JPH06111347A - Focus detector - Google Patents

Abstract

PURPOSE:To provide a focus detector which is simple in its constitution and has a high focus detection sensitivity. CONSTITUTION:An outgoing light 1 emitted from a laser beam source goes through a polarized beam splitter 2 and a lambda/4 plate 3 successively, is converged by an objective lens 4 and an information recording is performed by irradiating the surface of an optical information recording medium 5 and the information reproducing and a focus servo are carried out by using a reflected light 6 from the medium 5. The reflected light 6 from the medium 5 again goes through the objective lens 4 and the plate 3 successively and is splitted by the splitter 2 and goes to the different optical path from than that of the light 1. On this optical path, a bisected focus light receiving element 7 is disposed so that bisected lines 7a of light receiving surfaces (a) and (b) are deviated to an orthogoanl direction to the optical axis of the reflected light 6.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a focus detecting device used for exposure of an optical disc master.

[0002]

2. Description of the Related Art Conventionally, in an information recording / reproducing apparatus for recording / reproducing information by converging emitted light emitted from a laser light source by an objective lens and irradiating it onto the surface of an optical information recording medium, There are various methods for detecting the error signal, such as an astigmatism method and a knife edge method. As a specific example, for example, JP-A-59-127.
Japanese Patent No. 238 describes a beam decentering method under the title of "optical information recording device". this is,
The light emitted from the laser light source is obliquely incident on the optical axis of the objective lens and guided to the optical disc surface, and by detecting the deviation of the return light beam from the optical disc surface,
It is intended to calculate the focus error signal amount.

[0003]

In the focus detection method by oblique incidence by the beam eccentricity method as described above, since it is necessary to make the light incident on a position deviated from the optical axis of the objective lens, a large aperture is required. In addition, it is necessary to make the aberration of the objective lens equivalent to the tilted beam, which is expensive. In the case of exposing the master of an optical disc, N. If A is large and the aperture is large, the objective lens itself becomes a heavy object, which hinders focus servo control. Further, in this case, it is possible to reduce the incident light diameter and shift it, but if the diameter is made small, the depth of focus becomes large, and highly accurate focus control cannot be performed.

[0004]

According to a first aspect of the present invention, an outgoing light emitted from a laser light source is sequentially passed through a polarization beam splitter and a λ / 4 plate and then condensed by an objective lens. In the information recording / reproducing apparatus for recording information by irradiating the surface of the information recording medium and for reproducing information and focus servo using reflected light from the optical information recording medium, the optical information recording After the reflected light from the medium again passes through the objective lens and the λ / 4 plate in that order, the reflected light is on an optical path different from the optical path of the emitted light branched by the polarization beam splitter with respect to the optical axis of the reflected light. The two-divided focus light-receiving element is arranged with the dividing line of the light-receiving surface shifted in the right angle direction.

According to a second aspect of the present invention, in the first aspect of the invention, the reflected light from the optical information recording medium branched by the polarization beam splitter is in the optical path between the reflected light and the two-divided focus light receiving element. A condenser lens having the optical axis of as the central optical axis was arranged.

According to the invention of claim 3, claim 1 or 2
In the invention described above, two-division focus light reception is performed at an imaging position on an optical path of reflected light from the optical information recording medium branched by the polarization beam splitter, which corresponds to a set maximum distance between the objective lens and the optical information recording medium. The element was arranged.

[0007]

According to the first aspect of the invention, the light spot on the light receiving surface can be expanded and contracted in a concentric circle according to the distance between the optical information recording medium and the objective lens, and the expansion of these beams is performed. Since the focus position can be detected from the rate of increase or decrease of the focus error signal amount due to the reduction, focus control can be performed without using an objective lens having a large aperture as in the conventional case.

According to the second aspect of the invention, the focal length of the lens can be shortened, so that the detection sensitivity can be further enhanced.

According to a third aspect of the present invention, there are a spreading direction and a shrinking direction including a focused point of light incident on the two-division focus light receiving element, and a moving direction of a distance between the substrates between the objective lens and the optical information recording medium. It is possible to realize a light flux that spreads in one direction, and thus it is possible to freely set the control range without erroneous focus detection directions.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the present invention will be described with reference to FIGS. FIG. 1 shows an example of an information recording / reproducing apparatus.
1 shows a configuration of a focus detection device in an optical disc master exposure device. In this device, an outgoing light 1 emitted from a laser light source (not shown) is sequentially passed through a polarization beam splitter 2 and a λ / 4 plate 3 and then condensed by an objective lens 4 to be an optical disc master 5 as an optical information recording medium. By irradiating the surface of the optical disk, information is recorded, and the reflected light from the optical disk master 5 is used to reproduce information and perform focus servo.

In such an apparatus, in this embodiment,
The reflected light 6 from the optical disk master 5 is returned to the objective lens 4,
After being sequentially passed through the λ / 4 plate 3, the optical path of the outgoing light 1 reflected by the polarization beam splitter 2 is different from that of the outgoing light 1.
The split focus light receiving element 7 is provided. In this case, the two-division focus light receiving element 7 is arranged in a state in which the dividing line 7a of the light receiving surfaces a and b is displaced in the direction perpendicular to the reflected optical axis of the reflected light 6. The focus error signal Fo detected by such a two-divided focus light receiving element 7 drives an objective lens actuator (not shown) via the amplifier 8 to perform focus servo of the objective lens 4.

Here, the reason why the two-division focus light receiving element 7 is arranged in a state where the division line 7a is displaced in the direction perpendicular to the reflection optical axis of the reflection light 6 will be described. Now, in FIG. 2, the diameter of the reflected light 6 incident on the two-divided light receiving surfaces a and b is within a certain range (convergence in FIG. 4A) around the intersection of the cross lines indicated by the dotted lines p and q. (Range where light does not diffuse)
In the above, according to the distance between the optical disk master 5 and the objective lens 4, the optical disk master 5 is enlarged in a concentric circle. At this time, if the dividing line 7a is at the same position as the dotted line p, the difference value (focus error signal) of the outputs from the two light receiving surfaces a and b is always 0, and the difference between the objective lens 4 and the optical disc master 5 is set. It is constant regardless of distance.

However, assuming that the dividing line 7a is displaced to the position of the solid line r, the light receiving surfaces a and b in this displaced state.
The relationship between the value of the focus error signal Fo, which is the difference value of the outputs from the, and the distance between the objective lens 4 and the optical disc master 5, has a waveform as shown in FIG. Such a waveform is proportional to the light receiving area in FIG. 2, and the rate of change in the upper light receiving area is larger than the rate of change in the lower light receiving area of the dividing line 7a. Signal Fo
Has a large diameter on the upper side and a small diameter on the lower side, and the difference appears depending on the distance between the objective lens 4 and the optical disc master 5. Therefore, from such a fact, by using the focus error signal Fo as shown in FIG. 3, it is possible to detect whether the objective lens 4 is distant from or close to the optical disk master 5, or is at the in-focus position. You can

Next, the principle of operation for detecting the focus error signal Fo by using the two-division focus light receiving element 7 in which the division line 7a is displaced is described. The reflected light 6 from the optical disk master 5 passes through the λ / 4 plate 3 again, so that the polarization direction thereof is deviated by 90 ° in a reciprocating manner, and is reflected by the polarization beam splitter 2. Then, the reflected light 6 is divided by the dividing line 7a in FIG.
The light is incident on the two-division focus light receiving element 7 having the light receiving surfaces a and b as shown in FIG. At this time, the reflected light 6 diffuses differently depending on how the optical disc master 5 is separated from the focus position,
It is divided as shown in FIGS. That is, when the master is farther than the focus position, the incident light of parallel light is as shown in FIG.
The convergent light is as shown in FIG. 4A, and when the diffused light is incident, the convergent light is as shown in FIG. In addition, when the master is at the focal point, regardless of the incident form, it will be as shown in Fig. 5 (a).
The convergent light is as shown in (b). Further, when the master is closer to the focus position, the diffused light is as shown in FIGS. 6 (a) and 6 (b).

Here, since the dividing line 7a is arranged so as to be displaced in the direction perpendicular to the reflected light axis, all the reflected light 6 returning to the optical axis of the two-divided focus light receiving element 7 has a non-uniform light quantity. The difference in the amount of received light detected on the light receiving surfaces a and b does not become 0, and is output as a difference due to the size of the received light diameter. By thus detecting the difference value of the received light amount as the focus error signal Fo, the relationship as shown in FIG. 3 can be obtained from the detected signal value.
Focusing can be performed by driving and controlling the objective lens 4.

Therefore, since the focus control is performed using the same optical axis as the objective lens 4, it is not necessary to decenter the beam, and it suffices to shift the position of the dividing line 7a of the two-divided focus light receiving element 7. As a result, it is not necessary to use an objective lens having a large aperture, and an inexpensive device can be obtained. Further, since the reflected light 6 returns on the central axis of the optical axis, the influence of aberration due to the position of the reflected light can be reduced. Assembling and adjusting the two-divided focus light receiving element 7 can be done easily by simply removing the axis. Furthermore, since the focus position can be electrically set in voltage units, it is not necessary to adjust the optical system as in the related art.

Next, a second embodiment of the present invention will be described with reference to FIG. In the configuration of FIG. 1 described above, the spread of the light diameter on the light receiving surfaces a and b is slow with respect to the detected focus error signal Fo, and it cannot be said that the sensitivity is necessarily high. Therefore, in the present embodiment, in the optical path of the reflected light 6 from the optical disc master 5 split by the polarization beam splitter 2 toward the two-divided focus light receiving element 7, the optical axis of the optical path is used as the central optical axis. The optical lens 9 is provided.

By arranging the condenser lens 9 in this way, the image forming distance can be shortened, so that the detection sensitivity can be improved. Further, since the detection sensitivity can be changed by changing the lens focal length, the layout can be freely set. Furthermore, the condenser lens 9
Since it is not necessary to set the lens position with high precision, and the reflected light 6 is coaxial with the lens passing optical axis, the two-divided focus light receiving element 7 can be installed by shifting it from the passing optical axis. It will be easy.

Next, a third embodiment of the present invention will be described. Here, the two embodiments described above (see FIGS. 1 and 7)
In which a two-divided focus light receiving element 7 is arranged at an image forming position on the optical path of the reflected light 6 split by the polarization beam splitter 2 corresponding to the set maximum distance between the objective lens 4 and the optical disc master 5. Is. That is, the two-divided focus light-receiving element 7 is arranged at a position that spreads in one direction including the time of focusing.

Specifically, the shorter the distance between the objective lens 4 and the optical disc master 5, the more infinite the image forming position becomes, and thus the maximum distance between the objective lens 4 and the optical disc master 5 is set. The two-division focus light receiving element 7 is arranged at the image forming position. As a result, since the focus focus position is detected as a light beam that spreads in one direction, the light diameter increases as the master becomes closer, it is possible to prevent erroneous focus detection directions. In addition, the light fluxes incident on the light receiving surfaces a and b include the focal point, and the spreading direction and the contracting direction,
May be placed anywhere for the master distance is compatible, so that other that can take the focus control range freely, it is possible to reduce the like restricted location during assembly .

[0021]

According to the first aspect of the present invention, after the outgoing light emitted from the laser light source is sequentially passed through the polarization beam splitter and the λ / 4 plate, the light is condensed by the objective lens to obtain an optical information recording medium. Reflection from the optical information recording medium in an information recording / reproducing apparatus that performs information recording or the like by irradiating the surface and reproduces information or focus servo using reflected light from the optical information recording medium. After the light again passes through the objective lens and the λ / 4 plate in that order, the light on the light receiving surface with respect to the optical axis of the reflected light is on an optical path different from the optical path of the emitted light branched by the polarization beam splitter. Since the two-divided focus light-receiving element is arranged in a state where the division line is shifted in the right-angled direction, the light spot on the light-receiving surface is enlarged in a concentric circle according to the distance between the optical information recording medium and the objective lens. Shrink, The focus position can be detected from the rate of increase / decrease of the focus error signal amount due to the expansion / contraction of these beams, and thus focus control can be performed without using the conventional objective lens with a large aperture. It is possible to obtain an inexpensive device.

According to a second aspect of the present invention, in the first aspect of the invention, the reflected light from the optical information recording medium branched by the polarization beam splitter is in the optical path between the reflected light and the two-divided focus light receiving element. Since a condenser lens whose optical axis is the central optical axis is provided, the lens focal length can be shortened to further improve the detection sensitivity, and the layout can be freely set by changing the lens focal length. Is what you can do.

According to a third aspect of the present invention, in the first or second aspect of the invention, the optical information recording medium branched by a polarization beam splitter corresponds to the set maximum distance between the objective lens and the optical information recording medium. Since the two-division focus light receiving element is arranged at the image forming position on the optical path of the reflected light from
Divided focus A unidirectionally spread light flux is realized by associating a spreading direction and a shrinking direction including a focal point of light incident on a light receiving element with a moving direction of a distance between substrates between an objective lens and an optical information recording medium. Therefore, the focus control range can be freely set without mistaking the focus detection direction.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing a focus detection apparatus that is a first embodiment of the present invention.

FIG. 2 is a front view showing a two-division focus light receiving element.

FIG. 3 is a characteristic diagram showing a waveform of a focus error signal.

FIG. 4 is an optical path diagram showing an optical path state when a master is present at a position farther than a focal length.

FIG. 5 is an optical path diagram showing an optical path state when the master is at the focal position.

FIG. 6 is an optical path diagram showing an optical path state when a master disk is located at a position closer than a focal length.

FIG. 7 is a configuration diagram showing a focus detection device according to a second embodiment of the present invention.

[Explanation of symbols]

 2 Polarization beam splitter 3 λ / 4 plate 4 Objective lens 5 Optical information recording medium 6 Reflected light 7 Two-division focus light-receiving element 7a Dividing line 9 Condensing lens

Claims (3)

[Claims] 1. Information is obtained by sequentially passing outgoing light emitted from a laser light source through a polarization beam splitter and a λ / 4 plate, and then collecting the light by an objective lens and irradiating it onto the surface of an optical information recording medium. In the information recording / reproducing apparatus for performing the recording and the like, and performing the reproduction of information and the focus servo by using the reflected light from the optical information recording medium, the reflected light from the optical information recording medium is again the objective lens, After sequentially passing through the λ / 4 plate, the dividing line of the light receiving surface was shifted in a direction perpendicular to the optical axis of the reflected light on an optical path different from the optical path of the emitted light branched by the polarization beam splitter. A focus detection device characterized in that a two-divided focus light-receiving element is arranged in this state. 2. A condenser lens having an optical axis of the optical path as a central optical axis is disposed in an optical path between reflected light from the optical information recording medium branched by the polarization beam splitter and directed to a two-divided focus light receiving element. The focus detection device according to claim 1, wherein the focus detection device is provided. 3. Two-division focus light reception at an imaging position on the optical path of the reflected light from the optical information recording medium split by the polarization beam splitter, which corresponds to the set maximum distance between the objective lens and the optical information recording medium. The focus detecting device according to claim 1, wherein an element is provided.