JP3608046B2 - Optical disk device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical disc apparatus that irradiates an optical disc on which a recording signal is recorded with a light beam and reads the recording signal with reflected light.
[0002]
[Prior art]
An optical disk device such as a CD (compact disk) device, an LD (laser disk) device, a DVD (digital video disk) device or the like converges a light beam emitted from a light source such as a semiconductor laser onto a signal surface of the optical disk by an objective lens. The recording signal recorded on the optical disc is read out by detecting the reflected light reflected by the optical disc with a photodetector.
[0003]
Such an optical disc has a high recording density, and the pit size and track pitch of a recording signal are 0.83 μm and 1.6 μm for a CD, respectively, whereas 0.4 μm for a DVD, It is 0.74 μm. For this reason, it is necessary to make the spot diameter w of the light beam irradiated to the recording surface smaller in the DVD having a higher recording density.
[0004]
The spot diameter w is expressed by the equation (1) by the wavelength λ of the laser light to be used and the numerical aperture NA of the objective lens. In Equation (1), k is a constant. Therefore, a DVD device having a high recording density uses a laser beam having a short wavelength λ or an objective lens having a large numerical aperture NA as compared with a CD device.
[0005]
w = k · λ / NA (1)
[0006]
However, in recent years, for example, an optical disc apparatus capable of reproducing both a CD and a DVD has been put into practical use. This optical disk apparatus needs to include two optical pickups composed of optical systems having different laser light wavelengths λ and objective lens numerical aperture NA due to the difference in recording density between CD and DVD. However, if there are two optical pickups, the optical disk apparatus becomes large and the cost increases. For this reason, an optical disc apparatus capable of reproducing different optical discs by one optical system is disclosed in Japanese Patent Laid-Open No. 11-144307.
[0007]
FIG. 5 is a side view of the optical pickup disclosed in the publication, and photodetectors 56 and 57 are formed on the surface of the semiconductor substrate 53. On the semiconductor substrate 53, a prism 55 is disposed in contact with the photodetectors 56 and 57, and a light source unit 49 is disposed so as to face the prism 55.
[0008]
As shown in FIG. 6, the light source unit 49 is provided with first and second light sources 51 and 52 made of semiconductor lasers superimposed on a semiconductor substrate 50. An objective lens 60 is provided above the inclined surface 55 a of the prism 55.
[0009]
For example, when a CD is mounted on the optical disk device, a first light beam 51 a having a wavelength λ = 780 nm, for example, is emitted from the first light source 51. The first light beam 51 a is reflected by the inclined surface 55 a of the prism 55, and is converged on the CD recording surface by the objective lens 60. The reflected light reflected by the CD passes through the inclined surface 55 a of the prism 55 and is received by the photodetector 56, and the light beam reflected by the photodetector 56 is received by the photodetector 57. The recording signal is read out based on the intensity of light received by the photodetectors 56 and 57.
[0010]
When a high recording density DVD, for example, is mounted on the optical disc apparatus, the second light source 52 emits a second light beam 52a having a wavelength λ = 650 nm, for example. Similarly to the above, the second light beam 52a is reflected by the inclined surface 55a of the prism 55 and converged on the DVD recording surface by the objective lens 60.
[0011]
The reflected light reflected by the DVD passes through the inclined surface 55 a of the prism 55 and is received by the photodetector 56, and the light beam reflected by the photodetector 56 is received by the photodetector 57. The recording signal is read out based on the intensity of light received by the photodetectors 56 and 57.
[0012]
Accordingly, the first and second light beams 51a and 52a having different wavelengths from the first and second light sources 51 and 52 can be obtained from the first and second light sources 51 and 52 according to the optical disk by a small and low-cost optical disk apparatus using an optical pickup having one optical system. It is possible to read out optical discs having different recording densities upon ejection.
[0013]
[Problems to be solved by the invention]
However, according to the conventional optical disc apparatus described above, the first and second light sources 51 and 52 are arranged so as to overlap with each other, so that the optical paths of the first and second light beams 51a and 52a are different. For this reason, for example, when the chief ray of the first light beam 51 a coincides with the symmetry axis of the objective lens 60, the chief ray of the second light beam 52 a is tilted with respect to the symmetry axis of the objective lens 60. As a result, coma aberration or the like is generated, the optical characteristics are deteriorated, and there is a problem of deteriorating electric characteristics such as an S / N ratio of a signal due to an increase in crosstalk.
[0014]
An object of the present invention is to provide an optical disc apparatus that enables reading of optical discs having different recording densities at a small size and at low cost, and can prevent deterioration of optical characteristics.
[0015]
[Means for Solving the Problems]
In order to achieve the above object, the present invention provides a first and a second light beam that respectively emit a first light beam and a second light beam in an optical disk device that irradiates a light beam onto an optical disk on which a recording signal is recorded and reads the recording signal by reflected light. Two light sources, a beam splitter that enters the first light flux and emits it in a predetermined direction, and enters the second light flux and emits the first light flux so as to coincide with the optical path, and between the second light source and the beam splitter. comprising arranged and a concave lens, substrate thickness of the first thickness optical disc or the second thickness of the optical disc thickness of the substrate is thinner than the first thickness can be mounted, the thickness of the substrate is first thickness, The optical disk is selectively irradiated with the first light beam, and the optical disk with the second substrate thickness is alternatively irradiated with the second light beam . The invention of the present application is characterized in that, in the optical disc apparatus configured as described above, the first and second light sources are provided so that the second light beam has an optical path length longer than that of the first light beam.
[0016]
According to this configuration, when a predetermined optical disc is mounted, the first light beam is emitted from the first light source and irradiated onto the optical disc via the beam splitter. The reflected light reflected by the optical disk is captured by a photodetector or the like, and the recorded signal is read. When an optical disk having a recording density different from that of the optical disk is mounted, a second light beam is emitted from the second light source, and is irradiated onto the optical disk through the beam splitter through the same optical path as the first light beam. The reflected light reflected by the optical disc is captured by a photodetector or the like and the recorded signal is read as described above. In addition, the difference in optical path length between the first light flux and the second light flux is corrected by the concave lens and converges to a desired position. Further, since the second light beam has a longer optical path length than the first light beam, the correction amount by the concave lens can be reduced.
[0017]
The present invention is also characterized in that, in the optical disk apparatus having the above configuration, the first and second light beams have different wavelengths. According to this configuration, for example, when an optical disk with a high recording density is mounted, a light beam with a short wavelength is irradiated onto the optical disk, and when an optical disk with a low recording density is mounted, a light beam with a long wavelength is irradiated onto the optical disk.
[0018]
Further, the first and second light sources, the beam splitter, and the concave lens are housed in a housing together with a photodetector for receiving reflected light from the optical disk.
[0019]
According to the present invention, in the optical disc apparatus having the above-described configuration, a reflection mirror is disposed between the second light source and the beam splitter.
[0020]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing a configuration of an optical disc apparatus according to an embodiment. The optical disc apparatus 1 can be loaded with an optical disc D such as a CD or a DVD. The optical disk D is rotationally driven by a spindle motor 3, and the spindle motor 3 is rotationally controlled by a servo control circuit 6.
[0021]
The recording signal recorded on the optical disk D can be read out by the optical pickup 2 driven by the feed motor 4. The recording signal read by the optical pickup 2 is demodulated into a predetermined signal by the signal demodulator 5. Then, the data is output to the external computer 10 or the like via an interface (not shown). Further, a focusing amount and a tracking amount are detected by the signal demodulator 5, and the optical pickup 2 is moved in a predetermined direction by the servo control circuit 6 so that focusing and tracking are performed.
[0022]
2A and 2B are a main part plan view and a side view showing the optical pickup 2, respectively. The optical pickup 2 includes a light emitting / receiving unit 11 that emits a light beam and receives reflected light reflected by the optical disk D. The light beam emitted from the light emitting / receiving unit 11 is converted into parallel light by the collimator lens 14, reflected by the prism 13, and then transmitted through the transparent substrate of the optical disc D by the objective lens 12 and converged on the recording surface. The reflected light reflected by the optical disc D enters the light emitting / receiving unit 11 through the reverse path.
[0023]
The details of the light emitting / receiving unit 11 are shown in FIG. 3. The light receiving / emitting unit 11 is covered with a housing 20, and a terminal unit 28 that transmits and receives signals to and from the signal demodulator 5 is provided protruding from the housing 20. In the housing 20, first and second light sources 21 and 22 made of semiconductor lasers or the like for emitting first and second light beams 21a and 22a having different wavelengths are disposed.
[0024]
A beam splitter 25 is provided on the optical path of the first light source 21. The beam splitter 25 includes a prism having a reflecting surface 25a on which a laminated thin film that reflects or transmits light according to the wavelength of incident light is formed. The first light beam 21 a incident on the beam splitter 25 is transmitted through the beam splitter 25 and emitted.
[0025]
In the drawing of the beam splitter 25, a reflection mirror 23 is disposed on the left side, and the second light beam 21a is reflected by the reflection mirror 23 and the reflection surface 25a of the beam splitter 25 and emitted. Thereby, the optical path of the 1st, 2nd light beams 21a and 22a after passing the beam splitter 25 corresponds. As the beam splitter 25, a half mirror, a hologram element, a Wollaston prism, a Savart plate, or the like may be used.
[0026]
The first and second light beams 21a and 22a are alternatively emitted according to the recording density of the optical disk. For example, when the optical disk D is a CD, the first light source 21 is driven to irradiate the optical disk D with the first light beam 21 a having a wavelength λ of 780 nm. When the optical disk D is a DVD, the second light source 22 is driven to irradiate the optical disk D with a second light beam 22a having a wavelength λ of 650 nm.
[0027]
A hologram element 27 is supported on the housing 20 above the beam splitter 25 in the drawing. The hologram element 27 transmits the light beam applied to the optical disk D and diffracts the light beam reflected by the optical disk D. The light beam diffracted by the hologram element 27 enters a photodetector 26 provided in the housing 20.
[0028]
The photodetector 26 has a plurality of light receiving elements such as photodiodes, and an electrical signal based on the light intensity of the light beam captured by each light receiving element is sent to the signal demodulator 5 (FIG. 1). See). Thereby, tracking servo and focusing servo are performed, and a signal recorded on the optical disc D can be reproduced.
[0029]
According to the present embodiment, the optical disc apparatus 1 capable of mounting different types of optical discs D by irradiating the optical discs D having different recording densities using the same optical system with the first and second light beams 21a and 22a having different wavelengths. Can be provided in a small size and at low cost.
[0030]
Further, since the optical paths of the first and second light beams 21a and 22a after passing through the beam splitter 25 coincide with each other, the principal rays of the first and second light beams 21a and 22a are made to coincide with the symmetry axis of the objective lens 12. As a result, the occurrence of coma aberration can be prevented and deterioration of electrical characteristics such as signal-to-noise ratio of the signal due to increase of crosstalk can be prevented.
[0031]
In addition, since the light emitting / receiving section 11 is housed in one package integrated by the housing 20, the assembly of the optical disk apparatus can be simplified and the optical disk apparatus can be miniaturized.
[0032]
In FIG. 3, the second light beam 22a emitted from the second light source 22 has an optical path length longer than that of the first light beam 21a by a distance L between the reflection mirror 23 and the reflection surface 25a of the beam splitter 25. For this reason, the convergence position of the second light beam 22a is closer to the objective lens 12 (see FIG. 2B) than the convergence position of the first light beam 21a. Therefore, the optical path length difference can be corrected by disposing the concave lens 24 between the reflecting mirror 23 and the beam splitter 25. Thereby, the 1st, 2nd light beams 21a and 22a can be converged on the same position.
[0033]
Here, when the optical disc D is tilted, coma aberration occurs when the convergent light beam passes through the transparent substrate of the optical disc. The coma aberration Wc is proportional to the cube of the numerical aperture NA and the thickness d of the disk substrate, as shown in Expression (2).
[0034]
Wc∽d · (NA) ³ (2)
[0035]
Therefore, as shown in FIG. 2B, for example, the substrate thickness d ₁ of the CD is 1.2 mm, whereas the recording density is high, so an objective lens having a large numerical aperture NA is used depending on the optical disk apparatus. In the case of a DVD (indicated by D ′ in the figure), there is a substrate thickness d ₂ = 0.6 mm. This suppresses the generation of coma aberration due to tilt.
[0036]
On the other hand, when the light beam applied to the optical disc D passes through the substrate, spherical aberration occurs. Usually, the objective lens is formed so as to cancel out this spherical aberration. For this reason, for example, when the objective lens 12 (see FIG. 2B) is formed so that the spherical aberration is canceled when the CD is mounted, the spherical aberration W ₄₀ shown in the equation (3) is generated when the DVD is mounted. To do. In formula (3), n is the refractive index of the substrate.
[0037]
W ₄₀ = (n ² −1) (NA) ⁴ (d ₁ −d ₂ ) / (8n ³ ) (3)
[0038]
Accordingly, when the optical discs D and D ′ having different substrate thicknesses are mounted on the optical disc apparatus 1, the spherical aberration W ₄₀ is obtained by converging the first and second light beams 21a and 22a to different positions by the concave lens 24. Can be corrected. In this case, it is more desirable to irradiate the second light beam 22a having a long optical path length to an optical disk (DVD in the above example) having a thin substrate.
[0039]
That is, the correction by the concave lens 24 is required for the thickness difference (d ₁ −d ₂ ) of the substrate with respect to the optical disk having a thin substrate thickness. However, since the optical path length of the second light beam 22a is longer by the distance L, the second light beam 22a converges to a position closer to the objective lens 12 than the first light beam 21a, and the correction amount by the concave lens 24 can be reduced accordingly. Thereby, the generation of aberration due to the correction of the concave lens 24 can be reduced. As shown in FIG. 4, the concave lens 24 may be disposed between the reflection mirror 23 and the second light source 22.
[0040]
【The invention's effect】
According to the present invention, an optical disc apparatus capable of mounting a plurality of optical discs by irradiating optical discs having different recording densities by using first and second light beams having different wavelengths using the same optical system is provided at a small size and at low cost. Can do. Further, since the optical paths of the first and second light fluxes after passing through the beam splitter coincide with each other, it is possible to prevent the occurrence of coma aberration and to prevent the deterioration of electrical characteristics such as the signal-to-noise ratio of the signal due to the increase in crosstalk.
[0041]
Also, it is possible to converge the first since the arranged concave lenses between the beam splitter and the second light source, first by correcting a difference in optical path length of the second light flux, the second light flux in the same position . In addition, when the thicknesses of the substrates of the optical disks having different recording densities are different, the thickness difference can be corrected by the concave lens to prevent the occurrence of spherical aberration. Further, since the second light beam has a longer optical path length than the first light beam, the correction amount by the concave lens can be reduced.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a configuration of an optical disc apparatus according to an embodiment of the present invention.
FIG. 2 is a diagram showing an optical pickup of the optical disc apparatus according to the embodiment of the present invention.
FIG. 3 is a cross-sectional view showing a light emitting / receiving unit of the optical disc apparatus according to the embodiment of the present invention.
FIG. 4 is a diagram showing another configuration of the light receiving and emitting unit of the optical disc apparatus according to the embodiment of the present invention.
FIG. 5 is a cross-sectional view showing an optical pickup of a conventional optical disc apparatus.
FIG. 6 is a perspective view showing a main part of an optical pickup of a conventional optical disc apparatus.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Optical disk apparatus 2 Optical pick-up 3 Spindle motor 4 Feed motor 5 Signal demodulator 6 Servo control circuit 10 External computer 11 Light emitting / receiving part 12, 60 Objective lens 13 Prism 14 Collimator lens 20 Housing 21, 51 First light source 21a, 51a 1st Light beams 22, 52 Second light source 22a, 52a Second light beam 23 Reflection mirror 24 Concave lens 25 Beam splitters 26, 56, 57 Photo detector 27 Hologram element 28 Terminal portions D, D ′ Optical disc

Claims (5)

In an optical disc apparatus that irradiates a light beam onto an optical disc on which a recording signal is recorded and reads the recording signal by reflected light,
First and second light sources that respectively emit first and second light fluxes;
A beam splitter that enters the first light flux and emits the light in a predetermined direction, and enters the second light flux and emits the first light flux so that the optical path matches ;
A concave lens disposed between a second light source and the beam splitter ,
An optical disc having a substrate thickness of a first thickness;
Alternatively, an optical disc having a second thickness that is thinner than the first thickness can be mounted.
The first light flux is applied to the optical disk having the first substrate thickness,
An optical disc apparatus, wherein a second light beam is selectively irradiated to an optical disc having a substrate thickness of a second thickness . 2. The optical disc apparatus according to claim 1, wherein the first light source and the second light source are provided so that the second light beam has an optical path length longer than that of the first light beam. The first optical disc apparatus according to claim 1 or claim 2 and the second light flux, characterized in that it consists of different wavelengths. The first and second light sources, the beam splitter, and the concave lens,
The optical disk apparatus according to claim 1, wherein the optical disk apparatus is housed in a housing together with a photodetector for receiving reflected light from the optical disk. The optical disk apparatus according to claim 1, characterized in that arranged a reflecting mirror between the beam splitter and the second light source.

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