JPH07101519B2 - Optical head device - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical pickup head 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 using a pit-shaped pattern as a high-density and large-capacity storage medium has been put into practical use 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 the optical pickup head device (hereinafter abbreviated as OPU) are:
(I) Light-collecting property for forming a diffraction-limited minute spot, (i
i) focus control and pit signal detection of the optical system, and (i
ii) The tracking control is roughly classified into three types. These are realized by a combination of various optical systems and photoelectric conversion detection methods according to the purpose and application. Figure 8 shows
It is a schematic diagram which shows an example of the conventional OPU. Diverging wavefront from the semiconductor laser light source 1 that oscillates at normal TE ₀₀ mode (electric field:
Horizontally polarized light is made into a parallel beam by the collimator lens 2, and the left quarter wave plate (1 / 4λ) is made by the polarization beam splitter 109.
Selective reflection on plate 11. The circularly polarized wave front that has passed through the 1/4 λ plate is focused by the condenser lens system 3 into a spot of approximately 1 μm, reaches the optical storage medium surface 4, and irradiates the pit-shaped pattern 40. The light beam reflected and diffracted by the medium surface 6 goes through the condensing lens system 3 in the opposite direction and passes through the quarter-wave plate 11 again to become a vertically polarized parallel beam, which is a polarization beam splitter.
After passing through 10, the prism half mirror 12 splits it into two directions. One reflected light passes through the condensing lens 20 and the cylindrical lens 13 that imparts astigmatism, enters the four-division photodetector 14, and is converted into a focus control signal. The other transmitted light enters the two-divided photodetector 7 for detecting the tracking control signal while keeping the far field pattern.

Here, the 1/4 λ plate 11 enhances the utilization efficiency of the light quantity by combining with the polarization beam splitter 10 and, at the same time, suppresses the return to the semiconductor laser and increases 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 20) shown in FIG. 8 is formed 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. Further, the lens 3 that requires the highest performance of focusing cannot be replaced because of the limit of processing accuracy.

It has been recently reported that an attempt to arrange the hologram element 16 as shown in FIG. 7 close to the condenser lens 3 is made, assuming that the above reason is 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 formed on one half of the hologram surface, and the remaining one surface is formed). Wavefront 230
And 232 interference fringes) are formed on the ξ-η plane, and the design is based on the concept of a so-called lensless Fourier transform hologram system. As shown in Fig. 7b, the "wedge prism method" or the "double knife edge method" is used. The hologram element 16 is realized by electron beam drawing in the form of being divided into two parts 161 and 162 so as to have an effect equivalent to. FIG. 7C schematically shows a light spot on the photodetector 15. By doing so, it is possible to create an aberration-free hologram lens only when the design wavelength λ ₂ of the light source 1 is used, and the aberration of the beam detector (photodetector) 15 due to the slight variation of the spectral width of the light source is generated. Even if it appears on the conversion surface, the push-pull method using the four-divided photoelectric conversion surfaces 151, 152, 153, 154 can suppress the fluctuation within a practically acceptable range. However, although it is convenient to form the fine pattern of the element 16 of FIG. 7 by electron beam drawing, the pattern of the element 16 capable of electron beam drawing is in the case of a simple pattern such as a lattice or hyperbolic shape. A technique for forming a more general hologrom system with a limited accuracy is not disclosed at all. Further, in the conventional optical system, as generally seen from FIG. 7C, there is a problem that the positional accuracy of the photodetector 15 with respect to the beam is required to be on the order of several microns.

The present invention provides an optical head device using a simple diffraction element that stably realizes focus and tracking control of an OPU, without imposing restrictions such as electron beam drawing or recording / reproduction at a specific wavelength. A simplified optical system can be constructed by using a hologram element based on a more general optical principle.

The difference between the hologram element for a pickup disclosed heretofore and the diffractive (hologram) element according to the present invention will be specifically clarified in order in the following description. The following is a summary of the limitations of the conventional device from the aspect and the purpose of the present invention.

(1) Both function as an incident / reflected light separating means, but a conventional hologram element as a light beam used for detecting a focus error and a tracking error uses a “wedge prism method”, and this optical system is configured by a hologram system. As long as it is, the light beam emitted from the light source is diffracted by the hologram on the outward path, and the beams of a plurality of spots condensed by the objective lens are imaged on the disk with a power density that cannot be ignored. In order to suppress the diffracted light component, (i) the carrier frequency of the hologram is increased to increase vignetting at the objective lens aperture, and (ii) the distance between the hologram element and the objective lens is increased to enhance the same effect. , (Iii) A method of suppressing the diffraction efficiency of the hologram to be low can be considered, but (i) increases the influence of the wavelength fluctuation of the light source to impair the reliability of the photodetection system, and (ii) downsizes the device. (Iii) lowers the S / N ratio of signal detection and makes it difficult to achieve the function as an optical head for recording / reproducing. The present invention solves the above-mentioned problem by the structure capable of detecting the focus error in the foul field.

(2) Conventionally, the hologram element was configured as a "lensless Fourier transform type hologram" that suppresses the lens function as focusing power as much as possible, but a slight wavelength deviation from the design wavelength λ of the pickup light source (Δλ = ± 20 nm) However, it is necessary to provide a troublesome process of adjusting the position of the photodetector in the optical axis direction in order to generate a focus offset and adjust the wavelength shift due to the lot of the semiconductor laser. In the present invention, a long focus Fresnel zone plate is used for the diffractive element, but since the focusing power depends on another lens, the focus position variation due to the light source wavelength variation is small,
Moreover, a stable signal can be detected by the differential detection method in the far field.

(3) With respect to the adjustment of the photodetector, the conventional method strictly requires the positional accuracy in the photoelectric conversion plane and in the optical axis direction regardless of the presence or absence of the hologram system. The former often requires an order of 5 to 10 microns, and the latter often requires an order of several tens of microns. The present invention aims to realize an extremely simple adjustment process or manufacture of an optical head without adjustment by making it possible to relax the adjustment accuracy of the photodetector.

Means for Solving the Problems In order to solve the above problems, the present invention generates a coherent light source such as a semiconductor laser, an optical system for converging a coherent beam into a minute spot, and a plurality of wavefronts in an off-axis direction. By combining the diffractive elements, a desired beam control and reproduction information can be obtained on a photoelectric conversion surface having a predetermined shape and a relatively large area. The simplest structure of the diffractive element is the first off-axis Fresnel zone plate (off-axis Fresnel).
zone plate) and a second having a focal length equal to this
It is realized by a form in which the off-axis Fresnel zone plates of (1) and (2) are overlapped with each other by giving a predetermined optical axis distance. Further, as a more desirable configuration, the same wavefront as the target can be accurately generated by the lens Fourier transform hologram.

Function In the present invention, since the diffractive element for generating two focal points has a weak lens function and is used in combination with the focusing (or collimating) lens system, the focusing error signal is differential in the far field of the beam focused on the disk. Light is detected.

That is, the two Fresnel zone plates act as a weak convex lens and a weak concave lens to generate beams to be focused on the front and rear surfaces of the differential photodetector, respectively.
The focusing error can be detected as a difference between the two beam sizes on the light detection surface.

This configuration relaxes the adjustment system of the optical system and the photodetector.

Since the present invention is also configured to detect two beams in a simple form in the far field, (iii) the area division direction (boundary line) of the photodetector is centered on the 0th-order diffracted light convergence point from the diffraction element. It is possible to easily avoid wavelength fluctuations of the light source or signal deterioration due to multi-spectral components by designing so as to extend in the radial direction.

Embodiment FIG. 1 shows a schematic configuration of an OPU device according to an embodiment of the present invention. In the figure a, 1 is a semiconductor laser that emits a coherent beam in the infrared region or a wavelength range shorter than this region (for example, wavelength λ ₂ 800 nm), 2 is a collimating lens (focal length f _c 20 mm), 3 is for focusing The objective lens 4 is an optical storage medium (optical disk), and the beam emitted from the light source 1 is collimated by the collimator lens 2 and focused on the disc 4 by the lens 3. At this time, the diffractive element 6 is an off-axis bifocal diffractive element and is interposed between the lenses 2 and 3, and the 0th order transmitted light is focused on the disk 4 in the outward path. 42 is a substrate and 41 is a protective film. The beam reflected on the disk 4 passes through the lens 3 again on the return path, is made into substantially parallel light, and then enters the diffractive element 6,
In addition to the 0th-order transmitted light, two wavefronts 61 and 62 having different off-axis focal points are generated. The following structure is applied to the transmissive disc instead of the reflective disc. The two wavefronts are converged by the collimator lens 2, and a surface 111 including the convergence point of the 0th-order transmitted light (light emitting point 10 of the light source 1) and perpendicular to the optical axis of the lens 2
And are focused on the two surfaces of the front and rear positions. A detector 5 receives light and comprises detectors 51 and 52. Each focal plane and plane 1
The distance from 11 is designed to be δ ₁ = δ ₂ = δ, but for example, δ ₁ + δ ₂ can be about one digit larger than the astigmatic difference in the astigmatism method, so that even if δ ₁ ≠ δ _2. The error is tolerated to a greater extent than before.

FIG. 5b shows the first and second photodetectors 5 arranged on the surface 111.
The relationship between 1,52 and the light emitting point 10 is shown. In the figure, photodetector 5
The incident beams on the photoelectric conversion surface of 1,52 are as in 610,620, and are arranged on a straight line XX 'passing through the light emitting point.

FIG. 2 is a conceptual diagram showing another embodiment of the present invention. First
In contrast to the case where a transmission type diffraction element is used in this embodiment, a reflection type hologram element 66 is used in this embodiment and the optical axis is set to α.
It is bent at 90 °. Further, the image forming optical system is configured only by the objective lens system 30 without using the collimator lens, and the size is reduced and the number of parts is further reduced.

FIG. 3 illustrates still another embodiment of the present invention. The difference from the above two examples is that the polarization beam splitter 109 is provided so that the control beam can be obtained in the return path separated from the outward path from the light source 1. And that the wavelength plate 9 is provided, and the tracking detection is separately performed by the third photodetector 7. Here, the wave plate 9 has a wavelength of λ / 5 for the purpose of facilitating the performance balance with the polarization beam splitter 10.
Designed to the extent that the amount of return light is optimized for signal detection
The S / N ratio is maximized. The mirror 8 is for bending the optical path. In this case, the hologram 666 can be blazed to have maximum diffraction efficiency.

Now, the configuration of the photodetector in the above embodiment will be described in detail. FIG. 4 shows the photodetector 51 shown in FIG.
The relationship between the beams 610 and 620 detected in each divided region of 52 is schematically shown. FIG. 4b shows the case where a focused spot is formed on the disk, and both beams 610 and 620 are photoelectrically converted with the same diameter and the same optical power density. Therefore, the same output is detected from one of the three divided areas 510 and 520 of the detection areas 51 and 52, and the differential output thereof becomes zero.
That is, the focus error signal F _E is the output from each area j.
Expressed as S _j , F _E = S ₅₁₀ −S ₅₂₀ = 0 In FIG. 4c, the focused spot on the disk is in a defocused state, and the incident light on the diffraction element 6 is not a plane wave,
For example, it becomes a diverging wave, so that the focal point of the wavefront 61 included in the diffractive element and acting as a convex lens approaches the photodetector side. On the other hand, the concave lens action of the wavefront 62 focuses at a point further away from the photodetector. As a result, F _E = S ₅₁₀ −S ₅₂₀ > 0 If the optical disc moves to the opposite phase and enters the defocused state, the state is symmetrically reversed as shown in FIG. 4A, and F _E = S ₅₁₀ −S ₅₂₀ < It becomes 0. In the design example, W ₀ = W ₁ = W ₂ = 0.
05mm, the distance to the point 10 is l ₁ = 1mm, l ₂ = 0.65mm, the beam size at the time of focusing D = 0.10mmφ, δ ₁ ＝ δ ₂ ＝ 0.4mm, and the focal length of the collimator lens f ₂ = 20 mm and the lens aperture diameter was 5 mmφ.

In an actual disc device, the defocus amount on the optical disc surface is about ± 5 μm, and the focus shift amount on the photodetector surface is ±
It suffices to design the main operating range to be about 100 μ, and the beam diameter in FIG. 4 changes within a range of maximum 125 μmφ and minimum 75 μmφ.

Tracking error signal T _E, if the track by the first drawing is oriented in the direction parallel to the drawing (irregularities on the information recording surface of the first drawing shows a cross section of the signal pits along the track), T _E = ( _S511 + _S521 )-( _S512 + _S522 ). It can be seen that in both cases of T _F and T _E , in the case of the present invention, the photodetector adjustment accuracy is greatly relaxed as compared with the case of FIG. 7c.

When a semiconductor laser is used as a light source, there is a problem of wavelength shift due to temperature change or current change. In the present invention, the direction of the division boundary is made to coincide with the direction of the spatial carrier frequency of the hologram as shown in FIG. 4, so the beam transition (the beam of 611 is the beam of Δl ₁ , 621 is shown by the broken line in FIG. 4b. Causes Δl ₂ ) without causing any problems. The beam magnification change is the same for both beams with respect to the change from the central wavelength λ ₂ to λ ₂ + Δλ. Therefore, the abnormality of the differential output signal due to the change in beam diameter does not occur.

FIG. 5 illustrates another photodetector array configuration that can be commonly used in the embodiments of the present invention. Unlike the case of FIG. 4, the first and second photodetectors 51 and 52 are different from each other. Is equidistant from the light emitting point 10 and the dividing boundary line is formed at a constant angle θ on a line extending radially from the point 10. Since there is an advantage that the absolute value of the carrier frequency of the hologram can be made equal in the two beams, when used with a light source with a large wavelength variation, l can be designed to be a minimum to achieve the purpose. A point 100 that almost matches the emission point 10
By rotating the diffractive element (hologram) or the photodetector by a small angle (to 98 or 99) around, the adjustment with the beam positions 611 and 612 (which are separated from each other by the angle θ) is completed.

FIG. 6 is a conceptual diagram showing an example of a recording optical system for a lens Fourier transform hologram that can be used in common with the examples of the present invention. The gradient index rod lenses 901, 902 and 900 illuminated by the coherent plane wave 91 are arranged at predetermined spatial positions 1
Focus on 01,102,100, and further Fourier transform lens 9
Are superimposed on each other on the recording medium 60 via to form a lens Fourier transform hologram. 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 (1979.11.) (4) “Spec.
kle reduction in holography …… ", M.Kato et al; Applied Optics (Appl.Opt.), 14 (1975) 1093)
As reported and analyzed in detail, etc., it has been applied to recording and reproducing optical systems for general images ((5) "Optical Chinese character editing processing system" Sato et al .; IEICE Technical Committee, EC7)
8-53 (1978) 47), the present invention does not hinder practical use as a beam control means, as long as Fourier transform is established for the wavefront near the optical axis of the reproduction optical system, and the wavefront reproduction from the hologram element is possible. The lens used can be replaced by a collimator lens, or a desired reproduced image can be obtained on the converging surface by simply irradiating the hologram element with a convergent spherical wave. Now, in the recording optical system of FIG. 6, the intervals Δ ₁ and Δ _{2 in} the z direction from the x axis of 101 and 102 are Can be designed as Here, M is the magnification, f ₁ is the focal length of the recording Fourier transform lens 99, and f ₂ is the focal length of the reproducing Fourier transform lens. In FIG. 1, the focal length of the collimator lens 2 is shown in FIG. Is the radius of curvature f ₂ of the convergent wave incident on the hologram. The distances L ₁ and L ₂ between the optical axes of the rod lenses are also ML ₁ = l ₁ and ML with respect to l ₁ and l _{2 in} FIG.
There is a relationship of ₂ = l ₂ . In order to achieve dimensional matching with the optical head optical system and the photodetection system described in FIGS. 1 to 3, if the focal length f ₁ of the recording Fourier transform lens is set to about 50 to 100 mm, L ₁ and L ₂ Is about 1 to 2 mm. Although it is difficult to obtain a desired hologram with a normal lens optical system, a rod lens having a diameter of about 1 mm can be easily realized.

EFFECTS OF THE INVENTION As described above, the optical head device according to the present invention uses the two-wavefront generated by the diffraction element for the light beam control, and the far field pattern of the spot focused on the optical disc is differentiated by the photodetector. By adopting the detection configuration, the following excellent effects are brought about.

(I) In the conventional method in which the diffractive element functions as a beam splitter, high-order diffracted components such as ± 1st order generated on the outward path are condensed on the optical disk. However, in the present invention, a defocus pattern is formed, so that the power density is conventionally. Compared to the example, it is smaller by two digits or more, and unnecessary recording and servo operation failure can be prevented.

(Ii) With respect to the wavelength fluctuation of the light source, which is a weak point when the control beam is generated by the diffractive element, the present invention makes the photodetector dividing line coincide with the spatial carrier frequency direction of the hologram, and The problem is solved by the operation output.

(Iii) Conventionally, position adjustment of the photodetector has generally required strict accuracy in each of the three axial directions, but this can be greatly alleviated in the present invention, and the diffraction element or the photodetector can be rotated. A highly reliable optical head device can be manufactured by a simple process of adjustment.

(Iv) The diffractive element according to the present invention is a simple zone plate type element and can be easily mass-produced with extremely high precision and high diffraction efficiency.

As described above, the optical head device of the present invention provides a new technology that is excellent in miniaturization, weight reduction, high reliability, mass productivity, and economy.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of an optical head device showing an embodiment of the present invention, FIG. 2 is a principle explanatory diagram for explaining another embodiment of the present invention, and FIG. 3 is another embodiment of the present invention. FIG. 4 is a schematic configuration diagram of an optical head device to be described, FIG. 4 is a principle diagram of the present invention for explaining the operation of the photodetector, and FIG. 5 is a configuration conceptual diagram relating to still another embodiment of the photodetector of the present invention. FIG. 6 is a block diagram of an embodiment for explaining a recording optical system for realizing the diffraction element of the present invention as a hologram, FIG. 7 is a conceptual diagram of a configuration of an optical head optical system using a conventional hologram element, and FIG. It is a block diagram which shows the structural example of the conventional optical head optical system. 1 ... Semiconductor laser or equivalent coherent light source,
2 ... Collimating lens, 3 ... Condensing optical system, 4 ... Optical disc, 5 ... Photodetector, 51 ... First photodetector, 52 ... Second photodetector, 6 ... Diffraction element, 66
...... Reflective hologram element, 7 ... Third photodetector, 8 ... Reflecting mirror, 9 ... 1/5 wavelength plate, 10 ... Light source emission point, 11 ... 1/4 wavelength plate, 99 ... Fourier conversion Lens, 60 ... Recording medium, 900, 901, 902 ... Gradient index type rod lens, 16 ... Conventional multi-function hologram element.

Continuation of the front page (56) Reference JP-A-53-121644 (JP, A) JP-A-60-212835 (JP, A) JP-A-60-171644 (JP, A) JP-A-62-97141 (JP , A) JP 62-172538 (JP, A) JP 62-277640 (JP, A)

Claims (7)

[Claims] 1. A light source that emits a coherent beam or a quasi-monochromatic beam, a condensing optical system that converges the coherent beam or the quasi-monochromatic beam into a minute spot, and the coherent beam or the quasi-monochromatic beam via the optical system. Are arranged in a reciprocal optical path that is reflected by a predetermined optical storage medium and travels backward to the condensing optical system and the light source, and generates two wavefronts having the same order and different focal points in the off-axis direction of the optical system optical axis. And a rectangular light receiving element having at least one pair of two sides parallel to an extension line extending from a light emitting point of the light source, which coincides with a convergence point of 0th-order transmitted light from the diffractive element, on the return path of the round-trip optical path. And a first photodetector and a second photodetector that receive the two wavefronts on the same plane near the focal point, respectively, and the photodetector is provided near the light source. Two Fresnel zone plates are superposed over the entire surface of the diffractive element in order to generate the two wavefronts, respectively, and the two wavefronts have respective focal points before and after the photodetector. And an intensity distribution shape of the two wavefronts is substantially circular on the photodetector. 2. The optical head device according to claim 1, wherein the diffractive element is a Fourier transform hologram. 3. The hologram element according to claim 1, wherein the diffractive element is a reflection hologram, and a hologram surface is provided so as to form an angle of about 45 degrees with a beam emitted from the radiation source. Optical head device. 4. The optical head device according to claim 1, wherein each of the first and second photodetectors is divided into three rectangular light receiving portions. 5. A light source that emits a coherent beam or a quasi-monochromatic beam, a condensing optical system that converges the coherent beam or the quasi-monochromatic beam into a minute spot, and the coherent beam or the quasi-monochromatic beam via the optical system. Are arranged in a reciprocating optical path that is reflected by a predetermined optical storage medium and travels backward to the condensing optical system and the light source, and first and second focal points having the same order and different in the off-axis direction of the optical system optical axis. Of the diffractive element for generating the wavefront of the light source, and at least one of two sides parallel to the extension line extending from the light emitting point of the light source, which coincides with the convergence point of the 0th-order transmitted light from the diffractive element, in the return path of the round-trip optical path. A pair of rectangular photodetectors, and first and second photodetectors that respectively receive the two wavefronts on the same plane near the focus, and the photodetector is the optical detector. The Fresnel zone plate is disposed over the entire surface of the diffractive element in order to generate the two wavefronts, and the two wavefronts each have a focal point for the photodetection. Before and after the detector, the shapes of the two wavefronts are substantially circular on the photodetector, and an extension line extending from the light emitting point of the light source in the rectangular light receiving portions of the first and second photodetectors. The optical head device is characterized in that the parallel sides are provided so as to be substantially coincident with or parallel to the carrier frequency direction of the diffraction element pattern for generating the first and second wavefronts. 6. An angle formed by a side parallel to an extension line extending from a light emitting point of the light source in the rectangular light receiving portions of the first and second photodetectors is smaller than 90 degrees. An optical head device according to claim 5. 7. The optical head device according to claim 5, wherein the first and second photodetectors are photodetectors, and the photodetectors are integrated on a single substrate.