JPH0782652B2 - 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 used for focus position control of an optical head in an optical disc device.

2. Description of the Related Art In recent years, according to the demand for cost reduction of optical disk devices,
Focus error detection devices that do not require precise optical axis adjustment have been studied.

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

13 and 14 are block diagrams of a conventional focus error detection device. In FIG. 13, reference numeral 10 designates an optical recording medium having a concave-convex shape which means a recorded information string or an information pit having a reflectance different from that of the surroundings (forming an information track). . Reference numeral 1 is a light emitting source that emits coherent light, and 3 is an optical recording medium 10 that emits coherent light.
It is an objective lens that focuses light on the top. Reference numeral 4 denotes a light receiving means for converting light into an electric signal, which is divided into two mutually independent light receiving elements 4a and 4b which will be described later. Reference numeral 2 denotes a spectroscopic unit that transmits a part of the light incident on the optical recording medium 10 and reflects a part of the light reflected by the optical recording medium 10. Reference numeral 5 is a phase comparison means for detecting the phase difference between the outputs of the light receiving elements 4a and 4b, and 6 is a low-pass means for removing the high frequency component of the output of the phase comparison means. The optical recording medium 10 is generally composed of a recording surface 10a and a protective layer 10b. Protective layer 10
b is provided to prevent the information on the recording surface from being lost due to external damage. Therefore, the light emitted from the objective lens 3 reaches the recording surface 10a after passing through the protective layer 10b.

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

FIGS. 3A, 3B and 3C show the far field image of the information track projected on the light receiving element 4. FIG. 3A shows a far-field image when the recording surface 10a is within the depth of focus of the objective lens 3. The light diffracted at the pit end interferes with the light reflected at the flat surface (FIG. 4), resulting in the pattern shown in FIG. Diffracted light is generated in directions symmetrical to each other with respect to the optical axis. Therefore, the interference region can be formed on both sides of the optical axis. (B) and (c)
Indicates that the objective lenses 3 are too close and too far, respectively. At this time, the spatial frequency of the far-field image light amount distribution increases as the optical recording medium 10 shifts from the focus position of the objective lens 3.

Next, FIGS. 3 (d), (e) and (f) show how the objective lens 3 is moved by a minute amount along the information track provided on the optical recording medium 10. When the optical recording medium 10 is at the focal position of the objective lens 3, the light quantity distribution inside the interference area between the pit-edge diffracted light and the planar partially reflected light does not change, but only the total light quantity in the entire interference area changes (d ). When the objective lens 3 is too close to the recording medium 10 and too far away from the recording medium 10, the light amount distribution moves as shown in FIGS. Moreover, the movement directions are opposite to each other depending on whether the focus error is positive or negative (far or near).

The light receiving elements 4a and 4b are installed so as to divide one interference region in the track circumferential direction (FIG. 14). FIG. 5 shows an example of output signals of the light receiving elements 4a and 4b. (A),
(B) and (c) are within the depth of focus, respectively, when they are too close,
It corresponds when it is too far. It is assumed that the pit continuously moves in the track circumferential direction. As is clear from this, the focus error amount and its direction are
It can be seen that it corresponds to the phase of the 4b output signal. Therefore, a focus difference signal can be obtained by detecting this phase difference (FIG. 6). 6 (a), (b), (c)
Correspond to FIG. 5, respectively. Here, a circuit for outputting the absolute amount of the detected phase difference with the pulse width and the phase advance and the delay with the electrical polarity is used as the phase comparison means 5. Outputs (a), (b) of the low-pass means 6,
(D), (e), (f) corresponding to (c). The optical recording medium 10 is generally disc-shaped, and the pits are constantly moving in the track circumferential direction at a constant speed, so that a stable focus error signal can be detected.

The focus error detection method described above is generally called a phase difference method, and is characterized in that it does not require a precisely adjusted detection optical system such as the astigmatism method or the Foucault method.
(For example, Japanese Patent Publication No. 56-31651).

Problems to be Solved by the Invention However, the above-mentioned configuration has a problem that if the surface of the optical recording medium protective layer has a scratch, an offset is added to the focus error signal.

This is shown in FIGS. 7 and 8. That is, the protective layer 10
Assuming that the scratch on the surface of b is d, the image d ₁
And d ₂ appear. The images d ₁ and d ₂ are point-symmetric with respect to the optical axis of the objective lens 3. This is because, on the optical recording medium 10, the light flux passing through the scratch d when incident and the light flux passing through the scratch d when reflected are simultaneously projected on the light receiving means 4. Moreover, the images d ₁ and d ₂ of the scratches move as the optical recording medium 10 moves at a constant speed, and this causes the light amount distribution to move, and therefore, even when there is no focus error, it is shown in FIG. As described above, an apparent phase difference occurs in the outputs of the light receiving elements 4a and 4b. That is, this is the offset in focus error detection. Here, (a) and (b) are the light receiving elements 4a and 4a, respectively.
4b output, (c) output of the phase comparison means 5, (d)
Represents the output of the low-pass means 6.

In view of the above problems, the present invention provides a focus error detection device that uses a focus error detection method using a phase difference method and that does not cause an offset in a focus error signal due to a scratch on the surface of an optical recording medium.

Means for Solving the Problems In order to solve the above problems, the focus error detection device of the present invention includes a first light receiving element, a second light receiving element, a third light receiving element, and a fourth light receiving element. Provided along the track far-field image,
Phase comparison means for detecting the phase difference between the sum output of the first light receiving element and the third light receiving element and the sum output of the second light receiving element and the fourth light receiving element is provided.

Effect The present invention has the above-described configuration and cancels the phase difference due to the scratch on the surface of the optical recording medium without sacrificing the detection sensitivity of the phase difference due to the focus error.

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 showing the main part of a focus error detecting apparatus according to the first embodiment of the present invention, and FIG. 2 is an overall block diagram. In FIG. 1, reference numeral 40 denotes a light receiving means, which is a light receiving element 40.
It is divided into a, 40b, 40c and 40d. Reference numerals 51 and 52 denote addition means, which calculate the sum signal of the output signals of the light receiving elements 40a and 40c and the sum signal of the light receiving elements 40b and 40d, respectively. Reference numeral 53 is a phase comparison means, which compares the phases of the outputs of the addition means 51 and 52. Reference numeral 6 is a low-pass means, and a phase comparison means 53
Low pass filter the output signal of.

In FIG. 2, the light emitting means 1, the spectroscopic means 2, the objective lens 3, and the optical recording medium 10 are the same as those described in the conventional example.

The operation of the focus error detection device configured as described above will be described below with reference to FIG. 1, FIG. 2, FIG. 10, FIG. 11 and FIG.

First, FIG. 1 shows the positional relationship between the light receiving elements 40a, 40b, 40c, 40d and the information track far-field image. It has already been described that the reflected light of the optical recording medium has a plane reflection component and a diffracted light component by the pit edge, and the diffracted light always occurs in a direction symmetrical with respect to the plane reflection light optical axis. That is, the interference area between the diffracted light and the plane reflected light occurs on both sides of the optical axis in the track circumferential direction. The light receiving elements 40a and 40b are provided on one side with the light receiving elements 40c and 40d.
Is located on the other side of it. That is, the light receiving elements 40a, 40b
And the light receiving elements 40c and 40d are arranged symmetrically with the optical axis in between.

The outputs of the light receiving elements 40a, 40b, 40c, 40d are shown in FIG. FIG. 7A shows the output of each element when there is no focus error, FIG. 8B shows the output when each element is close, and FIG. In (b), the output of the light receiving element 40a leads the output of the light receiving element 40b, and the output of the light receiving element 40c leads the phase of the output of the light receiving element 40d. Also, in (c), it can be seen that the phase relationships are reversed. That is, regarding the phase of each light receiving element output with respect to the focus error, the outputs of the light receiving elements 40a and 40c and the outputs of the light receiving elements 40b and 40d are in phase. Therefore, even if the respective two outputs are added, the focus error can be detected with the same degree of sensitivity as the conventional one without losing the phase difference information which is the focus error information. FIG. 11 (a) Adding means 51 when there is no focus error,
52 shows the output, and (b) shows the output when the addition means 51, 52 is near, and (c) shows the output when it is far. A phase comparison of these outputs is exactly the same as in the conventional example. However, the function of the phase comparison means 53 is the phase comparison means 5 in the conventional example.
It does not differ from the function of.

Next, the case where there is a scratch on the surface of the protective layer 10b of the optical recording medium 10 will be described. As already described in FIG. 8, the images d ₁ and d _{2 of the} scratch d have the symmetry point at the optical axis center of the far-field image,
Move in opposite directions. Light receiving elements 40a, 40 at this time
The outputs of b, 40c and 40d are shown in FIGS. 12 (a), (b), (c),
As shown in (d), the phase difference between the light receiving elements 40a and 40b and the light receiving element
It can be seen that the phase differences of 40c and 40d occur in opposite directions. Therefore, the outputs of the light receiving elements 40a, 40c and the light receiving elements 40b, 4c
Adding the output of 0d cancels the effect. ((E), (f)).

As described above, according to the present embodiment, the light receiving elements 40a, 40b and the light receiving elements 40c, 40d are provided symmetrically with respect to the optical axis along the track far-field image, and the adding means for obtaining the sum of the outputs of the light receiving elements 40a, 40c is added.
51 and an addition means 52 for obtaining the sum of the outputs of the light receiving elements 40b and 40d,
By providing the phase comparison means 53 for detecting the phase difference between the output signals of the respective addition means, it is possible to cancel the influence of scratches on the surface of the optical recording medium.

In the first embodiment, the expression that the pit itself is an information signal is used, but the pit does not necessarily have to be an information signal. It may be a pit dedicated to servo signal detection.

EFFECTS OF THE INVENTION As described above, the present invention can cancel the focus offset due to the scratches on the surface of the optical recording medium by providing the light-receiving means divided into four parts symmetrically about the optical axis along the track far-field image.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a main part of a focus error detection apparatus according to an embodiment of the present invention, FIG. 2 is an overall configuration diagram thereof, FIG. 3 is a schematic diagram showing a state of a far field image of an information track, and FIG. Is an explanatory view showing the generation principle thereof, FIGS. 5 and 6 are operation explanatory views of a conventional focus error detection device, and FIGS. 7, 8 and 9 are explanatory views showing the problem, FIG. 10, FIG. 11 and FIG. 12 are operation explanatory views of an embodiment of the present invention, and FIG. 13 and FIG. 14 are a main part configuration diagram and an overall configuration diagram of a conventional example, respectively. 40a, 40b, 40c, 40d ... Light receiving element, 51, 52 ... Addition means, 53 ... Phase comparison means.

Claims (2)

[Claims] 1. An optical recording medium having concave and convex portions on a recording surface, or a minute region having a reflectance or a transmittance different from that of a surrounding area provided along a track, and light for irradiating the optical recording medium. A light emitting means, and an objective lens for focusing the light emitted from the light emitting means on the recording surface of the optical recording medium,
A focus error detecting device comprising means for detecting light reflected or transmitted through the optical recording medium, the first light receiving element, the second light receiving element, the second light receiving element in the optical path of the reflected light or the transmitted light of the optical recording medium. The third light-receiving element and the fourth light-receiving element are sequentially arranged in a line along the far-field image of the track, and the far-field image of the track is divided into two regions with the optical axis interposed between the two regions. In the light receiving element and the second light receiving element, the other area is arranged so as to be divided by the third light receiving element and the fourth light receiving element, respectively, and the first light receiving element and the first light receiving element First adding means for outputting the sum of the outputs of the three light receiving elements, second adding means for outputting the sum of the outputs of the second light receiving element and the fourth light receiving element, and the first adding means And phase comparison for detecting the phase difference between the outputs of the second adding means Focus error detection system, characterized in that it comprises a stage. 2. The first light receiving element is provided symmetrically with respect to the fourth light receiving element, and the second light receiving element is provided symmetrically with respect to the third light receiving element with an optical axis interposed therebetween. The focus error detection device according to claim 1.