JPS63201924A - Optical information pickup device - Google Patents

Abstract

PURPOSE:To stabilize tracking control by setting the direction of the carrier frequency of a hologram element to agree to that of the prescribed parting line of photodetectors and differentially detecting signals from a couple of photodetectors. CONSTITUTION:A titled device is provided with the lens Fourier transform type hologram element 12 which is disposed in an optical path and which has recorded an astigmatic wave surface for beam control, and a couple of photodetectors 11 and 17 which photoelectrically convert a couple of beams diffracted from the hologram element. The direction of the carrier frequency of the hologram element 12 is set to agree with that of the prescribed parting line of the photodetectors 11 and 17, and the signals from a couple of the photodetectors 11 and 17 are differentially detected, whereby they are signal- processed for focusing or tracking. Namely, the direction of a hologram carrier frequency is provided in the direction of the parting line of a couple of four- dividing detectors 11 and 17, and a differential detection is executed even if the oscillation mode of the semiconductor optical source is unstable and a multi spectral oscillation is executed, whereby accurate signals can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a pickup device for recording and reproducing optical information stored on an optical or magneto-optical medium such as an optical disk or an optical card.

Conventional technology Optical memory technology that uses pit-like patterns as a high-density, large-capacity storage medium is being put into practical use with its applications expanding to include digital audio discs, video discs, document file discs, and even data files. . The mechanism by which information is recorded and reproduced successfully with high reliability through a light beam focused on the micron order lies in the structure for picking up the optical information, especially the optical system. The basic functions of the optical information pickup device (hereinafter abbreviated as OPU) are (1)
It is broadly classified into type 2a: light focusing ability to form a diffraction-limited minute spot, (11) focus control and pit signal detection of the optical system, and tracking control of 01D.

These are realized by combining various optical systems and photoelectric conversion detection methods depending on the purpose and use. 7th
The figure is a schematic diagram showing an example of a conventional OPU. Normal T
A diverging wavefront (electric field: water polarization) from a semiconductor laser light source 1 oscillating in E0° mode is converted into a parallel beam by a collimating lens 2, and a left quarter-wave plate (Kλ plate) 4 is converted by a polarizing beam splitter 3. Reflect selectively. The circularly polarized wavefront that has passed through the Kλ plate is condensed into a spot of approximately 1 μm by a condenser lens system 6, reaches the optical disk medium surface 6, and irradiates a bit-like pattern. The light beam reflected and diffracted by the medium surface 6 travels back through the condensing lens system 6 and passes through the quarter-wave plate 4.
When it passes through, it becomes a vertically polarized parallel beam, passes through a polarizing beam splitter 3, and is split into two directions by a prism half mirror 7. One reflected light passes through a condensing lens 9 and a cylindrical lens 10 that imparts astigmatism, enters a four-segment photodetector 11, and is converted into a focus control signal.The other transmitted light is transmitted through a far-field pattern. The signal then enters the two-part photodetector 8 for detecting the tracking control signal.

Here, the λ plate 4 is used in combination with the polarization beam splitter 3 to increase the efficiency of using the amount of light, and at the same time suppresses the return to the semiconductor laser, so that unnecessary noise does not increase in the signal light component. This is a device for this purpose. but,
With the OPU for playback-only discs, there is some leeway in light intensity design.)
, it is possible to omit the λ plate and polarizing beam splitter, especially for miniaturization and cost reduction.

Compounding is being planned.

Problems to be Solved by the Invention However, even in a read-only OPU, it is necessary to construct a beam splitting means, a focus control means using astigmatism or the Knifezi method, and a tracking control means independently or in combination. For this purpose, the optical components conventionally used for this purpose, such as beam splitters, lenses, and prisms, are difficult to fabricate, assemble, and adjust in large quantities, and it is difficult to manufacture, assemble, and adjust them in large quantities. There was a problem〇The common reason why these problems occur is that firstly, optical parts that require a high-precision flat or hemispherical surface require many steps to achieve the desired processing, so pressing means etc. are used. Second, in order to combine a large number of parts to achieve a desired overall performance, assembly and adjustment requires a lot of time and complicated inspection and measurement equipment. 3. Since there is a limit to the miniaturization of components, there are also major restrictions on miniaturization of the entire optical system.

As a method to partially solve these problems, for example, a technique has been developed in which the collimating lens 2 (or 9) shown in FIG. 8 is constructed of a Fresnel lens and press-molded using a mold.

However, this does not reduce the number of parts, and components with a large number of surfaces, such as the prism beam splitter, remain unreplaced. Furthermore, due to the limitations of processing accuracy, the lens 6, which requires the highest performance light focusing, cannot be replaced.

The above-mentioned reason can be solved by introducing an optical element having multiple functions, and an attempt to arrange the hologram element 21 close to the condensing lens 6 as shown in FIG. 8 has recently been reported. There is.

(0) Wood, Ono, Sugama, Otaji 61 Autumn Proceedings of the Japan Society of Applied Physics, 30p-ZE-1, p. 227 (19
86). @) Same: 22nd Micro-Optics Research Group Lecture Paper:
Vol. 4 (1986) p-38) o Conventionally, devices have been created using a wavelength range (λ1: 400 to 500 nm) suitable for hologram recording, and a near-infrared or red laser (λ2: ~800 nm) suitable as an OPU light source.

633 nm), significant aberrations occurred due to the lens action of the hologram, and it was difficult to correct them. Therefore, the hologram element is constructed at two points P4 and 2 using an optical system as shown in the figure. Interference fringes between P2 and reference light source R (
In reality, wavefronts 230 and 23 are on one half of the hologram surface.
1. It is designed based on the concept of a so-called lensless Fourier transform hologram system, in which interference fringes of wavefronts 230 and 232 are formed on the remaining one side by a Mieg surface, and is equivalent to the ``wedge prism method'' or the ``double knife seven-dimensional method.'' In order to achieve this effect, the hologram element 21 is divided into two parts 211 and 212, and is realized by electron beam drawing. By doing this, you can be sure that the design wavelength λ2 of the light source 1 to be used
In this case, it is possible to create an aberration-free hologram lens 21, and even if aberrations due to slight variations in the spectral width of the light source appear on the photoelectric conversion surface of the beam detector (photodetector) 220, the hologram lens 21 can be created without any aberrations. .222,
By using the push-pull method using 223°224, it is possible to suppress fluctuations within a range that does not cause any problems with soil. However, the eighth
In the figure, the pattern of an element 21 that can be drawn with an electron beam is limited to a simple pattern such as a lattice or a hyperbolic shape, and no technique for forming a more general hologram system with high precision is disclosed at all.

The present invention provides a multi-functional hologram element that stably realizes focus and tracking control of an OPU, and also provides a more general hologram element without imposing restrictions such as electron beam lithography or recording/reproducing at a specific wavelength. To make it possible to construct a simplified optical system using a hologram element based on optical principles.

The differences between the conventionally disclosed hologram element for pickup and the hologram element of the present invention will be explained in detail in the following explanation, but here, we will discuss them particularly from the perspective of multiple functions. Let us compare and summarize the limitations of conventional elements and the objectives of the present invention.

0) Although both function as means for separating incident and reflected light, conventional hologram elements use the "wedge prism method" (or "double knife seven edge method") as the light beam used to detect focus errors and tracking errors.
It is also limited to one specific optical system configuration. The present invention focuses on the optical system functions conventionally developed in W4, especially the "astigmatism method".
The purpose is to realize a hologram element that can basically replace the hologram element according to the needs of the pickup design.

(2) Conventional hologram elements were configured as "lensless Fourier transform holograms" that minimized the lens function as focusing power, but there was a slight wavelength deviation from the design wavelength λ of the pickup light source (Δλ = ±20 am). Also, a focus offset occurs, and in order to reduce the wavelength shift due to the loft of the semiconductor laser to 1 gIgM, it is necessary to provide a troublesome process of adjusting the position of the photodetector in the optical axis direction. In the present invention, the hologram element can be designed and manufactured at any wavelength as a lens Fourier transform type that does not impart focusing power to the hologram element, and the position adjustment in the optical axis direction is not required, but the hologram element is based on minute wave fluctuations. This enables output errors to be corrected operationally.

Means for Solving the Problems The optical information pickup device of the present invention includes a light source that emits a coherent beam or a quasi-monochromatic beam in the infrared region or a wavelength region shorter than this region, and a light source that emits a coherent beam or quasi-monochromatic beam in the infrared region or a wavelength region shorter than this region, and a micro spot of the coherent beam or quasi-monochromatic beam. A converging optical system, and a lens Fourier transform that is arranged in an optical path through which the coherent beam or quasi-monochromatic beam is reflected or diffracted by a predetermined optical storage medium, and records an astigmatic wavefront for beam control. hologram element, and a pair of photodetectors each photoelectrically converting a pair of beams diffracted from the hologram element, the carrier wave frequency direction of the hologram element is aligned with a predetermined dividing line direction of the photodetector, and , differentially detecting signals from the pair of photodetectors and performing signal processing for focusing or tracking.

Regarding the characteristics of working lens Fourier transform holograms, please refer to the literature "Kanji memory using holography".

Kato, Fujito, Sato: Japan Society of Image Electronics Engineers Research Conference Proceedings 79-0
4-1 (1979, 11,) ←) “B peakle
r@duction in holography (speckled lida) As detailed and analyzed, results applied to general image recording and reproducing optical systems ((b) "Optical Kanji editing processing system" Sato et al.; Naruko Communication Society research Although there are additional materials, EC78-53 (1978) 47), in the present invention, as long as the beam control means does not cause any hindrance, it is sufficient that Fourier transform is established for the wavefront near the optical axis of the reproducing optical system, and The lens used for wavefront reconstruction can be replaced by a collimating lens, or simply by irradiating the hologram element with a convergent spherical wave,
It is possible to obtain a desired optical axis symmetrical reconstructed image on the light condensing surface.

The present invention solves the following problems by providing the above-described configuration.

- In the process of creating a hologram element, a lens Fourier transform type hologram recording system with an astigmatism wavefront is used using the wavelength λ of a coherent light source suitable for hologram recording. For a given light source wavelength λ2 such as a laser, a pair of desired wavefronts necessary for focus and trapping control can be set on the photoelectric conversion surface by using a converging lens with no aberration or by irradiating an equivalent convergent wavefront onto the hologram element. (G3) Furthermore, as a countermeasure against coherent fluctuations of the light source, the reference wave light source position of the hologram recording optical system is set so that the carrier wave spatial frequency direction of the hologram coincides with the direction of the photodetector area boundary of the photoelectric conversion element. It can be stabilized by setting and outputting the photoelectric conversion signals of a pair of conjugate reconstructed images, and in order to achieve the composite function, For focus control or tracking control, the zero-order transmitted light component from the hologram 4 can also be used for direct tracking control at the far field position.

Embodiment FIG. 1 shows a schematic configuration of an OPU device configured as a model in comparison with FIG. 7 in order to explain the present invention in detail. 1 is a short wavelength semiconductor laser (wavelength λ2 is 800 nm)
, 2 is a collimating lens (focal length f 220m1B)
, 3 is a prism-type polarizing beam sinter, in which all the incident light from the light source is reflected by the λ plate 4 on the left side, and becomes a condensing optical system. focal length f
p=4■), a minute spot of about 1 μmφ is formed on the bit surface (for example, on the bit 60) on the optical disk e. The light beam reflected and diffracted by the bit surface passes through the lens 6 and the λ plate 4 again, and then passes straight through the polarizing beam splitter 3 with the plane of polarization rotated by 90' from the previous optical path. The light is incident on the hologram element 124C. Element 1
2 is a lens 7-Lie transform type hologram with an astigmatic wavefront as described in the first order, and the first-order diffracted light passes through the condenser lens 9.
(Focal length f2 = 2 oss, NA = O-1)
One of the reproduced wavefronts that has been focused or diffracted is subjected to 7-Lier transformation by the lens 9, and a spot image including astigmatism is generated on the photoelectric conversion surface of the quadrant photodetector 110. On the other hand, the 0th order diffracted light transmitted through the element 12 forms a far field pattern on the two-part photodetector a.

The operation and characteristics of the OPU of the present invention, which will be described later as a development of the above configuration, will become clear by explaining and explaining the optical system for manufacturing the hologram element 12.

FIG. 3 is a conceptual diagram of an optical system that is realized as a hologram that can accurately record and reproduce an astigmatic wavefront, which is the first point of the present invention. As shown in figure a, a cylindrical lens 10 (the cylinder axis is in the x2 direction, and x2 is provided parallel to X, which will be described later) is placed in the optical path in which a coherent parallel beam 13 with wavelength λ is focused by a condenser lens 51. However, linear focused beams 101 and 103 oriented in directions perpendicular to each other and a circular beam 102 at an intermediate position are obtained. beam 101
It is also possible to correct the spherical aberration of the cylindrical lens 10 by inserting a slit 1010 (slit width ξ and ξa) in the cylindrical lens 10 . Assume that the center of the beam 102 is located at the front focal plane x, -y of the Fourier transform lens 6o, and at the origin on the coordinate plane. This optical system converts an element similar to that conventionally used to generate astigmatism in an optical pickup optical system into a single hologram element, but the important thing here is that the first Then, the Fourier transform wavefront of the circular beam 102 including the astigmatism is taken to the rear Fourier transform surface of the lens 6° (represented by ξ1" 1 coordinate) via the 7-Lier transform lens 60 (focal length f,). The reference wave diverges from a predetermined position point 16 on the front focal plane of the Fourier transform lens 60. An aberration-free spherical wave is used.The reference wave can be easily obtained by converging the parallel beam 13 and the mutually coherent parallel beam 14 with a lens 16, but here, as a second component, the points 16 and xl -y,
A-A' force direction Y connecting the origin of, angle θ with the axis = 46
The carrier wave frequency direction of the hologram is made to coincide with the photodetector dividing direction, which will be described later. When irradiated with a parallel beam of 1022 are reproduced in a mutually symmetrical positional relationship with respect to the x2-Y2 coordinate point, and linear patterns 1011, 1031 and 1012 in the horizontal (x2 axis direction) or vertical Y2 axis direction are displayed in front and behind each spot image. 10
32 is obtained. Since they are conjugate wavefronts, one has a vertical linear image 1031 close to the lens 6,
On the other hand, horizontal images 1o12 appear side by side.

FIG. 4 explains the operating principle of a model optical system based on the optical system shown in FIG. 3 from the perspective of the beam shape generated on the focus control photoelectric conversion element 11. That is, in FIG. 4a, the single wavelength λ focused by the condensing lens. When the beam 1o21 or 1022 is diffracted through the hologram element 12 (with the optical axis of the lens 90) The conjugated image crystals diffracted in the directions forming the angle β respectively have a shape as shown in the figure or d on the photodetector.
This shows what it looks like when it comes into focus (x2.Y-
Each x2. coordinate axis parallel to Y2). The focus control signal C is
The signal output components corresponding to each of the sectors 111 to 114 of the quadrant photodetector are respectively s4. s2. s3. As s4, t = (S, +83) - (S2 + 84) ・
...O), and focus control can be performed according to the condition of g % O.

The tracking signal T is obtained from T=S2-84 when the tracking servo direction of the optical disk is the x'2-axis direction, but if servo stability is given priority,
As shown in FIG. 6 and 8, detection may be performed from another photodetector using a far field pattern (0th-order diffracted light component of the hologram element 12).

In the above explanation, the astigmatism was caused by a cylindrical lens, but the hologram element may be created using another suitable aspherical element. In addition, in the model optical system, a semiconductor laser is assumed as a light source, and other coherent light sources, such as a He-Ne laser, or light waves based on harmonic generation obtained by guiding a semiconductor laser into a nonlinear medium, or In some cases, a light emitting diode or the like that emits quasi-monochromatic light may be used. Depending on the structure of the semiconductor laser, the oscillation wavelength fluctuates over a spectral width of about ±esnm due to injection current or temperature fluctuation during oscillation. As a countermeasure for such cases, the semiconductor laser oscillation is driven at a high frequency (about several hundred megahertz), so that the spectrum of the light source becomes as shown in Figure 6a.
Stable operation is possible by using a y-symmetric distribution over a plurality of oscillation spectrum widths 1Δλ1 with respect to the center wavelength A0, as shown in FIG. That is, the direction of the carrier wave frequency of the hologram element 12 is shown in FIG. 6, a and d.
As shown in the figure, since the photodetector 110 is aligned with the dividing line 1160, the beam focused on the photodetector 11 is aligned with the detector dividing line 116 in accordance with the wavelength width.
It will be distributed above. On the photodetector 11, there is another dividing line 1 perpendicular to this dividing line 116.
16 are provided. FIG. 6 explains how the beams are focused corresponding to each wavelength component. A spectrum λ as shown in a of the figure. For a pair of light sources that can be regarded as simultaneously oscillating ±Δλ, the focused beam obtained on the photodetector is at the detector dividing line 11 as shown in C when it is in focus.
6 is the "major axis" and the other dividing line 116 is the "minor axis", forming an elliptical pattern, and 1=0 in equation O) is maintained.

Similarly, even if Δλ is 0 for equation (@), there will be no problem in tracking control.

The model optical system as described above was devised in the process of developing the embodiment of the present invention, and based on these operating principles, an embodiment of the present invention can be constructed as shown in FIG. That is, in this embodiment, the diverging spherical wave from the light source 1 is directly incident on the Fourier transform hologram element 12 of the astigmatic wavefront explained in FIG. The reflected and diffracted light from the optical disk passes through the condensing lens 6 and lens 9 again and becomes a convergent spherical wave. Since the hologram element is of the lens Fourier transform type, the carrier wave of the hologram has the effect of deflecting the tip axis at a certain angle with respect to the spherical wave incident on it, thus,
The pair of diffraction components using the convergent spherical wave as reproduction light are successfully reproduced on the photoelectric conversion surfaces of the pair of photodetectors 11 and 17.

Now, FIG. 1 explains the principle of differential photoelectric conversion using a pair of conjugate images incident on a pair of photodetectors 11 and 17. The wavelength component of the light source is now the center wavelength λ. λ before and after. Consider distribution or fluctuation like ±Δ. The figure shows the wavelength λ.

The wavelength component λ is placed on the photodetector 11°17 whose position is adjusted relative to the wavelength component λ. The image position of the astigmatic wavefront (at the time of focus) reproduced by +Δλ1 is explained. In the figure, the shift amount of the center position of each reconstructed image from the intersection of the dividing lines of the four-part photodetector is equal to δ1, the distance between the hologram element 12 and the photoelectric conversion output is δ2, and the period corresponding to the carrier frequency of the hologram element 12 is p. Then, as a design example, d2 = 20jl, p = 0.0
11°J = 800nm = 51X10-'xa,
When Δλ, = 20nm = 2X10-'as, δ1"4
X10 m10 Also, the distance 1 between the centers of the photodetectors is 41 =
2d27 at 2d2sin7 = 1.6js Here, the wavefront (not shown in FIG. 1) diffracted from the hologram element 12 on the outward path is determined by appropriately selecting the distance d1 between the collimating lens 9 and the condensing lens 5. As a result, the amount of light is suppressed to a substantially harmless level.

Further, in the hologram element 12, by finely adjusting the distance d2, the distance between conjugate images can be adjusted instead of finely adjusting the distance between the centers of the photodetectors 11 and 17. In the configuration of the present invention, according to the principle of differential signal detection, the setting of 2 may be approximately based on the wavelength used, but if a light source that differs significantly from the design wavelength is implemented, δ2 may be adjusted and L may be fixed. There is an advantage to leaving it as is.

The differential detection circuit 18 shown in FIG. 〜184, and terminals 181, 182, 183, and 184
If the outputs of are again set to S, ˜S4, beam control equivalent to that obtained using FIG. 6 and equation (1) can be realized. Here, a pair of conjugate images reproduced from the hologram element 12 are
As shown in Figure 3, the defocused images are 1012 and 1031.
The correspondence between the addition areas is determined based on the principle of reproduction in mutually orthogonal directions such as 1032 and 1011. Moreover, in the case of Fig. 5, stable oscillation of the spectral distribution shown in Fig. 5a is assumed, but in this example, any spectral distribution (single, multi) can be used regardless of temperature, current, etc. This has a great effect in that symmetrical and differential photoelectric conversion outputs can always be obtained even if there are spectral fluctuations caused by this.

FIG. 2 shows, as a second embodiment of the present invention, a schematic configuration of an OPU that does not use the single collimator lens 9 compared to FIG. 1a and achieves the same effect with an optical system having fewer parts. However, the numerical aperture of the condensing lens 6 needs to be designed to be slightly larger than that of the lens shown in FIG. Beam 1 obtained by diffracting the beam from the light source 1 by the outgoing telehologram element 12
By appropriately selecting the distance d between the element 12 and the lens 6, the amounts of light 21 and 122 can be made harmless as in the case of FIG. Furthermore, the position where the conjugate image (focused point) reproduced on the return path is incident on the photoelectric conversion surface is on the circular arc 100 having the center on the element 12, and is ±α from the optical axis of the lens 6.
This is the same as in the first embodiment. Since the spread angle of the astigmatic wavefront reproduced from the hologram element 12 is small, good image reproduction is possible without using a special 7-Lier transformation lens.
Of course, image reproduction using a lens is also possible; for example, the first
The configuration can be changed by exchanging the positions of the lens 9 and the element 12 in the figure. Also a pair of photodetectors 11.17
can be integrated and installed close to the light source 1, and can be housed and mounted together with the hologram element 12 in a small and lightweight container, similar to the embodiment shown in FIG. This is based on the effect of the present invention, which eliminates the need for fine adjustment. Another advantage of the present invention regarding mass production is that once recorded, the lattice pattern of the elements is
By transferring it to a mold that is easy to reproduce, and then manufacturing a replica using resin or glass material, a large number of uniform elements can be obtained at low cost, which has a great effect on the design and production of optical information pickup devices. It is something. Since the pattern to be replicated is of the Fourier transform type, it is possible to blaze the mold by ion beam processing, etc., and Fresnel zone plate blaze technology (how many units).

Kubota, Nishida; 18th Micro-Optics Research Conference Lecture Paper; vol.
3 (1985) p, 33), the diffraction efficiency of the replica element can be easily blazed. In other words, the entire surface of the hologram consisting of a substantially parallel lattice pattern can be irradiated with an oblique beam.

The present invention has been described in detail using two embodiments above, but if another photodetector optical system is added to the configuration shown in FIG. 6, differential detection similar to that shown in FIGS. 1 and 2 will be possible. It will be easily understood.

That is, FIG. 6 also constitutes a part of the apparatus configuration according to another embodiment of the present invention.

Although the embodiments of the present invention have been mainly described as beam control systems for optical disk optical systems, it can also be applied to optical systems with both recording and reproducing functions, and furthermore as optical information pickup devices in magneto-optical storage and reproducing systems. Of course. In addition, the recording density of the optical information storage medium is 6 to 1 in bit size.
For optical cards, etc., which require a wavelength of about 0 μm or more, it is also possible to use a quasi-monochromatic light source with a relatively wider spectrum width than conventional semiconductor lasers.

Effects of the Invention As is clear from the above explanation, the present invention uses a lens-Fourier transform disk hologram element in combination with other optical elements in the optical system of the OPU, and the hologram has an astigmatic wavefront. (1) hologram creation wavelength λ,
Even if the wavefront is reproduced at a wavelength λ2 different from Correct control signals can be obtained by installing the split detector in the direction of the split line and performing differential detection. By placing a hologram element in the optical system that returns the hologram, it is possible to effectively divide and use beams for focus control and tracking control from the hologram diffracted light.

(4) Similar to optical disks, hologram elements can be easily produced in large quantities as replicas through a transfer process from a mold that serves as a mask.

(6) uses a lens-Fourier transform hologram that records a parallel grating pattern, so in order to improve the diffraction efficiency of the element, it is possible to perform placement processing using an ion beam, etc. on the entire surface simultaneously, resulting in high performance. A mask hologram mold can be manufactured.

[Brief explanation of the drawing]

FIGS. 1a and 1b are perspective views and principle explanatory diagrams of an optical information pickup device showing an embodiment of the present invention; FIGS. 2a and 2b are
3A and 3B are principle explanatory diagrams and schematic perspective views of an apparatus according to another embodiment of the present invention, and FIGS.
Figure 4a is a perspective view of the main parts showing an example of reproduction, and Figure 4a is a perspective view of a model optical system of the present invention that records and reproduces an astigmatic wavefront.
c and d are conceptual diagrams explaining the state of the beam reproduced on the photoelectric conversion surface, Figure 6 a is a spectrum distribution diagram of the light source, Figure 6 c and d are conceptual diagrams of the reproduced beam, and Figure 6 is A schematic and configuration perspective view of a model optical system for detailed explanation of the present invention,
FIG. 7 is a schematic perspective view showing an example of the configuration of a conventional pickup optical system, and FIGS. 8a and 8b are conceptual diagrams and schematic perspective views of an optical pickup optical system using a conventional hologram element. 1... Semiconductor laser or equivalent coherent light source, 2... Collimating lens, 3...
...Polarizing beam splitter, 4... Quarter wavelength plate, 5... Focusing optical system, 6... Optical disk, 12... Lens Fourier Conversion hologram element, 8... Photodetector, 11, 17...
...Photodetector, 10...Cylindrical lens, 18...Differential detection circuit. Name of agent: Patent attorney Toshio Nakao and 1 other person No. 1
Figure 2 (b) Figure 3 Figure 4 Figure 9 to Figure 5 HI3-←-4λ→ n Figure 6 Figure 7

Claims (1)

[Claims] a light source that emits a coherent beam or quasi-monochromatic beam in the infrared region or a wavelength region shorter than this region; an optical system that converges the coherent beam or quasi-monochromatic beam to a minute spot; A lens Fourier transform type hologram element arranged in an optical path in which a quasi-monochromatic beam is reflected or diffracted by a predetermined optical storage medium and records an astigmatic wavefront for beam control, and a pair of beams diffracted from the hologram element. at least a pair of photodetectors each photoelectrically converting the hologram element, the carrier wave frequency direction of the hologram element is aligned with a predetermined dividing line direction of the photodetector, and signals from the pair of photodetectors are differentially detected. An optical information pickup device that performs signal processing for focusing or tracking.