JP2008117444A - Optical head and optical information processor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To perform proper operation to a read-only recording medium with respect to an optical head which is compatible with both a read-only recording medium and a recordable recording medium. <P>SOLUTION: Provided is the optical head which is mounted on an optical information processor which has a controller and read/writes data selectively to/from the optical recording medium. The controller outputs a first instruction signal when a recordable optical recording medium is loaded, and outputs a second instruction signal when a read-only optical recording medium is loaded. The optical head is provided with; a light source to emit an optical beam; an optical element for reducing a light amount of the optical beam; an element drive part which drives an optical element according to the kind of the instruction signal, and adjusts a light amount of the optical beam by switching optical characteristics of the optical beam; and an optical system which condenses the optical beam to the optical recording medium, and detects return light from the optical recording medium. The optical head emits the optical beam with a first power based on the first instruction signal, and emits the optical beam by a second power which is smaller than the first power based on the second instruction signal. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an optical head and an apparatus including the optical head. More specifically, the present invention relates to the operation of an optical head when reading data from an optical recording medium using an optical head capable of writing and reading data.

  In recent years, digital versatile discs (DVDs) have attracted attention as large capacity (high density) recording media. The reason is that digital data can be written on a DVD at a recording density about six times that of a compact disc (CD). Digital data is written to or read from a DVD using a laser beam having a wavelength of about 660 nm and an objective lens having a numerical aperture (NA) of about 0.6.

  The amount of data to be recorded tends to increase as the quality of images and the like increases. Therefore, a higher density recording medium is desired. In order to make the recording density higher than that of the DVD, it is necessary to shorten the wavelength of the laser light and increase the NA of the objective lens. Therefore, for example, a Blu-ray Disc (BD) in which data is read and written using a blue semiconductor laser beam having a wavelength of 405 nm and an objective lens having an NA of 0.85 has been put into practical use. The recording capacity of BD is about 5 times the recording capacity of DVD.

  Since the recording capacity of one optical disk is proportional to the number of recording layers, not only an optical disk having a single recording layer (so-called single-layer medium) but also a multi-layer medium having a plurality of recording layers has been developed. For example, a BD having two recording layers has been developed, and has a storage capacity about 10 times that of a single-layer DVD. Note that the output of the blue semiconductor laser emitted to the multilayer medium needs to be larger than the output of the laser emitted to the single-layer medium, but this can be realized as the output of the laser increases in recent years.

  When a blue semiconductor laser is used to read / write data from / to a high-density optical disk such as a BD, the reproduction margin is very strict, and quantum noise of the light source becomes a problem. Thus, Patent Document 1 discloses an optical head that outputs a reproduction signal while suppressing the quantum noise of a semiconductor laser when reading data from a recordable optical disc (recording disc). Furthermore, this optical head prevents the optical disk from being deteriorated, data erased, etc. by suppressing the light amount (power) of light actually radiated to the recording disk.

  FIG. 3 shows a configuration of a conventional optical head. The conventional optical head can write data on the optical disc 107 or read the written data using the GaN-based blue-emitting semiconductor laser light source 101. The power of the laser light emitted from the light source 101 is set in advance to a value that is sufficiently small for quantum noise and necessary and sufficient for data writing. This power is constant regardless of when data is written or read.

  On the optical path between the light source 101 and the beam splitter 103 in the optical head, an intensity filter 102 having an absorption film is provided so as to be able to be taken in and out. The intensity filter 102 is inserted into the optical path when reading data, and is out of the optical path when writing data.

  The operation of the optical head at the time of data reading is as follows. The laser light emitted from the light source 101 first passes through the intensity filter 102. At this time, a part of the laser light is absorbed by the absorption film of the intensity filter 102, so that the amount of light is attenuated. Thereafter, the laser beam is focused on the recording layer of the optical disc 107 through the beam splitter 103, the collimator lens 104, the mirror 105, and the objective lens 106. The presence of the intensity filter 102 on the optical path makes it possible to keep the power relatively low so as not to cause deterioration of the optical disk or data erasure.

  On the other hand, the light reflected by the optical disk 107 reaches the beam splitter 103 through the objective lens 106, the mirror 105, and the collimator lens 104. Then, the light passes through the beam splitter 103 and the multi lens 108 and is condensed on the photodiode 109. The photodiode 109 outputs a reproduction signal of data on the optical disc 107 from the waveform of the reflected light. Since the quantum noise is set sufficiently small in advance, a reproduction signal with relatively good quality can be obtained.

  On the other hand, when writing data, the optical head retracts the intensity filter 102 from the optical path. Then, the light source 101 emits laser light, causes the laser light to reach the optical disc 107 through the same path as during reproduction, and writes data. Since the intensity filter 102 does not exist, the laser beam can be emitted with a power sufficient for writing. However, the optical head needs to read the address information of the optical disc 107 in order to specify the data writing position. Therefore, a strict reproduction margin equivalent to the reproduction margin at the time of data reading described above is required.

Next, Patent Document 2 discloses an optical head that reads / writes data using an intensity filter for each optical disc having one and two recording layers. This optical head emits a laser from a light source with power for a two-layer disk. However, the laser power required to read / write data from / to the single-layer disc is about one half of the laser power required for the dual-layer disc. Therefore, this optical head determines the type of the optical disk according to the number of layers, and inserts an intensity filter when it is determined that a single-layer disk is loaded. This keeps the quantum noise of the light source sufficiently low.
JP 2000-195086 A JP 2004-272949 A

  Each of the above-mentioned patent documents discloses the configuration and operation of an optical head corresponding to a recording disk. Both are described on the assumption that they are recordable discs, and there is no mention of the operation when a read-only disc is loaded. At present, there are many optical disc standards, and optical heads and devices that support as many standards as possible are required. Therefore, it is important that optical heads support not only recordable discs but also read-only discs. is there.

  At this time, it should be noted that the read-only disc and the recordable disc differ greatly in terms of optical characteristics, and the configuration and operation of the optical head must be determined in consideration of the difference.

  For example, a read-only disc has a sufficiently higher reflectance than a recordable disc, so when attempting to read data from a read-only disc using the power when reading data from a recordable disc, laser noise due to the return light will be generated. It becomes extremely large and the quality of the reproduction signal is deteriorated. In particular, in Patent Document 2 described above, since the operation is switched only by the number of recording layers of the optical disc, the strength filter is not inserted when a two-layer read-only disc is loaded. Therefore, there is a possibility that the quality of the reproduction signal is greatly deteriorated due to the laser noise due to the return light, and the data cannot be read out.

  Note that it is unclear whether or not the above-described optical disc apparatus of Patent Document 1 can be realized. According to Patent Document 1, it is necessary to put in and out an intensity filter when switching between a data read operation and a write operation. However, when data is written instantaneously immediately after reading out the address information, the speed at which the intensity filter is taken in and out becomes a problem. For example, in a high-density optical disc such as BD, it is necessary to complete the insertion and removal of the strength filter in about 100 nanoseconds. However, it is extremely difficult to put in and out a mechanical strength filter as mentioned in Patent Document 1.

  An object of the present invention relates to an optical head corresponding to both a read-only recording medium and a recording-type recording medium, and is to perform an appropriate operation on the read-only recording medium.

  The optical head according to the present invention is mounted on an optical information processing apparatus having a control unit, and selectively performs data writing to the optical recording medium and data reading from the optical recording medium. The control unit outputs a first instruction signal when a recordable optical recording medium is loaded, and outputs a second instruction signal when a reproduction-only optical recording medium is loaded. The optical head drives a light source that emits a light beam, an optical element that reduces the amount of light of the light beam, and an instruction signal, and switches the optical characteristics of the light beam. An element drive unit that adjusts the amount of light beam and an optical system that collects the light beam on the optical recording medium and detects reflected light from the optical recording medium. The optical head emits a light beam with a first power based on the first instruction signal, and emits a light beam with a second power smaller than the first power based on the second instruction signal.

  The element driving unit drives the optical element so that the light amount of the light beam is not reduced based on the first instruction signal, and reduces the light amount of the light beam based on the second instruction signal. The optical element may be driven.

  The optical head emits a light beam at the first power to read data from the recordable optical recording medium, and emits a light beam at the second power to read data from the read-only optical recording medium. You may radiate.

  The optical element is provided with a coating for reducing light transmittance, and the element driving unit retracts the optical element from the optical path of the light beam based on the first instruction signal, and The optical element may be moved on the optical path of the light beam based on two instruction signals.

  The optical element includes a liquid crystal, and the element driving unit drives the liquid crystal so that the optical element transmits the light beam based on the first instruction signal, and based on the second instruction signal. The liquid crystal may be driven so that the optical element reduces the amount of the light beam.

  The light source may emit a light beam having a wavelength of 415 nm or less.

  An optical information processing apparatus according to the present invention outputs a first instruction signal when a recordable optical recording medium is loaded, and outputs a second instruction signal when a read-only optical recording medium is loaded; One of the optical heads described above is provided.

  The optical head detects light reflected by the optical recording medium by radiating the light beam with a predetermined power, and the control unit detects the light based on the amount of emitted light and the amount of reflected light. One of the first instruction signal and the second instruction signal may be output.

  The controller may change the second power of the light beam emitted from the optical head according to the number of recording layers of the optical recording medium specified based on the reflected light.

  The control unit may change the second power relatively small as the number of the recording layers is small.

  The controller may change the second power by changing an output power of a light source of the optical head.

  The optical information processing apparatus may adjust the second power to 0.25 milliwatts or more for a read-only recording medium having one recording layer.

  According to the optical head according to the present invention and the optical disc apparatus equipped with the optical head, when the light beam is emitted to the optical recording medium, depending on whether the optical recording medium is a recording type or a read-only optical recording medium, The optical characteristic of the light beam is adjusted by switching the optical characteristics of the light beam using an optical element. The power radiated to the read-only optical recording medium is made smaller than the power radiated to the recording type optical recording medium in consideration of the reflectance according to the type of the optical recording medium.

  Thereby, it is possible to reduce the quantum noise when reading data from the read-only optical recording medium, and to further improve the reproduction signal quality of the optical head. Thus, the reliability of the device can be improved.

  In particular, when a light beam having a wavelength of 415 nm or less is emitted, the reproduction margin is very strict and the influence of the quantum noise of the light source is large. Therefore, the radiation depends on whether it is a recording type optical recording medium or a reproduction-only optical recording medium. Adjusting the power of the emitted light beam can greatly reduce the influence on the quantum noise of the light source.

  Embodiments of an optical head and an optical information processing apparatus including the optical head according to the present invention will be described below with reference to the accompanying drawings.

  FIG. 1 shows a functional block configuration of the optical disc apparatus 20 according to the present embodiment. The optical disk device 20 includes an optical head 30, an optical disk controller 40, and a motor 50. Although FIG. 4 shows the optical disc 60, this is for convenience of explanation and is not a component of the optical disc apparatus 20.

  In this embodiment, it is assumed that the optical disc apparatus 20 is loaded with a read-only optical disc that can only be read and cannot be written, or a recordable optical disc that can write data. Both types of optical discs are so-called Blu-ray discs (BD) in which data is read / written using blue laser light having a wavelength of about 405 nm.

  First, an outline of the operation of the optical disc apparatus 20 will be described. When a read-only disc is loaded, the optical disc apparatus 20 reads data written on the disc. Further, when a recordable disc is loaded, the optical disc apparatus 20 writes data on the optical disc or reads data written on the optical disc. When reading data, the optical disk device 20 changes the power of the laser (light beam) emitted depending on whether the loaded optical disk 60 is a read-only disk or a recordable disk.

  Hereinafter, the overall configuration of the optical disc apparatus 20 and the configuration of the optical head 30 will be described in detail. Thereafter, the operation of the optical disc apparatus 20 will be described to clarify how the power of the light beam emitted from the optical disc apparatus 20 is changed.

  In this specification, the term “power” of a light beam is synonymous with “light quantity” of the light beam. Further, it is assumed that the power of the light beam immediately after being emitted from the optical head 30 and the power of the light beam when reaching the recording layer of the optical disc 60 are substantially the same.

  The optical head 30 emits a light beam to the optical disc 60 to detect the reflected light from the optical disc 60, and outputs a light amount signal corresponding to the detection position of the reflected light and the detected light amount. The light quantity signal is sent to the optical disk controller 40.

  The optical disk controller 40 is a so-called control unit, and controls overall processing related to reading of data from the optical disk 60 by the optical disk device 20 and writing of data to the optical disk 60.

  The optical disk controller 40 includes a signal processing circuit 41, a servo control circuit 42, and a random access memory (RAM) 43. The control operation of the optical disk controller 40 is realized by executing a program stored in the RAM 43 by a signal processing circuit 41 or a servo control circuit 42 as a computer.

  In FIG. 1, the signal processing circuit 41 and the servo control circuit 42 are described as independent circuits, but they may be realized as one central processing unit (CPU). It is only necessary that the functions of the signal processing circuit 41 and the servo control circuit 42 can be realized by the CPU executing the program stored in the RAM 43.

  The signal processing circuit 41 determines whether the optical disc 60 is a recordable disc or a read-only disc according to the light amount signal of the reflected light output from the optical head 30. The servo control circuit 42 transmits an instruction signal corresponding to the type of the loaded optical disk 60 to an actuator 10 (described later) in the optical head 30 according to the determination result by the signal processing circuit 41. The actuator 10 is driven based on the instruction signal.

  When receiving an instruction signal corresponding to the recording disk, the actuator 10 drives the light attenuating element 3 (described later) to retract from the optical path, and does not attenuate the light beam from the light source. As a result, the optical head 30 emits a light beam with a predetermined first power. This first power is, for example, the power required for reading data from a recordable disc. Note that the power required to write data is even stronger.

  On the other hand, when an instruction signal corresponding to a read-only disk is received, the actuator 10 drives the light attenuating element 3 and inserts it on the optical path to attenuate the light beam from the light source. As a result, the light beam is emitted with a second power smaller than the first power.

  Since the read-only disk has a higher reflectance than the recording disk, data can be read out even with a relatively weak second-power light beam. Note that if data is read from the read-only disc with the first power applied to the recordable disc, the quantum noise of the light source increases and the signal quality deteriorates.

  When the power to be radiated to the optical disk 60 is determined, the optical disk device 20 radiates a light beam with the determined power in order to read data from the optical disk 60 or write data to the optical disk 60.

  As a result, the signal processing circuit 41 controls the rotation control signal for controlling the rotation of the motor 50, the power control signal for controlling the laser power of the light source 1, and the light on the optical disc 60 in accordance with the light amount signal output from the optical head 30. A focus error (FE) signal indicating the focused state of the beam, a tracking error (TE) signal indicating the positional relationship between the focal position of the light beam and the track of the optical disc 60, and the like are generated and output. The FE signal and the TE signal are collectively referred to as a servo signal because they are used in servo control for focusing a light beam on a desired recording layer and track of the optical disc 60. The servo control circuit 42 converts the servo signal and the control signal into a signal format suitable for each control target and outputs a drive signal. Control targets are the motor 50, the light source 1, the actuator (not shown) of the objective lens 9, and the like.

  The motor 50 rotates the optical disc 60 at a rotation speed corresponding to the recording speed / reproduction speed based on the drive signal. The actuator of the objective lens 9 adjusts the position of the objective lens 9 based on the drive signal. Thus, the focal point of the light beam emitted to the optical disc 60 is controlled so as not to deviate from the recording layer. The optical disc apparatus 20 may have a traverse motor (not shown) for moving the optical head 30 in the radial direction of the optical disc 60. The servo control circuit 42 described above can also output a control signal for driving the traverse motor.

  In a state where the focal point of the light beam is controlled so as not to deviate from the recording layer, the signal processing circuit 9 outputs a reproduction signal based on the light amount signal. The reproduction signal indicates data written on the optical disc 60. Thereby, reading of data from the optical disc 60 is realized.

  Next, the configuration of the optical head 30 will be described in detail.

  The optical head 30 includes a light source 1, a beam shaping element 2, a light attenuating element 3, a beam splitter 4, a collimator lens 5, a rising mirror 6, a quarter wavelength plate 7, a diffraction lens 8, The objective lens 9, the actuator 10, the detection hologram 11, the detection lens 12, the first photodetector 13, the condenser lens 14, and the second photodetector 15 are provided.

  The light source 1 is composed of a GaN blue semiconductor laser element and outputs a coherent light beam for writing and reading data. It is assumed that the wavelength of the light beam is about 405 nm. However, when a slight error is taken into consideration, the wavelength may be 415 nm or less.

  The beam shaping element 2 shapes the incident light beam into a circle.

  The light attenuating element 3 is a glass plate having a coating for reducing light transmittance, for example, a multi-coat. The light attenuating element 3 will be described in detail later.

  The beam splitter 4 separates the light beam incident from the light source 1 and directs it to the collimator lens 5 and the condenser lens 14. The beam splitter 4 receives the reflected light from the optical disk 60 via the collimator lens 5 and directs it to the detection hologram 11.

  The collimator lens 5 is a lens that converts divergent light emitted from the light source 1 and reflected by the beam splitter 4 into parallel light. The rising mirror 6 reflects incident light and directs it to the optical disc 60. The quarter wave plate 7 converts a linearly polarized light beam into circularly polarized light, or vice versa. The diffractive lens 8 suppresses a change in the focal length of the objective lens 9 due to a change in refractive index when the wavelength changes. The objective lens 9 condenses the light beam and focuses the light beam at a predetermined focal length. The numerical aperture (NA) of the objective lens 9 is set to 0.85. The position of the objective lens 9 can be adjusted by an actuator (not shown) and is focused on the recording layer of the optical disc 60.

  The actuator 10 is coupled to the light attenuating element 3 and has a drive function for changing the position of the light attenuating element 3 in accordance with a drive signal. In this embodiment, the actuator 10 arranges the light attenuating element 3 on the optical path between the beam shaping element 2 and the beam splitter 4 or retracts the light attenuating element 3 from the position on the optical path.

  The detection hologram 11 diffracts an incident light beam and divides it into three light beams including a sub beam in order to obtain a tracking error signal by, for example, a three-beam method. The detection lens 12 condenses the light beam on the first photodetector 13. The first photodetector 13 receives the light reflected by the recording layer of the optical disc and converts the received light beam into an electrical signal.

  The condensing lens 14 condenses the light beam on the second photodetector 15. The second photodetector 15 receives the light beam emitted from the light source 1 and outputs an electrical signal corresponding to the light amount of the received light beam. This electrical signal is used to monitor the amount of light beam.

  Next, the operation of the above-described optical head when mounted on the optical disc apparatus 20 will be described with reference to FIG.

  FIG. 2 shows an operation procedure of the optical disc apparatus 20. First, in step S21, the optical disc apparatus 20 detects the loading of the optical disc 60. For example, when it is detected that a disk loading mechanism (not shown) is operated to open / close the disk tray, the optical disk device 20 detects the loading of the optical disk 60.

  In the next step S22, the optical head 30 emits a light beam and measures the amount of light reflected from the optical disk. In the next step S23, the signal processing circuit 41 of the optical disk controller 40 specifies whether the type of the loaded optical disk 60 is read-only or recording type. Since the reflectivity of the read-only disc and the reflectivity of the recordable disc are different from each other, the signal processing circuit 41 determines the type of the optical disc 60 based on the amount of emitted light and the amount of reflected light from the optical disc 60. Can be identified.

  When the optical disc 60 is read-only, the process proceeds to step S24, and when it is a recording type, the process proceeds to step S26. Here, “the optical disc 60 is a read-only disc” means that the optical disc 60 is a BD-ROM. Further, “the optical disc 60 is a recordable disc” means that the optical disc 60 is a rewritable BD-RE or a BD-R that can only be additionally written. Note that the number of recording layers is one. An example when the number of recording layers is plural will be described later.

  Note that the method of specifying the type of the optical disc 60 is not limited to the method described above. For example, an area for recording the type of the disk is provided in a part of the recording area of the optical disk 60, and information indicating the type of the disk is written in this area before the disk is produced and delivered to the user. Then, when the optical disk 60 is loaded, this information may be read using the optical head 30, and the signal processing circuit 41 may specify the type of media.

  In step S24, the servo control circuit 42 outputs an instruction signal corresponding to the read-only disk to the actuator 10 based on the determination result of the signal processing circuit 41. Based on this instruction signal, the actuator 10 inserts the light attenuating element 3 into the optical path.

  In the next step S25, the optical head 30 emits a light beam to the optical disc 60 to read data and reproduce information. Thereby, the process ends.

  On the other hand, in step S <b> 26, the servo control circuit 42 outputs an instruction signal corresponding to the recording disk to the actuator 10 based on the determination result of the signal processing circuit 41. Based on this instruction signal, the actuator 10 retracts the light attenuating element 3 to a position that does not exist in the optical path.

  In the next step S27, the optical head 30 emits a light beam to the optical disc 60 to write or read data. Thereby, the process ends.

  Here, with reference to FIG. 1 again, a specific operation for the optical disc apparatus 20 to read data from the read-only optical disc 60 will be described. This operation is performed in step S25 described above.

  The light source 1 emits a light beam so that the power of the light beam immediately after being emitted from the optical head 30 is 0.25 milliwatts (mW). This numerical value of “0.25 mW” is a numerical value determined by the present inventors based on various experimental results as the optimum power for reading data from the read-only disk 60.

  Now, the optical transmittance of the optical head 30 with the light attenuating element 3 retracted from the optical path is set to 25%, and the transmittance of the light attenuating element 3 is set to 50% as described later. Then, the optical transmittance of the optical head 30 when the light attenuating element 3 is inserted in the optical path is 12.5%. Therefore, the light source 1 may radiate a light beam with a power of about 2 mW. These numerical values are preferable examples. It may not be exact and other numerical values may be used.

  The linearly polarized light emitted from the light source 1 enters the beam shaping element 2 to be shaped into a circle, and then enters the light attenuating element 3 disposed on the optical path by the actuator 10.

  The light attenuating element 3 attenuates the amount of incident light beam to about 50%. The light that has passed through the light attenuating element 3 enters the beam splitter 4, a part of which is transmitted as it is, and enters the second photodetector 15 through the condenser lens 14.

  The second photodetector 15 converts the received light into an electrical signal corresponding to the amount of light and outputs it. The signal processing circuit 41 receives the electric signal and monitors whether the light amount of the light beam attenuated by about 50% is appropriate for reading data on the read-only disk 60. Then, the light source 1 is controlled via the servo control circuit 42 so that the amount of light is appropriate. Thus, the light beam emitted from the optical head 30 is adjusted to have a power equivalent to 0.25 mW on the optical disk 60.

  The light reflected by the beam splitter 4 enters the collimator lens 5. The divergent light beam is converted into parallel light by the collimator lens 6. The light transmitted through the collimator lens 6 is reflected by the rising mirror 6 and travels in a direction bent 90 degrees from its traveling direction, converted into circularly polarized light by the quarter-wave plate 7, and the diffractive lens 8 having a convex lens action. The light passes through and enters the objective lens 9. Since the diffractive lens 8 is provided in the path, it is possible to suppress a change in the focal length of the objective lens 9 due to a change in refractive index when the wavelength changes. The light beam is condensed on the optical disk 60 by the objective lens 9.

  Next, the light reflected from the optical disk 60 is transmitted through the objective lens 9 and the diffraction lens 8, converted into linearly polarized light orthogonal to the forward path by the quarter wavelength plate 7, reflected by the rising mirror 6, and its traveling direction. Proceed in the direction bent 90 degrees. The light passes through the collimator lens 5, is reflected in the direction of the detection hologram 11 by the beam splitter 4, and enters the detection hologram. The reflected light is diffracted by the detection hologram 11, collected by the detection lens 12, and incident on the first photodetector 13.

  The first photodetector 13 has a plurality of light receiving regions, receives light in each of them, and outputs an electrical signal corresponding to the amount of light. The signal processing circuit 41 receives an electrical signal from each light receiving area, and outputs a focus error signal indicating the focused state of light with respect to the information surface on the optical disc 60 and a tracking error signal indicating the relationship between the track of the optical disc 60 and the focal position. To do. For example, the signal processing circuit 41 obtains a tracking error signal by using phase operation.

  The servo control circuit 42 controls the position of the objective lens 9 in the optical axis direction so that the light beam is always focused on the information surface of the optical disc 60 in a focused state based on the focus error signal. Generate a control signal. Further, the servo control circuit 42 controls the position of the objective lens 9 based on the tracking error signal so that the focal point of the light beam is positioned on a desired track on the optical disc 60. Information recorded on the optical disk 60 is also obtained from the second photodetector 15.

  During the reading of data from the optical disk 60, the light attenuating element 3 is always inserted on the optical path. Since the read-only disk has a very high reflectivity compared to the recordable disk, data can be read even with the power of the light beam described above. By inserting the light attenuating element 3 on the optical path, the return light to the light source 1 can be reduced, so that laser noise can be reduced. Thereby, a reproduction signal with high quality can be obtained.

  The operation when reading data from the recording disk is the same as the operation when reading data from the read-only disk, except that the light attenuating element 3 is controlled to be retracted from the optical path. When the light source 1 outputs a light beam with a power of about 2 mW, the light beam is emitted from the optical head 30 to the optical disc 60 with a power of about 0.5 mW (25%). Note that changes such as obtaining the tracking error signal by the three-beam method using the sub-beams generated by the detection hologram 11 may be appropriately performed.

  Next, the operation when the optical disk 60 having a plurality of recording layers is loaded will be described. The optical disk 60 is a so-called two-layer disk provided with two recording layers, but the following description is similarly applied even when the number of recording layers is three or more.

  It is easy to determine whether there is one or more recording layers. For example, when determining the type of the optical disc 60, the signal processing circuit 41 instructs the servo control circuit 42 to move the focal point of the light beam in a deeper direction of the optical disc 60. Then, the signal processing circuit 41 may count the number of times of focusing (the number of times the signal value of the focus error signal becomes 0) based on the waveform of the focus error signal of the reflected light. Then, the signal processing circuit 41 may determine that there are as many recording layers as that number.

  In recent years, high output of GaN-based blue semiconductor lasers has been actively reported, and those with peak outputs exceeding 50 mW have been developed. When the peak output of the light source exceeds 50 mW, the peak value of the power of the light beam emitted from the objective lens is also obtained in an optical head having a general optical transmittance (that is, a laser output efficiency of about 25%). 50 mW × 25% = 12.5 mW.

  On the other hand, the power of the light beam required to read data from the dual-layer disc is required to be about 0.8 mW or less from the viewpoint of preventing reproduction light deterioration. Therefore, when a blue semiconductor laser that has already been developed is used, it is possible to reproduce a double-layer disc.

  These powers are examples of values inherent to an optical information recording / reproducing system using a laser beam having a wavelength of 405 nm and an objective lens having a numerical aperture (NA) of 0.85 and a corresponding optical disk. In an optical disc system having a short wavelength and a large numerical aperture (NA), the energy density of spots on the optical disc is high. This means that data is recorded / reproduced with a very small power as compared with a system having a wavelength of 780 nm and a numerical aperture (NA) of 0.45 as in the conventional MD system.

  The present inventors set the optimum value of the power of the light beam radiated to the optical disc to 0.5 mW when reading data from the double-layer disc. The reason why the reproduction power of the double-layer disc is about twice that of the single-layer disc is because the transmittance of the layer closer to the objective lens of the double-layer disc (so-called L0 layer) is set to about 50%. is there. The power may be increased by the reciprocal of the transmittance of the L0 layer. In other words, the power may be changed relatively smaller as the number of recording layers is smaller.

  There is no degradation of reproduction light even in a reproduction-only dual-layer disc. Also, there is no need to write data. Therefore, even if the light attenuating element is always inserted in the optical path, the power output from the light source 1 may be about 4 mW. If the light source has a peak output power of 50 mW, the output is less than 1/10 of the light source. Therefore, it can be said that there is no burden on the light source 1.

  As described above, the power of the light beam required for the dual-layer disc is about twice that of the single-layer disc. The optical disk controller 40 of the optical disk apparatus 20 may control the light source 1 with power corresponding to the number of layers while controlling the optical attenuating element 3 to be inserted into the optical path in the case of a read-only disk. As a result, quantum noise can be suppressed even when data is read from a read-only double-layer disc, and the quality of the reproduced signal can be improved. On the other hand, in the case of a recordable disc, the light source 1 may be controlled with power corresponding to the number of layers while controlling the light attenuating element 3 to be retracted from the optical path.

  When writing data on a recordable double-layer disc, it is necessary to make the power of the light beam the strongest. However, since the light attenuating element 3 does not exist on the optical path, the power of the light beam may be slightly increased as compared with the case of the read-only double-layer disc. Since the burden on the light source 1 is still small, the influence on the product life of the light source 1 is small.

  In the description of the present embodiment, it is assumed that the optical disc 60 is a BD that uses laser light having a wavelength of about 405 nm, and the laser element 1 of the optical head 30 outputs laser light having a wavelength of about 405 nm. However, the optical disc 60 may be not only a BD but also a so-called DVD or a CD. Correspondingly, the optical head 30 may be capable of emitting laser light of another wavelength (for example, at least one of wavelengths 660 nm and 780 nm). At this time, whether or not to insert the light attenuating element 3 may be determined depending on whether or not the medium is a reproduction-only medium.

  In the present embodiment, the light attenuating element 3 is inserted on the optical path between the beam shaping element 2 and the beam splitter 4, but the insertion position is an example. The light attenuating element 3 may be inserted between the light source 1 and the beam shaping element 2, or may be inserted at other positions. Furthermore, since the light source 1 generally has a cover glass at a location where light is emitted, the cover glass can be used as the light attenuating element 3. At this time, the actuator 10 may be installed so as to drive the cover glass.

  Further, although the light attenuating element 3 is a glass plate, this is an example. As long as it has the function of the light attenuating element 3 described above, it can be realized by using, for example, an optical element (liquid crystal element) including liquid crystal. At this time, the element for driving the liquid crystal element may be provided with a drive circuit for applying a voltage to the liquid crystal element instead of the actuator 10. When the description of the above embodiment is applied, when a read-only disk is loaded, the drive circuit applies a predetermined drive voltage so that the transmittance of the liquid crystal element is about 50%. On the other hand, when a recording disk is loaded, the drive circuit may keep the transmittance of the liquid crystal element high (for example, about 100%) without applying a drive voltage. Note that a liquid crystal element in which the transmittance is kept low when no voltage is applied and the transmittance is increased when a voltage is applied may be used.

  An optical head according to the present invention and an apparatus equipped with such an optical head are useful when data is read out and reproduced using laser light from an optical recording medium such as a read-only optical disk or when the data is communicated. It is. Specifically, the output burden of the semiconductor laser can be greatly reduced, and the product life of the light source can be extended. In addition, the quality of the reproduction signal can be kept high by significantly reducing laser noise caused by the return light of the laser beam.

It is a figure which shows the structure of the functional block of the optical disk apparatus 20 by embodiment of this invention. 4 is a flowchart showing an operation procedure of the optical disc device 20. It is a figure which shows the structure of the conventional optical head.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Light source 2 Beam shaping element 3 Optical attenuation element 4 Beam splitter 5 Collimator lens 6 Standing mirror 7 1/4 wavelength plate 8 Diffraction lens 9 Objective lens 10 Actuator 11 Detection hologram 12 Detection lens 13 1st photodetector 14 Condensing Lens 15 Second optical detector 20 Optical disk device 30 Optical head 40 Optical disk controller 41 Signal processing circuit 42 Servo control circuit 43 RAM
50 motor 60 optical disk

Claims (12)

An optical head that is mounted on an optical information processing apparatus having a control unit and selectively writes data to an optical recording medium and reads data from the optical recording medium. When an optical recording medium is loaded, a first instruction signal is output. When a reproduction-only optical recording medium is loaded, a second instruction signal is output.
A light source that emits a light beam;
An optical element for reducing the amount of the light beam;
An element driving unit that drives the optical element in accordance with the type of the instruction signal, switches the optical characteristics of the light beam, and adjusts the light amount of the light beam;
An optical system for condensing the light beam on the optical recording medium and detecting reflected light from the optical recording medium, radiating the light beam with a first power based on the first instruction signal, and An optical head that emits a light beam with a second power smaller than the first power based on two instruction signals.   The element driving unit drives the optical element so that the light amount of the light beam is not reduced based on the first instruction signal, and reduces the light amount of the light beam based on the second instruction signal. The optical head according to claim 1, wherein the optical element is driven.   The light beam is emitted at the first power for reading data from the recordable optical recording medium, and the light beam is emitted at the second power for reading data from the read-only optical recording medium. The optical head according to 1. The optical element is provided with a coating for reducing light transmittance,
The element driving unit retracts the optical element from the optical path of the light beam based on the first instruction signal, and moves the optical element to the optical path of the light beam based on the second instruction signal. The optical head according to claim 3. The optical element includes a liquid crystal;
The element driving unit drives the liquid crystal so that the optical element transmits the light beam based on the first instruction signal, and the optical element determines the light amount of the light beam based on the second instruction signal. The optical head according to claim 3, wherein the liquid crystal is driven so as to be reduced.   The optical head according to claim 1, wherein the light source emits a light beam having a wavelength of 415 nm or less. A control unit that outputs a first instruction signal when a recordable optical recording medium is loaded, and that outputs a second instruction signal when a read-only optical recording medium is loaded;
An optical information processing apparatus comprising: the optical head according to claim 1. The optical head detects light reflected by the optical recording medium by emitting the light beam at a predetermined power;
The optical information processing apparatus according to claim 7, wherein the control unit outputs one of the first instruction signal and the second instruction signal based on a light amount of emitted light and a light amount of reflected light.   The control unit may change the second power of the light beam emitted from the optical head according to the number of recording layers of the optical recording medium specified based on the reflected light. The optical information processing apparatus described.   The optical information processing apparatus according to claim 9, wherein the control unit changes the second power relatively small as the number of the recording layers is small.   The optical information processing apparatus according to claim 10, wherein the control unit changes the second power by changing an output power of a light source of the optical head.   The optical information processing apparatus according to claim 11, wherein the second power is adjusted to 0.25 milliwatts or more for a read-only recording medium having one recording layer.

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