JPH01220133A - optical head device - Google Patents

Abstract

PURPOSE:To obtain the optical head of high reliability and to obtain the hologram element of high accuracy and high refracting efficiency by using two wave surfaces, which are generated from a refracting element, for optical beam control and differentially detecting the pattern on a condenser spot with an optical detector. CONSTITUTION:A beam from a light source 1 is made parallel by a lens 2 and condensed on a disk 4 through a lens 3. At such a time, a hologram element 6, which records the wave surface to include astigmatism, exists between the lens 2 and 3 and 0-th order transmitting light is condensed by the disk 4 in an outgoing path. A reflected beam is made parallel again by the lens 3 in an incoming path and incoming to the element 6. Then, in addition to the 0-th order transmitting light, the astigmatism reproducing image and wave surfaces 61 and 62, which are sharing images, are generated. One of the incoming beams in the photoelectric converting surfaces of an optical detector 5 goes to be the wave surface 62 and it is arranged on a straight line X-X', which goes through a light emitting point, together width the sharing image 61. Accordingly, by executing the differential detection of the beam 62 by detectors 51 and 52, the optical head of the high reliability can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an optical pickup device capable of recording, reproducing, or erasing 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 bit-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 for successfully recording and reproducing information with high reliability through a light beam focused on the micron order depends solely on the structure for picking up the optical information, and 9 in part on the optical system. There is. The basic functions of the optical pickup head unit (hereinafter abbreviated as OPU) are (1)
) Light focusing ability to form a diffraction-limited minute spot, (11)
Focus control of the optical system, bit signal detection, and (11
It is roughly divided into three types of tracking control. These are realized by combining various optical systems and photoelectric conversion detection methods depending on the purpose and use. Figure 10 shows
FIG. 1 is a schematic diagram showing an example of a conventional OPH. Usually Tieo
A diverging wavefront from a semiconductor laser light source 1 oscillating in the 0 mode (electric field: horizontally polarized Lt) is made into a parallel beam by the collimating lens 2 and selected as the left quarter-wave plate (V4λ@) 11 by the polarizing beam splitter 106. The circularly polarized wavefront that has passed through the Aλ plate is focused into a spot of about 1 μm by the condenser lens system 3, reaches the optical storage medium surface 4, and irradiates the bit-shaped pattern 40'j). The light beam reflected and diffracted by the medium surface travels in the opposite direction through the condenser lens system 3 again, passes through the quarter-wave plate 11, becomes a vertically polarized parallel beam, and passes through the polarizing beam splitter 106 to form a prism half. It is divided into two directions by a mirror 12. One reflected light passes through a condensing lens 2o and a cylindrical lens 13 that imparts astigmatism, and enters a four-part photodetector 14.
converted into a focusing control signal. The other transmitted light enters the two-split photodetector 7 for tracking control signal detection with the far field pattern intact.

Here, by combining the μλ plate 11 with the polarizing beam splitter 106, the μλ plate 11 increases 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. However, in the OPU for read-only discs, there is ample leeway in light intensity design, and it is possible to omit the %λ plate and polarizing beam splitter. is being planned.

Problems to be Solved by the Invention However, even in a read-only OPH, it is necessary to construct a beam splitting means, a focus control means using astigmatism or the Knifezi method, and a tracking control means, either independently or in combination. The optical components conventionally used for this purpose, such as beam splitters, lenses, and prisms, are difficult to manufacture, assemble, and adjust in large quantities, and it is difficult to manufacture, assemble, and adjust them in large quantities. There was a problem.

The common reasons why these problems occur are: First, optical components that require high-precision flat or aspherical surfaces require many processes to achieve the desired processing, so production using pressing means is generally not possible. Second, it requires a lot of time and complicated inspection and measurement equipment to assemble and adjust a large number of parts to achieve a desired overall performance. Third, the parts are small. Since there are limits to miniaturization of the entire optical system, there are also major restrictions on miniaturization of the entire optical system.To solve the above problem, a predetermined wavefront for focusing and tracking control is recorded on a single hologram element, and the optical head Techniques have recently been disclosed for directing each wavefront reproduced in the reading beam to a photodetector.

1) Japanese Patent Application No. 1989-108908 (Special Publication No. 62-336)
5? No.) (on the ceiling, Nagai) 2) Japanese Patent Application No. 18850 (Sho 62-1880)
3i) (on the ceiling, Nagai) 3) Japanese Patent Application No. 1983-79677 (Japanese Unexamined Patent Publication No. 62-2361)
No. 45) (Matsushita, Tatsumi) 4) Y, Kimuru at al” High
Par 7 Oh Man 7 Optical Herad Using Optimized Holographic Optical Element (High Performance
0ptica, IHead using Optimi
zed Holographic Optical El
ment)”, Proceedings of Opza International Symposium on Optical Memory (P
roc, of the International S
emposium on OpTicalMemory
) Tokyo, 5ept, 1e-1s,
1987 (P, 131).

s) K, Tatsumi et al, “A?/
L/Tif 77 functional reflection type
Grating Lens Forcer CD Optical Head (Multi-functional Re
reflectionType GratingLens
for the CD 0pticalHead)”
, Proceedings of the International Symposium on Optical Memory (Proc.
of the International Sympo
sium on 0ptical Memory)
Tokyo.

Sep, 16-18.1987 (p, 127).

Among the above, 4) is a focusing error signal (abbreviated as FE) using the double knife method, and a tracking error signal (abbreviated as TE) using the diffracted light from a slit grating provided on the far field (hologram element surface). This is a detection method based on intensity, and in all other methods, as shown in FIG. 9, an FF (TK) signal is detected by calculating an astigmatic wavefront from a signal received by a quadrant photodetector 14. FIG. 9 shows a conventional detection state, and (b) shows when a circular light beam 141 is incident on the detector 14,
(iL), (C) Elliptical light beams 140, 142
The amplifiers 30 and 31 and the signal processing circuit 33 shown in (b) are omitted in (&) and (Ct). However, each method has a serious problem,
When the wavelength of the light source deviates from the design standard wavelength λ by Δλ, the movement of each beam (14o → 1401° 141 → 14
11, 142 → 1421), and no technology for suppressing this effect has been developed under conditions of general use.

In other words, in a normal semiconductor laser, if the operating environment temperature changes by, for example, 60'C, the peak wavelength will shift by about 12 nm, and if the drive current changes by 40 mW, a wavelength shift of about snm will occur, and it is also necessary to consider the loft fluctuation width by about sonm. . As described above, in OPUs including write-once type or erasable type optical disks, there remains the problem of error signal detection that can cope with changes in the diffraction angle from the hologram element that occur due to large wavelength shifts.

Means for Solving the Problems In order to solve the above-mentioned problems, the present invention uses a coherent light source such as a semiconductor laser, an optical system that converges the coherent beam into a minute spot, and a hologram that records a wavefront including astigmatism. The element is combined with a band-shaped photodetector in which the region dividing direction (boundary line) of the photodetector is formed in a radial direction centered on the convergence point of the 0th order diffracted light from the diffraction element. be.

In the present invention, in order to stabilize FE signal detection, which is particularly susceptible to wavelength fluctuations, 1) the optical system is arranged so that the wavefront including astigmatism is incident on the band-shaped photodetector in a predetermined positional relationship; ) Furthermore, more preferably, by differentially detecting the outputs from multiple band-shaped photodetectors, the signal characteristics can be maintained at a level equal to or higher than that of the conventional four-division detector method, and 111)
By making the direction of beam movement on the Oto-detector match the direction in which the strip of the detector extends, wavelength fluctuations of the light source or signal deterioration due to multispectral components can be easily avoided.

Embodiment FIG. 1 shows a schematic configuration of an OPU device according to an embodiment of the present invention. In the same figure a, 1 is a semiconductor laser that emits a coherent beam (for example, wavelength λ2 = 800 nm)
, 2 is a collimating lens (focal length fc=20mm)
, 3 is an objective lens for condensing light, and 4 is an optical storage medium (optical disk). A beam emitted from a light source 1 is converted into a parallel beam by a collimating lens 2, and is focused onto a disk 4 by a lens 3.

At this time, reference numeral 6 denotes a hologram element recording a wavefront including astigmatism, which is interposed between lenses 2 and 30, and its 0°-order transmitted light is focused on the disk 4 on 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 trip.
After passing through the Here, the hologram element 6 is a Fourier transform type hologram, which will be described later, and these pair of conjugate wavefronts 61 and 62 are converged via the collimating lens 2, and when they are correctly focused on the disk 4, they become zero. The convergence point of the next transmitted light (light source 1
A plane 11 perpendicular to the optical axis of the lens 2 including the light emitting point 1o)
1 forms an astigmatic image in a direction orthogonal to two planes at the front and rear positions. Reference numeral 6 denotes a band-shaped detector that receives light and is composed of detectors 50, 51, and 52. The distance between each focal plane and the surface 111 is designed to be δ, = δ2 = δ. The relationship between the second and third photodetectors 50, 61, 52 and the light emitting point 10 is shown. In FIG. 1, an astigmatism image is shown on the surface 111 corresponding to the defocused state on the disk. In the figure, one of the incident beams on the photoelectric conversion surface of the photodetector 6 has a wavefront 62.
υ, the straight line x- passing through the light emitting point together with the conjugate image 61
Line up on x'. Tracking control error signal detection is omitted here and will be described later.

FIG. 2 is a conceptual diagram showing another embodiment of the present invention. 1st
In the embodiment, a transmission type hologram element is used, whereas in this embodiment, a reflection type hologram element 66 is used and the optical axis is bent at α:90'. In addition, the imaging optical system is configured only with the objective lens system 30 without using a collimating lens, thereby achieving miniaturization and reducing the number of parts.

FIG. 3 illustrates still another embodiment of the present invention, which differs from the previous two examples in that it uses a polarizing beam splitter so that a control beam 610o can be obtained on the return path separated from the outgoing path from the light source 1. 106 and the wavelength plate 9, and the fourth photodetector 530 allows the zero-order transmitted light 60 to be
The RF signal detection using 00'ii is now configured separately.

Here, the wavelength plate 9 is designed to have a diameter of approximately λ/6 in order to facilitate the performance balance with the polarizing beam splitter 106.
By optimizing the amount of returned light, the signal detection S/N ratio is extremely high. The mirror 8 is for bending the optical path. In this case, the hologram 666 can be blazed if necessary to provide maximum diffraction efficiency, and the conjugate image 6200 becomes a weak beam. Furthermore, a predetermined toroidal amplitude distribution is given to the astigmatism wavefront in order to improve the signal characteristics by a factor of FE.

The hologram will be described later.

Now, the configuration of the photodetector and the signal detection method in the above embodiment will be explained in detail. FIG. 4 shows a pair of reproduced wavefronts 610, which makes it possible to utilize not only the wavefront beam 62 detected in each divided area of the photodetectors 50, 61 and 52 already shown in FIG. 1, but also its conjugate image. 620 and 0th order light 6
The relationship of o00 is schematically and generally expressed. Fourth
The figure shows a case where a focused spot is formed on the disk, and both beams 610 and 620 have the same diameter (circular or donut shape) and are photoelectrically converted with equal optical power density.

Figures 4a and 4c each show a defocus state with opposite phases. The characteristics of focus control (so-called 8-character characteristics) are as follows: If a signal output such as ``person'' or ``A'' in Figure 7 is obtained from the photodetector 510 or 520 on one side, and ``person - person''
It can be seen that the operating range can be doubled by using the differential output as shown in the figure.

As another detection method, the central part of one conjugate image, for example, the output person 610, is placed on the photodetector 51 on both sides.
If 1,512 outputs are added with opposite signs, an output characteristic like B can be obtained. Similarly, an output symmetrical to B such as B' can be obtained from the other conjugate image, so again the working metal and n are B-
It is also possible to obtain a further expanded operating range from B'.

Here, FIG. 6 is a conceptual diagram of an optical system that realizes a hologram element capable of accurately recording and reproducing an astigmatic wavefront in the present invention. A cylindrical lens 70'f is provided in the optical path that condenses the coherent parallel beam 13 of wavelength λ with the condenser lens 51:
Astigmatism images, that is, linear focused beams 101 and 103 and a circular beam 102 at an intermediate position thereof are obtained. Beam 10 now
Assume that 2 is on the X, -Y coordinate plane. This optical system is similar to that conventionally used to generate astigmatism in optical pickup optical systems, but the important thing here is that the Fourier transform lens SO (focal length f,
)', the Fourier transform wavefront of the circular beam 102 is taken out to the rear Fourier transform surface (indicated by ξ, -η, coordinates) of the lens 8o, and superimposed with another plane wave that does not include aberration. , a so-called Nuns Fourier transform type hologram element 12'(i) is created. It is a well-known technique that the above reference wave can be easily obtained using an aberration-free spherical wave that diverges from a predetermined position 16 on the front focal plane of the Fourier transform lens 80. Here, the reference wave is a parallel beam 13 and a parallel beam 14 that is coherent with each other through a lens 15.
can be easily obtained by converging with .

When recording the reflection hologram used in the embodiment shown in FIG.
All you have to do is tilt it.

Now, when the hologram element 12 recorded in this way is placed in an optical system as shown in FIG. 6 and irradiated with a parallel beam of wavelength λ2, the rear focal point of the Fourier transform lens 6 (focal length f2) On the plane (displayed in X2-Y2 coordinates),
Fig. 6 Reconstructed image 1021 of the wavefront including the astigmatism shown above
Even under the conditions of λ and λ2, its conjugate image 1022 is X2
- Reproduced without distortion in a mutually symmetrical positional relationship with respect to the Y2 coordinate point, and horizontal or vertical linear patterns 1011, 1031 and 10 are displayed in the front and rear directions of each spot image.
12.1032 is obtained. Since they are conjugate wavefronts, one has a vertical linear image 1031 located close to the lens 5, and the other has horizontal images 1012 appearing side by side. If this hologram 12 is illuminated with a somewhat diverging spherical wave, the conjugate image 1012.103 will appear on the x2-Y2 plane.
The astigmatism image near 1 is formed, and the image shown in Fig. 4 &,
The state shown in Q is obtained. Conversely, if the hologram is illuminated with a convergent sphere, the wavefront 1032.1 in Figure 6
Astigmatism images in the vicinity of 011 are respectively detected.

In FIG. 6a, the spatial filter 1010 is a circular aperture,
It can be inserted at the position of the circle of least confusion of the astigmatism wavefront to give a beam shaping effect, and it is possible to reproduce a clear beam as shown in FIG. 4. -1fC, Fig. 3 The astigmatism wavefront having a donut-shaped amplitude distribution for improving the S-shaped characteristic described above can also be easily recorded by replacing the spatial filter in Fig. 6 with a donut-shaped amplitude pattern. Can be done. The spatial pattern used and the shape of the photodetector (fourth
The angle θ in the figure, the diameter of the circle of least confusion 610, etc.) are design issues, but unlike the conventional simple astigmatism image that is received as is with a four-segment detector, there is a greater degree of freedom in optimally designing the figure-of-eight characteristic. is an essentially important effect provided by the present invention.

In Figure 4, if the wavelength λ of the light source is shifted by Δλ to the longer wavelength side, the reproduced wavefront will become twice as large as 610, for example, and the output of the 7-otodetector will hardly change, but the light source wavelength will fluctuate by a few tens of nanometers. can be easily tolerated. Regarding the characteristics of lens Fourier transform holograms,
Literature ((1) Kanji memory using r-holography”, Kato, Fujito, Sato; Institute of Image Electronics Engineers Research Conference Proceedings 79-04
-1 (1979, 11,) (II) “5peckle
reductionin holography--
-----'', M, Kato at al; 7 Pride Obtics (pp1. opt, ),
14 (1976) 1093) etc.,
As analyzed, the track record of application to general image recording and reproducing optical systems (G [Dr optical kanji editing processing system])
Sato et al.; Materials of the Institute of Electronics and Communication Engineers, E078-53 (1)
97B) 47) However, in the present invention, as long as there is no practical problem as a means for beam control, 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 reproducing the wavefront from the hologram element is A collimating lens can be used instead, or by simply irradiating the hologram element with a convergent spherical wave, it is possible to obtain a desired reconstructed image on the condensing surface. Now, in the recording optical system of FIG. 6, the distance Δ1 between the beams 101 and 103 and 102 in two directions is
．． Δ2 can be designed as M2Δ = δ + I+ M2Δ = δ. Here, δ and +δ2 correspond to the astigmatism difference 2M in the reproduction optical system, the magnification, f corresponds to the focal length of the recording Fourier transform lens 80, f2 corresponds to the focal length of the reproduction Fourier transform lens 8o, and the first In the figure, it is the focal length of the collimating lens 2, and in FIGS. 2 and 3, it is the radius of curvature f2 of the convergent wave incident on the hologram. 1st~
In order to achieve dimensional matching with the optical head optical system and photodetection system described in Fig. 3, the focal length f of the recording Fourier transform lens should be approximately 50 to 100 mm, and the astigmatism difference δ, 10 δ
2 can be designed to be several hundred microns.

FIG. 5 is a conceptual diagram illustrating still another embodiment of the present invention. Figure a shows the functional area division of the hologram element 6660, with areas 691, 692 and 693 recording wavefronts including astigmatism for focusing control, and slit-like gratings recording simple grating patterns in different directions. It consists of areas 68 and 69. The figure shows the wavefront reproduced from the hologram element on the photodetector of the OPU. The configuration is such that 612.622 of the ±2nd-order light components are detected side by side. The diffracted light components from areas 68 and 69 are also ±1
The second and ±second order diffraction lights are detected, and tracking signals are differentially detected from photodetectors 912 and 922.
Furthermore, it is of course possible to utilize the differential output on the conjugate image side.

It goes without saying that the slit grating for tracking signal detection described above can be implemented in combination with all other embodiments of the present invention.

FIG. 8 shows a conceptual diagram of a more desirable configuration that can be generally applied to the first and second embodiments of the present invention. As is clear from the comparison with FIG. 1, FIG. 1A shows the cross-sectional structure of a device 105 in which the semiconductor laser 1 and the photodetector 5 are integrated in a hybrid or monolithic manner, and the emission and incidence states of the light beam. The figure schematically shows a plan view thereof. Conjugate image beam 61
The distance d between the center of the angle 62° and the emission center 10 of the surface emitting laser 100 can be precisely designed and manufactured, and the hologram head can be manufactured by accurately monolithically integrating the photodetector using photolithography technology and placing it on the same substrate. can be assembled very easily and in a short time.

That is, by finely adjusting the hologram element in the direction of rotational movement shown by the dashed arrow in FIG. 8, the relative positions of the beam and the photodetector can be determined correctly. Note that the photodetectors 501 and 502 may be constructed separately from the laser light source and may be a hybrid type in which they are assembled, or only one side 502 may be used instead of a pair, and the surface emitting laser 1oO uses a conventional laser and bends the beam direction by 90 degrees. Of course, it is also possible to implement it with a reflective or diffractive optical system.

Effects of the Invention As described above, the optical head device of the present invention uses two wavefronts generated from a diffraction element for light beam control, and differentially detects the far field pattern of a spot focused on an optical disk using a photodetector. By adopting the detection configuration, the following excellent effects are brought about.

(1) To deal with the wavelength fluctuation of the light source, which is considered to be a weak point when a control beam is generated by a diffraction element, the present invention uses a band-shaped photodetector in which the photodetector dividing line is aligned with the spatial carrier frequency direction of the hologram. More preferably, the problem is solved by a combination of differential calculation outputs.

(11) Conventionally, positional adjustment of a photodetector generally required strict precision in each axis in two axes, but with the present invention, this can be significantly alleviated, and the rotational adjustment of the hologram element or photodetector A highly reliable optical head device can be manufactured by a relatively simple process.

(m> The hologram element of the present invention has a Fourier transform pattern configuration almost like a simple lattice, and can be easily mass-produced with extremely high precision and high diffraction efficiency including blaze.

θ■) An optical disk drive device using the OPU of the present invention, which exhibits the above-described effects, can operate stably even in an environment with extremely rapid temperature changes while using a semiconductor laser light source.

[Brief explanation of the drawing]

FIG. 1a is a schematic configuration diagram of an optical head device showing one embodiment of the present invention, FIG. 1 is a layout diagram of a photodetector, and FIG. 2 is an optical head principle explaining another embodiment of the present invention. Explanatory diagram, Figure 3a
3 is a schematic configuration diagram of an optical head device explaining another embodiment of the present invention, FIG. 3 is a layout diagram of a photodetector, and FIG. 4 a, b, c
FIG. 6a is a general principle diagram of the present invention explaining the operation of a photodetector. 6b is a conceptual diagram of the configuration of still another embodiment of the hologram element and photodetector of the present invention; FIGS. FIG. 7 is a conceptual diagram of the S-shaped characteristic of an optical head optical system using the hologram element of the present invention, FIGS. 8a and 8b are sectional views and plan views of other examples of the detector section, and FIGS. c is a conceptual diagram showing the light receiving state of the conventional optical head optical system, 1st o
The figure is an exploded perspective view of a conventional optical head device. 1... Semiconductor laser or equivalent coherent light source, 2... Collimating lens, 3...
・・Condensing optical system, 4・・Optical disk, 6・River・
・Photodetector, 51...First photodetector,
62...27th Otoday Techta, 63...
...Third photodetector, 6... Hologram element, 66... Reflection type hologram element, 630
...4th photodetector, 8. River...Reflection mirror, 9...Z wave plate, 1o... Light source emission point, 11... Final wave plate , 5o...
Fourier transform lens for recording, 5... Fourier transform lens for playback, 1010... spatial filter,
14...4 division 7 otodaytekta. Name of agent: Patent attorney Toshio Nakao (1 person)
-Rede 2- Kotsumate Lens 3- Lens 4- Child Isk 5-゛Striped Photo≠Itekta 50-・81 Diitekku J4-11 Somura Type Porodilum Laozi 8-M-Mirror 9 --- 1/s Icho Board No. 4 ta+tb
ノ (C) Figure 5 (Q) (b) Figure 6 IυJl Figure 7 Figure 5Q1.5O2--Photonitector Figure 8 Figure 9 (a) <bノ
(C) 14/ +4-4 participle photodetector rot, --1-bit beams puzosota Figure 10

Claims (7)

[Claims] (1) A light source that emits a coherent beam or a quasi-monochromatic beam, a condensing optical system that converges the beam from the light source into a minute spot, and the beam is reflected by a predetermined optical storage medium through the optical system. An optical head device comprising at least a hologram element arranged in an optical path and recording a wavefront including astigmatism, and a band-shaped photodetector that receives light from the element. (2) The optical head device according to claim 1, wherein the hologram element is a lens Fourier transform hologram. (3) The optical head device according to claim 1, wherein the hologram element is a reflection hologram, and the hologram surface is provided at an angle of approximately 45 degrees with the direction of the beam emitted from the light source. (4) The photodetector has a plurality of band-like shapes, and the region division boundary line extends substantially parallel to the radial direction centered on the convergence point of the 0th-order diffracted light from the hologram element. An optical head device according to claim 1. (5) The optical head device according to claim 1, which uses a hologram element that records a wavefront obtained by applying a predetermined amplitude spatial filter to an astigmatic wavefront. (6) The optical head device according to claim 1, wherein the high-order diffracted light component from the hologram element and the first-order diffracted light component are detected by the same band-shaped photodetector. (7) The optical head device according to claim 1, wherein a pair of conjugate images are detected by a pair of band-shaped photodetectors, respectively, and a differential output thereof is obtained.