JPS61156536A - Optical head - Google Patents

Abstract

PURPOSE:To detect exactly a focus error by dividing the first and the second photodetectors into two in the same shape, respectively, and placing them at a specified position. CONSTITUTION:The first photodetector 30 and the second photodetector 34 are installed at a position where the light quantity of optical beams L', L'' irradiated to the first and the second photodetectors 30, 34 becomes equal to each other, at the time of focusing, and also in case when they have been divided into two in a shape of a concentric circle, at a position where spot sizes of the optical beams L', L'' irradiated to the first and the second photodetectors 30, 34 are larger than an area of the first photosensitive areas 30a, 34a, respectively, and also become smaller than an area of photosensitive areas 30a+30b, 34a+34b of the first and the second photodetectors 30, 34. A focus error is detected by detecting a difference of between the sum of the irradiated light quantity of the first photosensitive area 30a of the first photodetector 30 and the second photosensitive area 34b of the second photodetector 34, and the sum of the irradiated light quantity of the second photosensitive area 30b of the first photodetector 30 and the first photosensitive area 34a of the second photodetector 34.

Description

Detailed Description of the Invention [Technical Field of the Invention] The present invention relates to an improvement in an optical head capable of at least recording and reproducing information on an information recording medium using focused light. [Technical Background of the Invention] and its problems] In an optical head that can at least record and reproduce information using focused light, a beam quest (
When the most narrowed-down part) is not reached, that is, when a focus error occurs, normal information recording and reproduction cannot be performed. Therefore, in such an optical head, it is necessary to detect 7 orcus errors in order to record and reproduce information correctly.

Incidentally, the astigmatism method and the knife wedge method are known as typical conventional means for detecting focus errors.

FIG. 14 shows a conventional example of the astigmatism method, and its means will be explained with reference to this figure.

The light beam generated from the semiconductor laser 5o is collimated into a parallel beam by a collimator lens 52, and after passing through a polarization beam splinter 54, is turned into circularly polarized light by a quarter-wave plate 56.
), a spot is formed on an information recording medium 60 (hereinafter abbreviated as "recording medium") using an objective lens 58. The light beam reflected from the recording medium 60 becomes circularly polarized light with the opposite rotation to that of the light beam incident on the recording medium 6°.
After passing through the four-wavelength plate 56, the light beam becomes linearly polarized light whose vibration direction is perpendicular to the vibration direction of the incident light beam. Therefore, after being reflected by the polarizing beam sinter 54 and further passing through a spherical lens 62 and a cylindrical lens 64, it is transmitted to a photodetector 66.
is irradiated.

The photodetector 66 has four photodetection areas 66a, 66b, and 66c, for example, as shown in FIG. 815.

It is divided into four parts so as to have 66d. In this astigmatism method, the two sum signals of diagonally opposing photosensitive areas are subtracted ((66a+66d)
(66b+66c)) The focus error is detected by processing the signal amount obtained by the following.

However, according to this astigmatism method trap, the recording medium 60
Due to the brightness distribution of the light due to diffraction when it crosses the upper group or information pit, the pattern of this diffracted light is directly irradiated onto the photodetector 66, and the objective lens 58 and the recording medium 60 are brought into focus. Even at certain times, the amount of irradiation light detected by the photodetector 66 changes compared to when the focus is normally focused. Therefore, since the change in the amount of irradiation light detected by the photodetector 66 for detecting the 7-orcus error does not necessarily occur only in the out-of-focus state, it is difficult to accurately detect the focus error. The drawback was that it couldn't be done.

Next, a conventional example of the knife wedge method is shown in FIG. 16, and its means will be explained with reference to this figure.

The light beam generated from the semiconductor laser 70 sequentially passes through the collimator lens 72, the polarizing beam smitter 74°, the quarter-wave plate 76, and the objective lens 78 in the same manner as in the astigmatism method described above. It is guided to the recording medium 80.

Further, the light beam reflected from the recording medium 80 passes through the objective lens 78 and the quarter-wave plate 76, and then is reflected by the one-polarization beam sinter 74.

This reflected light beam is firstly amplitude-divided by a half prism 82. The light beam reflected by the half prism 82 has half of its wavefront cut by the knife 17 and the edge 84, and then passes through the trap and the focusing lens 86 and irradiates it to the photodetector 88. The photodetector 88 is, for example, As shown in FIG. 17, it is divided into two parts so as to have two light sensing areas 88a and 88b. In this tee wedge method, an out-of-focus state is detected when a spot is irradiated onto any one of the light sensing areas 88a and 88b, and as shown in FIG.
When a KM shot is made between 1 and 2, it is detected that the lens is in focus and focusing is performed.

On the other hand, the light beam that has passed through the half prism 82 is somewhat focused by the condensing lens 90 and is detected by the photodetector 9.
2. The amount of light beam incident on the photodetector 92 is used to detect tracking errors and information reproduction signals.

However, in this knife wedge method, the photodetector 88 that detects the focus error is.

It must be placed at or near the focal point of the light beam. Therefore, if the optical axis shifts due to a change in the external environment such as temperature or shock, or if the photodetector 88 shifts, the spot will be irradiated onto one of the photosensitive areas 88a and 88b. This method has the drawback of erroneously detecting orcus errors. Furthermore, since focus error is detected using one of the original beams whose amplitude has been divided by the half prism 82,
There was also a drawback that the N ratio deteriorated.

[Purpose of the invention]

The present invention was made based on the above-mentioned circumstances, and when detecting a focus error, a focus error may be incorrectly detected due to the influence of diffraction due to groups or information pits on the recording medium or changes in the external environment. To provide an optical head capable of accurately detecting a focus error at any time and maintaining a stable focus state without causing problems.

[Summary of the invention]

In order to achieve the above object, the present invention provides an optical head capable of at least recording and reproducing information using focused light, in which a light beam reflected from an information recording medium is separated into a plurality of beams. a first photodetector provided at a position closer to the separation means than the focus of the at least one light beam, and having at least a first photosensitive area and a second photosensitive area; 1. When the optical means and the information recording medium are in focus at a distance from the focus of at least one other light beam separated by the aiming separation means, the first photodetector is detected. A first photosensitive area and a second photosensitive area provided at a position equal to the amount of light beam irradiated onto the surface and having at least the same shape as the first photodetector. a second photodetector, the sum of the irradiation light amount of the tal photosensitive area of the first photodetector and the second photosensitive area of the second photodetector; A focus error signal is obtained by detecting a difference between a sum of irradiation light amounts of a second photosensitive area of the first photodetector and a first photosensitive area of the second photodetector. The present invention provides an optical head characterized in that a focus error is detected using the amounts of light beams irradiated onto two photodetectors.

[Embodiments of the invention]

The present invention will be described below with reference to an embodiment shown in FIG.

In FIG. 1, an optical head 10 capable of at least recording and reproducing information using focused light is provided so as to be movable in a radial direction 13 of a recording medium 120 by a linear actuator 11. This optical head 1
A semiconductor laser 14 that generates a KH1 light beam L is provided, and when writing information to the recording medium 12, the semiconductor laser 14 generates a diagonal beam L whose light intensity is modulated according to the information to be written. When reading information from the recording medium 12, a light beam having a constant light intensity is generated from the semiconductor laser 14.

The light beam L generated from the semiconductor laser 14 is converted into a parallel light beam by the collimator lens 16 and directed to the polarization beam splinter 18 . The light beam L that has not passed through the polarization beam subrifter 18 passes through a quarter-wave plate 20 and is incident on the objective lens 22.
is focused toward the recording medium 12 by. The objective lens 22 is supported movably in the direction of its optical axis 25 by a voice coil 24, and when the objective lens 22 is set at a predetermined position, the beam waist of the focused light beam L emitted from the objective lens 22 is also recorded. projected onto the medium 12;
A minimum beam spot is formed on the surface of the light reflective layer of the recording medium 12. In this state, the objective lens 22 is maintained in a focused state (hereinafter, this state will be referred to as "in-focus state" 0), and information can be written and read. When writing information, bits are formed in the tracking guide on the light reflection layer of the recording medium 12 by a light beam L modulated in light intensity, and when reading information, a light beam L having a constant light intensity is used to form bits. L is reflected by a bit formed in the tracking guide with a light intensity f.

The diverging light beam L reflected from the recording medium 12 is converted into a parallel light beam by the objective lens 22 when it is focused, passes through the quarter-wave plate 20 again, and is returned to the polarizing beam splitter 18 as a zero light beam L. By reciprocating through the quarter-wave plate 20, the plane of polarization of the light beam L is rotated by 90 degrees compared to when it passes through the polarizing beam splitter 18, and the plane of polarization of the light beam L rotated by this 90 degrees becomes polarized light. The light does not pass through the beam splitter 18, but is reflected by the polarizing beam sinter 18.

The light beam L reflected by this polarizing beam sinter 18
For example, the half prism 26 divides the amplitude of the light beam L without changing its shape. And this half prism 26
After passing through the condensing lens 28, the light beam L' transmitted by
The first photodetector 30 provided at a position close to is irradiated with light. - 10,000, no 1 - Light beam L reflected by Fugurism 26
# is this light beam L” after passing through the condensing lens 32
A second photodetector 34 provided further away than the focal point of
Here, the first photodetector 30 and the second photodetector 34 are each divided into at least two parts having the same shape, and when in focus, the first photodetector 30 and the second photodetector 34 are divided into two parts. Second photodetector 3
Light beam irradiated at 0.34 L/.

L# is installed at a position where the amount of light is equal to each other. The second photodetector 30.34 detects the light beam L
It is necessary to install the light beams at a position sufficiently far away from the focus of the light beams L/, L#.
, and the beam diameters W/ and W- of the light beams L' and L' are one light beam L'.
・Two photodetectors 30 and 34 are installed at positions where the diameter of the spot at the focal point of L# is larger than 5 times the diameter W.
Here depth of focus 2 and light beam L'.

The diameter W of the spot at the focal point of L' is determined as follows.

z == 1 (1) 2λ π a where a: radius of incident light on the condenser lens f: focal length of the condenser lens λ: wavelength of the incident light In this optical head 10, two photodetectors 30.
The focus error is detected by the amount of light beams L/LN irradiated onto the optical head 34. Therefore, with reference to the diagrams shown in FIGS. 2 to 11, the means for detecting the focus error in the optical head 10 will be explained in detail. explain.

FIG. 2 is a diagram showing a photosensitive area of the first photodetector 30 (Figure A) and a photosensitive area of the second photodetector 34 (Figure B). 1st. A second light region 308@34a and a second light sensing region 30b.

34b.

FIG. 3 shows the spot shape 301:8 (Figure A) of the light beam L' irradiated on the first photodetector 30 at the time of focusing, and the shape of the light beam irradiated on the second photodetector 34'N. FIG. 4 is a diagram showing the shape 348 of a spot L'.

When in focus, 1st. The first... A second photodetector 30.34 is provided so that the first photodetector 30.34 is provided as shown in this figure. Second photodetector 30°3
The shapes 308 and 348 of the light beams L' and L' irradiated on the light beams 4 and 4 are equal to each other. Also, the first
．． When the second photodetectors 30 and 34 are concentrically divided into two parts as shown in FIG. Second
The light beams L', L that are irradiated onto the photodetectors 30 and 34 of
The spot size of ' is the first photosensitive area 3, respectively.
0a, 34a, and the photosensitive areas 30a+30b, 34a+ of the first and second photodetectors 30.34.
The first photodetector 3 is located at a position smaller than the area of 34b.
Next, FIG. 4 shows that when the objective lens 22 of the optical head 100 is too close to the recording medium 12 compared to when it is in focus,
The shape of the spot 308' (Figure A) of the light beam L' irradiated on the first photodetector 30 and the shape 34S' (B) of the spot of the light beam L' irradiated on the second photodetector 34. FIG. (A) shows the in-focus state, and Fig. 5 (
B) shows a state in which the objective lens 22' is too close to the recording medium 12' compared to when it is in focus. That is, the light beam L"' reflected from the recording medium 12' does not become a parallel light beam even after passing through the objective lens 22', but instead becomes a divergent light beam. Therefore, the light beam I, which has passed through the condenser lens 29,
/// focuses at a position farther from the condensing lens 29 than when it is focused. Therefore, as shown in FIG. 4(A), the first photodetector 30 focuses the light beam The spot shape 308' is larger than the spot shape 308 at the focused point.

Further, as shown in FIG. 4(B), a second photodetector 34
In this case, the spot shape 348' of the light beam L# is smaller than the spot shape 34S at the focused point.

FIG. 6 shows the spot of the light beam L' irradiated on the first photodetector 30 when the objective lens 22 of the optical head 10 is too far away from the recording medium 12 compared to when it is in focus. Shape 30S' (Figure A) and second photodetector 3
The shape of the spot of the light beam L' irradiated on 4 348"
(Figure B).

Therefore, the shape of the spots of these light beams L' and L' is
This change will be explained in detail with reference to the diagram shown in FIG. FIG. 7(A) shows the state in focus, and FIG. 7(B) shows the state in which the objective lens 22' is too far away from the recording medium 12' compared to the state in focus. That is, the light beam L'" reflected from the recording medium 12' does not become a parallel light beam after passing through the objective lens 22', but instead becomes a convergent light beam. Therefore, the light beam L'" that has passed through the condenser lens 29
/II focuses at a position closer to the condenser lens than at the time of focusing 0 Therefore, as shown in FIG. 6(A), in the first photodetector 30.

The spot shape 308' of the light beam L' is smaller than the spot shape 308 at the focused point.

Further, as shown in FIG. 6(B), the second photodetector 34
In this case, the spot shape 348' of the light beam L' is larger than the spot shape 348 at the time of focusing.

As mentioned above, when a focus error occurs in the optical head IO, the first. Second photodetector 30.3
4, i.e. the size of the spot on the 1st. The amount of irradiation light detected by the second photodetector 30.34 has changed compared to when it is in focus.

Therefore, as means for detecting the focus error, the sum of the amount of light irradiated by the first photosensitive area 30a of the first photodetector 30 and the second photosensitive area 34b of the second photodetector 34,
This can be done by detecting the difference between the second photosensitive area 30b of the first photodetector 30 and the sum of the amounts of light irradiated by the second photodetector 34 and the first photosensitive area 34a. That is, the amount of light irradiated to each photosensitive area is set to 30a' and 30a'.
b', 34a', 34b'.

Then, each of these irradiated light amounts is converted into an electric signal, and F = (30a'+34b') - (30b'+3
4a') By calculating (1), the focus error signal below can be obtained. To explain this in detail, what are the first photodetector 30 and the second photodetector 34?

Since they are provided at positions where the amounts of irradiated light are equal when they are in focus, the result of this calculation is 0. Furthermore, when the objective lens 22 is too close to the recording medium 12,
Due to the previously described change in the size of the spot at each detector 30.34, 30a'+34b'(7) value')7j;
Since it is larger than the value of 6E30b'-1-343',
(2) A positive value is obtained for F as a result of the equation. Conversely, objective lens 2
When 2 is too far away from the recording medium 12, 3
Since the value of 0b'+34a' is greater than the value of 30a'+34b', a negative value is obtained for F as a result of equation (1). Then, the positive or negative focus error signal F obtained from the calculation result of equation (1) is processed, and this processed signal is sent to the drive coil 24, which drives the objective lens 22. By moving it to a predetermined position, the focused state is maintained.

As explained above, in this embodiment, the recording medium 1
Even if the influence of diffraction due to the groups and information bits above occurs, as shown in Figure 8, the shadows due to such diffraction 3
0(), 34G appear on the first and second photodetectors 30.34, symmetrically with respect to the center of the concentric circle of each of these detectors. Therefore, by performing the calculation of equation (1), the effects of this diffraction can cancel each other out, so accurate information can be obtained without being affected by the shadows caused by the diffraction of the groups and information pits on the recording medium 12. Focus error can be detected.

Also, if there are scratches or dust on the group on the recording medium 12 or for some other reason, the light beam l, /.

If the shape of L' is no longer a circle, the first . The light appears on the second photodetectors 30 and 34 in the form of a spot symmetrical with respect to the center of each concentric circle. Therefore, if the calculation of equation (1) is performed, these influences can be canceled out, so that there is no possibility of false detection of a focus error due to this influence trap.

In addition, if the optical axis shifts due to changes in the external environment such as temperature or impact, as shown in FIG. Each photosensitive area 30a, 30 of the second photodetector 30.34
The rate of change in the amount of irradiated light of 34b, 34a, and 34b is approximately equal to each other. Therefore, the influence of this optical axis deviation also cancels out each other by performing the calculation of equation (1), and erroneous detection of a focus error is unlikely to occur due to the influence of the optical axis deviation. Unfortunately, this change in the external environment may cause the optical axis to shift or the first. % In order to avoid the influence of the positional deviation of the second photodetector 30.34, this first photodetector 30. Second photodetector 3
0.34 means that the beam diameters w' and w' of the light beams L' and L" are at least the depth of focus Z from the focal point of the light beams L' and L", and that the beam diameters w' and w' of the light beams L' and L" are equal to It is provided at a position that is larger than 5 times.

Therefore, the effect of changes in the external environment is further eliminated, and there is an effect that more accurate focus error detection can be performed.

Furthermore, in this embodiment, the focus error signal is detected using all of the light beam L reflected from the recording medium 12, so the amount of signal is larger than that in the knife wedge method, and the S/N ratio is lower. This has the effect of improving focus error and allowing stable detection of focus errors.

Roughly speaking, in this embodiment, the state of the cross section of the light beam L' irradiated to, for example, the first photodetector 30 in FIG. 304' (Figure A) when the objective lens 22 is too close to the recording medium 12, and 30d' (Figure C) when the objective lens 22 is too far away from the recording medium 12.
As shown in the figure, the position of the boundary 30C of the two-divided photosensitive area of the first photodetector 30 varies depending on whether the cross section of the light beam L' changes between when it is in focus and when it is out of focus. By setting the first photodetector 30 at a place where the intensity is high, it is possible to detect orcus errors by detecting changes in the shape of the spot of the light beam L'. Since the change is divided into two parts, the first photosensitive area 30a and the second photosensitive area 30b, it is easy to detect the change, and focus error can be detected with high sensitivity. The above can also be said for the photodetector 34 of @2, so the size of the spot of the light beams L' and L' irradiated on the first photodetector 30 and the second photodetector is In this embodiment, the focus error is detected by calculating the change in the amount of irradiated light due to the change as shown in equation (1).
This method has the effect that it is possible to detect focus errors with higher sensitivity than the detection method using a photodetector (').

Furthermore, in the configuration of this embodiment, there is no need to use a compound lens or a cylindrical lens, so the configuration is relatively simple. Also, two photodetectors 30.3
Although it is conceivable to provide an aperture at position 4 and then provide a photodetector, the configuration of this embodiment has the effect of being single rather than the configuration in which an aperture is provided.

In the above embodiment, the condenser lens 28.32 is provided after the half prism 26, but the condenser lens 28.32 may be provided before the half prism 26. In addition, the photodetectors 30 and 34 of the first degree ta2 have been described as concentric two-split photodetectors, but in the optical head 10 in this embodiment, they are concentric three-split photodetectors shown in FIG. A similar effect can be obtained using other concentric photodetectors such as 36.38.

Furthermore, by using two-split photodetectors 40 and 42, each having a different area of the photo-sensing area shown in FIG. Second photodetector 40° 42 of 2:)+7) photosensitive areas 40a, 40b, 42a.

The shape of the light beam spot 408.4 on each of 42b
If the cross section No. 28 is set, substantially the same effect as this embodiment can be obtained when detecting a focus error.

Other 1 It goes without saying that the configuration of the present invention can be modified in various ways without departing from the gist of the present invention.

〔Effect of the invention〕

As explained above, according to the present invention, the optical head has a relatively simple configuration, and is free from the effects of diffraction due to the groups formed on the recording medium and information chips (K), as well as changes in the external environment such as temperature and impact. The first lens has sufficient tolerance against the effects of optical axis deviation due to By having the second photodetector, it is possible to accurately detect focus errors without being affected by these external conditions. Also, 1st. Since each of the second photodetectors is divided into at least two parts having the same shape, the size of the spot of the light beam changes between when the focus is focused and when the focus is not focused, depending on the light sensing area of each of the nine divided photodetectors. Since focus error can be detected with high sensitivity, focus error can be detected more stably.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the configuration of an optical head in an embodiment of the present invention, FIG. 2 is a diagram showing the configuration of a light sensing area of a photodetector provided in the optical head shown in FIG. 1, and FIG. is a figure showing the shape of the spot of the light beam formed on the photodetector shown in Fig. 2 when the focus is in focus, and Figs. 4 and 6 are the shapes of the light beam spots formed on the photodetector shown in Fig. 2 when the focus is out of focus. Figures 5 and 7 are diagrams showing the shape of the spot of the light beam formed above, and Figure 7 is a diagram showing the relationship between the focus positions of the projection lens when the focus is focused and when the focus is out of focus. ~Figure 10 is a diagram showing how the shape of the spot of the light beam formed on the photodetector shown in Figure 2 has changed depending on external conditions, and Figure 11 is a diagram showing the photodetector shown in Figure 2. Figures 12 and 13 are diagrams showing other embodiments of the photodetector provided in the optical head of the present invention, and Figure 14 is a diagram showing a conventional photodetector. A configuration diagram of an optical head showing an example of the astigmatism method, which is a means of detecting focus errors. FIG. 15 shows the configuration of a light sensing area of a photodetector provided in the optical head shown in FIG. 14. Figure 16 shows an example of the knife wedge method, which is a conventional means of detecting 7-orcus errors, and shows the configuration of the 9-optical head.
FIG. 17 is a diagram showing the configuration of the light sensing area of the photodetector provided in the optical head shown in FIG. 16.
- Optical head, 12... information recording medium. 14... Semiconductor laser, 16... Collimator lens, 18... Polarization, optical beam splitter, 20... 1
/4 wavelength plate, 22... Objective lens, 24... Drive coil, 26... Half prism, 28.32... Condensing lens, 30... First photodetector, 34...・Second
photodetector s 3Qa, 3ob... the first photosensitive area (a) and the second photosensitive area (b) of the first photodetector
), 34a, 34. b... The first photosensitive area (a) and the second photosensitive area (b) of the second photodetector. Agent Patent Attorney Noriyuki Chika (and 1 other person) Figure 1 Figure 2 Figure 4 t/Q tf 3) 5
Figure CB) Figure 6<A) (8) (A) Figure 8 (7'l)
(B) tlD,B. Figure 1θ Figure 11 (/4) (8J
(C) Fig. 12 (,4)<B) Fig. 13 (A) (B) Fig. 14a Fig. 16 Fig. 17

Claims (1)

[Scope of Claims] A light source section that generates a light beam, an optical means for guiding the light beam from the light source section to an information recording medium, and a separating means that separates the light beam reflected from the information recording medium into a plurality of beams. and a second photosensitive area, which is provided at a position closer to the separating means than the focal point of the at least one light beam separated by the separating means, and has at least a first photosensitive area and a second photosensitive area. 1 photodetector and at least one other light beam separated by the separation means, the amount of light of this light beam is equal to the first photodetector when the optical means and the information recording medium are in focus. a first photosensitive area and a second photosensitive area provided at a position equal to the light intensity of the light beam irradiated on the photodetector surface of the photodetector, and having at least the same shape as the first photodetector; and a second photodetector having an irradiation light amount between the first photosensitive area of the first photodetector and the second photosensitive area of the second photodetector. and the sum of the irradiation light amounts of the second photosensitive area of the first photodetector and the first photosensitive area of the second photodetector to generate a focus error signal. An optical head characterized by: