JPH10255290A - Objective lens for optical pickup - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a double group lens which holds a distance between first and second lenses contact in a simple structure without requiring a complicate structure such as a position servo or the like and exerts sufficient stability in focus servo. SOLUTION: A double group lens 1 is constituted of a bobbin 8 engaging an outer circumferential part 3a of a second lens 3 thereby holding the second lens 3, a yoke 10 arranged at an outer circumferential part of a second face 5 of a first lens 2 providing a space part 9 to the bobbin 8, a coil 11 wound on an outer circumferential part of the bobbin 8, a magnet 12 fitted to the yoke 10 and separated from the coil 11, a spring 13 mechanically coupling the bottin 8 and the yoke 10, a viscous fluid 14 sealed in the space part 9, and a protecting material 15 disposed at an outer circumferential part of a fourth face 7 of the second lens 3.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an objective lens for an optical pickup applied to a two-group lens comprising a first lens and a second lens. The present invention relates to an objective lens for an optical pickup in which a distance from a second lens is variable.

[0002]

2. Description of the Related Art Various types of optical recording media, such as optical disks, magneto-optical disks, and optical cards, have been proposed as information signal recording media. Then, there has been proposed an optical pickup that irradiates light from a light source onto the optical recording medium to write and read information signals on and from a signal recording surface of the optical recording medium.

In the above optical pickup, by increasing the numerical aperture (NA) of the objective lens, the beam diameter of the light can be reduced and condensed on the signal recording surface of the optical recording medium. Can be improved in information recording density.

For example, a so-called single lens requires a refracting power in order to obtain a high numerical aperture. However, when the refracting power is increased, the curvature of the lens surface decreases, and the positioning accuracy between the refracting surfaces becomes severe. For these reasons, the numerical aperture of the single lens is limited to about 0.6.

In an optical disk as an optical recording medium, when the thickness of a substrate for protecting a signal recording surface deviates from a specified value, spherical aberration changes greatly. On the other hand, spherical aberration is proportional to the fourth power of the numerical aperture, ignoring higher-order terms. Therefore, when the numerical aperture of the objective lens is increased,
The manufacturing tolerance of the disk substrate thickness becomes narrow.

[0006] The two-group lens can increase the design tolerance of the substrate thickness of the disk while enabling a high numerical aperture. As shown in FIG. 11, the second lens group includes a first lens 1 on which laser light from a semiconductor laser is incident.
01, and a second lens 102 into which the laser light transmitted through the first lens 101 is incident and which emits the incident laser light toward the optical disk facing the laser light.

With this configuration, the second group lens 10
0 indicates that the laser light from the semiconductor laser is
The light can be transmitted through the first and second lenses 102 and focused on the signal recording surface of the optical disc with a high numerical aperture.

In the second lens group 100, the distance between the first lens 101 and the second lens 102 is changed to cause an error in the substrate thickness, for example, an error in the radial direction of the optical disk. Laser light is focused on the signal recording surface of the optical disk. Furthermore, by making the distance between the first lens 101 and the second lens 102 variable,
The second group lens 100 can be used for optical disks having different substrate thicknesses.

A two-group lens 1 in which the distance between the first lens 101 and the second lens 102 is made variable.
00 is the first lens 101 and the second lens 101 in the normal state.
And a position servo for keeping the distance between the lens 102 and the lens 102 constant.

[0010] For example, the second group lens 100 is arranged such that the distance between the first lens 101 and the second lens 102 is appropriately changed in accordance with the optical disk on which information signals are recorded or reproduced, and then the position The distance between the first lens 101 and the second lens 102 is kept constant by the servo. At this time, the position servo is performed so that the spherical aberration is reduced and the focus servo is stabilized.

[0011]

Therefore, it is necessary to add a circuit for controlling the position servo to the second lens group 100. Further, in the second group lens 100, the mechanical structure of the position servo for keeping the distance between the first lens 101 and the second lens 102 constant is complicated.

Therefore, the second group lens 100 is provided with a position servo and a circuit for controlling the position servo in order to keep the distance between the first lens 101 and the second lens 102 constant. And the weight also increases.

The present invention has been made in view of the above circumstances, and is a two-group lens including a first lens and a second lens. It can be applied to an optical disk, and does not require a complicated structure like a position servo.
It is an object of the present invention to provide an objective lens for an optical pickup capable of maintaining a constant distance between the lens and the second lens at a level at which spherical aberration does not cause a problem and ensuring stability of focus servo.

[0014]

The objective lens for an optical pickup according to the present invention has been developed in order to solve the above-mentioned problems.
A second lens group that has a variable distance drive unit that supports the second lens in opposition to the first lens through the gap and changes the distance between the first lens and the second lens. Configure. Then, the attenuation rate of the variable distance driving means is set to 0.5 or more. With this configuration, the objective lens for the optical pickup keeps the distance between the first lens and the second lens constant, and ensures the stability of the focus servo at a level where spherical aberration does not matter.

Further, in order to solve the above-mentioned problems, the objective lens for an optical pickup according to the present invention supports the second lens in opposition to the first lens via a gap, and supports the first lens. It is constituted by a two-group lens provided with a variable distance driving means for changing the distance between the lens and the second lens. Then, the primary resonance frequency of the variable distance driving means is set to 5 kHz or more. With this configuration, the objective lens for the optical pickup keeps the distance between the first lens and the second lens constant, and ensures the stability of the focus servo at a level where spherical aberration does not matter.

[0016]

Embodiments of the present invention will be described in detail with reference to the drawings.

First, the first embodiment is applied to an objective lens for an optical pickup, which is incorporated in an optical pickup and focuses a laser beam from a semiconductor laser on a signal recording surface of an optical disk arranged opposite to the optical disk. This is a two-group lens.

As shown in FIG. 1, the second group lens includes a first lens 2 and a second lens 3 opposed to the first lens 2 with a gap 9 therebetween. The second lens group 1 includes a variable distance drive unit 16 that varies the distance between the first lens 2 and the second lens 3. Note that the attenuation rate of the variable distance drive unit 16 is 0.5 or more.

The first lens 2 emits the laser light incident from the first surface 4 to the second lens 3 from the first surface 4 on which the laser light from the semiconductor laser is incident. And a second surface 5. Also, the second lens 3
Is a third surface 6 on which the laser light transmitted through the first lens 2 is incident, and a fourth surface on which the laser light incident from the third surface 6 is emitted toward the optical disk opposed thereto. And 7.

The second group lens 1 composed of the first lens 2 and the second lens 3 can generate a numerical aperture of, for example, 0.8 or more.

The second lens group 1 holds the second lens 3 by fitting the outer peripheral portion 3a of the second lens 3 in addition to the first lens 2 and the second lens 3 described above. A gap 9 is provided between the cylindrical bobbin 8 and the bobbin 8, and a yoke 10 is provided around the outer periphery of the second surface 5 of the first lens 2.
And a coil 11 wound around the outer periphery of the bobbin 8.
And a magnet 12 attached to the yoke 10 and spaced apart from the coil 11, a spring 13 for mechanically connecting the bobbin 8 and the yoke 10, and sealed in the gap 9. It is composed of a viscous fluid 14 and a protective member 15 disposed on the outer peripheral portion of the fourth surface 7 of the second lens 3.

The bobbin 8 has a lens supporting portion 8a which fits and holds the outer peripheral portion 3a of the second lens 3, and a coil on which the coil 11 is wound around which is integrated with the outer peripheral portion of the lens supporting portion 8a. And a winding portion 8b.
It is shaped like a letter.

The bobbin 8 has a protection member 15 protruding from the fourth surface 7 of the second lens 3 fitted and held in the lens holding section 8a at the end of the coil winding section 8b. It is arranged. The protective material 15 is made of a resin material having a low hardness or a nonwoven fabric. The protection member 15 prevents the optical disc from colliding with the fourth surface 7 of the first lens 2 and damaging each other when the focus servo is deviated or the focus search fails.

The coil 11 is wound around the outer surface of the coil winding portion 8b of the bobbin 8. As will be described later, when a current is applied to the coil 11, a Lorentz force is generated by a magnetic field generated by the magnet 12 arranged opposite to the coil 11.

The yoke 10 has an inner wall portion 10a spaced apart from an inner surface of the coil winding portion 8b, and an outer wall portion 10b opposed to the outer surface of the coil winding portion 8b.
A gap is provided between the end portion of the coil winding portion 8b of the bobbin 8 and a lower surface portion 10c integrated with the inner wall portion 10a and the outer wall portion 10b. The yoke 10 is formed of, for example, a magnetic material.

The bin 8 and the yoke 10 are arranged without being in contact with each other. The distance between the inner wall portion 10a of the yoke 10 and the coil winding portion 8b of the bobbin 8 is approximately 50
μm, and the separated portion becomes the gap 9. The viscous fluid 14 is sealed in the gap 9 as described above. The viscous fluid 14 includes
For example, silicon-based grease is used.

The magnet 12 is disposed to face the coil 11 wound around the coil winding portion 8a of the bobbin 8, and is magnetized on the inner surface of the outer wall 10b of the yoke 10. This magnet 12 generates a magnetic field in the radial direction. The springs 13 are provided on the upper and lower portions of the yoke 10, respectively.

The spring 13 is a so-called Archimedes spring, which mechanically connects the bobbin 8 and the yoke 10 to elastically support the bobbin 8 with respect to the yoke 10.
Further, the spring 13 supports the bobbin 8 so as to be movable only in the optical axis direction.

In the second lens group 1, a so-called voice coil motor is constituted by the bobbin 8, the yoke 10, the coil 11, the magnet 12, and the spring 13.
A viscous fluid 14 is sealed in a gap 9 provided in the voice coil motor to form a variable distance driving unit 16.

That is, the second lens group 1 is moved relative to the first lens 2 and the first lens 2 by the variable distance drive unit 16 driven by the Lorentz force generated between the coil 11 and the magnet 12. Yoke 11, spring 13
The distance between the second lens 3 and the elastically supported second lens 3 having a degree of freedom in the optical axis direction via the bobbin 8 is variable.

The second lens group 1 provided with the variable distance drive unit 16 is supported by an objective lens drive actuator in the optical pickup so as to be movable in the optical axis of the laser light emitted from the semiconductor laser and in the direction perpendicular to the optical axis. ing.

The attenuation ratio of the second lens unit 1 is set to 0.5 or more by the variable distance drive unit 16.

The damping rate is determined mainly by the viscous fluid 14 sealed in the gap 9 of the variable distance drive 16. Further, the damping rate is a relation between the actual damping coefficient and the critical damping coefficient, that is, damping rate = (actual damping coefficient) /
(Critical damping coefficient), and is expressed in detail as in equation (1).

[0034]

(Equation 1)

Where c is the actual damping coefficient, m is the weight of the movable part, and k is the spring constant.

The second lens unit 1 includes a variable distance drive unit 1
6, the distance between the first lens 2 and the second lens 3 is changed to the fourth surface 7 of the second lens 3 as shown in FIG.
Signal recording surface 5 via an optical disc substrate facing
2 is set so that the laser light is focused and the spherical aberration at that time is minimized.

For example, the second lens group 1 includes a signal recording surface 52
The distance L ₁ between the first lens 2 and the second lens 3 is set such that spherical aberration having a polarity opposite to that of spherical aberration caused by a thickness error of the substrate 51 serving as a cover glass for protecting the lens is generated. Has been adjusted.

The spherical aberration W ₄₀ caused by the thickness error of the substrate can be obtained by the equation (2) ignoring higher-order aberrations. Here, n is the refractive index of the substrate, and Δd is the substrate thickness error.

[0039]

(Equation 2)

For example, when the spherical aberration generated by the substrate has a value as shown in FIG. 3A, the second lens group 1 is moved by the variable distance driving unit 16 between the first lens 2 and the second lens 3. by adjusting the distance L ₁ between, to generate a reverse polarity spherical aberration as shown in FIG. 3 (b). Then, FIG.
As shown in (c), the spherical aberrations of the second lens unit 1 and the substrate 51 cancel each other, and almost no spherical aberration occurs as a whole.

The attenuation ratio of the second lens group 1 in which the distance between the first lens 2 and the second lens 3 is adjusted by the variable distance drive unit 16 is set to 0.5 or more. Therefore, even when the objective lens drive actuator is driven, the distance L ₁ between the first lens 2 and the second lens 3 is provided without providing the position servo unlike the related art.
Is properly maintained, and stability of the focus servo can be secured.

Accordingly, the second group lens 1 can be maintained by a stable focus servo with little spherical aberration occurring even in a state where the disk surface oscillates.

For example, when the second group lens 1 is employed in an optical disk player capable of compatible reproduction, it works effectively when recording and reproducing information signals on optical disks having different substrate thicknesses.

That is, the second group lens 1 applied to an optical disc mounted on an optical disc player and having a suitably adjusted distance between the first lens 2 and the second lens 3 is:
Even if the objective lens drive actuator is driven during recording and reproduction, the distance between the first lens 2 and the second lens 3 can be kept constant by the variable distance drive unit 16.

Further, even if the objective lens drive actuator is driven by the unevenness of the substrate thickness in the radial direction of the optical disc, the second lens group 1 is moved by the variable distance drive unit 16.
The distance between the first lens 2 and the second lens 3 can be kept constant.

Accordingly, the optical disk player provided with the second group lens 1 can record and reproduce information signals on an optical disk having an uneven substrate thickness or an optical disk having a different substrate thickness without deteriorating information signals.

The second lens unit 1 is provided with a variable distance drive unit 1 without being affected by, for example, an external impact.
6 allows the distance between the first lens 2 and the second lens 3 to be kept constant.

Further, in the second group lens 1, since the variable distance drive unit 16 is constituted by simple parts, the same operation as that of the conventional position servo is provided at a low cost and provided by a lightweight structure. be able to.

The second group lens 1 slides the bobbin 8 and the yoke 11 to reduce the damping rate to 0.1 by the frictional force.
It can be 5 or more.

Next, the open-loop characteristic of the focus servo in which the above-mentioned second lens group 1 is applied to an optical pickup will be described.

In the second group lens 1 and the optical pickup including the second group lens 1, as shown in FIG. 4, the objective lens drive actuator 21 and the variable distance drive unit 16 are modeled using a spring and a dashpot. . By modeling in this way, it is possible to study, for example, a change in the open-loop characteristic of the focus servo with respect to a change in the attenuation rate by calculation.

First, as shown in FIG. 4, the second group lens 1
First lens 2, second lens 3, first lens 2
A second spring 22 and a second dashpot 23 are provided between the second spring 3 and the second lens 3.

The optical pickup 20 includes a second group lens 1, an objective lens driving actuator 21 including a first spring 24 and a second dashpot 25, and a semiconductor laser for emitting laser light toward the second group lens 1. 26,
A beam splitter 27 and a collimator lens 28 disposed between the second lens unit 1 and the semiconductor laser 26, and a condensing lens 29 to which light reflected from the signal recording surface of the optical disk 50 enters via the beam splitter 27. And a photodetector 30 for receiving the light condensed by the condensing lens 29
It is composed of

Although not shown, the optical pickup 20 includes a focus servo signal detection circuit that performs focus servo based on a signal obtained by a photodetector. The focus servo detection circuit applies an astigmatism method and feeds back a focus error signal to an objective lens driving actuator. At this time, it is assumed that there is no phase correlation between the focus servo and the control of the variable distance drive unit 16.

The objective lens driving actuator 21
And the displacement of the movable unit of the variable distance drive unit 16 of the second lens unit 1 is obtained by the following equation of motion.

With respect to the objective lens drive actuator 21, the equation of motion is as shown in equation (3). Here, k ₁ is the spring constant of the first spring 24, c ₁ is the attenuation coefficient of the _first dashpot 25, and m ₁ is the weight of the movable part of the objective lens drive actuator 21.

[0057]

(Equation 3)

Further, with respect to the variable distance drive unit 16, the equation of motion is as shown in equation (4). Here, c ₂ is the spring coefficient of the second spring 22, k ₂ is the damping coefficient of the _second dashpot 23, and m ₂ is the weight of the movable part of the distance variable drive unit 16.

[0059]

(Equation 4)

Note that, in equations (3) and (4), u
₁ and u ₂ are the displacement speeds of the movable portions of the objective lens drive actuator 21 and the variable distance drive unit 16.

Then, by subjecting the equations (3) and (4) to Laplace transform, the transfer functions T ₁ and T ₂ of the displacement of the first lens 2 and the second lens 3 with respect to the input signal f ₁ are given by Equations (5) and (6) are obtained.

[0062]

(Equation 5)

[0063]

(Equation 6)

On the other hand, the first lens 2 and the second lens 3
And a focus error level X, a relationship of a linear sum as shown in Expression (7) is established from optical calculations.
Here, α and β are constants.

[0065]

(Equation 7)

From the equations (5) to (7), the transfer function T of the focus error level X with respect to the input signal f _{1 is obtained} .
₃ is as shown in equation (8).

[0067]

(Equation 8)

By adding a phase compensation term to the above equation, s = j
By setting ω, the open loop characteristic of the actual focus servo is derived.

The calculation is performed under the conditions shown in Table 1.

[0070]

[Table 1]

The results regarding the open loop characteristics of the focus servo of the optical pickup 20 will be described below.

[0072] First, FIG. 5 (A) through FIG. 5 (C) attenuation factor xi] ₂ of the distance variable drive unit 16 indicates a phase change when 0.1.

FIG. 5A shows the transfer function of the displacement of the movable part with respect to the input current to the variable distance drive unit 16. In FIG. 5A, the horizontal axis represents the input frequency of the variable distance drive unit 16, and the vertical axis represents the amplitude or phase of the displacement of the variable distance drive unit 16.

FIG. 5A shows that the cut-off frequency of the objective lens drive actuator 21 is 1 kHz to
It is shown that the phase delay becomes approximately 80 ° around 2 kHz.

FIG. 5B shows a transfer function of the displacement of the movable portion of the distance variable drive section 16 when an input current is supplied to the objective lens drive actuator 21. In FIG. 5B, the horizontal axis represents the vibration frequency of the objective lens drive actuator 21, and the vertical axis represents the displacement amplitude or phase of the objective lens drive actuator 21.

According to FIG. 5B, 1 kHz to 2 kHz
Near z, the phase is shown to lag approximately 300 °.

FIG. 5C shows the open-loop characteristic of the focus servo obtained by the relationship between the effect shown in FIG. 5A and the effect shown in FIG. 5B. In the optical pickup 20, the focus servo is stabilized when the vibration frequency of the objective lens drive actuator 21 is 2 kHz or more, but the cutoff frequency is 600 Hz to 1.2 kHz.
When set to z, there is no phase margin and the servo becomes unstable.

Next, when the attenuation factor の of the distance variable drive unit 16 is 1.
A result regarding the open loop characteristic of the focus servo when the value is 0 or more will be described.

[0079] FIG. 6 (A) to FIG. 6 (C), the attenuation factor xi] ₂ of the distance variable drive section 16 shows the result of the phase change when, for example, 2.0. Here, FIGS. 6A to 6
FIGS. 5C to 5C correspond to FIGS. 5A to 5C so that FIGS. 5A to 5C can be compared with FIGS.

First, in FIG. 6A, for example, 1 kHz which is the cutoff frequency of the objective lens driving actuator 21 is used.
It shows that the order delay is reduced by about 40 ° in the vicinity of z to 2 kHz as compared with FIG. For example, FIG.
When compared to (A), 6 (A) is by increasing the attenuation factor xi] _2, obviously phase delay have been shown to decrease.

Further, in FIG. 6B, 1 kHz to 2 k
At around Hz, the phase lag shows a value around 200 °. For example, when compared with FIG. 5B, FIG.
That by increasing the attenuation factor xi] _2, clearly the phase delay is shown that reduction.

FIG. 6 (C) showing the open loop characteristic of the focus servo obtained from the relationship between the result of FIG. 6 (A) and the result of FIG. 6 (B), from 1 kHz to 2 kHz.
In the vicinity, the phase margin is approximately 50 °, and there is no point where the phase margin is delayed by 180 ° or more up to 10 kHz. For example when compared 5 (C), and FIG. 6 (C) by increasing the attenuation factor xi] _2, clearly increases the phase margin, further in the frequency domain of the 1KHz～2kHz, phase margin practical It shows a constant value at a level without any problem.

From the above results, the optical pickup 20
Is a variable distance drive unit 1 in which the attenuation factor ξ ₂ is 0.5 or more.
By providing the second lens group 1 having the value 6, the value of the phase margin of the open loop characteristic of the focus servo shows an appropriate value, so that the focus servo can be stably applied.

[0084] As an example, FIG. 7 shows the relationship between the damping factor xi] ₂ and phase margin of the distance variable drive unit 16. As shown in FIG. 7, the phase margin is shown to increase with attenuation factor xi] _2. That is, the second group lens 1
The decay rate xi] ₂ by 0.5 or more, it is possible to obtain a stable focus servo.

FIG. 8 shows the characteristics of a two-unit lens provided with a two-lens distance varying means as shown in FIG. 1 in an actual machine whose damping rate is increased by using a viscous fluid corresponding to FIG. 6 (A). .

As shown in FIG. 8, the cutoff frequency of the actuator for driving the objective lens is 1 kHz to 2 kHz.
In the region, the phase delay is 150 ° as calculated above.
It turns out that it is near. Therefore, the optical pickup can stably apply the focus servo by including the two-unit lens 1 configured as shown in FIG.

Next, the second embodiment will be described in detail. The second embodiment is configured to be applied to an objective lens for an optical pickup which is incorporated in an optical pickup and focuses a laser beam from a semiconductor laser on a signal recording surface of an optical disk arranged opposite to the optical disk. It is a group lens.

As shown in FIG. 1, the second group lens 1 has the same configuration as the second group lens 1 of the first embodiment, and the first lens 2 and the first lens And a second lens 3 facing the lens 2. The second lens group 1 includes a first lens 2 and a second lens 3
Is provided with a variable distance drive unit 16 for making the distance between the variable distance and the variable distance. The primary resonance frequency of the variable distance drive unit 16 is
It is 5 kHz or more.

More specifically, as described with reference to FIG. 1 in the description of the first embodiment, the second group lens 1
A first lens 2 composed of a surface 4 and a second surface 5;
A second lens 3 composed of a surface 6 and a third surface 7;
A substantially cylindrical bobbin 8 for holding the second lens 3 by fitting the outer peripheral portion 3 a of the lens 3, and a gap 9 between the bobbin 8.
And a yoke 10 arranged on the outer periphery of the second surface 5 of the first lens 2, a coil 11 wound on the outer periphery of the bobbin 8, and a coil 1 attached to the yoke 10.
1, a magnet 13, which is an Archimedes spring mechanically connecting the bobbin 8 and the yoke 10, and a fourth surface 7 of the second lens 3.
And a protective material 15 disposed on the outer peripheral portion of the.

The shape and the like of each part are as described in the first embodiment. The sealing of the viscous fluid into the gap 9 is determined by the value of the primary resonance frequency of the second lens group 1.

In the second lens group 1, a so-called voice coil motor is constituted by the bobbin 8, the yoke 10, the coil 11, the magnet 12, and the spring 13.
A viscous fluid 14 is sealed in a gap 9 provided in the voice coil motor to form a variable distance driving unit 16.

That is, the second lens group 1 is moved relative to the first lens 2 and the first lens 2 by the variable distance drive unit 16 driven by the Lorentz force generated between the coil 11 and the magnet 12. Yoke 11, spring 13
The distance between the second lens 3 and the elastically supported second lens 3 having a degree of freedom in the optical axis direction via the bobbin 8 is variable.

The second lens group 1 having the variable distance drive unit 16 is supported by an objective lens drive actuator in the optical pickup so as to be movable in the optical axis of the laser light emitted from the semiconductor laser and in the direction perpendicular to the optical axis. ing.

The primary resonance frequency of the second lens unit 1 is set to 5 kHz or more by the variable distance drive unit 16. The primary resonance frequency mainly depends on the distance variable drive unit 1.
6 is determined by the spring 13 and the weight of the movable part.

The two-unit lens 1 includes a variable distance drive unit 16
By setting the primary resonance frequency to 5 kHz or more, even when the objective lens driving actuator is driven, the first lens 2 and the second lens 3 can be connected to each other without providing the position servo unlike the related art. Distance L between
You can keep _one properly.

For example, when the second group lens 1 is employed in a compatible optical disk player, it effectively works at the time of recording and reproducing information signals on optical disks having different substrate thicknesses.

That is, the second lens group 1 in which the distance between the first lens 2 and the second lens 3 is appropriately adjusted by applying to the optical disc mounted on the optical disc player,
Even if the objective lens drive actuator is driven during recording and reproduction, the distance between the first lens 2 and the second lens 3 can be kept constant by the variable distance drive unit 16.

Further, even when the objective lens driving actuator is driven by the unevenness of the substrate thickness in the radial direction of the optical disk, the second lens group 1 is moved by the variable distance driving unit 16.
The distance between the first lens 2 and the second lens 3 can be kept constant.

Therefore, the optical disk player provided with the second lens group 1 can record and reproduce information signals on an optical disk having an uneven substrate thickness or an optical disk having a different substrate thickness without deteriorating information signals.

Further, the distance between the first lens 2 and the second lens 3 is changed by the distance variable drive unit 16 without being affected by, for example, an external impact. Can be kept constant.

The second lens unit 1 includes a variable distance drive unit 1
Since 6 is composed of simple components, the same operation as that of the conventional position servo can be provided at a low cost and provided by a lightweight structure.

As described above, the second lens group 1 maintains the distance between the first lens 2 and the second lens 3 constant even during focus servo by setting the primary vibration frequency to 5 kHz or more. This works effectively even at a damping rate at which a value of the phase margin to which the focus servo can be appropriately applied cannot be obtained, for example. This will be described by applying the second group lens 1 to an optical pickup and using an open loop characteristic of a focus servo obtained at that time.

Here, the open loop characteristic of the focus servo is obtained by an optical pickup modeled in the same manner as in the first embodiment.

That is, equations of motion, transfer functions, and calculation conditions relating to the modeling of the optical pickup use the same equations and Table 1 as in the above-described first embodiment. However,
The primary resonance frequency of the variable distance drive unit 16 is 5 kHz,
Attenuation factor xi] ₂ is set to 0.1.

FIG. 9 shows the open-focus state of the focus servo.
Shows the loop characteristics. In FIG. 9, the horizontal axis indicates the vibration frequency of the objective lens driving actuator 21, and the vertical axis indicates the gain and phase characteristics of the displacement of the objective lens driving actuator 21.

According to FIG. 9, the cut-off frequency of the objective lens driving actuator 21 is 1 kHz to 2 kHz.
It is shown that the phase margin is approximately 50 ° near z.

This is a result of shifting the value of the phase margin that is not appropriate for the focus servo to a frequency region higher than the cutoff frequency of the focus servo by setting the primary resonance frequency of the variable distance drive unit 16 to 5 kHz. Yes,
In FIG. 9, the phase change observed after the vibration frequency of 6 kHz is due to the shift.

Therefore, since the optical pickup 20 includes the second lens group 1 having the variable distance drive unit 16 having a primary resonance frequency of 5 kHz or more, the value of the phase margin of the open loop characteristic of the focus servo is appropriate. The value can be obtained, and the focus servo can be applied stably.

FIG. 10 shows one of the distance variable driving units 16.
The relationship between the next resonance frequency _f02 and the gain margin is shown.

FIG. 10 shows the result that the gain margin increases as the primary resonance frequency f ₀₂ increases. That there is a gain margin, but to indicate the stability of the control system, thus, the second group lens 1, a large value first resonance frequency f _02, for example, comprise a distance variable drive unit 16 which is set above 5kHz Thus, the focus servo can be stably applied.

[0111]

The objective lens for an optical pickup according to the present invention includes variable distance driving means for supporting a second lens opposed to the first lens via a gap, and the attenuation factor is controlled by the variable distance driving means. Is set to 0.5 or more, it is possible to maintain a constant distance between the first lens and the second lens even during focus servo or the like while enabling a high numerical aperture. Therefore, the objective lens for an optical pickup can provide a stable focus servo in the optical pickup.

Further, since the objective lens for the optical pickup does not require a complicated structure unlike the position servo, the objective lens can be manufactured at low cost and the weight can be reduced.

The objective lens for an optical pickup according to the present invention includes variable distance driving means for supporting a second lens opposed to the first lens via a gap, and the primary variable lens driving means uses the variable distance driving means. By setting the resonance frequency to 5 kHz or more, the distance between the first lens and the second lens can be kept constant even during focus servo or the like while enabling a high numerical aperture. Therefore, the objective lens for an optical pickup can provide a stable focus servo in the optical pickup.

Further, since the objective lens for an optical pickup does not require a complicated structure unlike the position servo, it can be manufactured at low cost and can reduce the weight.

[Brief description of the drawings]

FIG. 1 is a perspective sectional view of a two-unit lens according to a first embodiment, which is configured to be applied to an objective lens for an optical pickup according to the present invention.

FIG. 2 is a configuration diagram used for describing a setting of a distance between a first lens and a second lens of the second group lens.

FIG. 3 is a characteristic diagram showing spherical aberration when a distance between the first lens and the second lens is adjusted.

FIG. 4 is a configuration diagram showing a modeled second group lens and an optical pickup including the second group lens;

FIG. 5 is a characteristic diagram showing a phase change when an attenuation factor of a variable distance driving unit provided in the second group lens is 0.1.

FIG. 6 is a characteristic diagram showing a phase change when an attenuation factor of a variable distance drive unit provided in the second group lens is 2.

FIG. 7 is a characteristic diagram showing a relationship between the attenuation rate and a phase margin.

FIG. 8 is a characteristic diagram showing an actual phase change of the variable distance drive unit between the two lens units.

FIG. 9 is a characteristic diagram showing a phase margin when the primary resonance frequency of the variable distance drive unit included in the second group lens according to the second embodiment is 5 kHz.

FIG. 10 is a characteristic diagram showing a relationship between the primary resonance frequency and a phase margin.

FIG. 11 is a configuration diagram of a two-group lens used to describe a conventional two-group lens.

[Explanation of symbols]

Reference Signs List 1 2 group lens, 2 first lens, 3 second lens, 9 gap, 13 spring, 14 viscous fluid, 16 variable distance drive

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Shinichi Kai 6-7-35 Kita-Shinagawa, Shinagawa-ku, Tokyo Inside Sony Corporation (72) Inventor Akira Suzuki 6-7-35 Kita-Shinagawa, Shinagawa-ku, Tokyo Sony Corporation

Claims (6)

[Claims] A first lens on which light from a light source is incident; and a second lens having a surface facing the information recording medium, wherein the first lens and the second lens are driven by an objective lens. Means for supporting the second lens opposing the first lens via a gap, wherein the second lens is movably supported in the optical axis of the light and in the direction perpendicular to the optical axis. An objective lens for an optical pickup, comprising: variable distance driving means for changing a distance between the first lens and the second lens, wherein the variable distance driving means has an attenuation factor of 0.5 or more. 2. The objective lens for an optical pickup according to claim 1, wherein the attenuation factor is 1 or more. 3. The objective lens for an optical pickup according to claim 1, wherein the attenuation factor is set to 0.5 or more by filling a viscous fluid in the gap. 4. The variable distance driving means changes a distance between the first lens and the second lens by a Lorentz force between a current applied to a coil and a magnet. The objective lens for an optical pickup according to claim 1, wherein 5. A first lens on which light from a light source is incident, and a second lens having a surface facing the information recording medium, wherein the first lens and the second lens are driven by an objective lens. A second group lens supported movably in a direction perpendicular to the optical axis of the light by the means and supporting the second lens opposed to the first lens via an elastic body; A variable distance driving means for changing a distance between the first lens and the second lens, wherein a primary resonance frequency of a driving system by the variable distance driving means is 5 kHz or more. Objective lens. 6. The variable distance driving means uses a current applied to the coil and a Lorentz force between the magnet and
6. An objective lens for an optical pickup according to claim 5, wherein a distance between said first lens and said second lens is changed.