JPH09251657A - Device and method for recording and reproducing recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reproduce a DVD(digital video disk) of SD standard and a CD (compact disk) by using the same optical pickup. SOLUTION: An optical beam is converged to the information recording surface of the CD by using the optical pickup provided with an objective lens 41 (optimized against the disk having substrate thickness of 0.83mm) having 0.482 numerical apertures to make the crosstalk generated when the optical beam is converged to the information recording surface of the DVD of SD standard become-30dB, and designed so that the spherical aberration generated at this time become 0.07 λ rms of the allowable amount of marginal error. The deterioration of the modulation degree of the RF signal band due to the remaining spherical aberration caused at that time is corrected by the defocusing process in a focusing actuator 46.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a recording medium recording / reproducing apparatus and a recording medium recording / reproducing method, and in particular, information can be recorded or reproduced on discs having different substrate thicknesses without changing the numerical aperture or the like. The present invention relates to a recording medium recording / reproducing apparatus and a recording medium recording / reproducing method.

[0002]

2. Description of the Related Art In recent years, MPEG (MOVING PICTURE EXPER)
The technology of compressing video signals and audio signals at high density by using the T GROUP method and recording and reproducing them on a recording medium is being put to practical use. As an information recording medium of an optical disc, an AV (AUDI
In the field of O VISUAL, DVD (DIGITAL VIDEO
DISK), in the field of computers, HS (Hyper
Storage (trademark) and other HD (High Density) -CDRO
Each of the Ms has received attention as a next-generation recording medium. It has been proposed to record a video signal and an audio signal on a DVD in conformity with the MPEG system, and record a large volume of data on an HD-CDROM which has never been recorded.

Thus, for example, in a DVD, since the digital data is recorded on the disc at a high density, the spot size of the light beam is set to the conventional recording medium (eg, CD: COM).
It should be smaller than that for PACTDISK).

Assuming that the radius of the spot size of the light beam is R, the exit angle of the objective lens is θ, and the wavelength of the light beam is λ, the radius R of the spot size of the light beam is approximately represented by the equation (1). It R = 0.32λ / SINθ (1)

As can be seen from the equation (1), in order to reduce the spot size of the light beam, the wavelength λ of the light beam is reduced, and the numerical aperture (SINθ (= n * SINθ: in air, the refractive index For n, 1)) should be increased.

However, when the numerical aperture NA is increased, the coma aberration generated with respect to the disc skew (the angle at which the information recording surface of the optical disc and the optical axis of the optical pickup deviate from the vertical) is also increased. The coma aberration distorts the spot shape on the recording surface of the disk, which increases crosstalk, resulting in deterioration of the signal.

Since this coma aberration is proportional to the cube of the numerical aperture NA, the disc skew amount, and the thickness of the disc, the numerical aperture NA cannot be made too large. When a numerical aperture NA is large and a substrate such as polycarbonate having a disc skew of 0.5 to 1 degree is used, this coma aberration increases, the image spot on the disc becomes asymmetric, intersymbol interference increases, and reproduction RF (Radio Freque
ncy) A large distortion occurs in the signal.

Since the coma aberration is proportional to the thickness of the substrate as described above, if the thickness of the substrate is halved, the coma aberration can be halved. Therefore, the numerical aperture NA of the conventional CD (NA = NA =
0.45, the thickness of the substrate is 1.2mm) (NA = 0.6), which is larger than that in the first standard (for example,
DVD of SD standard) is considered.

Here, the structures of the CD and the DVD of the first standard will be compared. FIG. 27 shows a cross section of a CD.

In the CD, the substrate S1-1 made of transparent plastic such as polycarbonate is used.
And an information recording layer S2 composed of an aluminum thin film
-1 is formed, and the protective layer S3-1 is further formed on it.

The light beam from the light source is applied to the information recording layer S2-1 through the substrate S1-1 having a thickness of 1.2 mm, and the data of the information recording layer S2-1 is read from the reflected light. ing.

FIG. 28 shows a cross section of a DVD of the first standard.

The DVD of the first standard has a double-sided structure,
The information recording layer S2-21 is formed on the substrate S1-21, the protective layer S3-21 is formed thereon, and the information recording layer S2-22 is formed on the substrate S1-22.
The protective layer S3-22 formed thereon is formed by adhering the protective layers to each other.

The light beam from the light source is applied to the information recording layer S2-21 (or the information recording layer S2-22) through the substrate S1-21 (or the substrate S1-22) having a thickness of 0.6 mm, and the information is recorded. Recording layer S2-21 (or information recording layer S2-
22) to read the data from the reflected light (hereinafter, the thickness of the substrate S1-1, the substrate S1-21, or the substrate S1-22 (from the incident surface of the light beam to the information recording layer S2-1, The optical distance to the information recording layer S2-21 or the information recording layer S2-22) is simply referred to as the thickness of the substrate).

[0015]

However, for example, a conventional standard having an objective lens designed to be optimum for the DVD of the first standard (the thickness of the substrate is 0.6 mm and the numerical aperture NA is 0.6). Conventional optical CD with optical pickup
When the light beam is focused on the information recording layer (the thickness of the substrate is 1.2 mm and the numerical aperture NA of the standard value is 0.45), the optical condition changes due to the difference in the thickness of the substrate, resulting in a large spherical aberration. Occurs.

Since the spherical aberration generated at this time is proportional to the thickness of the disc, the fourth power of the numerical aperture NA, and other factors of higher order, it becomes a significant spherical aberration, making it difficult to read recorded information.

Therefore, this spherical aberration is equal to 4 of the numerical aperture NA.
Since it is proportional to the power, it is conceivable to reduce the numerical aperture NA to reduce the spherical aberration. However, when the numerical aperture NA is reduced to such an extent that the influence of the spherical aberration can be ignored, the cutoff spatial frequency Decrease, coupling efficiency (ratio between the laser emission power and the power of the light emitted from the objective lens), and the increase in cross size due to the increase in spot size. As a result, conversely The reproduced signal deteriorates.

Therefore, a plurality of types of discs having different disc thicknesses (for example, DVD, HS, and CD of the first standard (the substrate thicknesses are 0.6 mm, 0.8 mm, 1.2 m, respectively)
There is a problem that it is difficult to reproduce m)) with one optical pickup.

The present invention has been made in view of such a situation, and for example, an optical pickup can be provided with a numerical aperture corresponding to different substrate thicknesses without changing optical conditions such as changing the numerical aperture. It is possible to record or reproduce information on or from a recording medium having substrates of a plurality of thicknesses by using an optical pickup having a simple structure without providing a drive mechanism for changing .

[0020]

A recording medium recording / reproducing apparatus according to claim 1, wherein the numerical aperture NA of the objective lens is 0.55 when the refractive index of the first substrate is N and the wavelength of the light source is λ. It is characterized in that it is set so as to satisfy λ / TP1 ≦ NA t2-t1 ≦ 7.513 (λ / NA ^ 4) (N ^ 3 / (N ^ 2-1)).

In the recording medium recording / reproducing method according to a fifth aspect, when the numerical aperture NA of the objective lens is N, the refractive index of the first substrate is λ, and the wavelength of the light source is λ, 0.55λ / TP1 ≦ NA t2. It is characterized in that it is set to satisfy -t1≤7.513 (λ / NA ^ 4) (N ^ 3 / (N ^ 2-1)).

In the recording medium recording / reproducing apparatus according to the first aspect, the numerical aperture NA of the objective lens is 0.55λ / TP1 ≦, where N is the refractive index of the first substrate and λ is the wavelength of the light source. It is set to satisfy NA t2-t1 ≦ 7.513 (λ / NA ^ 4) (N ^ 3 / (N ^ 2-1)).

In the recording medium recording / reproducing method of the fifth aspect, the numerical aperture NA of the objective lens is 0.55λ / TP1 ≦ NA, where N is the refractive index of the first substrate and λ is the wavelength of the light source. It is set to satisfy t2-t1≤7.513 (λ / NA ^ 4) (N ^ 3 / (N ^ 2-1)).

[0024]

1 is a block diagram showing the configuration of an embodiment of a recording medium reproducing apparatus 1 to which the recording medium recording / reproducing apparatus of the present invention is applied.

The spindle motor 21 has a rotation control circuit 2
The disc 2 is rotated in accordance with the instruction from 2. The rotation control circuit 22 is the control circuit 2
The spindle motor 21 is driven in accordance with the instruction from 8.

The optical pickup 23 collects a light beam on the information recording layer of the disc 2, converts the reflected light from the information recording layer of the disc 2 into a reproduction signal, and passes it to the signal processing circuit 27 via the electric filter 30. It is designed to output.

The thread motor 24 is used for the optical pickup 2
3 is moved to a target track position on the information recording layer of the disc 2.

The sled motor control circuit 25 responds to the instruction from the control circuit 28 by sled motor 2
4 is designed to be driven.

The pickup control circuit 26 controls the focusing actuator and the tracking actuator in the optical pickup 23 in response to the instruction signal from the control circuit 28.

The signal processing circuit 27 performs demodulation processing, error processing, etc. on the reproduced signal from the optical pickup 23 input via the electric filter 30, and, for example, the thickness of the substrate of the disk 2 from the level of the RF signal. (Disc type) is detected and the detection signal is output to the control circuit 28.

The control circuit 28 outputs an instruction command to the above-mentioned various circuits in response to an instruction signal input via the external interface 29. Further, the control circuit 28 includes the signal processing circuit 2
The reproduction signal input from the device 7 is output to an external device via the external interface 29.

The electric filter 30 (correction means) is adapted to correct the residual spherical aberration generated corresponding to the disk having the substrate of the predetermined thickness.

FIG. 2 is a diagram showing the configuration of an embodiment of the optical pickup 23.

In the optical pickup 23, the light beam emitted from the laser diode 42 is reflected by the beam splitter 43, and the objective lens 41 causes the disc 2 to travel.
The light is focused on the information recording surface.

Further, in the digital data recorded on the information recording surface of the disk 2, the reflected light of the above-mentioned light beam is condensed on the photodetector 44 located at the conjugate point via the objective lens 41 and the beam splitter 43. By
It is adapted to be converted into a reproduction signal.

The objective lens 41 is designed so that the spherical aberration generated when the light having the numerical aperture NA of 0.482 is condensed on the information recording surface of the disk having the substrate having the thickness of 0.83 mm is minimized. There is.

The tracking actuator 45 and the focusing actuator 46 (correction means) are adapted to correct a focus error and a tracking error associated with the rotation of the disk 2. Further, the focusing actuator is also adapted to correct the residual spherical aberration generated corresponding to the type of disc by adding defocus.

FIG. 3 shows a disk 2-1 (having a substrate having a substrate thickness of 0.6 mm, a track pitch Tp of 0.725 μm, and a standard numerical aperture NA) using the optical pickup 23.
Shows the state in which the light beam is focused on the information recording surface of the first standard DVD) of 0.6.

In FIG. 3, parts corresponding to those in FIG. 2 are designated by the same reference numerals, and the description thereof will be omitted as appropriate.

The objective lens 41 has a numerical aperture NA of 0.482 with respect to a disk having a substrate having a thickness of 0.83 mm as described above.
Is designed to minimize the spherical aberration that occurs when a light beam that becomes Here, crosstalk and spherical aberration that occur when a light beam is focused on the information recording surface of the disk 2-1 (DVD of the first standard) using this objective lens 41 will be described. First, crosstalk included in the reproduction signal reproduced from the disc 2-1 will be described with reference to FIGS. 4 to 7.

FIG. 4 is a three-dimensional representation of the light intensity distribution when a light beam is focused on the information recording surface of the disc through a predetermined objective lens.

In FIG. 4, tracks (n-1) to track (n-1)
(n + 1) is shown. Then, in the state shown in this figure, the light beam is condensed on the track n, and the three-dimensional height shown in this figure represents the intensity of the light beam. From this figure, it can be seen that energy is concentrated near the center of the light beam.

FIG. 5 shows the amount of totally reflected light when the light beam is moved in the track direction of track n in FIG. 4 (amount obtained by integrating the intensity of the light beam in a predetermined pit in the track direction). Is a graph in which the vertical axis is plotted on the vertical axis, and the distance from the center of the track n (distance in the track direction and the vertical direction) is plotted on the horizontal axis.

As described above, since the spot size of the light beam is proportional to λ / NA, the distance on the horizontal axis in FIG. 5 is represented by the value that becomes the original distance when multiplied by λ / NA. .

As can be seen from this figure, the light quantity of the reflected light of the light beam sharply decreases with the peak of the center of the track n. The horizontal axis is 0.50 to 1.00 (or -1.00 to -0.
The second peak is present in the range of 50). In the range of -1.00 or less and 1.00 or more on the horizontal axis, the light amount of the total reflected light can be considered to be almost zero.

Therefore, when the crosstalk is considered, the reflected light from the track existing in the range represented by -1.00 to 1.00 on the horizontal axis may be considered.

FIG. 6 is a graph showing three tracks (n-1) to (n + 1) for taking crosstalk into consideration in the graph shown in FIG. In addition to the reflected light, there is shown a state of being composed of the reflected light from the adjacent track (n-1) and the adjacent track (n + 1). That is, in this case, the reproduction signal is composed of the reproduction signal from the original track n and the reproduction signals from the tracks on both sides thereof.

Let SM be the total amount of light reflected from within the pit width (track width) D of track n, and from within the pit width D of either one of track (n-1) or track (n + 1) If the total amount of light reflected is SC, the crosstalk K is expressed by equation (2). K = SC / SM (2)

Assuming that the ratio of the pit width D and the track pitch TP in the DVD of the first standard is almost equal to that in the CD, the crosstalk K expressed by the equation (2) is TP / (λ / It can be uniquely obtained from the value of (NA). Utilizing this, the horizontal axis shows TP / (λ / NA)
FIG. 7 shows a graph in which the crosstalk K is plotted on the vertical axis.

FIG. 7 shows the crosstalk K when the ratio TP / D is calculated from the values of the CD pit width D (0.5 μm) and the track pitch TP (1.6 μm), and the incident light is a uniform circular aperture. It is a plot of the calculated value of.

If the crosstalk K is, for example, -30 dB or less, it can be judged that the influence on the reproduction signal is small. From the graph of FIG. 7, the value of TP / (λ / NA) at which the crosstalk is -30 dB is 0.55. Utilizing this fact, the numerical aperture NA for the crosstalk generated in the state shown in FIG. 3 (the state in which the disc 2-1 is reproduced by the optical pickup 23) to be -30 dB or less can be obtained. TP / (λ / NA) = 0.55, the first standard DVD
By substituting the track pitch TP of 0.725 μm and the wavelength λ of the light beam of 635 nm, the numerical aperture NA at which the crosstalk K becomes -30 dB is calculated to be 0.482.

That is, even if the disc 2-1 (DVD of the first standard) is reproduced by using the optical pickup 23 (objective lens 41) having a numerical aperture NA of 0.482, it is caused by the size of the spot size. Crosstalk generated is -30dB
Therefore, the influence of crosstalk on the reproduced signal can be ignored.

Next, an objective lens 41 optimized for a disc having a numerical aperture NA of 0.482 and a substrate having a thickness of 0.83 mm (designed to minimize spherical aberration) has a diameter of 0.6 m.
The spherical aberration that occurs when the light beam is focused on the information recording surface of the disc 2-1 having the substrate with a thickness of m (the case shown in FIG. 3) will be described.

An objective lens having a numerical aperture NA optimized for a disk having a substrate thickness t0 and a substrate thickness t
Spherical aberration W40 that occurs when a light beam having a wavelength of λ is focused on the disk is expressed by equation (3), where N is the refractive index of the substrate. W40 = {1 / ((√5) * 6 * 8)} * {(((N) ^ 2) -1) / ((N) ^ 3)} * {(NA) ^ 4} * (t- t0) * (1 / λ) (3)

In equation (3), ^ represents a power.

This spherical aberration is, for example, 80% or more of the case where the central intensity of the image is completely aberration-free, which is a Marecha criterion.
(Marechal's allowable amount) 0.07λrms (root mean sq
If uare) or less, it can be determined that the influence on the reproduction signal is small. Therefore, using the above numerical aperture NA value (0.482), the light beam wavelength λ (635 nm), and the substrate refractive index N (1.58), the substrate thickness difference δ (= t- It is possible to obtain t0). Then, when δ satisfying this condition is calculated, it becomes 0.233 mm or less. That is, when the objective lens 41 optimized for a disk having a substrate with a numerical aperture NA of 0.482 and a thickness of 0.83 mm is used, the numerical aperture NA is 0.482 and the substrate thickness is 0.597 (=
Spherical aberration that occurs when light is irradiated on a disk of 0.83-0.233) mm to 1.063 (= 0.83 + 0.233) mm can be ignored.

Therefore, by using the optical pickup 23 having the objective lens 41, the disc 2 having a substrate having a thickness of 0.6 mm (the range of 0.597 <0.6 <1.063, that is, the range in which spherical aberration can be ignored) is used. The spherical aberration that occurs when a light beam having a numerical aperture NA of 0.482 is focused on the information recording surface of 1 (DVD of the first standard) (in the case shown in FIG. 3) can be ignored. Will be things.

That is, as shown in FIG. 3, when the disc 2-1 (DVD of the first standard) is reproduced by using the objective lens 41, the reproduction signal is sufficiently unaffected by crosstalk and spherical aberration. The reproduced signal can be practically used.

In this way, when reproducing the DVD of the first standard, instead of using the objective lens optimized to the standard value (the numerical aperture NA is 0.6 and the thickness of the substrate is 0.6 mm). Even if the objective lens 41 having a numerical aperture NA of 0.482 and a substrate thickness optimized to 0.83 mm is used, it is possible to obtain a reproduction signal that can be practically used.

Next, an objective lens 41 optimized for a disk having a numerical aperture NA of 0.482 and a substrate having a thickness of 0.83 mm (designed so that spherical aberration is minimized) is 1.2 m.
The spherical aberration that occurs when the light beam is focused on the information recording surface of the disk 2-2 (CD) having a substrate with a thickness of m will be described. FIG. 8 shows a state in which a light beam is focused on the information recording surface of the disc 2-2 by the optical pickup 23 having the objective lens 41.

In the state shown in FIG. 8, the numerical aperture NA is
Since this value is 0.482, which is larger than the numerical aperture NA (0.45) of the standard value of the disk 2-2 (CD), the crosstalk that occurs during reproduction is small and does not hinder the reproduction. Therefore, in the following, the spherical aberration that occurs during reproduction in the state of FIG. 8 will be described.

Here, the disk 2-1 (D of the first standard)
Regarding the objective lens capable of reproducing VD), the range of optimization of the numerical aperture and the thickness of the substrate will be described with reference to FIG.

FIG. 9 is a diagram showing the range of optimization of the numerical aperture of the objective lens and the thickness of the substrate in order to obtain a sufficient reproduction signal when reproducing the DVD of the first standard. is there. In FIG. 9, the horizontal axis represents the numerical aperture NA and the vertical axis represents the substrate thickness t.

As described above, it is necessary to set the numerical aperture NA to 0.482 or more in view of the crosstalk condition for reproducing the disc 2-1 (DVD of the first standard). on the other hand,
Assuming that the thickness of the substrate to be optimized is t0, the spherical aberration W40 generated in the substrate having the thickness t is equal to the thickness of the substrate represented by (t-t0) as shown in equation (3). Proportional to the difference,
Its value must be less than or equal to Marechal's allowable amount of 0.07λrms. In formula (3), the wavelength of the light beam is λ (635nm)
And the refractive index N (1.58) of the substrate, the relationship between the numerical aperture NA and the difference in the thickness of the substrate (t-t0) is obtained, and t0 = 0.6m
Substituting m gives equation (4). 0.01267 / ((NA ^ 4)) ≧ t-0.6 (4)

The range of the numerical aperture NA and the thickness t of the substrate satisfying these two conditions is the range shown by the shaded area in FIG.
In other words, the range of this shaded part represents the range in which the objective lens can be optimized when reproducing the DVD of standard 1.

That is, when optimizing the objective lens,
If the numerical aperture NA is the value of the numerical aperture NA of the shaded portion in FIG. 9 and the thickness t0 of the substrate is the value of the thickness t of the shaded portion in FIG. 9, the numerical aperture NA is 0.482 or more. The spherical aberration satisfies the condition that the spherical aberration is equal to or less than the Marechal's allowable amount, and a sufficient reproduction signal can be obtained when reproducing the DVD of the first standard.

For example, the example of FIG. 3 (numerical aperture NA is 0.482)
Then, when reproducing the DVD of the first standard by using the objective lens 41 optimized for the substrate thickness of 0.83 mm (= t0))
Corresponds to the point a in FIG. 9, and the point a is included in the shaded area. Therefore, when the DVD of the first standard is reproduced under this condition, a sufficient reproduction signal can be obtained.

Here, in the shaded area (D of the first standard)
Select some points of (range where VD can be reproduced without problems),
Under the condition of the range, CD (substrate thickness 1.2mm
Now, the spherical aberration that occurs when reproducing a standard numerical aperture of 0.45) will be described with reference to FIG.

In FIG. 10, the spherical aberration generated when a CD is reproduced under each condition indicated by points a to h is
In the figure, it is represented by a number surrounded by a square (unit is λrms).

For example, the condition indicated by point b (the numerical aperture is 0.482)
Then, the spherical aberration that occurs when a CD is reproduced under the condition that the objective lens whose substrate thickness is optimized to be 0.6 mm is used) is 0.18 λrms.

Here, the case where the disc 2-1 (DVD of the first standard) is reproduced under the condition of the point f and the disc 2-2
FIG. 11 and FIG. 12 show the case of playing (CD), respectively.
This will be described with reference to FIG. The condition of the point f is that the numerical aperture NA is 0.6.
In the case where the objective lens optimized for the substrate thickness of 0.6 mm is used, in FIG. 11, the objective lens 51 designed as described above is used, and the disk 2-1 (first This shows a state in which a light beam is focused on the information recording surface of a standard DVD).

In the state shown in FIG. 11, the objective lens 51 is optimized for the disc 2-1 (because the point f is included in the shaded portions in FIGS. 9 and 10), the best reproduction signal is obtained. Is obtained.

In FIG. 12, a light beam is projected onto an information recording surface of a disk 2-2 (CD having a substrate having a thickness of 1.2 mm and having a standard numerical aperture NA of 0.45) by using an objective lens 51. It shows a state of being condensed.

In the case shown in FIG. 12, since the objective lens 51 does not correspond to the disc 2-2 (CD),
Large spherical aberration occurs, and as a result, it becomes difficult to read the reproduction signal. This means that in the state of point f in FIG.
It is clear from the fact that the spherical aberration generated when D is reproduced is 0.43 (which is considerably larger than Marechal's permissible range 0.07).

Therefore, let us consider a case where the disc 2-2 (CD) is reproduced by using an objective lens optimized for a numerical aperture NA of 0.6 and a substrate thickness of 0.7 mm. The condition in this case is the condition indicated by point e in FIG. 10, and since point e is a point within the shaded area, disc 2-1 (DVD of the first standard) can naturally be satisfactorily reproduced. . Further, under this condition, the spherical aberration generated when the disc 2-2 (CD) is reproduced is 0.36. As can be seen by comparing the points f and e, when reproducing both discs, the condition of the point e is more effective than the condition of the point f in the spherical aberration generated when the disc 2-2 (CD) is reproduced. Can be reduced. That is, when the numerical aperture is the same (0.6), it is possible to further reduce the spherical aberration generated when reproducing a CD by using an objective lens optimized for a thicker substrate.

Next, the numerical aperture is 0.482 and the substrate thickness is 0.6.
Consider a case where the disc 2-2 (CD) is reproduced using an objective lens optimized for mm (= t0). The condition in this case is the condition shown by the point b in FIG. 10, and since it is the point within the shaded portion of the point b, the disk 2-1 (the DV of the first standard)
D) can be reproduced well. Under this condition, the spherical aberration generated when the disc 2-2 (CD) is reproduced is 0.18 as shown in FIG. As can be seen by comparing the points f and b, the condition of the point b can more reduce the spherical aberration generated when the disc 2-2 (CD) is reproduced than the condition of the point f. it can. In other words, when using an objective lens optimized for the same substrate thickness (0.6 mm), it is better to use a smaller numerical aperture for disc 2-
It is possible to further reduce spherical aberration that occurs when reproducing 2 (CD).

From these facts, the disc 2-1 (first disc
It is understood that the objective lens optimized for the thickest substrate with the smallest numerical aperture among the objective lenses capable of favorably reproducing the standard DVD) is the condition of point a. In other words, among the objective lenses capable of reproducing the disc 2-1 (DVD of the first standard) well, the spherical aberration generated when the objective lens 41 reproduces the CD can be most reduced.

This means that in a pickup capable of reproducing the disc 2-1 (DVD of the first standard), the optical pickup 23 having the objective lens 41 shown in FIG. 2 is used as shown in FIG. Use the disk 2-2 (C
By reproducing D), it means that the generated spherical aberration can be reduced most.

As described above, the disc 2-1 (first disc
In the case of reproducing both the standard DVD) and the disc 2-2 (CD), the optical pickup 23 (objective lens 4
By using 1), the generated spherical aberration can be reduced most.

However, as can be seen from the numerical value indicated by point a in FIG. 10, the optical pickup 23 (objective lens 4
Even if the method 1) is used, the spherical aberration generated when the disc 2-2 (CD) is reproduced (in the case of FIG. 8) is 0.11.
Therefore, the Marechal's allowable amount is larger than 0.07, and the influence on the reproduction signal cannot be ignored (spherical aberration remains).

FIG. 13 shows the relationship between the transfer function MTF (Modulation Transfer Function) of the optical system and the spatial frequency when the disc 2-2 (CD) is reproduced in the state of each point shown in FIG. The value obtained by multiplying the scale on the horizontal axis by 1154 shows the actual spatial frequency, and the unit is lp (line pair) / mm).

In the figure, a curve X is a CD (standard value is numerical aperture N
A is 0.45, wavelength λ is 780 nm, and is the characteristic of an optical pickup (numerical aperture NA is 0.366 (in the case of a CD, a sufficient numerical value is enough to obtain a reproduced signal), and the thickness is 1.2 mm. 3 is a curve showing an MTF when a CD is reproduced using an optical pickup having an objective lens optimized for the substrate of FIG. The curve Y is the DVD of the first standard.
(The standard values are an optical pickup for a numerical aperture NA of 0.6 and a wavelength λ of 635 nm) (an optical pickup having a numerical aperture NA of 0.6 and an objective lens optimized for a substrate having a thickness of 0.6 mm) 3 is a curve showing an MTF when a DVD of the first standard is reproduced by using.

FIG. 13 shows the MTF when a CD is reproduced under the conditions shown by points b, d, f, and h among the points shown in FIG. Has been done.

As can be seen from this graph, since the spherical aberration remains, the curve when the CD is reproduced under the conditions shown by the points b, d, f, and h deviates from the curve X. Since it is large, a reproduced signal of good quality cannot be obtained as it is.

Therefore, FIG. 14 shows a plot of the MTF value after the predetermined defocusing process in the state of each point (points a to h) shown in FIG. Further, enlarged views of the graph of FIG. 14 for the states of points a to h are shown in FIGS.

The curve a in FIG. 15 shows that in the state of the point a,
This is when the defocus W20 is set to 0.18 λrms. Similarly, the curves b to h in FIGS. 16 to 22 are
Defocus W20 is 0.29, 0.42, 0.56, 0.90, 1.08, 1.41, or 1.50 (λr
ms).

As can be seen from these figures, by performing the predetermined defocusing process, the MTF curve under the condition corresponding to each point deviates from the MTF curve X when the CD optical pickup is used. Becomes smaller. Especially,
In the state of the point a, the defocus W20 is 0.18, and the spherical aberration can be corrected with a relatively small value.

That is, when reproducing a CD and a DVD of the first standard by using a common optical pickup, the thickness of the substrate is thicker than 0.6 mm, for example, the numerical aperture NA is 0.482,
Objective lens 41 optimized for 83 mm substrate thickness
By using, the DVD of the first standard can be reproduced, and the disk 2-2 (CD) can be reproduced by applying a small defocus.

Therefore, in the optical pickup 23 shown in FIG. 2, when the disc 2-1 (DVD of the first standard) is reproduced, as shown in FIG. 3, no particular processing is performed on the optical system. Can be played without. Further, as shown in FIG. 8, when reproducing the disc 2-2 (CD), it is possible to reproduce by performing a small defocus (0.18) by the focusing actuator 46.

Disk 2-1 under conditions closer to the optimum value
When reproducing (DVD of the first standard), a higher level RF signal is obtained than when reproducing the disc 2-2 (CD). Therefore, the level of the RF signal is detected by the signal processing circuit 27, and when the control circuit 28 determines from the output of the signal processing circuit 27 that the disk 2-2 is being reproduced, the focusing actuator is operated via the pickup control circuit 26. An offset for providing a predetermined defocus to 46 is supplied.

Further, it is needless to say that HS or the like having a substrate having a thickness of 0.8 mm can be reproduced by using the optical pickup 23 (objective lens 41) without performing a specific process on the optical system. is there.

Residual spherical aberration (RF caused by it)
Degradation of the modulation degree of the signal band) can be corrected by setting the electric filter 30 to a predetermined characteristic without defocusing.

The spherical aberration when reproducing both discs (DVD and CD of the first standard) by using the objective lens optimized for the disc 2-1 (DVD of the first standard) is shown in FIG. 11 and FIG. 12, the disc 2-1 (DVD of the first standard) is then formed by using the objective lens optimized for the disc 2-2 (CD).
A case of playing back will be described with reference to FIGS. 23 and 24.

23 and 24, the numerical aperture NA is 0.45,
Objective lens 6 optimized for 1.2mm substrate thickness
1 is used to focus the light beam on the information recording surface of the disc 2-2 (CD) and to collect the light beam on the information recording surface of the disc 2-1 (DVD of the first standard). And a state in which they are respectively shown. In the state shown in FIG. 23, since the objective lens 61 is optimized for the disc 2-2 (CD), a good quality reproduction signal can be obtained, but in the state shown in FIG. Since it is not compatible with the disc 2-1 (DVD of the first standard), large spherical aberration occurs as in the case of FIG.

Next, applying the cases shown in FIGS. 3 and 8,
An objective lens capable of favorably reproducing a disc 2-2 (CD) optimized for a disc whose substrate thickness is less than 1.2 mm, for example, a numerical aperture NA is 0.482 and a substrate thickness is 0.97. Using an objective lens optimized for (≈ 0.967 = 1.20-0.233) mm, both discs (D of the first standard D
Consider the case of playing VD and CD).

In FIG. 25, an objective lens 71 optimized for a numerical aperture NA of 0.482 and a substrate thickness of 0.97 mm is used to direct a light beam to the information recording surface of a disc 2-2 (CD). It shows a state of being condensed.

Since the numerical aperture NA (0.482) at this time is larger than the numerical aperture (0.45) of the standard value of CD, the crosstalk generated at the time of reproduction is small and it does not hinder the reproduction. Also, the numerical aperture NA is 0.482 and the wavelength λ of the light beam is 6
When the refractive index N of the substrate is 35 nm and the refractive index N is 1.58, for the spherical aberration generated by the difference in the thickness of the substrate, the difference in the thickness of the substrate satisfying the Marechal's allowable amount is as already obtained.
It is 0.233 mm. That is, 0.967 (= 1.20-0.233) to 1.433
If an objective lens optimized for a substrate having a thickness of (= 1.20 + 0.233) is used, spherical aberration generated during reproduction can be ignored. Objective lens 7 shown in FIG.
1 satisfies this condition, and disc 2-2 (CD)
Can be reproduced well.

In FIG. 26, using this objective lens 71,
The state where the light beam is focused on the information recording surface of the disc 2-1 (DVD of the first standard) is shown.

The spherical aberration generated at this time is, for example,
Compared with the case shown in FIG. 24, the value is smaller. However, as can be seen from the graph of FIG. 9, the condition of the objective lens capable of reproducing the DVD of the first standard satisfactorily needs to be optimized such that the substrate thickness t0 is 0.83 mm or less, The objective lens 71 does not satisfy this condition.

Therefore, as in the case shown in FIG. 8, the residual spherical aberration generated during reproduction is corrected by adding a predetermined defocus.

That is, when the disc 2-2 (CD) is reproduced by using the objective lens 71, it can be reproduced without performing any specific processing on the optical system. When reproducing a standard DVD), FIG.
As shown in, reproduction can be performed by performing a relatively small defocus by the focusing actuator 46.

As described above, it is possible to record or reproduce data with respect to recording media of various different standards with one optical pickup without changing the numerical aperture or the like.

Further, since the optical pickup 23 does not have a special structure for supporting various standards, for example, a drive mechanism for changing the numerical aperture, etc.
It can be easily realized with a cheap and simple structure.

The present invention can be applied to the case of recording information and also to the case of recording or reproducing information on a recording medium other than the disc.

[0105]

As described above, according to the recording medium recording / reproducing apparatus of the first aspect and the recording medium recording / reproducing method of the fifth aspect, the numerical aperture NA of the objective lens is the refraction of the first substrate. When the ratio is N and the wavelength of the light source is λ, it is set to satisfy 0.55λ / TP1 ≦ NA t2-t1 ≦ 7.513 (λ / NA ^ 4) (N ^ 3 / (N ^ 2-1)) Therefore, it is possible to obtain a good image on the information recording layer of the recording medium by using an optical pickup having a simple structure for the individual recording media having different substrate thicknesses.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of an embodiment of a recording medium reproducing apparatus 1 to which a recording medium recording / reproducing apparatus of the present invention is applied.

FIG. 2 is a diagram showing a configuration of an embodiment of an optical pickup 23.

FIG. 3 shows a disc 2-1 using the optical pickup 23.
It is a figure which shows the state which condensed the light beam on (DVD of 1st specification).

FIG. 4 is a diagram showing a state in which a light beam is focused on a track n of a recording medium.

FIG. 5 is a graph showing the relationship between the amount of reflected light obtained by moving a light beam on track n in the track direction and the distance from the center of track n.

FIG. 6 is a diagram showing the amount of light reflected from tracks (n−1) to (n + 1) in the graph of FIG. 5.

FIG. 7 is a graph showing the relationship between crosstalk and TP / (λ / NA).

FIG. 8 shows a disc 2-2 using the optical pickup 23.
It is a figure which shows the state which condensed the light beam on (CD).

FIG. 9 is a diagram showing a range of optimization of a numerical aperture and a substrate thickness for an objective lens capable of reproducing a DVD of the first standard.

FIG. 10 is a condition C corresponding to each point in the hatched portion of FIG.
It is a figure which shows the spherical aberration produced when D is reproduced.

FIG. 11 is a diagram showing a state in which a light beam is focused on the disk 2-1 (DVD of the first standard) by the objective lens 51 optimized for the disk 2-1 (DVD of the first standard). is there.

FIG. 12 is a diagram showing a state where a light beam is focused on a disc 2-2 (CD) by an objective lens 51 optimized for the disc 2-1 (DVD of the first standard).

13 is a graph showing the relationship between MTF and spatial frequency when reproducing a CD under the conditions corresponding to the points in FIG.

14 is a graph showing the relationship between MTF and spatial frequency when defocusing is applied under conditions corresponding to the points in FIG.

15 is a graph showing the relationship between MTF and spatial frequency when defocusing is applied under the condition corresponding to point a in FIG.

16 is a graph showing the relationship between MTF and spatial frequency when defocus is applied under the condition corresponding to point b in FIG.

17 is a graph showing the relationship between MTF and spatial frequency when defocusing is applied under the condition corresponding to point c in FIG.

18 is a graph showing the relationship between MTF and spatial frequency when defocusing is applied under the condition corresponding to point d in FIG.

19 is a graph showing the relationship between MTF and spatial frequency when defocus is applied under the condition corresponding to point e in FIG.

20 is a graph showing the relationship between MTF and spatial frequency when defocus is applied under the condition corresponding to point f in FIG.

21 is a graph showing the relationship between MTF and spatial frequency when defocus is applied under the condition corresponding to the point g in FIG.

22 is a graph showing the relationship between MTF and spatial frequency when defocus is applied under the condition corresponding to point h in FIG.

FIG. 23 is a diagram showing a state in which a light beam is focused on the disc 2-2 (CD) by the objective lens 61 optimized for the disc 2-2 (CD).

FIG. 24 is a diagram showing a state in which a light beam is focused on the disc 2-1 (DVD of the first standard) by the objective lens 61 optimized for the disc 2-2 (CD).

FIG. 25 is a diagram showing a state in which a light beam is focused on the disc 2-2 (CD) by using the objective lens 71 capable of reproducing the disc 2-2 (CD) satisfactorily.

FIG. 26 is a diagram showing a state in which a light beam is focused on the disc 2-1 (DVD of the first standard) by using the objective lens 71 capable of favorably reproducing the disc 2-2 (CD).

FIG. 27 is a view showing a cross section of a CD.

FIG. 28 is a view showing a cross section of a DVD of the first standard.

[Explanation of symbols]

1 recording medium reproducing device, 2, 2-1 and 2-2 disc, 21 spindle motor, 22 rotation control circuit, 23 optical pickup, 24 thread motor, 25 thread motor control circuit, 26 pickup control circuit, 27 signal processing circuit, 28 control circuit, 29 external interface, 30 electric filter, 41 objective lens, 42 laser diode, 43 beam splitter, 44 photodetector, 45 tracking actuator, 46
Focusing actuator, 51, 61, 71
Objective lens

Claims (5)

[Claims] 1. A pickup comprising a light source for generating light and an objective lens for condensing light from the light source,
A substrate having a first thickness t1 and a track pitch T having a first width
A first recording medium having P1, a substrate having a second thickness t2 thicker than the first thickness t1, and a track pitch TP2 having a second width wider than the track pitch TP1 having the first width. In a recording medium recording / reproducing apparatus for selectively recording or reproducing information with respect to a second recording medium, the numerical aperture NA of the objective lens is N, the refractive index of the first substrate is N, and the wavelength of the light source is Is set to be λ, 0.55 λ / TP1 ≦ NA t2-t1 ≦ 7.513 (λ / NA ^ 4) (N ^ 3 / (N ^ 2-1)) is set. Recording medium recording / reproducing apparatus for 2. The recording medium recording / reproducing apparatus according to claim 1, further comprising a correcting unit that electrically corrects a residual spherical aberration that occurs when reproducing the second recording medium. 3. The recording medium recording / reproducing apparatus according to claim 1, further comprising a correction unit that corrects residual spherical aberration generated when reproducing the second recording medium by applying defocus. . 4. The first thickness t1 is 0.6 mm, the second thickness t2 is 1.2 mm, the first track pitch TP1 is 0.725 μm, and the second track pitch is 0.75 μm. The recording medium recording / reproducing apparatus according to claim 1, wherein TP2 is 1.6 μm. 5. A pickup comprising a light source that emits light and an objective lens that collects the light from the light source,
A substrate having a first thickness t1 and a track pitch T having a first width
A first recording medium having P1, a substrate having a second thickness t2 thicker than the first thickness t1, and a track pitch TP2 having a second width wider than the track pitch TP1 having the first width. In a recording medium recording / reproducing method of selectively recording or reproducing information with respect to a second recording medium, the numerical aperture NA of the objective lens, the refractive index of the first substrate is N, and the wavelength of the light source is Is set to be λ, 0.55 λ / TP1 ≦ NA t2-t1 ≦ 7.513 (λ / NA ^ 4) (N ^ 3 / (N ^ 2-1)) is set. Recording medium recording / reproducing method.