Optical pickup and optical disk apparatus using the same

Abstract: Laser light emitted from the wavelength-locked GaN semiconductor laser is collimated through a collimation lens, led through a polarization beam splitter and a 1/4 wavelength plate, and converged by a focus lens so as to be radiated on pits formed in an optical disk medium. The signal light from the optical disk medium is collimated by the focus lens, and has its polarization direction turned by the 1/4 wavelength plate by 90° relative to its polarization direction before being returned from the optical disk medium. As a result, the signal light is reflected from the polarization beam splitter so as to be converged on the optical detector by the focus lens.

Description: BACKGROUND OF THE INVENTION 1. FIELD OF THE INVENTION: The present invention relates to an optical pickup which utilizes coherent light for use in the field of optical information processing, and also to an optical disk apparatus incorporating the optical pickup. 2. DESCRIPTION OF THE RELATED ART: DVDs (digital video disks), which define one species of optical disks, can record 4.7 GB of data per disk medium with a diameter of 12 cm by utilizing a red semiconductor laser. Thus, signals equivalent to about 2 hours of motion pictures in the NTSC format can be recorded/reproduced on a single disk. A semiconductor laser which is employed as a light source in such a DVD apparatus typically generates red laser light with a wavelength of about 635 nm to about 650 nm, and has output power of about 5 mW. The relative intensity of noise (RIN) is generally -130 dB/Hz, thus satisfying the DVD requirement of -126 dB/Hz or smaller RIN. However, the following problem may occur in a conventional DVD apparatus incorporating the above-described red semiconductor laser when reproducing motion pictures of the HDTV (high definition TV) class, which is, one type of high quality TVs that requires high density recording/reproduction as necessitated by increase in the amount of information to be processed. Specifically, the use of a red semiconductor laser, i.e., a semiconductor laser having a relatively long wavelength, as light source permits data equivalent to only about 40 minutes of motion pictures to be recorded on one DVD medium. Therefore, it is impossible to record or reproduce a relatively long program, e.g., a movie, on one disk. In addition, since the amount of information to be recorded/reproduced increases with the enhancement in the quality of images, a higher transfer rate may be required at the time of recording/reproduction. This calls for a great increase in the rotation speed of the disk. EP 0 279 183 A2 (cf. the preamble of claims 1 and 4) discloses a magneto-optic read-out method and apparatus for optically reading pre-recorded magnetic information on a magneto-optic recording element. Undesired polarisation variations are sensed from the system's output signal, and an error signal representative of such variations is produced. Such an error signal is used to adjust the state of polarisation of the read-out beam prior to the detection by the system's detector elements. The read beam is directed through the combination of a quarter-wave plate and a Faraday rotator and the error signal is used to control the magnetic field of the Faraday rotator. US 5,477,527 discloses a high density optical disk having a track pitch of 0,85 µm, a shortest pit length 0,49 µm, a pit depth of 100 nm and pit width of 0,35 µm which is rotated at a linear velocity of 2,4 - 2,8 m/s such that the same is optically reproduced by a laser beam having a wavelength of 635 nm which is converged to a spot size of 0,9 µm by an objective lens having a numerical aperture of 0,6. According to the present invention, the optical pickup includes: a laser light source; an optical system providing light emitted from the laser light source with a prescribed optical path; an optical detector for detecting light from the optical system; and a first control member for controlling a light amount of an incident light onto the optical detector. The first control member controls the light amount of the incident light onto the optical detector by controlling a polarization direction of the incident light. Specifically, the first control member changes a polarization direction of a laser light emitted from the laser light source, thereby controlling the polarization direction of the incident light onto the optical detector. The laser light source is a semiconductor laser, and the first control member applies pressure to the semiconductor laser, thereby changing the polarization direction of the laser light emitted from the semiconductor laser. A second control member may be provided being a variable wavelength plate disposed between a polarizing optical element and a focus lens in the optical system. The incident light onto the optical detector may be a signal light from an optical disk medium. According to another aspect of the present invention, the optical disk apparatus includes: a laser light source; an optical system providing light emitted from the laser light source with a prescribed optical path; an optical detector for detecting light from the optical system; an optical disk medium having pits with a length of 0.3 µm or less; and a first control member for controlling a light amount of a signal light incident onto the optical detector from the optical disk medium. The first control member controls the light amount of the signal by controlling a polarization direction of the signal light. Specifically, the first control member may change a polarization direction of a laser light emitted from the laser light source, thereby controlling the polarization direction of the signal light. The laser light source is a semiconductor laser, and the controlling member may apply pressure to the semiconductor laser, thereby changing the polarization direction of the laser light emitted from the semiconductor laser. A second control member may be provided being a variable wavelength plate disposed between a polarizing optical element and a focus lens in the optical system. Thus, the invention described herein makes possible the advantage of providing, in the implementation of an optical pickup and an optical disk apparatus utilizing a short-wavelength laser as a light source, a high-performance optical pickup capable of high-density recording/reproduction and an optical disk apparatus incorporating such an optical pickup. The optical pickup according to the present invention, and the optical disk apparatus incorporating the same, utilize light having a relatively short wavelength, e.g., light of blue and other colors on the shorter end of the spectrum. As a result, it is possible to realize a high-performance, high definition DVD (HD-DVD) apparatus which is capable of recording/reproducing information in pits of an optical disk that are far more minute than those of an optical disk for a conventional DVD apparatus. Furthermore, by performing wavelength locking of the semiconductor laser used as a light source, an optical pickup and an optical disk apparatus which are immune to returned light and capable of excellent recording/reproduction performance with a low noise level can be provided. This and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a graph illustrating the relationship between the wavelength of a light source and recording density. Figure 2 is a graph illustrating the relationship between the wavelength of a semiconductor laser (a light source) and its RIN level. Figure 3 is a graph illustrating the shift amount of the focal point across the wavelengths of the light source responsive to a change in the light source wavelength of 2 nm between recording and reproduction. Figure 4 is a schematic diagram illustrating a configuration of an optical pickup and an optical disk apparatus according to Comparative Example 1. Figure 5 is a cross-sectional view illustrating a configuration of a GaN type semiconductor laser employed in Comparative Example 1. Figure 6 is a cross-sectional view illustrating a configuration of a GaN type semiconductor laser employed in Comparative Example 1. Figure 7 is a plan view illustrating a configuration of a GaN type semiconductor laser employed in Comparative Example 1. Figures 8A-1 to 8F-1 and Figures 8A-2 to 8F-2 are schematic diagrams illustrating manufacturing steps of a semiconductor laser employed in Comparative Example 1. Figure 9 is a cross-sectional view illustrating another configuration of a GaN type semiconductor laser employed in Comparative Example 1. Figures 10A to 10C are graphs illustrating the operational characteristics of a GaN semiconductor laser employed in Comparative Example 1. Figure 11 is a cross-sectional view illustrating a configuration of a GaN type semiconductor laser employed in Example 3 of the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the experiments and studies conducted by the inventors which led to accomplishing the present invention will be described prior to the descriptions of specific examples of the invention. An optical disk for the high-quality TV standard HDTV is required to have a recording capacity of at least about 10 GB or more even when the data is compressed at the maximum rate. This is because of the increased amount of information to be recorded corresponding to the increased number of pixels, which in turn is necessitated by the need for higher quality of images. Figure 1 illustrates the relationship between the wavelength of a light source and the recording density (recording capacity) of an optical disk. Specifically, the change in the recording density (recording capacity) responsive to the change in the wavelength of the light source is described as a ratio (capacity ratio) based on the recording density (recording capacity) of a conventional DVD in the case of employing a red semiconductor laser (having a wavelength of about 650 nm) as a light source, which was defined as 1. Figure 1 shows that it would be technically possible to increase the recording density to about 2.2 times that of the current DVD or higher (e.g., 10 GB) by selecting a wavelength of 435 nm or lower. The experiments and studies by the inventors also indicated that a semiconductor laser for use in such a HD (high definition)-DVD apparatus is required not only to be capable of generating a light beam with a small radius but also to have a RIN level which is equal to or lower than a predetermined level. This is because the sensitivity of a Si detector lowers in the blue region against the broad range of HDTV signals. Specifically, the inventors discovered through experiments and studies that the jitter must be reduced to less than 7%, which in turn requires a RIN level of -135 dB/Hz or less. The jitter will increase when the RIN level requirement is not met, thereby preventing sufficient signal reproduction. Figure 2 is a graph illustrating the relationship. between the wavelength of a semiconductor laser and its typical RIN level, as obtained through experiments and studies by the inventors. As seen from Figure 2 , the RIN level of a usual Fabry-Pérot semiconductor laser increases as the wavelength becomes smaller. For example, the RIN level at a light source wavelength of 400 nm is about -120 dB/Hz, far from meeting the above-mentioned RIN level requirement. Thus, the experiments and studies by the inventors have shown that achieving high-density recording for an optical disk apparatus such as a DVD apparatus requires more than selecting a short-wavelength semiconductor laser as a light source in order to attain recording/reproduction at a sufficiently high quality. Therefore, the present invention not only employs a short-wavelength semiconductor laser as a light source but also achieves wavelength locking of the short-wavelength semiconductor laser by optical feedback, thereby attaining a low noise level, as shown in Figure 2 . Specifically, a RIN level of about -140 dB/Hz is attained by the present invention, which is significantly lower than the above-mentioned RIN level requirement. Such low noise is a result of stabilizing the longitudinal mode by optical feedback, thereby reducing the generation of mode hop noise and/or mode competitive noise. Furthermore, the present inventors have confirmed that the wavelength locking by optical feedback also provides other advantages as set forth below. An optical lens typically employed as an element constituting a convergence lens in a conventional configuration is susceptible to a shift in the focal point between a recording operation and a reproduction operation. This is due to the change in refractive index that is experienced by the light, which in turn is due to the slight change in the wavelength of the light source. Such shift in the focal point is about 0.4 µm in the recording/reproduction in the red wavelength region and therefore is negligible in practice use. However, the experiments and studies of the inventors revealed that since the refractive index of glass has a larger wavelength dependency in the blue wavelength region, the focal point greatly shifts when switching operation from recording to reproduction, thereby preventing sufficient recording/reproduction in the blue wavelength region. The above point will be explained with respect to an exemplary optical lens BaCD6 (manufactured by HOYA Corporation), which is a common choice in the field. Figure 3 is a graph illustrating the shift amount of the focal point across the wavelengths of the light source responsive to a change in the light source wavelength of 2 nm between recording and reproduction, obtained by the present inventors. As shown by the graph, the refractive index (i.e., amount of change in the focal point) has small wavelength dependency in the red wavelength region such that only a 0.4 µm shift of the focal point results responsive to the 2 nm shift in the light source wavelength, as stated above. On the other hand, in the blue wavelength region, the refractive index (i.e., amount of change in the focal point) has large wavelength dependency, so that a substantially threefold shift in the focal point results, i.e., 1.2 µm. Thus, the desired signal reproduction operation becomes difficult to attain. The experiments and studies of the inventors revealed a similar phenomenon in other glass materials which are commonly employed for a convergence lens in the red wavelength region. Thus, it has been confirmed that the optical glass materials typical for the red wavelength region may not be appropriate choices for the blue wavelength region as they are. On the other hand, according to the present invention, a short-wavelength semiconductor laser is wavelength locked by optical feedback, thereby reducing the variation of the oscillation wavelength of the semiconductor laser used as the light source to a lower level. Thus, it has been confirmed that, according to the present invention, a typical conventional optical glass material for the red wavelength region, refractive index of which has large wavelength dependency, can also be employed for the blue wavelength region while reducing the shift amount in the focal point responsive to changes in the refractive index because the shift in the light source wavelength is in itself minimized. Hereinafter, several examples of the present invention achieved through the above discussion will be described with reference to the accompanying figures. (Comparative Example 1) An optical pickup and an optical disk apparatus 100 according to Comparative Example 1 will be described with reference to Figure 4 . The optical disk apparatus 100 at least includes a semiconductor laser 50 , an optical system including a lens, an optical disk medium 55 , and an optical detector 57 . A GaN type semiconductor laser 50 is used as a semiconductor laser 50 . The semiconductor laser 50 has a DBR (distributed Bragg reflector) structure with grating formed in its interior, and is subject to optical feedback for wavelength locking. The semiconductor laser has an oscillation wavelength of 420 nm for 2.4 times density as compared with that of conventional DVDs. Thus, the optical disk apparatus is capable of reproducing HDTV at a transfer rate of 12 Mpbs from 11.5 GB of information recorded on an optical disk medium (having a 12 cm diameter), Next, the operation of the optical pickup and the optical disk apparatus 100 is described. Laser light emitted from the GaN semiconductor laser 50 is collimated through a collimation lens 51 , led through a polarization beam splitter 58 and a 1/4 wavelength plate 59 , and converged by a focus lens 52 so as to be radiated on pits 56 formed in an optical disk medium 55 . The signal light from the optical disk medium 55 is collimated by the focus lens 52 , and has its polarization direction turned by the 1/4 wavelength plate 59 by 90° relative to its polarization direction before being returned from the optical disk medium 55 . As a result, the signal light is reflected from the polarization beam splitter 58 so as to be converged on the optical detector 57 by the focus lens 53 . The focus lens 52 for converging light onto the optical disk medium 55 has a numerical aperture (NA) of 0.6. The pits 56 formed in the optical disk medium 55 have a length of 0.26 µm and a track pitch of 0.49 µm. While an optical pickup incorporating a red semiconductor laser can only read pits that are 0.4 µm long or more, the optical pickup according to the present invention can read pits 56 that are 0.26 µm long. The experiments by the present inventors have shown that excellent HDTV images free of noise can be reproduced. This is in stark contrast with a conventional Fabry-Pérot type semiconductor laser which is not wavelength locked; an optical pickup incorporating such a laser is incapable of signal reproduction due to its high noise level. Figure 5 is a cross-sectional view of the semiconductor laser 50 taken at line 5-5 in Figure 6 (the cross section being parallel to the cavity end faces). Figure 6 is a cross-sectional view of the semiconductor laser 50 taken at line 6-6 in Figures 5 and 7 (the cross section being parallel to the cavity length direction). Figure 7 is a plan view of the semiconductor laser 50 . A substrate 1 is an n-SiC substrate. SiC is a suitable material for the substrate 1 of the GaN type semiconductor laser 50 because it has a similar lattice constant to those of GaN and AlGaN (lattice constant difference: about 3.4%) and allows formation of oscillation end faces through cleavage, with appropriate electrical conductivity and good thermal conduction. Sapphire (lattice constant difference: about 13.8%), which has a large difference in lattice constant from GaN or AlGaN, is also frequently employed as the material for the substrate 1 of the GaN type semiconductor laser 50 . However, since sapphire has poor electrical conductivity, an n-electrode 10 (see Figure 5 ) must be provided on an n-GaN layer 2 (see Figure 5 ), as shown in Figure 9 . Figure 9 is a cross-sectional view of a semiconductor laser 60 incorporating such a sapphire substrate, the cross section taken in parallel to the cavity end faces. In Figure 9 , the constituent elements which also appear in the semiconductor laser 50 illustrated in Figures 5 to 7 are indicated by the same reference numerals as used therein, with their descriptions omitted. Other materials for the substrate 1 include Si, GaAs, ZnO, LiAlO 2 , LiGaO 2 , MgAI 2 O 4 , and the like. A p-type substrate can be employed instead of an n-type substrate. It will be appreciated that, when using a p-type substrate, the conductivity types (i.e., p or n) of the respective layers in the multilayer structure formed on such a substrate must be reversed from their exemplary conductivity types in the following description. As shown in Figure 5 , on an n-SiC substrate 1 , the following layers are formed in this order: an n-GaN layer 2 (Si doped, having a carrier density of 1×10 18 cm and a thickness of 3 µm); an n-AlGaN cladding layer 3 (Al content 10%, Si doped, having a carrier density of 1×10 18 cm -3 and a thickness of 0.5 µm); an n-GaN optical waveguide layer 4 (Si doped, having a carrier density of 1×10 18 cm -3 and a thickness of 0.1 µm); an InGaN multiple quantum well (MQW) layer 5 (including 7 alternating layers of 4 nm thick In 0.02 Ga 0.98 N barrier layers and 3 nm thick In 0.15 Ga 0.85 N well layers; a p-GaN optical waveguide layer 6 (Mg doped, having a carrier density of 1×10 18 cm -3 and a thickness of 0.1 µm); a p-AlGaN cladding layer 7 (Al content 10%, Mg doped, having a carrier density of 1×10 18 cm -3 and a thickness of 0.5 µm); and a p-GaN layer 8 (Mg doped, having a carrier density of 1×10 19 cm -3 and a thickness of 0.3 µm). A buffer layer (not shown) is provided at the interface between the substrate 1 and the n-GaN layer 2 . An AlN layer (thickness: 20 nm) is employed as the buffer layer in the case where the substrate 1 is a SiC substrate, and a GaN layer (thickness: 30 nm) in the case where the substrate 1 is a sapphire substrate, thereby minimizing the lattice constant difference between the n-GaN layer 2 and the substrate 1 so as to obtain an excellent crystal with a low defect density. A mesa ridge is formed from the upper face of the p-GaN layer 8 to about the middle of the p-AlGaN layer 7 . The width of the mesa ridge varies along the cavity length direction as shown in Figure 7 , thereby forming a tapered beam shaping portion. In the narrowest portion (the left end in Figure 7 ) of the mesa ridge adjoining the cavity end face, the bottom width of the mesa ridge is 2 µm, and it gradually increases into the interior of the crystal (oscillator) to a maximum value of 5 µm. As a result, the horizontal spread angle of the laser light emitted from the end face (adjoining the narrow portion of the mesa ridge) becomes relatively broad (about 20°), so that a laser beam which is substantially circular with respect to the vertical spread angle (about 23°) is obtained. With reference to Figure 6 , the cavity length of the semiconductor laser 100 is typically 1 mm. In a region 100a which accounts for 0.6 mm, an MQW active layer 5 is provided. Neither the active layer 5 nor the upper and lower optical waveguide layers 4 and 6 are formed in the remaining region 100b; instead, a GaN layer 12 including a diffraction grating (distributed Bragg reflector: DBR) 13 with a period of 158 nm is provided in the region 100b . The GaN layer 12 does not absorb laser light with a wavelength of 410 nm, and the semiconductor laser 50 oscillates in a single longitudinal mode with the oscillation spectrum selected by the DBR 13 . A Si film 14 (thickness: 500 nm) is provided on the end face adjacent the DBR 13 so as to absorb the light component (accounting for about 10%) that was not reflected by the DBR 13 , thereby preventing the Fabry-Pérot cavity mode. On the other hand, four layers of alternating SiO 2 films 15 (thickness: 71 nm) and TiO 2 films 16 (thickness: 47 nm) are provided on the end face adjacent the MQW so as to increase the reflectance to 76%. As a result, instability in the laser spectrum due to returned light can be prevented, whereby low noise characteristics can be realized. Figures 8A-1 to 8F-1 and Figures 8A-2 to 8F-2 illustrate the manufacturing steps of the semiconductor laser 50 according to the present invention. Figures 8A- 1 to 8F-1 are cross-sectional views in parallel with the cavity end faces of the semiconductor laser 50 ; Figures 8A-2 to 8F-2 are cross-sectional views along the cavity length direction of the semiconductor laser 50 . First, the respective layers from the n-GaN layer 2 to the p-GaN layer 6 are formed on the n-SiC substrate 1 by MOVPE (metal organic vapor phase epitaxy), with an AlN buffer layer (not shown) interposed therebetween (Figures 8A-1 to 8A-2 ). During the growth process, TMG, TMI, TMA, and NH 3 are used as a Ga source, an In source, an Al source, and a N source, respectively; and H 2 or N 2 is used as a carrier gas to thermally decompose each material gas at a high temperature in the range of about 800°C to about 1100°C. thereby growing the respective layers on the substrate. In addition, SiH 4 or Cp 2 Mg is flowed to impart the layers with the p-type conductivity or the n-type conductivity. Next, after the SiO 2 layer 17 is deposited by CVD (chemical vapor deposition), a mask pattern is formed by photolithography and etching, and the p-GaN layers 6 to the n-GaN layer 4 are processed so as to leave predetermined patterns (Figures 8B-1 and 8B-2 ). In the case where milling technique is employed, a mask pattern of a resist layer is employed instead of the SiO 2 layer 17 . Next, the GaN layer 12 is grown by again using MOVPE. It should be noted that the GaN layer 12 does not grow in portions covered with the SiO 2 layer 17 . Since the GaN layer 12 is not doped with any impurities, the GaN layer 12 acquires high resistance so as to ensure that a current does not flow in the DBR region. After the crystal growth, a diffraction grating 13 (with a pitch of 158 nm) is formed on the GaN layer 12 by laser light interference exposition and etching (Figures 8C-1 and 8C-2 ). After removing the SiO 2 layer 17 , the p-type layers 7 and 8 are grown by using MOVPE (Figures 8D-1 and to 8D-2 ). Next, a mesa ridge is formed from the upper face of the layer 8 to about the middle of the layer 7 by photolithography and etching (Figures 8E-1 and 8E-2 ). Furthermore, Ni/Au is deposited on the mesa ridge so as to form the p-electrode 9 , and Ti/Au is deposited on the lower face of the substrate 1 so as to form the n-electrode 10 . As a result, the semiconductor laser 50 is formed. Figures 10A to 10C show the operational characteristics of the semiconductor laser 50 . Specifically, Figure 10A shows the current-optical output characteristics; Figure 10B shows the beam spreading distribution; and Figure 10C shows the oscillation spectrum. It will be seen from these figures that the semiconductor laser 50 has a threshold current of 45 mA with a beam aspect ratio (defined as the vertical spread angle/the horizontal spread angle) of 1.15, indicative of a single longitudinal mode oscillation. Since the aspect ratio is very close to 1, light can be radiated at an efficiency of 40% from a laser with an output level of 2 mW onto the surface of an optical disk medium, without even using a beam shaping element. As a result, the size and cost of the optical pickup can be minimized. Since the reflectance at the front end face of the cavity is increased while ensuring that the unreflected light at the rear end face of the cavity is absorbed, the laser can stably operate with low noise even under the condition of the 4% returned light. As described above, the optical pickup according to Comparative Example 1 is capable of reproducing/recording data in pits provided at higher densities than conventional pit densities. As a result, an optical disk apparatus capable of high-density reproduction/recording is realized. By employing a DBR semiconductor laser as the light source semiconductor laser 50, the noise level can be reduced to a RIN level of about -140 dB/Hz. By forming a tapered portion at the outgoing end face of the semiconductor laser 50 so as to bring the beam aspect ratio to the proximity of 1, the effective transmission ratio can be greatly improved. As a result, a low-output laser can be used as the semiconductor laser 50, thereby providing the device with a prolonged lifetime. Specifically, the conventional art techniques employ a 5 mW semiconductor laser, for example, with the peripheral portions of the emitted beam being cut off. In contrast, the present example requires the semiconductor laser 50 to have an output power of only 2 mW, implying a threefold increase in lifetime. According to the present example, an absorption film is provided on the rear end face of the DBR semiconductor laser 50 to prevent the occurrence of returned light. Alternatively, an absorption layer can be provided in the vicinity of the rear end face of the cavity (e.g., an upper portion in the figure). While the semiconductor laser 50 in the above example has an oscillation wavelength of about 420 nm, an oscillation at 380 nm is possible with a different composition of the material of the semiconductor laser 50 . A resultant optical disk apparatus would provide for a recording density 2.9 times as compared with that of a conventional red DVD, resulting in a disk capacity of about 13.8 GB. A disk capacity of about 20 GB becomes possible by further increasing the NA of the lens to 0.7. In such cases, a 2 hour HDTV program can be recorded/reproduced on a single disk at a transfer rate of 20 Mbps. A disk capacity of about 40 GB can be provided by a disk with a two layer structure. It is also applicable to induce optical feedback in the semiconductor laser 50 by employing a DFB (distributed feedback) structure, instead of a DBR structure as set forth above. However, a DBR laser is more preferable than a DFB laser in terms of production yield. According to the present example, the oscillation wavelength of the semiconductor laser 50 has a very small variation owing to the locking of the oscillation wavelength due to optical feedback. This substantially eliminates the constraints on the lens material (optical glass) due to wavelength variation. As a result, it becomes possible to employ high refractive index materials such as fluorine type glass material, thereby facilitating the realization of a lens having a high NA value. Furthermore, the oscillation wavelength locking configuration of the present example also makes it possible to employ a diffraction type lens, etc., which was not available to the conventional art techniques because of the variation in the focal point responsive to variation in the oscillation wavelength. (Comparative Example 2) Next, an embodiment will be described where recording is performed for an optical disk medium by employing an optical pickup incorporating the semiconductor laser according to the present invention as a light source. The fundamental structure of the optical pickup of the present example is similar to that described in Comparative Example 1. Specifically, a GaN type semiconductor laser having an oscillation wavelength of 400 nm and a beam aspect ratio of 1.5 is employed. However, in the present example, optical feedback is attained by means of an external grating. The present example also attains an aspect ratio which is very close to 1. As a result, light can be radiated at a 40% efficiency from a 2 mW laser onto the surface of an optical disk medium, without even using a beam shaping element. As a result, excellent recording can be performed on the surface of an optical disk medium with a recording power of about 8 mW. In contrast, an optical pickup incorporating a conventional semiconductor laser having an aspect ratio of 3 would only provide a light utility efficiency of 20%, so that a laser output of about 40 mW would be required to achieve a similar recording performance; this would reduce the lifetime of the apparatus and therefore is impractical. Moreover, according to the present example, RF superimposition at 600 MHz is performed for the semiconductor laser. Although this brings down the RIN level to about -137 dB/MHz, it also provides for increased tolerance against returned light. Such an RF superimposition technique is effective because it neither diversifies the oscillation mode nor substantially degrades the noise characteristics. In accordance with the optical disk apparatus of the present example, 12.5 GB of data can be recorded on an optical disk medium. Also according to the present example, the oscillation wavelength of the semiconductor laser 50 has a very small variation owing to the locking of the oscillation wavelength due to optical feedback. This substantially eliminates the constraints on the lens material (optical glass) due to wavelength variation. It also becomes possible to employ a diffraction type lens, etc., which was not available to the conventional art techniques because of the variation in the focal point responsive to variation in the oscillation wavelength. (Example 3) Polycarbonate is generally employed as a protection layer on an optical disk medium due to its low cost and high reliability. However, the birefringence of polycarbonate induces wavelength dispersion. In this connection, the present inventors performed experiments and studies to discover that the birefringence or wavelength dispersion of polycarbonate induce substantial problems in the blue wavelength region. Specifically, a polycarbonate protection layer on an optical disk medium may act in a similar manner to a 1/4 wavelength plate, especially in the wavelength region of 435 nm or below. It has been confirmed that the magnitude of such an action varies from the inner periphery to the outer periphery of the optical disk medium and becomes particularly problematic in the case of employing a polarization beam splitter or a polarization hologram device. The aforementioned problems were not recognized under the conventional art. In order to overcome the aforementioned problems, the present example controls polarization of a laser light emitted from a semiconductor laser 70 (corresponding to the semiconductor laser 50 in Example 1) in an optical pickup structure essentially similar to that shown in Figure 4 . Figure 11 is a schematic cross-sectional view illustrating a configuration of the semiconductor laser 70 (GaN type semiconductor laser) employed in the present example. In an active layer 30 of a GaN laser chip 34 in the illustrative structure, a DBR structure is formed (defining a DBR section 32 ) in the vicinity of end face of the laser chip 34 opposite the end face from which laser light P goes out. The remaining portion of the active layer 30 defines an active section 31 . Furthermore, after being bonded to a Si mount 33 , a pressure is applied to the semiconductor laser chip 34 on the side of the active section 31 via its substrate (not shown) or opposite the Si mount 33 . By varying the applied pressure, the polarization direction of the laser light P from the semiconductor laser chip 34 is rotated. In the present example, a piezo element 35 is used for the application of pressure. With a pressure of 400 g/cm 2 applied to the semiconductor laser chip 34 in the above structure, the polarization direction of the laser light P from the semiconductor laser chip 34 is rotated by about 60°. By thus controlling the polarization of the laser light P , it becomes possible to maintain substantially a constant amount of signal light being reflected from an optical disk medium and entering an optical detector. Specifically, the light amount of the signal light is monitored, and the monitoring results are fed back to the voltage applied to the piezo element 35 so as to vary the polarization direction of the laser light P . Consequently, the amount of incident signal light is kept constant. The apparatus according to the present example realizes excellent reproduction operation. Without performing such control for the signal light, the incident light amount would decrease, thereby making it difficult to correctly reproduce signals from the optical disk medium. It is preferable to restrain the application of pressure onto the semiconductor laser chip 34 (with the piezo element 35 , etc.) within the vicinity of the active section 31 because any pressure applied on the DBR section 32 might induce undesirable wavelength variation. Instead of the piezo element 35 , a torque motor or the like may also be used for the application of pressure.

Claims: An optical pickup comprising: a laser light source (70); an optical system (51, 52, 53, 58, 59) providing light emitted from the laser light source (70) with a prescribed optical path; an optical detector (57) for detecting light from the optical system (51, 52, 53, 58, 59); and a first control member (35) for controlling a light amount of an incident light onto the optical detector (57), wherein the first control member (35) controls the light amount of the incident light onto the optical detector (57) by controlling a polarization direction of the incident light, the first control member (35) changes a polarization direction of a laser light emitted from the laser light source (70), thereby controlling the polarization direction of the incident light onto the optical detector (57), characterized in that : the laser light source (70) is a semiconductor laser, and the first control member (35) applies pressure to the semiconductor laser (70), thereby changing the polarization direction of the laser light emitted from the semiconductor laser (70). An optical pickup comprising according to claim 1, wherein the incident light onto the optical detector (57) is a signal light from an optical disk medium (55). An optical pickup according to claim 1 or 2, further comprising: a second control member (61), said second control member (61) being a variable wavelength plate disposed between a polarization optical element (58) and a focus lens (52) included in the optical system. An optical disk apparatus comprising: a laser light source (70); an optical system (51, 52, 53, 58, 59) providing light emitted from the laser light source (70) with a prescribed optical path; an optical detector (57) for detecting light from the optical system (51, 52, 53, 58, 59); and a first control member (35) for controlling a light amount of a signal light incident onto the optical detector (57) by controlling a polarization direction of the signal light; characterized by an optical disk medium (55) having pits with a length of 0.3 µm or less; the first control member (35) controls the light amount of a signal light incident onto the optical detector (57) from the optical disk medium (55); and the laser light source (70) is a semiconductor laser, and the first control member (35) applies pressure to the semiconductor laser (70), thereby changing the polarization direction of the laser light emitted from the semiconductor laser (70). An optical disk apparatus according to claim 4, wherein the first control member (35) changes a polarization direction of a laser light emitted from the semiconductor laser (70), thereby controlling the polarization direction of the signal light. An optical disk apparatus according to claim 4 or 5, further comprising: a second control member (61), said second control member (61) being a variable wavelength plate disposed between a polarizing optical element (58) and a focus lens (52) included in the optical system.

Inventors: KITAOKA YASUO, KUME MASAHIRO, TSUJIMURA AYUMU, YAMAMOTO KAZUHISA, KITAOKA, YASUO, KUME, MASAHIRO, TSUJIMURA, AYUMU, YAMAMOTO, KAZUHISA
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1998
Main classification: G11B7
IPC codes: G11B7, H01S5