JPH076402A - Optical disk device - Google Patents

Abstract

PURPOSE:To reduce a crosstalk noise and to reduce an error occurrence rate in a signal component frequency multiplexed on a low band of a video signal by forming a beam spot converged on a disk in an elliptic shape and making the major axis direction coincide with a linear velocity direction. CONSTITUTION:A direction parallel to a polarization direction is made to coincide with the linear velocity direction (pit arrangement direction) of the disk D by turning and arranging a semiconductor laser 1 around a beam by 90 deg.. At this time, the length in the major direction of the ellipse is about 1.07mum, and the length in the minor direction is about 0.95mum. In such a case, when the length in the linear velocity direction is set to the same extent of length as the conventional extent of length, since the beam spot is converged more sufficiently since a wavelength is short as 630(nm), a numerical aperture NA of a collimate lens 2 is enlarged more than the conventional NA, and the utilization efficiency in the laser is improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical disk device for reproducing information on an optical disk by irradiating an optical disk with a laser beam spot and reading the intensity of the reflected light.

[0002]

2. Description of the Related Art A MUSE high-definition video disc (MUSE disc) in which a MUSE signal obtained by band-compressing a high-definition television signal by TCI multiplex sub-Nyquist Encoding system is FM-modulated and recorded is the current television system. Recording is performed at a density twice as high as that of a certain NTSC video disc (NTSC disc). This is because the MUSE signal has about twice the band of the NTSC signal and requires about twice the linear velocity at the time of reproduction.
Playback time similar to TSC disc (eg 120 on both sides)
This is because the high density recording of about twice is required to obtain (min.).

In order to read a MUSE disc recorded at high density, it is necessary to improve the resolution of the optical pickup. This can be achieved by reducing the spot diameter of the laser beam focused on the disk, and this spot diameter is proportional to the wavelength of the laser beam. For this reason,
78 in a conventional optical disk device for playing NTSC disks
The wavelength that was set to 0 [nm] is set to 670 [nm] in the recent optical disk device for reproducing MUSE & NTSC disks.

The numerical aperture NA of the objective lens, which was set to 0.5 in the conventional optical disk apparatus for reproducing NTSC disk, is set to 0.55 in the recent optical disk apparatus for reproducing MUSE & NTSC disk. This is because a laser with a wavelength of 670 nm is still insufficient to reduce the beam spot diameter to the level of the recording density of the MUSE disc, so increase the numerical aperture NA of the objective lens. It was made to correspond in.

As described above, by configuring the laser to have a short wavelength and increasing the numerical aperture NA of the objective lens,
It is possible to reproduce a MUSE disc that has approximately twice the high density recording. For example, when the laser wavelength is 670 [nm], the numerical aperture NA of the objective lens is 0.55, and the numerical aperture NA of the collimating lens is 0.1, the spot diameter of the laser beam is Is about 1.07 [μm], while the track pitch of the MUSE disc is 1.1.
Since it is approximately [μm], the above-mentioned configuration enables reproduction of the MUSE disc.

[0006]

In the optical disk device having the above structure, the spot diameter of the beam is about 1.07 [μm], whereas the track pitch of the MUSE disk is about 1.1 [μm]. Therefore, although reproduction is possible, the margin in the track pitch direction is small. For this reason,
When the influence of the warp of the disk and the thickness unevenness becomes large, the crosstalk noise becomes large.

In order to reduce this crosstalk noise, the laser wavelength is 630 [n] which is shorter than 670 [nm].
m] is proposed. Incidentally, it is extremely difficult to deal with the problem by making the numerical aperture NA of the objective lens larger than 0.55, since the requirements for the mechanical characteristics of the disk become very strict.

However, when the laser wavelength is set to 630 [nm], the following problems occur. MUSE disc and N
The pits of the TSC disc are information recorded with a signal formed by frequency division multiplexing low-frequency signal components (EFM signal, pilot signal, etc.) into a video signal. Also,
This pit has a laser wavelength of 6 on a MUSE disc.
When it is 70 [nm], the duty ratio is 1: 1 (optimum value)
The NTSC disk is standardized so that the duty ratio becomes 1: 1 when the laser wavelength is 780 [nm].

Therefore, the laser wavelength for reproduction is set to 630
When set to [nm], the duty ratio of the reproduction signal deviates from the optimum value, and as a result, the cross modulation with the signal component (EFM signal, etc.) frequency-multiplexed in the low frequency band of the video signal becomes large. , EFM signals and other demodulation error rates increase. The degree is 78
This is particularly remarkable in the case of an NTSC disk standardized to a wavelength of 0 [nm] (the difference from 630 [nm] is large).

For example, in a MUSE disc, 670 [n
The spot diameter at m] is 1.1 [μm] and 630 [μm]
The spot diameter is 0.95 [μm]. The numerical aperture NA of the objective lens is 0.55 and the same. Since the level that effectively acts on the duty ratio shift is about 80% of the peak of the Gaussian distribution, the duty ratio shift in this case is less than 3%. If this is corresponded to the block error rate at the time of demodulating the EFM signal according to FIG. 6, it is 5%.
However, the standard value of block error rate of 8% is cleared.

On the other hand, in the NTSC disk, since the spot diameter at the wavelength 780 [nm] / objective lens NA 0.5 is 1.35 [μm], the wavelength 670 [nm] /
Spot diameter at numerical aperture NA = 0.55 of objective lens 1.
The deviation of the duty ratio when reproduced at 1 [μm] is 3.7.
%become. If this is made to correspond to the block error rate at the time of demodulation of the EFM signal, it becomes about 7%. That is, in this case, 8% of the standard value is cleared. However, wavelength 630 [nm] / numerical aperture NA of the objective lens is 0.5
The deviation of the duty ratio when reproducing with the spot diameter of 0.98 [μm] at 5 is 5.4%, which corresponds to the block error rate during demodulation of the EFM signal according to FIG. And 13%, which greatly exceeds the standard value of 8%.

If the laser wavelength is set to 630 [nm], the block error rate of the EFM signal greatly exceeds the allowable value when reproducing the NTSC disk. The present invention is a compatible device capable of reproducing both a disc whose reproduction laser wavelength is standardized to a relatively long wavelength and a disc whose reproduction laser wavelength is standardized to a relatively short wavelength. E when playing standard discs
It is possible to suppress an increase in block error rate such as FM signals,
Moreover, the crossing during reproduction of a disk having a relatively short wavelength
The purpose is to further reduce the noise. For example, it is an object of the present invention to provide a compatible device capable of further reducing crosstalk noise when reproducing a MUSE disc and suppressing the increase of the block error rate of the EFM signal as described above when reproducing an NTSC disc.

[0013]

According to the present invention, information obtained by frequency-division-multiplexing other information as a low frequency component is recorded in predetermined information, and a laser wavelength for reproduction is standardized to a specific value. In an optical disk device for reproducing information recorded on the optical disk by scanning the optical disk with a laser beam spot and reading the reflected light, a laser beam having a wavelength shorter than the specified value is used. A laser emitting means for emitting a beam and a laser beam emitted from the laser emitting means, the beam spot of which has an elliptical shape on the optical disk, and And a means for collecting light such that the long axis thereof coincides with the scanning direction. The predetermined information may be video information as claimed in claim 2 or video information of the MUSE method as claimed in claim 3 (MUSE signal obtained by band-compressing a high-definition television signal by the TCI multiplex sub-Nyquist encoder method). Information) etc. The short wavelength is, for example, 610 as in claim 3.
The wavelength is in the range of 640 [nm], which corresponds to the case where the standardized specific value of the reproduction laser wavelength is about 670 [nm].

A means for making the beam spot on the optical disk elliptical and aligning its major axis with the beam scanning direction (pit arrangement direction) is, for example, a semiconductor with the beam as the central axis. It can be realized by rotating the laser by 90 °. It can also be realized by disposing an optical filter for dimming or blocking both sides in the scanning direction between the semiconductor laser and the beam splitter.

[0015]

The beam spot focused on the disk has an elliptical shape, and its major axis direction coincides with the linear velocity direction (pit arrangement direction). Here, the length of the major axis of the ellipse is approximately equal to the length in the linear velocity direction when the beam spot having the wavelength of the specific value (standard wavelength) is focused on the disk. Therefore, it is possible to prevent the duty ratio of the signal reproduced from the reflected light of the optical disc from deviating greatly from the duty ratio of the reproduced signal when the laser beam of the standard wavelength is irradiated. It Therefore, an increase in error rate during signal demodulation can be prevented.

[0016]

EXAMPLES Examples of the present invention will be described below. FIG. 1 shows the construction of the optical system of the apparatus of the first embodiment, and FIG. 2 shows the beam spots and pits focused on the optical disk D by the apparatus.

As shown in the figure, the laser beam having a wavelength of 630 [nm] emitted from the semiconductor laser 1 passes through a collimating lens 2, a deflecting beam splitter 3, and a 1/4 wavelength plate 4. , Is focused by an objective lens 5 having a numerical aperture NA of 0.55 and is focused as an elliptical beam spot S on the disk D.

The laser beam reflected from the disk D passes through the objective lens 5 and the quarter-wave plate 4 and is changed in direction by 90 ° by the deflecting beam splitter 4, and then the lens.
It is condensed by 6 and imaged on the sensor 7. The signal read by the sensor 7 is processed by a known processing circuit,
Is played.

Laser beam emitted from the semiconductor laser 1.
The divergence angle θp in the polarization direction is about 8 °, and the divergence angle θv in the direction perpendicular to the polarization direction is about 30 °. Conventionally, the semiconductor laser 1 has a direction perpendicular to the polarization direction (θv) because the frequency characteristics at the time of reproduction are improved because the light is better condensed in the direction perpendicular to the polarization direction (θv direction).
Direction) is aligned with the linear velocity direction (scanning direction) of the disk D. In this case, the beam spot on the disk D has a circular shape and the spot diameter is 1.
It is about 07 [μm]. However, the numerical aperture NA of the collimating lens 2 is 0.1 and the numerical aperture NA of the objective lens 5 is 0.5.
It is 5.

However, in the first embodiment, the semiconductor laser is used.
By arranging Z1 by rotating it 90 degrees around the beam, the direction parallel to the polarization direction (direction of θp) will be
Is aligned with the linear velocity direction (scanning direction) of. At this time, the beam spot on the disc has an elliptical shape, and its major axis is aligned with the linear velocity direction (scanning direction) of the disc D. The length of the ellipse in the major axis direction is about 1.07 [μm], and the length in the minor axis direction is about 0.95 [μm].
However, the numerical aperture NA of the collimating lens 2 is 0.13 to
The numerical aperture NA of the objective lens 5 is 0.16 and 0.55. In other words, if the length in the direction of linear velocity is set to the same level as the conventional one, the wavelength is as short as 630 [nm] and the light is well condensed. There is also an advantage that the utilization efficiency of the laser is improved.

In the first embodiment, the spot diameter of the disk D in the linear velocity direction is about 1.07 [μm], and the wavelength 670 is used.
Since it is about the same as in the case of [nm], the deviation of the duty ratio of the reproduction signal is also about the same as in the case of the wavelength of 670 [nm]. Therefore, the block error rate at the time of demodulating the EFM signal when reproducing the NTSC disc is about the same as the case of the wavelength of 670 [nm], and the standard value of 8% or less is sufficiently satisfied.

Further, since the spot diameter in the tracking direction (direction perpendicular to the linear velocity direction) is well focused due to the short wavelength and is about 0.95 [μm], crosstalk noise is also sufficiently reduced. It Further, as a result of the crosstalk noise being sufficiently reduced, the error occurrence rate of the EFM signal which is easily affected by the crosstalk noise is also reduced. Although the wavelength of 630 [nm] is described above, 610 to 64
Within the range of 0 [nm], the above effects can be sufficiently obtained.

Next, a second embodiment will be described with reference to FIGS. In the second embodiment, the collimating lens 2
An optical filter 9 is provided between the deflection beam splitter 3 and the beam splitter 3. The other structure is the same as that of the first embodiment. The optical filter 9 is a filter that dims or shields both sides of the transmitted laser beam (both sides in the linear velocity direction on the disk D). As a result, the numerical aperture NA of the objective lens 5 in the linear velocity direction is apparently reduced, and the beam spot on the disk has an elliptical shape as in the first embodiment.

The optical filter 9 may be composed of, for example, a liquid crystal shutter in which the filter portions 91 and 92 (FIG. 4) are turned on and off by the application of an electric field. You may comprise using the apotite filter which shows the characteristic of FIG. Further, the arrangement position of the optical filter 9 is not limited to the rear of the collimating lens 2. That is, it may be between the semiconductor laser 1 and the deflection beam splitter 3.

In the second embodiment, the laser wavelength 630 [n
m], the numerical aperture NA 0.1 of the collimating lens 2 and the numerical aperture NA 0.55 of the objective lens 5 are adopted, and the spot diameter in the linear velocity direction is set to about 1.07 [μm]. , The optical filter 9 above has been adjusted. As a result, even in the second embodiment, the tracking direction is sufficiently focused to reduce the crosstalk noise at the time of reproducing the MUSE disk, and the block error rate at the time of demodulating the EFM signal at the time of reproducing the NTSC disk is also improved. Also, the same effect as that of the first embodiment can be obtained.

[0026]

As described above, according to the present invention, the laser beam spot of the short wavelength focused on the disc has an elliptical shape, and the length in the major axis direction is the laser beam for reproduction of the disc. −
The wavelength is about the same as a specific value standardized as the wavelength (eg, 670 [nm] in the case of a MUSE disc). Further, the length of the ellipse in the minor axis direction (direction orthogonal to the scanning direction) is sufficiently smaller than that in the case where a laser spot having a wavelength of the above specific value is irradiated.

Therefore, for example, the deviation of the duty ratio of the reproduced signal at the time of reproducing the NTSC disc is MUS.
It is about the same as when the standard wavelength 670 [nm] of the E disk is used. Therefore, the block error rate at the time of demodulating the EFM signal can be suppressed to the same extent as when the standard wavelength 670 [nm] is used. Further, in the case of MUSE disc reproduction, crosstalk noise is sufficiently reduced, so that the S / N ratio of the video signal is improved. Further, as a result of the crosstalk noise being sufficiently reduced, the error occurrence rate of the EFM signal is also reduced.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a configuration of an optical system of a first embodiment device.

FIG. 2 is a diagram showing beam spots and pits focused on an optical disk by the first and second embodiments.

FIG. 3 is a schematic diagram showing a configuration of an optical system of a second embodiment device.

FIG. 4 is a structural explanatory diagram of an example of an optical filter used in the second embodiment device.

FIG. 5 is a characteristic diagram of an example of an optical filter used in the device of the second embodiment.

FIG. 6 is a characteristic diagram showing the relationship between the deviation of the duty ratio of pits and the block error rate during signal demodulation.

[Explanation of symbols]

 1 Semiconductor Laser 2 Collimating Lens 3 Deflection Beam Splitter 4 1/4 Wave Plate 5 Objective Lens D Disk

Claims (4)

[Claims] 1. An optical disc in which other information is frequency-division-multiplexed as a low-frequency component in predetermined information and a reproduction laser wavelength is standardized to a specific value -In an optical disk device that reproduces information recorded on the optical disk by scanning with a spot and reading the reflected light, a laser beam having a wavelength shorter than the specific value is emitted.
The laser emitting means and the laser beam emitted from the laser emitting means, the beam spot has an elliptical shape on the optical disk, and the long axis of the ellipse is the scanning direction. An optical disk device comprising: means for condensing light so as to match the direction of. 2. The optical disk device according to claim 1, wherein the predetermined information is video information. 3. The optical disk device according to claim 1, wherein the predetermined information is MUSE video information. 4. The laser beam having a wavelength shorter than the specific value according to claim 1, 2 or 3, wherein the laser beam has a wavelength of 610 nm.
An optical disk device which is a laser beam having a wavelength in the range of ˜640 nm.