increased. However, since the objective lens of the DVD optical pick-up is suitably designed for a thin optical disk, a spherical aberration increases when the objective lens is applied to an optical disk having a conventional thick substrate, thereby degrading imaging performance. For this reason, the DVD optical pick-up cannot perform a recording/reproducing operation to a CD. Further development has been made to prevent the above disadvantages. It is considered that a laser beam emitted from the optical pick-up is split into two focusing points to be converged, thereby coping with optical disks having different thickness. For example, JP-A-7-65407 describes the following technique. That is, a light flux emitted from a laser diode is split by adding a prism or a hologram to an optical system, and the light fluxes are converged through different optical lengths, respectively, thereby forming two beam spots. In JP-A-7-98431, a composite objective lens is formed by combining a deflective objective lens and a hologram lens to form two beam spots. However, in the optical pick-up described above, in order to make it possible to process optical disks having different thickness, one laser beam is split into two beam spots to be converged. For this reason, the energies of the beams converged to the respective beam spots decrease. Therefore, since the intensity of an reflected beam reflected from the optical disk decreases, the sensitivity of the light detector must be improved, or a laser diode having a large laser output power must be employed. For this reason, the optical pick-up increases in size and power consumption, and the manufacturing cost of the optical pick-up also increases. Although an optical disk having a high recording density requires a focus servo or tracking servo having higher precision, a large optical pick-up requires high power to remove a focusing error or a tracking error, and an actuator for performing these servo operations increases in size. In this manner, the optical pick-up having two focusing points and a large size requires high power consumption. For this reason, an optical recording system employing the optical pick-up is also large and expensive, and the power consumption of the optical recording system more increases. Since the conventional optical pick-up in which two focusing points are always formed by the objective lens always has two beam spots converged by the objective lens, for example, when a focus drawing operation is performed with a thick optical disk, the focus drawing operation may be erroneously performed at a beam spot for reproducing a thin optical disk. Therefore, a mechanism or control for preventing a focus drawing operation from being performed at the beam spot for reproducing the thin optical disk must be additionally arranged. Furthermore, when a thick optical disk is to be reproduced, there is a beam spot for reproducing a thin optical disk, this beam spot is diffused because the beam spot is not focused on the recording surface of the optical disk, and the beam spots may be finally incident, as stray rays, on photo detectors for signal detection to be noise. For these reasons, a focus drawing mechanism is complicated to cause the optical pick-up to increase in size and cost, and measures against an erroneous operation or stray rays must be made. These measures cause another factor that increases the optical pick-up in size and cost. By utilizing conventional techniques, laser beam sources and optical elements which are respectively suitable for an optical disk having a low recording density and an optical disk having a high recording density can be independently arranged, as a matter of course. However, such an optical pick-up requires a large number of independent laser beam sources and optical elements, and the optical pick-up increases even more in size and cost. An optical pick-up according to the pre-characterizing portion of claim 1 is disclosed in JP-A-06-259804. The two types of laser beam differ in their radiation angles thereby achieving different values of the effective numerical aperture of the optical element. JP-A-07-192291 discloses that both a laser diode used as light source of a pick-up head and a photo diode used for light detection may be formed on the same substrate by using a vertical cavity surface emitting laser structure. EP-A-0 742 554 (prior art according to Article 54(3) EPC) discloses an optical disc readout apparatus having an optical system which guides a laser beam onto a signal recording surface of a loaded disc through an objective and guides a reflected laser beam onto a photo detector. The effective numerical aperture of the objective is adjusted according to the distance between a substrate surface of the loaded disc and the signal recording surface, and the laser spot with a diameter suitable to a recording density of the loaded disc is focused on the signal recording surface thereof. Therefore, the data recorded on a plurality of optical discs with different thickness can be read out. The numerical aperture is controlled by means of a polarizing filter causing two different laser beams of S and p polarized light, respectively, to have different beam diameters. As an optical pick-up which is reduced in size, an optical pick-up using a surface emitting laser array as light sources is disclosed in JP-A-7-192291. By using the advantage that a surface emitting laser can be easily formed on a two-dimensional array, light sources in which five surface emitting lasers, i.e., a surface emitting laser for detecting an information signal, two surface emitting lasers for detecting a focusing error signal, and two surface emitting lasers for detecting a tracking error signal are formed on a single substrate in the form of an array is used. However, this optical pick-up is used only to obtain a small size, but is not used to reproduce optical disks having different substrate thickness or different track pitches. Therefore, it is an object of the present Invention to provide a compact optical pick-up which can be easily controlled and in which beam spots are appropriately formed on optical disks having a high recording density and a low recording density, respectively, and being different in thickness without splitting a laser beam into two focusing points to be converged. It is another object of the present invention to provide an optical pick-up which can stably perform a recording/reproducing process with respect to optical disks having different thickness, has a small size, a simple arrangement, and low power consumption, and can be supplied at low cost. These objects are achieved by an optical pick-up as claimed in claim 1. Preferred embodiments of the invention are subject-matter of the dependent claims. For this reason, in an optical pick-up according to the present invention comprising a light source for emitting laser beams, an optical element for converging laser beams on an optical recording medium, and light detectors for detecting reflected beams reflected from the optical recording medium, surface emitting lasers which can be decreased in size by being arranged in the form of an array are employed as the light source, and the surface emitting lasers constitute laser beam sources which can emit at least two types of laser beams which can change effective numerical apertures in the optical element. There is provided an optical pick-up which can appropriately form beam spots on optical disks having a high recording density and a low recording density, respectively, and being different in thickness without splitting the respective laser beams into two focusing points to be converged and has a small size, and which can be easily controlled. As means for controlling the numerical aperture of an optical element, an optical pick-up which switchably uses first or second laser beams having different radiation angles and which are emitted from first or second laser beam sources respectively will be described below. An optical pick-up in which the effective numerical aperture of an objective lens can be changed by the radiation angles of laser beams emitted from laser beam sources and which can perform a recording/reproducing process with optical recording media having different thickness can be realized. More specifically, a laser light source for emitting a laser beam having a large radiation angle is selected to perform a process with a thin optical recording medium having a high recording density, so that the effective numerical aperture of the objective lens can be increased. Therefore, the resolution of a laser beam irradiated on the optical recording medium can be Improved, and a recording/reproducing process can be performed at a high recording density. On the other hand, a laser light source for emitting a laser beam having a small radiation angle is selected for a thick optical recording medium having a low recording density, so that the effective numerical aperture of the objective lens according to the present invention can be decreased, and a beam spot can be converged to almost diffraction limit on the thick optical recording medium having a low recording density. Therefore, a process such as a recording/reproducing process can be reliably performed. When the numerical aperture of the optical element is limited by a radiation angle, an aperture reduction rate is preferably set to about 50 to 64%. In addition, when a manufacturing allowance or assembling allowance in manufacturing of the optical pick-up is considered, the aperture reduction rate is more preferably set to about 56 to 60%. In the optical pick-up according to the present invention, when laser beam sources having different radiation angles formed on a surface emitting laser array in which a plurality of laser beam sources can be formed at low cost are switchably used, a process can be reliably performed with optical recording media having different thickness. In addition, since the effective numerical aperture of an objective lens is controlled by a radiation angle, an optical element such as a prism, a hologram, or a hologram lens for controlling the numerical aperture of the objective lens is not required. Therefore, an inexpensive optical pick-up which has a smaller size and can perform a process with optical recording media having different thickness can be realized. In addition, since an optical element need not be newly added, an Inexpensive optical system which has been used for an optical recording medium and can obtain sufficient performance can be employed. The number of optical parts and other parts constituting the optical pick-up decreases, and simple optical adjustment can be performed. Therefore, with respect to this point, an inexpensive and highly reliable optical pick-up which can reliably perform a process such as a recording/reproducing process can be realized. A plurality of laser beam sources can be easily formed on the surface emitting laser array at low cost and high precision by a photolithographic technique, and the laser beam sources can be monolithically formed together with other circuit elements, photo diodes for detecting light, or photo transistors. For this reason, a compact and inexpensive optical pick-up can also be realized. As laser beam sources having different radiation angles, like the above arrangement, vertical cavity surface emitting lasers in which a pair of reflective layers, a clad layer, an active layer, and a pair of metal layers are at least stacked in the emitting direction of laser beams to form an optical resonator can be employed, and a large number of laser beam sources can be arranged in a small area. The radiation angle can be controlled by forming micro lenses having different refractive powers on the reflective layer on the side on which the laser beam is emitted. Furthermore, since a plurality of laser beam sources can be easily formed on the surface emitting laser array, when a first laser beam source group in which at least three laser beam sources for radiating first laser beams are aligned and a second laser beam source group in which at least three laser beam sources for radiating second laser beams are aligned are formed, by using the three-beams method which can perform tracking servo at high precision with a simple arrangement, a process can be performed with optical recording media having different recording densities, i.e., different track pitches. Since a thin optical recording medium having a high recording density has a small track pitch, a laser beam source group in which laser beam sources are arranged at a second pitch L2 corresponding to the small track pitch can be used. Since a thick optical recording medium having a low recording density has a large track pitch, a laser beam source group in which laser beam sources are arranged at a first pitch L1 being different from the second pitch L2 and corresponding to the large track pitch can be used. By using a surface emitting laser array in which a plurality of laser beam sources can be formed by a photolithographic technique, the first and second laser beam source groups can be easily formed at low cost. For this reason, a grating for generating three beams is not required, and an optical pick-up having a simple arrangement and high precision can be realized. In addition, when a beam is split by a grating, gratings must be arranged on optical disks having different thickness, respectively. However, since this arrangement cannot be realized, a different method of tracking error detection must be applied to one optical disk, and an optical pick-up Is complicated and expensive. Although the method of tracking error detection other than the three-beams method can be naturally applied to both optical disks, a signal detection element and a processing circuit tend to be more complex than those used in the three-beams method. When these first and second laser beam source groups are formed parallel to each other, the three-beams method can be applied to both optical disks having different thickness, and photo detectors for detecting light can be used with both optical disks. On the other hand, the first and second laser beam sources are arranged at different pitches in front of and behind a laser beam source which can control a polarization direction, so that the first and second laser beam source groups can be arranged in the same row. In order to set an interval between convergent spots of a main beam and a sub-beam in the three-beams method on the recording layer of an optical disk to an appropriate value of about 15 µm or less and keep the pitch of the laser beam sources at about 30 µm or more which is a minimum pitch at which the laser beam sources can be formed on the surface emitting laser array, a magnification m of the optical element is preferably set within the following range: m x (L2/L1) ≥ 2 where, L2 < L1. In consideration of facility for manufacturing a surface emitting laser array, the magnification m is more preferably set within the following range: m x (L2/L1) ≥ 8/3 Furthermore, when a diffraction unit for diffracting reflected beams from an optical disk into ±1-order lights to converge the ±1-order lights on light detectors which are arranged around light sources is arranged in the optical element, the laser beam sources and the light detectors can be formed on the same substrate, and the surface emitting laser array can be integrated together with the light detectors. For this reason, an optical pick-up in which photo detectors are Integrated with a light source can be realized. Therefore, several steps such as a position adjusting step required to assemble an optical pick-up can be omitted, and a compact and high-performance optical pick-up can be provided at low cost. Preferred embodiments of the invention will be described in detail below with reference to the drawings. FIG. 1 is a schematic view showing the arrangement of an optical pick-up . FIG. 2 is an enlarged view showing a polarizer used in the optical pick-up in FIG. 1 when viewed from an optical axis direction. FIGS. 3 are graphs showing a change in diameter of a convergent spot formed by an objective lens on the recording surface of a CD (FIG. 3(a)), a change in wave aberration of the convergent spot (FIG. 3(b)), and a result obtained by measuring a jitter value of a 3T signal corresponding to a shortest pit (FIG. 3(c)), respectively, when the CD is reproduced with changing the diameter of the transparent portion of the polarizer to change an effective numerical aperture. FIG. 4 is a view showing a convergent state of a reflected beam in an in-focus state of the optical pick-up shown in FIG. 7. FIG. 5 is a view showing a convergent state of a reflected beam in a state wherein the optical pick-up shown in FIG. 7 is close to an optical disk. FIG. 6 is a view showing a convergent state of a reflected beam in a state wherein the optical pick-up shown in FIG. 7 is far from an optical disk. FIG. 7 is a schematic view showing the arrangement of an optical pick-up according to an embodiment of the present invention. FIG. 8 is a view showing the arrangement of the surface of the integrated device in the optical pick-up shown in FIG. 7. FIG. 9 is a view showing an arrangement of a vertical cavity surface emitting laser used in the optical pick-up shown in FIG. 7. FIG. 10 is a plan view showing the vertical cavity surface emitting laser shown in FIG. 9 when viewed from the emitting direction of the laser beam. FIG. 11 is a view showing a state wherein a micro lens is arranged on the vertical cavity surface emitting laser shown in FIG. 9. FIG. 1 shows an example of an optical pick-up 2 which can perform a recording/reproducing process with a thick optical disk 1a having a low recording density according to the CD standard and a thin optical disk 1b having a high recording density according to the DVD standard, and the arrangement of an optical recording system 17 in which the optical pick-up 2 is employed. The optical pick-up 2 according to this example comprises an integrated device 10 in which a surface emitting laser 3 serving as a light source and a photo detector array for signal detection 4 using photo diodes serving as light detectors are monolithically formed on a single substrate (e.g., n-type GaAs substrate). A laser beam 7 emitted from the surface emitting laser 3 is converged on the recording layer of an optical disk 1 through a hologram 6, a polarization dependent optical element (polarizer) 9, and an objective lens 5 which constitute an optical element 15 of the optical pick-up 2 according to this example, and a reflected beam 8 from the recording layer passes through the optical element 15 again to be converged on the photo detectors. The laser beam 7 emitted with a polarization direction determined by a laser controller 11 passes through the hologram 6, and is converged on the recording layer of the optical disk 1 by the objective lens 5. The reflected beam 8 reflected from the recording layer of the optical disk 1 is incident on the objective lens 5 through the same optical path as described above to be diffracted by the hologram 6, and the ±1-order beams of the diffracted reflected beam are converged on the photo detector array 4 of the Integrated device 10 such that an conspicuous astigmatic aberration is generated. Signals obtained by the photo detector array 4 undergo an arithmetic operation by an arithmetic operation circuit 12. On the basis of the arithmetic result, the optical pick-up 2 is positionally controlled by an actuator using electromagnetic force such that a tracking error or a focusing error is minimized. In the integrated device 10, the surface emitting laser 3 serving as a laser beam source LD for emitting a laser beam is arranged at the center or almost the center, and photo detector arrays 4a and 4b arranged in two rows are formed on both sides of the surface emitting laser 3. Electrode pairs 32a and 32b are connected to the laser beam source LD. As will be described later, when the electrodes 32a and 32b are switched by the laser controller 11, the polarization direction of the laser beam 7 emitted from the laser beam source LD can be controlled. Here, in general, a pit recorded on an optical disk has a long and narrow shape along a track direction. When the pit width is shorter than the wavelength, linearly polarized light perpendicular to a pit length direction, i.e., perpendicular to the track direction (referred to as TM-polarized light) has a degree of modulation of a reproduced signal larger than that of linearly polarized light perpendicular to the TM-polarized light (referred to as TE-polarized light). Therefore, TM-polarized light is preferably used to perform a recording/reproducing process with the optical disk 1b having a high recording density, and TE-polarized light is preferably used for the optical disk 1a having large recording pits. For this reason, in the following description, the electrodes 32a are selected to perform a recording/reproducing process with the thick optical disk is having a low recording density according to the CD standard, and a TE-polarized laser beam 7a mainly having a TE-polarized light component is emitted from the laser beam source LD. When the electrodes 32b are selected to perform a recording/reproducing process with the optical disk 1b having a high recording density and a thin substrate according to the DVD standard, a TM-polarized laser beam 7b mainly having a TM-polarized light component is emitted from the laser beam source LD. As a matter of course, laser beams having the polarized light components opposite to the above light components can also be selected for the CD standard and the DVD standard. In the optical pick-up 2 of this example, the polarizer 9 is arranged on the optical path between the laser beam source LD and the objective lens 5. In this polarizer 9, as shown in the enlarged view in FIG. 2, a central portion 9a is a circular transparent portion (i.e., a portion from which a polarization separation film is removed) having high transmittance with respect to both the TE-polarized light and the TM-polarized light, and the peripheral portion 9b serves as polarization separation film in which the transmittance with respect to TE-polarized light is almost zero. Such a polarization dependent optical element can be constituted by using, e.g., an absorption type organic polarization filter. This filter is formed such that a polymer film containing an organic dye is extended In order to arrange the molecules of the organic dye in the same direction. A polarization dependent optical element can also be constituted by a polarization filter obtained by stacking films having birefringence, a polarization filter obtained by dispersing metal needles on a transparent substrate, a birefringent grating polarizer, or the like. In the optical pick-up 2 of this example, the TE-polarized laser beam 7a is emitted from the laser beam source LD onto the thick optical disk 1a according to the CD standard. In this case, a portion of TE-polarized laser beam 7a corresponding to the aperture 9a at the center of the polarizer 9 is transmitted, while a portion of the TE-polarized laser beam 7a corresponding to the peripheral portion 9b does not reach the objective lens 5. Therefore, the TE-polarized laser beam 7a decreases the effective numerical aperture NA of the objective lens 5, and the effective numerical aperture NA is set to about 0.35 in this example. On the other hand, the TM-polarized laser beam 7b is emitted from the laser beam source LD onto the thin optical disk 1b according to the DVD standard. Since the TM-polarized laser beam 7b almost entirely transmits through the polarizer 9, the numerical aperture NA of the objective lens 5 is equal to its design value. The objective lens of this example is optimally designed for the thin optical disk 1b according to the DVD standard, and the objective lens is designed such that the effective numerical aperture NA is set to about 0.8 with respect to the laser beam 7 having a wavelength of about 650 nm. In the integrated device 10, the photo detector arrays 4a and 4b are arranged in two rows on both sides of the laser beam source LD to detect the reflected beam 8. Four strip-shaped photo diodes 13a, 13b, 13g, and 13h and four strip-shaped photo diodes 13e, 13f, 13c, and 13d are formed on the photo detector arrays 4a and 4b, respectively, to obtain, from the reflected beam 8, a focusing error signal for performing focusing servo, a tracking error signal for performing tracking servo, and data signal for obtaining information recorded on an optical disk. The ±1-order lights of the reflected beam 8 divided in the tangential direction of the track of the optical disk 1 are converged on these photo diodes 13a to 13h to generate conspicuous astigmatic aberration by hologram 6. Therefore, the optical intensities of the beams are converted into signal strengths by the photo diodes 13a to 13h, and the signals are calculated by the arithmetic operation circuit 12 to be output. The arithmetic results of the arithmetic operation circuit 12 are transmitted to the position controller 16, and the optical pick-up 2 is positionally controlled such that a tracking error and a focusing error are minimum. For example, a focusing error signal FES Is obtained by arithmetic operation on the signal strengths obtained by the photo diodes 13a to 13h in the following manner: {(13a + 13b) + (13c + 13d)} - {(13g + 13h) + (13e + 13f)} On the other hand, a tracking error signal TES can be calculated by a push-pull method using the difference between the optical strengths of the reflected beams in the areas separated in the tangential direction. The tracking error signal TES is obtained by arithmetic operation in the following manner: {(13a + 13b) + (13e + 13f)} - {(13c + 13d) + (13g + 13h)} The tracking error signal TES can be also calculated by a differential phase tracking method using a signal strength representing the difference between light intensities of diagonal areas of the areas divided by 4 in the tangential direction and the radial direction with respect to the track of the optical disk. The tracking error signal TES Is obtained by arithmetic operation in the following manner {(13b + 13c) + (13e + 13h)} - {(13a + 13d) + (13f + 13g)} According to the push-pull method or the differential phase tracking method, a tracking error signal can be obtained regardless of the track pitch of the optical disk. Therefore, the optical pick-up 2 having compatibility with the optical disk 1b having a high recording density and the optical disk 1a having a low recording density can be realized. In addition, a data signal RFS can be obtained by the total sum of the photo diodes 13a to 13h. The data signal RFS is obtained by arithmetic operation in the following manner (13a + 13b + 13c + 13d + 13e + 13f + 13g + 13h) In the optical pick-up 2 according to the example described above, when the optical disk 1b having a high recording density and a thin substrate is set, the TM-polarized laser beam 7b Is emitted from the laser beam source LD of the surface emitting laser 3 and converged on the recording layer of the optical disk 1b by the objective lens 5. The objective lens 5 of this example is optimally designed such that light can be converged to diffraction limit on the recording layer of the thin optical disk 1b when the numerical aperture NA is 0.6. Therefore, by using the TM-polarized laser beam 7b, a laser beam emitted from the laser beam source LD almost entirely transmits through the polarizer 9 to be converged on the optical disk 1b with high focusing performance. Therefore, since the reflected beam 8 having high resolution can be obtained, the optical disk 1b according to the DVD standard can be stably positionally controlled at high precision, and a recording/reproducing process can be reliably performed. On the other hand, when a process is performed with the thick optical disk 1a having a low recording density by using the optical pick-up 2 of this example, the TE-polarized laser beam 7a is emitted from the laser beam source LD of the surface emitting laser 3. The laser beam 7a is stopped down by the aperture 9a of the central portion of the polarizer 9 to be incident on the objective lens 5. Therefore, for the objective lens 5 of this example, the optical system of an optical pick-up is designed such that the effective numerical aperture NA is set to 0.35 with respect to the TE-polarized laser beam 7a. Since the objective lens 5 is optimally designed for a thin optical disk having a substrate thickness of 0.6 mm and a high recording density, when the objective lens 5 Is used for a thick optical disk having a substrate thickness of 1.2 mm and a low recording density, a spherical aberration is generated, and imaging performance is degraded. However, in the optical pick-up 2 of this example, an effective numerical aperture NA Is set to a small value of 0.35 with respect to the thick optical disk 1a having a low recording density. A spherical aberration generated by the difference between substrate thicknesses is in proportion to the 4th power of the numerical aperture NA. Therefore, when the numerical aperture is decreased, a spherical aberration sharply decreases even if the substrate thickness are different from each other. For this reason, the optical pick-up 2 of this example can form a convergent spot obtained by converging the TE-polarized laser beam 7a to almost diffraction limit on the thick optical disk 1a having a substrate thickness of 1.2 mm, and an influence of the error of the substrate thickness on an spherical aberration can be considerably decreased. At this time, the diameter of the convergent spot is increased by a decrease in the numerical aperture. However, since the Information recording density of the thick optical disk 1a is lower than that of the thin optical disk 1b, such diameter is sufficiently small as the diameter of the convergent spot. In this manner, in the optical pick-up of this example, when the polarizer 9 is used for the thick optical disk 1a, the effective numerical aperture NA of the objective lens 5 is decreased, and the numerical aperture of the optical element 15 of the optical pick-up 2 according to this example is limited to a specific value. Therefore, the reflected beam 8 having high resolution can be obtained through the optical element 15. For this reason, a recording/reproducing process can be reliably performed with an optical disk having a low recording density. FIG. 3(a) shows a manner in which, when a beam (TE-polarized laser beam) for reproducing a CD is Incident on the objective lens 5 to change the diameter of the transparent portion 9a of the polarizer 9 thereby to change the effective numerical aperture, the diameter of a convergent spot formed by the objective lens 5 onto the recording surface of the CD 1a changes. FIG. 3(b) shows a manner in which the wave aberration of the convergent spot changes depending on the change in diameter. Here, the convergent spot diameter Is set to a diameter at which the strength of the main lobe of the convergent spot is set to 1/e 2 of the peak value. FIG. 3(a) shows a manner In which the convergent spot diameter changes as a function of the rate of aperture limit (aperture reduction rate) by the polarizer 9 plotted on the abscissa. When the aperture reduction rate Is increased to decrease the effective numerical aperture ,the convergent spot diameter decreases as shown in FIG. 3(a), and accordingly the wave aberration Increases as shown in FIG. 3(b). A wave aberration in an optical pick-up is generally regulated to 0.071 λ or less with the wavelength λ as rms (root mean square) average by the Marecial Criterion. Of the allowed wave aberration, about 0.05 λ is allowed as a wave aberration generated by the substrate of a CD. For this reason, the wave aberration of the optical system of the optical pick-up must be set to 0.05 λ or less. Therefore, with reference to the measurement result in FIG. 3(b), In order to satisfy this condition, the aperture reduction rate is desirably set to about 64% or less. FIG. 3(c) shows a result obtained by measuring a jitter value of a 3T signal serving as a shortest pit when the CD 1a is reproduced while an aperture reduction rate is changed. Here, as the jitter value, a value obtained by standardizing the value of the standard deviation of a distribution by an window width is set. A jitter value at which a BER (Bit Error Rate) of 10 -5 or less is obtained is 15%. For this reason, as is apparent from the measurement result shown in FIG. 3(c), the aperture reduction rate is preferably set to about 50% or more. Therefore, in consideration of changes In wave aberration and the jitter value as a function of the aperture reduction rate, it is understood that the aperture reduction rate by the polarizer 9 is preferably set to about 50 to 64%. In consideration of an allowance in manufacturing and assembling the optical pick-up 2, the aperture reduction rate is more preferably set to about 56% to 60%. According to these results, in the optical pick-up of this example, about 58% is employed as the reduction rate of the numerical aperture of the polarizer 9, and the numerical aperture of 0.6 set for a DVD is stopped down to 0.35 for a CD. In this manner, the optical pick-up 2 of this example Is an optical pick-up having high compatibility with the high-recording-density optical disk 1b and the low-recording-density optical disk 1a which are different from each other in thickness, track pitch, and the like. In addition, since the optical pick-up 2 can cope with the optical disks 1a and 1b by changing the polarization direction of the laser beam, the optical pick-up 2 is realized as an inexpensive and compact optical pick-up in which a simple optical system having high reliability is employed. Control for the polarization direction of a laser beam can be performed by only switching electrodes of the surface emitting laser 3 connected to the laser beam source LD. This control can be achieved by an extremely simple controller and has high reliability. FIG. 7 shows the an embodiment of an optical pick-up 2 according to the present invention, which can perform a recording/reproducing process with a thick optical disk 1a having a low recording density according to the CD standard and a thin optical disk 1b having a high recording density according to the DVD standard. The optical pick-up 2 according to this embodiment also comprises an Integrated device 10 in which a surface emitting laser array 3 serving as light sources and a photo detector array for signal detection 4 using photo diodes serving as light detectors are monolithically formed on a single substrate (e.g., n-type GaAs substrate). A laser beam 7 emitted from the surface emitting laser array 3 is converged on the recording layer of an optical disk 1 by an objective lens 5 through a hologram 6 of an optical element 15. A reflected beam 8 reflected from the recording layer of the optical disk 1 is incident on the objective lens 5 through the same optical path as described above and diffracted by the hologram 6, and ±1-order lights of the diffracted reflected beams are converged to generate a conspicuous astigmatic aberration in the photo detector array 4 of the integrated device 10. Therefore, the optical pick-up of this embodiment is suitable for an optical recording system 17 as shown in FIG. 1. FIG. 8 is an enlarged view showing a plane arrangement of the integrated device 10 of this embodiment. The integrated device 10 of this embodiment comprises the surface emitting laser array 3 in which a plurality of laser beam sources LD for emitting laser beams are formed in two rows in the center or almost the center. First and second laser beam source groups 3a and 3b are formed such that a tracking operation by the three beams method and a focusing servo operation for the optical disks 1a and 1b can be performed as in the above-described embodiments. The first laser beam source group 3a on the right in the drawing is a laser beam source group for performing a recording/reproducing process with the optical disk 1a having a thick substrate and a low recording density according to the CD standard. Three laser beam sources LD1a, LD2a, and LD3a constituting the laser beam source group 3a are arranged at a pitch L1 corresponding to the track pitch (1.6 µm) of the optical disk 1a according to the CD standard. A second laser beam source group 3b arranged parallel to the first laser beam source group 3a Is a laser beam source group for performing a recording/reproducing process with the optical disk 1b having a thin optical disk and a high recording density according to the DVD standard. Three laser beam sources LD1b, LD2b, and LD3b constituting the second laser beam source group 3b are arranged at a pitch L2 corresponding the track pitch (0.74 µm) of the optical disk 1b according to DVD standard. Therefore, the ratio of pitches L1 : L2 is almost equal to 1.6 : 0.74. Furthermore, the laser beam sources of the first laser beam source group 3a of this embodiment have a radiation angle different from that of the laser beam sources of the second laser beam source group 3b, thereby controlling the effective numerical aperture of the optical element 15. Therefore, in the optical element of the optical pick-up 2 of this embodiment, the polarization dependent optical element used in the optical pick-up of each of the above-described embodiments can be omitted. More specifically, the laser beam sources LD1a, LD2a, and LD3a constituting the first laser beam source group 3a are arranged such that a laser beam having a wavelength of about 650 nm can be Irradiated at a radiation angle Θ1, and the effective numerical aperture NA of the objective lens 5 of the optical element 15 is set to about 0.35. The laser beam sources LD1b, LD2b, and LD3b constituting the second laser beam source group 3b are arranged such that a laser beam having a wavelength of about 650 nm can be irradiated at a radiation angle Θ2 larger than the radiation angle Θ1, and the effective numerical aperture NA of the objective lens 5 is set to about 0.6. On both sides of the first and second laser beam source groups 3a and 3b, two rows of photo detector groups 4a and 4b for detecting reflected beams are arranged parallel to the first and second laser beam source groups 3a and 3b. Three photo diodes PD1a, PD2a, and PD3a and three photo diodes PD1b, PD2b, and PD3b are arranged In the photo detector groups 4a and 4b such that a tracking error signal TES by the three-beams method, a focusing error signal FES, and a data signal RFS are obtained in correspondence with laser beam sources LD1 to LD3. Since signal processes using data obtained by these photo diodes are the same as those in the above-described embodiments, a description thereof will be omitted. The manner in which reflected beams 8a or reflected beams 8b reflected from the optical disks 1a and 1b are converged on the photo diodes PD1a, PD1b, PD2a, PD2b, PD3a, and PD3b is described with reference to FIGS. 4, 5, and 6. FIGS. 4, 5, and 6 show a manner in which reflected beams 8b reflected from the optical disk 1b are converged on the photo detector arrays 4a and 4b In an in-focus state, a state wherein the objective lens 5 and the optical disk 1b are close to each other, and a state wherein the objective lens 5 and the optical disk 1b are far from each other, respectively. As is apparent from these drawings, signal strengths obtained from the photo detectors PD2a and PD2b on which ±1-order beams are converged are substantially equal in the In-focus state shown in FIG. 4. On the other hand, when the focal points are offset from each other, as shown in FIG. 5 or 6, the signal strengths obtained from the photo detectors PD2a and PD2b are different. For this reason, a convergent state is obtained as a focusing error signal FES, and a focusing servo operation is performed on the basis of this signal, and the servo operation can be reliably performed. When a tracking error occurs, signal strengths obtained from the photo diodes PD1a, PD1b, PD3a, and PD3b for detecting the sub-beams are different. For this reason, this state is obtained as the tracking error signal TES, and a tracking servo operation can be performed on the basis of this signal. In order to detect a focusing error signal, the hologram 6 pattern-designed in the following manner is used. That is, a main beam and sub-beams reflected from the optical disk 1a or 1b are diffracted to ±1-order diffraction, and the optical axes of these beams are changed to generate a conspicuous astigmatic aberration. In this embodiment, for example, a pattern along a hyperbola group or a corrected hyperbola group is used as the pattern of a grating, and the grating Is arranged such that ±1-order diffracted light is parallel to the longitudinal direction (x axis) of the device 10. Therefore, when the objective lens 5 is dose to the optical disk 1a or 1b, as shown in FIG. 5, the spot of +1-order diffracted light converged on the photo diodes PD1a, PD2a, and PD3a has an elliptical shape having a major axis parallel to the x axis, and the spot of -1-order diffracted light converged on the photo diodes PD1b, PD2b, and PD3b has an elliptical shape having a major axis parallel to the y axis. In contrast to this, when the objective lens 5 and the disk 1a or 1b are far from each other, as shown in FIG. 6, the spot of +1-order diffracted light has an elliptical shape having a major axis parallel to the y axis, and the spot of -1-order diffracted light has an elliptical shape having a major axis parallel to the x axis. Therefore, as described above, amounts of light received by the photo diodes PD2a and PD2b change, and the focusing error signal FES can be obtained. In addition, as shown in FIG. 8, the photo diodes PD2a and PD2b formed on the device 10 of this embodiment are arranged on the same straight line as that of the laser beam sources LD2a and LD2b to form a rectangle having long sides parallel to the straight line. The length of the long side of the rectangle is sufficiently longer than the Interval between the laser beam sources LD2a and LD2b. Therefore, when the thick optical disk 1a is to be reproduced, or when the thin optical disk 1b is to be reproduced, a focusing error signal can be detected by common photo detectors. In addition, when the wavelength of a beam emitted from the surface emitting laser array 3 or the distance between the surface emitting laser array 3 and the hologram 6 on the optical axis is subject to tolerances, the spots of ±1-order diffracted lights converged on the photo diodes PD2a and PD2b only move on the x axis. For this reason, the spots can be reliably caught. Therefore, as in the device 10, when the photo diodes PD2a and PD2b are arranged In a rectangle having long sides parallel to the x axis, the precision of the wavelength of a light source, the assembly precision of the light source, and the like can be moderated. This is advantageous to manufacturing an optical pick-up. The photo detection areas of the photo diodes PD1a, PD3a, PD1b, and PD3b for detecting a tracking error signal are sufficiently large. Each photo detection area has a square or almost square shape having a side whose length is equal to the length of the long side of the photo diode PD2a or PD2b. Therefore, as in detection for a focusing error signal, the photo diodes PD1a, PD3a, PD1b, and PD3b can be used to detect the tracking error signals from both the thick optical disk (disk according to the CD standard) 1a and the thin optical disk (disk according to the DVD standard) 1b. The precision of the wavelength of the laser beam source and the assembly precision of the laser source can be also moderated. The laser beam sources LD1b, LD2b, and LD3b are arranged in the surface emitting laser array 3b of this embodiment at intervals corresponding to the track pitch of an optical disk having a high recording density. In addition, the objective lens 5 of this embodiment is optimally designed such that the effective numerical aperture NA is set to 0.6 for the laser beam 7b emitted at a predetermined radiation angle from the second laser beam source group 3b, and the laser beam 7b can be converged to diffraction limit on the recording layer of the thin optical disk 1b. Therefore, since the laser beam 7b emitted from the second laser beam source group 3b Is converged on the recording layer to obtain a reflected beam having high resolution, a conspicuous tracking error signal TES is obtained, and a tracking servo operation can be reliably performed. For this reason, positional control can be stably performed at high precision with the thin optical disk 1b having a high recording density, and a recording/reproducing process can be reliably performed. On the other hand, when a process is performed by using the optical pick-up 2 of this embodiment to the thick optical disk 1a having a low recording density, the first laser beam source group 3a of the surface emitting laser array Is selected, and laser beams 7a are irradiated from the three laser beam sources LD1a, LD2a, and LD3a constituting the first laser beam source group 3a. The central laser beam source LD2a of these laser beam sources serves as a main beam source for outputting a data signal and a focusing error signal by an astigmatic aberration method, and the laser beam sources LD1a and LD3a arranged on both sides of the laser beam source LD2a serve as sub-beam sources for outputting tracking error signals by a three-beams method. The laser beams 7b emitted from the laser beam source group are converged on the recording layer of the optical disk 1b by the objective lens 5. A reflected beam 8a reflected from the optical disk 1a according to the CD standard is converged like the reflected beam 8b as indicated by the broken lines in FIGs. 4, 5, and 6, and a data signal and a focusing error signal can be obtained. In addition, in the first laser beam source group 3a, the laser beam sources LD1a, LD2a, and LD3a are arranged at a pitch corresponding to the optical disk 1a having a large track pitch and a low recording density. For this reason, a tracking error signal can also be obtained by the three-beams method with respect to the optical disk 1a. The objective lens 5 of this embodiment is designed such that the effective numerical aperture NA is set to about 0.35 when the radiation angle of TE-polarized light emitted from the first laser beam source group 3a is decreased. Therefore, although the objective lens 5 is optimally designed for a thin optical disk having a substrate thickness of 0.6 mm and a high recording density, the numerical aperture of the optical element 15 Is decreased for a thick optical disk having a substrate thickness of 1.2 mm and a low recording density so that the laser beam 7a can be converged to almost diffraction limit, and a focusing operation and a tracking servo operation can be performed with the optical disk 1a having a large track pitch and a low recording density by using the same control system as that used for the optical disk 1b having a small track pitch and a high recording density. In addition, the radiation angle of the laser beam 7a is decreased by the polarized hologram 18, so that the effective numerical aperture NA of the objective lens 5 can be decreased. For this reason, a spherical aberration by a substrate thickness is suppressed from being generated, and the reflected beam 7a having high resolution can be obtained. Therefore, a recording/reproducing process can be performed with the optical disk having a low recording density. In this manner, the optical pick-up 2 of this embodiment is also an optical pick-up having compatibility with the high-recording-density optical disk 1b and the low-recording-density optical disk 1a which are different from each other in thickness and track pitch. In addition, for example, when the optical pick-up 2 of this embodiment only selects the first or second laser beam source 3a or 3b on the basis of a selection signal from a laser controller 11 in the optical recording system shown in FIG. 1 to change the polarization direction of a laser beam irradiated on an optical disk, the optical pick-up 2 can cope with the optical disks 1a and 1b. For this reason, the optical system which can be prevented from being complicated and has high reliability can be employed, and an inexpensive and compact optical pick-up having high performance can be provided. In the optical pick-up of this embodiment, since laser power is not divided, a recording/reproducing process can be performed with low power consumption. Furthermore, in this embodiment, by utilizing the advantage of the surface emitting laser array in which a plurality of laser beam sources can be formed in a small area by using the photolithographic technique, a plurality of laser beam sources corresponding to the track pitches of the respective optical disks 1a and 1b can be formed, so that an optical pick-up which can obtain tracking error signals by using a three-beams method with respect to the optical disks 1a and 1b and has a very small size and a simple arrangement is realized. According to the three-beams method, a stable tracking error signal can be obtained, and a simple control system can be used. For this reason, the three-beams method is used in many optical pick-ups for CD. However, when an optical element such as a prism is used to form three required beams, it is impractical that a considerably complex optical system is required to cope with the track pitches of two different types of optical disks. Therefore, even if the three-beams method is applied to one optical disk, a different optical system must be applied to the other optical disk. For this reason, the control system is considerably complicated. In contrast to this, In the optical pick-up 2 of this embodiment, two groups of laser beam sources 3a and 3b corresponding to the respective optical disks 1a and 1b are formed in the surface emitting laser array, so that an optical element for forming three beams can be omitted. At the same time, the three-beams method can be applied to the different types of optical disks, and tracking and focusing control can be performed by a single control system. In addition, since the surface emitting laser array 3 is employed, a total of six laser beam sources constituting the two groups of laser beam sources 3a and 3b can be arranged in a very small space. In the optical pick-up of this embodiment, since the laser beam sources for outputting a main beam and sub-beams are separated from each other, a laser beam for detecting an information signal and a focusing error signal and a laser beams for detecting a tracking error signal can be emitted at different timings. When an optical disk is to be reproduced, the flow of processes is executed in the following rough order. That is, the optical disk is determined, the surface emitting lasers corresponding to the optical disk are turned on, focus drawing and focusing control are performed, tracking control is performed, laser beams are moved to a desired address, and an information signal is reproduced. Therefore, when only the main beam Is turned on until tracking control is started, a sub-beam can be prevented from being mixed in the photo diodes PD2a and PD2b, and highly precise focusing control can be performed. The arithmetically calculated tracking error and focusing error signals are supplied to, e.g., a position controller 13 shown In FIG. 1, the optical pick-up 2 is moved by an actuator in a direction perpendicular to the recording surface of the optical disk or a radial direction of the optical disk, and control for converging a laser beam to a target pit on the recording track of the optical disk to form a fine spot is performed. According to the three-beams method, the convergent spots of sub-beams are arranged in front of and behind the convergent spot of a main beam in the track direction. For this reason, since a time difference is generated between the signals of the two sub-beams, abnormality of a tracking error signal may occur when the beam spots pass through cracks or defects on the disk. In order to suppress an influence caused when the abnormality of the tracking error signal occurs, the interval between the convergent spots of the main beam and the sub-beam is preferably set to 15 µm or less. On the other hand, when the interval between the convergent spots is decreased, the pitches L1 and L2 of the laser beam sources decrease. In particular, the pitch L2 of the laser beam sources 3b corresponding the optical disk 1b according to the DVD standard considerably decreases. The range in which two adjacent emitting portions of the surface emitting laser array 3 can be closest to each other is determined by the cross sectional shapes of the columns constituting the optical resonators of the emitting portions parallel to the semiconductor substrate. Since the size of a general cross section is 10 to 30 µm, the minimum interval between the adjacent laser beam sources, i.e., the pitch L2 of the laser beam sources 3b is preferably set to 30 µm or more. Assume that a magnification m of the optical element 15 of this embodiment is a ratio (f1/f2) of an optical length f1 between an object surface (light source) and an object side principle point (after the optical elements such as the objective lens 5 and the polarized hologram 18 constituting the optical element 15 are combined to each other) of the optical element 15 and an optical length f2 of an image (optical disk 1) side principal point of the optical element 15. In this case, the relationship between the pitch L1 of the first laser beam source group 3a for CD In which an interval between convergent spots increases and an allowable interval (15 µm) between the convergent spots is as follows: 15 x m = L1 (µm) Therefore, when it is considered that the ratio of the pitch L1 of the first laser beam source group 3a and the pitch L2 of the second laser beam source group 3b is equal to the ratio of the track pitches of the optical disks 1a and 1b, the pitch L2 of the second laser beam source group 3b is given by: 15 x m x (L2/L1) ≥ 30 µm, i.e., m x (L2/L1) ≥ 2 Since the track pitch of the CD standard is 1.6 µm, and the track pitch of the DVD standard is 0.74 µm, the pitch ratio L2/L1 is given by 0.74/1.6. Therefore, it is understood that as an optical element for an optical pick-up having compatibility with both the DVD standard and the CD standard, an optical system having a magnification m of 4.4 or more is desirably used. Furthermore, in consideration of the facility of manufacturing a surface emitting laser array, the minimum interval between laser beam sources is more preferably set to about 40 µm or more. For this reason, it is more desirable that the magnification m of the optical system satisfies the following relationship: 15 x m x (L2/L1) ≥ 40 µm, i.e., m x (L2/L1) ≥ 8/3 Therefore, as an optical element for an optical pick-up having compatibility with both the DVD standard and the CD standard, an optical element having a magnitude m of 5.8 or more is more preferably used. The magnification m of the optical element 15 can be replaced with the magnification of the objective lens 5, because the influence of the optical elements such as polarized hologram 18 constituting the optical element 15 except for the objective lens 5 can be almost neglected. As a concrete example, when the wavelength of a laser beam emitted from the surface emitting laser array 3 is set to 650 nm, an objective lens which has a numerical aperture of 0.6 and a finite system having a magnification m of 6 and is optimally designed for DVD can be used. The surface emitting laser array 3 in which the pitch L1 of the first laser beam source group 3a for CD is 90 µm and the pitch L2 of the second laser beam source group 3b for DVD is 41.6 µm can be applied to the objective lens 5. Therefore, an optical pick-up having such preferable optical performance that the interval between the convergent spots of a main beam and a sub-beam for CD obtained by the objective lens 5 Is set to 15 µm and the interval between the convergent spots of a main beam and a sub-beam for DVD is set to 6.9 µm. The Interval (e.g., the distance between the laser beam sources LD2a and LD2b) between the first laser beam source group 3a and the second laser beam source group 3b is set to 40 µm. Furthermore, the objective lens 5 employed in the optical pick-up 2 of this embodiment is designed such that an aberration can be corrected to an Image height of ±50 µm to make it possible to moderate manufacturing precision or assembling precision. Since the magnification m of the objective lens 5 of this embodiment is 6, the objective lens 5 can allow the light source to be offset from the optical axis by a maximum of ±300 µm. Therefore, when the intermediate point obtained by connecting the laser beam sources LD2a and LD2b located at the centers of the first and second laser beam source groups 3a and 3b is defined as an optical axis, although it is considered that the laser beam source LD1a or LD3a which is maximally offset from the optical axis by 93 µm, laser beams emitted from all the light sources of all the first and second laser beam source groups can be converged by the objective lens 5 of this embodiment on the recording surface of a CD or DVD to almost diffraction limit. In the optical pick-up 2, a plurality of laser beam sources must be arranged to use the three-beams method to process disks having different specifications, and the surface emitting laser described above is optimally selected. In particular, in a vertical cavity surface emitting laser (referred to as a VCSEL hereinafter), the degrees density and in-plane arrangement are high. The VCSEL may be most suitable for the optical pick-up 2 of this embodiment. In the optical pick-up 2 of this embodiment, when the optical disk 1b having a high recording density and a thin substrate is set, the second laser beam source group 3b of the surface emitting laser array 3 is selected by a laser beam source selector 11 of the optical recording system shown in FIG. 1. The laser beams 7b are irradiated from the three laser beam sources LD1b, LD2b, and LD3b constituting the second laser beam source group 3b. The central laser beam source LD2b of these laser beam sources serves as a main beam source for outputting a data signal and a focusing error signal by an astigmatic aberration method, and the laser beam sources LD1b and LD3b arranged on both sides of the laser beam source LD2b serve as sub-beam sources for outputting tracking error signals by a three-beams method. These laser beams 7b emitted from the laser beam sources LD1b, LD2b, and LD3b are converged on the recording layer of the optical disk 1b by the objective lens 5 of the optical element 15. The objective lens 5 of this embodiment is optimally designed such that the laser beam 7b having a large radiation angle and emitted from the second laser beam source group 3b can be converged to diffraction limit on the recording layer of the thin optical disk 1b when the effective numerical aperture NA is 0.6. Therefore, positional control can be stably performed at high precision with the optical disk 1b according to the DVD standard, and a recording/reproducing process can be reliably performed. On the other hand, when a process is performed by using the optical pick-up 2 of this embodiment for the thick optical disk 1a having a low recording density, for example, the first laser beam source group 3a of the surface emitting laser array is selected by the laser beam source selector 11 of the optical recording system shown in FIG. 1, and laser beams 7a are irradiated from the three laser beam sources LD1a, LD2a, and LD3a constituting the first laser beam source group 3a. The central laser beam source LD2a of these laser beam sources serves as a main beam source for outputting a data signal and a focusing error signal by an astigmatic aberration method, and the laser beam sources LD1a and LD3a arranged on both sides of the laser beam source LD2a serve as sub-beam sources for outputting tracking error signals by a three-beams method. The laser beams 7a emitted from the laser beam source group 3a are converged on the recording layer of the optical disk 1a by the objective lens 5. The laser beam source 3a is designed to radiate a laser beam having a radiation angle smaller than that of the laser beam obtained from the laser beam source 3b, so that the effective NA is set to 0.35 when a laser beam radiated from the laser beam source 3a is incident on the objective lens 5. Therefore, by using the optical element 15 (objective lens 5) having a large numerical aperture and optimally designed for a thin optical disk having a substrate thickness of 0.6 mm and a high recording density, a convergent spot can be formed on a thick optical disk having a substrate of 1.2 mm and a low recording density while suppressing a spherical aberration caused by a substrate thickness error. In the optical pick-up 2 of this embodiment, the effective numerical aperture NA of the objective lens 5 is limited to 0.35 by the laser beam radiated from the laser beam source 3a, and the laser beam 7a can be converged to almost diffraction limit on the optical disk 1a having a substrate thickness of 1.2 mm and a low recording density. When the numerical aperture of the optical element is limited by the radiation angle of a laser beam, a wave aberration and a Jitter value are desirably set within the range described in the above embodiment, as a matter of course. Therefore, in the optical pick-up of this embodiment, the radiation angle Θ1 of the first laser beam source group 3a is preferably set with respect to the radiation angle Θ2 of the second laser beam source group 3b such that the reduction rate of the effective numerical aperture NA of the optical element (objective lens) is set to about 50 to 64%. In consideration of an allowance in manufacturing the optical pick-up and an allowance in assembling the optical pick-up, the aperture reduction rate is more preferably set to about 56% to 60%. In the optical pick-up 2 of this embodiment, as in the above-described embodiments, the radiation angles are selected such that the effective numerical aperture NA is set to about 58%. In this manner, the optical pick-up 2 of this embodiment can apply the three-beams method in which stable control can be performed with a simple arrangement to the high-recording-density optical disk 1b and the low-recording-density optical disk 1a which are different in thickness and track pitch, and a tracking operation and a focusing control operation can be performed with the same control system. Therefore, the optical pick-up has high compatibility with optical disks having different specifications. In addition, since the optical pick-up can cope with the optical disks 1a and 1b by changing the radiation angle of a laser beam, an optical system which can be prevented from being complicated and has high reliability can be employed, and the optical pick-up can be provided as an Inexpensive and compact optical pick-up. In order to form light sources for irradiating a plurality of laser beams having different radiation angles, a plurality of conventional edge emitting laser diodes may be arranged as a matter of course. However, a large number of semiconductor laser chips cannot be easily arranged in a small space to apply the three-beams method to a plurality of optical disks having different track pitches. In addition, a substrate for connecting these semiconductor chips to each other to operate them is required making the optical pick-up considerably large . When an optical pick-up is to be assembled, the relationship between the irradiation positions of these semiconductor chips and a photo detector array for detecting reflected laser beams must be adjusted. For this reason, the number of steps of assembling the optical pick-up increases, and the optical pick-up is very expensive. In contrast to this, according to the present invention, the surface emitting laser array 3 in which a plurality of laser beam sources can be two-dimensionally arranged to be Integrated Is employed as the laser beam sources. Therefore, a plurality of laser beam sources can be arranged in a small area, and a laser beam is not irradiated from an edge, but irradiated perpendicular to the substrate. For this reason, as described above, the laser beam sources and light detectors such as photo diodes can be easily monolithically formed on a single substrate. In particular, in a vertical cavity surface emitting laser (referred to as a VCSEL hereinafter) in which a resonant path is arranged in a direction perpendicular to the substrate and which is employed in the above-described embodiment, the concentration of the two-dimensional array and the degree of freedom of an in-plane arrangement are high. The VCSEL may be most suitable for the optical pick-up according to the present invention. FIG. 9 shows the arrangement of the VCSEL by using its cross sectional view. FIG. 10 shows the VCSEL of this embodiment when viewed from the emitting side of a laser beam. The VCSEL of the embodiment comprises an optical resonator 20 in which a plurality of semiconductor layers are stacked between a pair of metal layers 21 and 32. An n-type GaAs substrate 22, an n-type GaAs buffer layer 23, 60 pairs of semiconductor distributed Bragg reflectors 24 each constituted by an n-type AlAs layer and an n-type Al 0.5 Ga 0.5 As layer and together having a reflectance of 99% or more with respect to light having a wavelength of about 650 nm, an n-type Al 0.7 Ga 0.3 As clad layer 25, a multi-quantum well active layer 26 constituted by five n--type Ga 0.5 In 0.5 P well layers and an n - -type (Al 0.5 Ga 0.5 ) 0.5 In 0.5 P barrier layer, a p-type Al 0.7 Ga 0.3 As clad layer 27, and a p-type Al 0.2 Ga 0.8 As contact layer 28 are sequentially stacked on the optical resonator 20 from the bottom. And 8 pairs of dielectric multilayer reflectors 33 each constituted by a first layer, e.g., an SiO x layer such as SiO 2 and a second layer, e.g., a TaO x layer such as Ta 2 O 5 and having a reflectance of 98.5% or more with respect to light having a wavelength of about 650 nm are formed on an area so as to sufficiently cover an exposed surface 35 (referred to as an aperture hereinafter) of the contact layer 28 on the emitting side. In place of this, 50 pairs of semiconductor distributed Bragg reflectors each constituted by a p-type AlAs layer and a p-type Al 0.5 Ga 0.5 As layer and having a reflectance of 98.5% or more with respect to light having a wavelength of about 650 nm may be formed on the emitting side of the clad layer 27, and a p-type Al 0.3 Ga 0.7 As contact layer may be stacked on the semiconductor distributed Bragg reflectors as In the above-described embodiments. Epitaxial growth using the MOVPE method or epitaxial growth using the MBE method can be used to form these stacked layers. In the VCSEL of this embodiment, the contact layer 28 and a part of the clad layer 27 are etched into a columnar or almost columnar shape when viewed from the upper surface of the resonator 20, i.e., the emitting side, and this column 34 of diameter R serves as a resonant portion. The periphery of the column 34 is buried with a first insulative layer 29 constituted by a silicon oxide film (SiO x film) such as an SiO 2 film formed by a thermal CVD method and a metal layer 30 consisting of a metal such as a gold-zinc alloy. The first insulative layer 29 is continuously formed along the p-type Al 0.5 Ga 0.5 As clad layer 27 and the surface of the contact layer 28, and the metal layer 30 is formed to bury a portion around the first Insulative layer 29. A second insulative layer 31 constituted by a silicon oxide film (SiO x film) such as an SiO 2 film formed by a spattering method is formed on the surface of the metal layer 30, the metal layer (upper electrode) 32 consisting of, e.g., Cr and a gold-zinc alloy is formed on the second insulative layer 31 to be in contact with the contact layer 28, and the contact metal layer 32 serves as an electrode for current injection. The upper electrode 32 is attached such that a circular opening is formed In the column 34 on the emitting side, and the Inside of the upper electrode 32 serves as the aperture 35. Therefore, in the VCSEL of this embodiment, a current is injected from the upper electrode 32, the current is guided to the active layer 26 to be converted Into light, and the light is amplified by the column 34, and the laser beam is emitted from the aperture 35 to the outside. In the surface emitting laser array 3 used in the optical pick-up 2 of this embodiment, laser beam sources LD having different radiation angles are required. One way of controlling the radiation angle of a laser beam emitted from the aperture 35 Is by changing the diameter W of the aperture 35. According to this invention different radiation angles of laser beams are obtained by means of a micro lens. As shown in FIG. 11, a micro lens 39 Is arranged on the upper portion of the dielectric multilayer reflector 33 and controlled such that a laser beam having a desired radiation angle is emitted by the refractive power of the micro lens 39. The micro lens 39 is formed in the following manner. That is, an opening is formed in the reflector 33 by a photolithographic technique, the resultant structure is subjected to ion exchange treatment to control reflective distribution, thereby obtaining predetermined refractive power. Therefore, this structure has an extremely high arrangement concentration and an extremely high degree of freedom of an in-plane arrangement, and is suitable to constitute a lens array on the surface emitting laser VCSEL of this embodiment. In the optical pick-up 2 of this embodiment, micro lenses (e.g., concave micro lenses) 39 having negative refractive power may be arranged on the laser beam sources LD1b, LD2b, and LD3b of the surface emitting laser array 3b for Irradiating laser beams each having a large radiation angle, while no micro lenses 39 are arranged on the laser beam sources LD1a, LD2a, and LD3a for laser beams each having a small radiation angle. Naturally, the micro lenses 39 may be arranged on all the laser beam sources LD and the radiation angles controlled by making the refractive powers of the respective micro lenses different from each other. As has been described above, in the optical pick-up of this embodiment, laser beam sources which can irradiate laser beams having different radiation angles are employed, and a plurality of laser beam sources having different radiation angles can be formed in a small space by using a surface emitting laser array. For this reason, an optical pick-up which can control the effective numerical aperture NA of an optical system for optical disks having different thickness can be provided. Therefore, the optical pick-up according to the present invention can converge beam spots on any one of optical disks having different thickness, and a reflected beam having high resolution can be obtained. For this reason, by utilizing the servo function of the optical pick-up, a recording/reproducing process can be reliably performed with the optical disks having different thickness. In addition, In the optical pick-up of this embodiment, a plurality of laser beam sources are arranged in a surface emitting laser array at a pitch corresponding to the track pitch of each optical disk, a tracking servo operation can be performed with optical disks having different track pitches by using the three-beams method which can be reliably operated. Therefore, a data processing can be more reliably performed with a plurality of optical disks having different recording densities. As in the optical pick-up of this embodiment, processes such as the process of moving an optical element in an optical system, the process of switching optical elements, and the process of splitting one laser beam into a plurality of laser beams by the optical system need not be performed. Therefore, an inexpensive and highly reliable optical system which is used in a conventional technique can be applied without being changed. The laser beam source group of the surface emitting laser array may be selected depending on the thickness and track pitch of an optical disk. Since this control can be easily electrically performed, the control can be realized at low cost with a simple arrangement, and an optical pick-up having high reliability and high durability can be provided. Since a laser beam source group corresponding to each optical disk can be selectively used, the energy of the laser beam is not dispersed, and an optical pickup which can prevent electric power from being wasted and has low power consumption can be realized. Therefore, the optical pick-up of this embodiment is inexpensive and highly reliable, can form a convergent spot on an optical disk, and is suitable for a phase change recording medium, a programmable optical recording medium such as a magneto-optical recording medium, or a write-once optical recording medium. In addition, the optical pick-up has high compatibility with various types of optical recording media for various applications which will coexist in the future. Therefore, the optical pick-up is suitable for an optical recording device which can process all of the various types of optical disks. In this embodiment, although two rows of laser beam source groups having different pitches are arranged in the surface emitting laser array, the number of rows Is not limited to two, and three or more rows can be used as a matter of course. Although this embodiment exemplifies the three-beams method which is used by utilizing the characteristics of the surface emitting laser array which can freely two-dimensionally arrange a large number of laser beam sources, an optical pick-up which obtains a tracking error signal by using the push-pull method or the differential phase tracking method described in the previous embodiments can be used. In the optical pick-up described above, at least two laser beam sources having different radiation angles are desirably arranged in the surface emitting laser array. In this embodiment, the value of the numerical aperture NA is only an example. The present invention is not limited to the above embodiments, optical pick-ups according to the present invention include all optical pick-ups according to the accompanying claims, as a matter of course. As has been described above, in the optical pick-up according to the present Invention, the convergent performance of an optical system, especially the numerical aperture can be changed, and at least two types of laser beams can be emitted from a surface emitting laser array. These laser beams are emitted from the above laser beam sources having different radiation angles, and the radiation angle of the laser beam can be controlled by a micro lens. These elements can be easily Incorporated In the steps In manufacturing the surface emitting laser array, and the optical pick-up of this embodiment can be manufactured at low cost. In addition, the surface emitting laser array can be easily manufactured on a single semiconductor substrate on which a photo detector is formed. With respect to this, the manufacturing cost can be reduced. Since the problem that a laser element and a photo detector have to be positionally matched with each other does not occur, an inexpensive optical pick-up having higher precision can be provided. By using the optical pick-up according to the present invention, an optical pick-up having high compatibility with a conventional thick CD having a low recording density and a thin DVD having a high recording density and expected as a future recording medium can be realized, and an optical pick-up which is suitable for an optical recording system which can perform a process with all of these optical recording media can be provided. Since high focusing performance can be kept for every optical recording medium, an optical recording system or an information processing system which can perform a highly reliable process while preferably performing tracking control and focusing control and which can handle various types of optical disks can be provided.

Claims: An optical pick-up comprising a light source of first and second laser beam sources (LD1a-LD3a, LD1b-LD3b) for emitting at least two types of laser beam (7a, 7b), an optical element (5) for converging the emitted laser beams (7a, 7b) on an optical recording medium (1a, 1b), and light detection means (PD1a-PD3a, PD1b-PD3b) for detecting a reflected beam reflected from the optical recording medium (1a, 1b), said optical element (5) having different values of the effective numerical aperture for said at least two types of laser beam (7a, 7b), characterized In that said first and second laser beam (7a, 7b) sources (LD1a-LD3a, LD1b-LD3b) are vertical cavity surface emitting lasers constituted such that a pair of reflective layers(24, 33), a clad layer (25), an active layer (26), and a pair of metal layers (21, 32) are stacked in the emitting direction of the laser beam (7a, 7b) to form optical resonators; and at least one of said first and second laser beam (7a, 7b) sources (LD1a-LD3a, LD1b-LD3b) has a micro lens (39) formed on that one of said reflective layers provided on the side on which the laser beams (7a, 7b) are emitted. An optical pick-up according to claim 1, characterized In that said optical element (5) comprises an objective lens for converging the first and second laser beams (7a, 7b) on an optical recording medium (1a, 1b). An optical pick-up according to claim 2, characterized In that the radiation angle of the first laser beam is selected with respect to the radiation angle of the second laser beam such that the numerical aperture for one of said first and second laser beams (7a, 7b) is about 50 to 64% of that for the other laser beam. An optical pick-up according to claim 2, characterized In that the radiation angle of the first laser beam is selected with respect to the radiation angle of the second laser beam such that the numerical aperture for one of said first and second laser beams (7a, 7b) is about 56 to 60% of that for the other laser beam. An optical pick-up according to claim 1, characterized In that said surface emitting laser beam sources comprises a first laser beam source group In which at least three first laser beam sources (LD1a-LD3a) are arranged at a first pitch and a second laser beam source group in which at least three second laser beam sources (LD1b-LD3b) are arranged at a second pitch, and the first and second pitches are different from each other. An optical pick-up according to claim 5, characterized In that when the magnification of said optical element (5) is represented by m, the first pitch is represented by L1, and the second pitch is represented by L2, the following relationship is satisfied: m x (L2/L1) ≥ 2 where L2 < L1. An optical pick-up according to claim 5, characterized In that when the magnification of said optical element (5) is represented by m, the first pitch is represented by L1, and the second pitch Is represented by L2, the following relationship is satisfied: m x (L2/L1) ≥ 8/3 where L2 < L1. An optical pick-up according to claim 1, further comprising a diffractor (6) for diffracting the reflected beam into ±1-order lights to converge them on light detectors of said light detection means (PD1a-PD3a, PD1b-PD3b) arranged around said light source, said surface emitting lasers and said light detectors are formed on a single semiconductor substrate. An optical recording system characterized by comprising: an optical pick-up according to claim 1 laser beam control means (11) for controlling said surface emitting lasers; and position control means (12, 16) for positionally controlling said optical pick-up on the basis of signals from said light detection means (PD1a-PD3a, PD1b-PD3b).

Inventors: ARIMURA TOSHIO, KANEKO TAKEO, KITAMURA SHOJIRO, KONDO TAKAYUKI, MORI KATSUMI, ARIMURA, TOSHIO, KANEKO, TAKEO, KITAMURA, SHOJIRO, KONDO, TAKAYUKI, MORI, KATSUMI
Applicants: SEIKO EPSON CORP, SEIKO EPSON CORPORATION
Application year: 1997
Main classification: G11B7
IPC codes: G11B7