JP2000348369A - Optical pickup - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an optical pickup that is adaptable to two or more kinds of optical disks used for laser beams of different wavelengths, and that is capable of superior recording and reproduction, and that is also compact and inexpensive. SOLUTION: The optical pickup is equipped with a plurality of semiconductor lasers 2, 3 each emitting a laser beam with a wavelength different from each other, an anamorphic coupling lens for making a nearly parallel light flux out of each laser flux from the semiconductor lasers 2, 3, an objective lens 7 for converging such parallelized light flux of the laser beams onto the recording surface 8A of an optical disk 8 as an optical spot, and a return light flux detecting means 4, 9 for detecting light reflected by the recording surface as a return light flux. In the coupling lens 5, assuming that the optical axis direction to be the Z direction and that two directions orthogonally crossing the Z direction and intersecting with each other at right angles to be the X and Y directions, a focal length fX in the X direction of the coupling lens and a focal length fY in the Y direction satisfies and inequality 0<fX<fY, and the light emitting parts of the semiconductor lasers 2, 3 are arranged within the YZ plane.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to an optical pickup.

[0002]

2. Description of the Related Art In addition to conventionally known CD-Rs, DVDs (digital video discs) for recording / reproducing with shorter wavelength (650 nm) laser light have been put to practical use. Since the recording / reproducing density can be increased as the wavelength of the laser beam becomes shorter, the realization of an optical disk capable of recording / reproducing at a shorter wavelength is intended. So in the future,
It is anticipated that various types of optical disks used with laser beams of different wavelengths will coexist, and an optical pickup that can support these plural types of optical disks is intended. It is desirable that such an optical pickup capable of supporting various optical discs can perform recording and / or reproduction on or from various optical discs satisfactorily, and can be realized compactly and inexpensively.

[0003]

The present invention can cope with two or more kinds of optical disks used with laser beams of different wavelengths, and can perform recording and / or reproduction, or erasing with them satisfactorily, It is another object to realize a compact and inexpensive optical pickup.

[0004]

An optical pickup according to the present invention comprises: a plurality of semiconductor lasers; a coupling lens;
At least one objective lens and return light beam detecting means are provided. The “plurality of semiconductor lasers” emit laser beams having different wavelengths. "Coupling lens"
Is an "anamorphic lens" which has different focal lengths in two directions orthogonal to the optical axis and orthogonal to each other, and converts each laser beam from a plurality of semiconductor lasers into a substantially parallel beam.
The “one or more objective lenses” collects the laser light, which has been converted into a parallel light beam by the coupling lens, as a light spot on the recording surface of the optical disc. "Return light beam detecting means"
Light reflected by the recording surface is detected as a return light flux. The return light beam detecting means includes an optical path separating means for separating an optical path of the return light beam from an irradiation optical path from each semiconductor laser to the recording surface of the optical disk, and a light receiving unit for receiving the separated return light beam. Since the coupling lens converts each laser beam from the plurality of semiconductor lasers into a substantially parallel beam, it is common to a plurality of types of laser beams emitted from the plurality of semiconductor lasers. The objective lens may be “shared for a plurality of semiconductor lasers” as in the embodiment described later, or the plurality of objective lenses may be “switched and used according to the wavelength of the laser light”. Is also good. In the coupling lens, that is, when the optical axis direction is fixed to the coupling lens and the optical axis direction is a Z direction, and two directions orthogonal to the Z direction and orthogonal to each other are an X direction and a Y direction, these X, Y directions
The Y and Z directions are orthogonal to each other. X of coupling lens
When the focal length in the direction (focal length in a plane parallel to the XZ plane) is f _X and the focal length in the Y direction (focal length in a plane parallel to the YZ plane) is f _Y , 0 <f an _X <f _Y, the light-emitting portions of the plurality of semiconductor lasers are arranged in the YZ plane.

In the optical pickup according to the first aspect, the attitude of each of the plurality of semiconductor lasers is determined such that the "active layer is parallel to the YZ plane" of each semiconductor laser (claim 2). 3. The optical pickup according to claim 1, wherein a semiconductor laser that emits a laser beam having the shortest wavelength among the plurality of semiconductor lasers is arranged such that the light emitting portion is located on the optical axis of the coupling lens. Is preferable (claim 3). In the optical pickup according to the first, second, or third aspect, a "flat edge portion" may be formed on the outer peripheral portion of the coupling lens in a direction parallel or orthogonal to the X direction as a mounting reference ( Claim 4). Alternatively, a flat portion parallel or orthogonal to the X direction may be formed on the outer peripheral portion of the cell holding the coupling lens to serve as an attachment reference (claim 5). The optical pickup according to any one of the claims 1-5, the focal length of the coupling lens: f _X,
f _Y is: to satisfy "Condition _{1 <f Y / f X <} 5 " (claim 6). In the optical pickup according to any one of claims 1 to 6, the coupling lens may be configured as a “several sub-element configuration or a cemented lens” by a plurality of lenses, but may be configured by a single lens. (Claim 7). In the optical pickup according to any one of claims 1 to 6, the number of semiconductor lasers can be of course three or more. However, when two semiconductor lasers are used, the number of objective lenses is two. Common to semiconductor lasers, 2
Two semiconductor lasers may be provided in the same laser unit, and a hologram element for diffracting the returning light beam may be provided between the laser unit and the coupling lens. The hologram element constitutes "optical path separating means" in the return light beam detecting means. In this case, the light receiving section of the return light beam detecting means can be "deployed in the laser unit" (claim 9).

[0006]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In the embodiment shown in FIG.
The semiconductor lasers 2 and 3 (which are laser chips) and the light receiving element 9 are sealed in a common laser unit 1.
The semiconductor laser 2 and the semiconductor laser 3 radiate “laser beams having different wavelengths”. A hologram element 4 is provided on a laser light emitting surface of the laser unit 1.

In each laser beam emitted from the semiconductor laser 2 or 3, a “zero-order light component” that passes through the hologram element 4 is used as an illumination light beam. That is, the laser beam that has passed through the hologram element 4 is coupled to the coupling lens 5
Is converted into a “substantially parallel light beam”, enters the objective lens 7 via the deflecting prism 6, is converted into a converged light beam by the objective lens 7, passes through the substrate of the optical disk 8, and becomes a light spot on the recording surface 8 A. As light. The light beam reflected by the recording surface 8A of the optical disk 8 becomes a “return light beam”
Objective lens 7, deflection prism 6, coupling lens 5
Then, the light enters the hologram element 4 and is diffracted by the hologram element 4, and the diffracted light component is received by the light receiving element 9. A focus error signal, a track error signal, a reproduction signal, and the like are generated based on the output of the light receiving element 9, and focusing control and tracking control are performed by a known method based on the focus error signal and the track error signal. Therefore, in this embodiment, the hologram element 4
Is a "light path separating means" and the light receiving element 9 is a "light receiving section", and these constitute "return light beam detecting means". In addition,
A hologram element having a polarization characteristic is used, and all the laser light from the light source side is transmitted as it is, and an optical element which acts as a quarter-wave plate for the laser light of each wavelength is provided on the optical disk side of the hologram element. To make the polarization direction of the return light beam orthogonal to the polarization direction of the outward path (irradiation light path), so that the diffraction effect of the hologram element acts on the entire return light beam. There are two types of optical disks 8, each of which uses a different wavelength, and the one with a shorter used wavelength has a higher recording density. The wavelength of the laser light emitted from the semiconductor laser 2 is
It is shorter than the wavelength of the laser light emitted from the semiconductor laser 3. Therefore, the semiconductor laser 2 is used for recording / reproducing on an optical disk having a high recording density, and the semiconductor laser 3 is used for recording / reproducing on an optical disk having a lower recording density.
Is used. The coupling lens 5 is configured as a single lens, and has a different focal length between the X direction and the Y direction which are orthogonal to the optical axis direction, that is, the Z axis direction. Convert to luminous flux. As shown in FIG. 1, the light emitting portions of the semiconductor lasers 2 and 3 are located in the YZ plane, and the light emitting portion of the semiconductor laser 2 is located on the optical axis of the coupling lens 5. The orientation of the semiconductor lasers 2 and 3 is determined so that the “active layer” in the semiconductor lasers 2 and 3 is parallel to the YZ plane. Therefore, the divergence angle of the laser light emitted from the semiconductor lasers 2 and 3 is the maximum in the XZ plane (a plane including the Z direction and orthogonal to the drawing in FIG. 1).

FIG. 2 shows that the divergent laser light emitted from the light emitting portion of the semiconductor laser 2 (located on the optical axis of the coupling lens 5) is converted into a parallel light beam by the coupling lens 5. It shows the situation. FIG. 2 (a)
Indicates “the lens action in the XZ plane”, and (b) indicates “the lens action in the YZ plane”. FIG.
Surface indicated by -1 is "light source side of the main surface of the imaging in the direction parallel to the XZ plane", the focal length of the distance X direction between the light emitting portion of the main surface 2-1 and the light source: is f _X . FIG.
The surface indicated by −2 is “the main surface on the light source side of the image in the direction parallel to the YZ plane”, and the distance between the main surface 2-2 and the light emitting unit of the light source is a focal length in the Y direction: f _Y. . As is clear from the figure, 0 <f _x <f _Y , and the laser light from the semiconductor laser 2 passes through the coupling lens 5 to become a substantially parallel light beam.
The luminous flux diameter in the Y direction becomes substantially the same. Therefore, the light spot formed on the recording surface of the optical disk has a “substantially circular shape”. The same applies to the laser light emitted from the semiconductor laser 3. Therefore, the coupling lens 5 has a “beam shaping function” for making the light beam cross-sectional shape of the parallel light beam into a substantially circular shape. FIG. 3 (a)
A state where the coupling lens 5 is viewed from the Z-axis direction (optical axis direction) is shown. The “dashed circle” drawn inside the coupling lens 5 indicates the effective diameter of the coupling lens 5. The coupling lens 5 has a flat edge portion 10 on the outer peripheral portion of the lens outside the effective diameter. The edge portion 10 is formed in a direction orthogonal to the X direction (a direction parallel to the Y direction). The coupling lens 5 has an anamorphic optical action, and it is necessary to accurately set the X and Y directions in the relative positional relationship with the semiconductor lasers 2 and 3. Mounting of the coupling lens 5 becomes extremely easy. The edge surface may be formed in parallel to the X direction, like the edge surface 10 'shown in FIG. When the coupling lens 5 is held in the cell, as shown in FIGS.
Cell 11 or 1 holding coupling lens 5
1 ', the flat portion 12 or 1
By forming 2 ′ and setting these flat portions in a direction parallel or orthogonal to the X direction, it can be used as an attachment reference.

The optical pickup described in the above embodiment has a plurality of semiconductor lasers 2 and 3 which emit laser beams having different wavelengths from each other, and has different focal lengths in two directions orthogonal to the optical axis and orthogonal to each other. Semiconductor lasers 2 and 3
An anamorphic coupling lens 5 for converting each laser beam from the laser beam into a substantially parallel beam, and an objective lens for condensing each laser beam parallelized by the coupling lens 5 as a light spot on the recording surface 8A of the optical disc 8 7 and
Return light beam detecting means 4 and 9 for detecting the light reflected by the recording surface 8A as a return light beam; and in the coupling lens 5, the optical axis direction is orthogonal to the Z direction, and the two orthogonal directions are X directions. when the direction and the Y-direction, the focal length in the X direction of the coupling lens 5: f _X, Y-direction focal length: f _Y is 0 <f _X <f _Y, a plurality of semiconductor lasers 2 and 3 This is an optical pickup (Claim 1) in which each light emitting unit is arranged in the YZ plane. The orientation of each of the semiconductor lasers 2 and 3 is determined so that each active layer of each of the plurality of semiconductor lasers 2 and 3 is parallel to the YZ plane (claim 2). Among them, the one that emits the laser beam with the shortest wavelength (semiconductor laser 2) is arranged so that the light emitting portion is located on the optical axis of the coupling lens 5 (claim 3). The portion has a flat edge portion 10, and the edge portion 10 is formed in a direction orthogonal to the X direction and serves as an attachment reference (claim 4). Further, as described with reference to FIGS. 3C and 3D, the coupling lens 5 is held in the cell 11 or 11 'having a flat portion on the outer periphery of the cell, and the flat portion 12 or 12' is It can be formed in a direction parallel or orthogonal to the X direction and used as a mounting reference (claim 5). The coupling lens 5 is a “single lens” (claim 7), and two semiconductor lasers are used, and the objective lens 7 is shared by these two semiconductor lasers 2.3.
Two semiconductor lasers 2 and 3 are provided in the same laser unit 1, and a hologram element 4 for diffracting a returning light beam is provided between the laser unit 1 and the coupling lens 5 (claim 8). The light receiving section 9 of the return light beam detecting means for detecting the return light beam is provided in the laser unit 1 (claim 9).

The focal length of the coupling lens 5 is as follows:
f _X and f _Y are set so as to satisfy the condition: 1 <f _Y / f _X <5 (claim 6). “F _Y / f _X ” is a magnification (beam shaping magnification) at which the beam diameter in the X direction is enlarged in the beam shaping function of the coupling lens. If this value is 1 or less, the beam shaping function cannot be realized. If the divergence angle of the divergent light beam emitted from the semiconductor laser is θ _Y in a direction parallel to the active layer and θ _{X in} a direction perpendicular to the active layer, these divergence angles: θ _X and θ _Y are represented by There is "variation" for each individual. This variation is represented by θ _X <
32 degrees and θ _Y > 6.5 degrees. Therefore, the divergence angle:
The ratio of θ _X , θ _Y : θ _X / θ _Y is at most θ _X / θ _Y = 32 /
6.5 = 4.92. Therefore, in order to obtain a substantially circular light spot on the recording surface by performing beam shaping in such a case, the beam shaping magnification of the coupling lens must be 5
About twice is sufficient.

As described above, an appropriate beam shaping magnification can be set within a range satisfying the condition: 1 <f _Y / f _X <5 according to the divergence angle of the semiconductor laser. The “flat portion for attachment reference” formed on the outer periphery of the lens or the outer periphery of the cell may be replaced with a predetermined angle with respect to the X axis, for example, 45 degrees, 60 degrees, etc. It is also conceivable to form them so that

[0012]

EXAMPLES Specific examples will be described below. The emission wavelength of the semiconductor laser 2 is λ ₁ = 635 nm, and the emission wavelength of the semiconductor laser 3 is λ ₂ = 785 nm. The optical disk 8 has a wavelength:
DVD used at 635nm with a substrate thickness of 0.6mm
The one used at a wavelength of 785 nm has a substrate thickness of 1.
2 mm CD-R. The coupling lens 7 is optimized for the laser beams of the above two wavelengths. The coupling lens 5 is a single lens made of a single glass material, and is aspherical on both the light source side (hereinafter, the second surface) and the optical disk side (hereinafter, the first surface). Coupling lens: refractive index of glass material: n ₆₃₅ (wavelength: λ ₁
= 635 nm) = 1.72688, n ₇₈₅ (wavelength: λ ₂ = 785 nm)
= 1.718773, thickness on the optical axis of the lens: 9.237 mm.
The above-mentioned aspheric surface on both surfaces of the coupling lens is represented by the following formula (1). ^{_{Z = (R x X 2 +}} R y Y 2) / {1 + √ [1- (1 + K x) R x 2 X 2 - (1 + K y) R y 2 Y 2]} + AR [(1 -AP) X ² + (1 + AP) Y ² ] ² + BR [(1-BP) X ² + (1 + BP) Y ² ] ³ + CR [(1-CP) X ² + (1 + CP) Y ² ] ⁴ + DR [(1-DP) X ² + (1 + DP) Y ² ] ⁵ (1) where Z is a lens surface coordinate in the optical axis direction, R _x , R
_y is the paraxial curvature of the lens surface in the ZX plane and ZY plane, respectively, K _x and _Ky are conical constants, and AR, BR, CR and DR are
The rotationally symmetric components of the fourth, sixth, eighth and tenth order deformation coefficients from the cone, AP, BP, CP and DP, are the fourth, sixth, and
Represents the non-rotationally symmetric components of the eighth and tenth order deformation coefficients. Surface coefficient First surface Second surface Rx 0.031556 -0.138891 Ry 0.167308 0.334217 Kx -43.887151 0.519138 Ky 0.550925 0.5 AR -0.217241E-06 0.969748E-05 BR -0.235514E-08 0.393184E-03 CR -0.215347E-06 -0.321152 E-05 DR -0.377241E-11 0.192550E-06 AP 0.455692E + 02 0.923870E + 01 BP 0.169132E + 02 0.116785E + 01 CP -0.115189E + 01 0.600516E-01 DP 0.553829E + 01 0.157569E + 01 Center thickness: 9.533 mm, refractive index: n ₆₃₅ = 1.72688, n ₇₈₅ = 1.
718773 n ₆₃₅ is the refractive index for light having a wavelength of 635 nm;
₇₈₅ is a refractive index for light having a wavelength of 785 nm. In the above, for example, “E-08” means “10 ⁻⁹ ”, and this numerical value is related to the immediately preceding numerical value. The wavefront aberration with respect to the image height (corresponding to the light source position) when a parallel light beam (plane wave) is incident on the coupling lens from the left side (first surface side) in FIGS. 2A and 2B is as follows. It is as follows. Image height (mm) Wavefront aberration in XZ plane (λ) Wavefront aberration in YZ plane (λ) 0.0 0.009 0.009 0.1 0.043 0.013 0.2 0.087 0.024 0.3 0.138 0.044 0.4 0.197 0.071 As can be seen, when the light source position (corresponding to the above image height) deviates from the optical axis, the same in the XZ plane and in the YZ plane. As for the “shift amount”, the increase of the wavefront aberration is almost three times larger in the shift in the XZ plane than in the YZ plane. In consideration of this point, arranging a plurality of (lasers) semiconductor lasers (chips) is more likely to be in the YZ plane than in the XZ plane.
It is good to set it in the plane. That is, when the light-emitting portions of a plurality of semiconductor lasers are arranged in the XZ plane, the luminous flux from the light source located at a position shifted from the optical axis has a large “deformation from a plane wave” of the coupled wavefront. When light is condensed as a light spot, it is difficult to obtain a light spot having a good shape. On the other hand, if the light-emitting portions of a plurality of semiconductor lasers are arranged on the YZ plane, the state of the wavefront after coupling is good, and a good light spot can be formed even by laser light from a light source distant from the optical axis. Can be formed.

The coupling lens of this embodiment has f _x = 9 mm, f _y = 23.94 mm, and f _y / f _x = 2.
66, which satisfies the condition: 1 <f _Y / f _X <5. In order to increase the recording density of an optical disk, it is necessary to reduce the track pitch of the optical disk and shorten the mark length. To achieve this, it is necessary to reduce the size of the light spot formed on the recording surface, For that purpose, “shorter wavelength laser light” must be used. In other words, the shorter the wavelength of the laser light, the stricter the requirements on the quality of the light spot. Therefore, of the plurality of semiconductor lasers, the one that emits the laser light having the shortest wavelength (semiconductor laser 2)
Are arranged such that the light emitting portion is located on the optical axis of the coupling lens 5, thereby minimizing the wavefront aberration due to the angle of view and forming a minimum light spot satisfactorily.

As shown in FIG. 1, wavelength: λ = 635 nm
A semiconductor laser that emits a laser beam is disposed on the optical axis of the coupling lens 5 and a DV having a substrate thickness of 0.6 mm is provided.
The wavefront aberration when focused as a light spot on the recording surface of D is 0.002λ, which is sufficient for performing recording and reproduction. In addition, as shown in FIG. 1, the light emitting portion of the semiconductor laser 3 that emits laser light having a wavelength of λ = 635 nm is 0.2 m in the Y direction from the optical axis of the coupling lens 5.
m, and 0.5 mm in the Z direction from the light emitting portion of the semiconductor laser 2, the laser beam from the semiconductor laser 3 is applied to the CD-R recording surface having a substrate thickness of 1.2 mm. The wavefront aberration when condensed as
012λ, which is also sufficient for recording and reproduction.

[0015]

As described above, according to the present invention, a novel optical pickup can be realized. ADVANTAGE OF THE INVENTION The optical pickup of this invention can use a coupling lens common to several semiconductor lasers from which emission wavelength differs mutually, and since a coupling lens can have a beam shaping function, it can implement | achieve compactly and inexpensively. In addition, by arranging the light emitting units of the plurality of semiconductor lasers in the YZ plane where the wavefront aberration is hardly affected by the deviation from the optical axis of the coupling lens, the laser light of each wavelength can be used for a plurality of types of optical discs. On the other hand, recording and reproduction can be performed well.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining one embodiment of an optical pickup of the present invention.

FIG. 2 is a diagram for explaining a shape and a function of a coupling lens in the embodiment.

FIG. 3 is a diagram for explaining a characteristic portion of the invention according to claims 4 and 5;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Laser unit 2 Semiconductor laser with short emission wavelength 3 Semiconductor laser with long emission wavelength 4 Hologram element 5 Coupling lens 6 Deflection prism 7 Objective lens 8 Optical disk

Claims (9)

[Claims] A plurality of semiconductor lasers which emit laser beams having different wavelengths from each other; and a focal length in two directions orthogonal to the optical axis and orthogonal to each other is different. An anamorphic coupling lens, a laser beam converted into a parallel light beam by the coupling lens, one or more objective lenses for condensing the laser beam as a light spot on a recording surface of an optical disc, and a reflected light from the recording surface. Return light beam detecting means for detecting as a return light beam, wherein in the coupling lens, the optical axis direction is the Z direction,
When orthogonal to the Z direction, the two directions with X and Y directions orthogonal to each other, the focal length in the X direction of the coupling lens: f _X, Y-direction focal length: f _Y is, 0 <f _X <
f is _Y, the optical pickup, characterized in that the light-emitting portions of the plurality of semiconductor lasers, arranged in the YZ plane. 2. The optical pickup according to claim 1, wherein the orientation of each semiconductor laser is determined so that each active layer of the plurality of semiconductor lasers is parallel to the YZ plane. 3. An optical pickup according to claim 1, wherein the semiconductor laser emitting the shortest wavelength laser light among the plurality of semiconductor lasers is arranged such that the light emitting portion is located on the optical axis of the coupling lens. An optical pickup characterized by being arranged. 4. The optical pickup according to claim 1, wherein the coupling lens has a flat edge on the outer periphery of the lens, and the flat edge is formed in a direction parallel or orthogonal to the X direction. An optical pickup characterized by being a mounting reference. 5. The optical pickup according to claim 1, wherein the coupling lens is held by a cell, and the cell has a flat portion on an outer peripheral portion, and the flat portion is parallel or orthogonal to the X direction. An optical pickup characterized by being formed in a direction and serving as a mounting reference. 6. The optical pickup according to any one of claims 1 to 5, the focal length of the coupling lens: f _X, f _Y are the conditions: 1
An optical pickup characterized by satisfying <f _Y / f _X <5. 7. The optical pickup according to claim 1, wherein the coupling lens is a single lens. 8. The optical pickup according to claim 1, wherein two semiconductor lasers are used, and the two semiconductor lasers have a common objective lens. Is provided in the same laser unit, and a hologram element for diffracting a return light beam is provided between the laser unit and the coupling lens. 9. The optical pickup according to claim 8, wherein the light receiving section of the return light beam detecting means for detecting the return light beam is provided in the laser unit.