JPS63142540A - Optical head device - Google Patents

Abstract

PURPOSE:To obtain a sufficient focus sensitivity in a state that an S/N is not deteriorated, without adding a new optical member, by working in a concave shape an end face opposed optically to a photodetector of an optical member for leading a light beam which has passed through a recording medium, to the photodetector. CONSTITUTION:Reflected light containing a focus error signal from a recording medium is made incident on an optical waveguide 6 from the upper part of the paper surface, and a part thereof is diffracted by a diffraction grating 8a and made incident on photodetectors 7a, 7b in the optical waveguide 6. The end face 6a of the optical waveguide 6 opposed to the photodetectors 7a, 7b is worked in a concave shape, therefore, a composite focal length of a focal length of the diffraction grating 8a and a focal length of the concave part 6a becomes longer than a focal length of only the optical member in which a concave shape is not worked in the end face of the waveguide. Accordingly, in a state that a DC light (light which is not varied against a focal movement) level is constant, against a variation of a minute luminous flux, its variation can be detected with high sensitivity.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical head device that irradiates an information recording surface of an information carrier with light to read and/or write information.

[Prior Art] Conventionally, optical information reproducing devices have been put into practical use that read information by irradiating read light onto tracks recorded concentrically or spirally on a recording medium. In this device, the recording/reproducing light is formed on a spot on the recording medium to record/reproduce information, but the amount of focusing error based on the amount of defocus of the recording medium is accurately detected, and the recording/reproducing light is must be controlled so that the light is always focused on the recording medium within the depth of focus.

Conventionally, various methods such as astigmatic focusing and knife focusing have been proposed as means for detecting a focusing error signal.

The present applicant proposed a focusing method using a diffraction grating in JP-A-61-11947 and JP-A-61-11951.

FIG. 5 is a configuration diagram showing an example of a conventional optical head device, and FIG.
The figure is a configuration diagram showing the configuration of an optical waveguide having a diffraction grating described in the above patent application.

In FIG. 5, a light beam emitted from a semiconductor laser 1 is converted into a parallel light beam by a collimator lens 2, and a portion thereof is reflected by a beam splitter 3 toward an objective lens 4. The light focused on the recording medium 5 by the objective lens 4 is reflected from the recording medium 5, including signals such as focusing, tracking, and recorded information.

The reflected light from the recording medium 5 passes through the objective lens 4 and beam splitter 3 again, and is focused onto a photodetector 7 by a leading wavepath 6 including a diffraction grating.

FIG. 6 is a diagram of the configuration of a leading waveguide including a diffraction grating and a photodetector, viewed from the recording medium side. The axis AA' in the figure indicates a line parallel to the tracks on the recording medium.

In FIG. 6, reflected light from a recording medium 5 (not shown in FIG. 6) enters diffraction gratings 8a, 8b, and 8c from above the plane of the paper. The light beam incident on the diffraction grating 8a portion is guided in the guide waveguide 6 and is incident on the photodetectors 7a and 7b, and the out-of-focus signal of the recording medium is transmitted to the photodetectors 7a and 7b.
and 7b as the difference between the output signals Ia and Ib.

The light flux incident on the diffraction gratings 8b and 80 is transmitted through the optical waveguide 6.
and enter the photodetectors 7c and 7d, respectively.
The amount of track deviation on the recording medium is detected by photodetectors 7c and 7d.
It is obtained as the difference between the changes in the amount of light of the output signals Ic and Id. Also, output signals Ia from each of the photodetectors 7a, 7b, 7c, and 7d.

The sum of Ib, Ic, and Id is the information reproduction signal engineering R of the recording medium.
It is F.

Here, the principle of detecting the focus error signal will be explained in more detail.

It is assumed that the aforementioned photodetectors 7a and 7b are arranged as shown in FIG. That is, the photodetectors 7a and 7b are arranged so that when the minimum light spot is generated on the recording medium 5 (that is, in a focused state), the light amount distribution is as shown by the hatched area in FIG. 7(b). It is assumed that At this time, if the distance between the recording medium 5 and the objective lens 4 is too large (that is, the state is out of focus), the focal spot moves to the right side in the light receiving surface arrangement direction as shown in FIG. If the objective lens 4 comes too close to the object and becomes out of focus, the focal spot moves to the left as shown in FIG. 7(C).

Therefore, the focus error signal Iaf can be obtained by processing the output currents Ia and Ib of the photodetectors 7a and 7b using an arithmetic unit (not shown) according to the following equation.

Iaf=ia-Ib・・・・・・・・・・・・・・・
- ■ [Problems to be Solved by the Invention] Not only the optical head device shown in the above-mentioned conventional example, but also means for detecting a focus error signal using any method, need to have sufficiently high focus error sensitivity. Needless to say, this is because the amount of change in light amount at the photodetector must be sufficient as a focus control signal for minute movements of the recording medium in the focus direction (movements that deviate slightly from the depth of focus). Therefore, in the past, in order to obtain this sensitivity of the focus error signal, the length of the optical waveguide was lengthened, a new optical member was used, or the sensitivity was obtained by electrical means.

However, if a method of improving sensitivity using electrical means is adopted in the method shown in the conventional example above, the so-called DC
There is a problem in that light (light that does not change with focus movement) is also amplified, and if this DC light exceeds a certain level, the value exceeds the ability of the amplifier.

Further, if a new optical member is used, the number of parts increases, resulting in an increase in manufacturing costs and troublesome optical adjustment. Furthermore, if the length of the optical waveguide is increased, the device becomes larger, the amount of light in the waveguide decreases, resulting in deterioration of the S/N ratio, and furthermore, light is diffusely reflected within the waveguide, resulting in an increase in stray light.

[Means for Solving the Problems] In view of the problems of the prior art described above, it is an object of the present invention to provide sufficient focus sensitivity while keeping the DC light level constant, without adding any new optical members, and to improve the S/N ratio. An object of the present invention is to provide a small and lightweight optical head device that can be obtained without deterioration.

The above purpose is to irradiate a recording medium with a light beam emitted from a light source, detect the light beam passing through the recording medium on a photodetector, and detect the state of the irradiated light on the recording medium. In an optical head device for reproducing and/or recording information, the optical member guides the light passing through the recording medium to a photodetector for detecting the state of the irradiated light on the recording medium. This is achieved by an optical head device characterized in that the end face optically facing the container is machined into a concave shape.

[Function] According to the optical head device as described above, (1) the composite focal length of the focal length of the optical member and the focal length of the recess is longer than the focal length of the optical member alone; It is possible to detect changes in luminous flux with high sensitivity.

(2) This can be carried out by simply machining the end face of the optical member into a concave shape (including spherical and aspherical surfaces), so compared to adding a new member, there is no need to adjust the optical system, which is advantageous in terms of cost.

It has the following advantages.

[Examples] Hereinafter, the optical head device of the present invention will be described in detail based on specific examples.

FIG. 1 is a configuration diagram showing the configuration of an optical waveguide 6 and photodetectors 7a, 7b, 7c, and 7d in an optical head device of the present invention. Note that the overall configuration of the optical head device of the present invention is as follows.
This is the same as the conventional optical head device shown in the figure.

In FIG. 1, substantially the same members as in FIG. 6 described above are given the same numbers, and the functions of each part are as explained in FIG. 6.

The reflected light containing the focus error signal from the recording medium enters the optical waveguide 6, a portion of which is diffracted by the diffraction grating 8a, and guided within the optical waveguide 6 toward photodetectors 7a and 7b. The light guided to the photodetectors 7a, 7b is adjusted to the focal length fl of the diffraction grating 8a and the concave shape of the optical waveguide 6 by the end face 6a which is processed into a concave shape on the optical waveguide 6 facing the photodetectors 7a, 7b. The light is emitted according to the focal length F of the composite lens determined by the focal length f2 of the processed concave lens portion (the end surface 6a), and is emitted to the photodetector 7a.

Reach 7b. Here, the focal length F of the composite lens is determined by the following formula (■), where e is the distance between the diffraction grating 8a and the concave end surface.
It is expressed as

That is, by appropriately setting fl, f2, and e, it is possible to set the focal pressures fa and F of the composite lens to be larger than the focal length f3 of only the diffraction grating 8a when there is no concave portion.

Next, based on FIG. 2, the principle of detection of the focus error signal will be explained by comparing the improvement in sensitivity of the focus error signal with the case where there is no concave processing on the end face of the optical waveguide.

The photodetectors 7a and 7b are arranged as shown in Fig. 2, and Fig. 2 (t), (b), and (C) respectively show the state of the light spot although the end face of the optical waveguide is machined into a concave shape. FIG. Also, Fig. 2 (d), (e), (f)
is a diagram showing a case in which no concave processing is performed. Here, when the minimum light spot is generated on the recording medium (i.e. in the focused state), FIGS. 2(b) and (e)
), it is assumed that the photodetectors 7a and 7b are arranged so as to have a light intensity distribution as shown by the shaded area. At this time, if the distance between the recording medium 5 and the objective lens 4 becomes very small (that is, when the distance is slightly greater than the depth of focus), the waveguide end face with a concave shape becomes as shown in Fig. 2(a). In addition, in the case where the concave shape is not processed, the focal spot moves to the right side in the direction of arrangement of the light-receiving surfaces as shown in FIG. 2(d). In this state, the amount of movement to the right is large because the focal length of the composite lens in the case where the leading waveguide end face is processed into a concave shape is longer than that in the case where the end face is processed into a concave shape.

On the other hand, when the distance between the recording medium 5 and the objective lens 4 approaches by a very small distance (that is, when it becomes slightly closer than the depth of focus), the waveguide surface with a concave shape is shown in FIG. 2 (C).
Like, again. The one without concave processing is shown in Figure 2 (
The focal spot moves to the left in the light-receiving surface arrangement direction as shown in f). Even in this state, when the optical waveguide end face is processed into a concave shape, the focal length of the composite lens is longer than when the optical waveguide end face is not processed into a concave shape, so that the amount of movement to the left is large.

Therefore, the difference signal between the output currents of the photodetectors 7a and 7b is larger when the end face of the optical waveguide is processed into a concave shape than when it is not processed into a concave shape. In other words, the configuration of the present invention provides sufficient sensitivity to minute movements of the recording medium.

As mentioned above, by machining the end face of the optical waveguide facing the photodetector into a concave shape, the sensitivity of the focus error signal is improved, and the actuator etc. This makes it possible to obtain signals that can be quickly responded to.

Next, other embodiments of the present invention will be described.

FIG. 3 is a schematic configuration diagram showing the configuration of the optical head device of the present invention when the optical member that guides the light from the recording medium to the photodetector IO is a prism 9.

In this figure, members having the same functions as those in FIG. 6 are given the same numbers, and 9 is a prism and 10 is a photodetector.

A portion of the light beam emitted from the semiconductor laser l is reflected by the prism 9 toward the collimator lens 2 . This reflected light becomes a parallel light beam by the collimator lens 2, enters the objective lens 4, and is focused on the recording medium 5. Recording medium 5
The reflected light from the mirror contains signals such as focusing, tracking, and recorded information. This reflected light passes through the objective lens 4 and collimator lens 2 again,
A part of it passes into the prism 9. The light beam guided inside the prism 9 exits from the - face 9a of the prism 9, which has a concave end face, and reaches the photodetector 10.

FIG. 4 is a block diagram of the photodetector 10, and the principle of focus error signal detection will be explained using this diagram.

The photodetector lO is divided into four parts as shown in the figure, and each photodetector 1
0a, 10b, 10c, lod, the photodetector 10 is arranged so that when the minimum light spot is generated (that is, in the focused state), the light intensity distribution is as shown by the shaded area in FIG. 4(b). It is assumed that At this time, when the distance between the recording medium 5 and the objective lens 4 becomes slightly larger (that is, when the distance is slightly larger than the depth of focus), the light amount distribution changes as shown in FIG. 4(C). Each photodetector 10a, 10b.

The output currents of 10c and 10d are respectively Ia and Ib.

Assuming Ic and Id, the focus control signal Iaf is expressed by the following 2〇.

Iaf=(Ia+Ic)-(Ib+Id) ---■
This is a conventionally known method for so-called astigmatism autofocus. Here, if the end surface of the prism 9a facing the photodetector 10 in FIG. The focal pressure can be made sufficiently longer than the unprocessed focal pressure #f4, and the amount of change in the light amount distribution at the photodetector IO becomes large with respect to a minute movement of the recording medium in the focus direction. That is, the sensitivity of the focus error signal is improved, and it is possible to obtain a signal that allows the actuator and the like to quickly respond to minute movements of the recording medium in the focus direction.

The present invention is not limited to the above-mentioned embodiments, and various modifications and applications are possible.

In the above two embodiments, focus error signal detection using an optical waveguide including a diffraction structure and astigmatism autofocus detection using a prism were explained.
The present invention is applicable to other known focus error signal detection methods (e.g., beam eccentricity method, Foucault method, beam size method, etc.)
The same applies to

[Effects of the Invention] As explained above, according to the optical head device of the present invention, D
S/ without adding new optical components while keeping the C light level constant.
Sufficient focus sensitivity can be easily obtained without deterioration of the N ratio. A small and lightweight optical head device using various focus error detection principles can be provided.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the structure of an optical waveguide and a photodetector in an optical head device of the present invention. FIG. 2 is a diagram illustrating the improvement in the sensitivity of the focus error signal according to the present invention in comparison with the case where there is no concave processing on the end face of the optical waveguide. FIG. 3 is a schematic configuration diagram showing the configuration of the optical head device of the present invention when the optical member that guides the light from the recording medium to the photodetector is a prism. FIG. 4 is a configuration diagram of a photodetector of the optical head device in FIG. 3. FIG. 5 is a configuration diagram showing an example of a conventional optical head device, and FIG.
This figure is a configuration diagram showing the configuration of an optical waveguide used in the optical head device of FIG. 5. FIG. 7 is a diagram showing the configuration of the photodetector used in FIG. 6. 1: Semiconductor laser 2: Collimator lens 3: Beam splitter 4: Objective lens 5: Recording medium 6: Leading waveguide 7, IO: Photodetector 8a, 8b, 8c: Diffraction grating 9 Nibrism 6a, 9a: Optical member Formed Four-Party Agent Patent Attorney Sekihira Yamashita Fig. 3 Fig. 4 (a) (b) (C) Fig. 5, I 61m 6 Kamii1 Shock (:

Claims (3)

[Claims] (1) Irradiate the recording medium with a beam of light emitted from a light source,
In an optical head device that reproduces and/or records information while detecting a light beam passing through the recording medium on a photodetector and detecting the state of the irradiated light on the recording medium, An optical head characterized in that an end surface optically facing the photodetector of an optical member that guides light to a photodetector for detecting the state of the irradiated light on the recording medium is processed into a concave shape. Device. (2) The optical head device according to claim 1, wherein the optical member is an optical waveguide. (3) The optical head device according to claim 1, wherein the optical member is a prism.