Method for correcting influence of thickness unevenness of recording medium, information recording/reproducing apparatus using the same method and optical head unit (09-Mar-2010)

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 11/299,660, now U.S. Pat. No. 7,260,029, filed Dec. 13, 2005, which is a divisional of U.S. application Ser. No. 10/095,007, filed Mar. 12, 2002, now U.S. Pat. No. 7,145,846 and for which priority is claimed under 35 U.S.C. §121. This application is based upon and claims the benefit of priority under 35 U.S.C. §119 from prior Japanese Patent Application No. 2001-232633, filed Jul. 31, 2001.

The entire contents of each of the above-identified applications for which priority is claimed is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an information recording/reproducing apparatus, which is capable of, by focusing light onto a light reflecting layer or a recording layer in an information recording medium comprising a transparent substrate or a transparent protective layer and a single or plural light reflecting layers or recording layers from the transparent substrate or the transparent protective layer, reproducing information recorded in the light reflecting layer or the recording layer or recording information in the information recording medium, and further having a correcting mechanism capable of detecting a thickness unevenness from light incident side for use in reproducing or recording, stretched from the surface of the transparent protective layer or the transparent substrate to the light reflecting layer or the recording layer and a correcting mechanism capable of correcting an influence of the detected thickness unevenness, and a method for correcting the thickness unevenness employed therein, and more particularly to,

• 1) A method for correcting a thickness unevenness by the time when information is reproduced or recording of information is started, after an information recording medium is loaded on the information recording/reproducing apparatus, and startup of control of that method;
• 2) A method for correcting an influence of thickness unevenness by the time when information is reproduced or recording of the information is started, just after a layer, which light is converged, of the light reflecting layer or the recording layer is moved (the layer is switched) with respect to a recording medium having plural light reflecting layers or recording layers, and startup of control of that method, and
• 3) A method for detecting a jump between layers (irregular shift of light converging spot between layers) generated at random, when information is reproduced or information is recorded by converging light to any layer in an information recording medium having plural light reflecting layers or recording layers.

2. Description of the Related Art

Jpn. Pat. Appl. KOKAI Publication No. 2000-171346 has disclosed an example of detecting a defocus of an objective lens by means of a defocus detecting system for detecting the defocus according to a knife edge method, and detecting a spherical aberration or a thickness unevenness in a substrate of a transparent recording medium with a single photo detector 7.

In the aforementioned detecting optical system, a hologram 2 used for dividing a light converging spot including the optical axis center into two sections extracts half of light from the center, and separates it to light 2a near the optical axis center and light 2b far from the optical axis center. Then, a light converging spot P1 of the light 2a near the separated optical axis center and a light converging spot P2 of the light 2b far from the optical axis center are detected on division border lines of split detectors 7a-7b and 7c-7d disposed at focusing positions with respect to a photomagnetic disk 6 upon focusing.

As for the signal detection method, this publication indicates

• i) detecting a difference in detecting signals from any one (7a-7b or 7c-7d) of the 2-split detectors making a pair as a defocus detecting signal,
• ii) calculating a difference in differential value of the detection signals from the 2-split detectors (7a-7b and 7c-7d) making a pair so as to detect a spherical aberration.

Generally, in the information recording/reproducing apparatus (optical disk drive unit) including the aforementioned example disclosed in the Jpn. Pat. Application No. 2000-171346, the spot size D (diameter) of a light converging spot to be irradiated to the recording layer or the light reflecting layer of the information recording medium (optical disk) in order to record information in the recording medium or reproduce information therefrom has such a relation of D∝λ/NA between a numerical aperture NA of the objective lens and wavelength λ of light.

Because the recording density of information to be recorded in the information recording medium depends upon this spot size D largely, this spot size D needs to be reduced in order to improve the recording density.

In a widely prevailing CD type disk, the NA of the objective lens is substantially 0.47 and the wavelength of light for use is λ=780 nm.

In the DVD type disk, the NA of objective lens is substantially 0.60 and the wavelength of light for use is λ=650 nm. Currently, it has been proposed to obtain a recording density several times the DVD type by employing an objective lens whose NA is about 0.9 and light of wavelength of about λ=400 nm to meet a demand for higher density.

In the current CD type and DVD type disks, when converging light on the light reflecting layer or the recording layer, light is irradiated from the side of the substrate or supporting body (beyond the substrate). If it is intended to converge light on the light reflecting layer or the recording layer by intensifying the NA of the objective lens and shortening the wavelength of light, light is irradiated from the side of a covering layer (transparent protective layer) which functions as a protective film for the light reflecting layer or the recording layer provided opposite to the substrate (supporting body).

However, if a thickness unevenness occurs in the thickness of the transparent protective layer, spherical aberration occurs so that light spot converged on the light reflecting layer or the recording layer is expanded thereby providing a problem that recording or reproduction characteristic deteriorates. Meanwhile, the amount of thickness unevenness in the transparent protective layer or the amount of the spherical aberration with respect to the thickness unevenness increases proportional to the fourth power of the NA of the objective lens.

Further, to increase the recording capacity of the information recording medium, it has been already proposed to provide the DVD type disk with two layers of the recording layers or the light reflecting layers and converge light to only any one layer of the respective layers in the same direction. By providing the recording layer or the light reflecting layer with two layers each (or more), a distance from the transparent protective layer differs depending on each layer. Thus, there is generated such a problem that by using an objective lens with a higher NA than 0.6 (for DVD-disk), the degree of spherical aberration generated by the thickness unevenness exceeds its allowance largely in all the layers or some layer exceeding its allowance may occur.

For the reason, there exists a necessity of measuring a thickness unevenness in the transparent protective layer or a substantial thickness unevenness generated in the light reflecting layer or the recording layer so as to correct the spherical aberration in a real time.

Meanwhile, the aforementioned Jpn. Pat. Appl. KOKAI Publication No. 2000-171346 has not described anything about a correction method (control method) for removing an influence of the thickness unevenness based on the detected spherical aberration.

Further, if there are two or more recording layers or light reflecting layers, the light converging spot generated at random often may be moved to a different recording layer or light reflecting layer due to disturbance such as vibration (hereinafter referred to as abnormal jump between the recording layers), thereby leading to difficulty of focus control (generating defocus).

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to provide an information recording/reproducing apparatus ensuring a high reliability with respect to thickness unevenness in a transparent resin layer located between a recording layer or a light reflecting layer and an objective lens, and spherical aberration generated depending on light converging position with respect to a plural-layer film when light is focused on the recording layer or the light reflecting layer through an objective lens and a method of thickness unevenness correction control for removing an influence of the thickness unevenness.

According to an aspect of the present invention, there is provided an optical head unit comprising:

a light source supplies light of a predetermined wavelength;

an objective lens focus the light from the light source to the recording layer of the recording medium;

an objective lens moving mechanism moves the objective lens in the optical axis direction and in the direction intersecting a guide groove and a signal mark string formed in the recording medium preliminarily;

a defocus detecting system detects a defocus in the objective lens;

a thickness unevenness detecting system detects a thickness unevenness in a transparent resin layer of the recording medium provided nearest the objective lens; and

a thickness unevenness correcting mechanism changes a focusing characteristic of light impinging upon the objective lens from the light source based on a change in the thickness of the transparent resin layer of the recording medium detected by the thickness detecting system.

According to an other aspect of the present invention, there is provided an information recording/reproducing apparatus for reproducing information recorded in the recording layer or recording the information in a recording medium, the information recording/reproducing apparatus including an optical head unit comprising:

a light source

an objective lens focus light from the light source to a recording layer of the recording medium;

an objective lens moving mechanism moves the objective lens in the optical axis direction and in the direction intersecting a guide groove and a signal mark string formed in the recording medium preliminarily;

a defocus detecting system detects a defocus in the objective lens;

a thickness unevenness detecting system for detecting a thickness unevenness in a transparent resin layer of the recording medium provided nearest the objective lens; and

a thickness unevenness correcting mechanism changes a focusing characteristic of the light impinging upon the objective lens from the light source based on a change in the thickness of the transparent resin layer of the recording medium detected by the thickness detecting system, wherein

the thickness unevenness of the transparent resin layer is detected using a defocus detecting signal detected by the defocus detecting system so as to remove an influence of the defocus of the objective lens by removing an influence of the thickness unevenness of the transparent resin layer.

According to a still other aspect of the present invention, there is provided a method for removing an influence of thickness unevenness in a recording medium upon reproducing information recorded in the recording layer of the recording medium or recording information in the recording medium, including an optical head comprising: a light source for supplying light of a predetermined wavelength; an objective lens for focusing light from a light source to a recording layer of a recording medium; a defocus detecting system for detecting a defocus generated when light converged to the recording medium by the objective lens is not focused at a predetermined position; a thickness unevenness detecting system including at least two photo detecting regions and for detecting a thickness unevenness (spherical aberration) in a transparent resin layer of the recording medium provided nearest the objective lens; a thickness unevenness correcting mechanism for changing focusing characteristic of light impinging upon the objective lens from a light source based on a change in thickness of the transparent resin layer of the recording medium detected by the thickness unevenness detecting system; and a defocus correcting mechanism for correcting a defocus detected by the defocus detecting system, the method comprising:

when changing the recording layer which light is focused by the objective lens from a currently focused recording layer to another recording layer, terminating changing of the focusing characteristic of light by the thickness unevenness correcting mechanism, terminating defocus correction control by the defocus correcting mechanism, and moving a light converging position by the objective lens.

Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention, and together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the invention.

FIG. 1 is a schematic view for explaining an example of the basic structure of an optical head unit and an information recording/reproducing apparatus incorporating the optical head unit of the present invention;

FIG. 2 is a schematic view for explaining an example of the structure in more detail, of the optical head unit and the information recording/reproducing apparatus incorporating the optical head unit shown in FIG. 1;

FIG. 3 is a schematic view for explaining occurrence of a shift of a circle of least confusion due to spherical aberration, which is a phenomenon generated when the thickness of a transparent protective layer near an objective lens changes while a laser beam converges on a recording layer near a covering layer of an optical disk (information recording medium) in an optical disk unit;

FIG. 4 is a schematic view for explaining changes in light intensity of light spots of the laser beam at respective positions corresponding to shift positions of the circle of confusion, when a converging position or the minimum circle of confusion by a spherical lens in a direction along an optical axis of a detecting optical system shifts in the optical disk unit shown in FIG. 2;

FIG. 5 is a schematic view for explaining a shift amount along the optical axis of ±first-order light generated by a hologram device for defining an optimum range of a distance between A and O and a distance between B and O shown in FIG. 4, and detection characteristic thereof;

FIG. 6 is a graph showing a calculation result based on an expression (5);

FIGS. 7A and 7B are schematic views for explaining a principle that detection sensitivity can be improved by an increase of the occurrence of the spherical aberration described with reference to FIG. 3;

FIG. 8 is a schematic view for explaining a principle capable of providing a shift amount along the optical axis of ±first-order light generated by a hologram device for defining an optimum range of a distance between A and O and a distance between B and O shown in FIG. 4, and detection characteristic thereof;

FIG. 9 is a graph showing changes in a value Q when c and σ calculated according to equations (10-23) to (10-25) change;

FIG. 10 is a graph for comparing the frequency characteristics (transfer functions) of a defocus correction control circuit and a thickness unevenness correction control circuit;

FIGS. 11A to 11F are schematic views for explaining the characteristic of a defocus detecting signal and the characteristic of a thickness unevenness detecting signal in the optical disk unit 10 (optical head unit and information recording/reproducing apparatus using the same optical head unit shown in FIG. 1) shown in FIG. 2;

FIG. 12 is a schematic view for explaining another embodiment of the optical disk unit shown in FIG. 2;

FIG. 13 is a schematic sectional view for explaining still another embodiment of the optical disk unit shown in FIG. 2;

FIG. 14 is a schematic sectional view for explaining a further embodiment of the optical disk unit shown in FIG. 2;

FIG. 15 is a schematic sectional view for explaining a still further embodiment of the optical disk unit shown in FIG. 2;

FIG. 16 is a schematic sectional view for explaining a yet still further embodiment of the optical disk unit shown in FIG. 2;

FIGS. 17A to 17G show a relative position of defocus and changes in the output of the thickness unevenness detecting signal and thickness unevenness detecting sum signal;

FIGS. 18A and 18B show an example of a method for extracting a start timing of thickness unevenness detection/correction control by detecting a status having a small thickness unevenness amount using the sum signal of the thickness unevenness detecting signal as an applied example other than detection of abnormal jump between converging spot recording layers;

FIGS. 19A and 19B show a processing method for recording information across plural recording layers 3b and 3d from a side of the information recording medium 3 and for reproducing the same information, which is a modification of the method for extracting a start timing of thickness unevenness detection/correction control by detecting a status having a small thickness unevenness amount using the sum signal of the thickness unevenness detecting signal described with reference to FIGS. 18A and 18B; and

FIG. 20 is a schematic diagram (flow chart) for explaining a process for detecting a jump (an undesired shift of converging light between layers) between the recording layers on which the converging light is focused.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, the embodiments of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 is a schematic view for explaining an example of the basic structure of an optical head unit and an information recording/reproducing apparatus incorporating the optical head unit of the present invention.

As shown in FIG. 1 each of the optical head unit and the information recording/reproducing apparatus 1 incorporating the optical head unit includes a laser device 2 for emitting a laser beam of a predetermined wavelength, an objective lens 4 for focusing the laser beam emitted from the laser device 2 on an information recording medium or an arbitrary recording layer in an optical disk 3 capable of achieving a high density recording, in which two layers are provided on a single side thereof, and defocus correcting coils 5 for changing the position of the objective lens 4 so that a distance between an arbitrary recording layer of the optical disk 3 and the objective lens 4 matches a focal distance inherent of the objective lens 4.

A thickness unevenness (spherical aberration) correcting mechanism 101 for correcting the unevenness (spherical aberration) in thickness of a surface covering layer 3a provided on the side on which the laser beam incidence side of the optical disk 3 is provided between the laser device 2 and the objective lens 4.

A defocus detecting system 102 is provided between the optical disk 3 and the laser device 2. The defocus detecting system 102 detects a defocus, which is a deviation of the position of the objective lens 4 according to the laser beam directed from the laser device 2 to the optical disk 3 and a reflect laser beam which is reflected by an arbitrary recording plane of the optical disk 3 and split by a beam splitter 6 for splitting the laser beam reflected by the recording layer of the optical disk 3. The thickness unevenness correcting mechanism 101 brings the status of the laser beam incidence upon the objective lens 4 near a status having no thickness unevenness, based on a thickness unevenness component detected by a thickness unevenness (spherical aberration) detecting system 103, which picks up the thickness unevenness (spherical aberration) of the covering layer 3a in the optical disk 3, included in the defocus component detected by the defocus detecting system 102. Meanwhile, the position of the objective lens 4 is controlled independently by a defocus correcting circuit 105 corresponding to the defocus detected by the defocus detecting system 102.

According to the principle of thickness unevenness detection (spherical aberration detection), as shown in FIG. 1, a thickness unevenness detection (spherical aberration detection) signal is obtained only when the defocus correction is carried out completely (on focusing). This indicates a necessity of detecting the defocus at a very high precision. To meet that demand, the defocus is detected using all spot sections of a laser beam (if knife edge method is employed to detect the defocus, the defocus detection is carried out using half of detected light extracted by splitting along a straight line including the vicinity of an optical axis in which the detection accuracy is stabilized most). As a result, even if the laser beam contains much spherical aberration, the defocus can be detected very stably at a high precision.

FIG. 2 is a schematic view for explaining an example of the structure in more detail, of the optical head unit and the information recording/reproducing apparatus (hereinafter referred to as optical disk unit) incorporating the optical head unit shown in FIG. 1.

As shown in FIG. 2, an optical disk unit 10 records information in an arbitrary recording layer of the optical disk 3, which is an information recording medium, and reproduces information from the optical disk 3. More specifically, in the optical disk unit 10, a laser beam 12 is irradiated from a laser unit 11, which is a light source, to a predetermined recording layer 3d or 3b in the optical disk 3 and a reflect laser beam 12′ reflected from the arbitrary information recording layer 3d or 3b in the optical disk 3 is received so as to reproduce information recorded in the optical disk 3. When information is recorded in the optical disk 3, an intensity of the laser beam emitted from the laser device 2 changes intermittently by changing the magnitude of a laser driving current supplied to the laser unit 11 corresponding to data to be recorded (information) is irradiated to an arbitrary information recording layer of the optical disk 3. Meanwhile, recording of information into the optical disk 3 and reproduction of information from the optical disk 3 will be described in detail later. Although not shown, needless to say, a guide groove acting as a guide when information is recorded and a signal mark string, which is already recorded information, are formed in the information recording layers 3d and 3b of the optical disk 3.

In the optical disk unit 10 shown in FIG. 2, laser beam 12 emitted from the laser unit 11 is collimated by a collimate lens 13, irradiated into a polarization beam splitter 14 and passes through it toward the optical disk 3. The laser beam 12 passing through the polarization beam splitter 14 passes a λ/4 plate 15, a thickness unevenness correcting concave lens 16 and a thickness unevenness correcting convex lens 17 successively and then is guided to an objective lens 19. Meanwhile, the thickness unevenness correcting convex lens 17 is formed so as to be movable in the direction of the optical axis by means of a thickness unevenness correcting convex lens driving coil 18. The objective lens 19 is formed so as to be movable independently in the optical axis direction and in each of directions perpendicular to a track (guide groove) and the signal mark string (not shown) formed in the optical disk 3.

The laser beam 12 guided by the objective lens 19 is provided with predetermined convergence so that it is converged to a predetermined recording layer of the optical disk 3. In the optical disk (information recording medium) 3, a recording layer or an optical reflecting layer 3d is provided on one face of a substrate 3e such that it adjoins the substrate 3e (adjacent the substrate), followed by a space layer 3c transparent to the wavelength of the laser beam 12, a recording layer or an optical reflecting layer 3b (adjacent a covering layer) apart from the recording layer or the optical reflecting layer adjacent the substrate 3e and a transparent protective layer (light irradiation side covering layer) 3a, layered successively.

The laser beam 12 focused on any one of an arbitrary recording layers (or light reflecting layers) 3d and 3b in the optical disk 3 by the objective lens 19 forms a recording mark (pit) in that light focused recording layer by changing the characteristic of the phase of the recording layer while reflected laser beam 12′ generated slightly is returned to the objective lens 19. On the other hand, when information is reproduced, light intensity is changed depending on the status of the recording layer and reproduced laser beam (reflected laser beam) 12′ is returned to the objective lens 19. Because the reproduced laser beam 12′ and the reflected laser beam 12′ are handled substantially in the same manner in a signal reproducing system described below, hereinafter, the reproduced laser beam 12′ will be described below.

The reproduced laser beam 12′ returned to the objective lens 19 passes the thickness unevenness correcting convex lens 17, the thickness unevenness correcting concave lens 16 and the λ/4 plate 15 to be sent back to the polarization beam splitter 14. The polarization beam splitter 14 separates the reflected laser beam 12′ from laser beam 12 directed from the laser unit 11 toward the objective lens 19 (optical disk 3).

The reproduced laser beam 12′ separated from the laser beam 12 by the polarization beam splitter 14 is divided to substantially ½ each by a half prism 22.

After the separation, one laser beam 12′ is provided with predetermined convergence by a spherical lens 23 and then provided with predetermined focusing characteristic in a direction perpendicular to the optical axis (section of the laser beam 12′) by cylindrical lens 24 provided after the lens 23 so that it is focused on a light receiving plane of a first photo detector 25 for use in detecting a defocus and track shift. Meanwhile, the first photo detector 25 is a four-division photo detector having four light receiving regions 25a, 25b, 25c and 25d, produced by dividing with two straight lines passing the optical axis and perpendicular to each other. For explanation of a light receiving pattern, FIG. 2 shows a plan view of the condition in which the separated reproduced laser beam 12′ is focused.

Remaining separated reproduced laser beam 12′ passes a hologram device 26 in which a predetermined refraction pattern is formed and a sensitizing filter 27 for facilitating detection of a spherical aberration in order. After being provided with predetermined convergence by a spherical lens 28, the beam is focused on a light receiving plane of a second photo detector 29. The second photo detector 29 is a detector in which three light receiving regions 29a, 29b and 29c are disposed in series so that zero-order light and ±first-order light passing the hologram device 26 can be received in an arbitrary direction perpendicular to the optical axis. For explanation of the pattern of the light receiving plane, FIG. 2 shows a plan view of the condition in which the separated reproduced laser beam 12′ is focused. An optical system having the spherical lens 23, the cylindrical lens 24 and the first photo detector 25 corresponds to the defocus detecting system 102 in FIG. 1 while an optical system having the hologram device 26, the sensitizing filter 27 for detecting spherical aberration, the spherical lens 28 and the second photo detector 29 corresponds to the thickness unevenness (spherical aberration) detecting system 103 shown in FIG. 1.

An optical system including the thickness unevenness correcting concave lens 16, the thickness unevenness correcting convex lens 17 and the thickness unevenness correcting convex lens driving coil 18 corresponds to the thickness unevenness (spherical aberration) correcting mechanism 101. A current of a predetermined magnitude and polarity is supplied from a spherical aberration correcting circuit 104 shown in FIG. 1 to the thickness unevenness correcting convex lens driving coil 18 and consequently, the thickness unevenness correcting convex lens 17 is moved so as to change a distance between the thickness unevenness correcting convex lens 17 and the thickness unevenness correcting concave lens 16, thereby making it possible to correct an influence of the spherical aberration (unevenness of the thickness of the covering layer 3a in the optical disk 3).

The reproduced laser beam 12′ focused on the first photo detector (for detecting a defocus and a track shift) 25 is converted to electric signal (current) corresponding to the light intensity of irradiated laser beam 12′ by the four light receiving regions 25a, 25b, 25c and 25d and then converted to a voltage value by preamplifiers 41 (corresponding to the light receiving region 25a), 42 (corresponding to the light receiving region 25b), 43 (corresponding to the light receiving region 25c) and 44 (corresponding to the light receiving region 25d).

Outputs of the respective preamplifiers 41 to 44 are inputted to an adder 71 for summing an output of the preamplifier 41 with an output of the preamplifier 43, an adder 72 for adding an output of the preamplifier 42 with an output of the preamplifier 44, an adder 73 for summing an output of the preamplifier 42 with an output of the preamplifier 43 and an adder 74 for summing an output of the preamplifier 41 with an output of the preamplifier 44.

The outputs of the adders 71 and 72 are subtracted by a subtractor 81 in order to generate a defocus control signal to be supplied to the defocus correcting coil 20 for use in correcting a defocus of the objective lens 19 and amplified (seldom attenuated) to a predetermined level by a gain/band setting circuit 82. After the phase is compensated by a phase compensating circuit 83, the signal is outputted to an adder 85 at a predetermined timing by a switch 84.

A signal supplied to the adder 85 (after the gain and band are set up and the phase is compensated) is added to a reference voltage supplied from a reference voltage generating section 86 and amplified to a predetermined magnitude by an amplifier 87 and supplied to the focus coil 20 at a timing set up by the switch 84.

The outputs of the adders 73 and 74 are subtracted by a subtractor 75 in order to generate the track shift control signal to be supplied to a track shift correcting coil 21 for correcting the track shift of the objective lens 19 and amplified (seldom attenuated) to a predetermined level by a gain/band setting circuit 76. After the phase is compensated by a phase compensating circuit 77, the signal is amplified to a predetermined magnitude by an amplifier 78 and then supplied to the track coil 21.

The outputs of the adders 73 and 74 are summed up by an adder 91 in order to obtain a reproduction signal and a result thereof is supplied to a reproduction signal processing circuit 92.

The reproduced laser beam 12′ focused on the second photo detector (for detecting spherical aberration (thickness unevenness of the covering layer) 29 is converted to an electric signal (current) corresponding to the light intensity of irradiated laser beam 12′ by the light receiving region 29a for receiving the zero-order light and the light receiving regions 29b and 29c for receiving the ±first-order lights and then converted to a voltage value by preamplifiers 31 (corresponding to region 29b), 32 (corresponding to region 29a), and 33 (corresponding to region 29c).

The outputs of the preamplifiers 31 and 33 are supplied to a subtractor 50 and an adder 51 for carrying out addition so as to generate a difference signal and a sum signal between voltages signals obtained from the ±first-order light.

The difference signal obtained by the subtractor 50 is amplified (in some rare cases attenuated) to a predetermined gain by a gain/band setting circuit 52 and after that, a predetermined band is set up. After the phase is compensated by a phase compensating circuit 53, the signal is outputted to an adder 55 at a predetermined timing by a switch 54.

The difference signal supplied to the adder 55 (after the gain and band are set up and the phase is compensated) is added to a reference signal supplied from a reference voltage generating section 56 and amplified to a predetermined magnitude by an amplifier 57. After that, the signal is supplied to the thickness unevenness correcting convex lens driving coil 18 at a timing set up by the switch 54.

The sum signal obtained by the adder 51 is compared with a signal obtained in a comparator 59 by converting photo-electrically the zero-order light produced by attenuating the output of the preamplifier 32 to a predetermined level by means of an attenuator 58 in order to enable comparison with that sum signal, because the sum signal has an intensity base on the ±first-order light. The output of the comparator 59 is employed as a detection signal for detecting abnormal jump between recording layers (described later) 60.

A phenomenon generated when the thickness of the surface covering layer 3a with the laser beam 12 focused on the recording layer 3b (near the covering layer) in the optical disk (information recording medium) 3 will be described with reference to FIG. 3.

The objective lens 19 is designed so as to collect light most (the minimum circle of confusion coincides with the depth of the covering layer) when the thickness of the transparent protective layer (covering layer) 3a is of an ideal thickness).

For example, if the thickness of the surface covering layer 3a is thinner than ideally expected, the spherical aberration occurs so that a laser beam 12 passing outside of the objective lens 19 is focused frontward of a laser beam 12 passing inside of the objective lens 19 in the optical axis direction. Therefore, a position in which the light intensity (center intensity) in a spot section of the laser beam 12 maximizes at the position of the minimum circle of confusion (in the optical axis direction) is moved frontward by δ as compared to when there is no spherical aberration.

Conversely, if the thickness of the surface covering layer 3a is thicker than ideally expected, the position of the minimum circle of confusion is moved in an opposite direction to the example shown in FIG. 3 or deeper in the optical axis direction (rightward in this specification) although not shown.

When the objective lens 19 is moved a space between the recording layer 3d (near the substrate) and the layer 3b (near the covering layer) of the objective lens 19 as well as when the thickness of the surface covering layer 3a changes, the position of the objective lens 19 is corrected so that the spherical aberration becomes 0 while beam is focused on the recording layer 3b (near the covering layer) and then, if a spot of the laser beam 12 is moved to the recording layer 3d (near the substrate), the same phenomenon occurs.

If the change rate of the thickness of the surface covering layer 3a is relatively smaller than ideally expected, the change rate of the thickness and the moving distance δ shown in FIG. 3 can be regarded to be in an approximately proportional relation.

As shown in FIG. 2, according to the present invention, all the reproduced laser beam 12′ is received by the first photo detector 25 and used for detection of the defocus, even if a great deal of spherical aberration components are contained in the reproduced laser beam 12′, the defocus can be detected stably at a high precision.

Because a position where refracted light in the direction of the optical axis of the reproduced laser beam 12′ is focused using the hologram device 26 is shifted by a predetermined amount as shown in FIG. 2, the reproduced laser beam 12′ passing the spherical lens 28 and provided with predetermined convergence is converged by an activity of the hologram device 26 such that refracted light of the +first-order light is focused backward of the light receiving plane of the second photo detector 29 while the −first-order refracted light is focused frontward of the light receiving plane of the second photo detector 29. In other words, the second photo detector 29 is disposed at a position where the zero-order light of the reproduced laser beam 12′ passing the hologram device 26 is converged and a contrast position to the position where the ±first-order lights generated by the hologram device 26 are focused in the direction of the optical axis.

Consequently, the zero-order light of the reproduced laser beam 12′ focused by the second photo detector 29 is focused at a predetermined position of the second photo detector 29 as a small convergent spot 12a while the +first-order light and −first-order light are focused at each predetermined position thereof as larger spots 12b and 12c than the spot 12a.

As shown in FIG. 2, the amount of light of each the zero-order beam spot 12a and the ±first-order beam spots 12b and 12c is detected by the photo detecting cells 29a, 29b and 29c within the second photo detector 29. The photo detecting cells 29b and 29c for detecting the ±first-order beam spots 12b and 12c, can detect only the center portion of the beam spots 12b and 12c so as to detect the light intensity in the center of each of the beam spots 12b and 12c. Meanwhile, a direction in which the track direction (circumferential direction) of the information recording medium 3 is irradiated to the second photo detector 29 is a vertical direction relative to this specification paper in FIG. 2. By intersecting the length direction of each of the photo detecting cells 29b and 29c with the track direction, the light is unlikely to be affected by the refraction pattern contained in light reflected by a pre-groove (not shown) on the information recording medium 3.

More specifically, according to the method for detecting the spherical aberration, the center intensities of the ±first-order beam spots 12b and 12c, which are parts of the laser beams 12 separated by the hologram device 26, or distributions of brightness of the beam spots 12b and 12c or at least a spot size of the beam spots 12b and 12c is compared about two different positions (points A and B in FIG. 4 described later) in the direction of the optical axis of the laser beam 12. That is, by comparing about any one of the center intensities, brightness distributions or spot sizes of the ±first-order beam spots 12b and 12c, the magnitude (quantity) and direction of spherical aberration originated from the thickness unevenness of the surface protective layer (covering layer) 3a are detected.

FIG. 4 is a schematic view for explaining changes in light intensity of light spots of the laser beam 12 at respective positions corresponding to shift positions of the minimum circle of confusion, when a converging position or the minimum circle of confusion by a spherical lens 28 in a direction along optical axis of a detecting optical system shifts.

In FIG. 4, the position “O” indicates a position of a zero-order photo detecting cell 29a in the second photo detector 29 relative to zero-order light having no spherical aberration, the position “A” indicates a position of a photo detecting cell 29b relative to the +first-order light beam and the position “B” indicates a position of a photo detecting cell 29c relative to the −first-order light beam.

Although as evident from FIG. 4, the center intensities of the ±first-order light beams at the positions “A” and “B” coincide with each other in a condition having no spherical aberration or on a curve α, if a slight spherical aberration occurs as indicated on a curve β, the center intensity at the position “B” is stronger than the center intensity at the position “A”. A difference between these center intensities is obtained with the subtractor 50.

If such a large change as a shift of the spot of the laser beam 12 to the recording layer 3d occurs due to a disturbance or the like when the focal point of the spherical lens 28 is matched with the recording layer 3b near the covering layer (if an abnormal jump between the recording layers occurs), the magnitude of the spherical aberration is increased largely as indicated by a curve γ. As a result, the amounts of detected lights drop conspicuously both at the positions “A” and “B”.

That is, because the total amount of lights detected at the photo detecting cells 29b and 29c in the second photo detector 29 drops remarkably when an abnormal jump between recording layers occurs, output signal of the adder 51 shown in FIG. 2 drops largely. On the other hand, because drop of the light amount of the laser beam 12a irradiated by the photo detecting cell 29a is slight even if the abnormal jump between the recording layers occurs, the output signal of the attenuator 58 does not change so much. Thus, by detecting a difference between the both with the comparator 59, a detection signal 60 of detecting an abnormal jump between the recording layers 3d and 3b can be obtained.

Next, optimum ranges of moving amounts (a distance between A and O and a distance between B and O in FIG. 4) in a direction along the optical axis of the ±first-order light beams generated with the hologram device 26 will be described.

First, a consideration model for use in considering a detection characteristic will be described with reference to FIG. 5.

The laser beam 12 emitted from the laser unit 11 described with reference to FIG. 2 passes the collimate lens 13 and the objective lens 19 as shown on the left of FIG. 5 and are focused on the recording layer 3b near the covering layer of the optical disk 3 and the recording layer 3d near the substrate.

On the other hand, the reproduced laser beam 12′ reflected by the recording layer 3b near the covering layer of the optical disk and the recording layer 3d near the substrate passes the objective lens 19 and the spherical lens 28 successively as shown on the right side of FIG. 5 so as to be provided with predetermined focusing characteristic and are irradiated onto the second photo detector 29.

Assuming that the lateral magnification of the detecting system is M, if optical paths of laser beams are parallel between the objective lens 19 acting upon the laser beam 12 directed to the recording layer and the spherical lens 28 acting upon the reflected laser beam 12′, the lateral magnification is given as a ratio between the focal distance of the spherical lens 28 and the focal distance of the objective lens 19. Depth magnification is given as M² in the same optical system.

If a thickness unevenness δ t is generated in the transparent protective layer 3a as described previously with reference to FIG. 3, the position of the minimum circle of confusion of the laser beam 12 focused on the recording layers 3b and 3d of the optical disk 3 by the objective lens 19 shifts by only δ However, after reflected by the recording layer 3b or 3d, the laser beam 12 returned to the objective lens 19 is affected by the thickness unevenness δ t of the transparent protective layer 3a again. A shift amount ζ of the minimum circle of confusion of the laser beam 12′ focused on the second photo detector 29 is expressed by multiplying double a distance between the objective lens 19 and the recording layer of the optical disk 3 (reciprocation distance) with the depth magnification.
ζ=4M²δ  (1)

Assuming that a factor which changes the position of the minimum circle of confusion by only δ is ω, a numerical aperture number of the objective lens 19 is NA, refractive index of the transparent protective layer 3a is n and the amount of defocus is δ z, if when a defocus having a magnitude of δ z occurs, a defocus due to wave aberration is ω₂₀ and a defocus due to a thickness unevenness of the substrate protective layer 3a (that is, it is reasonable that this is originated from spherical aberration) is ω₄₀, the factor is ω=ω₂₀+ω₄₀. The ω₂₀ and ω₄₀ can be expressed in the same way as an expressions (A.1) and (A.2) described in H. Ando et. al.: Jpn J. Appl. Phys. Vol. 32 (1993) Pt. 1, No. 11B p. 5272.

ω 20 = 1 2 ⁢ NA 2 ⁢ δz ⁢ ⁢ and ( 2 ) ω 40 = n 2 - 1 8 ⁢ ⁢ n 3 ⁢ NA 4 ⁢ δ ⁢ ⁢ t ( 3 )

If the δ z in the expression (2) is substituted for δ in (1), the shift amount ξ of the minimum circle of confusion of the laser beam 12′ focused by the second photo detector 29 is expressed as

ζ = 8 ⁢ ⁢ ω 20 ⁡ ( M NA ) 2 ( 4 )

A change in the spot center intensity of the laser beam 12 relative to a spherical aberration coefficient ω₂₀ corresponding to the defocus amount δ z when the wavelength of the laser beam 12 is assumed to be λ, is expressed with following expressions obtained by substituting 0 for η in an expression (10) (here, expressed as an expression (M10) and the expression (M10) is marked at an end of the expression), described in the above quoted H. Ando et. al.: Jpn J. Appl. Phys. Vol. 32 (1993) Pt. 1, No. 11B p. 5272.

I ⁡ ( ω 20 ) ≈ { 1 - exp ⁡ ( - σ 2 ) } 2 + 4 ⁢ ⁢ exp ⁡ ( - σ 2 ) · sin 2 ⁡ ( k ⁢ ⁢ ω 20 2 ) σ 4 + ( k ⁢ ⁢ ω 20 ) 2 ⁢ ⁢ and ( 5 ) k = 2 ⁢ ⁢ π λ ( 6 )

When assuming that distribution of intensity in section of the laser beam 12 impinging upon the objective lens 19 in light transmitting system located on the left (on this paper) relative to the recording medium 3 in FIG. 5 is Gauss distribution, a region of a radius e⁻² from the center is regarded to be an effective beam diameter of the laser beam 12 or a diameter (W) of a beam spot, σ means a value A/W which is a ratio relative to the diameter of the objective lens 19 (here, effective aperture diameter (A)) (σ=A/W).

FIG. 6 shows a calculation result of the expression (5). As evident from FIG. 6, a region in which the change of the center intensity relative to aberration ω₂₀ corresponding to a defocus amount δ z is a range from 0.2 to 0.8 in vertical axis (a relative center intensity under a condition in which the maximum is normalized to “1”).

A defocus amount ω₂₀ corresponding to a defocus amount δ z in which the center intensity is 0.2 is ±0.65 λ on the horizontal axis (axis of “λ” times the wavelength of the laser beam 12) when σ=0 and ±0.65 λ when σ=0.8. On the other hand, the defocus amount ω₂₀ corresponding to the defocus amount δ z in which the center intensity is 0.8 is ±0.26 λ when σ=0 and σ=0.8.

Therefore, if the expression (4) is used, an optimum range of a distance from a focal point of the laser beam 12 by the objective lens 19 to the second photo detector 29 (a distance between A and O and a distance between B and O in FIG. 4) is given in the form of

ζ ≦ 5.2 ⁢ ⁢ λ ⁡ ( M NA ) 2 ⁢ ⁢ and ( 7 ) ζ ≧ 2.1 ⁢ ⁢ λ ⁡ ( M NA ) 2 ( 8 )

As described above, the principle of detecting a thickness unevenness (detecting a spherical aberration) of the transparent protective layer 3a according to the present invention has a feature that a defocus detecting optical system (a portion comprised of the spherical lens 23, a cylindrical lens 24 and the first photo detector 25 in FIG. 1) is prepared separately from a detecting optical system for detecting a thickness unevenness of the transparent protective layer 3a and output originated from the thickness unevenness is corrected using a detecting signal of the thickness unevenness detecting system (spherical aberration detecting system) in a condition that the defocus correcting control is carried out (when focusing is attained).

Next, the structure of the sensitizing filter for detecting the spherical aberration shown in FIG. 2 and sensitizing principle will be described.

The sensitizing filter 27 for detecting the spherical aberration intensifies actual sensitivity upon detecting the spherical aberration by dividing a section of the reproduced laser beam 12′ to at least two sections [dividing a region along an optical path section is generally called “wavefront splitting”] and changing any one or both of i) transmission light amount or ii) phase characteristic with respect to part of light subjected to the wavefront splitting. Intensifying the spherical aberration detection characteristic using the sensitizing filter 27 means a different content of the invention (specific feature of the present invention) independent from the content of the invention described up to here.

Hereinafter, the sensitizing principle will be described in detail.

As described previously with reference to FIG. 3, if the spherical aberration occurs, in the section spot of the reproduced laser beam 12′ returned to the objective lens 19, a component passing outside a component in the optical axis center (region of a predetermined radius including the optical axis) is converged forward of a component passing inside or the region including the optical axis (FIG. 7A shows the same as FIG. 3 again to facilitate a comparison with FIG. 7B). If from the optical axis center to the radius r of the section spot of the reproduced laser beam 12′ impinging upon the objective lens 19 is shielded using this phenomenon as shown in FIG. 7B, the position of the minimum circle of confusion is moved from δ to ε.

According to the present invention, detection characteristic upon detecting the spherical aberration is intensified using an amount of shift from δ to ε of the position of the minimum circle of confusion as shown in FIG. 7A (FIG. 3) and FIG. 7B.

To consider the condition of r for intensifying the spherical aberration detection most by characteristic analysis, the system shown in FIG. 8, which facilitates comparison with FIG. 5 described before will be used as a calculation model.

If the thickness of the transparent protective layer 3a changes by only δ t in the optical disk unit 10 shown in FIG. 2, as shown in FIG. 5, the laser beam 12 reciprocates between the objective lens 19 and the recording layer 3d, so that a spherical aberration equivalent to 2δ t is generated.

After that, when the laser beam 12 passes the sensitizing filter 27 for detecting the spherical aberration before it impinges upon the spherical lens 28 of the detecting optical system, the characteristic of the laser beam 12 changes partly.

“A pseudo spherical aberration generating/sensitizing filter function provided device 127”, which has a spherical aberration and functions as a sensitizing filter by converging a place where this spherical aberration is generated and a portion in which the characteristic of the laser beam 12 is changed to a portion just before the laser beam 12 impinges upon the objective lens 19, will be described.

After the laser beam 12 passes the pseudo spherical aberration generating/sensitizing filter function provided device 127 in FIG. 8, the thickness of the transparent protective layer 3a maintains its ideal status. If the converging spot characteristic in the optical disk 3 is enlarged with a detection optical system having a lateral magnification M, the detection characteristic of the second photo detector 29 shown in FIG. 5 coincides with the detection characteristic of a second photo detector 129 shown in FIG. 8, which is a calculation model for analysis. Meanwhile, the enlargement characteristic (magnification) of the model shown in FIG. 8 can be converted easily according to the expression (1).

FIG. 7B shows an example of the sensitizing filter which shields laser beam impinging upon the objective lens 19 from the optical axis center to its radius r. In FIG. 8, as the characteristic of the sensitizing portion of the pseudo spherical aberration generating/sensitizing filter function provided device 127, laser beam impinging upon the objective lens 19 is split in terms of wavefront to three concentric regions with circumferences of a radius b and a radius “a” as borders and by subjecting only a ring region surrounded by the radius b and the radius “a” to attenuation of transmission light amount and phase change, the change of a position ε in which the center intensity is maximized is analyzed.

Because an optical device which gives attenuation of transmission light amount and phase change to only the ring region surrounded by the radius “a” and the radius b is called apodizer in a special field of optics, a portion acting as a sensitizing filter for detecting spherical aberration in the pseudo spherical aberration generating/sensitizing filter function provided device 127 shown in FIG. 8 will be called “apodizer” in a following description.

Expressions (A-1) and (A-15) to be quoted in a following calculation are quoted from respective expressions described in H. Ando: Jpn. J. Appl. Phys. Vol. 38 (1999) Pt. 1 No. 2A p. 764 Appendix A and will be introduced later. A detailed description for the introduction is omitted.

Coordinates on a pupil on a predetermined surface of the objective lens 19 are defined to be (X, Y) while coordinates on a light converging surface of the optical disk 3 are defined to be (x, y). Because complex amplitude distribution G (x, y) of the light converging spot is in the relation of Fourier transformation relative to a pupil function g (X, Y) of the pupil on a predetermined surface of the objective lens 19, when it is assumed that P₀ is a range of Fourier integration on the pupil on a predetermined surface of the objective lens 19, α is a standardization constant, f is a focal distance of the objective lens 19 and NA is the numerical aperture of the objective lens 19, the wavelength of the laser beam 12 is expressed in λ, so that it can be described as follows.
G(x,y)=αF{g(X,Y)}_P0  (2-1)

If the intensity distribution of laser beam impinging upon the objective lens 19 is approximated to Gauss distribution and it is assumed that the A/W value in the X-axis direction is σx (=(A/W)_x) and the A/W value in Y-axis direction is σy (=(A/W)_Y) and a deviation amount of the center intensity due to lens shift of the objective lens 19 is X₀, a following expression can be introduced.

( X , Y ) = ⁢ exp ⁢ { - ( σ X f · NA ) 2 ⁢ ( X + Xo ) 2 - ( σ Y f · NA ) 2 ⁢ Y 2 } ≈ ⁢ exp ⁢ { - ( ( σ X f · NA ) 2 ⁢ X 2 + ( σ Y f · NA ) 2 ⁢ Y 2 ) - ( σ X f · NA ) 2 · 2 ⁢ ⁢ Xo ⁢ ⁢ X } ( 2 ⁢ - ⁢ 2 )
where in the expression (2-2), X₀ is regarded to be so small that it can be approximated to X₀²≈0.

If the above-described orthogonal coordinate system representation is transformed to polar coordinate system using

r = X 2 + Y 2 f ⁢ ⁢ NA ( 2 ⁢ - ⁢ 3 ) ϕ = tan - 1 ⁡ ( Y X ) ( 2 ⁢ - ⁢ 4 ) ρ = NA ⁢ X 2 + Y 2 λ ( 2 ⁢ - ⁢ 5 ) φ = tan - 1 ⁡ ( y x ) ⁢ ⁢ it ⁢ ⁢ comes ( 2 ⁢ - ⁢ 6 ) σ 2 ≡ σ ⁢ ⁢ x 2 + σ ⁢ ⁢ y 2 2 ⁢ ⁢ and ( 2 ⁢ - ⁢ 7 ) σ _ 2 ≡ σ ⁢ ⁢ x 2 - σ ⁢ ⁢ y 2 2 . ⁢ Here , ⁢ if ( 2 ⁢ - ⁢ 8 ) Δ OL ≡ Xo f ⁢ ⁢ NA ( 2 ⁢ - ⁢ 9 )
is used, the expression (2-2) is transformed to
g(r,φ)=exp{−σ²r²−σ_—²r² cos(2φ)−2σ_x²Δ_OLr cos φ}  (2-10)

The pupil function of the pupil on a predetermined surface of the objective lens 19 when wavefront aberration ω(r,φ) occurs is formulated as follows
g(r,φ)=exp{−σ²r²−σ_—²r² cos(2φ)−2σ_x²Δ_OLr cos φ−ikω(r,φ)}  (2-11)
and
k=2π/λ  (2-12)

Meanwhile, when in the optical disk system, up to quartic or lower term is considered by polynomial development of the wavefront aberration ω(r,φ), it is desirable to consider in a condition that the spherical aberration ωs is
ωs(r,φ)=ω₄₀(r⁴−Qr²+R)  (2-13)
and the defocus ωd is
ωd(r,φ)=ω₂₀r²  (2-14).

In the aforementioned expression (2-13), Q means an optimum value in which the center intensity is maximized according to movement theorem. In the expression (2-13), R is a phase term, which affects a mixing ratio between a real part and an imaginary part of the complex amplitude distribution although it does not affect the center intensity.

In Fourier transformation in polar coordinate system, Henkel's transformation expression is applied and
G(ρ,φ)=αH{g(r,φ)}_P0  (2-15)
is expressed with respect to the expression (2-1).

Then, if the expression (2-11) is transformed to

g ⁡ ( r , φ ) =

1. An optical head unit comprising: a light source configured to supply light of a predetermined wavelength; an objective lens that focuses the light from the light source to the recording layer of a recording medium; an objective lens moving mechanism configured to move the objective lens in the optical axis direction and in the direction intersecting a signal mark string formed in the recording medium; a defocus detecting system configured to detect a defocus in the objective lens; a thickness unevenness detecting system configured to detect a thickness unevenness in a transparent resin layer of the recording medium provided nearest to the objective lens, the thickness unevenness detecting system including a photodetector divided into at least two detecting areas; a thickness unevenness correcting mechanism configured to change a focusing characteristic of light impinging upon the objective lens from the light source based on a change in the thickness of the transparent resin layer of the recording medium detected by the thickness unevenness detecting system; and wherein the thickness unevenness correcting mechanism operates with a smaller gain relative to a servo gain of a DC level of the frequency characteristic of a defocus correcting control circuit.

2. An optical head unit according to claim 1, wherein the detecting areas of the photodetector are arranged separate from each other, and each of the detecting areas of the photodetector detects light that is split by a half mirror.

3. An optical head unit according to claim 1, wherein a pair of detectors for detecting defocus is arranged in between the detecting areas of the photodetector, with a predetermined distance between the pair of detectors for detecting defocus and each of the detecting areas of the photodetector.

4. An optical head unit according to claim 1, wherein the detecting areas of the photodetector are arranged in a 2×2 matrix, and subtraction is performed using a sum of outputs of first two non-adjacent detecting areas of the photodetector and a sum of outputs of second two non-adjacent detecting areas of the photodetector.

5. An optical head unit comprising: a light source configured to supply light of a predetermined wavelength; an objective lens that focuses the light from the light source to the recording layer of a recording medium; an objective lens moving mechanism configured to move the objective lens in the optical axis direction and in the direction intersecting a signal mark string formed in the recording medium; a defocus detecting system configured to detect a defocus in the objective lens; a thickness unevenness detecting system configured to detect a thickness unevenness in a transparent resin layer of the recording medium provided nearest to the objective lens, the thickness unevenness detecting system including a photodetector divided into at least two detecting areas; wherein the thickness unevenness correcting mechanism operates with a smaller gain relative to a servo gain of a DC level of the frequency characteristic of a defocus correcting control circuit; and wherein both of the detecting areas of the photodetector are arranged at substantially equal distances from a center of a minimum circle of confusion.

6. An optical head unit according to claim 5, wherein the detecting areas of the photodetector are arranged separate from each other, and each of the detecting areas of the photodetector detects light that is split by a half mirror.

7. An optical head unit according to claim 5, wherein a pair of detectors for detecting defocus is arranged in between the detecting areas of the photodetector, with a predetermined distance between the pair of detectors for detecting defocus and each of the detecting areas of the photodetector.

8. An optical head unit according to claim 5, wherein the detecting areas of the photodetector are arranged in a 2×2 matrix, and subtraction is performed using a sum of outputs of first two non-adjacent detecting areas of the photodetector and a sum of outputs of second two non-adjacent detecting areas of the photodetector.

9. An information recording/reproducing apparatus for reproducing information recorded in a recording layer or recording information in a recording medium, the information recording/reproducing apparatus including an optical head unit, the optical head unit comprising: a light source configured to supply light of a predetermined wavelength; an objective lens that focuses the light from the light source to the recording layer of a recording medium; an objective lens moving mechanism configured to move the objective lens in the optical axis direction and in the direction intersecting a signal mark string formed in the recording medium; a defocus detecting system configured to detect a defocus in the objective lens; a thickness unevenness detecting system configured to detect a thickness unevenness in a transparent resin layer of the recording medium provided nearest to the objective lens, the thickness unevenness detecting system including a photodetector divided into at least two detecting areas; a thickness unevenness correcting mechanism configured to change a focusing characteristic of light impinging upon the objective lens from the light source based on a change in the thickness of the transparent resin layer of the recording medium detected by the thickness unevenness detecting system; wherein the thickness unevenness correcting mechanism operates with a smaller gain relative to a servo gain of a DC level of the frequency characteristic of a defocus correcting control circuit; and wherein both of the detecting areas of the photodetector are arranged at substantially equal distances from a center of a minimum circle of confusion.

10. An information recording/reproducing apparatus according to claim 9, wherein the detecting areas of the photodetector are arranged separate from each other, and each of the detecting areas of the photodetector detects light that is split by a half mirror.

11. An information recording/reproducing apparatus according to claim 9, wherein a pair of detectors for detecting defocus is arranged in between the detecting areas of the photodetector, with a predetermined distance between the pair of detectors for detecting defocus and each of the detecting areas of the photodetector.

12. An information recording/reproducing apparatus according to claim 9, wherein the detecting areas of the photodetector are arranged in a 2×2 matrix, and subtraction is performed using a sum of outputs of first two non-adjacent detecting areas of the photodetector and a sum of outputs of second two non-adjacent detecting areas of the photodetector.

13. An information recording/reproducing method for reproducing information recorded in a recording layer or recording information in a recording medium, the information recording/reproducing apparatus including an optical head unit, the method comprising: supplying light of a predetermined wavelength; focusing the light to the recording layer of a recording medium; moving an objective lens in the optical axis direction and in the direction intersecting a signal mark string formed in the recording medium; detecting a defocus in the objective lens; detecting a thickness unevenness in a transparent resin layer of the recording medium provided nearest to the objective lens, by a photodetector divided into at least two detecting areas are arranged at substantially equal distances from a center of a minimum circle of confusion; and changing a focusing characteristic of light impinging upon the objective lens from the light based on a change in the thickness of the transparent resin layer of the recording medium and operating with a smaller gain relative to a servo gain of a DC level of the frequency characteristic of a defocus correcting control circuit.