JPH01220134A - Optical head device - Google Patents

Abstract

PURPOSE:To independently and stably execute each detection of focusing and tracking and to obtain a compact optical head by using a hologram, for which respective wave surfaces are written overlapped to a coherent. CONSTITUTION:An outgoing light beam L5 of a light source 6 is made to be parallel light by a lens 7 and transmitted through a hologram 8. After that, the beam is condensed on a disk 10 by an objective lens 9 and the reflected light is incoming through the lens 9 to the phologram 8. A primary refracting light L6 in the hologram 8 is incoming to a detector 11. The refracting light 6 from areas e1-e3 of the hologram 8 is incoming over detectors S1-S4 and a focus error signal can be obtained. A tracking error signal can be obtained from detector S5 and S6.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an optical head for recording or reproducing information in an optical information device.

The optical system of the optical head was simplified using a conventional technology hologram.
is a semiconductor laser light source. A beam L5 emitted from this light source passes through the hologram 2, enters the objective lens 3, and is focused onto the disk 4. The light reflected by the disk retraces its original optical path and enters the hologram 2. The hologram is divided into four parts, Hl-H4, and each
It is recorded independently as interference between spherical waves emitted from each point light source located at positions corresponding to P1 to P4 and spherical waves emitted from a point corresponding to the position of the semiconductor laser light source. At this time, from the beam incident on H1 to H4 of the hologram, P
Diffracted light L6 that forms an image at points 1 to P4 is generated and enters the detector 5. The detector is divided into six parts S1 to S6. FIG. 12 schematically shows the light spot on the detector of 5l-S4, showing the case (b) at the just focus position, and (a) and (C) before and after the just focus position. Therefore, the focus error signal FE is obtained by the calculation E=(S 1 +54)−(S2+S3). Also, Fig. 5 schematically shows the light spots (PI and P2) on the detectors S5 and S6, and shows the state (b) where tracking control is applied accurately, and
(A) and (C) before and after that are shown (note that the diffracted light (diffraction image) for detecting a focus error signal in the center is not shown). Therefore, the tracking error signal TE used for tracking is TE=
It is obtained by the calculation S5-96.

Problems to be Solved by the Invention However, according to the optical system configuration using such a hologram element, the light beam L5 emitted from the light source is diffracted toward the disk 4 side as in the 13th time even on the outward path passing through the hologram. Light is focused. Even if a considerable portion of this diffracted light is blocked by the objective lens aperture, it forms focused spots P5 and P6. In the conventional example, the ±1st-order diffractometer 7 is about a fraction of the size of the 0th-order diffracted light, and there is a risk that the disc 4 may be partially recorded by the diffracted light L7. Therefore, it is unsuitable as an optical head for recording and reproduction. Furthermore, if the school carrier frequency of the hologram is increased so that unnecessary diffraction components are largely blocked by the objective lens aperture, the influence of the wavelength fluctuation of the light source becomes large, and the configuration in which the distance between the hologram and the objective lens is made large contains similar difficulties.

Means for Solving the Problems In the present invention, in order to solve the above-mentioned problems, the respective wavefronts used to separate and detect the tracking error signal and the focus error signal as predetermined diffracted lights are modified to reduce the characteristic deterioration of the focus error signal. In order to prevent this from occurring, holograms written in a coherent manner are used. For example, a hologram recording an astigmatic wavefront can be used to detect a focus error signal. Further, as a wavefront for detecting a tracking error signal, a simple spot or information on astigmatism is coherently superimposed on a wavefront for detecting a focus error signal and written on the hologram.

Function: Due to the hologram and objective lens optical system for reproducing astigmatic wavefronts of the present invention, the diffracted light condensed onto the disk in the outward path has astigmatism, so that the diffracted light component is transmitted to the disk device for recording and reproducing. There is no risk of erroneous recording, and since the astigmatism wavefront is coherently recorded in the hologram area for tracking error signal generation,
Complete astigmatism with no omissions is reproduced as the focus error signal, and the rereading signal can be accurately detected.

Example The present invention will be described in detail below using the drawings.

FIG. 1 and FIG. 2 are diagrams for explaining the principle of an embodiment of the present invention. The light source 6 usually uses a semiconductor laser,
In some cases, an optical system for wavefront correction is included, but since it is not directly related to the present invention, a description thereof will be omitted.

The light beam L5 emitted from the light source 6 passes through the collimating lens 7
The parallel light passes through the hologram 8 and is focused onto the disk 10 by the objective lens 9 . The light beam reflected by the information recording/reproducing surface of the disk passes through the objective lens 9 again and enters the hologram 8. As shown in FIG. 2, the +1st-order diffracted light (or -1st-order diffracted light) L6 (L6-1 to L6-3) diffracted by the hologram 8 is condensed by a collimating lens 7, and then sent to a detector placed near the light source 6. 11. L6-1. L6-
3 is a diffracted light for tracking error signal detection.

The hologram 8 has area divisions as shown in FIG.
The diffracted light L6-2 emitted from e2 and e3 enters the detectors S1 to S4 as described later. The focus error signal FE is obtained from the signals S1 to S4. FIG. 4 shows the state of diffracted light on 5l-94. (b) shows the state of just focus. (a)
and (C) represent the states before and after that. Therefore, the focus error signal FE is obtained as FE=(S1+94)-(S2+53).

Also, the tracking error signal is (S1+52)-(
Although it is possible to detect it as S3+54), it is more stable to separate it from the S5 and S6 signals. S5
The arrangement of diffracted light L6-1°L6-3 on S6 is the first
This is the same as the case shown in FIG. 1, and is again shown in FIG. When (b) is tracked, (a)
and (C) show the states before and after that. However, in the figure, for explanatory purposes, both 85 and S6 are shown enlarged compared to the astigmatism image (not shown) detected in the central portion. Therefore, the tracking error signal TE is obtained from TE=S5-S6.

Since three diffracted light components are used for both or one of the ±1st-order diffracted lights obtained from the hologram 8, this diffracted light is generated on the outward path as well, as shown in Figure 6, but one of the diffracted lights has astigmatism. One (L7-1), the other two (L?
-2) is a plane wave that is diffracted from a portion of at most 20 to 30% of the total area of the hologram, and its intensity is relatively weak. Therefore, no matter which type of diffracted light is used, unnecessary recording is not made on the recording/reproducing surface during recording/reproducing, unlike the conventional example.

Of course, the astigmatism wavefront may also be used for tracking error signal detection if necessary.

As another embodiment, if it is necessary to further reduce the size of the optical head, it is of course possible to use the configuration shown in FIG. 7 and omit the collimating (condensing) lens 7 shown in FIG. 1. 21 is a lens.

Next, a method for creating the hologram 8 is shown in FIGS. 8 and 9. First, in FIG. 8, Ll-L4 are all mutually coherent plane waves. 12 to 17 are Fourier transform lenses. Ll passes through the Fourier transform lens 12 and the cylindrical lens 13 to generate an astigmatism wavefront, and is adjusted to become the circle of least confusion at point C. Also L2
~L4 are focused on points a, s, and b by Fourier transform lenses 14 to 16, respectively. Figure b shows the positional relationship of points A, B, c, and s on the xy plane. All the light up to Ll-L4 above passes through the Fourier transform lens 17 and is sent to the dry plate 8.
Causes interference fringes. When creating the hologram 8 in Fig. 1, only Ll and L2 are irradiated when recording the parts el, e2, and e3 in Fig. 3, and Ll, L2, and L4 are irradiated when recording the part C. When recording the portion d, only Ll, L2, and L3 are irradiated.

Furthermore, another method of creating the hologram 8 will be described.

In FIG. 9, Ll and L2 are mutually coherent plane waves. First, Ll generates a pair of diffracted lights (for example, +1st order diffracted lights) L8 of the same order by a double grating 24 as shown in FIG. 1O. Other diffracted light components are blocked. These two diffracted lights L8 and transmitted lights L9 all pass through the Fourier transform lens 12 and the cylindrical lens 13 to produce astigmatic wavefronts, which are adjusted to form circles of least confusion at points A', B', and C. . The light beams L4 are each focused on a point S by a Fourier transform lens 15.

Figure b shows the positional relationship of points A', B', C, and S on the Xy plane. All of these lights pass through the Fourier transform lens 17 and produce interference fringes on the dry plate 8.

As another embodiment, if a configuration as shown in FIG. 7 is used, a similar Fourier transform hologram may be recorded, and the detector position may be selected as shown in FIG. 7 for optical detection.

Finally, the positions where C and d are created in the hologram 8 of FIG. 3 will be described. When the position of the light beam on the disk changes in a direction perpendicular to the track, the light intensity of the far field pattern of the light beam incident on the hologram changes. By creating C and d in the hologram 8 at positions where the light intensity of this far field pattern changes most sensitively, a tracking error signal can be efficiently obtained.

Effects of the Invention As shown above, the present invention provides a hologram that generates a wavefront of astigmatism and diffracted light for tracking error signal detection.
By coherently superimposing the optical head, focusing and tracking detection can be performed independently and stably, and an optical head that is small, inexpensive, and usable for recording and reproducing can be developed.

[Brief explanation of the drawing]

FIG. 1(a) is a schematic diagram of a head according to an embodiment of the present invention, FIG. 1(b) is a front view of a part of the detector, FIG. 2 is a perspective view of the structure of the device, and FIG. 3 is a front view of a hologram. Figure 4 (a
) to (c) are diagrams showing the state of diffracted light for focus error signal detection on the detector of the present invention, and FIG. 5(a)
~(c) are diagrams showing the state of the diffracted light for tracking error signals on the detector in the present invention and the conventional example, and FIGS. 6(a) and 6(b) are the diffracted light on the outward path in the embodiment of the present invention. 7(a) is an enlarged view of the X portion of FIG. 7(b), and FIG. 7 is a schematic diagram of a head according to another embodiment of the present invention.
8(a) and (b) are diagrams showing an example of the hologram creation method of the present invention, FIGS. 9(a) and (b) are diagrams showing another example of the hologram creation method, and FIG. is a diagram showing the double grating used in FIG. 7, FIG. 11 is a schematic diagram of a conventional head, and FIGS. 12 (a) to (C) are diagrams on a diffracted light detector for detecting a focus error signal in the conventional example. FIG. 13 is a diagram showing the state of diffracted light on the forward path of a light beam in a conventional example. 6... Light source, 8... Hologram, 10... Disk, 11... Detector, 51-S4... Focus error signal detection element, Ss, Ss... Tracking error signal detection element, L6...Diffracted light. Name of agent Patent attorney Toshio Nakao (1 person) Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Fang Figure 9 Figure 10 Figure 11 Figure 12 ( Cn Cb)
(C-) Figure 13 3 171st order @exclusion

Claims (3)

[Claims] (1) a synchrotron radiation source; an objective lens that receives a light beam emitted from the synchrotron radiation source and converges it onto an information carrier; A hologram in which wavefronts used to generate and separate and detect tracking error signals and focus error signals as predetermined diffracted lights are coherently superimposed; An optical head device comprising a photodetector configured to generate an output proportional to the amount of light obtained by dividing the light or each diffracted light into a plurality of parts. (2) The optical head device according to claim 1, wherein the wavefront written on the hologram for separating the focus error signal is a wavefront including a predetermined astigmatism. (3) Of the beams incident on the hologram, the far-field diffraction pattern portion that changes sensitively in response to the position of the convergent beam on the information carrier generates diffracted light for tracking error signal separation from the incident hologram portion. An optical head device according to claim 1.