JP2008052778A - Optical pickup - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To make it possible to keep the focusing diameter small to reproduce signals with less noise by not lowering the power and lowering the pupil limb intensity when reproducing but increasing it when recording to improve the efficiency of the laser beam to secure the power. <P>SOLUTION: This optical pickup has two light sources to emit almost the same wave-length laser beams, and a recording means to record by focusing the first laser beam from the first light source 1 on the recording medium through the objective lens. It disposes a concave lens 3 in the light path to guide the second laser beam from the second light source 2 to the recording medium, and makes the pupil limb intensity of the second laser beam in the objective lens higher than the limb intensity of the first laser beam to reproduce signals. Thus, it can secure the recording power on the recording medium by improving the efficiency of the laser beam when recording, while making the focusing diameter small on the recording medium when reproducing, and can reproduce signals by setting the output power of the light source 2 to the power sufficiently low in RIN. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an optical pickup for recording and reproducing information on a recording medium in an optical information recording apparatus.

  In an optical information recording apparatus such as an optical disk or optical tape that optically records and reproduces information using laser light, the recording material formed on the recording medium is focused and irradiated with laser light. A general method is to record information by raising the temperature of a recording material to cause a physical or chemical change. When reproducing information from a recording medium, a laser beam with a lower power than that used during recording is focused and irradiated to prevent physical or chemical changes in the recording material so that the recorded information is not destroyed or erased. Information is reproduced by detecting the presence or absence as a change in optical characteristics such as reflectance.

  In the history of the development of such an optical information recording apparatus, a technique for shortening the wavelength of the laser beam to be used has been taken for the purpose of improving the recording density of the recording medium. This is because when the laser beam is condensed by a lens or the like, the condensed diameter is proportional to the wavelength of the laser beam. For example, in an optical disk device, a semiconductor laser is used as a light source suitable for mounting on a player or a drive device, and the shortest wavelength semiconductor laser that was mass-produced when each optical disk was standardized was used. I came. For this reason, it is assumed that an infrared semiconductor laser having a wavelength of about 780 nm is used for a CD (Compact Disc) and a red semiconductor laser having a wavelength of about 650 nm is used for a DVD (Digital Versatile Disc). In recent years, Blu-ray standards and HD DVD (High Density Digital Versatile Disc) standards have been proposed in which the recording density is increased by using a blue semiconductor laser having a wavelength of about 405 nm.

Further, even when the wavelength of the laser beam is the same, the focused diameter changes depending on the intensity distribution of the beam incident on the lens that collects the laser beam. As shown in FIG. 2, a laser beam emitted from a semiconductor laser generally used for optical recording has a cross-sectional intensity distribution 12 that has a maximum value (I ₀ ) on the optical axis and decreases as the distance from the optical axis increases. When the laser beam 11 is condensed by the lens 13, the relative intensity I ₁ / I ₀ of the laser beam at the pupil diameter end of the lens 13 (rim intensity: pupil
As the limb intensity increases, the diameter d of the laser light condensing region 14 can be reduced. For example, FIG. 3 shows the relationship between the rim intensity and the focused spot diameter when a laser beam having a wavelength of 405 nm and an objective lens having a numerical aperture of 0.85 are used. However, as the rim intensity is increased, the light away from the optical axis of the laser light is not used, and the use efficiency of the laser light is lowered.

  When recording information on a recording medium, relatively large power is required. Therefore, it is preferable to reduce the rim strength to increase the utilization efficiency of laser light. If the rim strength is reduced, the laser beam condensing diameter will be increased accordingly. However, as described above, in optical recording, the recording material is caused to undergo physical and chemical changes by utilizing the heat generated by laser light irradiation. Thus, since the information is recorded, the laser beam intensity can be adjusted so that the recording is performed only in the high temperature region near the center of the focused laser beam. Therefore, it is more important to ensure power than to reduce the light collection diameter during recording. In particular, in a semiconductor laser that emits a laser beam having a short wavelength, it is generally more important to secure power because a large output cannot be obtained. On the other hand, when reproducing information from a recording medium, it is necessary to reduce the power on the recording medium to the extent that information already recorded is not destroyed or erased, and to reduce the influence of crosstalk from adjacent tracks. It is desirable to keep the condensed spot diameter as small as possible.

  In addition, semiconductor lasers have quantum noise (RIN: Relative Intensity Noise), and this noise generally tends to become larger when semiconductor lasers are used at low power, especially blue semiconductor lasers, specifically GaN-based semiconductors. In the case of laser, this tendency is more remarkable. FIG. 4 shows an example of the relationship between the output of a semiconductor laser emitting blue-violet laser light and RIN. In general, in an optical information recording / reproducing apparatus, a good recording / reproducing signal cannot be obtained if RIN is larger than −125 dBm / Hz. Therefore, for example, in the case of a semiconductor laser having the characteristics shown in FIG. 4, a good signal cannot be obtained unless it is used at an output of 7 mW or more.

  Therefore, if the laser emission power is simply lowered with the recording configuration during reproduction, the RIN increases and the condensing diameter remains large because the rim intensity is originally small, making it difficult to obtain a good reproduction signal.

As a method of not reducing the laser output power during reproduction, for example, a method of inserting a neutral density filter in the optical path during reproduction (see Patent Document 1), or a method of reducing the transmittance of laser light during reproduction using a liquid crystal (patent) Document 1 and Patent Document 2) are known.
JP2002-260272 JP2003-115109

  In the method of inserting / removing the neutral density filter in the optical path, a driving mechanism for that purpose is required separately, which makes the configuration complicated and causes problems in accuracy and reliability. Even if the method of changing the transmittance of laser light using liquid crystal is used, the RIN can be reduced by not having to lower the laser emission power during reproduction, but the ratio of rim intensity does not change, so the concentration of laser light is not changed. The light diameter cannot be reduced, and the configuration is not suitable for obtaining a good reproduction signal.

  The present invention secures power by reducing the rim intensity during recording to increase the utilization efficiency of laser light, and also increases the rim intensity and reducing the utilization efficiency of laser light without reducing the laser emission power during reproduction. It is an object of the present invention to provide an optical pickup that can be reproduced with low noise while keeping the light diameter small.

  The optical pickup of the present invention has two light sources that emit laser beams having substantially the same wavelength, and the first laser beam emitted from the first light source is condensed and irradiated onto the recording medium via the objective lens. A means for performing recording, and a lens or the like in the optical path for guiding the second laser light emitted from the second light source to the recording medium, the rim intensity of the second laser light in the objective lens is set to the first laser light. It is characterized by having a means for reproducing with a larger rim strength.

  In the optical pickup of the present invention configured as described above, the recording power is ensured by increasing the use efficiency of the laser light by using the laser light having a reduced rim intensity in the objective lens at the time of recording, and the objective lens at the time of reproduction. Increases the rim intensity of the laser beam in the recording medium to reduce the condensing diameter on the recording medium, and sets the output power of the semiconductor laser as the light source to a power that sufficiently reduces RIN, while reproducing power on the recording medium Can be reproduced at a level that does not cause destruction or erasure of information.

According to the optical pickup of the present invention, the output of the semiconductor laser is more than necessary by using two laser light sources and using laser light with reduced rim intensity in the objective lens during recording to increase the use efficiency of the laser light. The recording power is ensured without increasing the laser power, and the laser light rim intensity in the objective lens is increased during playback to reduce the condensing diameter on the recording medium. It is possible to perform reproduction while setting the power to be low and suppressing the reproduction power on the recording medium to a level that does not cause destruction or erasure of information.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 shows an example of an optical pickup according to an embodiment of the present invention. When information is recorded on a recording medium 8 such as an optical disk with this optical pickup, the first semiconductor laser 1 is selectively driven as a light source by the laser drive switching circuit 10 and the laser beam is emitted from the semiconductor laser 1 according to the recording signal. Is emitted. Laser light emitted from the semiconductor laser 1 passes through the polarization beam splitter 4, the beam splitter 5, and the collimator lens 6, and is condensed and irradiated onto the recording medium 8 by the objective lens 7. In FIG. 1, a polarizing beam splitter 5 has a characteristic of transmitting when the polarization direction of incident laser light is parallel to the paper surface and reflecting when polarized light is orthogonal (perpendicular to the paper surface) and is emitted from the semiconductor laser 1. It is assumed that the laser beam is arranged in a direction (parallel to the paper surface) that transmits the polarized beam splitter 4. Further, the output of the semiconductor laser 1 is set to a power that is sufficiently low for RIN to be sufficiently low and to perform recording on the recording medium 8 through the optical path described above.

  Next, when reproducing the information recorded on the recording medium 8, the laser drive switching circuit 10 selectively drives the second semiconductor laser 2 as a light source. Here, it is assumed that the semiconductor laser 2 is arranged so that the direction of polarization of the emitted laser light is reflected by the polarization beam splitter 4 (perpendicular to the paper surface). The laser light emitted from the semiconductor laser 2 has its radiation angle widened by the concave lens 3, reflected by the polarizing beam splitter 4, transmitted through the beam splitter 5 and collimating lens 6, and condensed and irradiated onto the recording medium 8 by the objective lens 7. The Furthermore, the laser beam reflected by the recording medium 8 traces the objective lens 7 and the collimating lens 6 in reverse, and the component reflected by the beam splitter 5 enters the photodiode IC 9 where it is converted into an electrical signal. Information recorded on the recording medium 8 is reproduced.

  As described above, the laser beam emitted from the semiconductor laser 2 at the time of reproduction has its radiation angle widened by the concave lens 3 immediately after the emission, so that when it reaches the objective lens 7, it is emitted from the semiconductor laser 1 at the time of recording. Compared with the state in which the laser beam that has reached the objective lens 7, the beam intensity distribution perpendicular to the optical axis is expanded. As a result, the ratio of the rim intensity of the laser light incident on the objective lens 7 from the semiconductor laser 2 during reproduction becomes larger than that during recording, and the spot diameter focused on the recording medium 8 can be made relatively small. At the same time, the laser beam utilization efficiency is reduced compared to recording due to an increase in the amount of scattered light away from the optical axis while the laser beam is guided from the semiconductor laser 2 to the objective lens 7 during reproduction. The power that reaches the medium 8 is relatively attenuated. Therefore, it is possible to reproduce the information recorded on the recording medium 8 without destroying or erasing it without setting the output of the semiconductor laser 2 so low that RIN adversely affects the reproduction signal.

For example, in the case of a magneto-optical disk, the ratio of power in the disk recording film during recording and reproduction is about 8 to 10 times. When a semiconductor laser having RIN characteristics as shown in FIG. 4 is used, it is necessary to emit light with an output of 7 mW or more in order to use it with RIN being −125 dBm / Hz or less. Therefore, if the proper reproduction laser power on the recording medium is 0.7 mW, the efficiency of the optical system from the semiconductor laser to the recording medium can be adjusted to 10% and reproduction can be performed with a laser output power of 7 mW. In this case, however, a laser output power of 56 to 70 mW is required for recording only by changing the power. It is not preferable to output this recording power with a blue-violet semiconductor laser in view of power consumption and laser durability. Therefore, if the pickup is configured as shown in FIG. 1, which is the present embodiment, the ratio of the laser output power during recording and during reproduction can be reduced by lowering the efficiency of the optical system during reproduction using the concave lens 3. it can. If the intensity distribution of the outgoing beam of the semiconductor laser 2 is a Gaussian distribution, for example, if the rim intensity is changed from 15% to 58%, the utilization efficiency of the laser light will be reduced from 85% to 42% (approximately 1/2). The ratio of power during recording and playback is reduced to 4-5 times. That is, if the rim intensity at the objective lens is changed from 15% to 58% by using the concave lens in the above example, and the efficiency of the optical system excluding the concave lens is set to 20%, the output power of the laser 2 during reproduction is 7 mW. The output power of the laser 1 at the time of recording is 28 to 35 mW. In addition, as shown in FIG. 3, since the condensed spot diameter during reproduction is reduced by about 7% from 0.44 μm during recording to 0.41 μm, good reproduction characteristics can be ensured.

  As mentioned above, although embodiment of this invention was described concretely, this invention is not limited to the above-mentioned embodiment and numerical value, The various deformation | transformation based on the technical idea of this invention is possible. For example, in the above-described embodiment, FIG. 1, by reducing the efficiency of the optical system at the time of recording by additionally arranging a neutral density filter between the semiconductor laser 2 for reproduction and the polarization beam splitter 4, It is also possible to further reduce the ratio of laser output power during reproduction.

1 is a schematic diagram showing an optical system of an optical pickup according to an embodiment of the present invention. Illustration of laser light intensity distribution and rim intensity for lens Data showing the relationship between rim intensity and collection diameter Data showing the relationship between RIN output power and blue-violet semiconductor laser

Claims (6)

An optical pickup for recording / reproducing information on / from a recording medium using laser light, comprising two laser light sources, wherein the wavelengths of the laser lights output from the two light sources are substantially equal. Optical pickup to be used. 2. The optical pickup according to claim 1, wherein the two laser beams share one objective lens for condensing on the recording medium, and have different intensities at the pupil ends of the objective lens. 3. The optical pickup according to claim 1, further comprising means for reducing the utilization efficiency of the laser beam in at least one of the optical paths of the two laser beams. 4. The optical pickup according to claim 3, further comprising a concave lens as means for reducing the utilization efficiency of the laser beam. 5. The optical pickup according to claim 1, wherein the light source is a semiconductor laser capable of oscillating in a blue to ultraviolet wavelength region. One of the two light sources is used for recording or rewriting or erasing information on a recording medium, and the other is used for reproducing information from the recording medium. Item 5. The optical pickup according to Item 5.

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