JP2007193916A - Method of reproducing optical recording medium and reproducing device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve both reproducing characteristics and tilt margin when increasing recording capacity of an optical recording medium. <P>SOLUTION: When an information recording layer of the optical recording medium is irradiated with a laser beam to reproduce information, the wavelength of the laser beam is 400-410 nm, and the numerical aperture NA of an objective lens condensing the laser beam is 0.70-0.90. Further, the power of the laser beam is set to 0.45 mW or more, and the reproduction signal obtained by the irradiation with the laser beam is decoded by PRML discrimination system. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a reproducing method and an apparatus for reproducing an optical recording medium signal, and more particularly to a reproducing method suitable for increasing recording density.

  Conventionally, CDs and DVDs are widely used as optical recording media. The recording capacity required for this type of optical recording medium has been increasing year by year, and various attempts have been made to meet the demand. Recently, in order to further increase the recording capacity, New standards such as disks have been proposed. In the Blu-ray Disc standard, it is possible to reduce the beam spot diameter of laser light used for data recording / reproduction. Specifically, the numerical aperture (NA) of the objective lens that focuses the laser light is increased, and the wavelength λ of the laser light is shortened. As a result, 25 GB of information can be recorded on the information recording layer.

  The reproduction of a signal in the Blu-ray Disc standard is detected based on whether or not a reproduction signal obtained by irradiation with a laser beam spot intersects a bit determination level (slice level).

In addition, in order to reproduce a signal based on this bit determination level, the C / N (carrier to noise ratio) of the reproduced signal is important. For example, if the recording mark is made small in order to increase the recording density of the optical recording medium, the C / N tends to deteriorate because it approaches the resolution limit determined by the beam spot diameter. Thus, a technique has been proposed for improving the C / N by appropriately setting the reproduction power of the laser beam even when the size of the recording mark approaches the resolution limit.
JP 2005-209299 A JP 2003-6872 A

  At present, even in the Blu-ray disc, an information recording amount exceeding 25 GB is required for the information recording layer. However, when the recording density of the information recording layer is increased, there is a problem that the quality of the reproduction signal is deteriorated and it is difficult to determine the binary value by detecting the slice. Further, in the recording density region where binary determination by slice detection is difficult, there is a problem that it is difficult to determine the signal reproduction quality only by the C / N characteristic. As a result, in order to cope with further increase in density, it is necessary to improve the signal reproduction quality even if the C / N characteristic is deteriorated.

  Furthermore, when the recording density of the information recording layer is increased, there is a problem that the tilt margin of the laser beam with respect to the optical recording medium, that is, the tolerance of the tilt angle error of the optical axis with respect to the optical recording medium becomes extremely small. That is, in the Blu-ray Disc standard and the like, it is required to increase the signal quality by suppressing the deterioration of the tilt margin while increasing the information recording amount.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a reproduction method and the like for improving both signal reproduction quality and tilt margin in an optical recording medium.

  The present inventor has clarified that the reproduction characteristics of the signal can be improved by increasing the reproduction power while decoding the reproduction signal of the optical recording medium by the PRML identification method through intensive research. At the same time, it has been clarified that increasing the reproduction power significantly improves the tilt margin during high-density recording. That is, the above-described object is achieved by the following means made by intensive research by the present researchers.

  (1) When reproducing information by irradiating an information recording layer of an optical recording medium with a laser beam, the wavelength of the laser beam is set to 400 to 410 nm, and the numerical aperture NA of the objective lens for condensing the laser beam is An optical recording characterized in that the reproducing power of the laser beam is set to 0.70 to 0.90 and is set to be larger than 0.45 mW, and a reproducing signal obtained by irradiation with the laser beam is decoded by a PRML identification method. Media playback method.

  (2) The method for reproducing an optical recording medium according to (1), wherein the reference class of the PRML identification method is a constraint length of 5 (1, 2, 2, 2, 1).

  (3) The reproducing method for an optical recording medium according to (1) or (2), wherein the reproducing power is 0.50 mW or more.

  (4) The reproducing method for an optical recording medium according to (1) or (2), wherein the reproducing power is 0.60 mW or more.

  (5) The optical recording medium according to any one of (1) to (4) above, wherein the optical recording medium having a distance from the light incident surface to the information recording layer of 0.100 mm or less is reproduced. Playback method.

  (6) The method for reproducing an optical recording medium according to any one of (1) to (5), wherein the shortest mark length recorded in the information recording layer is 125 nm or less.

  (7) The method for reproducing an optical recording medium according to any one of (1) to (6), wherein the shortest mark length recorded in the information recording layer is 112 nm or less.

  (8) The method for reproducing an optical recording medium according to any one of (1) to (7), wherein the composition of the information recording layer includes Bi and O.

  (9) A laser light source that generates a laser beam having a wavelength of 400 to 410 nm, a laser controller that controls the laser light source so that the reproduction power of the laser beam is greater than 0.36 mW, and the laser beam is condensed Objective lens having a numerical aperture NA of 0.70 to 0.90, a photodetector for detecting reflected light of the laser beam, and a PRML processing device for decoding a reproduction signal detected by the photodetector by a PRML identification method And an optical recording medium reproducing device.

  According to the reproducing method and the reproducing apparatus of the present invention, when the recording capacity or recording density of the optical recording medium is increased, it is possible to achieve both the suppression of reproduction signal quality deterioration and the improvement of the tilt margin. Can be played.

  Next, exemplary embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 shows a playback apparatus 100 that realizes a playback method according to an embodiment of the present invention. The reproducing apparatus 100 includes a laser light source 102 that generates laser light Z used for reproduction, a laser controller 104 that controls the laser light source 102, an optical mechanism 106 that guides the laser light Z to the optical recording medium 1, and reflected light of the laser light Z. , A PRML processing device 110 that decodes detection information of the light detection device 108 using a PRML identification method, a spindle motor 112 that rotates the optical recording medium 1, and a spindle driver 114 that controls the rotation of the spindle motor 112. In addition, a signal processing device 116 is provided for exchanging data after decoding with a CPU (central processing unit) (not shown).

  The laser light source 102 is a semiconductor laser and is controlled by the laser controller 104 to generate the laser light Z. The optical mechanism 106 includes a half mirror 106A and an objective lens 106B, and can appropriately focus the laser beam Z on the information recording layer. The half mirror 106A takes out the reflected light of the information recording layer and guides it to the light detection device 108. The light detection device 108 is a photodetector, receives the reflected light of the laser light Z, converts the received light into an electrical signal, and outputs it as a reproduction signal to the PRML processing device 110. The PRML processing device 110 decodes the reproduced signal and outputs the decoded binary digital signal to the signal processing device 116.

  Further, in the reproducing apparatus 100, the wavelength of the laser beam Z is set to 400 to 410 nm, and the reproducing power of the laser beam Z is set to be larger than 0.45 mW. Specifically, the reproduction power is 0.60 mW. The numerical aperture NA of the objective lens 106B in the optical mechanism 106 is set to 0.70 to 0.90. Therefore, in order to reproduce the information on the optical recording medium 1, the laser light Z is generated from the laser light source 102 and the information recording layer of the optical recording medium 1 is irradiated with the laser light Z. The laser beam Z is reflected by the information recording layer, taken out via the optical mechanism 106, and converted into an electronic signal by the light detection device 108. This electronic signal is converted into a digital signal through the PRML processing device 110 and the signal processing device 116 and provided to the CPU.

  Next, the optical recording medium 1 used for reproduction of the reproduction apparatus 100 will be described. As shown in FIG. 2A, the optical recording medium 1 is a disk-shaped medium having an outer shape of about 120 mm and a thickness of about 1.2 mm. As shown in an enlarged view in FIG. 2B, the optical recording medium 1 is configured by laminating a substrate 10, a single-layer information recording layer 20, a cover layer 30, and a hard coat layer 35 in this order. Is done.

  The cover layer 30 and the hard coat layer 35 are light transmissive and transmit the laser light Z incident from the outside. Accordingly, the laser beam Z incident from the light incident surface 35A passes through the hard coat layer 35 and the cover layer 30 in this order, reaches the information recording layer 20, and reproduces the information held in the information recording layer 20. . In this optical recording medium 1, the recording capacity of the information recording layer 20 is set to 33.3 GB.

  The substrate 10 is a disk-shaped member having a thickness of about 1.1 mm, and various materials such as glass, ceramics, and resin can be used as the material, but here, polycarbonate resin is used. In addition to the polycarbonate resin, an olefin resin, an acrylic resin, an epoxy resin, a polystyrene resin, a polyethylene resin, a polypropylene resin, a silicone resin, a fluorine resin, an ABS resin, a urethane resin, or the like can be used as the resin. Of these, polycarbonate resins and olefin resins are preferred because of their ease of processing and molding. Further, grooves, lands, pit rows, and the like are formed on the surface of the substrate 10 on the information recording layer side according to applications.

  Although various materials can be used for the cover layer 30, it is necessary to use a light-transmitting material in order to transmit the laser beam Z as described above. For example, it is also preferable to use an ultraviolet curable acrylic resin. In this optical recording medium 1, the thickness of the cover layer 30 is set to 98 μm, and the thickness of the hard coat layer 35 is set to 2 μm. Therefore, the distance from the light incident surface 35A to the information recording layer 20 is about 100 μm. The optical recording medium 1 conforms to the current Blu-ray Disc standard except for the recording capacity (currently 25 GB at the time of this application).

  The information recording layer 20 is a layer for holding data. As a data holding form, there are a read-only type in which data is written in advance and cannot be rewritten, and a recording type in which a user can write. In addition, when the data holding form is a record type, a write-once type in which data cannot be written again in the area where data has been written once, and a rewritable type in which data can be erased and written again in the area where data has been written. There is. In the present embodiment, either a reproduction-only type or a recording type may be used.

  As shown in FIG. 3A, when the data holding form of the information recording layer is a read-only type, a spiral pit row 40 is formed on the substrate 10, thereby holding information. In this case, a reflective film is formed on the information recording layer 20. The laser beam Z at the time of reproduction is reflected by the reflective film of the information recording layer 20, and the light reflectance of the reflective film varies depending on the pit row 40 that contacts the information recording layer 20. Therefore, the data of the bit string 40 can be read by measuring the change state of the reflected light.

  Further, when the data holding form of the information recording layer 20 is a recording type, a spiral groove 42 (land 44) is formed on the surface of the substrate 10 as shown in FIG. In this case, a recording film capable of forming the recording mark 46 by the energy of the laser beam Z is formed on the information recording layer 20. The groove 42 serves as a guide track for the laser beam Z at the time of data recording. The energy intensity of the laser beam Z traveling along the groove 42 is modulated, so that the information recording layer 20 on the groove 42 is modulated. A recording mark 46 is formed. When the data holding mode is a write-once type, the recording mark 46 is formed irreversibly and cannot be erased. On the other hand, when the data holding mode is a rewritable type, the recording mark 46 is formed reversibly and can be erased and re-formed. Although the case where the recording mark 46 is formed on the groove 42 is shown here, it may be formed on the land 44 or on both the groove 42 and the land 44.

  The recording capacity of the information recording layer 20 is determined by a combination of the size of the recording area (area) and the recording density. Since there is a physical limit in the recording area, in this embodiment, as shown in FIG. 3B, the linear density of each recording mark 46, that is, the length of the unit recording mark 46 in the spiral direction is reduced. To increase the recording density. In other words, if the shortest mark length T in the spiral direction of the recording mark 46 formed on the information recording layer 20 is reduced, the recording capacity is increased. In the present embodiment, the shortest mark length T is set to 124.3 nm to 106.5 nm, specifically, 111.9 nm. When the shortest mark length T is 124.3 nm, the information recording layer 20 can hold 30 GB of information. When the shortest mark length T is 106.5 nm, the information recording layer 20 20 can hold 35 GB of information.

  Next, a PRML (Partial Response Maximum Likelihood) identification method in the PRML processing apparatus 110 will be described. This PRML identification method estimates binary data recorded in the information recording layer 20 based on an electrical analog signal detected by the light detection device 108. In this PRML identification method, it is necessary to appropriately select a PR (Partial Response) reference class characteristic according to the reproduction characteristic. Here, a constraint length of 5 (1, 2, 2, 2, 1) is used as the PR reference class characteristic. ) The characteristic is selected. The characteristic of the constraint length 5 (1, 2, 2, 2, 1) means that the reproduction response to the sign bit “1” constrains 5 bits, and this reproduction response waveform can be expressed by the sequence “12221”. ing. It is presumed that the reproduction response of various code bits actually recorded is formed by the convolution operation of this sequence “12221”. For example, the response to the code bit sequence 00100000 is 00122210. Similarly, the response to the code bit sequence 00010000 is 00001221. Therefore, the response of the code bit sequence 00110000 is the convolution operation of the above two responses and becomes 00134431. The response of the code bit sequence 001110000 is 001356531.

  The response obtained by this PR class characteristic assumes an ideal state. In this sense, the above response is called an ideal response. Of course, since the actual response includes noise, there is a deviation from this ideal response. Therefore, an actual response including noise and an ideal response assumed in advance are compared, and an ideal response that minimizes the difference (distance) is selected, and this is used as a decoded signal. This is called ML (Maximum Likelihood) identification. When a reproduced signal approximating “12221” is obtained by reproducing the recorded code bit “1”, reproduction is performed by performing PRML identification processing with a constraint length of 5 (1, 2, 2, 2, 1). It can be reproduced as signal → ideal response “12221” → decoded signal “1”.

In ML identification, the Euclidean distance is used to calculate the difference between the ideal response and the actual response. For example, the Euclidean distance E between the actual reproduction response sequence A (= A0, A1,..., An) and the ideal response sequence B (= B0, B1,..., Bn) is E = Σ (Ai − Bi) Defined in ² . Therefore, the actual response and various ideal responses assumed in advance are compared using the Euclidean distance, and the ideal response having the smallest Euclidean distance is selected and decoding is performed.

  The playback apparatus 100 according to the present embodiment employs a PRML identification method in which the reference class has a constraint length of 5 (1, 2, 2, 2, 1) as the processing of the playback signal. At the same time, the reproduction power of the reproduction laser beam Z is set to 0.6 mW, and the reproduction power standard of Blu-ray Disc at the time of this application is 0.35 mW ± 0.1 mW (0.25 to 0.45 mW). It is set to exceed.

  As described above, when the PRML identification method having the constraint length of 5 (1, 2, 2, 2, 1) is adopted and the laser power is increased, the bit error rate (bER) in the reproduction signal is reduced and the tilt margin is reduced. It is possible to achieve both improvements. In particular, the effect of reducing the bER and improving the tilt margin becomes significant when the recording capacity of the information recording layer 20 is 30 GB or more, preferably 33.3 GB or more, and more preferably 35 GB or more. That is, even if the recording capacity is increased, both the error rate and the tilt margin can be converged within a reasonable range.

  For example, when the recording capacity is 25 GB, the tilt margin is hardly improved even when the reproduction power is set to 0.45 mW or more. Therefore, in the conventional recording capacity (25 GB) environment, there is little need to increase the reproduction power. However, when the recording capacity exceeds 30 GB, an increase in reproduction power contributes to an improvement in tilt margin. In particular, when reproducing the optical recording medium 1 having a recording capacity of 33.3 GB or more, a sufficient tilt margin cannot be obtained with the conventional power (0.45 mW or less), but when the laser power exceeds 0.45 mW, the tilt is increased. The margin is remarkably increased and the target tilt margin (0.2 deg or more) can be cleared.

For example, even when the recording capacity is 35 GB or more, the bit error rate can be within an allowable range (3.1 × 10 ⁻⁴ or less) by setting the laser power to 0.5 mW or more.

  [Example]

  Four types (25 GB, 30 GB, 33.3 GB, and 35 GB) of optical recording media 1 having different storage capacities are created, and the playback signal quality and tilt margin are changed using the playback apparatus 100 described above while changing the playback power. The results of examining the state are shown below.

  First, in order to produce the optical recording medium 1, the substrate 10 was manufactured by an injection molding method. A spiral groove having a track pitch of 0.32 μm was formed on the surface of the substrate 10. A polycarbonate resin was used as the material of the substrate 10 and the thickness was set to 1.1 mm and the diameter was 120 mm.

  Next, the substrate 10 was set in a sputtering apparatus, and the information recording layer 20 having a thickness of 50 nm was formed on the surface on the side where the grooves were formed. The information recording layer 20 contained bismuth (Bi) and oxygen (O), and the composition ratio (atm%) was set to Bi: O = 32: 68.

  The substrate 10 on which the information recording layer 20 was formed was set in a spin coater, and an acrylic ultraviolet curable resin was dropped while being rotated, and this was spin coated. The cover layer 30 having a thickness of 98 μm was completed by irradiating this with ultraviolet rays. Further, an ultraviolet / electron beam curable hard coating agent is applied on the cover layer 30 by a spin coating method, and then heated in the air for 3 minutes to remove the diluting solvent in the coating, and an uncured hard coating material layer Formed. A surface material solution was applied to the uncured hard coat material layer by a spin coat method. This surface material solution was prepared by adding perfluoropolyether diacrylate (0.33 parts by weight, molecular weight: about 2000) and 3-perfluorooctyl-2-hydroxypropyl to a fluorinated solvent (99.5 parts by weight). It is prepared by adding acrylate (0.17 parts by weight). Thereafter, the hard coat material layer was dried at 60 ° C. for 3 minutes and further irradiated with an electron beam under a nitrogen stream to simultaneously cure the hard coat material layer and the surface material solution, thereby completing the hard coat layer 35. In addition, about the irradiation of an electron beam, the electron beam irradiation apparatus Curetron (made by Nissin High Voltage Co., Ltd.) was used, the electron beam acceleration voltage was 200 kV, and the irradiation dose was 5 Mrad. The oxygen concentration in the irradiation atmosphere was 80 ppm. In this way, an optical recording medium 1 was obtained.

  Four optical recording media 1 (samples Nos. 1 to 4) were prepared, and random data was written at recording densities corresponding to 25 GB, 30 GB, 33.3 GB, and 35 GB, respectively. The shortest recording mark length of the 25 GB optical recording medium 1 was greater than 120 nm, and the shortest mark length of the 30 GB to 35 GB optical recording medium 1 was 125 nm or less, specifically 124.3 nm to 106.5 nm.

  Next, the optical recording media 1 of Sample Nos. 1 to 4 were set in the reproducing apparatus 100, and reproduction was performed with the data transfer rate set to 72 Mbps (2X). While changing the reproduction power during reproduction, the quality (bit error rate) of the reproduction signal was examined by the SbER evaluation method. Note that SbER (Simulated bit Error Rate) uses the difference between the first ideal response of the first Euclidean distance and the second ideal response of the second Euclidean distance determined when decoding is performed by the PRML identification method. This is an evaluation method. The first ideal response is used as the decoded signal, but if the difference between the first Euclidean distance and the second Euclidean distance (this is called the SAM value) is small, the signal may be misrecognized. High nature. Therefore, the SAM values are calculated for a plurality of reproduction signals, and the probability of occurrence of erroneous recognition is evaluated based on the average and standard deviation of the normal distribution obtained from the plurality of SAM values. Here, an SbER measuring unit manufactured by Pulstec Industrial Co., Ltd. was used.

  Although not particularly shown here, for example, a PRSNR evaluation method can be adopted in addition to the SbER evaluation method. PRSNR (Partial Response Signal to Noise Ratio) is an evaluation method that can simultaneously represent the signal-to-noise ratio (S / N ratio) of the reproduced signal and the linearity of the actual reproduced signal and the ideal response. It is possible to evaluate using a PRSNR measurement board.

FIG. 4 shows the dependency between the error rate and the reproduction laser power obtained from the above evaluation results. In the case of a reproduction method using the PRML identification method with a constraint length of 5 (12221), it can be seen that the error rate improves as the laser power increases. In particular, when the laser power is set to 0.5 mW or more, the error rate decreases in the optical recording medium 1 of 30 GB or more. On the other hand, when it is 0.6 mW or more, the error rate is reduced in all the optical recording media 1 including 25 GB. In the case of the 35 GB optical recording medium 1, it is possible to make the error rate below the allowable limit (3.1 × 10 ⁻⁴ ) by setting it to 0.5 mW or more.

  FIG. 5 shows the dependency between the tilt margin of the optical recording media 1 of sample Nos. 1 to 4 and the reproduction laser power. As is clear from this result, with 25 GB, which is the recording capacity of the current Blu-ray Disc standard, the tilt margin does not improve even if the upper limit (0.45 mW) of the reproduction laser power defined in this standard is exceeded. . On the other hand, when the recording capacity is 30 GB or more, it can be seen that the tilt margin is improved even when the laser power exceeds 0.45 mW. That is, it can be seen that when the recording capacity is increased, the increase in laser power contributes to the improvement of the tilt margin. In particular, when the recording capacity is 33.3 GB or more, if the reproduction laser power is 0.45 mW or less, the signal reproduction becomes difficult because it falls below the target value of 0.2 deg. Is significantly improved. For example, in the case of 33.3 GB, the target value can be exceeded at 0.5 mW, and in the case of 35 GB, the target value can be exceeded at 0.6 mW.

  According to the above results, when the PRML identification method in which the reference class has a constraint length of 5 (1, 2, 2, 2, 1) is adopted, the reproduction power of the reproduction laser beam Z is increased from 0.45 mW. It can be seen that the reduction of the bit error rate (bER) in the reproduction signal and the improvement of the tilt margin can be rationally achieved. In particular, the effect of reducing the bER and improving the tilt margin becomes significant when the recording capacity of the information recording layer 20 is 30 GB or more, preferably 33.3 GB or more, and more preferably 35 GB or more. That is, even if the recording capacity is increased, both the error rate and the tilt margin can be converged within the target range.

  As described above, the laser power during reproduction in the embodiment of the present invention means the power supplied to the information recording layer. In the present embodiment, the information recording layer in the optical recording medium is shown only when it is laminated to 100 μm from the light incident surface. However, the present invention is not limited to this and may be laminated at different places. good.

  Note that the reproducing method and optical recording medium of the present invention are not limited to the above-described embodiments, and it goes without saying that various changes can be made without departing from the scope of the present invention.

  According to the present invention, it is possible to improve the quality of a reproduced signal even when the recording capacity or recording density of an optical recording medium is increased.

1 is a block diagram showing a reproducing apparatus for an optical recording medium according to an example of an embodiment of the present invention. A perspective view and an enlarged sectional view showing the structure of the optical recording medium An enlarged perspective view showing a data holding form in the information recording layer of the optical recording medium Table showing dependency between error rate and playback laser power during playback by the playback device Table showing the dependency between tilt margin and playback laser power during playback by the playback device

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Optical recording medium 10 ... Board | substrate 20 ... Information recording layer 30 ... Cover layer 35 ... Hard-coat layer 35A ... Light incident surface 100 ... Reproducing apparatus 102 ... Laser Light source 104 ・ ・ ・ Laser controller 106 ・ ・ ・ Optical mechanism 108 ・ ・ ・ Photodetection device 110 ・ ・ ・ PRML processing device 112 ・ ・ ・ Spindle motor 114 ・ ・ ・ Spindle driver 116 ・ ・ ・ Signal processing device

Claims (9)

When reproducing information by irradiating a laser beam to the information recording layer of the optical recording medium,
The wavelength of the laser beam is 400 to 410 nm,
The numerical aperture NA of the objective lens for condensing the laser beam is 0.70 to 0.90,
The reproduction power of the laser beam is set to be larger than 0.45 mW,
A reproduction method of an optical recording medium, wherein a reproduction signal obtained by the laser beam irradiation is decoded by a PRML identification method.   2. The method of reproducing an optical recording medium according to claim 1, wherein the reference class of the PRML identification method is a constraint length of 5 (1, 2, 2, 2, 1).   3. The method for reproducing an optical recording medium according to claim 1, wherein the reproduction power is 0.50 mW or more.   3. The method for reproducing an optical recording medium according to claim 1, wherein the reproduction power is 0.60 mW or more.   5. The method of reproducing an optical recording medium according to claim 1, wherein the optical recording medium is reproduced such that a distance from a light incident surface to the information recording layer is 0.100 mm or less.   6. The method for reproducing an optical recording medium according to claim 1, wherein the shortest mark length recorded in the information recording layer is 125 nm or less.   7. The method for reproducing an optical recording medium according to claim 1, wherein the shortest mark length recorded in the information recording layer is 112 nm or less.   The method for reproducing an optical recording medium according to any one of claims 1 to 7, wherein the composition of the information recording layer includes Bi and O.   A laser light source that generates a laser beam having a wavelength of 400 to 410 nm, a laser controller that controls the laser light source so that the reproduction power of the laser beam is greater than 0.36 mW, and a numerical aperture that focuses the laser beam An objective lens having an NA of 0.70 to 0.90, a photodetector for detecting reflected light of the laser beam, and a PRML processing device for decoding a reproduction signal detected by the photodetector by a PRML identification method; An optical recording medium reproducing device comprising:

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