JPH0727653B2 - Optical pickup device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a pickup device for recording / reproducing optical information stored on an optical or magneto-optical medium such as an optical disc or an optical card.

2. Description of the Related Art Optical memory technology that uses a pit-shaped pattern as a high-density and large-capacity storage medium has been implemented while expanding its applications with digital audio disks, video disks, document file disks, and even data files. . The mechanism by which recording / reproducing of information is successfully performed with high reliability via a light beam focused on the order of micron is due to the structure for picking up the optical information, especially the optical system. The basic functions of an optical pickup device (hereinafter abbreviated as OPU) are (i) light-collecting ability to form a minute spot of diffraction limit, (ii) focus control and pit signal detection of the optical system, and (iii) same function. There are three types of tracking control. These are realized by a combination of various optical systems and photoelectric conversion detection methods according to the purpose and application. Figure 7 shows the conventional OPU
It is a schematic diagram which shows an example. Divergence front (electric field: horizontal polarization) from the semiconductor laser light source 1 that normally oscillates in ET ₀₀ mode
Is made into a parallel beam by the collimator lens 2 and is selectively reflected by the polarization beam splitter 3 to the left quarter wavelength plate (1 / 4λ plate) 4. The circularly polarized wave front that has passed through the 1 / 4λ plate is focused by the condenser lens system 5 into a spot of about 1 μm, reaches the optical disk medium surface 6, and irradiates a pit-shaped pattern. The light beam reflected and diffracted by the medium surface 6 goes through the condensing lens system 5 again in the reverse direction and passes through the quarter-wave plate 4 to become a vertically polarized parallel beam, which passes through the polarization beam splitter 3 and becomes a prism. It is divided into two directions by the half mirror 7.
One of the reflected lights passes through a condenser lens 9 and a cylindrical lens 10 which imparts astigmatism, and a four-division photo detector 11
And is converted into a focus control signal. The other transmitted light enters the two-divided photodetector 8 for tracking control signal detection while keeping the far field pattern.

Here, the 1/4 λ plate 4 is used in combination with the polarization beam splitter 3 to improve the utilization efficiency of the light quantity and at the same time suppress the returning to the semiconductor laser and increase unnecessary noise in the signal light component. It is a device for not doing it. However, in the OPU of a read-only disc, there is a margin in the light quantity design, and it is possible to omit the 1 / 4λ plate and the polarization beam splitter. In particular, in order to reduce the size and cost, parts are omitted and combined. Is being measured.

Problems to be Solved by the Invention However, even in the reproduction-only OPU, it is necessary to configure the beam splitting means, the focus control means using astigmatism or the knife edge method, and the tracking control means independently or in combination. For this reason, it is not easy to mass-produce, assemble, and adjust the beam splitter, lens, prism, and other optical components that have been used in the past, and in terms of downsizing, cost reduction, mass productivity, and high reliability. There was a problem.

As a common reason for these problems, firstly, since optical parts that require a highly accurate flat surface or aspherical surface can be processed as desired only after many steps, production is generally performed using a pressing means or the like. It is difficult, secondly, it takes a lot of time for assembly and adjustment and complicated inspection and measurement equipment in order to combine a large number of parts to exert a predetermined total performance, and thirdly, the small size of the parts. Since there is a limit to miniaturization, there is a big restriction on downsizing of the all optical system.

As a method of partially solving these problems, for example, a technique has been developed in which the collimator lens 2 (or 9) shown in FIG. 7 is composed of a Fresnel lens and is press-molded using a mold. However, this does not reduce the number of parts, and the prism type beam splitter, which is a part having a larger number of surfaces, is left unreplaced.

It has been recently reported that an attempt is made to dispose the hologram element 21 as shown in FIG. 6b close to the condenser lens 5, assuming that the above reason can be solved by introducing an optical element having a composite function. ((1) Kimura, Ono, Sugama, Ota; Autumn 1986 Proceedings of the Japan Society of Applied Physics, 30P-ZE-1, P.227.
(1986). (2) Same; 22nd Micro Optics Research Conference Lecture Paper; v
ol.4 (1986) P.38) Conventionally, elements were created in the wavelength range (λ ₁ : 400 to 500 nm) suitable for hologram recording, and a near infrared or red laser (λ ₂ : to 800 nm, suitable as an OPU light source was prepared. 633n
When reproduced at m), a remarkable aberration was generated due to the lens action of the hologram, and it was difficult to correct the aberration. Therefore,
The hologram element uses an optical system as shown in FIG. 1A to form interference fringes between two points P ₁ and P ₂ and the reference light source R (actually, the wavefronts 230 and 231 are on one side of the hologram surface, and the remaining one side is on the other side). Wavefront 230 and
It is designed based on the concept of a so-called lensless Fourier transform hologram system in which interference fringes with 232) are formed on the ξ-η plane, and it has an effect equivalent to that of the “wedge prism method” or “double knife edge method”. Hologram element
21 is divided into two parts 211 and 212, and is realized by electron beam drawing. By doing so, it is possible to create a hologram lens 21 without aberration only when the design wavelength λ ₂ of the light source 1 is used, and moreover, the aberration with respect to a slight fluctuation of the spectral width of the light source is a beam detector (photodetector).
Even if it appears on the photoelectric conversion surface of 22, the four-division photoelectric conversion surface 221,
With the push-pull method using 22,223,224, it is possible to suppress fluctuation within a range that does not hinder practical use. However, in FIG. 11, the pattern of the element 21 capable of electron beam drawing is
It is limited to the case of a simple pattern such as a lattice or a hyperbolic shape, and a technique for forming a more general hologram system with high accuracy is not disclosed at all. In addition, the conventional hologram element
Although it was configured as a "lensless Fourier transform hologram" that suppressed the lens function as focusing power as much as possible, a focus offset occurred even for a slight wavelength deviation (Δλ = ± 20 nm) from the design wavelength λ of the pickup light source. In order to adjust the wavelength shift depending on the lot of the semiconductor laser, it is necessary to provide a troublesome process of adjusting the position of the photodetector in the optical axis direction.

The present invention firstly provides a multi-functional hologram element that stably realizes focus and tracking control of the OPU, and is more general without imposing restrictions such as electron beam drawing and recording / reproducing at a specific wavelength. A stable and simplified optical system can be constructed by using a hologram element based on the conventional optical principle. In that case, in the present invention, a hologram element is designed at an arbitrary wavelength as a lens Fourier transform type in which focusing power is not applied to the hologram element.
It can be manufactured, and the position adjustment in the optical axis direction is unnecessary.

Secondly, the present invention provides an optical system that suppresses a coherence reaction of a light source necessary for stable operation of a thin film element such as a hologram element or a Fresnel lens within a range that does not hinder practical use. As a basis, the desired coherent beam and high signal-to-noise ratio can be obtained without making the optical system complicated and expensive.

Means for Solving the Problems In order to solve the above-mentioned problems, the present invention comprises (i) a semiconductor laser, a collimating optical system, and a diffractive element, which constitutes a light source optical system having optical feedback. Stabilize the coherence, and (ii) guide the non-returned beam of the diffracted light from the diffractive element to an optical system that converges in the form of a minute spot on a predetermined storage medium, and for example, a focusing and tracking beam control optical system for the minute spot. Stable signal detection is possible by connecting to (ii
i) Desirably, a light source system and a beam control optical system are optically completely separated by using a 1/4 wavelength plate and a diffraction element provided with a polarization selective reflection film, and more stable signal detection can be realized. iv) The optical system can be simplified by using a lens-Fourier conversion hologram element together.

Action For the characteristics of the lens-Fourier transform hologram, refer to (3) “Kanji memory by holography”, Kato, Fujito, Sato; IEICE Technical Committee 79-04-1 (197).
9.11.) (4) “Speckle reduction in holography…
… ", M.Kato et al; Applied Optics (Appl.Op
t.), 14 (1975) 1093), etc., and have been applied to recording / reproducing optical systems for general images as detailed and analyzed ((5) “Optical Kanji editing processing system” Sato et al .; Electronics) The Institute of Communication Engineers, EC78-52 (1978) 47) is included, but in the present invention, as long as there is no practical problem as a beam control means, it is sufficient that the Fourier transform is established for the wavefront near the optical axis of the reproduction optical system. The lens used for reproducing the wavefront from the element can be replaced with a collimating lens, or a desired reproduced image can be obtained on the converging surface by simply irradiating the hologram element with a convergent spherical wave.

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

(1) In the process of producing a hologram element, a lens Fourier transform type hologram recording system having an astigmatic wavefront or a predetermined wavefront such as a knife edge optical system using a wavelength λ ₁ of a coherent light source suitable for hologram recording is used. (2) In the OPU optical system, a focusing lens with almost no aberration for a given light source wavelength λ ₂ such as a semiconductor laser is used, or the equivalent converging wavefront is applied to the hologram element for focus and tracking control. A desired desired wavefront can be formed on the photoelectric conversion surface (photodetector). (3) Furthermore, as a measure against coherent fluctuations of the semiconductor laser light source, the diffraction grating surface of the diffraction element is arranged with an inclination with respect to the collimated beam. A part of the diffracted wave is fed back to the semiconductor laser, and a non-feedback light component,
For example, the zero-order diffracted light is guided to the converging optical system by an optical system configuration, and if necessary, a 1/4
By providing the wave plate, it is possible to suppress the generation of return light noise. Here, the diffraction grating also plays the role of a beam splitting means. Further, in order to exert the composite function, (4) one reproduction image from the hologram element, for example, a convergent beam having astigmatism is used for focus control, and The 0th-order transmitted light component from the hologram can be directly used for tracking control at the far field position. (5) The back surface of the hologram element substrate can also be used as a reflection surface, and a multilayer dielectric for polarization plane selection. Since it is easy to form a thin film, by applying a polarization plane selective film on the diffractive element described in (3), the polarization beam splitter, the optical feedback beam spectrum separation, and the beam control can be performed with one composite diffractive element. The function of the optical element can be combined.

Embodiment FIG. 1a shows a schematic configuration of an OPU device according to an embodiment of the present invention. 1 is a short wavelength semiconductor laser (wavelength λ ₂ 800n
m), 2 is a collimating lens (focal length f ₂ 20mm), 30
Is a diffractive element (diffraction grating) having a grating spacing d = λ ₂ / sin 45 ° 1.131μ in one design example with respect to the wavelength λ ₂ of the semiconductor laser, the surface of which is coated with a polarization selective reflection film. The polarization selection film is a dielectric multilayer film used for the polarization beam splitter,
You may substitute by coating in a semi-transparent state such as Au and Al. In that case, it is advantageous in terms of diffraction efficiency to make the collimated light incident on the diffraction element 30 at an angle close to the total reflection angle. The parallel beam incident through the collimator lens 2 is diffracted by the diffractive element 30, but a part of the first-order diffracted light (or higher-order diffracted light) is reflected by the polarization selection film and moves backward in the collimator optical system to travel through the semiconductor laser. It is returned to 1 as a return beam. On the other hand, a non-returned beam, for example, the 0th-order diffracted light travels as it is, passes through the 1/4 wavelength plate 4, and is a circularly polarized beam in the state of a condenser lens 5 (numerical aperture NA = 0.5, focal length f = 4 mm) of about 1 μm A minute spot is formed on the pit surface of the optical disk 6 (for example, on the pit 60). The light beam reflected and diffracted by the pit surface is again lens 5, 1 / 4λ
After passing through the plate 4, the polarization plane is rotated by 90 ° with respect to the forward optical path, the light beam is directly transmitted through the polarization beam splitter (diffraction element) 30, and is split by the beam splitter 7. The far-field pattern of the spot is placed on the photodetector 8, and the other split beam is passed through a condenser lens 9 and a cylindrical lens 10 to a photodetector 11
An astigmatism pattern is formed on the above and converted into tracking and focusing control signals. At this time, the diffraction element 30 is provided with the diffraction grating 301 and the polarization selective reflection film 302 on one surface of the substrate 300 as shown in FIG.
(I) Wavelength selective optical feedback optical element (diffraction grating) and (ii)
It plays the role of a multi-functional device that also has a polarization selective reflection film (polarization beam splitter). Further, an antireflection film 100 for the oscillation wavelength is provided on one side of the semiconductor laser,
The semiconductor laser is an external resonator type and forms a resonator system between the end face 101 and the diffraction grating surface 301 to perform narrow band oscillation. Although the antireflection film is not always necessary, more stable oscillation can be obtained by designing the reflectance to be several% or less. Regarding external cavity type semiconductor laser, refer to (6) “Oscillation Center Frequeney Tuning, Quan
tum FM Noise, and Direct Frequency Modulation Chara
cteristics in External Grating Loaded Semiconducto
r Lasers, "S.Saito et al, IEEE Journal of Quantum Electronics (IEEE.J.Quan
t.Elect.) vol.QE-18 (1982) 961, (7) “External-cavity semiconductor laser using holographic grating”, Asakura, Hagiwara, Nagaoka; In the present invention, (i) the optical element necessary for the optical pickup optical system configuration is provided with a composite function to reduce the number of parts / elements, and at the same time ( ii) The oscillation mode of the semiconductor laser is stabilized and the noise is suppressed. Therefore, (iii) it is possible to apply the technique of recording / reproducing a desired beam control wavefront by the hologram. In FIG. 1, the end face 101 on one side of the semiconductor laser 1 may be a perfect reflection surface, but the beam 1010 can be used for monitoring the current feedback to the laser 1. In the present embodiment, the pickup optical system for the reflective optical disk 6 is shown, but for a transmissive optical storage medium, the beam control optical system may be arranged on the side of the transmitted light from the medium. It is a matter of design.

FIG. 2 shows a schematic configuration of an OPU device according to another embodiment of the present invention. Compared with the case of FIG. 1, in the light source optical system, similar optical feedback is performed through one side 301, 302 of the composite diffractive element 31, but the beam control optical system is the back surface 313 of the composite diffractive element.
By incorporating it as a hologram in the, the number of parts and elements is further reduced. Further, in the embodiment shown in FIG. 1, lens systems are used for the collimating optical system 2 and the condensing optical system 5, but in the present embodiment, the thin film Fresnel lenses 20, 50, 90 are replaced to form a further layer. It is said to be compact and lightweight.

In the above configuration, the operation and characteristics of the OPU of the present invention will be completely clarified by explaining the optical system for manufacturing the hologram element 12. FIG. 4a is a conceptual diagram of an optical system that is realized as a hologram capable of accurately recording / reproducing an astigmatic wavefront. The cylindrical lens 10 is arranged in the optical path where the coherent parallel beam 13 having the wavelength λ ₁ is narrowed by the condenser lens 51, and the linear focused beams 10 oriented in the directions perpendicular to each other are arranged.
A substantially circular beam 102 is obtained at 1, 103 and its intermediate position. Now, assume that the beam 102 is on the X ₁ -Y ₁ coordinate plane. This optical system is similar to the one conventionally used to generate astigmatism in an optical pickup optical system, but the important point here is the Fourier transform lens 50.
Taking out the Fourier transform wavefront of the circular beam 102 to the rear Fourier transform (indicated by ξ-η coordinates) of the lens 50 via (focal length f ₁ ), and superimposing it on another plane wave that does not include aberration. Thus, a so-called lens Fourier transform type hologram element 313 is created. It is a well-known technique that the above-mentioned reference wave can be easily obtained by using an aberration-free spherical wave diverging from a predetermined position 16 on the front focal plane of the Fourier transform lens 50. Here, the reference wave can be easily obtained by converging the parallel beam 13 and the parallel beam 14 which interfere with each other with the lens 15. The reference wave may of course directly guide the plane wave to the hologram surface without passing through the lens 50.

Now, the hologram element 313 recorded in this way
Is arranged in an optical system as shown in FIG. 4b and is irradiated with a parallel beam of wavelength λ ₂ , the Fourier transform lens 5 (focal length f) rear focal plane (indicated by X ₂ -Y ₂ coordinates) Shows a reproduced image 1021 of a wavefront including astigmatism and its conjugate image 1022 at X ₂ −
Reproduced in a symmetrical positional relationship with respect to the Y ₂ coordinate origin,
Horizontal or vertical linear patterns 1011, 1031 and 1012, 1032 are obtained in the front and rear directions of each spot image. Since they are conjugate wavefronts, one is a vertical linear image 10
31 is near the lens 5, and the other is a horizontal image 10
Twelve appear side by side.

FIG. 5 explains the operation principle of the optical system shown in FIG. 2 from the aspect of the beam shape generated on the focus control photoelectric conversion element 11. That is, in FIG. 5A, when the beam focused by the condenser lens 5 is away from the pit surface 60 of the optical disc by a minute distance Δf (causing a focusing error), the hologram element 313 is turned on. The beam 1021 or 1022 that is diffracted through has a shape as shown in FIG. FIG. 13C shows the state when the focus is just in focus. The focus control signal ε is ε = (S ₁ + S ₃ ) − (S ₂ + S ₄ with the signal output components corresponding to the sectors 111 to 114 of the four-division photodetector as S ₁ , S ₂ , S ₃ , and S ₄ , respectively. ), (1), The focus control can be executed according to the condition of.

The tracking signal J is obtained from J = S ₂ −S ₄ (2), but considering the stability of the servo, it is detected from another photodetector using the far field pattern as shown in FIG. The method is more desirable.

In the above description, the cylindrical lens is used for the astigmatism, but another optical system, for example, a parallel plate is obliquely inserted into the optical axis of the convergent spherical wave, or another suitable spherical surface is used. Elements may be used.

In the above, the method including astigmatism as the beam control wavefront has been described focusing on the recording optical system of the lens Fourier transform hologram and the wavefront reproducing optical system of the information pickup optical system, but the present invention is based on these principles. The present invention is not limited to the device that stands up to, but it is also possible to replace a more general conventional pickup optical system with a single hologram element.

That is, in addition to the above-mentioned beam control optical system, various conventionally developed optical systems or a beam control optical system more suitable for the purpose can be recorded in advance as a lens Fourier transform hologram element.
The grid pattern of the element once recorded can be transferred to a mold that can be easily duplicated, and a large number of uniform elements can be obtained at low cost by replica manufacturing using a resin or glass material. It has a great effect on the design and production. Since the pattern to be reproduced is the Fourier transform type, it is possible to blaze the mold by ion beam processing, etc., and the blaze technology for Fresnel zone plates (Kawai, Kubota, Nishida; 18th Micro Optics Workshop) The diffraction efficiency of the replica element can be easily blazed compared to that of the lecture paper; vol3 (1985) P.33). That is, it is possible to obliquely irradiate the beam onto the entire surface of the hologram having a substantially parallel lattice pattern.

Although the embodiments of the present invention have been described focusing on the beam control system of the optical disc optical system, the present invention can be applied to an optical system having both recording and reproducing functions, and also as an optical information pickup device in a magneto-optical storage / reproducing system. Of course.

FIG. 3 illustrates, as another embodiment of the present invention, the principle of recording / reproducing a diffraction grating by a hologram element that is equally applicable to the optical systems of FIGS. In FIG. 10A, one of two mutually coherent light beams is guided to a long-focus condenser lens 101 to spread the widths δ ₁ and δ ₂ provided on the x-axis on the front focal plane of the Fourier transform lens 50. Small aperture with 10
Illuminate 02, and other light fluxes are condensed by the condenser lens 150 as a reference light source 160, are superimposed on the rear focal plane ξ-η of the lens through the Fourier transform lens 50, and are recorded as a hologram 3011. . Here, the minute aperture 1002 is provided with some diffusing surface, for example, a diffraction grating 1003 as shown in FIG. 3c, so that the diffraction efficiency of the hologram 3011 is sufficiently increased. The diffraction grating 3011 thus obtained is coated with a polarization selective film and then provided on a surface ξ-η ′ rotated by an angle θ from the ξ-η surface as shown in FIG. Beam 16 emitted from a point light source (semiconductor laser active layer)
Hologram 3 with 1 collimated lens 2 as a parallel beam
When 011 is radiated, a part of the diffracted light travels backward to the collimator lens 2 and returns to the point light source, which is a "return beam". This beam is a reconstructed image of the reconstructed minute aperture 1002 from the hologram 3011. 1003, the predetermined spread Cause Now, as one design example, λ ₁ = 442 nm, λ ₂ = 80
0 nm, When δ ₁ = 20 μm, δ ₁ ′ 9 μm. This beam divergence width has the effect of relaxing the mechanical accuracy (up to 1 μm or less) required for adjusting and holding the diffraction element 30 in the optical feedback optical system by about one digit.

Assuming that the grating pitch (average) of the diffractive element 3011 is P = 1.13 μm and the aperture is D = 8 mm, the number of gratings N7000, and therefore the resolution as a diffraction grating is δ ₁₀ mm, It is a degree. The actual oscillation wavelength can be selectively oscillated in a single mode within the gain width of the semiconductor laser 1 by finely adjusting the tilt angle of the diffraction element 30. If the minute aperture widths δ ₁ and δ ₂ are expanded more than necessary, the optical power returned to the vicinity of the semiconductor laser active layer will be reduced, and the spectral width contained in the feedback light will be expanded proportionally, so that it is in an appropriate range. Must be set to.

EFFECTS OF THE INVENTION As is apparent from the above description, the present invention provides the optical system of the OPU with (i) a light source system for stable single-mode oscillation by giving optical feedback to the semiconductor laser by the diffraction element and having a high signal-to-noise ratio. (Ii) The lens Fourier transform hologram element is used in combination with another optical element, and the astigmatism wavefront or the diffraction wavefront from the knife edge is recorded in the hologram. 1) Even if the wavefront is reproduced at a wavelength λ ₂ different from the hologram creation wavelength λ ₁ , the hologram itself does not generate aberration, and (2) focus control is performed by using a Fourier transform lens.
It is possible to effectively divide and use the beam for tracking control from the hologram diffracted light.

(3) Furthermore, the back surface of the hologram element substrate can be used as a reflection / diffraction surface for optical feedback beam diffraction and beam division, keeping the number of parts to a minimum, downsizing the optical system, lowering cost, and improving reliability. It can be greatly advanced.

(4) As with the optical disc, the hologram element can easily produce a large number of replicas from the master die through the transfer process.

(5) Since a lens Fourier transform hologram in which nearly parallel grating patterns are recorded is used, it is possible to carry out the whole surface simultaneous with the ion beam in order to improve the diffraction efficiency of the element. A hologram mold can be manufactured.

In addition, (iii) since the light source oscillates in a single mode, it is possible to exert high performance on a thin film optical element such as a Fresnel lens in addition to the hologram.
v) In the optical feedback optical system, the diffracted light that does not contribute to the return is extracted to form the beam of the optical pickup optical system, so that the optical power can be effectively used.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of an optical information pickup device showing one embodiment of the present invention, FIG. 2 is a principle explanatory diagram for explaining another embodiment of the present invention, and FIG. 3 is an explanation of an embodiment of the present invention. FIG. 4 is a schematic diagram showing the principle of the present invention for explaining an example of recording / reproducing an astigmatic wavefront, and FIG. 4 is an example of the present invention for recording / reproducing an astigmatic wavefront. FIG. 6 is a conceptual diagram illustrating a state of a reproduced beam, FIG. 6 is a conceptual diagram of an optical pickup optical system using a conventional hologram element, and FIG. 7 is a diagram showing a configuration example of a conventional optical pickup optical system. 1 ... Semiconductor laser, 2 ... Collimating lens, 30 ...
Polarizing beam splitter / diffraction element, 4 ... Quarter wave plate, 5 ... Condensing optical system, 6 ... Optical disk, 313 ...
Lens Fourier transform hologram element, 8 …… photodetector, 11 …… photodetector, 10 …… cylindrical lens,
20 …… Fresnel lens, 302 …… polarized light selective reflection surface, 100…
... Anti-reflection film.

Claims (5)

[Claims] 1. A semiconductor laser, a collimating optical system, a diffractive element as a means for optical feedback and beam splitting to the semiconductor laser, and a non-returned beam diffracted by the diffractive element in the form of a minute spot on a predetermined storage medium. 2. An optical pickup device comprising: an optical system that converges at 1 .; and focusing and tracking control means for the minute spots. 2. The optical pickup device according to claim 1, wherein the semiconductor laser has an antireflection film on the end face for optical feedback. 3. The optical pickup device according to claim 1, further comprising a diffractive element having a predetermined wavefront recorded on a lens Fourier transform hologram as a focusing and tracking control means. 4. The optical pickup device according to claim 1, wherein the diffraction element as the optical feedback and beam splitting means is formed by a Fourier transform hologram having a minute aperture. 5. A semiconductor laser having an antireflection film on its end face for optical feedback, a collimating optical system, and a diffractive element provided with a polarization selective reflection film as means for returning light to the semiconductor laser and beam splitting means. A quarter-wave plate, an optical system for converging a non-returning beam diffracted by the diffraction element into a fine spot on a predetermined storage medium, and a focusing and tracking control means for the fine spot. Optical pickup device.