JP2005078697A - Optical recording medium, its driving device, and its reproducing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical recording medium wherein digital signals recorded with high density by a PRML (partial response and maximum likelihood) decoding method can be stably reproduced and to provide its deriving device and its reproducing method. <P>SOLUTION: Square wave response of a system comprising the recording medium and the driving device and PR (Partial Response) response are made even to suppress noise increase in PR equalization by controlling the amount of perpendicular birefringence of a substrate of the recording medium so that the ratio of the amount to the numerical aperture NA of an object lens of an optical pickup head PUH mounted on the driving device is in the range of 1.3×10<SP>-4</SP>/NA to 3.9×10<SP>4</SP>/NA. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an optical recording medium, a driving apparatus capable of reproducing the optical recording medium, and a reproducing method thereof, and in particular, a high-density optical recording medium that performs decoding using PRML (Partial Response and Maximum Likelihood) technology and a driving capable of reproducing the same. The present invention relates to an apparatus and a reproducing method thereof.

  In recent years, as a standard for DVD (Digital Versatile Disc) recording media and driving devices thereof, the laser wavelength (λ): 650 nm (for professional authoring use) is commonly used for read-only type, write once type, and rewritable type. 635 nm for the write-once type), numerical aperture (NA) of the objective lens: 0.6, thickness of one substrate on which the recording layer is formed: 0.6 mm, and storage capacity per recording layer: 4.7 GB It has been. With this storage capacity, it is possible to reproduce 133 minutes of video, audio, and subtitles that can accommodate almost one movie. On the other hand, development for reproducing or recording / reproducing high definition (HD) moving images for 2 hours is also underway, and the storage capacity required for this is estimated to be approximately 15 GB.

The following method is effective for realizing such a large storage capacity.
(1) Shortening the wavelength of the laser light source (see, for example, Patent Documents 1 and 2)
(2) Higher objective lens NA (Same as above)
(3) Application of signal processing technology that enables improvement of recording mark density, particularly PRML technology (see, for example, Patent Document 3)
The methods (1) and (2) utilize the fact that the diameter of the focused beam from the laser light source is proportional to λ / NA. By using these methods, it is possible to form a smaller recording mark by using a narrowed focused beam.

  The method (3) is to employ a reading method that is resistant to intersymbol interference that occurs when the recording mark length becomes shorter than the focused beam diameter. Conventionally, when a signal is reproduced from a DVD recording medium, a method called a level slice method for comparing a threshold voltage and a read voltage value has been used. However, a PR (Partial Response) method and an ML (Maximum Likelihood; When the PRML method combined with the (maximum likelihood) decoding method is used, it is possible to perform more stable reproduction than the level slice method even when the recording density is high.

Below, the reproduction | regeneration procedure using PRML method is outlined. A read signal read from an optical pickup head (hereinafter referred to as “PUH”) is recorded due to intersymbol interference of signals from adjacent recording marks when the recording mark length is short and the distance between the recording marks is narrow. The reproduction voltage corresponding to the mark and the space between the marks is reduced, and the peak position is shifted on the time axis. The degree of this intersymbol interference is determined by the linear density of the recording marks when the reproducing optical system is the same. Therefore, the Viterbi decoder reverses the intersymbol interference during reproduction when the linear direction density of the recording marks is high. Corresponding processing is performed to obtain a desired signal. Further, for the purpose of absorbing individual differences between PUHs and recording media, the read signal is passed through a PR equalizer before ML decoding. This plays a role of adjusting the waveform when the interference observed in the actual signal deviates from the assumed intersymbol interference due to individual differences in PUH or media.
Japanese Patent Laid-Open No. 06-187667 Japanese Patent No. 003116986 JP 2000-187945 A

  The NA of the objective lens is given by the product of the refractive index of the objective lens and the sine of the angle at which the entrance pupil radius is stretched with respect to the focal point (object point). For example, the incident angle of the condensed beam that passes through the outer periphery of the objective lens and enters the substrate surface is increased. That is, the oblique incident component of the focused beam increases. When the oblique incident component of the focused beam increases, the effect of the difference between the refractive index in the direction perpendicular to the disk substrate surface and the refractive index in the in-plane direction (hereinafter referred to as “vertical birefringence amount”) increases and reproduction is performed. Signal quality deteriorates. For example, astigmatism causes waveform distortion in the reproduction signal, and jitter occurs. And the shorter the wavelength, the greater the effect.

  In addition, for incident light having an oblique incident component, as the vertical birefringence amount of the substrate increases, the polarization state of the return light changes greatly from circularly polarized light to elliptically polarized light. A phenomenon occurs in which the amount of light received by the detector is partially transmitted and the amount of light received by the photodetector decreases. Further, since the diffraction angle of the short mark is larger than that of the long mark, the amount of light received is greatly affected by the amount of vertical birefringence of the substrate, so that the reduction in the amount of received light becomes remarkable, leading to a reduction in resolution.

  Further, when the influence of the amount of vertical birefringence is large and the resolution of the system composed of the recording medium and the reproducing apparatus is low, it becomes difficult to perform stable reproduction when reproducing using the PRML method. There is a problem. FIG. 1 shows a rectangular wave response of one system composed of a recording medium and a drive device (reproducing device or recording / reproducing device) (not passing through a PR equalizer and ML decoder: hereinafter referred to as “system response”). Is shown. A curve A is a system response (hereinafter referred to as “PR response”) on the assumption (calculation) that is assumed to be an optimum reproduction signal when passing through the PR equalizer in this reproduction system. However, as shown by the curve C, when the influence of the amount of vertical birefringence is strong and the resolution is low, the response of the system deviates from the curve A, and when passing through the PR equalizer, SNR (Signal to Noise Ratio (signal-to-noise ratio) decreases. Conversely, as shown by curve B, if the influence of vertical birefringence is weak and the resolution becomes too high, the response of the system deviates from curve A, and the SNR decreases when the PR equalizer is passed. End up.

  The present invention has been made in view of the above problems, and an object thereof is high-density optical recording that can be stably reproduced when a recording signal of an optical recording medium is reproduced using the PRML method. An object is to provide a medium, a driving device thereof, and a reproducing method thereof.

In order to achieve the above object, according to the present invention, light is emitted from a driving device having an optical pickup head having an objective lens having a numerical aperture NA from the surface of the substrate opposite to the recording layer on which the recording mark is formed. An optical recording medium that receives a light beam and reads a read signal from the recording mark, and a refractive index in a direction perpendicular to a substrate surface on which the light beam is incident and an in-plane refractive index of the substrate surface, the maximum value of the difference (hereinafter, referred to as "vertical birefringence") dn _⊥ is a feature that meets the ^{1.3 × 10 -4 / NA ≦ dn} ⊥ ≦ 3.9 × 10 -4 / NA An optical recording medium is provided.

  In order to achieve the above object, according to the present invention, there is provided an optical pickup head having an objective lens having a numerical aperture of 0.6 or more, which reads a read signal from a recording mark formed on the optical recording medium. There is provided a drive device characterized by having a PR (Partial Response) equalizer and an ML (Maximum Likelihood) decoder.

In order to achieve the above object, according to the present invention, light is emitted from a driving device having an optical pickup head having an objective lens having a numerical aperture NA from the surface of the substrate opposite to the recording layer on which the recording mark is formed. Is a reproduction method for reproducing a modulated digital signal recorded on the recording mark by inputting the light beam, and reading the read signal from the record mark and digitizing the read signal to obtain a digital signal A process of obtaining PR waveform equalized data from the digital signal, and a process of obtaining a reproduction signal of the modulated digital signal by decoding the data using the ML method, numerical aperture NA is 0.60 or more, the vertical birefringence dn _⊥ of the ^{substrate, 1.3 × 10 -4 / NA ≦} dn ⊥ ≦ 3.9 × 10 -4 Play wherein the NA satisfy a, is provided.

  The high-density recording medium, the driving device thereof, and the reproducing method thereof according to the present invention control the vertical birefringence amount of the recording medium substrate to make the system response close to the PR response. Noise rise is suppressed, and thus stable reproduction is possible.

  As described above, when reproducing an optical recording signal using the PRML method, there are upper and lower limits on the amount of vertical birefringence of a substrate that can be stably reproduced. The inventor has found that the upper and lower limits of the amount of vertical birefringence of the substrate have a certain relationship with the numerical aperture of the objective lens, and will be described in detail below.

  FIG. 1 is a flowchart for explaining a reproduction procedure of a reproduction method according to the present invention. In step S101, the PUH is irradiated with a laser beam on the optical recording medium, and the modulated digital signal recorded on the optical recording medium is read as an analog signal. The read analog signal is passed through an amplifier, a low-pass filter, etc., and then digitized by an AD converter in step S102. Next, in step S103, the digitized read signal is processed by a PR equalizer to obtain PR waveform equalized data. Next, in step S104, the waveform-equalized data is decoded as a reproduction signal of the modulated digital signal recorded on the optical recording medium by the Viterbi decoder which is an ML decoder. Finally, in step S105, the subsequent process is performed to complete the playback procedure of the playback method according to the present invention. The subsequent processing in step S105 includes error correction, audio / video image decoding, bit error rate (hereinafter referred to as “bER”) measurement, and the like.

As the optical recording medium, a read-only optical disk was used. As the substrate material, a polycarbonate resin was used, and the disc thickness was 0.6 mm. A recording layer was formed on this substrate by embossing pit rows as recording marks. The interval between the pit rows in the radial direction of the disc was set to 0.4 μm. Random data modulated based on the (1-7) modulation method was used as the recording data of the pit train. In the following examples, the relationship between the amount of vertical birefringence of the substrate, the resolution, and the bER was examined by changing the numerical aperture of the objective lens. The resolution is the shortest mark and the longest mark (hereinafter referred to as “2T mark” and “8T mark”, respectively) having lengths of 2T and 8T (T is a clock cycle) in terms of reproduction time from the read voltage of the reproduction signal. The amplitude ratio was calculated in decibels. The larger the vertical birefringence, the smaller the amplitude of the read voltage of the reproduction signal of the 2T mark having a shorter mark length, and the smaller the resolution value. The bER was performed by comparing the original data recorded on the optical disc [random data modulated based on the (1-7) modulation method] and the data binarized by Viterbi decoding. The amount of vertical birefringence was changed by changing the molecular weight of the polycarbonate resin. Further, the in-plane birefringence amount (difference in refractive index between the two axial directions in the substrate surface) was set to 1.0 × 10 ⁻⁵ . All substrates were made using the same stamper. A semiconductor laser with a wavelength of 405 nm was used as the PUH light source, and its reproduction power was set to 0.5 mW. However, the wavelength and reproduction power are not limited to these values, and the same result is obtained in the range of at least 400 to 410 nm with respect to the wavelength. The rim strength is approximately 60% in the radial direction and 62% in the tangential direction. However, at the rim strength of at least 55-70%, there was no significant change in the measurement results. Also, adaptability in several PR systems was examined by changing the recording density of the pit rows.

As specifically shown in the following examples, the substrate of the recording medium according to the present invention has a vertical birefringence of 1.3 × 10 ⁻⁴ / NA with respect to the numerical aperture NA of the objective lens used for reproduction. Those having the above and 3.9 × 10 ⁻⁴ / NA or less and having substantially no absorption with respect to the wavelength of light used for reproduction are preferable. When PR (12221) is used as the PR class, the recording density of the recording marks is preferably in the range of 0.120 to 0.150 μm / bit. Beyond this recording density range, the PR response is closer to the system response when using another PR class.
The recording medium is not limited to the read-only type, and may be a write-once type or a rewritable type recording medium. The substrate thickness is not necessarily 0.6 mm, and may be 1.2 mm, 0.1 mm, or other thickness.

In the present invention, F (dn _⊥ ) defined by the following formula can be used as an evaluation index for evaluating the recording medium.
_{F (dn ⊥) = ∫ [} Smp (dn ⊥) -Smp PRclass] 2 dt
Here, Smp (dn _⊥ ) and Smp _PRclass are a system response and a PR response to the used PR class, respectively. F (dn _⊥ ), which is defined as the time integral of the square of the residual of these two quantities, becomes smaller as the system response is closer to the PR response, and can be used as an evaluation index for evaluating the recording medium. is there.

  As Example 1 of the present invention, the NA of the objective lens was 0.65, PR (12221) was used as the PR class, and the relationship between the amount of vertical birefringence and the resolution and bER was examined. The length of the 2T mark, which is the shortest mark formed on the substrate, was 0.2 μm. Reproduction was performed at a linear speed of 6.6 m / s. In this case, the clock cycle T is 15.15 ns and the recording density is 0.13 μm / bit. Table 1 shows the relationship between the amount of vertical birefringence obtained in this example, the resolution, and the bER.

垂直複屈折量と分解能およびｂＥＲとの関係（実施例１）

Relationship between amount of vertical birefringence and resolution and bER (Example 1)

From Table 1, when the threshold value of bER is 5.0 × 10 ⁻⁵ , the allowable birefringence amount of the objective lens used in this example is NA: 0.65 and PR (12221) class. It is clear that the range is 2.0 × 10 ^{−4 to} 6.0 × 10 ⁻⁴ . If the amount of vertical birefringence exceeds the above range, reproduction signal deterioration due to the PR equalizer occurs, and bER exceeds the threshold value.

  As Example 2 of the present invention, the NA of the objective lens was 0.75, and PR (12221) was used as the PR class. The recording density is 0.15 μm / bit. Table 2 shows the relationship between the amount of vertical birefringence obtained in this example, the resolution, and the bER.

垂直複屈折量と分解能およびｂＥＲとの関係（実施例２）

Relationship between amount of vertical birefringence and resolution and bER (Example 2)

From Table 2, assuming that the threshold value of bER is 5.0 × 10 ⁻⁵ , the range of allowable vertical birefringence in the combination of the objective lens NA: 0.75 and PR (12221) class used in this example. Is clearly 1.7 × 10 ^{−4 to} 5.2 × 10 ⁻⁴ .

The allowable range of the vertical birefringence amount of the present embodiment is narrower than that of the first embodiment. This is because the effect of the amount of vertical birefringence appears larger as NA increases. Therefore, signal degradation becomes more prominent as NA increases. However, in the converting the range of the vertical birefringence per NA, and the scope thereof, and ^{1.3 × 10 -4 / NA~3,9 × 10} -4 / NA, Example 1 and Example 2 Match exactly.

  As Example 3 of the present invention, the recording density was changed, waveform equalization was performed in several PR classes, and the bER at that time was examined. The vertical birefringence of the substrate was the same. The NA of the objective lens is 0.65 as in the first embodiment. Table 3 shows the measurement results.

記録密度およびＰＲクラスとｂＥＲとの関係（実施例３）

Relationship between recording density and PR class and bER (Example 3)

From Table 3, when the bER threshold is 5.0 × 10 ⁻⁵ , the PR class is PR (12221), and the recording density range is 0.120 to 0.150 μm / bit. Table 3 also shows that by optimizing the PR class, stable reproduction can be achieved over a wide recording density within the range of the vertical birefringence and NA of the present invention.

  In the above embodiment, the numerical apertures are 0.65 and 0.75, and the PR class is PR (12221). However, the numerical aperture and the PR class are not limited to these. If the numerical aperture and the PR class are different, a suitable range of the amount of vertical birefringence is determined accordingly.

The comparison figure of the response of a reproduction system, and PR response for demonstrating the resolution (vertical birefringence amount) dependence of the response of a reproduction system. The flowchart for demonstrating the reproduction | regeneration procedure of the reproduction | regeneration method which concerns on this invention.

Claims (12)

A light beam emitted from a driving device having an optical pickup head having an objective lens with a numerical aperture NA is incident from the surface of the substrate opposite to the recording layer on which the recording mark is formed, and a read signal is read from the recording mark. The maximum value of the difference between the refractive index in the direction perpendicular to the substrate surface on which the light beam is incident and the refractive index in the plane of the substrate surface (hereinafter referred to as “vertical birefringence amount”). _Dn ) satisfies the condition of 1.3 × 10 ⁻⁴ / NA ≦ dn _≦≦ 3.9 × 10 ⁻⁴ / NA. The optical recording medium according to claim 1, wherein the numerical aperture NA of the objective lens is 0.60 or more. The optical recording medium according to claim 1, wherein a recording density of the recording mark is 0.120 μm / bit or more and 0.150 μm / bit or less. 4. The optical recording medium according to claim 1, wherein the amount of vertical birefringence dn _条件を_満た_す satisfies a condition of 1.7 × 10 ⁻⁴ ≦ dn _≦≦ 6.0 × 10 ^−4. . The optical recording medium according to claim 1, wherein the substrate has a polycarbonate resin as a material. A drive device comprising an optical pickup head having an objective lens having a numerical aperture of 0.60 or more, which reads a read signal from a recording mark formed on the optical recording medium according to any one of claims 1 to 5. , PR (partial response) equalizer and ML (maximum likelihood) decoder. The driving apparatus according to claim 6, wherein the PR equalizer is a PR (12221) class PR equalizer. 8. The driving apparatus according to claim 6, wherein the ML decoder is a Viterbi decoder. The drive device according to claim 6, wherein the numerical aperture is 0.65 or more and 0.75 or less. The drive device according to claim 6, wherein the wavelength of the light beam emitted from the optical pickup head is 400 nm or more and 410 nm or less. The drive device according to any one of claims 6 to 10, wherein the rim strength is 55% or more and 70% or less. A light beam emitted from a driving device having an optical pickup head having an objective lens having a numerical aperture NA is incident on the recording mark from the surface of the substrate opposite to the recording layer on which the recording mark of the recording medium is formed. A reproduction method for reproducing a recorded modulated digital signal, a process of reading a read signal from the record mark, a process of digitizing the read signal, and PR waveform equalization from the digitized read signal Obtaining the data, and decoding the data using the ML method, the numerical aperture NA of the objective lens is 0.60 or more, and the vertical birefringence dn _{の of the} substrate is A reproduction method characterized by satisfying a condition of 1.3 × 10 ⁻⁴ / NA ≦ dn _≦≦ 3.9 × 10 ⁻⁴ / NA.

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