JP2007141377A - Spherical aberration correction method for optical recording/reproducing device, and the optical recording/reproducing device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a spherical aberration correction method which properly corrects a spherical aberration in a short period of time without using the amplitude value of the "S curve" of a focus error signal, in an optical recording/reproducing device for correcting a spherical aberration by using a spherical aberration correcting means. <P>SOLUTION: An aberration control means 32 sets a spherical aberration correcting means 16 in s state where a focus loop gain is the maximum in a first step, gives an offset value to a focus actuator to set the maximum amplitude value of a tracking error signal T1 in a second step, and then executes the first step again. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a spherical aberration correction method for correcting spherical aberration caused by a thickness error of a cover layer of an optical recording medium in an optical recording / reproducing apparatus for recording or reproducing information on an optical recording medium such as an optical disk, and such spherical aberration. The present invention relates to an optical recording / reproducing apparatus capable of executing a correction method.

  Conventionally, in a low recording density optical disk represented by an optical recording medium such as a DVD (Digital Versatile Disc) or a CD (Compact Disc), the influence of spherical aberration caused by the thickness error (thickness deviation) of the cover layer of the disk is I was able to ignore it.

  However, in a high recording density optical disc using a short wavelength laser beam and a high numerical aperture objective lens that have been proposed in recent years, the tolerance of the disc cover layer thickness error is small, and the thickness error deviates from the tolerance. In the light beam condensed on the recording layer of the optical recording medium, non-negligible spherical aberration exceeding the allowable amount occurs. Then, the shape of the laser light condensing spot in the recording layer is disturbed, and the recording / reproducing characteristics are deteriorated.

  Since this spherical aberration is inversely proportional to the wavelength of the light beam applied to the optical recording medium and proportional to the fourth power of the numerical aperture of the objective lens, the shorter the wavelength of the light beam and the larger the numerical aperture of the objective lens, the more the recording / reproducing characteristics are affected. The tolerance of disc cover layer thickness error is reduced. Therefore, in an optical recording / reproducing apparatus in which the wavelength of the light beam used for increasing the recording density is shortened and the numerical aperture of the objective lens is increased, in order not to deteriorate the recording / reproducing characteristics, the thickness error of the cover layer of the optical disc It is necessary to correct the resulting spherical aberration.

  Conventionally, reflected light that has been focused on a recording layer of an optical recording medium by an objective lens, reflected by the recording layer, and passed through the objective lens is passed through a beam expander that is a spherical aberration correction unit, and the beam expander is moved on the optical axis. Thus, an optical recording / reproducing apparatus in which spherical aberration is corrected has been proposed.

  In such a recording / reproducing apparatus, reflected light that has passed through the beam expander is received by a photodetector, a focus error signal is obtained by an arithmetic circuit based on an output signal from the photodetector, and the beam expander is By moving on the axis and moving the objective lens in the optical axis direction at each position of the beam expander, a so-called “S-curve” of the focus error signal obtained by the arithmetic circuit is expressed. Then, the position of the beam expander where the amplitude value of the “S-curve” is maximized is searched, and the beam expander is set to that position, thereby correcting the spherical aberration.

Japanese Patent Laid-Open No. 2003-141766 JP 2004-241100 A

  By the way, in the optical recording / reproducing apparatus as described above, in order to measure the amplitude value of the “S-shaped curve”, it is necessary to obtain the maximum value and the minimum value in the “S-shaped curve”. It is necessary to move the objective lens from the focal point in the optical axis direction until ## EQU2 ## appears.

  In general, an actuator that moves the objective lens has a frequency characteristic in which the sensitivity decreases as the frequency becomes higher, so it is difficult to move the objective lens at high speed. For this reason, there is a limit in reducing the time required to express the “S-curve”. Therefore, it takes a lot of time to measure the amplitude of the “S curve”, and it is difficult to shorten the time until the correction of the spherical aberration is completed.

  In addition, when using a multi-layer disc in which two or more recording layers are laminated, it is necessary to discriminate which recording layer the “S-curve” to be expressed belongs to. Considering the surface deflection of the disc, it is necessary to perform complicated processing to distinguish which recording layer the “S-curve” belongs to, and this cannot be done with a simple system.

  Therefore, the present invention has been proposed in view of the above-described circumstances, and in an optical recording / reproducing apparatus that performs spherical aberration correction using spherical aberration correction means, the amplitude value of the “S-curve” of the focus error signal. To provide a spherical aberration correction method capable of properly correcting spherical aberration in a short time without using the optical recording apparatus, and to provide an optical recording / reproducing apparatus capable of executing such a spherical aberration correction method. Objective.

  In order to solve the above-described problems and achieve the above object, a surface light source device according to the present invention has any one of the following configurations.

[Configuration 1]
A spherical aberration correction method in an optical recording / reproducing apparatus according to the present invention includes a light source, an objective lens that focuses laser light emitted from the light source on an optical recording medium, and a spherical surface of reflected light that is reflected by the optical recording medium and passes through the objective lens. Spherical aberration correcting means for correcting aberrations, a photodetector for receiving reflected light passing through the spherical aberration correcting means, an arithmetic circuit for obtaining a focus error signal and a tracking error signal based on an output signal from the photodetector, and a focus error In an optical recording / reproducing apparatus having a focus actuator for controlling the position of the objective lens based on the signal and an aberration control means for controlling the spherical aberration correction means, the focus servo is performed by the focus actuator based on the focus error signal. Set spherical aberration correction means at different N points (where N is an integer of 3 or more). The spherical aberration correction means that maximizes the focus loop gain value based on N (where N is an integer of 3 or more) focus loop gain values obtained while changing the state to obtain the focus loop gain value at each point. The first process of setting the spherical aberration correcting means to the setting state obtained by the approximate expression of the setting state, and different N points (where N is an integer of 3 or more) offset values are given to the focus actuator at each point. Determine the amplitude value of the tracking error signal and obtain the offset value of the focus actuator that maximizes the amplitude value of the tracking error signal based on the N (where N is an integer of 3 or more) tracking error signal amplitude values obtained. A second value that gives the offset value obtained by the approximate expression to the focus actuator. The second step is executed after the first step, and the first step is executed again, so that the spherical aberration amount in the optical recording medium is minimized by setting the spherical aberration correcting means. The setting state is as follows.

[Configuration 2]
The spherical aberration correcting method in the optical recording / reproducing apparatus according to the present invention is the spherical aberration correcting method in the optical recording / reproducing apparatus having the configuration 1, and the approximate expression used in the first and second processes is the [N−1] th order. It is a simultaneous equation of functions.

[Configuration 3]
The optical recording / reproducing apparatus according to the present invention corrects the spherical aberration of the light source, the objective lens for condensing the laser light emitted from the light source onto the optical recording medium, and the reflected light reflected by the optical recording medium and passing through the objective lens. Spherical aberration correction means, a photodetector that receives reflected light that has passed through the spherical aberration correction means, an arithmetic circuit that obtains a focus error signal and a tracking error signal based on an output signal from the photodetector, and a focus error signal A focus actuator for controlling the position of the objective lens, and a spherical aberration correction means and an aberration control means for controlling the focus actuator. The aberration control means is in a state in which focus servo is performed by the focus actuator based on the focus error signal. Change the spherical aberration correction means to three or more different setting states at each point The setting state of the spherical aberration correcting means that obtains the maximum focus loop gain value based on the three or more obtained focus loop gain values is obtained by an approximate expression based on the focus loop gain value, and the spherical aberration correcting means is set in this setting state. Is performed, and then, three or more different offset values are given to the focus actuator to determine the amplitude value of the tracking error signal at each point, and the amplitudes of the three or more tracking error signals obtained are obtained. Based on the value, the offset value of the focus actuator that maximizes the amplitude value of the tracking error signal is obtained by an approximate expression, the second process of giving this offset value to the focus actuator is executed, and the first process is executed again. By using the optical recording medium, It is characterized in that the set state of the spherical aberration becomes minimum.

[Configuration 4]
The optical recording / reproducing apparatus according to the present invention is characterized in that, in the optical recording / reproducing apparatus having the configuration 3, the spherical aberration correcting means is any one of a beam expander, a collimator lens, and a liquid crystal optical element. is there.

  In the spherical aberration correcting method in the optical recording / reproducing apparatus according to the present invention, having the configuration 1, the spherical aberration correcting means is set to the setting state in which the focus loop gain value is maximized by the first process, and the second process is performed. Since the offset value that maximizes the amplitude value of the tracking error signal is given to the focus actuator and the first process is executed again, the optical recording medium to be used is either a single-layer disc, a dual-layer disc, or a multi-layer disc. Regardless of this, the correction value of the spherical aberration caused by the disc cover layer thickness error can be easily obtained by measuring several points (three or more points), and the correction and adjustment of the spherical aberration can be performed accurately in a short time. be able to.

  Further, in this spherical aberration correction method, since the configuration 2 is used, simultaneous equations of the [N−1] degree function are used as approximate equations in the first and second processes, so that the focus loop gain value is maximized. The offset value that maximizes the setting state and the amplitude value of the tracking error signal can be easily obtained.

  In the optical recording / reproducing apparatus according to the present invention, having the configuration 3, the spherical aberration correcting means is set to the setting state in which the focus loop gain value is maximized by the first process, and the focus actuator is set by the second process. Since the offset value at which the amplitude value of the tracking error signal is maximized is given and the first process is executed again, regardless of whether the optical recording medium to be used is a single-layer disc, a dual-layer disc, or a multi-layer disc. In addition, the correction value of the spherical aberration caused by the disc cover layer thickness error can be easily obtained by measuring several points (three or more points), and the correction and adjustment of the spherical aberration can be performed accurately in a short time. .

  In addition, since this optical recording / reproducing apparatus has the configuration 4, since it has any of a beam expander, a collimator lens, or a liquid crystal optical element as a spherical aberration correction means, the configuration is not complicated. Correction and adjustment of spherical aberration can be performed accurately.

  That is, the present invention appropriately corrects spherical aberration in a short time without using the amplitude value of the “S-shaped curve” of the focus error signal in an optical recording / reproducing apparatus that performs spherical aberration correction using spherical aberration correction means. It is possible to provide a spherical aberration correction method that can be performed, and to provide an optical recording / reproducing apparatus that can execute such a spherical aberration correction method.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, the best mode for carrying out the invention will be described in detail with reference to the drawings.

[Configuration of optical recording / reproducing apparatus]
FIG. 1 is a block diagram showing a configuration of an optical recording / reproducing apparatus according to the present invention.

  As shown in FIG. 1, the optical recording / reproducing apparatus according to the present invention has, for example, a semiconductor laser element 4 as a light source, and the laser beam L1 emitted from the semiconductor laser element 4 is used as an optical recording medium. For example, it has an optical system that constitutes an optical pickup that leads to the optical disc D and detects reflected light from the optical disc D.

  That is, in this optical recording / reproducing apparatus, the laser light L 1 emitted from the semiconductor laser element 4 is converted into a parallel light beam by the collimator lens 8, enters the polarization beam splitter 12 through the grating 10. The grating 10 diffracts the incident laser beam L1 and splits it into three or more light beams. The polarization beam splitter 12 includes a polarizing film that transmits a certain polarization component (P polarization component) of the incident laser light L1 and reflects a polarization component (S polarization component) in a direction orthogonal to the polarization component. Yes. Here, a part of the laser beam L 1 reflected by the polarization beam splitter 12 is incident on the laser monitor detector 14. The laser monitor detector 14 is for monitoring the output of the emitted light from the semiconductor laser element 4 by receiving a part of the laser light L1.

  The laser beam L1 transmitted through the polarization beam splitter 12 is incident on a beam expander 16 serving as a spherical aberration correction unit. The beam expander 16 is composed of a first lens 16A and a second lens 16B arranged on the optical axis, and is incident by adjusting the distance between the first lens 16A and the second lens 16B. The diameter of the laser beam L1 is adjusted and transmitted.

  The laser beam L 1 that has passed through the beam expander 16 is incident on the rising mirror 18. The raising mirror 18 reflects and raises the incident laser beam L1. The laser beam L1 that has passed through the rising mirror 18 is made into a circularly polarized state by passing through the quarter-wave plate 20, and is incident on the objective lens 22. The objective lens 22 condenses and irradiates the incident laser beam L1 on the recording layer of the optical disc D to form a light spot.

  The reflected light reflected by the recording layer of the optical disc D is converted into a parallel light beam through the objective lens 22, converted into a linearly polarized state through the quarter-wave plate 20, passed through the rising mirror 18 and the beam expander 16, Return to the polarization beam splitter 12. Here, since the reflected light is S-polarized light, it is reflected by the polarization beam splitter 16 and enters the detection lens 24. The detection lens 24 causes the incident reflected light to enter the cylindrical lens 26 as a convergent light beam. The cylindrical lens 26 causes astigmatism in the incident reflected light to enter the photodetector 28. The photodetector 28 receives the reflected light and generates and outputs a detection output corresponding to the reflected light. The photodetector 28 is, for example, a so-called “four-divided sensor” in which the light receiving surface is radially divided into four.

  The detection output from the photodetector 28 is sent to the arithmetic circuit 30. The arithmetic circuit 30 generates necessary signals such as a reproduction signal from the optical disc D, a focus error signal F1, and a tracking error signal T1 based on the detection output sent from the photodetector 28. The focus error signal F1 obtained by the arithmetic circuit 30 is sent to the focus actuator drive circuit 35. The focus actuator drive circuit 35 controls a focus actuator (not shown) based on the focus error signal F1, moves the objective lens 22 in the optical axis direction, and executes a focus servo operation.

  The optical recording / reproducing apparatus includes an aberration control unit 32. The aberration control means 32 includes a signal processing device (CPU) 50 such as a microcomputer, for example. In this aberration control means 32, the focus error signal F 1 obtained by the arithmetic circuit 30 is sent to the focus actuator drive circuit 35 through the phase compensation circuit 51 and the adder 52. At the time of executing the normal focus servo operation, the adder 52 does not add any signal. The focus error signal F1 is branched before passing through the adder 52, and is sent to the measurement circuit 54 through the filter 53. The measurement circuit 54 sends the sampling value of the focus error signal to the A terminal of the signal processing device 50. The focus error signal F <b> 1 branches after passing through the adder 52, and is sent to the measurement circuit 56 through the filter 55. The measurement circuit 56 sends the sampling value of the focus error signal to the B terminal of the signal processing device 50.

  The signal processing device 50 controls the sine wave generation circuit 33 to generate a sine wave. This sine wave is sent to the adder 52 and added to the focus error signal F1.

  In this aberration control means 32, the tracking error signal T1 obtained by the arithmetic circuit 30 is sent to a tracking actuator drive circuit (not shown) via the phase compensation circuit 57. The tracking actuator drive circuit controls the tracking actuator to move the objective lens 22 in a direction orthogonal to the optical axis and the recording track of the optical disc D. The tracking actuator and the focus actuator are configured as one actuator.

  The tracking error signal T1 is sent to the amplitude measurement circuit 58. The amplitude measurement circuit 58 measures the amplitude value of the tracking error signal T1 and sends the measurement result to the T terminal of the signal processing device 50.

  Then, the signal processing device 50 outputs an instruction signal S2 to the beam expander 16 in order to execute the aberration correction method according to the present invention by performing an operation as described later. The instruction signal S2 output from the signal processing device 50 is sent to the driving means 34.

  The drive unit 34 includes a drive circuit 36 that receives the instruction signal S2 and outputs the drive signal S3, and a drive mechanism 38 that drives the beam expander 16 based on the drive signal S3. The drive mechanism 38 has a stepping motor 42 that operates according to the drive signal S3. A lead screw 40 is connected to the stepping motor 42. A thread portion of a lens holder 44 that holds the second lens 16 </ b> B, which is one of the lenses constituting the beam expander 16, is screwed into the lead screw 40. The lens holder 44 is moved along the lead screw 40 together with the second lens 16 </ b> B by forward and reverse rotation of the lead screw 40.

  The first lens 16A of the beam expander 16 is fixed to one end side of a guide rod 46 that guides the movement of the second lens 16B. The guide rod 46 is provided with an origin detector 48 for detecting the origin position of the second lens 16B.

  FIG. 2 is a graph showing the relationship between the position of the second lens 16B and the focus loop gain.

  By the way, in this optical recording / reproducing apparatus, when the position of the second lens 16B of the beam expander 16 is changed, the spherical aberration changes. At this time, as shown in FIG. The measured value changes. This focus loop gain measurement value is a sine wave input to the A terminal of the signal processing device 50 when a single frequency sine wave (disturbance) is added for a certain time in the adder 52 in the path constituting the closed loop. A numerical value A obtained by integrating the sampling value of the focus error signal before addition for a certain period of time and a numerical value B obtained by integrating the sampling value of the focus error signal after addition of the sine wave input to the B terminal of the signal processing device 50 for a certain period of time are obtained. , Measured value [C = A / B] or measured value [C = A−B]. This focus loop gain measurement value takes a maximum value when the amount of spherical aberration becomes minimum. At this time, the amplitude of the tracking error signal also takes a maximum value.

  FIG. 3 is a graph showing the relationship between the offset value of the focus actuator and the amplitude of the tracking error signal.

  However, as shown in FIG. 3, the amplitude of the tracking error signal changes due to defocus caused by the offset amount of the focus actuator. That is, in the state where defocusing occurs in the focus actuator, the amplitude of the tracking error signal does not become the maximum value when the spherical aberration amount is minimized.

  FIG. 4 is a graph showing the relationship between the position of the second lens 16B and the amplitude value of the tracking error signal.

  Therefore, when there is defocus in the focus actuator, as shown in FIG. 4, the position of the second lens 16B at which the focus loop gain measurement value becomes the maximum value does not coincide with the position at which the amplitude of the tracking error signal becomes the maximum value. . Therefore, the aberration control means 32 of this optical recording / reproducing apparatus performs spherical aberration correction as a coarse adjustment in the following first process, and then in a state where the defocus amount is removed in the second process. The servo operation is performed, and the first process is executed again as fine adjustment, so that an optimal correction amount that minimizes the spherical aberration amount can be obtained.

  In the first process, first, as shown in FIG. 2, in the state in which the focus servo operation is performed, the second lens 16B of the beam expander 16 is placed at N points on the optical axis (where N is an integer of 3 or more). ) To measure the focus loop gain at each position. In the first process, the second lens of the beam expander 16 in which the measurement value C is maximized based on the N measurement values C (N is an integer of 3 or more) thus obtained. The position of 16B is calculated by an approximate expression and set. As this approximate expression, simultaneous equations of [N−1] degree function can be used.

  In this way, the position of the second lens 16B obtained by the first first process does not coincide with the position where the amplitude of the tracking error signal becomes the maximum value, and is also the position obtained when there is no defocus. There is a slight deviation.

  In the second process, first, as shown in FIG. 3, an N-point (where N is an integer of 3 or more) offset value is given to the focus actuator to generate defocus. In the second process, the amplitude value of the tracking error signal is obtained in each defocus state, and based on the obtained amplitude values of N tracking errors signals (where N is an integer of 3 or more). Then, the offset value of the focus actuator that maximizes the amplitude value of the tracking error signal is calculated and calculated by an approximate expression. That is, the second process is a process for performing an optimum focus servo operation with the defocus removed.

  Next, by executing the first process again, an optimal correction amount with a minimum amount of spherical aberration can be obtained.

  In practice, as described above, after the first process, the second process may be performed and the first process may be performed again. However, in order to further improve the accuracy, After repeating the second process several times, the first process may be finally executed.

  In this optical recording / reproducing apparatus, the spherical aberration correction means is not limited to the beam expander 16, and the collimator lens 8 may be moved in the optical axis direction, or a liquid crystal optical element may be used. it can.

[Aberration correction method (operation of optical recording / reproducing apparatus)]
Next, the operation (aberration correction method) of the optical recording / reproducing apparatus configured as described above will be described. The procedure when N is 4 will be described below.

  First, after the operation power of the optical recording / reproducing apparatus is turned on, the initial preparation is executed. In this initial preparation, the position of the second lens 16B is moved to the origin. At this time, it is moved until the origin detector 48 detects it, and then moved in the reverse direction until the origin detector 48 becomes undetected. At this position, the moving step number n is set to “0”. The second beam expander 16B is moved from this origin position to the optimum position with respect to the standard disk (cover layer thickness error is 0) obtained in advance.

  FIG. 5 is a flowchart showing the spherical aberration correcting operation (aberration correcting method) of the optical recording / reproducing apparatus according to the present invention.

  Then, the optical recording / reproducing apparatus starts the spherical aberration correction operation as shown in step st1 in FIG. And it progresses to step st2 and performs a 1st process.

  FIG. 6 is a flowchart showing a first process in the spherical aberration correction operation (aberration correction method) of the optical recording / reproducing apparatus according to the present invention.

  As shown in step st7 of FIG. 6, when the first process is started, the process proceeds to step st8 to perform focus offset and focus gain calibration, and to set parameters necessary for performing the focus servo operation. Perform focus servo operation. Next, the process proceeds to step st9, the moving step number n is set to “1”, and the process proceeds to step st10.

  In step st10, the second lens 16B is moved to the position of the nth point, and the process proceeds to step st11. When step st10 is first executed, n = 1. In step st11, focus loop gain is measured, and the process proceeds to step st12. In step st12, the measured value is stored and the process proceeds to step st13.

  In step st13, it is determined whether the moving step number n is equal to N (= 4). If the moving step number n is equal to N (n = N = 4), the process proceeds to step st14, and the moving step number n is set to N. If not equal (n ≠ N = 4), the process proceeds to step st15. In step st15, the moving step number n is increased by 1 (n = n + 1), and the process returns to step st10.

  By returning to step st10 in this way, the second lens 16B is sequentially moved to the positions of the second point, the third point, and the fourth point, and each time the focus loop gain is measured, the measured value is obtained. Save each one.

When the moving step number n becomes equal to N and the process proceeds to step st14, based on the four measured values C, the position of the second lens 16B at which the measured value C is maximum is obtained from an approximate expression. Here, when the position Xn of four points and the measured value Yn are (X ₁ , Y ₁ ), (X ₂ , Y ₂ ), (X ₃ , Y ₃ ), (X ₄ , Y ₄ ), Y is The value Xmax of X when the maximum value is obtained from coefficients A, B, and C obtained from the following four expressions (Expression 1) to (Expression 4) (simultaneous equations of the [N-1] degree function): Calculated by equation 5).

y ₁ = AX ₁ ³ + BX ₁ ² + CX ₁ + D (Formula 1)
y ₂ = AX ₂ ³ + BX ₂ ² + CX ₂ + D (Formula 2)
y ₃ = AX ₃ ³ + BX ₃ ² + CX ₃ + D (Equation 3)
y ₄ = AX ₄ ³ + BX ₄ ² + CX ₄ + D (Formula 4)
Xmax = {− B−√ (B ² −3AC)} / (3A) (Formula 5)

  Then, the process proceeds to step st16, and based on the position obtained in step st14, the instruction signal S2 is output to the drive circuit 36, the second lens 16B is moved, the process proceeds to step st17, and the first process is completed. To do.

  FIG. 7 is a flowchart showing a second process in the spherical aberration correction operation (aberration correction method) of the optical recording / reproducing apparatus according to the present invention.

  Next, as shown in step st3 of FIG. 5, the second process is executed.

  As shown in step st18 of FIG. 7, when the second process is started, the process proceeds to step st19, the number of steps n is set to “1”, and the process proceeds to step st20.

  In step st20, the nth offset value is set in the focus actuator, defocused, and the process proceeds to step st21. When step st20 is executed for the first time, n = 1. In step st21, the amplitude of the tracking error signal is measured in a state where the nth offset value is set, and the process proceeds to step st22. In step st22, the measurement value is stored, and the process proceeds to step st23.

  In step st23, it is determined whether or not the step number n is equal to N (= 4). If the step number n is equal to N (n = N = 4), the process proceeds to step st24, and if the step number n is not equal to N. (N ≠ N = 4), the process proceeds to step st25. In step st25, the moving step number n is increased by 1 (n = n + 1), and the process returns to step st20.

  By returning to step st20 in this way, the second, third, and fourth offset values are sequentially set in the focus actuator, and each time the amplitude of the tracking error signal is measured and the measured values are stored.

When the number of steps n is equal to N and the process proceeds to step st24, an offset value that maximizes the measured value is obtained from the approximate expression based on the four measured values. Here, when the four offset values Xn and the measured value Yn are (X ₁ , Y ₁ ), (X ₂ , Y ₂ ), (X ₃ , Y ₃ ), (X ₄ , Y ₄ ), Y is The value Xmax of X at the maximum is obtained from the following four equations (Equation 6) to (Equation 9) (coefficients A, B, and C of the [N-1] degree function) as follows: Calculated by equation 10).

y ₁ = AX ₁ ³ + BX ₁ ² + CX ₁ + D (formula 6)
y ₂ = AX ₂ ³ + BX ₂ ² + CX ₂ + D (formula 7)
y ₃ = AX ₃ ³ + BX ₃ ² + CX ₃ + D (Equation 8)
y ₄ = AX ₄ ³ + BX ₄ ² + CX ₄ + D (Equation 9)
^{Xmax = {- B-√ (} B 2 -3AC)} / (3A) ·· ( Formula 10)

  Then, the process proceeds to step st26, and based on the offset value obtained in step st24, the focus actuator instruction signal F2 is output toward the focus actuator, the optimum focus servo with the defocus removed is performed, and the process proceeds to step st27. The second process is terminated.

  Next, as shown in step st4 of FIG. 5, the first process described above is executed again. After execution of the first process for the second time, the process proceeds to step st5, where the obtained position of the second lens 16B is stored as a final value, and an instruction signal S2 is output to the drive circuit 36 based on this final value. Then, the second lens 16B is moved, and the process proceeds to step st6 to complete the spherical aberration correction. In this optical recording / reproducing apparatus, after the spherical aberration correction is thus completed, a normal recording / reproducing operation is performed.

  In the optical recording / reproducing apparatus and aberration correction method, the number of measurement points in the first process and the second process may be 3 or more, and the number is not limited. When the number of measurement points is N, the optimum value in each process can be obtained by using simultaneous equations of the [N-1] degree function as an approximate expression. In addition, as described above, after the first process, the second process is performed and the first process is performed again, but there is no practical problem. However, in order to further improve the accuracy, the first process And the second process may be repeated several times, and finally the first process may be executed.

It is a block diagram which shows the structure of the optical recording / reproducing apparatus which concerns on this invention. It is a graph which shows the relationship between the position of the 2nd lens in the said optical recording / reproducing apparatus, and a focus loop gain. 4 is a graph showing a relationship between an offset value of a focus actuator and an amplitude of a tracking error signal in the optical recording / reproducing apparatus. It is a graph which shows the relationship between the position of the 2nd lens in the said optical recording / reproducing apparatus, and the amplitude value of a tracking error signal. It is a flowchart which shows the spherical aberration correction operation | movement (aberration correction method) of the said optical recording / reproducing apparatus. 3 is a flowchart showing a first process in a spherical aberration correction operation (aberration correction method) of the optical recording / reproducing apparatus. It is a flowchart which shows the 2nd process in the spherical aberration correction operation | movement (aberration correction method) of the said optical recording / reproducing apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 4 Semiconductor laser element 8 Collimator lens 10 Grating 12 Polarization beam splitter 14 Laser monitor detector 16 Beam expander 16A 1st lens 16B 2nd lens 18 Rising mirror 20 1/4 wavelength plate 22 Objective lens 24 Detection lens 26 Cylindrical lens 28 Photodetector 30 Arithmetic circuit 32 Aberration control means 33 Sine wave generating circuit 34 Drive means 36 Drive circuit 38 Drive mechanism 40 Lead screw 42 Stepping motor 44 Lens holder 46 Guide rod 48 Origin detector 50 Signal processing device 51 Phase compensation circuit 52 Addition 53 Filter 54 Measuring circuit 55 Filter 56 Measuring circuit 57 Phase compensation circuit 58 Amplitude measuring circuit D Optical disc (optical recording medium)
F1 Focus error signal F2 Focus actuator instruction signal T1 Tracking error signal S2 Instruction signal S3 Drive signal

Claims (4)

A light source, an objective lens for condensing laser light emitted from the light source on an optical recording medium, spherical aberration correction means for correcting the spherical aberration of reflected light reflected by the optical recording medium and passing through the objective lens, and A photodetector that receives the reflected light that has passed through the spherical aberration correction means, an arithmetic circuit that obtains a focus error signal and a tracking error signal based on an output signal from the photodetector, and the objective that is based on the focus error signal In an optical recording / reproducing apparatus having a focus actuator for controlling the position of a lens and an aberration control means for controlling the spherical aberration correction means,
In a state where focus servo is performed by the focus actuator based on the focus error signal, the spherical aberration correction means is changed to a different setting state of N points (where N is an integer of 3 or more), and a focus loop at each point. The gain value is obtained, and the setting state of the spherical aberration correcting means that maximizes the focus loop gain value is calculated by an approximate expression based on the obtained N (where N is an integer of 3 or more) focus loop gain values. Determining and setting the spherical aberration correcting means in the determined setting state;
The focus actuator is given offset values at different N points (where N is an integer of 3 or more), and the amplitude value of the tracking error signal is obtained at each point, and N obtained (where N is an integer of 3 or more). ) To obtain the offset value of the focus actuator that maximizes the amplitude value of the tracking error signal based on the approximate expression, and to give the obtained offset value to the focus actuator; Have
After the first step, the second step is executed, and the first step is executed again, so that the spherical aberration amount in the optical recording medium is minimized by setting the spherical aberration correcting means. A spherical aberration correction method in an optical recording / reproducing apparatus, characterized by being in a set state.   2. The spherical aberration correction method in an optical recording / reproducing apparatus according to claim 1, wherein the approximate expression used in the first and second steps is a simultaneous equation of an [N-1] degree function. A light source;
An objective lens that focuses laser light emitted from the light source onto an optical recording medium;
Spherical aberration correction means for correcting the spherical aberration of the reflected light reflected by the optical recording medium and passed through the objective lens;
A photodetector that receives the reflected light that has passed through the spherical aberration correction means;
An arithmetic circuit for obtaining a focus error signal and a tracking error signal based on an output signal from the photodetector;
A focus actuator that controls the position of the objective lens based on the focus error signal;
An aberration control means for controlling the spherical aberration correction means and the focus actuator,
The aberration control means changes the spherical aberration correction means to three or more different setting states in a state where focus servo is performed by the focus actuator based on the focus error signal, and obtains a focus loop gain value at each point. Based on the obtained three or more focus loop gain values, a setting state of the spherical aberration correction unit that maximizes the focus loop gain value is obtained by an approximate expression, and the spherical aberration correction unit is set to this setting state. Then, three or more different offset values are given to the focus actuator to obtain the amplitude value of the tracking error signal at each point, and based on the obtained amplitude values of the three or more tracking error signals. The maximum value of the tracking error signal amplitude value. The second process of obtaining the offset value of the slack actuator by an approximate expression and giving this offset value to the focus actuator is executed, and the first process is executed again, thereby changing the setting state of the spherical aberration correcting means. An optical recording / reproducing apparatus characterized in that a spherical aberration amount in an optical recording medium is set to a minimum state.   4. The optical recording / reproducing apparatus according to claim 3, wherein the spherical aberration correcting means is one of a beam expander, a collimator lens, or a liquid crystal optical element.

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