JPH073704B2 - 2 beam type optical pickup - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Outline] Two grating lenses, a first grating lens that crosses an incident laser beam in an axially symmetrical manner and a second grating lens that focuses the crossed beam on an optical disk, are combined. In this way, the first grating in the newly developed grating lens system, which is capable of achieving a good beam spot with less aberration and less influence of wavelength fluctuation of the incident laser beam and stable focusing performance without deviation. The lens is divided into a plurality of regions having different spatial frequencies by the surface including the optical axis, and the two laser beams are made incident on the grating lens system through the respective first regions of the first grating lens and reflected from the optical disc. The two signal lights thus generated go back in the grating lens system in an axially symmetric manner and go back to the first grating. It is caused to exit from the grating lens system through the second region of each of coating lenses. As a result, the signal light is focused on a position different from the outward path by the second grating lens. Therefore, this signal light is split into two beams by, for example, a half mirror, one of which is guided to the focusing photodetector, and the other of which is guided to the tracking photodetector. It is possible to realize an optical disk pickup device with high detection accuracy and high reliability, which reduces deterioration of the focal beam diameter.

[Industrial application field]

The present invention relates to an optical pickup, and more particularly to a pickup device for a magneto-optical type and a write-once type optical disc using a grating lens system.

[Conventional technology]

In a write-once type optical disk device in which information can be written on the user side of an optical disk, it is necessary to record the information in the form of pits on the disk and then read the information in order to confirm whether the information has been correctly written. A two-beam optical head has been researched in which a reading beam is arranged beside a writing beam spot so that the disk does not have to wait for one revolution.

On the other hand, there is a demand for a drastic reduction in the size and weight of the optical head and a reduction in the price of the optical head in order to reduce the size and weight of the optical disk device and shorten the access time.

In order to satisfy such a demand, development of an optical pickup using a hologram element has been advanced in recent years, and the applicant of the present application has previously described in Japanese Patent Application No. 61-43702, a novel two-beam optical system using a hologram lens. Suggested a pickup.

This two-beam type optical pickup, as shown in FIG.
Two focusing positions P ₁ and P _{1 which} are separated by a predetermined distance (d) in the groove direction of the track on the optical disc medium 107 by correcting the aberrations of the respective laser beams 110 and 111 from the two semiconductor lasers 101 and 102
₂ has two hologram lenses 103 and 104 for focusing respectively, and further has a configuration in which reflected lights 112 and 113 from the focusing positions P ₁ and P ₂ are received by the hologram lenses 104 and 103 opposite to each other and guided to the detectors 105 and 106. . The laser light 110 from the semiconductor laser 101 is focused by the hologram lens 103 at the focus position P ₁ on the optical disc medium surface 107. Then, the reflected light 112 from there is received by the other hologram lens 104 and guided to the detector 105. On the contrary, the laser beam 111 from the semiconductor laser 102 is the hologram lens 10
The reflected light 113 is focused at the focusing position P ₂ at 4 and is received by the opposite hologram lens 103 and guided to the detector 106. At this time, since the wavelengths of the laser lights 110 and 111 are different,
The reflected lights 112 and 113 do not return to the positions of the semiconductor lasers 102 and 101 and are guided to the adjacent detectors 105 and 106.

Thus, since the respective optical paths are separated in space, there is no need for a polarization beam splitter or a λ / 4 plate as in the conventional case, and the optical pickup can be significantly reduced in weight and size.

[Problems to be solved by the invention]

However, in the hologram, the diffraction angle changes with the change of the wavelength, so that the hologram (hologram lens) having a lens function is the case of the in-line hologram lens when the wavelength of the light source changes from λ ₀ to λ (λ ₀ <λ). As shown in Fig. 14, aberrations occur when the focal length (position in the optical axis direction) changes, and in the case of an off-axis hologram lens, changes in focal length and aberrations as shown in Fig. 15 occur. In addition to the occurrence of, the focus position is deviated from the optical axis.

The semiconductor laser used for the optical pickup is ± 12 within the ambient temperature (0 to 50 ° C) due to temperature changes, driving method changes, and variations in the central oscillation wavelength due to individual differences.
The oscillation wavelength is changed by about nm or more. That is, it can be considered that the oscillation wavelength of the laser is substantially constantly changing, and when such a change in the oscillation wavelength of the semiconductor laser occurs, the above-mentioned problem occurs in the optical pickup using the hologram, and the beam spot Deteriorates. Therefore, it becomes difficult to read and write the information of the optical signal. The off-axis type hologram lens shown in FIG. 15 is particularly susceptible to the wavelength change of the semiconductor laser.

By the way, the applicant of the present application has previously found that, in Japanese Patent Application No. 61-220870, even if there is a wavelength variation of incident light as described above, it is not affected by the variation and always obtains a good beam spot without aberration. We have proposed a grating lens optical system that is capable of accurate and stable focusing performance.

The present invention solves the above problems by using this grating lens optical system, satisfies the requirements of light weight, small size, and low cost, and yet has high accuracy and high reliability that is not easily affected by wavelength fluctuation. An object is to provide a pickup device.

[Means for solving problems]

In order to achieve the above object, according to the present invention, two laser beams having different wavelengths from two semiconductor lasers are condensed at different positions on an optical signal recording medium and each reflected beam is received. In a two-beam type optical pickup that records and reads information by means of a first grating lens that crosses the laser beams from the two semiconductor lasers in axial symmetry, and diffracted light that has passed through the first grating lens. A second grating lens for focusing on one point on the optical signal recording medium is arranged on the optical axis, and the first grating lens is divided into at least two regions having different spatial frequencies by a surface including the optical axis, The two laser beams pass through the respective first regions of the first grating lens and are converged on the optical signal recording medium by the second grating lens. The two signal lights that have been swallowed and reflected by the optical signal recording medium are axially crossed by the second grating lens and guided to the photodetector through the respective second regions of the first grating lens. It is a feature of.

[Work]

The divergent light of the semiconductor laser is incident on the first grating lens, diffracted by the first grating lens so as to intersect with the optical axis symmetrically, and then incident on the second grating lens, and then on the optical disc by the second grating lens. Focus without aberration. As will be described later, this grating lens system focuses light on the optical disk without aberration even if the oscillation wavelength of the semiconductor laser changes from the center by at least ± 12 nm.

In this grating lens system, the divergent light from the semiconductor laser is incident on the first region of the first grating lens, is diffracted in the axially symmetric direction according to the spatial frequency of the first region of the first grating lens, and is then diffracted. It is incident on the second grating lens, diffracted according to its spatial frequency, and condensed on the optical disc. The incident optical axis at this time is inclined with respect to the plane of the optical disc. The reflected signal light from the optical disc follows a different path (axial symmetry position) from the incident light and is incident on the second grating lens, is diffracted in the axially symmetric direction according to the spatial frequency thereof, and is reflected by the second grating lens It is incident on the region, diffracted according to its spatial frequency, and is incident on the photodetector. At this time, if the spatial frequency distributions of the first and second regions of the first grating lens are equal, signal light returns to the semiconductor laser and signal detection becomes impossible, but the spatial frequency distribution of the second region is different from that of the first region. Is led to the photodetector.

Therefore, the light emitted from the second region of the first grating lens is split into two by a half mirror, etc., and the reflected light (or transmitted light) is guided to a focusing photodetector to perform focusing control, and transmitted light (or reflected light). An optical pickup can be realized by guiding (light) to a tracking detector to perform tracking control.

As will be described later, the grating lens system is hardly affected by the wavelength variation of the laser beam, and can focus on one point on the optical disk within a certain wavelength range (for example, 830 ± 12 nm).

〔Example〕

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

First, the structure of a grating lens system that plays an important role in the present invention will be briefly described with reference to FIGS. The detailed structure of this grating lens system is disclosed in Japanese Patent Application No. 61-220870.

In FIG. 16, the grating lens system includes the first and second grating lens systems.
In-line type grating lenses 11 and 12 are arranged on the same optical axis (dashed line), and a point P on the optical axis
The spherical wave diverging from the (coherent light source) is diffracted to the optical axis side by the first grating lens 11 and once intersects with the optical axis, and then is focused at a predetermined point Q on the optical axis by the second grating lens 12. It was made to let.

The first grating lens 11 has a predetermined spatial frequency distribution that is rotationally symmetric with respect to the optical axis, and diffracted light from arbitrary two points that are symmetrical with respect to the optical axis intersect on the optical axis. . The second grating lens 12 has a predetermined spatial frequency distribution that is rotationally symmetric with respect to the optical axis, and the crossed diffracted light is focused at one point Q on the optical axis.

Consider the paths of two lights having different wavelengths λ ₀ and λ (λ ₀ <λ) that are incident on one arbitrary point of the first in-line type grating lens in the same direction in the above-mentioned configuration. First, the first in-line type grating lens diffracts the light of wavelength λ at a larger angle than the light of wavelength λ ₀ , and after all of these diffracted lights intersect the optical axis, the second in-line type Reach onto the grating lens. As for the distance of the arrival point of these lights from the optical axis, the light of wavelength λ is farther than the light of wavelength λ ₀ . next,
These lights are diffracted by the second in-line type grating lens. At this time, since the light of wavelength λ is diffracted at a larger angle than the light of wavelength λ ₀ , the distance between the two lights gradually narrows. Iki, and finally meet at one point. Therefore, by giving the two in-line type grating lenses a predetermined spatial frequency distribution, the point where the two lights intersect can be placed at a designated point on the optical axis.

The above can be said for light incident on any point of the first in-line type grating lens, and since the spatial frequency distribution is rotationally symmetric with respect to the optical axis, the wavelength of incident divergent spherical wave light changes. Even if it is done, it is focused on the above-mentioned predetermined one point on the optical axis, so that aberrations and focal position shifts do not occur.

Next, a specific method of determining the spatial frequency distribution of the grating lenses 11 and 12 will be described below with reference to FIG.
This will be described in (i) to (iv). The distance between the point P and the first grating lens 11 is l1, two grating lenses
The distance between 11, 12 is d, and the distance between the grating lens 12 and the point Q is l2.

(I) First, consider a ray having a wavelength λ ₀ that emits a point P and reaches a point R1 on the outermost periphery of the grating lens 11. This ray is diffracted at the point R1, reaches the center point r1 (= 0) of the grating lens 12, and is further diffracted there to reach the point Q (solid line a in FIG. 17). Then, by assuming the above-mentioned optical path (P → R1 → r1 → Q), the point R1, r
The spatial frequencies F1, f1 at 1 are determined.

(Ii) Next, consider the case where the wavelength changes from λ ₀ to λ (> λ ₀ ). A ray of wavelength λ that has traveled from point P to point R1 is diffracted at point R1 at a larger angle than when the wavelength is λ ₀ and reaches point r2 on grating lens 12 (broken line b). Here, the spatial frequency f2 at the point r2 is determined under the condition that the light is focused on the point Q even when the wavelength is λ.

(Iii) Returning to the case where the wavelength is λ ₀ , it is possible to reversely determine from which point on the grating lens 11 the ray that is diffracted at the point r2 and reaches the point Q comes from (solid line c). If the point on the grating lens 11 is R2, the spatial frequency F2 at the point R2 is determined under the condition that the diffracted light at the point R2 reaches the point P.

(Iv) Considering the case where the wavelength becomes λ again, the point r3 (not shown) on the grating lens 12 and its spatial frequency f3 are obtained in the same manner as (ii) above. Then, the wavelength is returned to λ ₀ and the grating lens 1 is used in the same manner as in (iii) above.
Find the point R3 (not shown) on 1 and its spatial frequency F3.
In this way, by repeating the above steps (ii) and (iii) until the point Rn (n = 1,2,3, ...) Reaches the center of the grating lens 11,
The radial spatial frequency distribution at 2 is determined. The diameter of the second grating lens 12 is determined by the position of the point rn.

By determining the spatial frequency distributions of the grating lenses 11 and 12 as described above, the light emitted from the point P is
Even if the wavelength λ is different from the reference wavelength λ ₀ , it can be focused on the point Q without aberration.

The present invention realizes an optical pickup device by using the above-mentioned grating lens optical system, and an embodiment of the present invention will be described with reference to FIG.

According to the present invention, for each beam 110, 111 (Fig. 13), the first grating lens 11 is divided into two by the plane containing the optical axis as shown in Figs. This is the third and fourth
This will be described in detail with reference to the drawings. In FIG. 3, the first grating lens 11 and the second grating lens 12 each have two regions having different diffraction angles, that is, different spatial frequencies.
It is divided into 11A ₁ , 11B ₁ , 12A ₁ and 12B ₁ . The first and second regions 12A ₁ and 12B ₁ of the second grating lens 12 are divided into first and second regions for convenience according to the first grating lens 11, and the first and second regions 12A 1 _The spatial frequencies of ₁ and 12B ₁ may be the same. That is, the second grating lens 12 does not necessarily have to be divided into two regions (the reason will be described later).

When the beam 110 is applied from the first semiconductor laser 101 to the first region 11A of the first grating lens 11 of the grating lens system configured as described above at an angle inclined with respect to the optical axis Z (FIG. 3), The beam is bent so as to intersect in an axially symmetric manner and is incident on the first region 12A ₁ of the second grating lens 12. Then, this beam is further bent by the second grating lens 12, and the optical disc 107
It can be converged to one point O above. The spatial frequencies of the first and second grating lenses 11 and 12 are designed so that the laser beam from the semiconductor laser 101 follows the above optical path.
The beam 112 obliquely incident on the point O is reflected there in the axially symmetric direction, and the second region 12 of the second grating lens 12 is reflected.
It is incident on B ₁ . Then, it is bent in a completely symmetrical manner with respect to the outgoing beam 110, and the second region of the first grating lens 11 is bent.
It is incident on 11B ₁ .

Here, if the first region 11A ₁ and the second region 11B _{1 of} the first grating lens 11 have the same spatial frequency, the reflected beam (signal light) 112 incident on the second region 11B ₁ of the first grating lens 11 Returns to the semiconductor laser 101, and the signal light cannot be detected. Therefore, the second region 11B ₁ of the first grating lens 11 is designed to have a spatial frequency different from that of the first region 11A ₁ . As a result, the signal light 112 incident on the second region 11B ₁ of the first grating lens 11 is located at a position different from that of the semiconductor laser 101.
Projected to 5. The spatial frequency of the second region of the first grating lens 11 is determined according to the position of the photodetector 105.

As is apparent from the above description, in the second grating lens 12, the forward beam 110 and the reflected beam 112 may be completely axisymmetric, and therefore the first and second regions of the second grating lens 12 may have the first and second regions. The part through which the forward beam 110 passes is named as the first region and the part through which the reflected beam 112 passes is named as the second region in accordance with one grating lens, but the same grating lens having exactly the same spatial frequency may be used.

The above is the second beam 1 from the second semiconductor laser 102.
The same applies to 11 as well. That is, as shown in FIG. 5, the first grating lens 11 has an optical axis Z (FIG. 3).
First and second regions 11 having different spatial frequencies due to a plane including
It is divided into A ₂ and 11B ₂ , and the second grating lens 12 is also divided into the nominal first and second regions 12A ₂ and 12B _{2 accordingly} . The signal light 113 reflected at the point O ′ of the optical disc 107
Is the second region 11B ₂ of the first grating lens 11 through the second region 12B ₂ of the second grating lens 12
It is bent toward the second photodetector 106 provided at a position different from the semiconductor laser 102.

As described above, the greatest feature of the present invention resides in that in the two-beam optical pickup, the beam convergence of the beam 111 and the beam 112 and the signal light detection follow different paths.

As can be seen from the above description, the subscript A corresponds to the forward beam and the subscript B corresponds to the reflected beam. Also, subscript 1
Corresponds to the first grating lens 11, and the subscript 2 is the second
Corresponds to the grating lens 12 respectively. Therefore, for example, 11A ₁ is the forward beam 11 of the first grating lens 11.
It indicates the first area through which 0 passes.

1 and 2 show the basic structure of the present invention based on the above basic concept.

FIG. 1 shows a structure for outgoing beams 110 and 111 from two semiconductor lasers 101 and 102, and FIG.
The configuration for the reflected beams 112, 113 from is shown.

Generally, as a method of writing and reading information to and from an optical disc by a two-beam type optical pickup, a wavelength λ _{1 is used.}
Writing, erasing, and reading (reproducing) with the second beam of wavelength λ ₂ after the distance d in the track direction.
And check if the writing was done correctly. In the case of reading only, any wavelength can be used. Therefore, if the horizontal direction (x direction) of the paper surface in FIG. 1 is the track groove direction, the first beam 110
The focal point on the optical disk for the second beam 111 is O, and the focal point on the optical disk for the second beam 111 is designed to be a point O ′ deviated from the point O by d in the track direction. d
Is generally d = 10 to 20 μm.

Therefore, for the first beam 110 (λ ₁ ) with respect to the center of division (corresponding to O) of the first and second regions 11A ₁ and 11B ₁ of the first grating lens 11 passing through the optical axis Z, the second beam 111 (λ
₂ ) for the first grating lens 11 for the first and second
The division center (corresponding to O ') passing through the optical axis Z'of the area is shifted by d in the track direction (x direction) of the optical disc. This positional relationship as seen from a plan view is shown in FIG. Incidentally, in FIG. 6, the first and second regions 1 for the second beam 111 are shown.
1A ₂ and 11B ₂ are applied to the first beam 110 and the first and second regions 11A ₁ and
Although the arrangement is orthogonal to 11B ₁ , it is sufficient if the division centers O and O ′ are deviated by d in the track direction (x direction), and it is not always necessary to make them orthogonal.

The above shift arrangement in the track direction is exactly the same for the second grating lens 12.

At the time of actual manufacturing, an appropriate mask or the like is used on the substrate on which the first grating lens 11 is formed and the four regions 11 are formed.
A ₁ , 11B ₁ , 11A ₂ , 11B ₂ may be formed in an arrangement as shown in FIG. 6, or four grating lenses corresponding to 11A ₁ , 11B ₁ , 11A ₂ , 11B ₂ are shown in the same plane. You may arrange like this. The same applies to the second grating lens 12. First areas 11A ₁ and 12A ₁ and second areas 11B ₁ and 12B ₁
Is designed with a center wavelength of 830 nm, and the spatial frequencies of the first areas 11A ₂ and 12A ₂ and the second areas 11B ₂ and 12A ₂ are designed with a center wavelength of 780 nm, for example.

If the first beam 110 and the second beam have the same wavelength, the first region 11A ₁ and the second beam 1 for the first beam 110
The spatial frequency of the second region 11A ₂ for 11 may be the same,
Spatial frequencies of the second region 11B ₂ for the second region 11B ₁ and the second beam 111 with respect to the first beam 110 similarly may be the same.

In FIG. 1, the first divergent light 110 (λ ₁ ) from the first semiconductor laser 101 is incident on the first region 11A ₁ of the first grating lens 11 at a predetermined angle and is diffracted in the axially symmetric direction according to its spatial frequency. First of the second grating lens 112
It is incident on the area 12A ₁ . Then the second grating lens
The light is diffracted according to the spatial frequency of the first region 12A ₁ of 12 and is focused on the first focus O of the optical disc 107. In the same manner, the second divergent light 111 (λ ₂ ) from the second semiconductor laser 102 passes through the first region 11A ₂ of the first grating lens 11 and the first region 12A ₂ of the second grating lens 12 of the optical disc 107. The light is focused on the second focus O '.

In FIG. 2, the focused light from the first semiconductor laser 101 obliquely enters the first focus O of the optical disc 107 as described above, and is reflected there to become the reflected signal light 112. The signal light follows another path that is axially symmetric with the focused light and is incident on the second region 12B ₁ of the second grating lens, is diffracted in the axially symmetric direction according to the spatial frequency thereof, and is incident on the second region 11B ₁ of the first grating lens 11. It is incident, diffracted according to its spatial frequency, and guided to the first photodetector 105. Second semiconductor laser 1
The focused light 111 (λ ₂ ) from 02 is reflected by the second focus O ′ of the optical disc 107 to become reflected signal light 113. Signal light 113
Is incident on the second region 12B ₂ of the second grating lens,
The light is diffracted in the axially symmetric direction according to the spatial frequency, enters the second region 11B ₂ of the first grating lens 11, is diffracted according to the spatial frequency, and is guided to the second photodetector 106.

Next, specific track error and focus error detection will be described.

Although the wavelength of the beam from the semiconductor laser changes substantially constantly, the aberration caused by the wavelength change can be absorbed as described above by using the grating lens system.

As is well known in the optical disk device, if there is a difference in the intensity of the reflected light from the pits on the optical disc 107 and the intensity of the reflected light from the lands, the intensity of the totally reflected light that interferes is weakened according to the degree of the difference. Since there is no such interference in the non-land area, the reflected light intensity cannot be weakened.By using the beam spot narrowed to near the diffraction limit, the reflected light intensity can be detected to determine the presence or absence of pits. It detects an optical signal.

FIG. 7 shows a signal detection method during information reproduction (second semiconductor laser 102). Second region 11 of first grating lens 11
The diffracted light (signal light) 113 diffracted by B ₁ passes through a so-called polarization beam splitter hologram (PBS hologram) 27 having polarization characteristics, where the magneto-optical signal is converted into a light amount difference signal.

That is, the PBS hologram 27 limits the amount of transmitted light according to its diffraction angle, guides only the transmitted light to the photodetector, and detects the magneto-optical signal by the difference in the amount of light, that is, the intensity of the amount of light.

The transmitted light (error detection light) of the PBS hologram 27 is separated into two by a half mirror 29, one of which, for example, the transmitted light is guided to a focusing photodetector 37, and the other, reflected light is a tracking photodetector 39. Lead to.

The focusing photodetector 37 and the tracking photodetector 39 are composed of, for example, a two-division or four-division photodetector composed of a divisional PIN photodiode known per se.

The presence or absence of pits on the optical disk, that is, the signal of the channel bit is discriminated by the strength of the output of these divided photodetectors.

FIG. 8 (1) shows a photodetector 39 for tracking formed by 2
An example of the arrangement of the split photodetectors is shown. For example, in a magneto-optical disc, tracking is performed by a guide groove of the optical disc. The split surfaces of the two-divided photodiodes 39a and 39b constituting the tracking photodetector 39 are located parallel to the guide groove of the optical disc 107, that is, the track direction (x direction). In FIG. 8 (1), when the beam focused on the magneto-optical disk 107 is located at the center of the guide groove, the beam spot is the photodiode 39a, as shown in FIG. 8 (1) (a). It is located at the center of the dividing plane of 39b. Incidentally, (b)
Shows the case where the beam is shifted to the right side (y direction) of the guide groove. In this case, the amount of light received by one photodiode 39b becomes partly dark, and conversely, the beam is on the right side of the guide groove (-y
If it is shifted in the direction), the amount of light received by the other photodiode 39a becomes partly dark as shown in (c). Since the laser beam is crossed axially symmetrically in the grating lens system, the dark portion due to the positional deviation of the aberration beam is inverted and appears in the vicinity of the split surface of the photodiodes 39a and 39b. The tracking error detection method shown in FIG. 2 uses the push-pull method for detecting the light amount difference between the two photodiodes 39a and 39b.

FIG. 8 (2) shows an example of the arrangement of the four-division photodetectors that form the focusing photodetector 37. The four-division photodetector 37 is composed of PIN photodiodes 37a, 37b, 37c, 37d whose light-receiving areas are divided into four. The four-division photodetector 37 is shown in (a) of FIG. 8 (1). As shown, the photodiodes 37a, 37b, 37c, 37d are arranged so that the beam spots strike the centers of the photodiodes 37a, 37b, 37c, 37d in the focused state. 4 when the optical disk 10 approaches the second grating lens 12 from the in-focus position
The spot of the signal light on the split photodetector 37 becomes a horizontally long ellipse as shown in FIG. 8 (1) (b), and conversely becomes a vertically long ellipse as shown in (c).

Therefore, the four regions 37a, 37b, 37c, 37 of the four-division photodetector 37
If the difference in the optical output of d, that is, (I1 + I2)-(I3 + I4) is measured, it is "0" in the focused state, <0 when the optical disk is close,
When it goes away, it becomes> 0. Therefore, focusing can be performed by vertically controlling the optical system by an actuator (not shown) so that the error signal (I1 + I2)-(I3 + I4) becomes zero. This focusing control is based on the astigmatism method utilizing the astigmatism generated by the half mirror 29.

FIG. 9 shows another embodiment of the signal detection method during information reproduction. In this embodiment, the reflected light from the half mirror 29 is the focusing detection light, and the transmitted light from the half mirror 29 is the focusing detection light. The tracking detector 39 is a push-pull method using a two-division photodetector as in the case of FIG. FIG. 10 (1) shows on-track, and FIGS. 10 (b) and 10 (c) show the case where the beams are deviated in the -y direction and the y direction, respectively. Also in this case, the dividing line is installed so as to be parallel to the track. FIG. 10 (2) shows the light distribution in the photodetector 37 for focusing. In the second embodiment, focusing uses the knife edge method known per se using the knife edge 36. 10th
(A) of FIG. (2) shows on-focus, (b),
(C) shows the case where the disks are near and the case where the disks are far. The edge of the knife edge 36 is arranged so as to be on the x axis, for example. The focusing photodetector 37 is of a two-division type and is installed so that the division line is parallel to the knife edge.

FIG. 11 shows a signal detection method at the time of recording / erasing information (using the first semiconductor laser 101). In the case of a two-beam type optical pickup, it is said that signal detection at the time of recording / erasing information is sufficient if only tracking is performed. Therefore, the light emitted from the second area 11B ₁ of the first grating lens 11 (signal light)
It is not necessary to divide 112 into two by a half mirror, and it is sufficient to directly lead to the photodetector 37 'for tracking. Therefore,
In this case, the tracking photodetector 37 'corresponds to the photodetector 105 shown in FIG. Photodetector 37 'for tracking
Is composed of a two-divided photodetector exactly like the photodetector 37 for tracking shown in FIG.
It is exactly the same as Fig. 10 (1).

FIG. 12 shows an example of a specific structure of the optical pickup according to the present invention. In this embodiment, the first grating lens
11 and the second grating lens 12 are arranged orthogonally, and a bending mirror 51 is interposed between them to reduce the thickness of the entire device.

Incidentally, T indicates a track T on the optical disc 107.

Incidentally, the adjustment of the optical system when a focusing error or a tracking error is detected by the focusing photodetector or the tracking photodetector, for example, the entire optical system is adjusted with respect to the optical disc 107 by an appropriate actuator (not shown). It can be performed by moving in a predetermined direction.

In each of the above embodiments, the three photodetectors are not limited to the above single, two-divided or four-divided photodiodes, and it goes without saying that other photodetectors can be used.

〔The invention's effect〕

As described above, according to the present invention, by using the grating lens optical system which is less susceptible to the influence of the change in the oscillation wavelength and has less aberration, it is difficult to cause the focal position shift and the deterioration of the focal beam diameter. It is possible to realize a high-performance two-beam optical pickup device that is highly reliable in operation, lightweight, compact, and inexpensive.

[Brief description of drawings]

FIGS. 1 and 2 are diagrams showing a basic configuration of a two-beam type optical pickup device according to the present invention. FIG. 1 shows a case of beam convergence, FIG. 2 shows a case of signal light convergence, and FIG. FIG. 4 is a diagram showing a basic concept of the present invention, FIG. 4 is a diagram showing a basic configuration of the present invention for a first beam, FIG. 5 is a diagram showing a basic configuration of the present invention for a second beam, and FIG. FIG. 7 is a diagram showing a geometrical plane layout relationship between the first beam and the second beam in the first grating lens, FIG. 7 is a diagram showing a signal detecting method at the time of reproducing information, and FIG. 7
FIG. 8 is a diagram showing three types of beam detection states of the two-division type photodetector for tracking, and FIG. 8 (2) is a diagram of four-division photodetector constituting the photodetector for focusing shown in FIG. FIG. 9 is a diagram showing three kinds of beam detection states, FIG. 9 is a diagram similar to FIG. 7 showing another embodiment of the present invention, and FIG. 10 (1) is a two-division type tracking light shown in FIG. FIG. 10 is a diagram showing three types of beam detection states of the detector, FIG. 10 (2) is a diagram showing three types of beam detection states of the two-division type photodetector for focusing shown in FIG. 9, and FIG. 11 is information. FIG. 12 is a diagram showing a signal detecting method such as writing and erasing, FIG. 12 is an illustrative view showing one example of a concrete structure of an optical pickup according to the present invention, and FIG. 13 is a two-beam hologram pickup disclosed in the applicant's prior application. Fig. 14 shows the basic configuration of Fig. 14 shows the defocus of the inline hologram lens. FIG. 15, FIG. 15 is a diagram showing a defocus of an off-axis hologram lens, FIG. 16 is a diagram showing a basic configuration of a grating lens optical system used in the present invention, and FIG. 17 is a grating shown in FIG. The figure explaining the determination method of the spatial frequency of a lens. 11 ... 1st grating lens, 12 ... 2nd grating lens, 37 ... Focusing photodetector, 39 ...
… Tracking photo detector, 101,102 …… Semiconductor laser, 105,106 …… Detector, 110,111 …… Laser light, 112,11
3 …… Reflected light, 11A ₁ , 11A ₂ …… First area, 11B ₁ , 11B ₂ ……
Second area.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Hiroyuki Ikeda 1015 Kamiodanaka, Nakahara-ku, Kawasaki City, Kanagawa Prefecture, Fujitsu Limited (72) Inventor, Yushi Inagaki 1015, Kamiodanaka, Nakahara-ku, Kawasaki City, Kanagawa Prefecture, Fujitsu Limited

Claims (1)

[Claims] 1. Two laser lights (110, 111) having different wavelengths from two semiconductor lasers (101, 102) are condensed at different positions (O, O ') on an optical signal recording medium (107), respectively. In a two-beam type optical pickup that records and reads information by receiving reflected light (112, 113), the first laser beam (110, 111) from the two semiconductor lasers (101, 102) is crossed axially symmetrically. A grating lens (11) and a second grating lens (12) for focusing the diffracted light transmitted through the first grating lens on one point on the optical signal recording medium (107) are arranged on the optical axis, and The first grating lens has at least two regions (11A ₁ , 1
1B ₁ , 11A ₂ , 11B ₂ ) and the two laser beams are divided into the first regions (11A ₁ , 11A ₂ ) of the first grating lens.
The two signal lights which have passed through the second grating lens and are converged on the optical signal recording medium by the second grating lens, and which are reflected by the optical signal recording medium are crossed axially symmetrically by the second grating lens so as to cross each other of the first grating lens. 2 area (11B _1, 11B ₂₎ 2-beam optical pickup, wherein the guided as light detector (37,39,37 ') a.