JP4563444B2 - Optical recording / reproducing device - Google Patents

Description

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

  First, the layer spacing of recording layers in a recording medium having a plurality of recording layers and the thickness of a light transmission layer generally called a cover glass layer or a protective layer will be described.

  As a factor for determining the layer interval between the two recording layers, first, as a factor for determining the minimum value of the recording layer interval, there are crosstalk of signals between the two layers and interference of the FES signal. Second, the recording layer As a factor for determining the maximum value of the interval, there is an increase in the diameter of the focused spot due to the occurrence of spherical aberration when the focused spot is formed for each of the two recording layers.

  Further, as a factor for determining the optical thickness from the objective lens side surface (incident surface) of the recording medium to the recording layer, a skew margin of the recording medium is ensured.

  First, the distance between the two recording layers will be described.

  If the distance between the two layers is narrow (especially when there is a recording layer close within the focal depth of the focused spot), the information signal from the recording layer that is recording / reproducing information is The information signal is mixed.

  Further, the dynamic range of the FES signal is usually about 10 to 20 μm. In this case, if the distance between the two layers is not at least 10 to 20 μm, the objective lens is focused on the recording layer. Since the interference of the FES curves of the two recording layers is detected, the focus lock may be unstable. For example, in a DVD-ROM, the interval between the two layers is 55 ± 15 μm.

  When an objective lens having a high numerical aperture is used, the depth of focus is very shallow. Therefore, interference of the FES signal is dominant as a factor for determining the minimum value of the recording layer interval.

  Furthermore, the objective lens is designed so that the spherical aberration of the condensed spot that is transmitted through and condensed through a light transmitting body having a specific optical thickness is minimized. That is, 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 focused spot on each recording layer Also grows.

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

  The recording medium is rotated by a 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 is generated due to an attachment error to the spindle motor or warping of the recording medium.

  When the recording medium is tilted with respect to the optical axis, coma aberration mainly occurs on the condensing spot condensed on the recording layer of the recording medium, and the condensing spot diameter becomes large.

  Since the relationship between the tilt of the recording medium and the amount of coma generated is proportional to the optical thickness from the incident surface of the recording medium to the recording layer, the recording medium is reduced by reducing the optical thickness from the incident surface to the recording layer. Generally, a method of reducing the amount of coma generated by the inclination of the image, that is, increasing the skew margin is taken.

  As described above, usually, the optical thickness from the objective lens side surface of the recording medium to the first recording layer is mainly set to an optical thickness that can assure a skew margin of the recording medium and is as thick as possible. The interval between the recording layers is determined so that there is no interference of the FES signal and the interval between the two layers is the thinnest. However, the dynamic range of the FES signal is generally about 10 to 20 μm, and if the dynamic range is further reduced, there is a possibility that the focus servo may be pulled in unstable. As a result, the recording layer interval should be greatly reduced. It is difficult.

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

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

  FIG. 10 shows a combined objective lens barrel in which the spherical aberration at the focused 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 disposed on the light source side (not shown), and the second lens 101 is disposed on the recording medium side with respect to the first lens 100. A 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 changing actuator 103 drives the second lens 101 in the focus direction, thereby changing the interval between the first lens 100 and the second lens 101, and reducing the spherical aberration of the focused spot focused on the recording medium. It has a function to correct.

  Light emitted from a light source (not shown) is guided to a combined objective lens 106 by an optical component (not shown), and is condensed 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 with only one lens because the numerical aperture of the lens is increased.

  When there is 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 by the spherical aberration correction mechanism that changes the lens interval in this way, or a plurality of recordings When information is recorded / reproduced in / from each recording layer of a recording medium having layers, spherical aberration can be corrected by changing the lens interval.

  The optical thickness referred to here is a thickness determined by the thickness and refractive index of the light transmitting layer through which light is transmitted. Even if the thickness is different, the optical thickness of the condensed spot that has been transmitted through the light transmitting layer and collected. It is assumed that the optical thickness is equal when the magnitudes of the spherical aberrations coincide.

  The error in the optical thickness from the incident surface of the recording medium to the recording layer refers to the optical thickness of the light transmitting member assumed as the combined objective lens design value and the actual recording / reproducing information on the recording medium. This is the difference from the optical thickness.

As a driving method for changing the lens interval, an electromagnetic force is generated by passing a positive or negative current through the coil, and the second lens 101 is moved in the vertical direction in the focus direction by a thrust generated with the magnet. A method called a voice coil motor that displaces the motor is proposed (see Patent Document 2).
JP-A-10-188301 Japanese Patent Laid-Open No. 10-255290

  In order to reduce the spherical aberration that occurs in each recording layer when performing recording and reproduction on a recording medium having a plurality of recording layers as described above, a method for 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 of the recording layer cannot be ignored.

The present invention has been made in view of such circumstances, to reduce the influence of the magnitude of the difference of the wavefront aberration caused by the thickness of the layers of the recording layer of the recording medium, in particular, alleviate skew margin An object of the present invention is to provide an optical recording / reproducing apparatus capable of performing the above.

  The present invention has been made in order to achieve the above-described object, and condenses light from a light source on a recording medium having N (N ≧ 2) recording layers through an objective lens to record information. An optical recording / reproducing apparatus for performing recording / reproducing, wherein each recording layer is first layer,..., Nth layer in order from the recording layer on the objective lens side, and is condensed on a recording medium through the objective lens. A feature of the present invention is that the spherical aberration occurring in the Nth recording layer is reduced with respect to the spherical aberration occurring in the other recording layer in the focused spot.

  The optical recording / reproducing apparatus of the present invention is characterized in that, in the above-described optical recording / reproducing apparatus, the numerical aperture of the objective lens is larger than 0.85.

  Further, the optical recording / reproducing apparatus of the present invention is the above-described optical recording / reproducing apparatus, wherein the objective lens has the smallest spherical aberration of the condensed spot that is transmitted through the light transmitting body having the optical thickness t. When the optical thickness tN from the objective lens side surface of the recording medium to the Nth layer is the optical thickness tN, the optical thickness tN and the optical thickness t substantially coincide with each other.

The optical recording / reproducing apparatus of the present invention is the above-described optical recording / reproducing apparatus, wherein the objective lens is a combined objective lens having two lens groups each including at least one lens, and is 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 correction mechanism for changing the distance between the lens groups by electric drive, and corrects the spherical aberration of the focused spot formed on the recording layer of the first layer. When the interval is dl, the input current is il, the lens group interval at the time of correcting the spherical aberration of the condensing spot formed in the Nth recording layer is dN, and the input current is iN,
| Il | = | iN |
And the lens group interval dc when the input current is 0 is
dc = (dl + dN) / 2
It is characterized by being.

  The optical recording / reproducing apparatus of the present invention is the above-described optical recording / reproducing apparatus, wherein information is recorded / reproduced on a recording medium composed of a single recording layer. Assuming that the distance between the lens groups when recording and reproducing is dl ′, dc and dl ′ are substantially the same.

  According to the present invention, when recording / reproducing information 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.

  In addition, according to the present invention, since the distance between the recording layers can be sufficiently separated by the spherical aberration correction mechanism, signal crosstalk and FES between the recording layers can be obtained even when the numerical aperture of the lens is large. Generation of signal interference and the like can be avoided.

  Further, according to the present invention, when information is recorded / reproduced on / from a recording medium having a plurality of recording layers by a combination objective lens equipped with 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.

  According to the present invention, in the optical recording / reproducing apparatus, when the information is recorded / reproduced on / from a recording medium having a single recording layer, the input current to the spherical aberration correction mechanism is substantially zero, and the power consumption Can also be reduced. In that case, the environmental resistance of the combined objective lens can be improved.

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

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

  The combination objective lens 1 includes two lenses, which are 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 applying a positive or negative current to the coil 7 by the voice coil motor 9 including the magnet 4, the leaf spring 6, and the coil 7. That is, the distance between the first lens 2 and the second lens 3 can be changed.

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

  As shown in FIG. 2, the combined objective lens 1 is designed so that the lens interval is d, and the spherical aberration of the condensed spot that is transmitted through the light transmitting body 12 having the optical thickness t is minimized. Has been.

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

  The magnitude of the spherical aberration of the condensing spot collected by the combined objective lens 1 varies depending on the thickness and refractive index of the light transmitting body (or light transmitting layer) and the lens interval in the combined objective lens. Therefore, when the distance between the combined objective lenses is fixed, and the spherical aberration in the condensed spot that is collected through the different light transmitting bodies is the same, the optical thickness of those light transmitting bodies is Suppose they are the same.

  In addition, the recording medium includes a light transmission layer, a recording layer, and a light transmission layer between the recording layers, but in the description, from the surface of the recording medium combined objective lens side to the position where the condensing spot is formed on the recording medium. In other words, the light transmission layer, the recording layer, and the light transmission layer between the recording layers may be collectively referred to as a light transmission layer.

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

  FIG. 3 shows a combination objective lens designed so that the spherical aberration is minimized when a condensing spot is formed on the first layer 13 of the two recording layers close to the combination objective lens side of the recording medium 11. The state which formed the condensing spot in the 1st layer 13 is shown. The first layer 13 and the second layer 14 are arranged at optical thickness positions of 100 μm and 115 μm (refractive index is 1.58) from the incident surface of the recording medium, respectively.

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

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

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

  In general, it is known that in the optical recording / reproducing apparatus, the magnitude of the wavefront aberration needs to be smaller than the Marechal criterion (0.07λrms), and the allowable value of the wavefront aberration for determining the skew margin is 0.07λrms. In this case, as shown in FIG. 5, the skew margins when recording and reproducing the recording layers of the first layer and the second layer are 0.98 degrees and 0.80 degrees. However, normally, 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, which has a smaller skew margin, is the skew margin of the recording medium. It turns out that.

  FIG. 6 shows that the second layer is formed by a lens designed to minimize the spherical aberration when a condensing spot is formed on the second layer 14 far from the combined objective lens side of the recording medium 11. 14 shows a state in which a condensing spot is formed. The first layer 13 and the second layer 14 are arranged at optical thickness positions of 100 μm and 115 μm (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 transmission layer and the wavefront aberration that are transmitted to the position where the focused spot is formed on the recording medium. The combined objective lens is designed to pass through the second layer, that is, the light transmission layer having an optical thickness of 115 μm, and to minimize the wavefront aberration (spherical aberration in this case) of the focused spot. When a condensing spot is formed in one layer, a wavefront aberration of 0.016 λrms occurs even after correction of spherical aberration. The NA of the combination objective lens was 0.85.

  FIG. 8 shows the amount of tilt of the recording medium and the amount of wavefront aberration that occurs at the focused spot focused on the first and second layers.

  Thus, the skew margin can be reduced to 0.85 degrees by optimizing the combination objective lens for the second layer in which larger coma aberration occurs with respect to the tilt 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 transmission layer that passes through the recording medium up to the position where the condensed spot is formed and the spherical aberration. In this case, in order to correct the spherical aberration, the lens interval is corrected. Is changing. FIG. 7B shows the relationship between the optical thickness of the light transmission layer that passes through the recording medium to the position where the condensed spot is formed and the lens interval. 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, the lens interval when recording / reproducing the first layer is dl, the lens interval when recording / reproducing the Nth layer is dN, and dc = (dl + dN) / 2.

  The relationship between the optical thickness of the light transmission layer of the recording medium and the lens interval when correcting the spherical aberration caused by the thickness is generally linear as shown in FIG. Since the drive current for driving the lens interval variable mechanism that changes the lens interval can be designed to have a linear relationship as shown in FIG. 7C, the lens interval is dc. The driving current of the spherical aberration correction mechanism is set to 0 (neutral point), and the driving current il for setting the lens interval to dl and the driving current iN for setting the lens interval to dN are approximately | If il | = | iN |, the maximum value of the input current input to the spherical aberration correction mechanism can be reduced.

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

  Further, in the case of the spherical aberration correction mechanism including the voice coil motor 9 and the combination objective lens 1, the current is passed through the coil 7 because the voice coil motor 9 is housed inside the combination objective lens barrel 10. In order to reduce the influence of heat generation, it is desirable that the power consumption is lower and the maximum input current is small.

  When the coil 7 generates heat, the coil support 8 and other parts are thermally expanded, and the lens interval is changed. That is, the initial value of the lens interval changes from the design value. Then, when a method of correcting the spherical aberration of the focused spot focused on each recording layer of the recording medium 11 by flowing an input current having a predetermined magnitude to the voice coil motor 9 is employed. The spherical aberration of the focused spot is increased, which affects the recording / reproducing of information. In particular, in the case of a combination objective lens having a high numerical aperture, the influence becomes large.

  Further, when the influence of thermal expansion is not uniform in the combined objective lens barrel 10, tilt and decentering occur between the combined objective lenses, which causes coma and affects information recording / reproduction as well as spherical aberration. However, with this configuration, the influence can be reduced.

Further, in a recording medium having a plurality of recording layers, dl = (dl + dN) where the lens interval of the combined objective lens when recording and reproducing the first layer is dl, and the lens interval when recording and reproducing the Nth layer is dN. / 2, and as shown in FIG. 9, when recording and reproducing information on a recording medium having a single recording layer, the lens interval is dl ′, and dc and dl ′ are substantially coincided with each other. When recording / reproducing on a recording medium having a layer, the current applied to the spherical aberration correction mechanism can be reduced to approximately 0, and the same effect as in recording / reproducing information on a recording medium having a plurality of recording layers is obtained. .

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

  The spherical aberration correction mechanism is not necessarily required, and may be a combined objective lens with a fixed lens interval. Furthermore, the skew margin can be increased even with an objective lens composed of only one lens.

  In addition, the combination objective lens has been described as having two lenses. However, in the case where spherical aberration is corrected by changing the lens interval between the first lens unit and the second lens unit, which is composed of many lenses. Produces the same effect.

  Further, although the recording medium having two recording layers has been described, the same effect can be achieved in a recording medium having a plurality of recording layers.

  Further, as a 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 described in this order from the combined objective lens side. For example, the same effect can be obtained even with a so-called bonded disc in which two such recording media are bonded. However, in that case, it is necessary to record and reproduce from both sides of the recording medium. The same applies to a recording medium having one recording layer.

  In addition, the same effect can be obtained in any of the recording layers, ie, a read-only layer, a write-once type, and a recordable layer.

  The optical thickness from the incident surface to the recording layer of the recording medium depends on the refractive index and the thickness from the incident surface to the recording layer (for example, a thickness measured by focusing an objective lens on each layer). It is possible to define. In the actual production of the recording medium, the above values vary due to manufacturing errors. Therefore, in the standard of the recording medium, the above values are within a certain range. 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 the refractive index and thickness of the light transmitting layer from the incident surface of the recording medium to the Nth layer. It suffices if the value is within the allowable range.

  Further, 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 condensing spot is formed on the recording layer farthest from the incident surface with respect to other recording layers. Is to reduce the spherical aberration, that is, to reduce the wavefront aberration. 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.

It is explanatory drawing about the spherical aberration correction mechanism of this invention. It is explanatory drawing about the design specification of the combination objective lens of this invention. It is explanatory drawing at the time of designing and optimizing a combination objective lens for the 1st recording layer. It is explanatory drawing about the combination objective lens of FIG. 3, and a spherical aberration correction mechanism. It is explanatory drawing of the wavefront aberration in the combination objective lens of FIG. It is explanatory drawing at the time of designing and optimizing the combination objective lens of this invention for a 2nd recording layer. It is explanatory drawing about the combination objective lens and spherical aberration correction mechanism of this invention. It is explanatory drawing of the wavefront aberration in the combination objective lens of this invention. It is explanatory drawing of the recording medium which has one recording layer of this invention. It is explanatory drawing about the combination objective lens in a prior art.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Combination objective lens 2 1st lens 3 2nd lens 4 Magnet 5 Magnet support body 6 Leaf spring 7 Coil 8 Coil support body 9 Voice coil motor 10 Combination objective lens barrel 11 Recording medium 12 Light transmitting body 13 1st layer 14 1st Two layers 15 Light transmission layer 16 Light transmission layer 17 Substrate layer 18 Recording layer 19 Recording medium 100 First lens 101 Second lens 102 FR actuator 103 Lens interval variable actuator 104 Recording medium 105a First recording layer 105b Second recording Layer 106 combined objective lens

Claims (5)

An optical recording / reproducing apparatus for recording / reproducing information by condensing light from a light source via a objective lens on a recording medium having N (N ≧ 2) recording layers,
The objective lens is designed so that the spherical aberration of the condensed spot transmitted through the light transmitting body having the optical thickness t and condensed is minimized.
When each recording layer is the first layer,..., Nth layer in order from the recording layer on the objective lens side,
In the focused spot focused on the recording layer via the objective lens,
The magnitude of spherical aberration at the focused spot on each recording layer after correction of spherical aberration is different,
The optical thickness t of the light transmitting member in the objective lens design is set so that the spherical aberration occurring in the Nth recording layer is smaller than the spherical aberration occurring in other recording layers. An optical recording / reproducing apparatus.   2. The optical recording / reproducing apparatus according to claim 1, wherein the numerical aperture of the objective lens is 0.85. When the optical thickness from the objective lens side surface of the recording medium to the Nth layer is tN,
3. The optical recording / reproducing apparatus according to claim 2, wherein the optical thickness tN and the optical thickness t substantially coincide with each other. The objective lens is a combined objective lens having two lens groups each including at least one lens;
In order to correct the spherical aberration of the focused spot formed on each recording layer, a spherical aberration correction mechanism that changes the lens group interval by electrical drive,
The lens group interval at the time of correcting the spherical aberration of the condensing spot formed in the first recording layer is dl, and the input current is il,
When the lens group interval at the time of correcting the spherical aberration of the condensing spot formed in the Nth recording layer is dN and the input current is iN,
| Il | = | iN |
And
The lens group interval dc when the input current is 0 is
dc = (dl + dN) / 2
The optical recording / reproducing apparatus according to claim 2 or 3, wherein When recording and reproducing information on a recording medium consisting of a single recording layer,
5. The optical recording / reproducing apparatus according to claim 4, wherein dl 'is substantially coincident with dl' when the lens group interval when recording / reproducing information on the recording medium comprising the single recording layer is dl '. .

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