JP2001043553A - Optical pickup and optical information reproducing device using it - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the number of components and cost by making up an optical element of a condensing optical system wherein a recording medium is selectively irradiated with one light beam of two semiconductor laser beam sources with a parallel flat plate and transmitting the light beam in a position where the light beam is in a convergent state. SOLUTION: A reflecting film reflecting a laser beam of e.g. 650 nm wavelength is formed on the surface of a dichroic mirror 4 being an optical element consisting of a parallel flat plate. A holgram unit 3 is composed of a semiconductor laser 10 of e.g. 650 nm wavelength, a hologram 11 and photodetectors 12a, 12b, outgoing optical flux is reflected by the reflecting film of the dichroic mirror 4, and a light spot is formed on an information recording surface of an optical disk 1 such as a DVD-ROM. A hologram unit 9 is composed of the semiconductor laser 13 of e.g. 785 nm wavelength, a hologram 14 and photodetectors 15a, 15b, the outgoing optical flux transmits the dichroic mirror 4, and the light spot is formed on the information recording surface of an optical disk 2 such as a CD-ROM.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical information reproducing apparatus (hereinafter, referred to as an optical disk apparatus) used for reproducing an information signal recorded on an optical information recording medium (hereinafter, referred to as an optical disk).

[0002]

2. Description of the Related Art An optical disk apparatus is an information recording / reproducing apparatus characterized by a non-contact, large-capacity, high-speed access, and low-cost medium. By utilizing these characteristics, it is used as a digital audio signal recording / reproducing apparatus or a computer. It is used as an external storage device.

At present, there are various types of optical disks depending on the difference in substrate thickness and the corresponding wavelength. For example, while the wavelength of a laser beam optimal for recording and reproduction is 1.2 mm in a disk substrate such as a CD or a CD-R in a 780 nm band, a recently standardized DV
A D-ROM or DVD-RAM has a disk substrate thickness of 0.6 mm and a corresponding wavelength in the 650 nm band. For this reason, optical pickups for DVDs that have begun to spread in recent years are mainly equipped with semiconductor lasers having two wavelengths of 780 nm and 650 nm in consideration of compatibility with CD optical disks that have already spread.

[0004] On the other hand, with the expansion of the use of these optical discs, miniaturization and cost reduction of optical disc apparatuses are being promoted, and in order to do so, techniques for downsizing and simplifying optical pickups are indispensable. For miniaturization and simplification of optical pickup,
It is effective to reduce the number of components constituting the optical system or to adopt a configuration using low-cost components. In particular, when considering compatibility with a plurality of types of optical discs, an optical system corresponding to each optical disc is required.However, simplification of the optical system or reduction in the number of components by sharing optical components is one of the requirements of optical pickups. This is effective for miniaturization and cost reduction. As an example, Japanese Patent Application Laid-Open Nos.
Japanese Patent Application Laid-Open No. 54977/1990 discloses a technique in which laser beams of two different wavelengths are combined on an optical path in the middle to enable reproduction of information by one objective lens in order to enable reproduction of a plurality of types of optical discs. Have been.

[0005]

As described above, in an optical pickup equipped with two semiconductor lasers, the purpose is to reduce the size and cost of the optical pickup itself.
Many optical systems have a common optical system configuration such as an objective lens and a collimating lens, and a wavelength-selective prism is used as an optical element for synthesizing an optical path to make two light beams a common optical system. Often used. FIGS. 1A and 1B show such a conventional optical system configuration. The wavelength-selective prism is an element that transmits a laser beam of a specific wavelength in almost the entire amount of light and acts as a mirror for laser beams of different wavelengths by reflection on an internal reflection film. However, since the wavelength-selective prism is a laminated component and requires a high component cost, it has been a major obstacle to further reducing the cost of the optical pickup.

In view of the above situation, an object to be solved by the present invention is to provide an optical pickup which is compatible with a plurality of types of optical disks by using a plurality of semiconductor lasers, and has an optical system configuration advantageous for miniaturization. It is still another object of the present invention to provide an optical pickup and an optical disk device which achieve further cost reduction.

[0007]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention provides two semiconductor laser light sources and one of the semiconductor laser light sources that transmits a light beam emitted from one of the semiconductor laser light sources while simultaneously transmitting the other light beam. Optical element for reflecting a light beam emitted from a semiconductor laser light source, and a condensing optical system for condensing a light beam transmitted or reflected by the optical element and irradiating a light spot on a predetermined position on an optical information recording medium In the optical information reproducing apparatus, the optical element is formed of a parallel plate, and at the same time, the optical element is arranged so that a light beam emitted from the semiconductor laser light source is transmitted at a position in a divergent light state.

In order to solve the above-mentioned problems, according to the present invention, two semiconductor laser light sources, and a light beam emitted from one of the semiconductor laser light sources is transmitted in a divergent light state and at the same time the other. An optical element comprising a parallel plate for reflecting a light beam emitted from a semiconductor laser light source, and condensing a light beam transmitted or reflected by the optical element to irradiate a light spot on a predetermined position on an optical information recording medium. An optical information reproducing apparatus including a condensing optical system, wherein between the optical element and the semiconductor laser light source, astigmatism and / or coma caused by transmission of a light beam through the optical element are reduced. An optical element to be corrected is arranged.

In the present invention, the optical element for correction is constituted by a cylindrical lens, a parallel plate, or an aspherical lens.

Further, in the present invention, the plate thickness of the optical element composed of the parallel flat plate is set to 1 mm or less.

Further, in the present invention, the optical element is arranged such that a meridional plane of a normal to a transmission surface of the optical element formed of a parallel plate and a direction parallel to an active layer of the semiconductor laser light source are parallel.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The configuration and operation of an optical pickup according to a first embodiment of the present invention will be described below with reference to the drawings.

FIGS. 2A and 2B are schematic structural views of an optical pickup according to a first embodiment of the present invention, showing a state in which different types of optical disks 1 and 2 are being reproduced. I have. In FIG. 2A, the hologram unit 3 is configured such that a semiconductor laser 10 oscillating at a wavelength of, for example, 650 nm, a hologram 11, and a photodetector 12 are integrally formed. The light beam emitted from the semiconductor laser 10 in the hologram unit 3 is
1 is transmitted. Dichroic mirror 4
Is an optical element having the shape of a parallel plate, and 650 is provided on the surface.
A reflecting film for reflecting a laser beam having a wavelength of nm is formed.
The light beam emitted from the hologram unit 3 is reflected by a reflective film on the surface of a dichroic mirror 4 disposed at an angle of 45 ° with respect to the optical axis, and then converted into a parallel light beam by a collimating lens 5 to obtain an objective light. The lens 6 is reached. The objective lens 6 is integrally held by an actuator 7, and by energizing a drive coil 8, a light beam can be focused on an information recording surface of an optical disc 1 such as a DVD-ROM to form a light spot. It is.
The light beam reflected from the optical disk 1 follows the same optical path as the outward light in the reverse direction, and reaches the hologram unit 3 via the objective lens 6, the collimator lens 5, and the dichroic mirror 4. A grating groove (not shown) is formed in the hologram 11, and the light beam reflected from the optical disk 1 is diffracted by the grating groove and guided to the photodetectors 12a and 12b. Note that the individual components constituting the hologram unit 3 may be the same as existing components, and thus detailed description is omitted here.

In FIG. 2B, the hologram unit 9 has a semiconductor laser 13 oscillating at a wavelength of, for example, 785 nm, a hologram 14, and a photodetector 15 integrally formed. The light beam emitted from the semiconductor laser 13 in the hologram unit 9 is configured to pass through the hologram 14. Since the dichroic mirror 4 has a property of transmitting a laser beam having a wavelength of 785 nm, the light beam emitted from the semiconductor laser 13 of the hologram unit 9 passes through the dichroic mirror 4 and is converted by the collimating lens 5 into a parallel light beam. Reach 6. The objective lens 6 is a lens capable of condensing a light beam emitted from the semiconductor laser 13 on the information recording surface of the optical disk 2 such as a CD-ROM. A light spot is formed.

The light beam reflected by the optical disk 2 follows the same optical path as that of the outward light in the reverse direction, and reaches the hologram unit 9 via the objective lens 6, the collimator lens 5, and the dichroic mirror 4. The hologram 14 has a grating groove (not shown)
Are formed, and the light beam reflected from the optical disk 2 is diffracted by the grating grooves and guided to the photodetectors 15a and 15b. It should be noted that the individual components constituting the hologram unit 9 may be the same as the existing components, and thus detailed description is omitted here.

In the first embodiment of the present invention, in order to reduce the number of components of the optical pickup, six optical systems from the collimate lens 5 to the objective lens are shared for two different laser wavelengths. Configuration. Therefore, as described above, the dichroic mirror 4 that combines the two laser beams on the optical path reflects the light beam from the first semiconductor laser 10 and transmits the light beam from the second semiconductor laser 13. In both cases, a divergent light beam is incident.

Here, it is known that an inclined parallel plate in the divergent light causes astigmatism and coma. Therefore, astigmatism and coma are generated in the light beam transmitted through the dichroic mirror 4 in the state shown in FIG. Using the thickness t of the dichroic mirror 4, the refractive index N of the medium, the angle θ with the optical axis, the spread angle α of the divergent light transmitted through the dichroic mirror 4, and the laser wavelength λ,
Astigmatism ΔZ, astigmatism AS, coma wavefront aberration coefficient W3
1. It is known that coma aberration COMA can be expressed by the following equation.

[0018]

(Equation 1)

In general, a semiconductor laser has an astigmatic difference Δ
Z exists. FIG. 3 shows a schematic configuration of a semiconductor laser and a state of emitted laser light. As shown in FIG. 3, the intensity distribution 16 of the light beam emitted from the semiconductor laser 13 includes a direction perpendicular to the bonding surface of the active layer 17 (hereinafter, this direction is referred to as θ⊥), and this θ⊥. , That is, in a direction parallel to the bonding surface (hereinafter, this direction is referred to as θ‖), and has different intensity distributions.
The intensity distribution is such that the θ⊥ direction is the longitudinal direction. The reason why the intensity distribution 16 occurs is that the position of the beam waist of the emitted light is on the resonator end face of the active layer 17 in the θ⊥ direction and is within the resonator of the active layer 17 in the θ‖ direction. The astigmatic difference ΔZ of the semiconductor laser 13 is
It is defined as the distance between the beam waist in the θ⊥ direction and the beam waist in the θ‖ direction.

FIG. 4 shows the positional relationship between the semiconductor laser 13 and the dichroic mirror 4 according to the first embodiment of the present invention. As shown in FIG. 4, in the first embodiment, the plane including the optical axis and the θ‖ direction of the semiconductor laser 13 and the meridional plane of the normal to the inclined surface of the dichroic mirror 4 are the same. Because of the setting, the direction of the astigmatic difference ΔZ in the semiconductor laser 13 and the direction of the astigmatic difference in the dichroic mirror 4 match. Therefore, the dichroic mirror 4 depends on the set values of the thickness t of the dichroic mirror 4 and the inclination angle θ.
Can be changed to change the astigmatism of the light beam transmitted through the dichroic mirror 4.

Next, the state of the light spot on the optical disk will be described. FIG. 5 shows an intensity distribution 16b of the light spot on the optical disk 2. The pits 18 are arranged side by side on the information recording surface on the optical disk 2 and form a recording track row in a tangential direction of the optical disk. According to the first embodiment of the present invention, the optical disc 2
The angle of attachment of the semiconductor laser 13 to the optical axis is set so that the θ‖ direction substantially coincides with the radial direction of the disk with respect to the intensity distribution 16b of the light spot with respect to. This is intended to reduce the influence of crosstalk from a recording track adjacent to a recording track from which information is to be reproduced.

Here, the astigmatism in the light spot on the optical disk and the reproduction performance will be described. For example, when an optical disk such as a CD is reproduced in the state of a light spot as shown in FIG. 5, astigmatism on the optical disk and 3T
Experiments have shown that the jitter has a relationship as shown in FIG. 6 as an example. As shown in FIG. 6, the jitter performance is best when there is astigmatism of about 0.04 λrms in the θ⊥ direction. As a condition for obtaining a good reproduction signal for the optical disk device for 25 ns or less, astigmatism on the optical disk is set to be 0. 0 in the θ⊥ direction.
It can be seen that it suffices if it is 1λrms or less or 0.02λrms or less in the θ‖ direction.

As described above, when the relationship between the plate thickness t and the inclination angle θ of the dichroic mirror 4 and the amount of aberration of the light spot is calculated by using the equations (1) to (4), FIGS. )become that way. FIG. 7A shows astigmatism, and FIG.
(B) shows the calculation result of the coma aberration. Here, the wavelength of the semiconductor laser = 785 nm and the NA of the laser (sin
α) = 0.1. The astigmatic difference ΔZ of the semiconductor laser is considered to be about 0 μm to 35 μm, but is calculated as a representative value of 12 μm. As described above, the calculation is performed such that the astigmatism of the semiconductor laser 13 and the astigmatism of the dichroic mirror 4 cancel each other. According to FIGS. 7A and 7B, astigmatism and coma can be reduced by reducing the thickness t of the dichroic mirror 4 or reducing the inclination angle θ of the dichroic mirror 4. is there. Further, as described with reference to FIG.
rms or less, the thickness t of the dichroic mirror 4 is 1 m if it is in a range where no problem such as deformation occurs.
This can be realized by setting the value to m or less. Regarding coma, even when 0.02λrms is set as an allowable value,
The thickness of the dichroic mirror, t = 1.5 mm, can be secured.
In the first embodiment, the coma aberration can be corrected by moving the light emitting point of the semiconductor laser 13 on the transmission side in a plane orthogonal to the optical axis.

As described above, according to the first embodiment of the present invention, as compared with the conventional optical system configuration shown in FIG. The mirror 4 can be arranged, and a good reproduction signal can be obtained.

Next, a second embodiment of the present invention will be described with reference to FIGS.

FIG. 8 differs from FIG. 2 in that the dichroic mirror 4 is arranged so as to form an angle of 45 ° or less with respect to the optical axis of the divergent light to be transmitted. As described above, the astigmatism and the coma generated by the inclined dichroic mirror 4 during the diverging light are substantially proportional to the inclination angle θ of the dichroic mirror 4, and therefore occur on the optical disk as compared with FIG. Aberration can be set small. Therefore, if the inclination angle does not cause any problem in the configuration of the optical system using the dichroic mirror 4 on the reflection side, the plate thickness t of the dichroic mirror 4 can be set to be large, and the same effect as in the first embodiment can be obtained. be able to.

Next, a third embodiment of the present invention will be described with reference to FIG.

FIG. 9 differs from FIG. 2B in that a cylindrical lens 20 is additionally arranged between the dichroic mirror 4 and the semiconductor laser. Since the cylindrical lens 20 in the diverging light can generate astigmatism in the direction with the lens action and the direction without the lens action, the astigmatism generated in the dichroic mirror 4 can be canceled. It is possible. This makes it possible to easily cancel astigmatism even when the thickness t of the dichroic mirror 4 is secured as in the conventional case. Further, the coma can be canceled by moving the light emitting point of the semiconductor laser 13 as in the first embodiment.

Next, a fourth embodiment of the present invention will be described with reference to FIG.

FIG. 10 shows that a parallel flat plate 21 is provided instead of the cylindrical lens 20 in FIG.
This is a different point. The direction of inclination of the parallel plate 21 with respect to the optical axis is set in a plane orthogonal to the direction of inclination of the dichroic mirror 4 with respect to the optical axis. As previously mentioned,
Since astigmatism occurs in a parallel plate in the divergent light,
By combining two parallel flat plates together with the dichroic mirror 4, the amount of astigmatism can be optimized.
Thereby, the degree of freedom of the inclination angle θ and the plate thickness t of the dichroic mirror 4 can be increased.

Next, a fifth embodiment of the present invention will be described with reference to FIG.

FIG. 11 shows that an aspheric lens 22 is provided instead of a cylindrical lens.
This is a different point. The aspheric lens 22 has a lens shape capable of canceling astigmatism and coma generated by the dichroic mirror 4. Therefore, it is possible to optimize the astigmatism on the optical disk, and at the same time, it is possible to configure an optical system free from coma.

Next, a sixth embodiment of the present invention will be described with reference to FIG.

In FIG. 12, for example, a light beam emitted from a CD semiconductor laser 23 is transmitted through a diffraction grating 24, a correction lens 25, and a dichroic mirror 4 and then reflected by a half mirror 26. Then, the collimating lens 5
Is converted into parallel light, and the objective lens 6 causes the optical disk 2
Is condensed. The light reflected from the optical disk 2 follows a path reverse to the outward path, passes through the half mirror 26, and is then focused on the photodetector 30. Here, the diffraction grating 24 is for generating three light spots on the optical disk 2, and the correction lens 25 is for correcting the occurrence of aberration due to the movement of the objective lens 6. further,
A part of the light beam emitted from the semiconductor laser 23 passes through the half mirror 26 as it is, and the light intensity is detected by the front monitor 29. On the other hand, a light beam having a wavelength different from that of the semiconductor laser 23 and emitted from the semiconductor laser 27 for DVD, for example, passes through the diffraction grating 28, is reflected by the dichroic mirror 4, and follows the same optical path as that of the CD system. The configuration reaches the detector 30. Here, the diffraction grating 28 is placed on the optical disc 1 (not shown).
For generating two light spots. here,
Since the astigmatism generated by the divergent light passing through the dichroic mirror 4 and the astigmatism generated by the semiconductor laser 23 are arranged to cancel each other, the astigmatism on the optical disk 2 is explained as shown in FIG. , It is possible to obtain a good light spot.

Next, a seventh embodiment of the present invention will be described with reference to FIG. In FIG. 13, the front monitor 29
Is disposed at a position reflecting the transmission surface side of the dichroic mirror 4, and this point is a configuration different from FIG. The dichroic mirror 4 is designed so that about 10% of the light is reflected on the transmission side surface of the dichroic mirror 4, whereby the laser intensity of the semiconductor laser 13 can be measured by the front monitor 29. This makes it possible to set an optimum recording power especially for a recordable optical disc such as a CD-R.

Although the present invention has been described with reference to a plurality of embodiments, the number of objective lenses is not limited to one in an optical system of an optical pickup using the present invention. A lens may be used. Further, the number of semiconductor lasers may be three or more.

[0037]

As described above, according to the present invention, in an optical system configuration of an optical pickup that combines two different semiconductor lasers in the middle of the optical path, the optical component of the portion combining the two optical paths is a parallel plate. By using a dichroic mirror, the number of parts and cost can be reduced, and at the same time, the astigmatism on the optical disk can be optimized by canceling the astigmatic difference generated by the semiconductor laser, and a good reproduction signal can be obtained. It is possible to get.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a conventional optical pickup.

FIG. 2 is a configuration diagram of an optical pickup according to the first embodiment of the present invention.

FIG. 3 is a diagram showing an astigmatic difference in a semiconductor laser.

FIG. 4 is a diagram showing a positional relationship between a dichroic mirror and astigmatic difference according to the present invention.

FIG. 5 is a diagram showing an intensity distribution of a light spot on an optical disk.

FIG. 6 is an experimental result showing a relationship between astigmatism of a light spot on an optical disk and 3T jitter.

FIG. 7 is a calculation result of a relationship between an aberration and an inclination angle and a plate thickness of a dichroic mirror.

FIG. 8 is a configuration diagram of an optical pickup according to a second embodiment of the present invention.

FIG. 9 is a configuration diagram of an optical pickup according to a third embodiment of the present invention.

FIG. 10 is a configuration diagram of an optical pickup according to a fourth embodiment of the present invention.

FIG. 11 is a configuration diagram of an optical pickup according to a fifth embodiment of the present invention.

FIG. 12 is a configuration diagram of an optical pickup according to a sixth embodiment of the present invention.

FIG. 13 is a configuration diagram of an optical pickup according to a seventh embodiment of the present invention.

[Explanation of symbols]

1, 2, optical disk 3, 9, hologram unit 4, dichroic mirror 5, collimating lens, 6, objective lens, 7, actuator, 8, driving coil, 10, 13, 23, 27
... semiconductor laser, 11, 14 ... hologram,
15, 30 ... photodetector, 16 ... intensity distribution, 1
7 Active layer 18 Pit 19 Prism 20 Cylindrical lens 21 Parallel plate 22 Aspherical lens 24 28
Diffraction grating, 25 correction lens, 26 half mirror, 29 front monitor

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Hideo Suenaga 1st Kitano, Makino, Mizusawa-shi, Iwate Inside Hitachi Media Electronics Co., Ltd. (72) Inventor Hiroshi Ogasawara 1st Kitano, Makino, Mizusawa-shi, Iwate Hitachi, Ltd. Inside Media Electronics (72) Inventor Tohru Sasaki 1 Kitano, Majo, Mizusawa-shi, Iwate Inside Hitachi Media Electronics Co., Ltd. (72) Inventor Masayuki Inoue 292 Yoshida-cho, Totsuka-ku, Yokohama-shi, Kanagawa Prefecture Digital Media Hitachi, Ltd. Development Headquarters (72) Inventor Kuniichi Onishi 292 Yoshida-cho, Totsuka-ku, Yokohama-shi, Kanagawa Prefecture Hitachi, Ltd. Digital Media Development Headquarters F-term (reference) 5D119 AA03 AA05 AA41 BA01 DA05 EC02 EC04 EC47 FA05 FA08 JA08 JA17

Claims (7)

[Claims] An optical element that transmits a light beam emitted from one of the semiconductor laser light sources and simultaneously reflects a light beam emitted from the other semiconductor laser light source; A light condensing optical system for converging a light beam transmitted or reflected by the optical element and irradiating a light spot to a predetermined position on an optical information recording medium, wherein the optical element is a parallel plate And an optical information reproducing apparatus using the optical pickup, wherein the optical element is arranged so that a light beam emitted from the semiconductor laser light source is transmitted at a position in a divergent light state. . 2. A semiconductor laser light source, wherein a light beam emitted from one of the semiconductor laser light sources is transmitted in a divergent light state and simultaneously reflects a light beam emitted from the other semiconductor laser light source. Optical information reproducing apparatus comprising: an optical element formed of a parallel flat plate; and a condensing optical system that condenses a light beam transmitted or reflected by the optical element and irradiates a light spot to a predetermined position on an optical information recording medium. Wherein an optical element for correcting astigmatism and / or coma generated by transmitting a light beam through the optical element is disposed between the optical element and the semiconductor laser light source. An optical pickup and an optical information reproducing apparatus using the same. 3. An optical pickup according to claim 2, wherein said optical element for correction is an optical element comprising a cylindrical lens. 4. The optical pickup according to claim 2, wherein the optical element for correction is an optical element made of a parallel plate. 5. An optical pickup according to claim 2, wherein said optical element for correction is an optical element comprising an aspherical lens. 6. A semiconductor laser light source, and a light beam emitted from one of the semiconductor laser light sources is transmitted in a diverging light state and simultaneously reflects a light beam emitted from the other semiconductor laser light source. Optical information reproducing apparatus comprising: an optical element formed of a parallel flat plate; and a condensing optical system that condenses a light beam transmitted or reflected by the optical element and irradiates a light spot to a predetermined position on an optical information recording medium. The optical pickup according to any one of claims 1 to 5, wherein a thickness of the optical element formed of the parallel flat plate is set to 1 mm or less. 7. Two semiconductor laser light sources, and a light beam emitted from one of the semiconductor laser light sources is transmitted in a divergent light state and simultaneously reflects a light beam emitted from the other semiconductor laser light source. Optical information reproducing apparatus comprising: an optical element formed of a parallel flat plate; and a condensing optical system that condenses a light beam transmitted or reflected by the optical element and irradiates a light spot to a predetermined position on an optical information recording medium. In Claim 1, The said optical element is arrange | positioned so that the direction parallel to the active layer of the said semiconductor laser light source and the meridian of the transmission surface normal of the said optical element may become parallel.
And an optical information reproducing apparatus using the same.