Abstract: An optical disc drive includes an objective lens for focusing light on an optical disc with five or more stacked data storage layers and a tilt control mechanism for controlling a tilt angle defined between the optical axis of the light and a normal to the storage layers. The drive reads and/or writes data from/on a selected one of the storage layers by focusing the light on the selected layer. The numerical aperture NA of the objective lens is defined so as to fall within the range of 1.3009×D3?2.9315×D2+2.3133×D?0.0502 to 1.3009×D3?2.9315×D2+2.3133×D+0.2028, where D is a distance (mm) from a light incident side of the disc to the deepest one of the storage layers, which is located most distant from the light incident side.

Abstract: The present invention relates to an optical pickup and an optical disc drive that records or reproduces data signals on or from optical discs different in specifications, such as recording density and thickness. One objective lens (17) is used for optical discs (22a, 22b) of two types that differ in specifications. The optical pickup and the optical disc drive comprise two light source sections (24a, 24b) and a lens-supporting mechanism (15). The light source sections emit two laser beams of different wavelengths, respectively. The lens-supporting mechanism (15) controls the position of the objective lens (17) in the radial direction of the discs and along the optical axis.

Abstract: A device for canceling a land-groove offset component of a focusing error signal in an optical storage system includes first and second circuits, and an adder. The first circuit processes a radio (or wobble) frequency ripple signal to obtain a first processed signal that is multiplied by a first gain value (G1) so as to result in a first signal. The second circuit processes a tracking error signal to obtain a second processed signal that is multiplied by a second gain value (G2) so as to result in a second signal. The adder is coupled to the first and second circuits, and adds the first and second signals to produce a compensative land-groove offset component that is to be fed into a focusing device of the optical storage system so as to enable the focusing device to cancel out the land-groove offset component of the focusing error signal.

Abstract: To provide an optical pickup device and optical disk device capable of realizing at least one of thickness reduction, size reduction and suppression against characteristic deterioration even where coping with various wavelengths of laser including a blue laser. An optical pickup device comprising light sources for respectively emitting a plurality of different wavelengths of light, unit structured for causing at least a part of the light emitted from the light sources to pass a same optical path; and focusing unit for focusing the light. The focusing unit includes at least first and second focusing parts, the first focusing part being to focus mainly a wavelength of light different from a wavelength of light to be mainly focused by the second focusing part.

Abstract: A coma aberration correcting apparatus of an optical pickup includes an optical pickup main body having a photo diode, and an actuator mounted on an objective lens focusing a beam emitted from the photo diode onto a recording medium. The coma aberration correcting apparatus includes a main supporting unit, a holding unit, an optical system, a driving part, and a controller. The main supporting unit detachably supports the optical pickup main body. The holding unit holds and releases the actuator on the optical pickup main body supported by the main supporting unit. The optical system magnifies and photographs the beam emitted from the photo diode through the objective lens of the actuator held by the holding unit. The driving part adjusts a position of the actuator relative to the optical pickup main body. The controller controls the driving part to correct the coma aberration of the objective lens.

Abstract: A method and a device for adjusting control parameters of a servo system of an optical disk drive. A first sinusoidal signal is introduced into a control loop of the servo system, while a band-pass filter is used to extract a second sinusoidal signal. Gain and/or phase variations of the control loop are derived by comparing signal parameters of both the first and second sinusoidal signals. A compensator is then employed to adjust the derived gain and/or phase variations of the control loop so as to overcome the disadvantage of system instability after a long-term working.

Abstract: There are provided an optical disk device and tilt correction processing method for performing recording/reproduction by rapidly acquiring an optimal tilt correction value for an optical disk having a plurality of recording/reproducing layers. In an optical disk device capable of recording or reproducing information by irradiating a laser beam to each of a plurality of recording/reproducing layers of an optical disk, the tilt value is stepwise set within a predetermined range at a predetermined radius position of the recording/reproducing layer located at the farthest distance from the laser beam irradiation side so that an optimal tilt correction value can be obtained from the quadratic function by the method of least squares of the jitter or the like obtained from the disk. By applying the optimal tilt value obtained to the other recording/reproducing layer, it is possible to start recording or reproduction in a short time.

Abstract: Provided are an apparatus and method for detecting a type of an optical disc inserted into an optical disc system and/or adjusting a track balance in the optical disc system by detecting voltages of a focus error signal and a tracking error signal. For detecting a type of an optical disc inserted into an optical disc system, the apparatus comprising: an analog-to-digital converter that converts a focus error signal into n-bit voltage data; and a duty measurer that compares the n-bit voltage data with a positive noise voltage level and a negative noise voltage level, upcounts by a predetermined value if the n-bit voltage data is higher than the positive noise voltage level or lower than the negative voltage level, and outputs the upcounted result as a duty of the focus error signal.

Abstract: An optical disc drive includes an optical head having a stationary portion and a movable portion. The movable portion supports an objective lens for converging a beam emitted from the stationary portion on an optical disc. The movable portion moves radially across an optical disc in order to irradiate the beam on a desired track of the optical disc. The optical disc drive is further provided with an aberration correcting lens that corrects aberration caused by the objective lens. The aberration correcting lens is mounted on the movable portion so as to be movable in a direction substantially orthogonal to a movable direction of the movable portion, which may parallel or perpendicular to the optical disc. The aberration correcting lens is moved in accordance with a variation of the aberration caused by the objective lens.

Abstract: A light pickup device includes a light projection optical system comprising a light source which emits light, and an objective lens which condenses the light on an optical disk, a light detection optical system comprising a light detector, and a condenser lens which condenses light reflected from the optical disk on the light detector, and an adjustment optical element for coinciding a center axis of an intensity distribution of the light condensed on the optical disk by the light projection optical system with an optical axis of the objective lens.

Abstract: An objective lens in which a first wavelength is used in reading a first optical disk, a second wavelength is used in recording or reading a second optical disk, and a phase shifter provided with annular step portions having a center coincident with an optical axis is formed in a single surface of the objective lens, the phase shifter producing a phase difference with respect to light having the first wavelength and light having the second wavelength, is presented. The objective lens can read well both a CD and a DVD.

Abstract: An optical disc device of the present invention includes spherical aberration changing means for changing spherical aberration occurring on a converging position of a light beam converged by a lens, an actuator for moving the spherical aberration changing means in a relatively precise manner, and an actuator for moving the spherical aberration changing means in a relatively rough manner. The optical disc device drives a second actuator and a third actuator based on a signal of spherical aberration detecting means and performs control so that spherical aberration is almost 0. The third actuator moves the spherical aberration changing means based on a direct current component included in a signal of the spherical aberration detecting means, and the second actuator moves the spherical aberration changing means based on an alternating current component included in the signal of the spherical aberration detecting means.

Abstract: A tilt compensating device and method of performing normal tilt compensation in the event of a servo emergency which may occur during reproduction of data from an optical recording medium are provided. The tilt compensating device includes: an optical pickup; a tilt adjusting unit that adjusts a tilt angle of the optical pickup; a jitter detecting unit that detects an amount of jitter in a reproduction signal output from the optical pickup; and a controlling unit that monitors speed at which the optical recording medium operates using a phase-locked loop (PLL) signal generated from the reproduction signal and the degree of focus of the optical pickup, performs an operation to return to a normal state if any abnormality occurs, and controls operation of the tilt adjusting unit by comparing an amount of jitter detected at regular intervals against a reference.

Abstract: An optical head performs recording and/or reproducing of a signal to an optical recording medium and comprises a light source, an objective lens converging the light emitted from the light source to the optical recording medium, and objective lens tilting mechanism for tilting the objective lens in order to correct aberration generated when the optical recording medium is tilted, and the optical head is assembled so that a direction of the coma aberration of the objective lens itself perpendicularly crosses a direction tilted by the objective lens tilting mechanism.

Abstract: An optical-disc driving apparatus for an optical disc includes a pickup; a detection unit for detecting a traverse signal by differential push-pull (DPP) detection for every radial tilt angle within a first range, the radial tilt angle being measured between the optical disc and the pickup; a determination unit for determining the radial tilt angle corresponding to a traverse signal within a second range including a maximum value among the detected traverse signals; and a control unit for controlling the orientation of the pickup with respect to the optical disc based on the determined radial tilt angle. The optical-disc driving apparatus controls driving of the optical disc based on the traverse signal by DPP detection generated from a preceding beam, a main beam, and a succeeding beam.

Abstract: An optical disk unit of this invention includes a defocus detecting system for detecting a defocus of an objective lens, a thickness unevenness detecting system for detecting a thickness unevenness of a transparent resin layer provided nearest the objective lens of a recording medium, and a thickness unevenness correcting mechanism for changing the focusing characteristic of light impinging upon the objective lens based on a change in the thickness of the transparent resin layer detected by the thickness detecting system.

Abstract: The invention relates to a method and an arrangement for a replay or recording appliance for optical recording media having an inclination control system for vertical alignment of the scanning beam with respect to optical recording media. In order to exclude other factors which, in addition to the inclination of the scanning beam, influence the radio-frequency signal which is detected by the recording medium, without any additional inclination sensors from the disadvantageous influence of the inclination control, an inclination control system is provided which uses the lower envelope curve signal of a radio-frequency signal which is detected by the optical recording medium and represents the dark value of the detected radio-frequency signal.

Abstract: A tilt compensating device and method of performing normal tilt compensation in the event of a servo emergency which may occur during reproduction of data from an optical recording medium are provided. The tilt compensating device includes: an optical pickup; a tilt adjusting unit that adjusts a tilt angle of the optical pickup; a jitter detecting unit that detects an amount of jitter in a reproduction signal output from the optical pickup; and a controlling unit that monitors speed at which the optical recording medium operates using a phase-locked loop (PLL) signal generated from the reproduction signal and the degree of focus of the optical pickup, performs an operation to return to a normal state if any abnormality occurs, and controls operation of the tilt adjusting unit by comparing an amount of jitter detected at regular intervals against a reference.

Abstract: An objective lens actuator is provided with an objective lens holder and a tilting coil that drives the objective lens holder in a tilting direction. The tilting coil is inclined with respect to the horizontal direction, so that a rotation center when the objective lens holder is driven in the tilting direction, is located on extension lines of sides that effectively contribute to driving the objective lens holder in the tilting direction by interaction between a magnetic field formed by a magnet and current that is supplied and flows in the coil among sides of the tilting coil.

Abstract: An optical element feeding device driving method and an optical disk apparatus capable of detecting biting in a stepping motor and conducting restoration. The optical disk apparatus includes an optical element feeding device including a movable frame for supporting a spherical aberration correcting lens to move the lens in optical axis direction, a spring for applying force to the frame in the direction, a sensor for detecting a lens reference position, a stepping motor, and a lead screw and a nut for converting motor rotation into the direction, a motor driving circuit, a decision circuit for judging abnormality when an output of the sensor does not change even if the driving circuit outputs a driving signal to the motor after reference position detection, and a changeover circuit for lowering a driving frequency than in ordinary operation, based on an abnormality signal.

Abstract: In an optical disk apparatus applicable to super thin-sizing thereof, and further a sliding drive mechanism for an optical pickup, having a tilt adjustment mechanism, being suitable to such the optical disk apparatus, the sliding drive mechanism for an optical pickup slides the optical pickup 330 in the radial direction of an optical disk, wherein the optical pickup is attached to be movable on a pair of main and counter sliding shafts 331 and 332, being disposed in parallel with, and in neighboring to the main sliding shaft is disposed a lead screw 336, which is rotationally driven by a stepping motor 337, in parallel, while being engaged with a rack 335 formed in a part of the optical pickup.

Abstract: A method for calibrating tilt control values of a PUH in real time is applied to an optical disk drive and includes the steps of: creating a look-up table, the look-up table including several optical disk data/physical addresses of track zones and the corresponding tilt control values; reading tilt control values as reference values from the look-up table corresponding the track position of a PUH; calculating a current tilt control value according to the reference values; controlling the PUH according to the current tilt control value; reading a decoding error generated during a decoding procedure of the optical disk drive; calibrating the tilt control values and updating the look-up table. When the decoding error is larger than a decoding error threshold, the tilt control values of the track zone is calibrated. Hence, even an irregularly deformed optical disk can be calibrated to obtain better tilt control values.

Abstract: An objective lens drive having an optical axis adjustment function. An objective lens is supported by a lens holder, and the lens holder is supported on a suspension holder by way of a suspension. The suspension holder is supported so as to be rotatable about an axis extending in a direction in which the suspension extends, whereby the suspension holder is supported laterally by means of multilayer piezoelectric elements. When a drive voltage is applied to the multilayer piezoelectric elements, the piezoelectric elements are displaced vertically, whereupon the suspension holder is rotated about the axis, to thus adjust the optical axis of the objective lens.

Abstract: A method for measuring a tilt angle of a pickup head. The method includes the steps of: moving the pickup head to a track to be measured; radiating a laser beam with a constant power; defining a proper range of tilt control values; controlling the tilt angle of the pickup head according to the range of tilt control values, acquiring a RF signal corresponding to each tilt control value, and storing an associated value of a associated signal; detecting the maximum among the stored associated values; selecting and storing the tilt control value corresponding to the maximum as the optimum tilt control value for the track if there is a maximum; and modifying the range of tilt control values and executing the previous steps if there is no maximum. The so-called associated signal includes a tracking error signal, a reading/writing RF signal, an S-curve signal, or the RF signal.

Abstract: A controller includes a servo-controller for performing nonlinear and linear control of the read/write position according to an error signal to drive an actuator for driving the pickup; a switch for switching the servo-controller from open-mode to closed-mode according to the error signal; and an adjuster for adjusting the gains of the nonlinear and linear control so that the nonlinear control component is larger than the linear control component at the time of switching and the linear control component is larger than the nonlinear control component after the elapse of a predetermined time from the time of switching.

Abstract: A focus control process (S2), a spherical aberration correcting process (S4), and a focus offset adjusting process (S7 to S9) are performed in this order. In the focus control process, an output of a focus error signal obtained by detecting focal point displacement that occurs in a direction of an optical axis of a light beam focused through a two-element object lens is controlled so that the output becomes close to zero. In the spherical aberration correcting process, spherical aberration that occurs with respect to the light beam is corrected. In the focus offset adjusting process, offset of the focus error signal is adjusted. In this way, it is possible to provide a focal point adjusting method and an optical pickup device in which focus control can be performed stably by eliminating the offset.

Abstract: An optical disk unit of this invention includes a defocus detecting system for detecting a defocus of an objective lens, a thickness unevenness detecting system for detecting a thickness unevenness of a transparent resin layer provided nearest the objective lens of a recording medium, and a thickness unevenness correcting mechanism for changing the focusing characteristic of light impinging upon the objective lens based on a change in the thickness of the transparent resin layer detected by the thickness detecting system.

Abstract: An optical disk unit of this invention includes a defocus detecting system for detecting a defocus of an objective lens, a thickness unevenness detecting system for detecting a thickness unevenness of a transparent resin layer provided nearest the objective lens of a recording medium, and a thickness unevenness correcting mechanism for changing the focusing characteristic of light impinging upon the objective lens based on a change in the thickness of the transparent resin layer detected by the thickness detecting system.

Abstract: An apparatus and method of estimating the quality of an input signal, and an optical disc driver including the apparatus for estimating the quality of the input signal, the signal quality estimating apparatus including a level value detection unit that detects level values of an input signal according to a binary signal of the input signal, an input signal composing unit that composes a plurality of ideal input signals by using the level values and a plurality of pre-defined binary signals, and a quality calculation unit that obtains a quality of the input signal according to a calculation between the plurality of ideal input signals.

Abstract: An optical disk apparatus having a high reliability by evading collision between a condensing lens and an optical disk regardless whether the focus control is in the operative state or in the inoperative state, thereby preventing damage to the condensing lens and the optical disk. When the focus control is in the inoperative state, by using a reflection light amount from the optical disk (1), it is ascertained that the focus of the light beam is in the vicinity of the information surface and the condensing lens (15) is driven to be apart from the optical disk (1).

Abstract: An optical recording/reproducing apparatus includes an optical pickup recognizing information of an optical disc; a main shaft and a sub-shaft oppositely installed on a base plate, for supporting the optical pickup; a fixing portion for movably supporting one end of the main shaft; a support portion installed on the base plate at a certain distance from the fixing portion, for elastically supporting the other end of the main shaft; a prop portion installed on the base plate, for supporting a bottom middle surface of the main shaft; and a skew adjustment screw rotatably installed at the base plate, for upwardly pushing an end of the main shaft supported by the support portion.

Abstract: The method and a device to measure the repeatable and non-repeatable runouts of a rotating component of a spindle motor with the aid of at least one probe measuring changes in the distance between the probe and the rotating component depending on the angle of rotation. The probe records a first test signal in a first test cycle from which a signal defining the RRO is determined and stored. In a second test cycle, a command signal proportional to the amplitude and phase of the RRO signal is generated and fed to an electromechanical actuator supporting the probe. Depending on the command signal derived from the first test signal, the actuator changes the distance between the probe and the outer perimeter of the rotating component, thus compensating the runout defined by the RRO signal, while the probe records a second test signal defining the NRRO.

Abstract: Test writing is executed while skipping areas where deviations are present on an optical disk to thereby improve accuracy and reliability of an optimum recording condition to be derived. A deviation detecting unit detects the deviations of a guide groove on the optical disk on the basis of a fluctuation amount of a focus error signal or a tracking error signal, and registers information of the detected deviations onto a memory. A test writing processing unit refers to the memory, and records and reproduces a test signal while skipping the areas where the deviations are detected.

Abstract: Disclosed here is a method for solving a conventional problem that no spherical aberration signal can be detected from any push-pull signal that is easy to adjust an object photodetector when a differential push-pull signal is obtained. To achieve the above object, sub-beams used to detect a differential push-pull signal are defocused from the main beam and spherical aberration signals are detected and the compensation of the spherical aberration is started while tracking controlling is off and detection of spherical aberration signals is held while the tracking controlling is on. The present invention can thus obtain both of the spherical aberration signal and the differential push-pull signal at the same time with use of the conventional detection system.

Abstract: An image entering from the left side of the figure passes through a correcting lens unit, and forms an image on an image pickup device on the right side of the figure. The correcting lens unit includes a cylindrical outer frame and a lens arranged therein, and at coupling points, coupled to one end of each of electrostrictive coils and suspended thereby. Electrostrictive coils strain in response to voltages applied from a voltage applying unit, causing displacement of electrostrictive coils. The displacement is transmitted to correcting lens unit through the coupling points, and therefore, in accordance with the displacement from electrostrictive coils, position and/or direction of correcting lens unit changes.

Abstract: A highly accurate and stable tilt servo system is provided. A liquid crystal device for phase correction is driven based on drive data stored in a lookup memory to thereby make a tilt correction. Upon pre-processing, a pickup is moved to a predetermined position on the inner circumference of a disc for a tilt correction to employ tilt error data delivered from a tilt sensor as reference tilt error data and drive data served for the tilt correction as a reference tilt correction quantity. After the pre-processing, when the pickup is moved, the difference between the tilt error data delivered from the tilt sensor at each position of movement and the reference tilt error data is employed as a relative correction quantity. The drive data equivalent to the relative correction quantity is acquired from the lookup memory. The drive data acquired is added to the reference correction quantity to thereby determine a tilt correction quantity.

Abstract: A collimator lens located in a light path from a laser light source to an optical information recording medium. The collimator lens is constituted by a cemented lens which is composed of a biconvex lens and a meniscus lens, and an aspheric plastic lens in the form of a transparent thin synthetic resin film which is laminated on a lens surface of the biconvex lens on the side of or facing toward an optical recording medium. When calculated on the basis of a composite focal length f=10 mm, the collimator lens has an epaxial chromatic aberration correction rate (a) and a numerical aperture NA in the ranges of (1) (a) <2.2 ?m/nm and (2) 0.4>NA?0.12, respectively.

Abstract: An optical head comprising a light source, an objective lens, a light splitting means, a light receiving element, a tracking error signal detection means, and a spherical aberration detection means, wherein the light splitting means has six regions that are divided by a first splitting line that is substantially parallel to a longitudinal direction of an information tracks, and by second and third splitting lines perpendicular to the first splitting line.

Abstract: A method of one embodiment of the invention is disclosed that determines optical disc vertical displacement information at a number of positions on an optical disc. The method interpolates optical disc vertical displacement information at a number of other positions on the optical disc from the vertical displacement information determined.

Abstract: An objective lens used in an optical head having a convex lens and a chromatic aberration correction optical element wherein there is provided a positioning means that positions the center of gravity of the objective lens used in an optical head substantially on a straight line connecting supporting points for the objective lens used in an optical head.

Abstract: Apparatus and method for exposing an optical disk master having a laser source, a deflector for deflecting a recording laser beam based on the laser beam of the laser source and an objective lens for focusing the recording laser beam on an optical disk master, including a liquid crystal plate provided between the laser source and the deflector, a parallel flat plate provided between the deflector and the objective lens, a photodetector that detects reflected light from the parallel flat plate, and an adjusting device for adjusting a voltage applied to the liquid crystal plate based on information on wavefront fluctuation of the recording laser beam detected by the photodetector.

Abstract: A shock detector for an optical disc recorder and method for controlling the recording using a shock signal. The shock detector includes a first detecting unit for detecting the amplitude of a first kind of reference signal passing through a band-pass filter and outputting a first detecting signal, a second detecting unit for detecting the level of a second kind of reference signal and outputting a second detecting signal, a third detecting unit for detecting the revolution of a third kind of reference signal and outputting a third detecting signal, and a judging unit for receiving the first detecting signal, the second detecting signal and the third detecting signal, and enabling a shock signal when the first detecting signal, the second detecting signal and the third detecting signal are simultaneously enabled. Therefore, the shock detector does not need an additional shock sensor.

Abstract: A stabilization part stabilizes surface vibration of a flexible optical disk along a rotation axis direction of the optical disk by means of pressure difference of air flow created according to Bernoulli's law at a portion on which information writing/reading is performed, provided on a side of the optical disk opposite to a side on which information recording/reproducing is performed. In this case, areas are provide on the upstream side and down stream side along the disk rotation direction of the portion of the optical disk which is stabilized by said stabilization part, said areas of the optical disk not having pressure difference created thereon by the air flow.

Abstract: An apparatus which stabilizes an operation of a disc driver in a mode conversion setting section includes a pickup which reads information from a disc that is inserted into the disc driver, a filter which performs a low-pass filtering of a servo error signal that is generated during an operation of the pickup, a selector which selects one of the servo error signal and the low-pass filtered servo error signal from the filter and outputs the selected servo error signal to drive the pickup using the selected servo error signal, and a control signal generator which generates a signal to control the selector to select the low-pass filtered servo error signal in a mode conversion setting section of the disc driver. Accordingly, a defocus and a detrack can be prevented in the mode conversion setting section of the disc driver.

Abstract: An apparatus and method for detecting a tilted disc, and an apparatus and method for reproducing data from the tilted disc are provided.

Abstract: A thickness servo system using optical aberrations, wherein a photodetector detects an aberration amount corresponding to thickness irregularity, and a thickness irregularity correction section operates to minimize the detected aberration amount. The thickness irregularity of a transparent layer can also be calculated from the detected aberration amount.

Abstract: The direction in which an information disk apparatus (for example, a DVD-ROM player) is installed is judged without employing a sensor. A controller measures a number of track crossings when a reading unit is moved by a driving unit in a direction corresponding to the radius of an optical disk for a predetermined period of time, and judges a direction in which the DVD-ROM player is installed on the basis of the measured number of track crossings.

Abstract: A device for delivering a data signal at a data rate has crosstalk cancellation. The crosstalk reducing unit (14) has an adaptive filter (15) to generate a crosstalk signal corresponding to a track adjacent to a track being scanned. A subtractor (16) subtracts the crosstalk signal from a read signal. A calculating unit (17) calculates filter coefficients for the adaptive filter. The adaptive filter (15) and subtractor (16) are coupled to an asynchronous clock (18) for operating at an asynchronous sample rate. The crosstalk reducing unit (14) has a sample rate converter (19) coupled to a synchronous clock for converting the output of the subtractor to the data signal (8) at a synchronous sample rate. A timing recovery unit (11) is coupled to the data signal (8) for retrieving the synchronous clock corresponding to the data rate.

Abstract: An optical disc drive is used to read and/or write data from/on an optical disc having a data storage layer. The drive corrects electrical offset while reading and/or writing data. The drive updates a correction value based on an electrical offset detected at correcting operation or a value derived using electrical offset values that has been detected and stored, without detecting an electrical offset at correcting operation. Therefore, the optical disc drive can correct the electrical offset less often.

Abstract: There is employed a configuration to detect, by making use of an optical pick-up (104) when tracking servo is performed along wobble grooves of an optical disc (102), phase errors between main wobble signal outputted as the result of the fact that reflected light beams corresponding to main spot of the optical disc (102) are received at a main light receiving section (321) of light detecting means (32) and side wobble signals outputted every side spot as the result of the fact reflected light beams corresponding to respective side spots of the optical disc (102) are received at a first sub-light receiving section (322) and a second sub-light receiving section (323) to calculate difference between the both phase errors at a difference calculating circuit (59) to generate, from this difference, a skew signal including inclination direction and inclination quantity of the optical disc (102).