JPS63173233A - Focal error detector - Google Patents

Abstract

PURPOSE:To always attain stable focus control by changing suitably a gain balance of each photodetector in accordance with a shift in the track scanning direction of a far field image. CONSTITUTION:Each output of photodetectors 1a-1d is denoted by A-D, respectively, and an adding means 31 and 32 calculates A+B and C+D from outputs of each photodetector. In such a case, if the center of a far field image is positioned on an intersection of parting lines, outputs of both output of both are equal, and if it is displaced, output of both become unbalanced by a portion corresponding to its quantity, and A+B<C+D or A+B>C+D depends on the displacement direction of the far field image. Accordingly, the outputs of the adding means 31 and 32 have information related to the shift direction and the shift quantity of the far field image. A comparing means 4 generates a control signal for amplifying the outputs of the photodetectors 1a-1d by the respective different amplification factors in accordance with said information, and variable amplifying means 5a-5d can change the amplification factor in accordance with its control signal.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a focus error detection device for detecting the focal point position of an objective lens actuator in an optical disk device (particularly one having a continuous track).

2. Description of the Related Art In recent years, optical disk devices with large capacity and high information retrieval capabilities have been gradually put into practical use. An optical disk device is capable of recording and reproducing high-density information by irradiating a recording medium with a laser beam focused on a spot with a diameter of 1 to 2 μm using an objective lens.

Optical disk devices are composed of various technical elements, among which focus control technology that always maintains the distance between the objective lens and the recording medium at the focal length of the objective lens records and reproduces high-density information. It is no exaggeration to say that this is the most important technology. Among focus control technologies, focus error detection technology that detects the amount of deviation from focus is a technology that particularly requires high precision and reliability. Up to now, excellent methods with high detection sensitivity have been devised, such as the Astigma detection method and the Foucault (Naifetsugi) detection method.

An example of the conventional focus error detection device described above will be described below with reference to the drawings.

FIGS. 7A and 7B are block diagrams of a conventional focus error detection device. In FIG. 7(A),
10 is a recording medium. 1) is a laser light source, 12 is a half mirror, 1) 3 is an objective lens, 14 is a convex lens, 15
is a cylindrical lens.

1 is a light receiving means. The light-receiving means l has four light-receiving elements (light-receiving parts) la, lb,
It is divided into lc and ld, and one of the dividing lines coincides with the track direction provided on the recording medium 10. Further, the cylindrical lens 15 is installed so that its generatrix is at an angle of 45° with respect to the dividing line (not explicitly shown in the figure). 2 is a focus error detection means for calculating a focus error from the outputs of the light receiving elements 1a to 1d.

The operation of the focus error detection device configured as described above will be explained below.

The focus error is detected by an optical system composed of an objective lens 13, a convex lens 14, a cylindrical lens 15, and a light receiving means l. That is, the relative position between the recording medium 10 and the objective lens 13 can be determined by producing astigmatism using the convex lens 14 and the cylindrical lens 15 and detecting the two focal positions obtained from the astigmatism using the light receiving means 1. can. Now, when the recording medium 10 is located at the focal point of the objective lens 13, the reflected light passes through the convex lens 14 and the cylindrical lens 15, and then focuses on two points on the optical axis (meridian image and spherical image). . These two
If the light receiving means 1 is provided near the center of the focal point, the image on the light receiving means 1 will be approximately circular. When the recording medium 10 is displaced from the focal position of the objective lens 13, the two focal points also move along the optical axis (at this time, the image on the light receiving means 1 extends in the diagonal direction, that is, in the 45'' or 1456 direction from the dividing line. It becomes an ellipse.The light receiving element 1a.

The outputs of lb, lc, and ld are A, B, and C, respectively.

If D, a focus error signal can be obtained by electrically calculating (A+C)-(B+D). The focus error detection means 2 is for performing such processing. This method is called astigma detection method, and is disclosed in Japanese Patent Application Laid-open No. 53-1003 or Japanese Patent Application Laid-open No. 53
-19806 etc. in detail.

Problems to be Solved by the Invention However, with the above configuration, if the far field image on the light receiving means 1 is shifted in the trunk scanning direction due to the surface runout of the recording medium 100 or the misalignment of parts constituting the optical head, This has the problem that a tracking error component is mixed into the focus error signal as a disturbance.

Figure 6 shows this situation. When the center of the far field image (at the time of focus) is on the intersection of the dividing lines of the light receiving means 1 as shown in FIG. 6(A), the trunk is The statue is canceled.

However, when the far field image shifts in the track scanning direction as shown in Figure (B), the trunk image is distributed asymmetrically with respect to the dividing line in the track orthogonal direction, so even if the difference in the diagonal sum is taken, it cannot be canceled, and the tracking error component A part of it will be mixed into the focus error signal.

In digital audio discs and video discs where the tracks are discontinuous, the amplitude of the tracking error signal itself is relatively small, so any interference with the focus control system can often be ignored. However, recordable or erasable recording media often have continuous trunks (and the influence of tracking error components on focus control becomes a problem).

In view of the above-mentioned problems, the present invention provides a focus error detection device that is capable of always performing stable focus control, with almost no tracking error components mixed in even if the far field image shifts in the track scanning direction. be.

Means for Solving the Problems In order to solve the above problems, the focus error detection device of the present invention has two methods that add the outputs of two appropriate light receiving elements.
The light receiving element is constructed of two adding means, a comparing means for comparing the outputs of these adding means, and a variable amplifying means for amplifying the output of each light receiving element with an amplification factor corresponding to the output of the comparing means. be.

Operation The present invention uses the above-described configuration to electrically cancel the tracking error signal by appropriately changing the gain balance of each light receiving element in accordance with the shift of the far field image in the track scanning direction.

Embodiment Hereinafter, a focus error detection device according to an embodiment of the present invention will be described with reference to the drawings.

FIG. 1 shows a block diagram of a focus error detection device in a first embodiment of the present invention. In Figure 1,
1 is a light receiving means, 2 is a focus error detection means, 31.32 is (
first, second) addition means, 4 is comparison means, 5a, 5b,
5c and 5d are variable amplification means. The input of the adding means 31 is the light receiving element 1a among the light receiving elements constituting the light receiving means 1;
The input of the adding means 320 is connected to the output of the light receiving elements 1c and 1d. The input of the comparison means 4 is connected to the output of the addition means 31.32. The variable amplification means 5a, 5b, 5c, and 5d each have a light receiving element 1a.
, lb, lc, and ld are amplified, and the amplification factor is determined by the output of the comparing means 4.

The input of the focus error detection means 2 is connected to the corresponding variable amplification means 5.
They are connected to the outputs of a to 5d, respectively.

The operation of the focus error detection device configured as described above will be described below.

The light receiving means 1 has exactly the same structure as that described in the conventional example. In this embodiment, the respective outputs of the light receiving elements 1a, lb, 1c, and ld are assumed to be A, B, C, and D, respectively. The adding means 31 and 32 calculate A+B and C+D from the outputs of the respective light receiving elements of the light receiving means 1. Here, if the center of the far field image is located on the intersection of the dividing lines, the outputs of both are equal. If the far field image is in the track scanning direction (
If the displacement occurs in the vertical direction (in the figure), the balance between the two outputs will be disrupted by an amount corresponding to the displacement. A+B<C+D or A
Whether +B>C+D depends on the direction of displacement of the far field image. Therefore, the outputs of the adding means 31 and 32 have information regarding the direction and amount of deviation of the far field image.

The comparison means 4 has a function of issuing control signals for amplifying the outputs of the light receiving elements 1a to 1d with different amplification factors in accordance with this information. The variable amplification means 5a to 5d can change their amplification factors according to their control signals. In this embodiment, the amplification factors of the variable amplification means 5a and 5b and those of the variable amplification means 5c and 5d are made equal to each other.

As an example, consider a case where the far field image shifts as shown in FIG. 6(B). At this time, the output of the adding means 31 is A+B, and the output of the adding means 32 is kX (C+D)(
k>1). In contrast, the comparison means 4 calculates that, for example, the amplification factors (Ga-Cd) of the variable amplification means 5a to 5d are Gc (=
Send a control signal such that Gd)=kxGa (=Gb). As a result, the unbalanced amount of light in the trunk scanning direction caused by the image shift is electrically corrected. At this time, the amount of unbalance of the track images irradiated onto the light receiving elements 1a, lb, lc, and ld is also alleviated, so that the mixing of tracking error components is drastically reduced.

However, the function of the comparing means 4 is not limited to this. For example, an appropriate gain balance f (k):Gc (=Gd)=f (k
)xGa (Gb) can completely eliminate the mixing of tracking error signals.

The reason why Ga=Gb and Gc=Gd in this embodiment is to prevent an offset from being added to the focus error detection signal by changing the gain balance. The focus error detection means 2 uses the outputs GaXA and Gb of the variable amplification means 5a to 5d.
Focus error signal FE=G from xB, GcXC, Gdx[)
It is obtained by calculating aXA+GcXC-(GbXB+GdXD). As mentioned in the conventional example, when the focus is on, the image on the light receiving means 1 is circular, so if there is no deviation in the direction perpendicular to the track, then when the focus is on, A=B, C.
=D. The focus error signal at this time is FE=
(Ga-Gb)XA+ (Gc-Gd)XC,
Here, if Ga=Gb and Gc=Gd, FE=O and no offset occurs.

If there is a deviation in the direction perpendicular to the track, Ga=Gb
=Gc=Gd, resulting in the normal Astigma method.

No offset occurs in this case either (Japanese Patent Application Laid-Open No. 53-100
3, JP-A-53-19806, etc.).

A drawback of the astigma detection method is that an offset occurs when a deviation occurs in the track scanning direction and the orthogonal direction, but this can be alleviated by using the present invention. That is, since the deviation in the track scanning direction is electrically corrected, the deviation is substantially only in the direction perpendicular to the track.

A problem in this embodiment occurs when A+B and C+D contain many tracking error components. However, if the duty ratio between the trunk and the track is not far greater than 1:1, the tracking error component included in the re-output is extremely small. Even if there is some distortion, the tracking error components included in A+B and C+D are in phase with each other, so it is possible to cancel them out differentially. Band separation using a filter is also possible.

As described above, according to this embodiment, there are two adding means for adding the outputs of two appropriate light-receiving elements, a comparison means for comparing the outputs of these adding means, and a comparison means for comparing the outputs of the respective light-receiving elements. A focus error detection device that does not cause disturbance due to tracking error signals by electrically canceling the shift of the far field image in the trunk scanning direction using variable amplification means that amplifies the output with an amplification factor corresponding to the output of the comparison means. can be realized.

A second embodiment of the present invention will be described below with reference to the drawings.

FIG. 2 is a block diagram of a focus error detection device showing a second embodiment of the present invention. In the figure, the input to the addition means 31 is from the outputs of the variable amplification means 5a and 5b.
The inputs are obtained from the outputs of variable amplification means 5c and 5d, respectively. The following configuration is the same as that shown in FIG. However, in this embodiment, the track scanning direction is set to the left and right for convenience of illustration. The operation of the focus error detection device configured as described above will be described below.

In this embodiment, the amplification factors of the variable amplification means 5a to 5d are determined by a feedback loop.

FIG. 3 is a circuit diagram showing an example of the comparison means 4.

In FIG. 3, 41 and 42 are low pass filters. 43 and 44 are differential amplifiers, and a reference voltage source 45 is connected to their positive phase input terminals. The response frequency of the feedback loop is determined by the time constant of the low pass filter 41.42 and the gain of the differential amplifier 43.44. When the feedback loop is configured as described above, the output of the variable amplification means 53 to 5d is always GaxA+G
bxB=Eo, GcxC+GdXD=Eo
(Eo: reference voltage value generated by the reference voltage source 45). In this embodiment, as in the previous embodiment, Ga-Gb and Gc=Gd.

As described above, according to this embodiment, the variable amplification means 5a to 5
Since the amplification factor of d is determined by a feedback loop, the comparator stage 4 can be simplified.

Next, a third embodiment of the present invention will be described with reference to the drawings. FIGS. 4(A) and 4(B) are block diagrams representing a third embodiment of the present invention. Figure 4 (
In A), 10 is a recording medium, 1) is a laser light source,
12 is a half mirror, 3 is an objective lens, 14 is a convex lens, and 16 is a roof prism. 1 is a light receiving means. The light-receiving means 1 is divided into four light-receiving elements 1a, lb, 1c, and ld in a line, as shown in FIG. 1B.

The output of each element is connected to addition means 31, 32 and focus error detection means 2 via variable amplification means 5a, 5b, 5c, and 5d. Focus error detection means 2 includes light receiving element 1a+1
A focus error signal is obtained by synthesizing sum signals from the outputs of the light receiving elements d and the light receiving elements 1b and lc, and then calculating the difference between the two. This method is called the Foucault detection method (for example, Sakurai et al.: Comprehensive collection of optical memory and magneto-optical memory technologies, Science Forum, pp. 95-96, 19
83).

The outputs of the variable amplification means 5b and 5d are connected to the addition means 31, and the outputs of the variable amplification means 5a and 5c are connected to the addition means 32, respectively. Further, the outputs of the adding means 31, 32 are fed via the comparing means 4 to variable amplifying means 5b, 5d and variable amplifying means 5a, 5c, respectively.

The comparison means 4 are identical to those shown in FIG.

The operation of the third embodiment of the present invention configured as described above will be described below. FIG. 5 is an explanatory diagram thereof.

As shown in FIG. 5, the roof prism 16 is installed so that its ridgeline divides the light beam into two in the track scanning direction. Therefore, the light transmitted through the roof-shaped prism 16 travels in two directions along the ridgeline, and is further imaged at two locations by the convex lens 14, as shown in FIG. 2(B).

If the dividing line of the light receiving means 1 is adjusted to be at the center of each image when the recording medium 10 is at the focal position of the objective lens 13, then the output of the focus error detecting means will be O. If the recording medium 10 deviates somewhat from the focal position,
The two images move closer or further apart depending on the direction of the shift. Since the focus error detection means 2 detects the difference between the output of the light receiving element placed on the outside and the output of the light receiving element placed on the inside, the output of the focus error detection means 2 takes a negative or positive value at this time.

In this embodiment, the sum of the outputs of variable amplification means 5b and 5d (Gbx
B+GdxD) and the sum of the variable amplification means 5a and 5C output (G
Feedback is applied so that both aXA + Gc Even if it moves parallel in the orthogonal direction, Ga (=G
c) This can be canceled by setting <Gb (=Gd). Therefore, in this case as well, it is possible to prevent track images from being mixed in, as in the previous embodiment.

Effects of the Invention As described above, the present invention includes two adding means for adding the outputs of two appropriate light-receiving elements, a comparison means for comparing the outputs of these adding means, and a comparison means for comparing the outputs of each light-receiving element. By providing variable amplification means that amplifies the output with an amplification factor corresponding to the output of the means, it is possible to realize a focus error 5 detection device that does not cause disturbance due to tracking error signals.

Further, by configuring the displacement detecting means to obtain the input from the output of the variable amplifying means, it is possible to simplify the configuration of the controlling means.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a focus error detection device in an embodiment of the present invention, FIG. 2 is a block diagram of a focus error detection device in a second embodiment of the present invention, and FIG. 3 is a block diagram of a focus error detection device in a second embodiment of the present invention. Circuit diagrams showing one embodiment of the control means, FIGS. 4(A)(B)
is a block diagram of a focus error detection device in a third embodiment of the present invention, and FIGS. 5(A), 5(B), and 5(C) are explanatory diagrams thereof,
FIGS. 6(A) and 6(B) are explanatory diagrams showing the problems of the conventional technique, and FIGS. 7(A) and 7(B) are block diagrams of the conventional focus error detection device. 1... Light receiving means, 2... Focus error detection means, 31°32... Adding means, 4...
- Comparison means, 5a to 5d...variable amplification means. Name of agent: Patent attorney Toshio Nakao (1 person) Figure 1 Figure 2! Figure 3 Figure 4 Figure 5 Figure 6 Cc) ζB)/l /C
/d /' Figure 7

Claims (9)

[Claims] (1) A focus error detection device comprising a light receiving means divided into a plurality of light receiving sections, and an optical means for projecting an image of an information surface of a recording medium onto the light receiving means, wherein each of the light receiving sections first and second adding means for adding signals obtained from the outputs of at least two light-receiving parts; a comparison means for comparing the outputs of the first and second adding means; and a comparison means for comparing the outputs of each light-receiving element. A focus error detection device comprising: variable amplification means for amplifying the output of the means with an amplification factor according to the amplification factor; and focus error detection means for obtaining a focus error signal from the output of the variable amplification means. (2) The focus error detection device according to claim (1), wherein the optical means includes an optical member that generates astigmatism. (3) The focus error detection device according to claim (1), wherein the optical means includes an optical member that splits and disperses the light beam transmitted or reflected by the optical means in at least two directions. (4) The light receiving means is characterized by being composed of a first light receiving section, a second light receiving section, a third light receiving section, and a fourth light receiving section, and the focus error detection means is substantially the first light receiving section. The focus error signal is obtained by calculating the difference between the sum of the first light receiving part output and the third light receiving part output and the sum of the second light receiving part output and the fourth light receiving part output. The means is characterized in that it outputs substantially the sum of the first light receiving section output and the second light receiving section output, and the second adding means substantially outputs the sum of the third light receiving section output and the fourth light receiving section output. The focus error detection device according to claim 1, wherein the focus error detection device outputs the sum of . (5) The recording medium is characterized by having a track on the information surface, and is substantially the sum of the output of the first light receiving section and the output of the fourth light receiving section, the output of the second light receiving section and the output of the third light receiving section. 4. The focus error detection device according to claim 4, wherein the tracking error detection signal is obtained by calculating the difference between the sum and the tracking error detection signal. (6) The recording medium is characterized in that it has a track on the information surface, and the track is arranged so that its scanning direction is mapped approximately parallel to one of the dividing lines of the light receiving means. A focus error detection device according to claim (1). (7) Claim (6) characterized in that the first and second adding means obtain the respective sum outputs of two sets of light receiving sections separated by a dividing line in a direction perpendicular to the track scanning direction. Focus error detection device as described in . (8) The focus error detection device according to claim (1), wherein the output of each light receiving section is input to the first and second addition means via variable amplification means. (9) It is assumed that the amplification factors of the two variable amplification means that amplify the first light receiving section output and the second light receiving section output are equal to each other, and the third light receiving section output and the fourth light receiving section output are respectively amplified. The focus error detection device according to claim 4, wherein the amplification factors of the two variable amplification means are equal to each other.