JPH0740370B2 - Focus error detector - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a focus error detection device for detecting the in-focus position of an objective lens actuator in an optical disk device (particularly one having continuous tracks).

2. Description of the Related Art In recent years, optical disc devices with large capacity and high information retrieval capability have been gradually put into practical use. The 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 having an objective lens diameter of 1 to 2 μm. The optical disc device is composed of various technical elements. Among them, the focus control technology that keeps the distance between the objective lens and the recording medium always the focal length of the objective lens is suitable for recording and reproducing high density information. It is no exaggeration to say that this is the most important technology in. Among the focus control technologies, the focus error detection technology that detects the amount of deviation from the focus is a technology that requires particularly high accuracy and high reliability. Excellent methods with high detection sensitivity, such as the Astigma detection method and the Foucault (knife edge) detection method, have been developed so far.

Hereinafter, an example of the conventional focus error detection device described above will be described 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. 11 is a laser light source, 12 is a half mirror, 13 is an objective lens, 14 is a convex lens, and 15 is a cylindrical lens. Reference numeral 1 is a light receiving means. The light receiving means 1 is divided into four light receiving elements (light receiving portions) 1a, 1b, 1c, 1d at right angles as shown in FIG. 1B, and one of the dividing lines is provided on the recording medium 10. Matches the track direction. Further, the cylindrical lens 15 is installed so that its generatrix is 45 ° with respect to the dividing line (not shown in the figure). Reference numeral 2 is a focus error detecting 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 described below.

The focus error is detected by the optical system including the objective lens 13, the convex lens 14, the cylindrical lens 15, and the light receiving means 1. That is, the convex lens 14 and the cylindrical lens 15
The astigmatism is generated by the above, and the relative positions between the recording medium 10 and the objective lens 13 can be known by detecting the positions of the two front and rear focal points obtained by the light receiving means 1. Now, the recording medium 10 is placed on the focal position of the objective lens 13.
When there is, 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 aspherical image). If the light receiving means 1 is provided near the center of these two focal points, the image on the light receiving means 1 becomes substantially circular. When the recording medium 10 is displaced from the focal position of the objective lens 13, the two focal points also move on the optical axis accordingly. At this time, the image on the light receiving means 1 becomes an ellipse extending diagonally, that is, in the direction of 45 ° or −45 ° from the dividing line. Light receiving element 1a, 1b, 1c, 1d
If the output of each is A, B, C, D, then (A + C)-(B
A focus error signal can be obtained by electrically calculating + D). The focus error detecting means 2 is for performing such processing. This method is called the astigmatism (astigma) detection method, and is described in detail in JP-A-53-1003 or 53-19806.

Problems to be Solved by the Invention However, in the above-described configuration, when the far-field image on the light receiving means 1 is shifted in the track scanning direction due to surface wobbling of the recording medium 10 or positional deviation of the portion forming the optical head, etc. However, there is a problem that a tracking error component is mixed into the focus error signal as a disturbance. This is shown in FIG. As shown in FIG. 6 (A), when the center of the far field image (at the time of focusing) is located on the intersection of the dividing lines of the light receiving means 1, the track is obtained by taking the differential of the diagonal sum of each light receiving element. The statue is canceled. However, when the far-field image shifts in the track scanning direction as shown in FIG. 6B, the track image is asymmetrically distributed with respect to the dividing line in the direction orthogonal to the track, and therefore the diagonal sum differential cannot be canceled and the tracking error component A part will be mixed in the focus error signal.

In the case of digital audio discs and video discs having intermittent tracks, the amplitude itself of the tracking error signal is relatively small, and therefore some mixing in the focus control system can often be ignored. However, the write-once or recording-erasable type recording medium often has continuous tracks, and the influence of the tracking error component on the focus control becomes a problem.

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

Means for Solving the Problems In order to solve the above problems, the focus error detection device of the present invention adds the outputs of two appropriate light receiving elements to each other.
One adding means, further comparing means for comparing the outputs of these adding means, and variable amplifying means for amplifying the output of each light receiving element with an amplification factor according to the output of the comparing means. is there.

Action The present invention has 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 is a block diagram of a focus error detecting device in a first embodiment of the present invention. In FIG.
Reference numeral 1 is a light receiving means, 2 is a focus error detecting means, 31 and 32 are (first and second) adding means, 4 is a comparing means, and 5a, 5b, 5c and 5d are variable amplifying means. The input of the adding means 31 is connected to the outputs of the light receiving elements 1a and 1b of the light receiving elements constituting the light receiving means 1, and the input of the adding means 32 is connected to the outputs of the light receiving elements 1c and 1d. The input of the comparison means 4 is connected to the outputs of the addition means 31, 32. The variable amplifying means 5a, 5b, 5c, 5d amplify the outputs of the light receiving elements 1a, 1b, 1c, 1d, respectively, and the output of the comparing means 4 determines the amplification factor. The inputs of the focus error detection means 2 are connected to the outputs of the corresponding variable amplification means 5a-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 configuration as that described in the conventional example. Also in this embodiment, the outputs of the light receiving elements 1a, 1b, 1c and 1d are A, B, C and D, respectively. The adding means 31 and 32 are the light receiving means 1
A + B and C + D are calculated from the output of each light receiving element. If the center of the far field image is located on the intersection of the dividing lines, the outputs of the both are equal. If the far-field image is displaced in the track scanning direction (vertical direction in the figure), the balance between the outputs of both is disrupted by the amount. Whether A + B <C + D or A + B> C + D depends on the displacement direction of the far field image. Therefore adding means
The outputs of 31 and 32 have information on the direction and amount of the far field image.

The comparing means 4 has a function of issuing a control signal for amplifying the outputs of the light receiving elements 1a to 1d at different amplification factors according to this information. The variable amplification means 5a-5d can change the amplification factor according to the control signal. In the present embodiment, the amplification factors of the variable amplification means 5a and 5b and the amplification factors of 5c and 5d are made equal to each other.

As an example, consider a case where the far field image is shifted 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 k × (C + D) (k>
It becomes 1). On the other hand, in the comparison means 4, for example, the amplification factor (Ga to Gd) of the variable amplification means 5a to 5d is Gc (= Gd) = k × Ga (=
Gb) control signal. As a result, the unbalanced amount of light in the track scanning direction caused by the image shift is electrically corrected. At this time, the light receiving elements 1a, 1b,
Since the unbalance amount of the track images irradiated on 1c and 1d is also relaxed, the mixing of tracking error components is drastically reduced.

However, the function of the comparison means 4 is not limited to this. For example, by setting an appropriate gain balance f (k): Gc (= Gd) = f (k) × Ga (Gb) for the unbalance amount k, it is possible to completely eliminate the mixing of the tracking error signal. .

In the present embodiment, Ga = Gb and Gc = Gd are set to prevent an offset from being added to the focus error detection signal by changing the gain balance. The focus error detecting means 2 uses the outputs Ga × A, Gb × B, Gc × C, and Gd × D of the variable amplifying means 5a to 5d to obtain the focus error signal FE = Ga × A + Gc × C− (Gb × B + Gd ×
It is obtained by calculating D). As described in the conventional example, since the image on the light receiving means 1 is circular at the time of focusing, if there is no deviation in the track orthogonal direction, A = B, C = D at the time of focusing. . At this time, the focus error signal is FE = (Ga-Gb) * A + (Gc-Gd) * C, but if Ga = Gb and Gc = Gd, FE = 0 and no offset occurs.

Ga = Gb = Gc when there is a deviation only in the track orthogonal direction
= Gd and the normal Astigma method is used. In this case also, no offset occurs (Japanese Patent Laid-Open No. 53-1003, Japanese Patent Laid-Open No. 19806/53, etc.).

It is a drawback of the stigma detection method 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 track orthogonal direction.

The problem in this embodiment is that A + B and C + D contain many tracking error components. However, if the duty ratio between tracks is not far apart from 1: 1, the tracking error component contained in both outputs is extremely small. Even if it is included to some extent, the tracking error components included in A + B and C + D are in phase with each other, and therefore can be canceled differentially. Also, band separation by a filter is possible.

As described above, according to this embodiment, two adding means for adding outputs of two appropriate light receiving elements, a comparing means for comparing outputs of these adding means, and an output of each light receiving element are described above. A focus error detection device that does not cause a disturbance due to a tracking error signal by electrically canceling the shift of the far-field image in the track scanning direction with a variable amplification means that amplifies with a gain according 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 of the adding means 31 is obtained from the outputs of the variable amplifying means 5a and 5b, and the input of the adding means 32 is obtained from the outputs of the variable amplifying means 5c and 5d. The following is the same as the configuration of FIG. However, in the present embodiment, the track scanning direction is set to the left and right for the convenience of the layout of the drawing. 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-5d are determined by the feedback loop. FIG. 3 shows comparison means 4
It is a circuit diagram showing an example. In FIG. 3, 41 and 42 are low-pass filters. Reference numerals 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 constants of the low pass filters 41 and 42 and the gains of the differential amplifiers 43 and 44. When the feedback loop is configured as described above, the outputs of the variable amplification means 5a-5d are always Ga × A.
+ Gb × B = E ₀ , Gc × C + Gd × D = E ₀ (E ₀ : reference voltage source 45
, Gb and Gc, so that
Gd is determined. Also in this embodiment, Ga = g as in the previous embodiment
b, Gc = Gd.

As described above, according to this embodiment, the amplification factors of the variable amplifying means 5a to 5d are determined by the feedback loop, so that the gain can be set accurately with a simple configuration.

Next, a third embodiment of the present invention will be described with reference to the drawings. 4 (A) and 4 (B) are block diagrams showing a third embodiment of the present invention. Fig. 4 (A)
In the above, 10 is a recording medium, 11 is a laser light source, 12 is a half mirror, 13 is an objective lens, 14 is a convex lens, and 16 is a roof prism. Reference numeral 1 is a light receiving means. The light receiving means 1 has four light receiving elements 1a, 1b, 1c, 1d as shown in FIG.
It is divided into one line. The output of each element is variable amplification means
It is connected to the addition means 31, 32 and the focus error detection means 2 via 5a, 5b, 5c, 5d. The focus error detection means 2 is a light receiving element
A focus error signal is obtained by synthesizing sum signals from the outputs of 1a and 1d and light receiving elements 1b and 1c, respectively, and taking the difference between them. This method is called the Foucault detection method (for example, Sakurai et al .: Optical Memory Magneto-Optical Memory Comprehensive Technology Collection, Science Forum, pp. 95-96, 1983).

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 back to the variable amplifying means 5b, 5d and the variable amplifying means 5a, 5c via the comparing means 4, respectively. The comparison means 4 is the same as that shown in FIG.

The operation of the third embodiment of the present invention configured as 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 type prism 16 advances in two directions with its ridge line as a boundary, and is further imaged by the convex lens 14 at two positions as shown in FIG.
If the dividing line of the light receiving means 1 is located at the center of each image formation when the recording medium 10 is at the focus position of the objective lens 13, the output of the focus error detecting means becomes 0 at this time. If the recording medium 10 is slightly deviated from the focus position, the two images will move closer or further apart depending on the direction of the deviation.
Since the focus error detecting means 2 detects the difference between the output of the light receiving element arranged outside and the output of the light receiving element arranged inside, the output of the focus error detecting means takes a negative or positive value at this time.

In this embodiment, the sum of the outputs of the variable amplification means 5b.5d (Gb × B + Gd
× D) and the sum of the outputs of the variable amplification means 5a and 5c (Ga × A + Gc ×
C) a feedback so is both equal E ₀ is applied. That is, Ga (= Gc) <Gb (= Gd) even if the image on the light receiving means 1 is translated in the direction orthogonal to the dividing line as shown in FIG. You can cancel it. Therefore, also in this case, it is possible to prevent the mixture of track images as in the previous embodiment.

As described above, according to the present invention, two adding means for adding outputs of two appropriate light receiving elements, a comparing means for comparing the outputs of these adding means with a reference potential, and an output of each light receiving element are provided. By providing the variable amplification means for amplifying with the amplification factor according to the output of the comparison means, it is possible to realize a focus error detection device in which no disturbance due to the tracking error signal occurs.

Further, the input of the adding means is obtained from the output of the variable amplifying means, and the output of the comparing means is fed back to the variable amplifying means, so that the gain of the variable amplifying means can be accurately determined. You can

[Brief description of drawings]

FIG. 1 is a block diagram of a focus error detecting device according to an embodiment of the present invention, FIG. 2 is a block diagram of a focus error detecting device according to a second embodiment of the present invention, and FIG. 3 is a block diagram of FIG. Circuit diagram showing an embodiment of the control means, FIG. 4 (A) (B)
Is a block diagram of a focus error detecting device in a third embodiment of the present invention, and FIGS. 5 (A), (B) and (C) are explanatory diagrams thereof.
6 (A) and 6 (B) are explanatory views showing conventional problems, and FIGS. 7 (A) and 7 (B) are block diagrams of a conventional focus error detection device. 1 ... Light receiving means, 2 ... Focus error detecting means, 31, 32 ...
Adder means, 4 ... Comparison means, 5a-5d ... Variable amplification means.

Claims (9)

[Claims] 1. A focus error detecting device comprising a light receiving means divided into a plurality of light receiving portions and an optical means for projecting an image of a recording medium information surface on the light receiving means, the same number as the light receiving portions. At least two of the light receiving means are provided, each of which is provided with variable amplifying means for amplifying the output of the light receiving portion with an arbitrary amplification factor and focus error detecting means for calculating a focus error signal from the output of the variable amplifying means. The first and second adding means for adding the signals obtained from the partial outputs, and the comparing means for comparing the low-frequency components of the outputs of the first and second adding means with the reference potential, respectively. The focus error detection is characterized in that the output of the comparing means is fed back to the variable amplifying means so that the low frequency components of the outputs of the first and second adding means and the reference potential are substantially equal to each other. Location. 2. The focus error detecting device according to claim 1, wherein the output of each light receiving portion is input to the first or second adding means via the variable amplifying means. 3. The light receiving means comprises first, second, third and fourth light receiving means.
Of the first and second, third, and fourth variable amplifying means, respectively, and the focus error detecting means includes the first and third variable receiving means. The focus error signal is generated by calculating a difference between the output signal of the variable amplification means and the output signals of the second and fourth variable amplification means, and the first addition means is further provided with the first and the first addition means. Focus error detection according to claim 1 or 2, characterized in that the output of the two variable amplification means is added, and the second addition means adds the outputs of the third and fourth variable amplification means. apparatus. 4. The amplification rate of the first variable amplification means and the amplification rate of the second variable amplification means are equal to each other, and the amplification rate of the third variable amplification means and the amplification rate of the fourth variable amplification means. Are equal to each other, The focus error detecting device according to claim 3. 5. The focus error detection device according to claim 1, wherein the optical means includes an optical member that produces astigmatism. 6. The focus error detecting device according to claim 1, wherein the optical means comprises means for splitting the light beam into at least two beams. 7. A track is formed on the information surface of the recording medium,
The tracking error signal is generated from a difference signal between the sum signal of the outputs of the first and fourth variable amplification means and the sum signal of the outputs of the second and third variable amplification means. A focus error detection device according to the item. 8. A track is formed on the information surface of the recording medium,
Moreover, the focus error detecting device according to claim 1, wherein the track is arranged so that the scanning direction thereof is mapped substantially parallel to one of the dividing lines of the light receiving means. 9. The first and second adding means obtain the sum output of each of the two sets of light receiving portions divided by a dividing line in the direction perpendicular to the track scanning direction. A focus error detection device according to the item.