JPH0353699B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a focus error detection device suitable for application to an optical reproduction device for video or PCM audio.

BACKGROUND TECHNOLOGY AND PROBLEMS Conventionally, as an optical reproducing device, a laser beam from a laser light source is focused onto the recording surface of an optical disk using an objective lens, and this reflected beam is irradiated onto the detection surface of a photodetector. Accordingly, there are devices in which a reproduced signal is obtained from a photodetector.

In such an optical reproducing device, the light detection surface of the conventional photodetector is made circular, and this detection surface is
The focus error detection device is constructed by dividing the photodetector into four quadrants of detecting sections, and arranging a semi-cylindrical lens on the front side of the photodetector. , a focus error detection output is obtained from the difference between the sum of the detection outputs of the detection units in the second and fourth quadrants, and the position on the axis of the objective lens is controlled using this to perform focus servo. be. This will be explained below with reference to the drawings.

FIG. 1 shows a semi-cylindrical lens 1, 2 is its semi-cylindrical surface, and 3 is a rectangular plane. On the plane 3, the center is set as the origin O, and orthogonal coordinates are provided, which are composed of an X axis parallel to the generatrix of the semi-cylindrical surface 2, a Y axis perpendicular to the plane 3, and a Z axis perpendicular to the plane 3. Also, on the Z-axis, a point at a predetermined distance from the origin O on the side of the semi-cylindrical surface 2 of the lens 1 is set as the origin O', and within a plane perpendicular to the Z-axis there are 45 A rectangular coordinate is provided consisting of an x-axis forming an angle of .degree. and a y-axis perpendicular thereto. and,
The circular detection surface of the photodetector is aligned within the x and y planes, and the detection surface is divided into four equal parts along the x and y axes to divide the photodetector into four quadrants of photodetection sections.

Then, as shown in FIG. 2, on the plane 2 side of the lens 1, the optical axis coincides with the Z axis, and a spot 4 on the plane 2
A focused beam having a circular shape (FIG. 2 is a perspective view) is made incident. For circular spot 4,
Radial axes a to d forming angles of 45 degrees with the X and Y axes are temporarily provided in the first to fourth quadrants.

Now, as mentioned above, the XOZ cross section of lens 1 when the focused beam is incident on plane 3 of lens 1 and
Regarding the trajectory of optical axes 5 and 6 in the YOZ cross section,
This will be explained with reference to FIG. Since the YOZ cross section has a constant thickness, the light ray 6 that enters the YOZ cross section of the lens 1 travels parallel to the incident light, and the Z
It passes through point P on the axis. Since the XOZ cross section constitutes a convex lens, the light ray 5 incident on the XOZ cross section of the lens 1 is refracted toward the Z axis side and passes through a point P' in front of the point P on the Z axis.

Now, in Figure 3, a point midway between points P and P'
The detection surface DT of the detector is located at O' (see Figure 1),
The spot 4' of the irradiation beam on the detection surface DT is the fifth
When the beam becomes circular as shown in the figure, it is assumed that the focused beam from the objective lens is focused on the recording surface of the optical disk. Thus, the fact that the focus of the focused beam from the objective lens is focused on the front side of the optical disk means that the detection surface of the photodetector is positioned from point α in front of point O′ to point β on the opposite side. This is equivalent to the deviation of the detection surface of the photodetector.
The DT spot 4' has an elliptical shape as shown in FIGS. 4 and 6. That is, this elliptical spot 4' has its major axis in the direction of 45° to the x-axis in the first and third quadrants in the case of FIG. 4, and in the second and fourth quadrants in the case of FIG. , with respect to the x-axis
It has a major axis in the 45° direction. Note that in FIGS. 4 to 6, axes a' to d' correspond to axes a to d in FIG. 2, respectively.

Therefore, if the detection surface of the photodetector is larger than the spot 4', then among the four quadrants of photodetection parts,
The focus state of the objective lens on the optical recording medium is determined by the difference between the sum of the detection outputs of the photodetectors in the first and third quadrants and the sum of the detection outputs of the photodetectors in the second and fourth quadrants. Detected. Therefore, by moving the objective lens on its optical axis so that the above difference becomes zero, focus servo becomes possible.

However, since the beam distribution of the emitted beam from the semi-cylindrical lens 1 is not maintained, when the semi-cylindrical lens is combined with a photodetector, the correct tracking error due to the symmetry of the light distribution is detected from the photodetector. Obtaining the signal (separately from the focus error signal) is difficult.

Therefore, the applicant first developed a system that has axial symmetry in each of the four quadrants, maintains similarity in the beam distribution of the emitted beam, and produces different powers depending on the angle of the plane containing the optical axis. We have proposed a focus error detection device using an optical element (lens).

The focus error detection device will be described in detail below with reference to the drawings.

As shown in FIG. 7, the lens used in such a focus error detection device has four quadrant lens regions 7a to 7d separated by two mutually orthogonal planes, ie, the XZ plane and the YZ plane, both of which include the optical axis Z. , the focal lengths of the lens regions 7a and 7c in the first and third quadrants are equal to each other (denoted as f ₁ ), and the focal lengths of the lens regions 7b and 7d in the second and fourth quadrants are equal to each other (denoted as f ₂₎ . ), and the focal length f ₁ of the lens regions 7a and 7c in the first and third quadrants is different from the focal length f ₂ of the lens regions 7b and 7d in the second and fourth quadrants, and each lens region 7a to 7d is different from each other. have axial symmetry. Note that the focal lengths f ₁ and f ₂ are f ₁ ≠ f ₂
If so, it may be positive or negative, or one may be ∞.

Lens 7 is a plano-convex lens here, and the rectangular coordinates (with O as the origin) consisting of the above-mentioned X and Y axes
is provided on the plane 7A, and the X and Y axes and the Z axis on the optical axis constitute orthogonal coordinates.

There are two ways to make such a lens 7. One is to combine four quadrants of lens materials such as two types of glasses and plastics with different refractive indexes by adhesion or the like, and then polish the combined materials to obtain the lens 7. be.

The other method involves polishing lenses of different curvatures, that is, different focal lengths, using the same lens material such as glass or plastic, and dividing each lens into four quadrants.
Two of each lens area are combined by gluing, etc.
A lens 7 is obtained.

Also, on the Z-axis, a point at a predetermined distance from the origin O on the spherical surface 7B side of the lens 1 is O', and in a plane perpendicular to the Z-axis, x parallel to the X-axis and Y-axis, respectively.
A rectangular coordinate is provided consisting of an axis and a y-axis. and,
The circular detection surface of the photodetector is aligned within the x and y planes, and the detection surface is equally divided into four by the x and y axes to divide the photodetector into four quadrants of photodetection sections.

Then, a parallel beam whose optical axis coincides with the Z axis and whose cross section on the plane 7A is circular is made to enter the plane 7A side of the lens 7.

Now, when a parallel beam is incident on the plane 7A of the lens 7 as described above, the trajectories of the light rays 5 and 6 that are incident on the lens regions 7a and 7b in the first and second quadrants of the cross section on the x-axis are as follows. This will be explained with reference to FIG. The output light of the light ray 5 that enters the lens area 7a of the lens 7 is refracted toward the Z-axis side and reaches the focal point on the Z-axis.
Pass _F1 . The output light of the light ray 6 that enters the lens region 7b of the lens (7) is refracted toward the Z-axis side and passes through the focal point F ₂ on the opposite side of the focal point F ₁ on the Z-axis.

Now, in Fig. 8, when the detection surface DT of the detector is located at a point O' between the focal points F ₁ and F ₂ (see Fig. 7), the irradiation beam on the detection surface DT is When the spot 4' becomes circular as shown in FIG. 10, it is assumed that the convergent beam from the objective lens is focused on the recording surface of the optical disk. In this way, the fact that the focus of the focused beam from the objective lens is either in front of or on the opposite side of the optical disk means that the detection surface of the photodetector is positioned at point α in front of point O′ or point β on the opposite side. It is equivalent to a deviation,
The spots 4' on the detection surface DT of each photodetector are a combination of quarter circles having different radii, as shown in FIGS. 9 and 11. The spot 4' in FIG. 9 is composed of a 1/4 circle with a large diameter in the second and fourth quadrants and a 1/4 circle with a small diameter in the first and third quadrants. The spot 4' in FIG. 11 is composed of a 1/4 circle with a large diameter in the first and third quadrants and a 1/4 circle with a small diameter in the second and fourth quadrants.

In FIGS. 9 to 11, the amount of light in each quadrant of the spot 4' is equal, but when the radius of the quarter circle is different, the amount of light per unit area is different, resulting in brightness and darkness.

Therefore, as shown in Figures 9 to 11, the outline of the detection surface DT of the photodetector is made into a circle with a radius smaller than the radius of a quarter circle, and the detection surface
The DT is divided into four equal parts along the x-axis and the y-axis to form a four-quadrant detection section. Thus, the detection output of each detection section is proportional to the amount of light per unit area of the spot 4'.

When a focused beam is irradiated onto a certain surface, the relationship between the amount of light (%) in a circular area within the spot of the focused beam and the focused state of the beam is shown by the curves S ₁ and S in Figure 12. ₂ , _S3 . curve S ₁
is the case where the focused beam is focused on that plane, and curves S ₂ and S ₃ are the curves when the focused beam is focused before and after that plane. Therefore, by detecting the amount of light (encircled energy) in a circular region with a predetermined radius ρ ₀ , it is possible to detect the focusing state of the focused beam on that surface.

Therefore, among the photodetecting parts in the four quadrants of the photodetector,
The difference between the sum of the detection outputs of the photodetectors in the first and third quadrants and the sum of the detection outputs of the photodetectors in the second and fourth quadrants determines the focusing state of the objective lens with respect to the optical recording medium. Detected. Therefore, by moving the objective lens on its optical axis so that the above difference becomes zero, focus servo becomes possible.

Furthermore, since the beam distribution of the beam emitted from the lens 7 maintains similarity, when the lens 7 is combined with a photodetector, a correct tracking error signal can be obtained due to the symmetry of the light distribution. As a method for this, a focusing error signal (low frequency) and a tracking error signal (high frequency) can be obtained by frequency separation from the above-mentioned difference detection output. or,
Among the four quadrants of the photodetector, the sum of the detection outputs of the first and second (or first and fourth) quadrants and (,) photodetector, and the third and fourth (or The tracking state is detected based on the difference between the sum of the detection outputs of the photodetectors in the second and third quadrants (,). Therefore, tracking servo may be performed by controlling, for example, a tracking mirror so that this difference becomes zero. Note that the reproduced signal is obtained by the sum of the detection outputs of the photodetectors in all quadrants.

Next, with reference to FIG. 13, an example of an optical reproducing device to which the focus error signal detection device using the lens 7 of FIG. 7 described above is applied will be described.
A diverging laser beam from a laser light source 10 is made into a parallel beam by a collimator lens 11, and is sent to a galvanometer mirror (tracking mirror) 1 via a polarizing beam splitter 12-1/4 wavelength plate 13.
4 and then focused onto an optical disk 16 by an objective lens 15.
The reflected beam from the optical disk 16 is made incident on the beam splitter 12 via the objective lens 15 - galvano mirror 14 - 1/4 wavelength plate 13, and the reflected wave, which is a substantially parallel beam, is optically detected via the lens 7. The light is focused on the light detection surface DT of the device 17.
18 are four output terminals from each detection section of the photodetector 17.

By the way, when a photodetector having the configuration shown in FIGS. 9 to 11 is used as a photodetector that constitutes a focus error signal detection device in combination with the above-mentioned lens 7, there are the following drawbacks. Figures 9 to 11 above
There are actually band-shaped dead zones between the first to fourth quadrants of the photodetector surface DT of the photodetector described in the figure, as shown in FIG.
When the degree of defocus in an optical reproducing device is extremely large, as shown in FIG. 14, a spot portion with a small radius and a high optical density is selected among the spots 4' of the irradiation beam incident on the photodetection surface DT of the photodetector. Since most of the difference falls near the center of the photodetection surface DT in the dead zone ND, the focus error signal obtained from the focus error detection device includes a detection error.

That is, the characteristic curve (S-curve) of the level (relative value) of the focus error signal with respect to the amount of defocus (μm) based on the distance during focusing between the objective lens and the disk is as shown in FIG. When the absolute value of the defocus amount further increases after crossing the linear region between points B and B' that sandwich the range, the absolute value of the focus error signal level becomes small, but it temporarily increases at points C and C'. The sign is reversed.
Therefore, if such a focus error signal is supplied to the electromagnetic drive device that drives the objective lens, the focus control for the objective lens will malfunction.

Purpose of the Invention In view of the above, the present invention aims to solve the problem as the degree of defocus in optical reproducing devices has become extremely large.
Focus error detection Even if a spot with a small radius and high light density occurs among the beam spots incident on the photodetector surface of the photodetector, this spot is not affected by the dead zone and does not become a detection error for the focus error signal. This is an attempt to propose a device.

Summary of the Invention A focus error detection device according to the present invention includes a photodetector into which reading light from a disk is incident, and a lens provided on an optical path of the reading light to the photodetector. It has four quadrant lens regions separated by two mutually orthogonal planes including the axis, the focal lengths of the lens regions in the first and third quadrants are equal to each other, and the focal lengths of the lens regions in the second and fourth quadrants are equal to each other. The focal lengths of the lens regions in the first and third quadrants are equal to each other, and the focal lengths of the lens regions in the second and fourth quadrants are different from each other, and each lens region has axial symmetry. The photodetecting surface of the detector is formed in an annular shape consisting of photodetecting parts in the first to fourth quadrants divided into four parts corresponding to the lens areas in the first to fourth quadrants of the lens.
A focus error signal is obtained from the difference between the sum of the detection outputs from the photodetectors in the third quadrant and the sum of the detection outputs from the photodetectors in the second and fourth quadrants.

According to the present invention, the degree of defocus in the optical reproducing device becomes extremely large, and a spot portion with a small radius and high optical density is generated among the spots of the beam incident on the photodetection surface of the photodetector. However, it is possible to obtain a focus error detection device that is not affected by the dead zone and does not become a detection error for the focus error signal.

Embodiment An embodiment of the present invention will be described below with reference to FIG. The photodetector is formed into an annular shape whose photodetecting surface DT is divided into four parts corresponding to the lens regions 7a to 7d of the first to fourth quadrants of the lens 7, and consists of photodetecting parts in the first to fourth quadrants. Form. And the first
A focus error signal is obtained by the difference between the sum of the detection outputs from the photodetectors in the second and fourth quadrants and the sum of the detection outputs from the photodetectors in the second and fourth quadrants.

Also, inside the photodetecting sections in the first to fourth quadrants of the photodetecting surface DT of the photodetector, a central CT is placed as a photodetecting section (this may be omitted, but the efficiency of the reproduced signal is improved if ) will be provided. Incidentally, FIG. 16 shows the spot 4' at the time of focusing. The total amount of light incident on the beam to the plane containing the photodetector surface DT of the photodetector is I ₀ ,
1st to 4th quadrants and central part of photodetection surface DT
Assuming that the incident light amount of the beam to the CT is ₁ to ₄ and I _CT respectively, the information reproducing light amount I _p is I _p =I ₁ +I ₂ +I ₃ +I ₄ +I _CT ≈I ₀ . Also, the focus error light amount I _FE is I _FE = (I ₁ + I ₃ ) - (I ₂ + I ₄ ). Further, the tracking error light amount I _TE is I _TE = (I ₁ + I ₂ ) − (I ₃ + I ₄ ) or I _TE = (I ₁ + I ₄ ) − (I ₂ + I ₃ ). Incidentally, in the case of the photodetector shown in FIGS. 9 to 11, the information reproducing light amount I _p satisfies I _p <I ₀ .

According to the focus error detection device using such a photodetector, the degree of defocus in the optical reproducing device becomes extremely large, and the radius of the spot 4' of the beam incident on the photodetection surface DT of the photodetector increases. Even if a spot portion with a small amount of light and a high light density occurs, this spot portion is not affected by the dead zone and does not become a detection error for the focus error signal. That is, the characteristic curve (S-shaped curve) of the level (relative value) of the focus error signal with respect to the amount of defocus (μm) based on the distance during focusing between the objective lens and the disk is as shown in FIG.
When the amount of defocus increases further after exceeding the linear region between points B and B' that intersect the two points, the absolute value of the level of the focus error signal gradually decreases, and its sign temporarily reverses. Never.

Further, by providing a photodetection section in the center CT of the photodetection surface DT of the photodetector, the efficiency of the reproduced signal is increased.

Effects of the Invention According to the present invention, the degree of defocus in the optical reproducing device is greatly increased, and among the spots of the beam incident on the photodetection surface of the photodetector, the spot portion with a small radius and high optical density is Even if this occurs, it is possible to obtain a focus error detection device that is not affected by the dead zone and does not become a detection error for the focus error signal.

[Brief explanation of drawings]

Fig. 1 is a perspective view showing a conventional semi-cylindrical lens, Fig. 2 is a perspective view showing a beam spot, Fig. 3 is a line diagram showing the locus of the incident ray of the semi-cylindrical lens in Fig. 1, Fig. 4, Figures 5 and 6 are line diagrams showing the beam spot, Figure 7 is a perspective view showing an example of the previously proposed lens, and Figure 8 is a line diagram showing the locus of the incident ray of the lens in Figure 7. , FIG. 9, FIG. 10, and FIG. 11 are diagrams showing an example of a photodetector and the beam spot for each, FIG. 12 is a characteristic curve diagram, and FIG. 13 is an optical diagram to which the lens of FIG. 7 is applied. FIG. 14 is a perspective view showing an example of a photodetector and a line diagram showing a beam spot for the photodetector;
FIG. 15 is a characteristic curve of the amount of defocus versus focus error signal level of an example of a photodetector, FIG. 16 is a diagram showing an example of a photodetector used in an embodiment of the present invention and its corresponding spots, and FIG. FIG. 17 is a characteristic curve diagram of the amount of defocus versus focus error signal level of the photodetector. 7 is a lens, 7a to 7b are lens regions in the first to fourth quadrants, DT is a light detection surface of a photodetector, ~ is a first to fourth quadrant, and CT is a central portion.

Claims (1)

[Claims] 1. It has a photodetector into which the reading light from the disk enters, and a lens provided on the optical path of the reading light leading to the photodetector, and both of the lenses are arranged in two mutually orthogonal planes including the optical axis. comprising a lens area in four divided quadrants, the focal lengths of the lens areas in the first and third quadrants are equal to each other, and the focal lengths of the lens areas in the second and fourth quadrants are equal to each other,
and the focal lengths of the lens regions in the first and third quadrants are different from each other, and the focal lengths of the lens regions in the second and fourth quadrants are different from each other, and each of the lens regions has axial symmetry, and The light detection surface of the photodetector is formed in an annular shape consisting of photodetection parts in the first to fourth quadrants divided into four corresponding to the lens regions in the first to fourth quadrants of the lens, and 1st
A focus error signal is obtained by the difference between the sum of the detection outputs from the photodetectors in the third quadrant and the sum of the detection outputs from the photodetectors in the second and fourth quadrants. Focus error detection device.