JPH11110813A - Optical disk device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To eliminate an adverse effect due to useless reflected light from the other side recording layer for reflected light of reproducing laser light simultaneously emitted on plural tracks on one side recording layer to be reproduced by specifying an optical condition of a focus detection system. SOLUTION: A quadripartite optical sensor 10 detecting a focus error is made P<2.β.NA0 .df-H (where, P: a length from an outer periphery c/2 of a reflected light spot to the maximum position of the reflected light spot in the direction that anisotropy exists at an optimum operation point time based on the size C of the reflected light spot s1 on the quadripartite optical sensor 10 at the optimum operation point time, NA0 : a numerical aperture(NA) of an objective lens, β: mean image forming magnification from an optical disk to a multi-division sensor, df: 1.2.dDL/n, dDL: a gap between recording layers of optical disk, (n): a refractive index between recording layers of optical disk, H: a half of a variable of a width of the reflected light spot in the two directions that the anisotropy on the quadripartite sensor 10 exists).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical disk apparatus capable of simultaneously and accurately reproducing a plurality of tracks formed on an optical disk.

[0002]

2. Description of the Related Art At present, an optical disk apparatus capable of simultaneously reproducing a plurality of tracks formed on an optical disk is known (for example, Japanese Patent Application Laid-Open No. Hei 8-249720).

In other words, an optical disk device includes a wide area illumination source such as a laser array capable of simultaneously irradiating a plurality of tracks formed on an optical disk, and a light reflected from the optical disk by being illuminated by the wide area illumination source. Light detecting means for detecting the detected light, and means for synthesizing light in a direction along the track detected by the detecting means (radial direction of the optical disk around the detected track). In particular, as the light detection means described above,
The number of photodetectors in the radial direction of the optical disk is sufficiently larger than the number of tracks (about four times or more) so that the optical image reproduced from each track can be sufficiently resolved. It is configured using

As such a detector, PD (photo de
tector), PIN-PD (PIN-photo diode) or CCD (charge cuppled divice), not only one line of sensors in the radial direction, but also TDI (Time D
(Elay and Integration) / A detector, such as a CCD sensor, which performs integration processing in the time axis direction in a two-dimensional arrangement may be used. The focus servo in the optical disk reproducing apparatus for simultaneously reproducing a plurality of tracks on the optical disk described above,
This is the same as that of a general optical disk reproducing apparatus that reproduces only one track. Light reflected from an optical disk is detected by using, for example, a multi-divided photodetector (for example, a 4-divided photodetector). A focus error signal is generated by calculation between outputs of the divided light beams, and the position of the objective lens is adjusted accordingly.

[0005] More specifically, a focus error signal is generated from a detection signal light detected by a light detection unit different from an information signal detection unit for lightly detecting information signals of a plurality of tracks. Alternatively, a focus error signal is generated from a detection signal light detected by a split photodetector provided separately from a pickup optical system having a photodetector for detecting such an information signal.

[0006]

According to the above-described optical disk apparatus for simultaneously reproducing a plurality of tracks on an optical disk, as described below, when reproducing an optical disk having two or more recording layers, a focus is required. An offset occurs in the error.

Problems when playing back an optical disk having two recording layers The structure of an optical disk having two recording layers is, for example, the first and the second through a space of about 50 μm.
A second recording layer is formed, and tracks on which information is recorded concentrically or spirally are formed on the first and second recording layers.

Now, a photodetector (sensor) in the optical disk device is a known optical disk that receives light reflected from only one track in order to simultaneously receive light reflected from a plurality of tracks on the optical disk. The detection width (detection area) is larger than that of the device. Due to such a structure, the sensor receives two reflected lights from tracks in the first and second recording layers slightly separated from each other in an overlapping state.

Here, since the sensor in the optical disk device must have a large detection width (detection area), the sensor related to the reflected light from a plurality of tracks in the recording layer to be reproduced is required. There is a problem in that the focus error signal interferes with (combines with) the focus error signal components related to the reflected light from a plurality of tracks in the recording layer that is not reproduced, thereby causing an offset in the focus error signal.

The present invention has been made in view of the above points. For example, when reproducing an optical disk having two recording layers, reproduction is performed by simultaneously irradiating a plurality of tracks on one recording layer to be reproduced. Astigmatism optics that uses an optical sensor that can reliably eliminate the adverse effects of unwanted reflected light from multiple tracks on the other recording layer in order to obtain the maximum level of RF signal in the focused state of the reflected laser light It is an object of the present invention to provide an optical disk device having a system.

[0011]

In order to solve the above problems, the present invention provides an optical disk device having the following constitutions (1) and (2).

(1) On the basis of reflected light obtained by irradiating a reproduction light beam so as to straddle a plurality of tracks formed on a plurality of recording layers DD2 (D1) and DD5 (D2) of the optical discs D and DD, respectively. In an optical disc apparatus AA that performs information reproduction and detects a focus error using an astigmatism method, multi-divided optical sensors (four-divided sensors) 10A to 10D that detect a focus error are used when a focus error exists. , P <2 · β · NA ₀ · df-H (where P is the size of the reflected light spot s1 on the multi-segmented optical sensor (four-segment sensor) 10A to 10D at the time of the optimum operating point (focusing state)) With reference to (diameter C), from the outer circumference (C / 2) of the reflected light spot at the optimal operating point,
Reflected light spots s12 and s1 other than the optimum operating point
3, s14, s15 and the maximum position (10A1-10D) of the reflected light spot in the directions x, y where astigmatism exists.
Length up to 1) (both shown in FIG. 6) NA ₀ : numerical aperture (NA) of objective lens 4 β: multi-division optical sensor from optical disk D (four-division sensor)
Average imaging magnification from 10A to 10D df: 1.2 · dDL / n dDL: distance between recording layers DD2 and DD5 (resin layer DD4) of optical disk DD n: distance between recording layers DD2 and DD5 (resin layer) of optical disk DD The refractive index between DD4) H: The reflected light spots s12 and s13 on the multi-segment optical sensors (four-segment sensors) 10A to 10D are the focal line positions d
1, d2, two directions x, where astigmatism exists,
The relationship between the reflected light spot s1 (s12, s13, and s) is expressed by the following equation, where the change amount of the width of the reflected light spot at y (long axis b-short axis a) is 2H, and the length is half of the change amount.
14, s15) to be satisfied according to the direction of change,
An optical disk device characterized by being formed.

(2) In the multi-segment optical sensors (4-segment sensors) 10A to 10D used for focus error detection, the length P is P <2 · β · NA ₀ · df-H and P> H / The optical disk device according to the above (1), wherein the optical disk device is formed so as to satisfy the second condition.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The optical disk apparatus of the present invention determines the focus error outside the linear range (S curve opening) of the S curve and the convergence to zero (defocus amount Δ) in the focus error detection by the optical condition (focus detection). Reflection of an optical disc having two recording layers from one recording layer to be reproduced and another reflection layer from the other recording layer by optimally setting a sensor for use, for example, the size of a sensor of a four-division optical sensor. Even if there is light, the occurrence of a focus error due to the unnecessary reflected light is suppressed (offset of the focus error). As a result, when one of the recording layers to be reproduced is reproduced, a zero-cross operation point of the focus error (focus). Is the optimal operating point of the focus servo,
It is configured to obtain a maximum level RF signal.

Hereinafter, the optical disk device of the present invention will be described.
One embodiment will be described with reference to the accompanying drawings. FIG. 1 is a diagram for explaining the configuration of the optical system of the optical disk device of the present invention. Here, an example in which the focus error detection system and the information signal detection system are configured by different optical systems will be described.
In particular, as the light source, laser light emitted from a laser diode is incident on a multi-mode fiber, and is used as a surface light source. Incidentally, a multimode fiber is a bundle of optical fibers to form one optical fiber.

As shown in FIG. 1, an optical disc apparatus AA according to the present invention has a surface light source including a laser diode LD, a coupling lens L, and a multimode fiber F, a collimator lens 1, beam splitters 2, 5, mirrors. 3,
It mainly comprises an objective lens 4 and a detection optical system A. In the figure, D denotes an optical disk (for example, two recording layers (signal surfaces) D).
1 and D2).

Next, the operation of the optical disk device AA of the present invention having the above-described configuration is as follows. Hereinafter, the optical disc D
The reproduction of the information recorded on the signal surface D1 will be described. As shown in FIG. 1, the laser light emitted from the laser diode LD is incident on the incident end side Fi of the multimode fiber F via the coupling lens L. The light incident here is converted into a surface light source (reproducing laser light source) having the diameter of the core of the fiber F at the emission end Fo by the effect of the multimode fiber F (light dispersion effect). The light emitted from the multimode fiber F is converted into substantially parallel light by the collimator lens 1 as a secondary light source (surface light source).
Objective lens 4 via beam splitter 2 and mirror 3
To irradiate the optical disk D (signal surface D1 on which a number of tracks are formed). On the optical disk D, a circular spot s having a size determined by the imaging magnification of the optical system determined by the ratio of the focal length of the collimator lens 1 to the focal length of the objective lens 4 is obtained. On the optical disc D, the area of the spot s is illuminated.

As the multimode fiber F described above,
In the case where the optical system having a diameter of 60 μm is used, if the imaging magnification of the optical system is 4 times (focal length of the collimator lens 1: focal length of the objective lens 4 = 4: 1), the optical disc D
The diameter of the spot s illuminated upward is 15 μm.
The diameter of the spot s is equal to that of the DV
In the case of D (digital versatile disc), 2
The spread is equivalent to 0 tracks (it is about 20 tracks). Even if light unevenness at the periphery of the area of the spot s (a decrease in light intensity with respect to the center) is taken into consideration, a sufficient number of parallel signals (RF signals) of 10 or more tracks (up to about 15 tracks) can be obtained. Spread.
At this time, the focal length of the objective lens 4 is 3 mm, and the focal length of the collimator lens 1 is 12 mm.

The reflected light reflected from the optical disk D (signal surface D1) passes through the objective lens 4, the mirror 3, and the beam splitter 2 again, and is guided to the detection optical system A. In this example, in the detection optical system A, a focus error detection system A1 and an information signal detection system A2 are configured by different optical systems. Specifically, the reflected light guided to the detection optical system A via the beam splitter 2 is further divided into two by the beam splitter 5, and the focus error detection system A1
And the information signal detection system A2.

The information signal detection system A2 has a spread of, for example, about 10 tracks out of a large number of tracks formed on the signal surface D1 of the optical disk D by receiving the reflected light guided from the beam splitter 5 in parallel. It has a configuration that allows simultaneous detection of reflected light. The information signal detection system A2 mainly includes a condenser lens 6, a parallel light receiving element 7,
A, comprises a parallel input serial output conversion circuit 7B. The parallel light receiving element 7A is formed by, for example, assembling a large number of photodetection cells arranged in a mesh.

The information signal detection system A2 having such a configuration condenses the reflected light guided from the beam splitter 5 on all the photodetection cells constituting the parallel light receiving elements 7 via the condensing lens 6. I do. A large number of photodetection cells constituting the parallel light receiving element 7A receive the reflected light at the same time in parallel and collectively receive all the optics in the range (for example, spread of about 10 tracks) irradiated with the spot s. Images (optical images based on the reflected light from the pit trains constituting the respective tracks on the disk D, and optical images obtained by combining all the optical images based on the reflected light from the tracks) are detected and output in parallel. The parallel input / serial output conversion circuit 7B converts the parallel detection output output from the parallel light receiving element 7A into a serial signal, processes the reproduced signal, and outputs an RF signal.

On the other hand, the focus error detecting system A1 uses a multi-split optical sensor through a cylindrical lens to reflect the reflected light guided from the beam splitter 5 similarly to the focus error detecting system in the conventional pickup. This error is detected by the point aberration method. The focus error detection system A1 mainly includes a stop-down lens 8, a cylindrical lens 9, a focus detection sensor 10, which is a multi-segment optical sensor, and a comparator 10E.

A focus error detection system A having such a configuration
1 condenses the reflected light guided from the beam splitter 5 via a stop lens 8 and a cylindrical lens 9 onto, for example, four-divided optical sensors 10A to 10D constituting a focus detection sensor 10. A reflected light spot s1 is illuminated on the four divided optical sensors 10A to 10D. Then, the focus detection sensor 10 and the focus detection sensor 10 ((10A + 10D)-(1
0B + 10C)), the difference photoelectric conversion signal
A focus error signal extracted by a well-known astigmatism method, which detects a focus error by supplying the comparator 10E, is used as a focus / not-shown signal for performing a far / near (focus direction) braking of the objective lens 4 with respect to the optical disc D.
Supplied to the actuator. Thus, the focus error detection system A1 detects a focus error in the optical disc device AA of the present invention. Although not described in detail here, the parallel output signal (RF signal) from the information signal detection system A2 or a signal based on the parallel output signal may be appropriately superimposed on the focus error signal as needed. Of course.

Now, the above-mentioned focus error detection system A
The change in the shape of the reflected light spot s1 on the focus detection sensor 10 that constitutes No. 1 will be described. In the following description, a case where four-divided optical sensors 10A to 10D divided into a cross shape are used as the focus detection sensor 10, but the specific shape of the division is based on the astigmatism method. It goes without saying that other shapes may be used as long as focus errors can be detected. FIG. 2 shows a reflected light spot s on a focus detection sensor (four-division optical sensor).
FIG. 4 is a diagram for explaining a shape change of FIG.

The shape of the reflected light spot s1 formed on the four-divided optical sensors 10A to 10D is, as shown in FIG.
It changes as follows. That is, the reflected light spot s1 becomes a perfectly circular reflected light spot s11 having a diameter c when in a focused state. In addition, the vicinity of the in-focus state (focal line positions d1,
At d2), the reflected light spots s12 and s13 have an elongated elliptical shape with a short axis a and a long axis b. The time when the reflected light spot s12 (s13) is obtained is a time when the objective lens 4 is slightly deviated from the in-focus state and is slightly close (slightly distant) from the optical disk D (signal surface D1). This is when the reflected light spot shape s1 is the thinnest. Further, when the objective lens 4 is far from the focused state and is in a state close to (distant from) the optical disk D (signal surface D1), the above-mentioned reflected light spots s11 to s1
The reflected light spot s14 having an elliptical shape larger than 3
(S15) is obtained. Thus, the reflected light spot s
The fact that 1 has a different shape according to the movement of the objective lens 4 in the focus direction indicates astigmatism characteristics.

The above-mentioned reflected light spot s1 (s11-s
15) is reflected light corresponding to a spot s having a diameter due to surface irradiation that spreads out corresponding to 20 tracks among a large number of tracks in the recording layer to be reproduced on the signal surface D1 of the optical disc D, Unlike a reflected light spot from a conventional optical disc, which is reflected light corresponding to a single spot having a diameter that spreads over one track on the signal face D1, the signal face D1 of the optical disc D
It can be said that surface illumination (surface illumination) is performed above. Therefore, the aforementioned reflected light spot s1 (s11 to s15)
Is the position of the focal line (d1, d2 shown in FIG. 2) where the reflected light spot is thinnest in each direction, and the reflected light spot s1
2 and s13 are not straight lines (only the major axis b, minor axis a = 0), but have a spread (reflected light spots s12 and s13 where the minor axis a does not become zero). It is. The reflected light spot s12 corresponds to the focal line position d1, and the reflected light spot s13 corresponds to the focal line position d2.

The average imaging magnification in the optical system from the signal surface D1 of the optical disc D to the focus detection sensors (four-division optical sensors) 10A to 10D is defined as the average value of the astigmatism magnification in each focal line direction. . If you set this to 20 times,
The diameter of the reflected light spot s11 at the time of focusing is 300 μm. At this time, the average focal length of the focus error detection system A1 is 60 mm. Here, the meaning of “average” in “average focal length” is based on the above-mentioned “average” in “average imaging magnification”.

FIG. 3 is a diagram showing the shape of an S curve in focus error detection. In the figure, a point 0 indicates a focal point, and corresponds to the position in the focused state shown in FIG. −
Δ and Δ indicate the defocus amounts when the focus error becomes almost 0 outside the S curve. Points e and f where the S curve has the minimum value and the maximum value are the positions d1 and d of the focal line shown in FIG.
2 is supported. An area between the points e and f around the focal point 0 is referred to as an S-curve opening Spp. S curve opening Spp
In, the focus error detection signal changes linearly (linear range).

Here, the focus error is the position d of the focal line.
1, d2, the reflected light spot s1 on the focus detection sensor (quadrant optical sensor) 10A to 10D
2, the amount of change in the width of s13 is 2H. 2H = b-
a. Furthermore, approximately, the following equation holds. bc = ca = H (1) where a is the length of the minor axis a of the elliptical reflected light spots s12 and s13 b: is the length of the elliptical reflected light spots s12 and s13 Long axis b
C: Length of the diameter c of the reflected light spot s11 having a perfect circular shape

FIG. 4 is a diagram for explaining the structure of an optical disk having two recording layers. The structure of the optical disc DD having these two recording layers (signal surfaces) is such that the first recording layer DD is provided on the transparent substrate DD1 on which the reproduction laser beam is incident.
2, semi-transmissive film DD3, resin layer DD4, second recording layer DD
5, the reflective film DD6 and the protective layer DD7 are sequentially laminated (the signal surfaces D1 and D2 shown in FIG. 1 correspond to the first and second recording layers DD2 and DD5). .
The first and second recording layers DD2 and DD5 have a typical value of 50 μm.
Tracks on which information is recorded concentrically or spirally are formed on the first and second recording layers DD2 and DD5, respectively. The resin layer DD4 between the first and second recording layers DD2 and DD5 is filled with a medium having a refractive index of n. The thickness of the resin layer DD4 is defined as dDL. In addition, the semi-permeable membrane DD3
Is sufficiently smaller than the thickness of the resin layer DD4. From this, it can be said that the optical distance between the first and second recording layers DD2 and DD5 is dDL / n.

Here, a DVD is considered as a typical application example of the optical disk D. The structure of this DVD is substantially the same as the structure of the optical disk DD having two recording layers shown in FIG. Now, the optical disc apparatus A of the present invention described above.
Consider a case where a DVD is reproduced in A. Then, the focus detection sensors 10 (four-division optical sensors) 10A to 10D constituting the focus error detection system A1 simultaneously reflect the reflected light reflected from an area having a spread corresponding to 20 tracks from a plurality of tracks on the optical disc DD. In order to receive light in parallel, the detection width (detection area) on the focus detection sensors (four-division optical sensors) 10A to 10D is extremely larger than that of a known optical disk device that receives reflected light from only one track. large. Due to such a structure, a focus detection sensor (four-division optical sensor)
10A to 10D are slightly separated first and second recording layers DD
2, the two reflected lights from the track in DD5 are received in an overlapping state.

Therefore, a focus error signal component related to reflected light from a plurality of tracks in a recording layer not to be reproduced interferes with a focus error signal related to reflected light from a plurality of tracks in a recording layer to be reproduced ( Therefore, there is a problem that an offset occurs in the focus error signal corresponding to the recording layer to be reproduced. The condition under which the signal crosstalk is the worst is when the thickness dDL of the resin layer DD4 is the thinnest. In a DVD, dDL = 40 μm is the minimum. Also, the interlayer (resin layer DD4) generally has n = 1.5
Filled with resin. At this time, the first and second
Distance between the recording layers DD2 and DD5 (dDL / n)
Is Becomes

FIG. 5 shows a conventional optical disk apparatus.
FIG. 7 is a diagram comparing the shape of an S curve in focus error detection obtained when reproducing an optical disk DD having two recording layers shown in FIG. 4 with the waveform of a reproduced RF signal.

A focus point RF at which the RF signal obtained when the first recording layer DD2 is reproduced and scanned is the maximum level.
At the focus point F1 corresponding to 1, the focus error signal is not zero (not in a focused state), and a significant offset F1O has occurred. This offset F1O is added to the focus error signal obtained from the reflected light from the first recording layer DD2 to be reproduced and the second recording layer DD5 not to be reproduced.
This is caused by interference (combining) of focus error signal components obtained from reflected light from the camera.

Similarly, the focus point F2 corresponding to the focus point RF2 at which the RF signal obtained when the second recording layer DD5 is reproduced and scanned has the focus error signal is not zero (in the in-focus state). No), a significant offset -F2O has occurred. This offset-
In F2O, a focus error signal component obtained from reflected light from the first recording layer DD2 that is not reproduced interferes (combines) with a focus error signal obtained from reflected light from the second recording layer DD5 to be reproduced. It is caused by this.

The two waveforms of the S curve and the RF signal shown in FIG. 5 correspond to two recording layers (first and second recording layers DD2 and DD).
The optical disk having the above item 5) is obtained by reading and scanning a conventional optical disk device which does not particularly consider the reproduction of such a multi-layer disk, and actually measuring it. Note that this S
The origin of the waveform shape is one layer (the second recording layer DD).
5) The optical distance dDL of the thickness of the interlayer (resin layer DD4)
It can be considered that the same S curve is virtually considered at a position separated by / n and obtained by superimposing both.

FIG. 6 is a view for explaining the shape of the reflected light spot s11 in a focused state on the focus detection sensors (four-division optical sensors) 10A to 10D. The focus detection sensor 10 shown in FIG.
0D. At focusing, 4-split optical sensor 10A-
A four-division optical sensor in the direction x (or y) of the axis of astigmatism from the position corresponding to the radius (c / 2) of the reflected light spot s11 irradiated so that the center coincides with the center of 10D. Far end 10A1-10 of 10A-10D
The length up to D1 is defined as P (here, 4
The divided optical sensors 10A to 10D are assumed to be square.)

FIG. 7 is a diagram for explaining a change in the shape of the reflected light spot on the focus detection sensors (four-division light sensors) 10A to 10D. As shown in FIG.
Reflected light spot s1 on quadrant optical sensors 10A to 10D
Changes as shown in FIG. Focused state (position "focus" shown in FIG. 2, "focus 0" shown in FIG. 3)
At this time, the reflected light spot s11 has a perfect circular shape. Further, when it is in the vicinity of the focused state slightly close to the optical disk D (signal surface D1) (the position d1 of the focal line shown in FIG. 2 and the point e which becomes the minimum value shown in FIG. 3), the reflected light spot s12 having an elongated elliptical shape Becomes Further, the optical disk D (signal surface D1)
(Point g indicating the defocus amount-Δ shown in FIG. 3), the reflected light spot s14 has a large elliptical shape. At this time, the quadrant optical sensors 10A to 10A
Since substantially the same amount of light is incident on each of the sensors D, the focus error is substantially zero. In addition, as shown in FIG. 7, in the reflected light spot s14, not all of them can be collected on the four-divided optical sensors 10A to 10D. Although not shown here, the optical disk D
The reflected light spot s13, which is an elongated elliptical shape when near the in-focus state (a position d2 of the focal line shown in FIG. 2 and a point f where the local maximum value is shown in FIG. 3) is slightly distant from the (signal plane D1), and the optical disk D ( The reflected light spot s15 having a large elliptical shape when in a state far from the signal surface D1) (point h indicating the defocus amount + Δ shown in FIG. 3) also has a shape similar to the above-described reflected light spots s12 and s14. Split optical sensor 10A-10
It goes without saying that an image is formed on D.

Here, the defocus amounts -.DELTA. And .DELTA. At points g and h at which the focus error signal level becomes zero for the first time outside the S curve are determined by the optical disc D having two recording layers.
Thickness dD between the first and second recording layers DD2 and DD5 of D
It has been found that if the optical distance of L is less than half of the optical distance (dDL / n), the following is obtained. That is, the defocus amount Δ is the optical distance (dDL / n) of the thickness of the interlayer (resin layer DD4).
Is less than half, the focus offsets F1O and -F2O shown in FIG. 5 do not occur during reproduction. In other words, the focus error signal of the focus point F1 corresponding to the focus point RF1 at which the RF signal obtained when the first recording layer DD2 is reproduced and scanned becomes the maximum level becomes zero (in a focused state). At the focus point F2 corresponding to the focus point RF2 at which the RF signal obtained when the second recording layer DD5 is reproduced and scanned is the focus error signal is zero (in a focused state). Of course, in this case, the shape of the linear range of the S curve (the S curve opening Spp shown in FIG. 3) is not adversely affected. This condition can be expressed as Δ <(dDL / n) / 2 where Δ is the defocus amount when the focus error signal becomes substantially zero. From this equation, it can be seen that it is necessary to bring the positions g and h, where the defocus amount Δ becomes as close as possible, to the in-focus point 0 side (both are shown in FIG. 3). One approach to this approach is to use four-divided optical sensors 10A-1A-1A.
This is to make the size of 0D itself uniform and small. As described above, the time when substantially the same amount of light enters each of the four-divided optical sensors 10A to 10D after passing through the focal point is the time when the defocus amount Δ is obtained (focus error = 0). 4-split optical sensor 10A-10
Needless to say, even if the size of each sensor D is made uniform and small, it is not so small as to adversely affect the shape of the linear range of the S curve (S curve opening Spp shown in FIG. 3).

Given the above, next, the size of the four-divided optical sensors 10A to 10D will be examined.
That is, the optical system (the four-divided optical sensor 10
A to D) and length P are analyzed. Incidentally, the length P is, as shown in FIG. 6 described above, a position corresponding to the radius (c / 2) of the reflected light spot s11 radiated to the center of the four-split optical sensors 10A to 10D during focusing. To the far ends 10A1 to 10D1 of the four-split optical sensors 10A to 10D in the axial direction x (or y) of the astigmatism axis. As a result, the length P is: P <2 · β · NA ₀ · dDL / 2n−H (2) where β: optical sensor 1 divided from disks D and DD
Imaging magnification NA from 0A to 10D NA ₀ : NA (numerical aperture) of objective lens 4 dDL: Thickness between first and second recording layers DD2 and DD5 n: First and second recording layers DD2 and DD5 (The refractive index of the resin layer DD4) If the size of each of the four-divided optical sensors 10A to 10D is made uniform and smaller than the length P determined by the relational expression (2), The defocus amount Δ is equal to or less than half the optical distance (dDL / n) having the thickness dDL between the first and second recording layers DD2 and DD5. As a result, the first
In the focus point F1 corresponding to the focus point RF1 at which the RF signal obtained when the recording layer DD2 is reproduced and scanned becomes the maximum level, the focus error signal becomes zero (in a focused state), and similarly, the second recording layer DD5 In the focus point F2 corresponding to the focus point RF2 at which the RF signal obtained when the reproduction scanning is performed at the maximum level, the focus error signal becomes zero (focused state). Of course, in this case, the shape of the linear range (the S-curve opening Spp shown in FIG. 3) is not adversely affected without disturbing the shape of the S-curve. Note that the focus offset F1O, -F described above when a focus error is detected.
Of course, the detection of 2O can be omitted.

Actually, the defocus amount Δ is equal to the optical distance (dDL) between the first and second recording layers DD2 and DD5.
Even if it is slightly longer than half of (/ n) or less, there is no offset at the operating point of the focus servo, and the disturbance of the shape of the S curve is small. Taking this into account, the following relationship is obtained. P <2 · β · NA ₀ · df-H (3) where df = 0.6 · dDL / n

In this example, β = 20, NA ₀ = 0.6, dDL / n = 26.67
(Μm) H is 144 μm when the amount of astigmatism is set so that the S-curve frontage Spp becomes 6 μm. In addition,
Here, there is a relation of H = 2 * β * Spp * NAo (4) between the S curve frontage Spp and H. In this case, if P <240 μm, focus detection without interference can be performed.

P, which determines the shape of the four-split optical sensors 10A to 10D, may be shorter than the length determined by the above relational expression, but if it is too short, even when a focus error occurs,
The change in the shape of the reflected light spot s1 cannot be sufficiently captured.

This state is shown in FIG. FIG. 8 shows a focus detection sensor (four-division optical sensors 10A to 10A).
D) is a diagram for explaining a shape change of the reflected light spots s11 and s12 in the focused state and the out-of-focus state on D). The reflected light spot s12 shown in FIG. 8 is a focus point which becomes narrowest in the direction x of the axis of astigmatism in FIG. 8, but in the illustrated case, the reflected light spot s12 is a four-divided optical sensor 10A to 10A. Not all of it can be focused on 10D. In the figure, in the direction y of the axis of astigmatism, 4
The split light sensors 10A to 10D are sufficiently reflected light spots.
12, the focus error signal of a sufficient level cannot be obtained. In view of this, the following second conditional expression has been obtained. P> H / 2
If the length P is smaller than this condition, the reflected light spot s11 in the focused state to the reflected light spot s12 in the unfocused state
This makes it impossible to sufficiently capture the change in the shape. This is clear when Equation (1) and FIGS. 2 and 6 are considered together.

This condition is 72 μm <P in this example. The waveform obtained by the optical system satisfying the above conditions is
This is shown in FIG. FIG. 9 shows the shape of the S curve and the waveform of the reproduced RF signal in the focus error detection obtained when reproducing the optical disk DD having two recording layers shown in FIG. 4 using the optical disk device of the present invention. It is the figure which compared.

The focus point RF at which the RF signal obtained when the first recording layer DD2 is reproduced and scanned is the maximum level.
At the focus point F1 corresponding to 1, the focus error signal becomes zero (in a focused state), and the offset as shown in FIG. 5 described above does not occur. Similarly, the second recording layer DD5
Focus point F2 corresponding to the focus point RF2 at which the RF signal obtained when the reproduction scan is performed is
Indicates that the focus error signal is zero (in focus),
The offset as shown in FIG. 5 does not occur. Further, no disturbance due to interference occurs in the shapes of the two S curves (there is no adverse effect on the shape of the S curve opening Spp). Thus, the first and second recording layers D
It can be seen that focus error detection in which D2 and DD5 are separated is performed. It should be noted that the aforementioned equation (3) and (4)
Thus, P <2 · β · NAo (df-Spp). If this expression is negative, it has no practical meaning. Therefore, df> Spp must be satisfied. In other words, it can be seen that the frontage Spp of the S curve must be narrower than this equation. In this example, Spp <16
μm.

As another example, when the astigmatism detection (focus detection) and the signal detection are performed by the same sensor, the generation of the astigmatism by the astigmatism method is performed by the objective lens 4.
From the focus detection sensor (four-division optical sensor 10A-
In the optical system up to 10D), it is necessary to suppress the occurrence of aberration, so that it is generated by the light projecting optical system.

Specifically, astigmatism can be generated by installing a cylindrical lens 9 near the collimator lens 1. In this case, the parallel light receiving element 7 and the focus detection sensor 10 (four-division optical sensors 10A to 10A)
10D) can be configured on the same photodetector by in-plane division. Also in this configuration, stable focus detection on a multilayer disc is possible under exactly the same conditions. Furthermore, what has been described above is the use of a surface light source having a perfect circle as the light source. However, the light source used in the present invention is not limited to this, and may have a shape other than a perfect circle (for example, It is needless to say that even in the case of a light source using an elliptical shape, a multilayer disc can be favorably reproduced based on the spot shape at the optimum operating point.

[0049]

According to the optical disk apparatus of the present invention having the above-described configuration, for example, when reproducing an optical disk having two recording layers, the reproducing laser irradiates a plurality of tracks on one recording layer to be reproduced simultaneously. An astigmatic optical system that uses an optical sensor that can reliably remove the adverse effects of unnecessary reflected light from a plurality of tracks on the other recording layer in order to obtain the maximum level of RF signal in a state where the reflected light is in focus The optical disk device provided with the above can be provided.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining a configuration of an optical system of an optical disk device according to the present invention.

FIG. 2 is a diagram for explaining a shape change of a reflected light spot on a focus detection sensor (four-division light sensor).

FIG. 3 is a diagram illustrating the shape of an S curve in focus error detection.

FIG. 4 is a diagram for explaining a structure of an optical disc having two recording layers.

FIG. 5 is a diagram comparing the shape of an S-curve in focus error detection obtained when reproducing an optical disk having two recording layers using a conventional optical disk device and the waveform of a reproduced RF signal.

FIG. 6 is a diagram illustrating a shape of a reflected light spot in a focused state on a focus detection sensor (four-division optical sensor).

FIG. 7 is a diagram for explaining a shape change of a reflected light spot on a focus detection sensor (four-division light sensor).

FIG. 8 is a diagram for explaining a shape change of a reflected light spot in a focused state and an unfocused state on a focus detection sensor (four-division optical sensor).

FIG. 9 shows an S-curve shape and a reproduced RF in focus error detection obtained when an optical disk having two recording layers is reproduced using the optical disk apparatus of the present invention.
FIG. 3 is a diagram comparing with a signal waveform.

[Explanation of symbols]

Reference Signs List 4 Objective lens, 10 Focus detection sensor 10A to 10D 4-split optical sensor, multi-split optical sensor A1 Focus error detection system A2 Information signal detection system AA optical disk device d1, d2 Focus line position D, DD Optical disk D1, D2 Signal surface DD2, DD5 Recording layer dDL Distance between recording layers n Refractive index NA ₀ Numerical aperture P Length s1, s11 to s15 Reflected light spot x, y Direction where astigmatism exists β Imaging magnification

Claims (2)

[Claims] 1. An information reproducing apparatus that reproduces information based on reflected light obtained by irradiating a reproducing light beam so as to straddle a plurality of tracks formed on a plurality of recording layers of an optical disc, and uses an astigmatism method. In an optical disc device that performs focus error detection, a multi-segment optical sensor that detects a focus error uses P <2 · β · NA ₀ · df-H (where P: the optimal operating point). With reference to the size of the reflected light spot on the multi-segmented optical sensor at the time, from the outer periphery of the reflected light spot at the optimal operating point, it is a reflected light spot other than the optimal operating point and has astigmatism. length NA to the maximum position of the direction of the reflected light spot _0: numerical aperture of an objective lens (NA) beta: mean imaging magnification df from the optical disk to the multi-split photosensor: 1.2 · DDL / dDL: distance between recording layers of the optical disc n: refractive index between recording layers of the optical disc H: reflected light spot in two directions where astigmatism exists when the reflected light spot on the multi-segmented optical sensor is at the position of the focal line When the amount of change in the width of
An optical disc device formed so as to satisfy the relational expression (half the length of H) according to the direction of change of the reflected light spot. Multi-split photosensor for use in wherein the focus error detection, as the length P is, P <and a _{2 · β · NA 0 · df} -H, P> satisfying H / 2 condition, The optical disk device according to claim 1, wherein the optical disk device is formed.