JP2596993B2 - Information reading method for optical disk - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an information reading method for an optical disk for reading information recorded on an optical disk such as a compact disk and a laser disk.

[Prior Art] FIG. 2 is an explanatory diagram of a conventional information reading method for an optical disk.

As shown in FIG. 2, the reflected light from the recording surface 2 of the optical disk on which the information pits 1 are formed is converted into a light spot on the detection surface 3 by a well-known optical system including an objective lens and a cylindrical lens (not shown). 4 are formed. conventionally,
The light spot 4 is divided into four areas having the same area, and the light intensity of each area is detected by four light detectors A, B, C, and D, and information reading is performed while performing focusing control of the objective lens. I was going.

3 and 4 are block diagrams showing a conventional information reading method and a conventional focusing control method, respectively.

As shown in FIG. 3, in the information reading, the sum (A + B) of the outputs of the photodetectors A and B is shifted by a predetermined angle by a phase shift circuit PS, and the outputs of the photodetectors C and D are added thereto. (C + D) is vector-added by the adder circuit 5 to obtain an information signal output. In this case, the phase of the sum (A + B) of the outputs of the photodetectors A and B is shifted, as shown in FIG. 2, by the phase of the sum (A + B) of the outputs of the photodetectors A and B and the light. Since the phase of the sum (C + D) of the outputs of the detectors C and D is shifted by about 80 to 90 °, if this is added as it is, the merit of the addition is reduced. Is used to extract an information signal having a large amplitude.

Also, as shown in FIG. 4, the photodetectors A and C
The difference between the sum (A + C) of the outputs (A + C) and the sum (B + D) of the outputs of the photodetectors B and D is calculated by a subtractor 6 to obtain a focusing error signal, and the position of the objective lens is determined based on the focusing error signal. This is controlled so that the focal point of the objective lens is always located on the recording surface 2. That is, if the optical system that forms the light spot 4 is:
When the focus of the objective lens is on the recording surface 2, as shown by the dotted line in FIG.
On the other hand, when it is not on the recording surface 2, it is elliptical so as to become an elliptical spot 42 or 43 as shown in FIG. Therefore, by controlling the position of the objective lens so that the output of the subtractor 6 becomes zero, the focal point of the objective lens can always be located on the recording surface 2.

[Problems to be Solved by the Invention] By the way, the above-mentioned conventional method uses an MTF (Modulation Tr
ansfer Function) has a certain limit,
There was a certain limit to the improvement in / N. In other words, to improve the MTF, the resolution (NA) of the objective lens should be increased to reduce the laser spot formed on the recording surface,
Alternatively, a method of narrowing the light spot on the detection surface is conceivable.However, the improvement of the resolution of the objective lens is already near the limit at present, and the light output is reduced by merely narrowing the light spot on the detection surface. No improvement in S / N can be obtained. Further, as a method of narrowing the light spot on the detection surface, a method of cutting a part of the spot on the detection surface, for example, a skirt portion of the Gaussian distribution of the light spot can be considered. In the method, when this foot is cut, there is a disadvantage that the focusing control cannot be performed, so that this method cannot be applied.

Furthermore, when one information reading apparatus to which the conventional method is applied reads information on a plurality of types of optical disks such as a compact disk and a laser disk, the MTF for reading one type of optical disk is reduced. If the setting is made to keep the level at a certain level, there is a disadvantage that the MTF when reading the other type of optical disk is significantly reduced. This is due to the fact that in the conventional method, in order to obtain an information signal, the light spot 4 is divided, the phases of the respective light intensities are aligned, and then the vectors are added. That is, since the recorded information format (pit length or interval, etc.) differs depending on the type of the optical disk, the amount of phase shift differs depending on the type of the optical disk, and the phases are aligned in accordance with the amount of shift of one of the optical disks. This is because if they are set in this way, they will not be aligned in the case of the other optical disk.

The present invention has been made under the above-mentioned background,
It is an object of the present invention to provide an optical disk information reading method that can improve the MTF by a relatively simple method and can obtain the same MTF even when reading information from a plurality of types of optical disks. I do.

Means for Solving the Problems In order to achieve the above object, the present invention focuses a laser beam on an information recording surface of an optical disc via an objective lens, and transmits the laser beam to information formed on the recording surface. In an information reading method for an optical disc, which reads information recorded on the optical disc by scanning along the pit row and detecting reflected light or transmitted light thereof through the objective lens, the method includes the steps of: Light or transmitted light is condensed to form a light spot of the reflected light or transmitted light on the detection surface, and the shape of the light spot on the detection surface is such that the focal point of the objective lens is located on the recording surface. Sometimes, the detection surface has a substantially circular shape, and has an elliptical shape whose axis is inclined in a direction in which the oblique angle between the major axes increases as the recording surface is shifted toward or away from the recording surface. At least one of a pair of portions where the swing trajectory of the long axis end of the ellipse overlaps with the periphery of the upper circular spot and its vicinity is divided into at least one portion substantially along the bisector of the oblique angle And
Focusing control is performed on the objective lens so that the difference between the received light intensity in the two divided regions or the difference between the sum of the received light intensity in the two divided regions located at diagonal positions of the pair of portions is zero, and Information reading is performed by detecting light receiving intensity in a region other than the region of the light spot on the detection surface, and detecting the presence or absence of the information pit from a change in the light receiving intensity.

[Operation] According to the above-described method, at least one of the pair of portions where the swing locus of the long axis end of the ellipse overlaps with the periphery of the circular spot on the detection surface and the vicinity thereof is substantially inclined to the oblique direction. The light beam is divided into two along the bisector of the intersection angle, and the difference in the received light intensity in the divided region or the difference in the sum of the received light intensity in the two divided regions located at diagonal positions between the pair of portions is zero. Focusing control of the objective lens, and also detects the light reception intensity in a region other than the region of the light spot on the detection surface, and detects the presence or absence of the information pit from a change in the light reception intensity. Read information.

Since the detection area of the light spot for performing information reading is an area in which one end of the light spot is truncated, the light intensity is reduced in a state where the foot of the Gaussian distribution of the light spot formed on the detection surface is truncated. It is possible to detect the presence or absence of information pits more sensitively than in the conventional method of detecting the light intensity in a state that also includes the foot part of the Gaussian distribution, thereby improving the MTF.

In addition, since information is read by one photodetector, there is no need to align the phases as in the conventional method. Therefore, the same MTF can be used when reading information from a plurality of different types of optical discs. It is possible to secure.

Furthermore, in the case of configuring an apparatus for performing this method, since means for aligning phases is not required, a circuit configuration associated therewith is not required, and the configuration of the apparatus can be simplified.

[Embodiment] FIG. 1 is a view for explaining a characteristic portion of an embodiment of the information reading method for an optical disk according to the present invention, and FIG. 5 is an embodiment of an apparatus to which the information reading method for an optical disk according to the present invention is applied. FIG. Hereinafter, an embodiment of the present invention will be described with reference to these drawings.

In FIG. 5, a laser beam L emitted from a laser oscillation device (not shown) is divided into three light beams separated from each other by a slight angle by a diffraction grating 51.
2. Three light spots are formed on the information recording surface of the optical disk 58 via the polarizing prism 53, the quarter-wave plate 54, the first mirror 55, the second mirror 56, and the objective lens 57. Then, the reflected lights of these three light spots are arranged on the detection surface through the objective lens 7, the second mirror 56, the first mirror 55, the quarter-wave plate 54, the polarizing prism 53, and the cylindrical lens 59, respectively. (A, b, c, d, e, f, g) (only the central light beam among the three reflected light beams is shown in the figure). In this case, of the three reflected lights, the central light beam is used for information reading and focusing control, and the remaining left and right light beams are used for tracking control, thus forming a well-known three-beam system. That is, the right and left light spots among the three light spots formed on the recording surface 2 of the optical disk are scanned so as to sandwich the information pit, and the reflected light is detected by the photodetectors f and g. The difference between the outputs of the photodetectors is obtained as a tracking signal, and tracking control of the objective lens 57 is performed on the basis of the tracking signal. The pits are scanned.

Next, a characteristic portion of the information reading method of this embodiment will be described with reference to FIG. The reflected light for reading information reflected from the recording surface of the optical disk 58 reaches a circular detection area formed by the photodetectors a to g, and the light spots 401, 40
2,403 etc. are formed.

The light spot 401 has a substantially circular shape and is formed when the focus of the objective lens 57 is located on the recording surface of the optical disc. The light spot 402 and the light spot 403 are substantially elliptical, and are formed when the focus of the objective lens 57 is located on one side or the other side from the recording surface of the optical disk 58. is there. That is, when the focus of the objective lens 57 is located on the recording surface of the optical disk 58, the light spot formed in the detection area becomes circular, and gradually becomes elliptical as the distance from the recording surface increases. . Specifically, as the focus of the objective lens 57 shifts toward or away from the recording surface of the optical disc, the light spots on the detection surface become oblique angles of the long axes of each other as shown by a dotted line in FIG. Has an elliptical shape whose axis is inclined in the direction in which it expands. Therefore, a pair of portions where the swing trajectory of the long axis end of the ellipse and the periphery of the circular spot on the detection surface and the vicinity thereof are overlapped, are each bisected substantially along the bisector of the oblique angle, Photodetectors a, b, c, d are provided respectively. The sum of received light intensities in the two divided regions at diagonal positions between a pair of portions, that is, the sum (a + c) of outputs of photodetectors a and c and photodetector b
The difference between the two outputs (b + d), that is, (a + c)-(b + d), is used as a focusing error signal, and the objective lens is set so that this signal becomes zero.
The position of 57 is controlled by a well-known focusing servo control method or the like. Thereby, control can be performed such that the focal point of the objective lens 57 is located on the recording surface of the optical disk 58.

On the other hand, the information recorded on the optical disk 58 can be read by detecting the output signal of the photodetector e. In this case, the light intensity detected by the photodetector e is the received light intensity of both ends of the light spot 401 formed on the detection surface, that is, the portion where the upper and lower ends in FIG. 1 are cut. Therefore, the light spot 401
The presence or absence of information pits on the recording surface of the optical disk 58 can be detected more sensitively as compared with the case where the entire light receiving intensity of the optical disk 58 is detected. Moreover, since the cut portion is a foot portion of the Gaussian distribution of the light spot 401, the light detection intensity is slightly reduced by cutting the light spot. Therefore, it is possible to improve the resolution of MTF or information reading as compared with the conventional method in which information is read by detecting the entire light receiving intensity of the light spot 401. In this case, it is preferable that about 70% of the entire area of the light spot 401 is detected by the light detector e.

In addition, according to the method of this embodiment, since there is one photodetector for reading information, it is necessary to add outputs of a plurality of photodetectors and to align the phases as in the related art. No, therefore, it is possible to secure the same MTF even when reading information from different types of optical discs, and to simplify the circuit configuration of the device for implementing this method compared to the conventional method. can do.

In the above-described embodiment, the case where four photodetectors a, b, c, and d are used to perform the focusing control is described as an example.However, the present invention is not limited to this. As shown in FIG. 6, using two photodetectors a ₁ and b ₁ , the difference (a ₁ −b ₁ ) between these outputs a ₁ and b ₁ is used as a focusing error signal to perform focusing control. You may make it.

[Effects of the Invention] As described in detail above, the present invention provides at least one of a pair of portions in which the swing trajectory of the long axis end of the ellipse overlaps the periphery of the circular spot on the detection surface and the vicinity thereof. Is divided substantially along the bisector of the oblique angle, and the difference in the received light intensity in the two divided regions or the light reception in the divided region at a diagonal position between the pair of portions is obtained. Focusing control of the objective lens so that the difference between the intensity sums becomes zero, and also detects light reception intensity in a region other than the region of the light spot on the detection surface,
Since the presence or absence of the information pit is detected based on the change in the received light intensity, the information pit is detected using the received light intensity detected in an area of the light spot formed on the detection surface excluding an area for detecting a focusing error. The presence or absence of information pits can be detected, whereby the presence or absence of information pits on the recording surface of the optical disk can be more sensitively detected as compared with the case where the light receiving intensity of the entire light spot is detected, and the focusing error can be reduced. Since the area to be detected is at the foot of the Gaussian distribution of the light spot, the decrease in the received light intensity due to the cut of this foot is small.Therefore, information is detected by detecting the received light intensity of the entire light spot. MT compared to the conventional method of reading
F or information reading resolution can be improved,
In addition, since information can be read with one photodetector, there is no need to align the phases as in the conventional method.
Therefore, it is possible to secure the same MTF even when reading information from a plurality of different types of optical discs, and a device for implementing this method does not require a means for aligning phases or a circuit therefor. This provides excellent effects such as simplification of the configuration.

[Brief description of the drawings]

FIG. 1 is a view for explaining a characteristic part of an embodiment of an information reading method for an optical disk according to the present invention, FIG. 2 is an explanatory view of a conventional information reading method for an optical disk, and FIG. FIG. 4 is an explanatory view of a conventional focusing control method, FIG. 5 is a schematic perspective view showing an embodiment of an apparatus to which the information reading method of an optical disk of the present invention is applied, and FIG. FIG. 10 is a view for explaining another embodiment of the information reading method of the optical disk of the present invention. DESCRIPTION OF SYMBOLS 1 ... Information pit 2 ... Recording surface 3 ... Detection surface 4 ... Light spot 5 ... Adder 6 ... Subtractor 51 ... Diffraction grating 53 ... Polarizing prism 55 ... First gmirror 57 ... light spot a on the objective lens 58 ...... optical disc 41,42,43,401,402,403 ...... detection surface, B, C, D, a , b, c, d, a 1, b 1 ...... photodetector

Claims (1)

(57) [Claims] 1. A laser beam is focused on an information recording surface of an optical disk via an objective lens, and the laser beam is scanned along an information pit row formed on the recording surface to reflect reflected light or transmitted light. In an information reading method for an optical disk for reading information recorded on the optical disk by detecting through the objective lens, the reflected light or transmitted light is condensed through the objective lens and the reflected light or transmitted light is reflected on a detection surface. Form a light spot of light or transmitted light,
The shape of the light spot on the detection surface is substantially circular when the focal point of the objective lens is located on the recording surface,
The oblique angle of the long axis of each other is increased so as to be shifted toward the front or the back of the recording surface so as to form an elliptical shape that is axially inclined, and the periphery of the circular spot on the detection surface and the vicinity thereof and the ellipse At least one of the pair of portions where the swing trajectory of the long axis end overlaps is divided into two substantially along the bisector of the oblique angle, and the difference between the received light intensity in the two divided regions is Or, focusing control of the objective lens, so that the difference of the sum of the received light intensity in the two divided regions located at diagonal positions of the pair of portions is zero,
Information on an optical disc, wherein information is read by detecting light receiving intensity in a region other than the region of the light spot on the detection surface, and detecting the presence or absence of the information pit from a change in the light receiving intensity. Reading method.