JPH01298539A - Optical pickup head device - Google Patents

Abstract

PURPOSE:To attain stable focus control by maintaining parallel relation among a space carrier frequency axis, the symmetrical axis of a far field pattern relation to the tracks of an optical storage medium and the symmetrical axis of a diffracted light detecting area of a photodetector. CONSTITUTION:The space carrier frequency axis 80 and the symmetrical axis 82 of the diffracted light detecting area of the photodetector at the time of focusing a beam on the optical storage medium are arranged so as to maintain parallel relation. Thereby, when the wavelength of a light source is changed to the long wavelength side, a reproduced wave face is moved from 70 to 73. At that time, the output intensity of a focus error (FE) signal is weakened, but no offset is generated in the EF signal. In addition, the symmetrical axis 81 of the far field pattern with the diffracted light 71 on the photodetector about a track on the medium 4 is arranged so as to be parallel with the axes 80, 82. Consequently, stable focus control can be attained.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an optical vibrator or tube head device capable of recording, reproducing, or erasing optical information stored on a source medium or magneto-optical medium such as an optical disk or an optical card. It is.

Conventional technology Optical memory technology that uses pit-like patterns as a high-density, large-capacity storage medium is being put into practical use with its applications expanding to include digital audio discs, video discs, document file discs, and even data files. . The mechanism by which information is recorded and reproduced successfully with high reliability through a light beam focused on the micron order is entirely dependent on the optical system. The basic functions of an optical pickup head unit (hereinafter abbreviated as OPU) are (I) light focusing ability to form a minute spot with a diffraction limit, (■) focus control of the optical system and pit signal detection, and (III) Tracking control 3
Broadly divided into types. These are realized by combining various optical systems and photoelectric conversion detection methods depending on the purpose and use.

FIG. 8 is a schematic diagram showing an example of a conventional OPU. Normally, a diverging wavefront (electric field: horizontally polarized wave) from a semiconductor laser light source 1 that oscillates in TE mode is converted into a parallel beam by a collimating lens 2, and a left quarter-wave plate (1/4λ plate) 18 for selective reflection. 1/
The circularly polarized wavefront that has passed through the 4λ plate 18 is narrowed down to a spot of approximately 1 μm by the condenser lens system 3, reaches the surface of the optical storage medium (optical disk) 4, and irradiates the pit-like pattern 40. 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 18λ plate 18, becomes a vertically polarized parallel beam, passes through the polarizing beam splitter 106, and is transmitted to the beam splitter 19. It is divided into two directions.

One of the reflected lights passes through a condensing lens 20 and a cylindrical lens 21 imparting astigmatism, enters the four-part photodetector 5, and is converted into a focus error (hereinafter abbreviated as FE) signal. The other transmitted light enters a two-part photodetector 22 for tracking error (hereinafter abbreviated as TE) signal detection with the far field pattern intact.

Here, the 1/4λ plate 18 is the polarizing beam splitter 10
By combining it with E3, it is possible to increase the efficiency of using the amount of light and at the same time suppress the return light to the semiconductor laser, so that unnecessary noise does not increase in the signal light component. However, in the OPU for read-only discs, there is leeway in the light intensity design, and it is possible to omit the 1/4λ plate and polarizing beam splitter.In particular, in order to reduce the size and price, it is necessary to omit parts and combine them. is planned.

Problems to be Solved by the Invention However, even in a read-only OPU, it is necessary to construct a beam splitting means, a focus control means using astigmatism or the Knifezi method, and a tracking control means independently or in combination. Optical components conventionally used for this purpose include beam splitters, lenses,
It is not easy to manufacture, assemble, and adjust prisms in large quantities, and there are problems in miniaturization, cost reduction, mass productivity, and high reliability.

The common reasons for these problems are: First, optical parts that require highly accurate 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, there were also major constraints on miniaturizing the entire optical system.

As a solution to the above problem, a technique has recently been disclosed in which predetermined wavefronts for focus and tracking control are recorded on a single hologram element, and each wavefront reproduced by the reading beam of an optical head is guided to a photodetector.

11-6) 1) Patent application No. 108908, 1982, Oinoue.

Nagai 2) Patent application No. 16850, 1982, on the ceiling.

Nagai 3) Patent Application No. 1987-79877, Matsushita, Tatsumi 4) Y, KImura et al, “Old gh P
performance 0ptlcal Head
uslng 0ptls1zed Hologr
aphlc 0ptlcal Element”, Proceedings of the International Symposium on ψ Optical 0 Memory (Proc.
of the International Symp
osium on 0ptlca! Memory),
Tokyo, 5ept, + [1i-18, 1987 (p
, 131) 5) K, Tatsuml et al+
”A Multi-functional Refle
ctlon Type Grating Lens f
or the CD0ptlcal Head”, Proceedings on the International Symposium on Optical Memory (Proc.
f the International Sympo
sium on OptlcalMemory), T
OKYO, 5ept, IB-18, 1987 (p, 12
7) Of the above, 4) is the FE multiplied signal double knife method, which detects the TE multiplied signal by the intensity of diffracted light from a slit grating provided on the far field (hologram element surface). As shown in FIG. 9, astigmatism wavefronts 140, 141.degree. 142 are calculated from the signals received by the quadrant photodetector 5 to detect FE and TE multiplied signals. For example, after adding the outputs of each detector of an FE multiplied signal quadrant photodetector in adder circuits 31 and 32, the difference is taken in a differential circuit 33, and the signal processing circuit 3
4 by signal processing. The FE multiplied signal characteristics thus obtained are shown in FIG. 10A. However, a serious problem with each method is that when the wavelength of the light source deviates from the design standard wavelength λ by δλ, each beam on the photodetector changes (for example, from 140 to 1401).
As a result, an offset occurs in the FE multiplied signal as shown in FIG. 10B. Furthermore, since the FE multiplied signal tracking signal component is mixed in, it becomes difficult to perform accurate focus control. In a normal semiconductor laser, if the operating environment temperature changes by, for example, 60°C, the laser oscillation wavelength will change by 1.
When the laser output is changed by 40 mW, the wavelength changes by about 8 nm. There remains the problem of stable error signal detection that can cope with changes in the diffraction angle from the hologram element that occur due to such wavelength fluctuations.

Means for Solving the Problems In order to solve the above-mentioned problems, the present invention provides an axis (spatial carrier frequency axis) that connects the diffracted light from the hologram element and the conjugate image of the diffracted light, and the beam from the light source. The axis of symmetry of the far-field pattern of the diffracted light on the photodetector surface with respect to the track of the optical storage medium when the beam from the light source is focused on the optical storage medium, and the diffracted light receiving area of the photodetector when the beam from the light source is focused on the optical storage medium. The axes of symmetry are arranged in a parallel relationship with each other.

In the present invention, with the above-described configuration, even if the diffraction angle of the diffracted light from the hologram element changes due to wavelength fluctuation in the light source, the TE multiplied signal is not mixed into the FE multiplied signal, and furthermore, the FE multiplied signal offset is prevented. Since this does not occur, stable focus control is possible.

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, a wavelength λ2"800n
m), 2 is a collimating lens (for example, focal length 1fc
'X20mm) 3 is an objective lens for focusing, 4 is an optical storage medium (optical disk), the beam emitted from the light source 1 is made into a parallel beam by the collimating lens 2, and is focused onto the disk 4 by the lens 3. Ru. At this time, reference numeral 6 denotes a hologram element recording a wavefront including astigmatism, which is interposed between the lenses 2 and 3, and its zero-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 and becomes almost parallel light, and then passes through the hologram element 6.
In addition to the zero-order transmitted light, two wavefronts 71 and 72 of an astigmatism reconstructed image and its conjugate image are generated off-axis. Here, the hologram element 8 is a Fourier transform type hologram, which will be described later, and these pair of conjugate wavefronts 71 and 72 are converged via the collimating lens 2, and when the focus is correctly set on the disk 4, zero-order transmission occurs. 111, which is perpendicular to the optical axis of lens 2, including the convergence point of light (light emitting point 10 of light source 1)
forms an astigmatic image in the direction orthogonal to each of the two planes at the front and rear positions. Reference numeral 5 denotes a four-part photodetector that receives light and is composed of detectors 51, 52, 53, and 54. Each focal plane and plane 11
1 is designed to be δ1=δ2=δ.

The figure (b) shows the first... placed on the surface 111. 2nd゜3rd.
and the relationship between the fourth photodetector 51.52°53.54 and the light emitting point 10. This figure shows a reproduced image corresponding to the cane state, which is correctly focused on the disk. In the figure, one of the incident beams on the photoelectric conversion surface of the photodetector 5 is shown as 71, and is aligned with the conjugate image 72 on the straight line XX- passing through the light emitting point.

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 68 is used to bend the optical axis at αy90°. 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(a) illustrates yet another embodiment of the present invention. FIG. 3(b) shows the state of the diffracted light from the hologram element 668 on the photodetector 55, and 551 is a four-part photodetector that receives the astigmatism reconstructed image; detectors 5511, 5512, and 5513
．． Consists of 5514. The configuration shown in FIG. 3 differs from the previous two examples in that the polarizing beam splitter 1
7 and a wavelength plate 9, and a configuration in which a fifth photodetector 5515 separately performs high frequency information (RF) signal detection using the zero-order transmitted light 70.

Here, the wavelength plate 9 has a λ15100 design in order to facilitate the performance balance with the polarizing beam splitter 107,
S/N of signal detection by optimizing the amount of light returned to light source 1
The ratio is maximized. Mirror 8 is for bending the optical path. In this case, the hologram element 666 can be blazed if necessary to provide maximum diffraction efficiency, and the conjugate image 72 becomes a weak beam.

Now, the structure of the photodetector and the signal detection method related to the present invention in the above embodiment will be explained in detail. Fourth
The figure shows a four-part photodetector 51 shown in FIG. 1(b).
, 52, 53, and 54 schematically and generally represent the relationship between the beam 71 and the light emitting point 10 detected in each of the divided regions. FIG. 4(b) shows a case where a focused spot is formed on the disk, and FIGS. 4(a) and (C) each show a defocused state in opposite phases. The focus control characteristic (so-called 8-character characteristic) can be obtained by calculating the astigmatism wavefront from the signal received by the quadrant photodetector as shown in FIG. 9, and this is a well-known technique. The FE multiplied signal characteristics obtained at this time are shown in FIG. 5A. An axis (spatial carrier frequency axis) 80 connecting the diffracted light 71 from the hologram element and the conjugate image 72 of the diffracted light, and the symmetry axis of the diffracted light receiving area of the photodetector when the beam from the light source is focused on the optical storage medium. If they are arranged in a parallel relationship with 82, when the wavelength λ2 of the light source changes by δλ toward the long wavelength side, the reproduced wavefront will move from 71 to 73. At this time, the FE multiplied signal characteristics change as shown in FIG. 5B, and the output intensity becomes weaker, but no FE multiplied signal offset occurs. Furthermore, when the beam from the light source is focused on the optical storage medium, the symmetry axis 81 of the far field pattern with respect to the track of the optical storage medium 4 in the diffracted light 71 on the photodetector surface is also connected to the spatial carrier frequency axis 80 of the hologram element and the photodetector. If they are arranged parallel to the axis of symmetry 82 of the light source, stable focus control is possible because even if wavelength fluctuation occurs in the light source, the TE multiplied signal will not be mixed in with the FE multiplied signal.

FIG. 6 is a conceptual diagram of an optical system that realizes a hologram element capable of accurately recording and reproducing an astigmatic wavefront according to the present invention. A cylindrical lens 17 is arranged in an optical path in which a coherent parallel beam 13 with a wavelength λ1 is narrowed down by a condensing lens 501,
Astigmatism images, that is, linear convergent beams 101° 103 oriented in directions perpendicular to each other, and a substantially circular beam 102 at an intermediate position are obtained. Now, the circular beam 102 is
+ −Y Assume that it is on the I coordinate plane. This optical system is similar to that conventionally used to generate astigmatism in an optical pickup optical system, but what is important here is that the Fourier transform lens 500 (focal length f
+), the Fourier transform wavefront of the circular beam 102 is taken out to the rear Fourier transform surface (indicated by ξ1-η1 coordinates) of the Fourier transform lens 500, and superimposed with another plane wave that does not include aberration. This is to create a so-called lens Fourier transform type hologram element 12.

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 500. Here, the reference wave is easily obtained by converging the parallel beam 13 and the parallel beam 14 which are mutually coherent with each other using the lens 15. In recording the reflection type hologram used in the embodiment of FIG. 2, the hologram recording surface 12 may be tilted by 45° with respect to two axes and the η axis.

Now, the hologram element 12 recorded in this way is placed in an optical system as shown in FIG. 6(b), and the wavelength λ2 is
When irradiated with a parallel beam of
0 (focal length f2), the circular beam 1021 and its conjugate image 1022 are symmetrical with respect to the origin of the X2-Y2 coordinates even under the condition of λ, ≠λ2. Convergent beams 1011, 1031 and 1 are reproduced without distortion due to the positional relationship and exhibit horizontal or vertical linear patterns in the front and rear directions of each spot image.
012°1032 is obtained. Convergent beam 1012.1
032 are conjugate images of the convergent beams 1031 and 1011, respectively, so one convergent beam 1031 exhibits a linear pattern in the vertical direction is located close to the Fourier transform lens 5000, and the other exhibits a linear pattern in the horizontal direction. Convergent beams 1012 appear side by side. If this hologram element 12 is illuminated with a slightly diverging spherical wave,
- Convergent beam 1012.10 in a conjugate relationship on the Y2 plane
The astigmatism image in the vicinity of 31 is formed, and the image shown in Fig. 4 (
The states shown in a) and (C) are obtained. Conversely, when the hologram element 12 is illuminated with a convergent spherical surface, astigmatism images near the convergent beams 1032 and 1011 in FIG. 6 will be detected.

In FIG. 6(a), if the spatial filter 1010 has a circular aperture and is inserted at the position of the circle of least confusion of the astigmatism wavefront, a beam shaping effect can be provided and a clear beam can be reproduced.

The characteristics of the lens Fourier transform hologram are reported and analyzed in detail in references 6) to 7), and have been applied to general image recording and reproducing optical systems8). As long as there is no practical problem as a control means, it is sufficient that the 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 can be replaced by a collimating lens, or simply by focusing the hologram element. Just irradiate with spherical waves,
It is possible to obtain a desired reconstructed image on the light collection surface.

6) “Kanji memory using holography” Kato.

Fujito, Sato; Proceedings of the Society of Image Electronics Engineers Research Group 79-04-1 (
1979, November) 8) "Optical Kanji Editing Processing System" Sato et al.; Institute of Electronics and Communication Engineers Study Group Materials, EC78-53 (1978) 47
Now, with the recording optical system shown in Fig. 6, 101. 103 of 102
The distance in the Z direction from Δ1. Δ2 can be designed as M2Δ1=δI M2Δ2=δ2 δ,=δ2 M=λ2/λ1·f 2/f + . Here, 61 + δ2 corresponds to the astigmatism difference 9M in the reproduction optical system, which is the magnification + fI is the focal length of the recording Fourier transform lens 500 + f2 corresponds to the focal length of the reproduction Fourier transform lens 5000, and in Fig. 1, the collimation Focal length of lens 2, Fig. 2,
In Figure 3, the radius of curvature f2 of the convergent wave incident on the hologram
It is. In order to achieve dimensional matching with the optical head optical system described in FIGS. 1 to 3, the focal length f1 of the recording Fourier transform lens is set to 50 to 100 mm, and the astigmatism δ is
1+δ2 can be designed to be several hundred microns. Note that since the hologram element according to the present invention has a pattern configuration almost similar to a simple lattice of the Fourier transform type, it is also possible to create a hologram element by synthesizing patterns using a computer.

FIG. 7 is a conceptual diagram explaining another embodiment of the present invention,
For example, in the optical system shown in FIG. 1(a), a reproduced image 74 obtained on the photodetector 550 is obtained by using a hologram element such as a Fresnel zone plate in which a wavefront having two conjugate focal points is recorded as the hologram element 6. .75 and the light emitting point 10. Figure 7 (
b) shows a case where a focused spot is formed on the disk 4, and FIGS. 7(a) and 7(C) each show a defocused state in reverse phase. FE double signal photodetector 5
The FE multiplied signal output strength can be obtained by taking the differential between 501 and 5504, and further using the outputs of photodetectors 5500, 5502, 5503 and 5505. This detection method is called spot size detection and is a well-known technique. Now, the axis (
If the spatial carrier wave frequency axis) 80, the symmetry axis 81 of the far-field pattern regarding the track in the diffracted light, and the symmetry axis 82 of the diffracted light receiving area of the photodetector are arranged in a parallel relationship, the wavelength λ2 of the light source can be varied. Even if the diffracted light is moved in parallel on the photodetector (for example, from 74 to 741), the FE multiplication signal characteristics do not change and stable focus control is possible. Furthermore, in this case, even if the diffracted light moves in a direction perpendicular to the spatial carrier frequency axis 80 of the hologram and the symmetry axis 82 of the photodetector, the FE multiplied signal output intensity decreases, but an offset occurs or the TE multiplied signal is mixed. Therefore, it is possible to perform extremely stable focus control, and the permissible range of not only the wavelength fluctuation of the light source but also the set position of the photodetector is greatly expanded.

Effects of the Invention As described above, the axis (spatial carrier frequency axis) connecting the diffracted light from the hologram element of the present invention and its conjugate image and the axis on the photodetector surface when the beam from the light source is focused on the optical storage medium. The axis of symmetry of the far-field pattern with respect to the track of the optical storage medium in the diffracted light at The optical head device placed in

(I) In order to address the deterioration of the stability of focus control due to the wavelength fluctuation of the light source, which is considered to be a weak point when the control beam wavefront is generated by the diffracted light from the hologram element, in the present invention, the spatial carrier frequency axis of the hologram element and the track By arranging the symmetry axis of the far field pattern and the symmetry axis of the photodetector so that they are parallel to each other, there is no offset in the focus error (FE) signal or mixing of the tracking error (TE) signal. Therefore, extremely stable focus control can be performed.

(II) Conventionally, the position adjustment of a photodetector has generally required strict precision, but the present invention can significantly alleviate this requirement.

(III) The adjustment process in the manufacturing process of optical head devices is greatly simplified by expanding the tolerance range for the wavelength fluctuation of the light source and the position adjustment of the photodetector.
This makes it possible to lower prices.

(IV) By expanding the tolerance range for the wavelength fluctuation of the light source and the position adjustment of the photodetector, the optical pickup head device of the present invention is stable and reliable even in an environment with extremely rapid temperature changes while using a semiconductor laser as the light source. Highly condylar movements are possible.

[Brief explanation of the drawing]

FIG. 1(a) is a schematic configuration diagram of an optical head device showing one embodiment of the present invention, FIG. 1(b) is a diagram of the relationship between a photodetector and a light emitting point, and FIG. 2 is a diagram showing another embodiment of the present invention. FIG. 3(a) is a schematic configuration diagram of an optical head device for explaining an example, and FIG. 3(b) is a schematic configuration diagram of an optical head device for explaining another embodiment of the present invention.
4(a), (b), and (c) are diagrams showing the relationship between the photodetector and the diffracted light from the hologram element, and FIGS. 4(a), (b), and (c) are diagrams illustrating an implementation of a recording/reproducing optical system that realizes a general hologram element to explain the present invention. Example configuration diagram, Figure 7 (a), (
b) and (C) are diagrams showing the relationship between diffracted light from a photodetector and a hologram element to explain other embodiments of the present invention; a configuration diagram showing an example of an astigmatism wavefront detection system of an optical pickup head device; The figure shows a conventional optical pickup head device using a hologram element. ...Four-section photodetector, 6...
Hologram element, 9...115 Wave plate, 10... Light emitting point, 511... First detector, 52... Second detector, 53... Third detector, 54... Fourth detector, 55 ...Photodetector, 70...0
Next light, 711@ψ astigmatism reproduction wavefront, 72... astigmatism reproduction wavefront (conjugate image), 73... astigmatism reproduction wavefront,
80 @ φ fist spatial carrier wave frequency axis, 81...Symmetry M of far field pattern with respect to the track, 82...
Symmetry axis of photodetector, 107...Polarizing beam splitter. Name of agent: Patent attorney Toshio Nakao (1 person) Figure 1 IJ hologram cloth Figure 2 δ--Mirror'? -113 sill Nagasai En・°-Bolojiram's sky M Chrysanthemum road favorite broken narrow & +---
Track Farfield Bataso Coupling Axis 82 - Photo Neidekta's ttsm Fig. 4 (σ) (bJ (C) Fig. 5 Fig. 6 (Q) ! - Light source --- Collimation death '
3° = jt! IF lens 4- Optical recording medium (first lens) 5-4 minutes t1 Photonometer 18°-1/4 Dip plate 19- Do-Muspri νri, T-11 front lens zI--Cylindrical lens? - 2-minute photonitector ω-t'' call 1% - - polarization - polarization - 1 - differential circuit・-0 points!! Jll chewing surface Figure 9 (aν mountain) (C) Figure 10

Claims (2)

[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 a light source that emits a coherent beam or a quasi-monochromatic beam; An axis (spatial carrier frequency axis) that includes at least a hologram element that generates diffracted light and a photodetector that receives the diffracted light from the hologram element, and connects the diffracted light from the hologram element and a conjugate image of the diffracted light. and an axis of symmetry of a diffracted light receiving area of the photodetector when the beam from the light source is focused on the optical storage medium are in a parallel relationship. (2) 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 onto a minute spot; and a light source that emits a coherent beam or a quasi-monochromatic beam; An axis (spatial carrier frequency axis) that includes at least a hologram element that generates diffracted light and a photodetector that receives the diffracted light from the hologram element, and connects the diffracted light from the hologram element and a conjugate image of the diffracted light. , an axis of symmetry of a far field pattern with respect to the track of the optical storage medium in the diffracted light on the photodetector surface when the beam from the light source is focused on the optical storage medium, and a beam from the light source is focused on the optical storage medium. An optical pickup head device characterized in that the symmetry axes of the diffracted light receiving areas of the photodetector when the medium is in focus are parallel to each other.