JP2009099176A - Optical pickup and optical disk device using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain various kinds of satisfactory signals by reducing a noise component in a signal caused by an interference between stray light incident from one recording layer and return light to a light receiving part from other recording layer used for recording or reproduction to/from an optical disk. <P>SOLUTION: An optical pickup 3 recording and/or reproducing information to/from an optical disk 2 having a plurality of recording layers is provided with: a light source 31; an objective lens 32 converging light in one recording layer of the optical disk 2; an optical path separating means 33 separating the optical path of return light from that of a light beam emitted from the light source 31; a photodetector 35 having a light receiving part 34 receiving return right, and an azimuth rotator 36 arranged between the optical path separating means 33 and the light receiving part 34 and rotating the polarization direction of the incident optical beam. The azimuth rotator 36 reduces coherency of the polarization direction of the light beam reflected by the one recording layer at which the light beam is converged and the light beam made incident at an angle different from the light beam reflected by the other recording layer and reflected by the one recording layer. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an optical pickup used for recording an information signal on an optical disc and reproducing the information signal recorded on the optical disc, and an optical disc apparatus using the optical pickup.

  Conventionally, optical discs such as CDs (Compact Discs) and DVDs (Digital Versatile Discs) have been used as recording media for information signals. Information signals are recorded on this type of optical discs, or information signals recorded on optical discs are reproduced. There is an optical disc device for performing the above-mentioned, and this optical disc device is provided with an optical pickup that is moved in the radial direction of the optical disc and irradiates the optical disc with a light beam.

  This optical pickup generally has a light source, a beam splitter, an objective lens, a light receiving element, etc., and a light beam emitted from the light source passes through the beam splitter and is condensed by the objective lens. A light beam spot is formed on the information recording layer (hereinafter also referred to as “recording layer”). The light beam condensed on the recording layer of the optical disc is reflected and incident on the beam splitter again, the optical path is changed by the beam splitter and incident on the light receiving element.

  There are two types of optical discs: a single-layer type having a single recording layer and a multi-layer type having a plurality of recording layers. As an example of this multi-layer type, there are two recording layers, and when reading / writing signals of the optical pickup, the objective lens is moved up and down to focus the light beam on the recording layer to be read / written. Thus, there is a standard such as a so-called two-layer medium in which the two recording layers are distinguished and read / written. Furthermore, standardization of multilayer media for increasing the recording information capacity per medium by increasing the number of recording layers from two layers is also being studied.

  When recording and / or reproducing information on such a multilayer optical disc, for example, when the light beam is focused on one recording layer for recording or reproduction, the focus of the light beam is reduced. The light beam is reflected by a recording layer that is not suitable, that is, a recording layer different from the recording layer to which information is read and written, and the effect of the reflected light beam (reflected stray light) from such another recording layer is a problem. (Hereinafter, reflected stray light from another recording layer is also referred to as “stray light”, and reflected light from the recording layer on which the light beam is focused is also referred to as “signal light”).

  Part of the stray light as described above reaches the light receiving portion of the photodetector, and the light intensity is converted as a reproduction signal by the photodetector together with the signal light received by the light receiving portion. turn into. In particular, this disturbance is remarkably increased by causing interference between stray light and signal light on the photodetector and forming interference fringes on the light receiving portion. As described above, when information is recorded and / or reproduced on a multi-layered optical disk, there is a problem that the disturbance causes unnecessary fluctuation in the servo signal or increases the error of the reproduction signal. there were.

  In order to reduce the influence of the stray light as described above, for example, a diffractive optical element for directing a portion of the stray light that is irradiated to the photodetector in a direction without the photodetector, or a photodetector of the stray light is irradiated. An optical pickup that includes an optical component having a shielding portion for shielding a portion that does not reach the light detector in the return optical path of the optical system of the optical pickup has also been studied. However, in such an optical pickup, a part of the light beam which is signal light does not reach the photodetector, and this causes a problem of deterioration of the signal light.

JP 2005-63595 A

  An object of the present invention is to prevent interference caused by stray light from another recording layer incident on return light from a recording layer that performs recording or reproduction of an optical disc to a light receiving unit. An object of the present invention is to provide an optical pickup and an optical disc apparatus that can prevent noise components from being generated in a detected signal.

  An optical pickup according to the present invention includes a light source that emits a light beam of a predetermined wavelength in an optical pickup that records and / or reproduces information with respect to an optical disc having at least a plurality of recording layers in the incident direction of the light beam; An objective lens for condensing the light beam emitted from the light source on one recording layer of the optical disk, and an optical path of the return light reflected by the optical disk, disposed between the light source and the objective lens. An optical path separating means for separating the optical path of the emitted light beam, a photodetector having a light receiving portion for receiving the return light separated by the optical path separating means, and between the optical path separating means and the light receiving portion. An optical rotator arranged to rotate the polarization direction of the incident light beam, the optical rotator comprising: a light beam reflected by one recording layer on which the light beam is collected; and another recording In the polarization direction of the light beam incident at an angle different from that of the reflected light beam reflected by the recording layer of the one, a state of reducing the coherence.

  An optical disc apparatus according to the present invention includes an optical pickup that records and / or reproduces information with respect to an optical disc having at least a plurality of recording layers in the incident direction of the light beam, and a rotation driving unit that rotates the optical disc. An optical disk device provided with the above-described optical pickup used in the optical disk device.

In the present invention, the optical rotator provided in the optical path of the light beam reflected by the optical disc and received by the light receiving unit is reflected by the recording layer for recording or reproducing, and the light reflected by another recording layer. rotating the polarization direction of the beam, respectively, these polarization directions emitted from the polarization rotator, by a state for reducing coherence, signals these light beams are detected by the interfering light detector Therefore, it is possible to prevent the noise component from being generated and to realize good recording and / or reproduction.

  Hereinafter, an optical disk apparatus using an optical pickup to which the present invention is applied will be described with reference to the drawings.

  An optical disc apparatus 1 to which the present invention is applied is a recording / reproducing apparatus that records and / or reproduces an information signal with respect to an optical disc or the like in which recording layers are laminated. Specifically, as shown in FIG. 1, the optical disc apparatus 1 includes an optical pickup 3 that records and reproduces information from the optical disc 2, a spindle motor 4 as a driving unit that rotates the optical disc 2, and the optical pickup 3. And a feed motor 5 that moves in the radial direction.

  The optical disk used here is, for example, an optical disk such as CD (Compact Disc), CD-R (Recordable), CD-RW (ReWritable) using a semiconductor laser having an emission wavelength of about 785 nm, or an emission wavelength of about 655 nm. Optical discs such as DVD (Digital Versatile Disc), DVD-R (Recordable), DVD-RW (ReWritable), DVD + RW (ReWritable) using semiconductor lasers, and semiconductor lasers with a shorter emission wavelength of about 405 nm (blue-violet) It is a high-density recording optical disk such as a BD (Blu-ray Disc (registered trademark)) that can be used for high-density recording.

  In the following description, the optical disk used in the optical disk apparatus 1 will be described as an optical disk 2 having two recording layers as a multilayer type as shown in FIG. Thus, the optical disk that records or reproduces the information signal is not limited to this, and may be an optical disk formed by laminating one or a plurality of recording layers in the incident direction of the light beam. Specifically, in the optical disc 2, a protective layer (cover layer) 2a, a recording layer L1, and a recording layer L0 are formed in order from the light beam incident side. Here, the thickness of the protective layer 2a is 75 μm, and the distance between the recording layer L1 and the recording layer L0 is 25 μm.

  In the optical disc apparatus 1, the spindle motor 4 and the feed motor 5 are driven and controlled at a predetermined rotational speed corresponding to the disc type by a servo control unit 9 that is controlled based on a command from a system controller 7 that also serves as disc type discriminating means. The

  The optical pickup 3 irradiates the recording layer of the optical disc with a light beam having a predetermined wavelength from the protective layer side, and detects the reflected light of the light beam on the recording layer. The optical pickup 3 outputs a signal corresponding to each light beam from the detected reflected light.

  The optical disc apparatus 1 generates a focus error signal, a tracking error signal, an RF signal and the like based on a signal output from the optical pickup 3, and demodulates a signal from the preamplifier 14 or a signal from an external computer 17 or the like. A signal modulator / demodulator and error correction code block (hereinafter referred to as a signal modulator / demodulator & ECC block) 15, an interface 16, a D / A / A / D converter 18, and an audio / visual processing unit 19. And an audio / visual signal input / output unit 20.

  The preamplifier 14 generates a focus error signal by an astigmatism method or the like based on an output from the photodetector, generates a tracking error signal by a DPP method or the like, further generates an RF signal, and generates an RF signal. Is output to the signal modulator / demodulator & ECC block 15. Further, the preamplifier 14 outputs a focus error signal and a tracking error signal to the servo control unit 9.

  When recording data on the optical disc 2, the signal modulator / demodulator & ECC block 15 performs LDC-ECC and LDC-ECC on the digital signal input from the interface 16 or the D / A / A / D converter 18, for example. Error correction processing is performed by an error correction method such as BIS, and then modulation processing such as the 1-7PP method is performed. Then, the signal modulator / demodulator & ECC block 15 outputs the modulated data to the laser controller 21. Further, when reproducing each optical disk, the signal modulator / demodulator & ECC block 15 performs demodulation processing based on the RF signal input from the preamplifier 14, further performs error correction processing, and converts the interface 16 or data to D / D. Output to the A, A / D converter 18.

  When data is compressed and recorded, a compression / decompression unit may be provided between the signal modulator / demodulator & ECC block 15 and the interface 16 or the D / A / A / D converter 18. In this case, the data is compressed by a method such as MPEG2 or MPEG4.

  The servo controller 9 receives a focus error signal and a tracking error signal from the preamplifier 14. The servo control unit 9 generates a focus servo signal and a tracking servo signal so that the focus error signal and the tracking error signal become zero, and an objective lens such as a biaxial actuator that drives the objective lens based on these servo signals. Drive control of the drive unit. Further, a synchronization signal or the like is detected from the output from the preamplifier 14, and the spindle motor is servo-controlled by CLV (Constant Linear Velocity), CAV (Constant Angular Velocity), or a combination of these.

  The laser control unit 21 controls the laser light source of the optical pickup 3. In particular, in this specific example, the laser control unit 21 performs control to vary the output power of the laser light source between the recording mode and the reproduction mode.

  The optical disk apparatus 1 described here is described as performing recording and reproduction on an optical disk using a light beam of one wavelength, but the present invention is applied to an apparatus that realizes multiple wavelength compatibility. In this case, a disc type discriminating unit may be provided so that the laser control unit 21 switches the laser light source of the optical pickup 3 in accordance with the type of the optical disc 2 detected by the disc type discriminating unit. That's fine. The disc type discriminating unit 22 provided in such a case can detect a different format of the optical disc 2 by detecting a change in the amount of reflected light from the surface reflectance, the shape and the external difference between the plural types of optical discs.

  The system controller 7 performs recording and reproduction based on address information and table information (TOC) recorded in premastered pits and grooves in the innermost periphery of the optical disc in response to an operation input from the user. The recording position and reproduction position of the optical disc are specified, and each unit is controlled based on the specified position.

  The optical disc apparatus 1 configured as described above rotates the optical disc 2 by the spindle motor 4, drives and controls the feed motor 5 in accordance with a control signal from the servo control unit 9, and controls the optical pickup 3 on the optical disc 2. Information is recorded on and reproduced from the optical disc 2 by moving to a position corresponding to a desired recording track.

  Specifically, when recording / reproducing is performed by the optical disc apparatus 1, the servo control unit 9 rotates the optical disc 2 by CAV, CLV, or a combination thereof. The optical pickup 3 irradiates a light beam from a light source, detects a returning light beam from the optical disc 2 by a photodetector, generates a focus error signal and a tracking error signal, and based on these focus error signal and tracking error signal Then, the objective lens is driven by the objective lens driving mechanism to perform focus servo and tracking servo.

  When recording is performed by the optical disc apparatus 1, a signal from the external computer 17 is input to the signal modulator / demodulator & ECC block 15 through the interface 16. The signal modulator / demodulator & ECC block 15 adds a predetermined error correction code as described above to the digital data input from the interface 16 or the A / D converter 18, and further performs a predetermined modulation process and then a recording signal. Is generated. The laser control unit 21 controls the laser light source of the optical pickup 3 based on the recording signal generated by the signal modulator / demodulator & ECC block 15 and records it on a predetermined optical disk.

  When the information recorded on the optical disc 2 is reproduced by the optical disc apparatus 1, the signal modulator / demodulator & ECC block 15 demodulates the signal detected by the photodetector. If the recording signal demodulated by the signal modulator / demodulator & ECC block 15 is for data storage of a computer, it is output to the external computer 17 via the interface 16. Accordingly, the external computer 17 can operate based on the signal recorded on the optical disc 2. Further, if the recording signal demodulated by the signal modulator / demodulator & ECC block 15 is for audio visual, it is digital-analog converted by the D / A converter 18 and supplied to the audio / visual processing unit 19. Audio visual processing is performed by the audio / visual processing unit 19 and output to an external speaker or monitor (not shown) via the audio / visual signal input / output unit 20.

  Here, the recording / reproducing optical pickup 3 described above will be described in detail.

  As shown in FIG. 2, the optical pickup 3 to which the present invention is applied has a light source 31 that emits a light beam of a predetermined wavelength and the light beam emitted from the light source 31 on a selected recording layer of the optical disc 2. The optical path of the return light (return light beam) reflected by the optical disc 2 is separated from the optical path of the light beam emitted from the light source 31. A polarizing beam splitter 33, an optical detector 35 having a light receiving part 34 for receiving the return light separated by the polarizing beam splitter 33, and a polarizing beam splitter 33 and the light receiving part 34, An optical rotator 36 is provided as optical rotation means for rotating the polarization direction of the incident light beam in accordance with the incident angle.

  The optical pickup 3 diffracts and splits the light beam emitted from the light source 31 into at least three light beams on the optical path between the light source 31 and the polarization beam splitter 33 in order to obtain a tracking error signal or the like. And a collimator lens 39 that converts the divergence angle of the incident light beam divided by the diffraction grating 38 into substantially parallel light.

  The optical pickup 3 adjusts the wavefront curvature of the incident light beam on the optical path between the polarization beam splitter 33 and the objective lens 32, and the spherical surface of the light beam condensed on the optical disk 2 by the objective lens 32. As a spherical aberration correction means for correcting aberration, a beam expander 40, a rising mirror 41 that reflects the light beam emitted from the beam expander 40 and emits it toward the objective lens 32, and a rising mirror 41 And a quarter-wave plate 42 for imparting a quarter-wave phase difference to the light beam that has been raised and reflected by the above-described method.

  Further, the optical pickup 3 includes a condenser lens 43 that condenses the incident light beam on the light receiving unit 34 of the photodetector 35 on the optical path between the polarization beam splitter 33 and the optical rotator 36, and a focus error signal. And a cylinder lens 44 for providing astigmatism to obtain the above.

  The light source 31 is, for example, a semiconductor laser that emits a light beam having a wavelength of about 405 nm. The wavelength of the light beam emitted from the light source 31 is not limited to about 405 nm, but corresponds to an optical disk for recording or reproduction. For example, a light beam having a wavelength of about 650 nm or 780 nm is emitted. You may comprise. In the following, a case where a light beam having a single wavelength is emitted will be described. However, one or a plurality of light source units that emit light beams having two or more types of wavelengths may be provided.

  The diffraction grating 38 has a predetermined diffraction structure, diffracts the incident light beam emitted from the light source 31 to generate a tracking error signal, zero-order light (hereinafter also referred to as “main beam”), and The light is divided into three beams composed of ± first-order diffracted light (hereinafter also referred to as “sub-beam”). With this diffraction grating 38, a tracking error signal can be obtained by a plurality of divided light beams.

  The collimator lens 39 is provided between the diffraction grating 38 and the polarization beam splitter 33, converts the divergence angle of the light beam incident from the diffraction grating 38 side, and emits it as the substantially parallel light to the polarization beam splitter 33 side.

  The polarization beam splitter 33 reflects the outgoing light beam incident as parallel light by the collimator lens 39 and emits the light beam toward the beam expander 40 and the objective lens 32, and is reflected by the recording layer of the optical disc 2. A return light beam (also referred to as “return light beam”) incident through the objective lens 32, the beam expander 40, etc. is transmitted and emitted toward the condenser lens 43 and the light receiving unit 34 side. . Specifically, the polarization beam splitter 33 includes a separation surface 33a on which an optical thin film having a polarization characteristic that transmits the P-polarized component and reflects the S-polarized component is formed, and the S-polarized component of the forward light beam. Is reflected and emitted toward the objective lens 32 side, and the return light beam reflected by the optical disc 2 as the P-polarized component is transmitted and emitted toward the condenser lens 43 side. Further, the polarization beam splitter 33 provided in the optical pickup 3 causes a light beam (signal light and stray light) passing therethrough to enter an optical rotator 36 described later in the same polarization state.

  The beam expander 40 includes, for example, at least a negative lens and a positive lens, and a driving unit that moves either one of the lenses in the optical axis direction, and emits a divergent state or a converged state by changing the divergence angle of the incident light beam. Can be made. The beam expander 40 can correct spherical aberration generated in the light beam focused on the recording layer of the optical disc by adjusting the divergence angle and emitting the light beam. That is, the beam expander 40 can correct a focus position shift due to vibration, impact, temperature change, and the like by a defocus component accompanying a change in divergence angle, and a spherical surface generated between recording layers in a multilayer optical disc. Since the aberration can be corrected, it is possible to collect the spots with the spherical aberration corrected for each recording layer.

  The rising mirror 41 reflects the light beam whose divergence angle has been adjusted by the beam expander 40 and emits the light beam toward the quarter-wave plate 42 in a state aligned with the optical axis of the objective lens 32.

  The quarter-wave plate 42 gives a quarter-wave phase difference to the light beam from the rising mirror 41 and emits it to the objective lens 32 side. Here, the ¼ wavelength plate 42 emits the forward light beam incident in the S-polarized state toward the objective lens 32 in the circularly polarized state, and the return light incident in the circularly polarized state. The beam is emitted toward the beam expander 40 and the polarization beam splitter 33 in the P-polarized state.

  The objective lens 32 focuses the incident light beam on the signal recording surface of the optical disc 2. The objective lens 32 is movably held by an objective lens driving mechanism such as a biaxial actuator (not shown). The objective lens 32 is moved and operated by a biaxial actuator or the like based on the tracking error signal and the focus error signal generated by the return light from the optical disk 2 detected by the photodetector 35, so that the optical disk 2 is moved in two axial directions, ie, in the direction of approaching and separating from 2 and the radial direction of the optical disc 2. The objective lens 32 focuses the light beam so that the incident light beam is always focused on the signal recording surface of the optical disc 2 and forms the focused light beam on the signal recording surface of the optical disc 2. Follow the recorded track.

  The condenser lens 43 is provided between the polarization beam splitter 33 and the cylinder lens 44, converts the divergence angle of the return light beam transmitted by the polarization beam splitter 33, and converts the light beam at a predetermined divergence angle. The light is emitted toward the cylinder lens 44 and the light receiving unit 34 in a state of being focused on the light receiving unit 34 of the photodetector 35.

  The cylinder lens 44 generates astigmatism for detecting the focus error signal by the astigmatism method in the light beam from the condenser lens 43 and emits the astigmatism to the optical rotator 36 side.

  The optical rotator 36 is a light beam incident at a different angle from the light beam reflected by one recording layer on which the light beam is condensed and the light beam reflected by another recording layer and reflected by the one recording layer. Is emitted as a polarization state that reduces coherence. Here, coherence means that two light beams (here, a light beam reflected by one recording layer and a light beam reflected by another recording layer) easily interfere with each other. In other words, it indicates that the polarization planes of the two light beams are aligned or that the angles formed by the polarization planes of the two light beams are close to each other. That is, the polarization state that reduces coherence means that the polarization planes (polarization directions) of the two light beams have an angle of about 90 °, which is a state in which interference is difficult.

  That is, the optical rotator 36 is an element having a property of rotating the polarization plane of the light beam that passes through the element (also referred to as “optical rotation”). The optical rotator 36 is composed of a liquid crystal, a polymer, or an inorganic compound crystal having a spiral structure (helical structure) when viewed on a microscopic scale. Specifically, the optical rotator 36 is made of, for example, crystal, cholesteric liquid crystal, smectic C * phase of liquid crystal, or the like. In general, there are optical rotators which have optical rotatory properties that are similar to those under specific conditions. However, as will be described later, the polarization planes of signal light and stray light are reduced to reduce interference. A phase shifter having an optical rotatory power that is rotated to an excessively large thickness is not realistic from the viewpoint of light utilization efficiency (light loss). In other words, the optical rotation means used in the optical pickup to which the present invention is applied is an optical rotation having sufficient optical rotation so that the difference in the vibration direction (polarization direction) between the stray light and the signal light becomes approximately 90 ° as will be described later. An element, specifically as described above. Further, the optical rotator used in the optical pickup to which the present invention is applied is an optical rotatory element having optical rotatory power with respect to any incident polarization state regardless of specific conditions such as the above-described phase shifter.

  The optical rotator 36 having such a spiral structure changes the optical rotation, that is, the rotation angle of the rotated polarization plane, by changing the direction (incident angle) of the incident light beam. Also, as shown in FIG. 3, recording and reproduction are performed with return lights BL12 and BL13 incident on the optical rotator 36 and reflected by a recording layer for recording and reproduction (hereinafter also referred to as “focus recording layer”). The incident angles γ and δ of the return light BL0 as stray light reflected by a recording layer different from the recording layer (hereinafter also referred to as “other recording layer”) to the optical rotator 36 are different. This is because the wavefront of the light beam from the focus recording layer is condensed by the condenser lens 43 so as to form a spot on the light receiving portion 34, and is a curved wavefront having a curved surface. The wavefront of the light beam from the recording layer is in a state (defocused state) that is not properly focused on the light receiving surface even after the divergence angle is converted by the condensing lens 43, and has a larger curvature than the curved surface described above. This is because the wavefront is close to a plane. 3 indicate the return light (signal light) of the sub beam (± 1st order diffracted light) reflected by the focus recording layer, and γ is the luminous flux of the return light of this sub beam. Is an incident angle of the outer peripheral portion of the light beam, BL0 indicated by a broken line indicates the return light (stray light) of the main beam and the sub beam reflected by another recording layer, and δ indicates the reception of these light beams. The incident angle of the outer peripheral part of the part which injects into the same part as the sub beam reflected in the focus recording layer on the parts 34b and 34c is shown. In FIG. 3, the illustration of the return light of the main beam (0th-order light) reflected by the focus recording layer is omitted, but as with the above-described γ and δ, the luminous flux of the main beam reflected by the focus recording layer is shown. The incident angle is incident on the optical rotator 36 at an angle different from the incident angles of the main beam and the sub beam reflected by another recording layer corresponding to the incident angle.

  Thus, the optical rotator 36 adjusts the polarization direction of the return light beam from the focus recording layer and the polarization direction of the return light beam from the other recording layer in different directions, and emits both light beams. That is, by causing the polarization directions to be orthogonal to each other and emitting the light to the light receiving unit 34 side, it is possible to prevent interference even if these light beams overlap on the light receiving unit 34.

  The light detector 35 includes a light receiving unit 34 having a plurality of light receiving regions that receive the return light from the optical disc 2 of each beam divided by the diffraction grating and can detect various signals. These light beams are received, and various signals such as a tracking error signal and a focus error signal are detected together with the information signal. Here, the tracking error signal is detected by the DPP method, and the focus error signal is detected by the astigmatism method.

  Specifically, as shown in FIG. 4, the light receiving unit 34 a has four divided light receiving regions A, B, C, and D that are divided into two in directions orthogonal to each other, and the light receiving units 34 b and 34 c are respectively The main push-pull signal MPP obtained from the signal intensities A, B, C, and D detected by the light receiving unit 34a that has the light receiving areas E and F and the light receiving areas G and H that receive the main beam return light BL11. = (A + D)-(B + C) and sub push-pull signal SPP1 = E−F, which is obtained from the signal intensities E, F, G, H detected by the light receiving portions 34b, 34c that have received the return beams BL12, BL13 of the sub beams. A so-called DPP tracking error signal can be detected by SPP2 = GH. That is, the tracking error signal DPP is obtained by DPP = MPP− (K1 × SPP1 + K2 × SPP2) (where K1 and K2 are correction coefficients). In order to detect this DPP tracking error signal, the sub-beam spot on the optical disc is shifted from the main beam spot by half the groove pitch.

  Further, the astigmatism focus error signal FE from the signal intensities A, B, C, D detected by the light receiving areas A, B, C, D of the light receiving unit 34a is expressed by the relational expression FE = (A + C)-(B + D ).

  The optical pickup 3 configured as described above moves the objective lens 32 on the basis of the focus error signal and the tracking error signal obtained by the photodetector 35, thereby moving the optical pickup 2 relative to the signal recording surface of the optical disc 2. The objective lens 32 is moved to the in-focus position, the light beam is focused on the signal recording surface of the optical disc 2, and information is recorded on or reproduced from the optical disc 2.

  Here, by providing the optical rotator 36 as described above, the return light (signal light) reflected by the recording layer (focus recording layer) for recording and reproducing and the recording layer (recording layer different from the recording layer for recording and reproducing) ( The coherence with the return light (stray light) reflected by another recording layer) will be described. Prior to the explanation of the principle of reducing coherence, the optical rotator constituting the optical pickup to which the present invention is applied. A problem of the conventional optical pickup that does not include 36 will be described. In the following description, the return light reflected by the recording layer that performs recording and reproduction is referred to as “signal light”, and the return light that becomes stray light reflected by a recording layer different from the recording layer that performs recording and reproduction. This is called “stray light”.

  In the conventional optical pickup having no optical rotator described above, the signal light and the stray light are in the polarization state having the highest coherence. That is, in general, when two light beams overlap to form an interference fringe, the polarization states of the two light beams (ellipticity, major axis direction of elliptically polarized light, polarization direction of linearly polarized light) match. Interference fringe contrast is maximized. Specifically, taking the case where the two light beams are in the state of linearly polarized light with intensity 1 as an example, when the polarization planes coincide, interference fringes whose intensity varies from 0 to 4 according to the phase difference are shown. However, when the planes of polarization are orthogonal, a uniform light beam with an intensity of 2 is obtained. In the conventional optical pickup, the polarization states of the signal light and the stray light coincide with each other, and the interference fringe has the highest contrast.

  On the other hand, as shown in FIG. 5, the wavefront of the signal light B1 from the focus recording layer L1 is refracted on the cylindrical surface with respect to the optical axis of the cylinder lens 44 and the generatrix of the cylinder lens 44. The cutting plane by the plane including the line where the force becomes zero) is a curved wavefront that is substantially curved in the vicinity of the photodetector 35, whereas the cylinder of the wavefront of the stray light B2 from the other recording layer L0. A cut surface formed by a surface including the optical axis and the generatrix of the lens 44 is a wave surface having a curved surface with a large radius of curvature close to a substantially flat surface in the vicinity of the photodetector 35. When the optical path difference between the two light beams is an integral multiple of the wavelength, the light intensity is strengthened to make a bright part, and when the optical path difference is shifted from the integral multiple of the wavelength by a half wavelength, the light intensity is weakened. As a result of matching, a dark portion is generated, and interference fringes are generated as shown in FIG. 6 according to the optical path difference. FIG. 6 shows the interference fringes when astigmatism is applied similarly to the optical pickup 3 described above, and concentric interference fringes are generated when no astigmatism is given. It becomes. FIG. 6 schematically shows a bright part and a dark part, but there is also an intermediate light intensity part between the bright part and the dark part according to a deviation from an integral multiple of the wavelength of the optical path difference. The brightness changes sequentially according to the optical path difference, and as a result, interference fringes are formed.

  In addition, there is a minute change in the distance between the focus recording layer and the other recording layer in accordance with the position in the plane of the optical disc, and the bright and dark state of the interference fringes changes due to this minute change or the like. That is, the bright and dark state sequentially changes between the case where the bright part and the dark part as shown in FIG. 6 are formed and the case where the bright part and the dark part are formed in an inverted manner. As the light and dark states change sequentially, the signal detected by the light receiving unit that receives the spot on which the interference fringes are formed changes.

  In particular, in such a conventional optical pickup, similarly to the optical pickup 3 described above, a detection signal using a sub beam (± first-order diffracted light of a diffraction grating) is used, for example, when a tracking error signal is obtained by a so-called DPP method. In this case, since the light intensity of the sub-beam signal light with a small amount of light and stray light of the main beam are close to each other, such interference becomes a problem. That is, the above-described interference fringes are formed on the light receiving portion for the sub beam, and the optical path length difference between the focus light and the stray light slightly changes due to, for example, a slight change in the interlayer thickness of the rotated optical disc 2. As a result, the bright and dark portions of the interference fringes change, which causes a problem that the detected signal varies.

  The present invention pays attention to such points and changes the polarization states of the signal light and the stray light so as not to coincide with each other using the optical rotator 36 as described above. Here, as described above, the optical rotator is an element having a property (optical rotation) of rotating the polarization plane of the light beam transmitted through the element. An optical rotator is composed of a material having a helical structure when viewed on a microscopic scale, and quartz, cholesteric liquid crystal, a liquid crystal smectic C * phase, and the like are known. When the direction of the incident light beam changes with respect to this spiral structure, the magnitude of the optical rotation changes as shown in FIG. FIG. 7 shows an example of changes in the polarization state when the light beam BA and the light beam BB having the same polarization state (electric field vibration directions PA1, PB1) are incident on the optical rotator 36 as described above. If the optical rotator has a spiral structure around the optical axis O1 in FIG. 7, the light beam BA and the light beam BB have different angles with respect to the optical axis direction O1 of this element, and the electric field vibration direction PA2 after passing through the element. , PB2 are different, that is, the angle α and the angle β with respect to the reference direction O2 are different.

  Next, it will be described that the optical pickup 3 having the optical rotator 36 can reduce the coherence between the signal light and the stray light based on the above-described contents. In the optical pickup 3 to which the present invention is applied, the signal light and the stray light have different wavefront curvatures as described above, and therefore the light rays incident on the optical rotator 36 are different. For this reason, when passing through the optical rotator 36, the polarization states of both the signal light and the stray light are different, and the coherence is reduced as compared with at least the state having the highest coherence like the conventional optical pickup described above. It will be.

  In the optical pickup 3 to which the present invention is applied, as shown in FIG. 3, the angle γ of the peripheral rays of the light beams of the sub-beams BL12 and BL13 reaches the same position on the light receiving portions 34b and 34c of the photodetector 35. Since the angle δ of the stray light beam BL0 is always different, the optical rotator 36 emits the light beams in a state where the polarization planes of the light beams do not coincide with each other. FIG. 8 shows a calculation example of the angle difference between the polarization planes of these rays. In FIG. 8, the horizontal axis indicates the pitch of the liquid crystal material used for the optical rotator 36 (liquid crystal helical pitch (μ)), and the vertical axis indicates the angle difference (degree) between the polarization planes of these rays. . As shown in FIG. 8, by using a liquid crystal material with an appropriate pitch of about 6 μm, the polarization planes of the outgoing signal light and stray light can be made orthogonal to each other, thereby realizing a significant reduction in coherence. To do.

  The optical pickup 3 including the optical rotator 36 as described above has a conventional problem when performing recording and / or reproduction on the optical disc 2 having a multilayer structure and capable of increasing the recording capacity. Alternatively, when unnecessary stray light from another recording layer is incident on each of the light receiving portions 34a, 34b, and 34c that receives the return light (signal light) that is collected and reflected by the focus recording layer that performs reproduction. Problems such as unnecessary fluctuations in the servo signal and increased errors in the reproduction signal are eliminated by reducing the coherence of the signal light and stray light.

  In particular, in the optical pickup 3, the return light (signal light) of the sub beam from the focus recording layer and the return of the main beam from the other recording layers are incident on the light receiving portions 34b and 34c having a high incident intensity and a large interference problem. Coherency with light (stray light) is greatly reduced by increasing the difference in the polarization direction of the emitted light to about 90 degrees corresponding to the difference in incident light angles γ and δ described above, that is, By reducing the interference of the light beams received on the light receiving portions 34b and 34c, noise components are included in various detection signals such as DPP tracking error signals detected by the light beams received by the light receiving portions 34b and 34c. Occurrence is prevented and good various detection signals are obtained to perform good tracking servo and the like, thereby enabling good recording and / or reproduction. The optical pickup 3 can also reduce the influence of sub-beam return light (stray light) from other recording layers incident on the light receiving portions 34b and 34c in the same manner as described above.

Further, the optical pickup 3 solves the problem not only with respect to the effects of the light receiving portions 34b and 34c described above but also with respect to the light receiving portion 34a in which the incident intensity is different and the influence of interference is not so great. The coherence between the main beam return light (signal light) from the focus recording layer and the main beam and sub-beam return light (stray light) from the other recording layers is incident on the light receiving section 34a. By increasing the difference in the polarization direction of the outgoing light in response to the difference in the light beam angle, the light receiving section 34a reduces the interference of the light beam received on the light receiving section 34a. For example, it is possible to prevent noise components from being generated in various detection signals such as tracking error signal, focus error signal, and RF signal, which are detected by the received light beam. It performs better servo, etc. by obtaining various detection signals, to prevent the error of the reproduced signal, thereby enabling good recording and / or reproducing.

  As described above, the optical pickup 3 includes the light beam reflected by the focus recording layer and the optical rotator 36 provided in the optical path of the light beam reflected by the optical disc 2 and received by the light receiving portions 34a, 34b, and 34c. By rotating the polarization direction of the light beam reflected by the other recording layer in accordance with the incident angle, and reducing the coherence of these light beams emitted from the optical rotator 36, the photodetector 35 It prevents the noise component from being generated in the detected signal.

  In the case of the optical pickup 3 according to the present embodiment described above, the difference between the beam angles of the signal light and the stray light near the optical axis is small, and the effect of suppressing interference is mainly the peripheral part (outer peripheral part) of the light beam. ). In this example, the optical rotator 36 is arranged between the photodetector 35 and the cylinder lens 44. However, the place where the optical rotator constituting the optical pickup 3 to which the present invention is applied is arranged. However, the present invention is not limited to this. For example, the detection optical system may be arranged between the condenser lens 43 and the polarization beam splitter 33. In this case, the angle difference between the stray light and the signal light is reduced, and an optical rotator having a larger optical rotatory power than the above-described example is required, but the difference in interference suppression effect between the peripheral part of the light beam and the vicinity of the optical axis is As a result, the interference on the light receiving portions 34a, 34b, 34c of the main beam and the sub beam can be reduced.

  Next, the optical path of the light beam emitted from the light source unit in the optical pickup 3 configured as described above will be described with reference to FIG.

  The light beam emitted from the light source 31 is divided into three beams by the diffraction grating 38, converted into parallel light by the collimator lens 39, and reflected to the beam expander 40 side by the polarization beam splitter 33.

  The light beam reflected by the polarization beam splitter 33 has its divergence angle converted by the beam expander 40, reflected by the rising mirror 41 and raised toward the objective lens 32, and circularly polarized by the quarter wavelength plate 42. In this state, the light is condensed by the objective lens 32 and a spot is formed on the focus recording layer of the optical disc 2. The light beam condensed by the objective lens 32 is converted in divergence angle by the beam expander 40, so that spherical aberration is reduced and an appropriate spot is formed on the focus recording layer of the optical disc 2.

  The light beam condensed on the recording layer of the optical disc 2 (returned light beam) is reflected and is again returned to the polarization beam splitter via the objective lens 32, the quarter wavelength plate 42, the rising mirror 41, and the beam expander 40. 33 is incident. At this time, the returning light beam is incident on the polarization beam splitter 33 in the P-polarized state by way of the quarter-wave plate 42.

  The return light beam incident on the polarization beam splitter 33 is transmitted and guided to the condenser lens 43 side, and the divergence angle is converted by the condenser lens 43 so as to be converged on the light receiving unit 34. Astigmatism for focus detection is given to the lens 44, the polarization direction (polarization plane) is rotated by the optical rotator 36, and the light is condensed and received on the light receiving portions 34a, 34b, 34c.

  At this time, for example, when the light beam is focused on one recording layer L1 as the focus recording layer, as shown in FIG. 4, the main beam focused on the recording layer L1 as the focus recording layer Is condensed on the light receiving part 34a of the light receiving element by the condensing lens 43 to form a spot S11. The one sub beam condensed on the recording layer L1 is condensed on the light receiving part 34b of the light receiving element by the condenser lens 43, and a spot S12 is formed. Similarly, the other sub beam condensed on the recording layer L1 is condensed on the light receiving part 34c of the light receiving element by the condenser lens 43, and a spot S13 is formed.

  At the same time, the main beam and the first and second sub beams reflected by the other recording layer L0 are condensed in the same plane as the light receiving portions 34a, 34b, 34c of the light receiving element by the condenser lens 43, and the focus recording layer As a result, a spot S2 larger than the light beam from the recording layer L1 is formed.

  Here, as described above, the optical pickup 3 includes the optical rotator 36 to change the polarization state of the return light (signal light) from the focus recording layer and the return light (stray light) from other recording layers. A state where coherence is reduced, that is, a state in which the polarization directions of each other are close to each other can be obtained, and interference between signal light and stray light can be reduced. At this time, if the optical rotator 36 is not provided, the return sub-beam (± first-order diffracted light) from the focus recording layer condensed on the sub-beam light-receiving portions 34b and 34c, and the light-receiving portions 34b, 34b, Interference is a problem because the light intensity with the returning light beam from the other recording layer incident on 34c is close, and interference can be prevented.

  An optical pickup 3 to which the present invention is applied includes a light source 31 that emits a light beam of a predetermined wavelength, an objective lens 32 that condenses the light beam emitted from the light source 31 on one recording layer of the optical disc 2, and a light source 31. And a polarizing beam splitter 33 that separates the optical path of the return light reflected by the optical disc 2 from the optical path of the outgoing light beam emitted from the light source 31, and the polarizing beam splitter 33. The light detector 35 having the light receiving portions 34a, 34b, 34c for receiving the return light, and the polarization beam splitter 33 and the light receiving portions 34a, 34b, 34c are arranged to rotate the polarization direction of the incident light beam. The optical rotator 36 is provided with a light beam (signal light) reflected by the focus recording layer on which the light beam is condensed and reflected by the other recording layer. By making the polarization direction of the light beam (stray light) incident at a different angle from the light beam reflected by the recording layer into a state in which the coherence is reduced, the light receiving units 34a, 34b, 34c reflected by the optical disc 2 are used. The optical rotator 36 provided in the optical path of the light beam received at, rotates the polarization direction of the light beam reflected by the focus recording layer and the light beam reflected by the other recording layer according to the incident angle. By making these polarization directions emitted from the optical rotator 36 into a state in which the coherence is reduced, noise components are generated in the signal detected by the photodetector 35 due to interference of these light beams. Therefore, it is possible to obtain a good recording and / or reproduction by obtaining various good signals.

  As described above, the optical pickup 3 to which the present invention is applied has various signals caused by reflected stray light from a recording layer different from the recording layer on which recording or reproduction is performed when recording or reproducing an optical disc having a plurality of recording layers. It is possible to suppress disturbance of (reproduction signal, servo signal, etc.) and perform stable recording or reproduction.

  Furthermore, the optical pickup 3 to which the present invention is applied can be compared with the existing technology in which the portion of the stray light that is irradiated on the photodetector is removed by diffraction, shielding, etc. to prevent interference. Since the return light beam (signal light) is not partially lost, the characteristics of various signals such as playback signals and servo signals can be prevented from deteriorating, and there are no problems such as light loss. To obtain various signals. As described above, the optical pickup 3 has a multi-layered structure and enables good recording and reproduction by reducing the influence of stray light from a recording layer different from the focus recording layer in the optical pickup capable of increasing the recording capacity. .

  Further, the optical disc apparatus 1 to which the present invention is applied is provided with the optical pickup 3 described above, so that it is reflected by other recording layers on the light receiving portions 34a, 34b, 34c that receive the light beam reflected by the focus recording layer. As a result of the incident light beam, the noise component generated in the signal detected by the photodetector 35 can be reduced with a simple configuration such as providing the optical rotator 36 to obtain various good signals. At the same time, it is not necessary to add a conventional element that performs new diffraction, shielding, etc., and it is possible to eliminate a loss of light amount due to the addition of this new diffraction, shielding element, or the like. Furthermore, the optical disc apparatus 1 realizes good recording / reproducing characteristics by lowering the influence of stray light on an optical disc having a multilayer structure and realizing a large recording capacity.

1 is a block circuit diagram showing an optical disc apparatus to which the present invention is applied. It is an optical path diagram explaining an optical system of an optical pickup to which the present invention is applied. It is a figure explaining the incident angle of the stray light and signal light which inject into the optical rotator which comprises an optical pick-up, and is a side view which shows an optical rotator and a light-receiving part. It is a top view which shows each light-receiving area | region of each light-receiving part which comprises an optical pick-up, and the spot formed on each light-receiving part. It is a figure for demonstrating the optical path difference of the return light beam from the focus recording layer and other recording layers condensed on a light-receiving part. It is a figure which shows typically the interference fringe formed by the optical path difference of the return light beam from the focus recording layer and other recording layers condensed on a light-receiving part, and is a top view of the light-receiving part which comprises an optical pick-up is there. It is a figure for demonstrating the difference in the electric field vibration direction of the emitted light when the light ray of a different incident angle injects into the optical rotator which comprises an optical pick-up, and is a perspective view of an optical rotator. It is a figure which shows the change of the angle difference of the polarization plane on signal light, a stray light, and a light-receiving part when changing the helical pitch of an optical rotator.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 Optical disk apparatus, 2 Optical disk, 3 Optical pick-up, 4 Spindle motor, 5 Feed motor, 9 Servo control part, 31 Light source, 32 Objective lens, 33 Polarization beam splitter, 34 Light receiving part, 35 Photo detector, 36 Optical rotator, 38 Diffraction grating, 39 Collimator lens, 40 Beam expander, 41 Start-up mirror, 42 1/4 wavelength plate, 43 Condenser lens, 44 Cylinder lens

Claims (5)

In an optical pickup that records and / or reproduces information with respect to an optical disc having at least a plurality of recording layers in the incident direction of a light beam,
A light source that emits a light beam of a predetermined wavelength;
An objective lens for condensing the light beam emitted from the light source onto one recording layer of the optical disc;
An optical path separating unit that is disposed between the light source and the objective lens and separates the optical path of the return light reflected by the optical disc from the optical path of the light beam emitted from the light source;
A photodetector having a light receiving portion for receiving the return light separated by the optical path separating means;
An optical rotator disposed between the optical path separating means and the light receiving unit and rotating a polarization direction of an incident light beam;
The optical rotator is incident at a different angle from the light beam reflected by the one recording layer on which the light beam is condensed and the light beam reflected by the other recording layer and reflected by the one recording layer. An optical pickup in which the direction of polarization with the light beam is reduced.   The optical pickup according to claim 1, wherein the optical rotator is composed of a liquid crystal having a helical structure.   The optical pickup according to claim 1, wherein the optical rotator is composed of a polymer or an inorganic compound crystal.   The light path separating means is a polarization beam splitter that reflects or transmits the light beam according to a polarization state of an incident light beam, and guides the return light reflected by the optical disc to the light receiving unit side. Optical pickup. In an optical disc apparatus comprising: an optical pickup that records and / or reproduces information with respect to an optical disc having at least a plurality of recording layers in the incident direction of the light beam; and a rotation driving unit that rotates the optical disc.
The optical pickup includes a light source that emits a light beam having a predetermined wavelength,
An objective lens for condensing the light beam emitted from the light source onto one recording layer of the optical disc;
An optical path separating unit that is disposed between the light source and the objective lens and separates the optical path of the return light reflected by the optical disc from the optical path of the light beam emitted from the light source;
A photodetector having a light receiving portion for receiving the return light separated by the optical path separating means;
An optical rotator disposed between the optical path separating means and the light receiving unit and rotating a polarization direction of an incident light beam;
The optical rotator is incident at a different angle from the light beam reflected by the one recording layer on which the light beam is condensed and the light beam reflected by the other recording layer and reflected by the one recording layer. An optical disc apparatus in which the direction of polarization with a light beam is reduced in coherence.

Images