JP2004071150A - Optical recording and reproducing device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical recording and reproducing device in which influence caused by the difference of the magnitude of the wave front aberration caused by the correlation thickness of the recording layers of a recording medium is reduced, especially, the skew margin can be reduced. <P>SOLUTION: This device is an optical recording and reproducing device performing recording and reproducing of information by focusing light to a recording medium 11 having recording layers of N(N≥2) layers from a light source through an objective lens 1. The device is provided with a spherical aberration compensating mechanism compensating the spherical aberration of the spot focused on the recording medium 11, and when each recording layer is assumed to be a first layer to Nth layer in order from the recording layer of the objective lens 1 side, in the spot focused on the recording medium 11 through the objective lens 1, spherical aberration caused in the Nth recording layer is made smaller than the spherical aberration caused in the other recording layers. <P>COPYRIGHT: (C)2004,JPO

Description

The present invention relates to an optical recording / reproducing apparatus for recording / reproducing information with a lens having a high numerical aperture, and an optical recording / reproducing apparatus and an optical recording / reproducing apparatus for reducing a skew margin in recording / reproducing a recording medium having a plurality of recording layers. The reduction of power consumption.

First, a description will be given of a layer interval between recording layers in a recording medium having a plurality of recording layers, and a thickness of a light transmitting layer generally called a cover glass layer or a protective layer.

The factors that determine the layer spacing between the two recording layers are, first, the factors that determine the minimum value of the spacing between the recording layers include signal crosstalk between the two layers and interference of the FES signal. A factor that determines the maximum value of the interval is an increase in the diameter of the focused spot due to occurrence of spherical aberration when a focused spot is formed on each of the two recording layers.

要 因 Further, a factor that determines the optical thickness from the surface of the recording medium on the objective lens side (referred to as an incident surface) to the recording layer is to secure a skew margin of the recording medium.

{First, the spacing between two recording layers will be described.

If the distance between the two layers is small (especially if there is a recording layer close to the focal point of the focused spot), the information signal of the recording layer that is recording and reproducing information from the adjacent recording layer Information signal is mixed.

The dynamic range of the FES signal is usually about 10 to 20 μm. In such a case, the distance between the two layers must be at least 10 to 20 μm or more when focusing the objective lens on the recording layer. Since the interference between the FES curves of the two recording layers is detected, the focus lock may become unstable. For example, in a DVD-ROM, the interval between the two layers is 55 ± 15 μm.

(4) When an objective lens having a high numerical aperture is used, since the depth of focus is very shallow, the interference of the FES signal is dominant as a factor for determining the minimum value of the interval between the recording layers.

Furthermore, the objective lens is designed such that the spherical aberration of the condensed spot transmitted through and condensed through a light transmitting body having a specific optical thickness is minimized. In other words, in a recording medium having two recording layers, if the distance between the first recording layer and the second recording layer is large, the difference in the magnitude of the spherical aberration of the condensed spot on each recording layer will be large. Also gets bigger.

Next, the optical thickness from the incident surface of the recording medium to the recording layer and the skew margin will be described.

(4) The recording medium is rotated by the spindle motor in the optical recording / reproducing apparatus. At this time, an inclination with respect to the optical axis of the objective lens or the optical pickup occurs due to an attachment error to the spindle motor or a warp of the recording medium.

場合 When the recording medium is inclined with respect to the optical axis, coma aberration mainly occurs on the converging spot converged on the recording layer of the recording medium, and the converging spot diameter becomes larger.

The relationship between the inclination of the recording medium and the amount of coma aberration is proportional to the optical thickness from the incident surface of the recording medium to the recording layer. Therefore, by reducing the optical thickness from the incident surface to the recording layer, the recording medium is reduced. In general, a method of reducing the amount of coma caused by the inclination of the skew, that is, increasing the skew margin has been adopted.

As described above, the optical thickness from the surface of the recording medium on the objective lens side to the first recording layer is generally set to be as large as possible so as to ensure the skew margin of the recording medium. Further, the distance between the recording layers is determined so that there is no interference of the FES signal and the distance between the two layers is the smallest. However, the dynamic range of the FES signal is generally about 10 to 20 μm, and if the dynamic range is reduced further, the pull-in of the focus servo may become unstable. As a result, the distance between the recording layers may be significantly reduced. It is difficult.

Next, an example of a mechanism for correcting the spherical aberration of the light spot focused on the recording medium will be described.

Here, a conventional technique disclosed in Patent Document 1 will be described.

FIG. 10 shows a combined objective lens barrel in which the spherical aberration in the condensed spot on the recording medium is changed by changing the distance between the combined objective lens and the lenses constituting the combined objective lens.

In FIG. 10, the first lens 100 is a lens arranged on the light source side (not shown), and the second lens 101 is arranged on the recording medium side with respect to the first lens 100. The focus radial actuator (FR actuator) 102 is for displacing the first lens 100 and the second lens 101 in the focus and radial directions. The lens-interval varying actuator 103 changes the interval between the first lens 100 and the second lens 101 by driving the second lens 101 in the focusing direction, and reduces the spherical aberration of the condensed spot condensed on the recording medium. It has a function to correct.

Light emitted from a light source (not shown) is guided to the combination objective lens 106 by an optical component (not shown), and is focused on the first recording layer 105a or the second recording layer 105b on the recording medium 104. .

The reason why the lens is a combination objective lens composed of a plurality of lenses is that it is difficult to design and manufacture using only one lens because the numerical aperture of the lens is increased.

The spherical aberration correcting mechanism that changes the lens interval as described above causes an error in the optical thickness from the incident surface of the recording medium 104 to the first recording layer 105a or the second recording layer 105b, or a plurality of recordings. When recording and reproducing information on each recording layer of a recording medium having a layer, the spherical aberration can be corrected by changing the lens interval.

Note that the optical thickness here is a thickness determined by the thickness and the refractive index of the light transmitting layer through which light passes, and even if the thickness is different, the light spot of the light transmitted through the light transmitting layer and condensed When the magnitudes of the spherical aberrations match, it is assumed that the optical thicknesses are equal.

In addition, the error of the optical thickness from the incident surface of the recording medium to the recording layer means the optical thickness of the light transmitting body assumed as the design value of the combination objective lens and the actual thickness of the information recorded / reproduced on the recording medium. Is the difference from the optical thickness.

As a driving method for changing the lens interval, a positive or negative current is applied to the coil to generate an electromagnetic force, and the thrust generated between the second lens 101 and the magnet causes the second lens 101 to move in the vertical direction in the focusing direction. A system called a voice coil motor for displacing the motor has been proposed (see Patent Document 2).
JP-A-10-188301 JP-A-10-255290

In order to reduce the spherical aberration occurring in each recording layer when performing recording and reproduction on a recording medium having a plurality of recording layers, a method of correcting the spherical aberration by changing the lens interval has been proposed. . However, when the numerical aperture of the lens is high, the influence of coma generated due to the thickness between the recording layers cannot be ignored.

The present invention has been made in view of such a situation, and reduces the influence of the difference in the magnitude of the wavefront aberration generated due to the correlation thickness of the recording layer of the recording medium, and in particular, reduces the skew margin. It is an object of the present invention to provide an optical recording / reproducing apparatus which can perform the recording and reproducing.

SUMMARY OF THE INVENTION The present invention has been made to achieve the above object, and condenses light from a light source through an objective lens onto a recording medium having N (N ≧ 2) recording layers, thereby obtaining information. An optical recording / reproducing apparatus for performing recording / reproduction, comprising a spherical aberration correction mechanism for correcting spherical aberration of a condensed spot focused on a recording medium, wherein each recording layer is a first layer in order from a recording layer on an objective lens side, ... When the N-th layer is formed, in the condensed spot condensed on the recording medium via the objective lens, spherical aberration generated in another recording layer is generated in the N-th recording layer. The characteristic feature is that the spherical aberration that occurs is reduced.

In the optical recording / reproducing apparatus of the present invention, in the optical recording / reproducing apparatus described above, the objective lens has the smallest spherical aberration of a condensed spot transmitted through a light transmitting body having an optical thickness of t and condensed. The optical thickness tN from the object lens side surface of the recording medium to the Nth layer is substantially equal to the optical thickness tN.

Further, according to the optical recording / reproducing apparatus of the present invention, in the above-described optical recording / reproducing apparatus, the objective lens is a combination objective lens having two lens groups each including at least one lens, and the objective lens formed on each recording layer. In order to correct the spherical aberration of the light spot, the lens group is provided with a spherical aberration correcting mechanism for changing the lens group interval by electric driving, and the lens group at the time of correcting the spherical aberration of the condensed spot formed on the first recording layer When the interval is dl, the input current is il, the lens group interval at the time of spherical aberration correction of the condensed spot formed on the N-th recording layer is dN, and the input current is iN,
| Il | = | iN |
And when the input current is 0, the lens group interval dc is
dc = (dl + dN) / 2
It is characterized by being.

Further, in the optical recording / reproducing apparatus of the present invention, in the optical recording / reproducing apparatus described above, when recording / reproducing information on a recording medium having a single-layer recording layer, the information is recorded on the recording medium having the single-layer recording layer. Assuming that the distance between the lens groups when recording and reproducing is dl ', dc and dl' substantially match.

According to the present invention, when information is recorded / reproduced on / from a recording medium having a plurality of recording layers, the maximum value of the wavefront aberration generated in each recording layer can be reduced, and as a result, the skew margin is reduced. be able to.

Further, according to the present invention, the distance between the recording layers can be sufficiently increased by the spherical aberration correcting mechanism. Therefore, even when the numerical aperture of the lens is large, the crosstalk of the signals between the recording layers and the FES can be reduced. The occurrence of signal interference or the like can be avoided.

Further, according to the present invention, when recording and reproducing information on a recording medium having a plurality of recording layers using a combination objective lens having a spherical aberration correction mechanism, the maximum input current and power consumption can be reduced. The environmental resistance of the objective lens can be improved.

Further, according to the present invention, in the optical recording / reproducing apparatus, when recording / reproducing information on / from a recording medium having a single recording layer, the input current to the spherical aberration correction mechanism is set to almost 0, and power consumption is reduced. Can also be reduced. In that case, the environmental resistance of the combination objective lens can be improved.

Next, an embodiment of the present invention will be described.

A description will be given of a spherical aberration correction mechanism including a combination objective lens and a voice coil motor with reference to FIG.

The combination objective lens 1 is composed of two lenses, a first lens 2 on the light source side and a second lens 3 on the recording medium side, respectively. The second lens 3 is fixed to a magnet 4, and the magnet 4 is supported by a leaf spring 6 via a magnet support 5. The coil 7 is supported by a coil support 8. The second lens 3 is displaced in the focus direction by passing a positive or negative current through the coil 7 by a voice coil motor 9 including a magnet 4, a leaf spring 6, and a coil 7. That is, the distance between the first lens 2 and the second lens 3 can be changed.

The combination objective lens 1, the voice coil motor 9, and other supporting parts are housed inside the combination objective lens barrel 10. The combined objective lens barrel 10 is driven in the focus and radial directions of the recording medium 11 by an FR actuator (not shown).

The combination objective lens 1 is designed such that the lens interval is d as shown in FIG. 2 and the spherical aberration of the condensed spot transmitted through the light transmitting body 12 having the optical thickness t is minimized. Have been.

The light transmitting body 12 is made of a transparent material that transmits light, and corresponds to a light transmitting layer of a recording medium assumed when designing a lens. The light transmitting body or the light transmitting layer is called a cover glass layer or a protective layer, and includes various types such as polycarbonate (PC), glass, and UV curable resin.

球面 The magnitude of the spherical aberration of the condensed spot condensed by the combination objective lens 1 varies depending on the thickness and refractive index of the light transmitting body (or light transmission layer) and the lens spacing in the combination objective lens. Therefore, when the lens interval of the combination objective lens is fixed and the magnitude of the spherical aberration in the condensed spot transmitted and condensed through different light transmitting bodies is the same, the optical thickness of those light transmitting bodies is Let it be the same.

The recording medium has a light transmission layer, a recording layer, and a light transmission layer between the recording layers. In the description, the light transmission layer extends from the combined objective lens side surface of the recording medium to a position where a condensed spot is formed on the recording medium. In some cases, the light transmitting layer, the recording layer, and the light transmitting layer between the recording layers may be collectively referred to as a light transmitting layer.

Next, the present embodiment of a recording medium having two recording layers will be described.

FIG. 3 shows a second objective lens designed to minimize spherical aberration when forming a converging spot on the first layer 13 near the objective lens side of the recording medium 11 among the two recording layers. This shows a state where a condensing spot is formed on the first layer 13. The first layer 13 and the second layer 14 are arranged at positions having an optical thickness of 100 μm and 115 μm (the refractive index is 1.58) from the incident surface of the recording medium, respectively.

The recording medium 11 includes a light transmitting layer 15, a first layer 13, a light transmitting layer 16, a second layer 14, and a substrate layer 17 disposed between the recording layers in this order from the combined objective lens side of the recording medium 11. You.

FIG. 4 shows the relationship between the optical thickness of the light transmitting layer and the wavefront aberration that is transmitted up to the position where the condensed spot is formed on the recording medium. The combination objective lens is designed to transmit through the first layer, that is, the light transmitting layer having an optical thickness of 100 μm, and to minimize the wavefront aberration (here, spherical aberration) of the focused light spot. When a condensing spot is formed in two layers, a wavefront aberration of 0.013 λrms occurs even after the spherical aberration is corrected. The combined objective lens used had an NA of 0.85.

FIG. 5 shows the amount of tilt of the recording medium and the amount of wavefront aberration generated at each of the condensed spots condensed on the first and second layers.

Generally, in an optical recording / reproducing apparatus, it is known that the magnitude of the wavefront aberration needs to be smaller than the male criterion (0.07λrms), and the allowable value of the wavefront aberration for determining the skew margin is set to 0.07λrms. In this case, as shown in FIG. 5, the skew margins when recording and reproducing the first and second recording layers are 0.98 degrees and 0.80 degrees. However, usually, the skew margin is not defined for each recording layer, but is defined as a recording medium. In this case, the skew margin of 0.80 degrees is smaller than the skew margin of the recording medium. It turns out that.

FIG. 6 shows a second layer formed by a lens designed to minimize spherical aberration when forming a converging spot on the second layer 14 far from the combined objective lens side of the recording medium 11 among the two recording layers. FIG. 14 shows a state where a converging spot is formed. The first layer 13 and the second layer 14 are arranged at positions having an optical thickness of 100 μm and 115 μm (the refractive index is 1.58) from the incident surface of the recording medium, respectively.

FIG. 7A shows the relationship between the optical thickness of the light transmitting layer and the wavefront aberration that is transmitted up to the position where the condensed spot is formed on the recording medium. The combination objective lens is designed to transmit through the second layer, that is, the light transmitting layer having an optical thickness of 115 μm, and to minimize the wavefront aberration (here, spherical aberration) of the collected light spot. When a condensing spot is formed on one layer, a wavefront aberration of 0.016 λrms is generated even after the spherical aberration is corrected. The combined objective lens used had an NA of 0.85.

FIG. 8 shows the amount of tilt of the recording medium and the amount of wavefront aberration generated at the condensed spots converged on the first and second layers.

In this way, the skew margin can be reduced to 0.85 degrees by optimizing the combination objective lens with respect to the second layer in which the coma aberration is larger than the inclination of the recording medium. It became.

Next, the spherical aberration correction mechanism will be described.

FIG. 7A shows the relationship between the optical thickness of the light transmitting layer transmitted to the position where the condensed spot is formed on the recording medium and the spherical aberration. In this case, the lens spacing is used to correct the spherical aberration. Is changing. FIG. 7B shows the relationship between the optical thickness of the light transmitting layer and the lens interval, which is transmitted up to the position where the condensing spot is formed on the recording medium. FIG. 7C shows the relationship between the optical thickness of the light transmission layer and the drive current for driving the spherical aberration correction mechanism.

Here, let dl be the lens interval when recording and reproducing the first layer, and let dN be the lens interval when recording and reproducing the Nth layer, and let dc = (dl + dN) / 2.

The relationship between the optical thickness of the light transmitting layer of the recording medium and the lens spacing for correcting spherical aberration caused by the thickness is substantially linear as shown in FIG. Since the drive current for driving the lens-interval variable mechanism for changing the lens interval can be designed to have a linear relationship as shown in FIG. The drive current of the spherical aberration correction mechanism is set to 0 (the neutral point), and the drive current il for setting the lens interval to dl and the drive current iN for setting the lens interval to dN are substantially | If il | = | iN |, it is possible to reduce the maximum value of the applied current supplied to the spherical aberration correction mechanism.

If it is considered that information is recorded and reproduced at the same ratio with respect to the first layer 13 and the second layer 14 of the recording medium 11, the power consumption can be reduced in that case.

Further, in the case of a spherical aberration correction mechanism including the voice coil motor 9 and the combined objective lens 1, since the voice coil motor 9 is housed inside the combined objective lens barrel 10, when the current is applied to the coil 7, However, there is a problem that the coil 7 generates heat. In order to reduce the influence of the heat generation, it is desirable that the power consumption is lower and the maximum input current is smaller.

(4) When the coil 7 generates heat, the coil support 8 and other components thermally expand, and the lens interval changes. That is, the initial value of the lens interval changes from the design value. Then, a method of correcting the spherical aberration of the condensed spot condensed on each recording layer of the recording medium 11 by flowing a predetermined amount of input current to the voice coil motor 9 is adopted. In addition, the spherical aberration of the condensed spot increases, which affects the recording and reproduction of information. In particular, in the case of a combination objective lens having a high numerical aperture, the influence becomes large.

If the influence of thermal expansion is not uniform in the combined objective lens barrel 9, tilt and decenter between the combined objective lenses will occur, causing coma aberration, and affecting the recording and reproduction of information as well as spherical aberration. However, with this configuration, the effect can be reduced.

Further, in a recording medium having a plurality of recording layers, the lens interval of the combined objective lens when recording and reproducing the first layer is dl, the lens interval when recording and reproducing the Nth layer is dN, and dc = (dl + dN). / 2, and when the distance between lenses at the time of recording / reproducing information on a recording medium having a single recording layer is dl ′ and dc and dl ′ are substantially equal to each other, recording on a recording medium having one recording layer is possible. The current applied to the spherical aberration correction mechanism during reproduction can be substantially zero, and the same effect as in the case of recording and reproducing information on a recording medium having a plurality of recording layers can be obtained.

In the above description, the voice coil motor was used as the spherical aberration correcting mechanism for changing the lens interval. However, the mechanism for changing the lens interval by electrically driving a piezoelectric element or the like also changes the current supplied to the spherical aberration correcting mechanism. This has the effect of reducing power consumption.

The spherical aberration correction mechanism is not always necessary, and a combination objective lens with a fixed lens spacing may be used. Further, even if the objective lens includes only one lens, the skew margin can be increased.

Although the combination objective lens is described as being composed of two lenses, further, in the case where spherical aberration is corrected by changing the lens interval between the first group lens and the second group lens that are composed of many lenses. Has the same effect.

{Although the recording medium having two recording layers has been described, the same effect can be obtained in a recording medium having more plural recording layers.

Further, as the recording medium having a plurality of recording layers, a light transmitting layer, a plurality of recording layers and a light transmitting layer disposed between the recording layers, and a substrate layer are arranged in order from the combination objective lens side. For example, the same effect can be obtained even with a so-called bonded disk in which two such recording media are bonded. However, in that case, it is necessary to perform recording and reproduction from both sides of the recording medium. The same applies to a recording medium having one recording layer.

The same effect can be obtained with any of the recording layers that are read-only, write-once, and recordable.

The optical thickness from the incident surface of the recording medium to the recording layer is determined by the value of the refractive index and the thickness from the incident surface to the recording layer (for example, the thickness measured by focusing an objective lens on each layer). It is possible to define. In actual manufacturing of a recording medium, the above-mentioned values vary due to manufacturing errors. Therefore, in the standard of the recording medium, a certain range of the above-mentioned values is allowed. In such a case, the value of the optical thickness (refractive index and thickness) of the light transmitting body assumed when designing the objective lens is determined by the refractive index and the thickness of the light transmitting layer from the incident surface of the recording medium to the Nth layer. May be within the allowable range of the value of.

In addition, the gist of the present invention is that, in the design of an objective lens for recording and reproducing information on a recording medium having a plurality of recording layers, a case where a condensed spot is formed on a recording layer farthest from an incident surface with respect to other recording layers. Is to make the spherical aberration small, that is, to make the wavefront aberration small. Therefore, the optical thickness of the light transmitting body in the design of the objective lens may be set in consideration of this point.

The present invention can be applied to an optical recording / reproducing apparatus for recording / reproducing information on / from a recording medium having a plurality of recording layers, and is particularly suitable when a lens having a high numerical aperture is used.

FIG. 3 is an explanatory diagram of a spherical aberration correction mechanism of the present invention. It is explanatory drawing about the design specification of the combination objective lens of this invention. FIG. 4 is an explanatory diagram in a case where a combination objective lens is designed to be optimized for a first recording layer. FIG. 4 is an explanatory diagram of a combination objective lens and a spherical aberration correction mechanism of FIG. 3. FIG. 4 is an explanatory diagram of a wavefront aberration in the combination objective lens of FIG. 3. FIG. 9 is an explanatory diagram in the case where the combination objective lens of the present invention is designed by optimizing it for the second recording layer. It is explanatory drawing about the combination objective lens and spherical aberration correction | amendment mechanism of this invention. FIG. 4 is an explanatory diagram of a wavefront aberration in the combination objective lens of the present invention. FIG. 2 is an explanatory diagram of a recording medium having one recording layer according to the present invention. It is explanatory drawing about the combination objective lens in a prior art.

Explanation of reference numerals

REFERENCE SIGNS LIST 1 combination objective lens 2 first lens 3 second lens 4 magnet 5 magnet support 6 leaf spring 7 coil 8 coil support 9 voice coil motor 10 combination objective lens barrel 11 recording medium 12 light transmitting body 13 first layer 14 Two layers 15 Light transmitting layer 16 Light transmitting layer 17 Substrate layer 18 Recording layer 19 Recording medium 100 First lens 101 Second lens 102 FR actuator 103 Lens spacing variable actuator 104 Recording medium 105a First recording layer 105b Second recording Layer 106 Combination objective lens

Claims (4)

An optical recording / reproducing apparatus for recording / reproducing information by condensing light from a light source on a recording medium having N (N ≧ 2) recording layers via an objective lens,
Equipped with a spherical aberration correction mechanism that corrects the spherical aberration of the light spot focused on the recording medium,
When each recording layer is a first layer,..., An N-th layer in order from the recording layer on the objective lens side,
At the focused spot focused on the recording medium via the objective lens,
An optical recording / reproducing apparatus, wherein the spherical aberration generated in the Nth recording layer is made smaller than the spherical aberration generated in another recording layer. The objective lens is designed such that the spherical aberration of a condensed spot transmitted through a light transmitting body having an optical thickness of t is minimized,
Assuming the optical thickness tN from the objective lens side surface of the recording medium to the Nth layer,
2. The optical recording / reproducing apparatus according to claim 1, wherein the optical thickness tN substantially coincides with the optical thickness t. The objective lens is a combination objective lens having two lens groups including at least one lens,
In order to correct the spherical aberration of the condensed spot formed on each recording layer, a spherical aberration correction mechanism that changes the lens group interval by electrical driving is provided.
The distance between the lens groups at the time of spherical aberration correction of the condensed spot formed on the first recording layer is dl, the input current is il,
When the distance between the lens groups at the time of correcting the spherical aberration of the condensed spot formed on the N-th recording layer is dN and the input current is iN,
| Il | = | iN |
And
Further, the lens group interval dc when the input current is 0 is
dc = (dl + dN) / 2
The optical recording / reproducing apparatus according to claim 1 or 2, wherein When recording and reproducing information on a recording medium consisting of a single recording layer,
4. The optical recording / reproducing apparatus according to claim 3, wherein dc and dl 'substantially coincide with each other, assuming that a distance between lens groups when recording / reproducing information on / from the recording medium having the single recording layer is dl'. .

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