JPS59221834A - Optical head - Google Patents

Abstract

PURPOSE:To detect stably an out-of-focus quantity by spreading a light out of a light sensing region on a detector very much to reduce sufficiently the sum of a detection signal if the out-of-focus quantity is very large. CONSTITUTION:The value of an optical parameter is so determined that the sum of the quantity of laser light irradiated onto light detecting cells 16-1 and 16-2 on a photodetector 16 is small when the outer-of-focus quantity is very large. That is, the output voltage of an adding circuit 25 is reduced when the out-of- focus quantity is very large; and therefore, a certain reference value is determined in a CPU27, and it is judged that the out-of-focus quantity is very large if the output voltage of the adding circuit 25 is lower than this value. When the out-of-focus quantity is very large, the laser spot on the photodetector 16 is spread out of light detecting cells 16-1 and 16-2 very much for the purpose of reducing the sum of the quantity of laser light irradiated onto light detecting cells 16-1 and 16-2.

Description

Detailed Description of the Invention [Technical Field of the Invention] The present invention relates to an apparatus capable of reading at least information from an information storage medium using focused light.
It is used for playback-only information storage media such as D (contact disc) and video disc, and image files and still image files COM (computer output memory). The present invention relates to an optical head used when at least reproducing or recording information on an information storage medium capable of completely recording and reproducing information by causing a state change such as fully opening, and furthermore, on an erasable information storage medium.

[Technical background of the invention and its problems]

The functions of the read-only optical head include automatic focus correction, tracking of tracks on the information storage medium, and reading of information from the information storage medium. ``Additional recording of information to an information storage medium'' is added. ``Automatic defocus correction J'' specifically detects the amount of defocus with a photodetector, provides full electrical feedback, and drives a voice coil to direct the objective lens toward the information storage medium. The goal is to move to the optimal position.

By the way, when starting defocus correction, it is more stable to connect the feedback loop when the distance between the information storage medium and the objective lens is arbitrary. Feedback loops are easy to connect. Furthermore, even when the feedback loop is connected and the focus correction is automatically performed, the following inconvenience is likely to occur.

l) The shifted field pack looseness may be disturbed by optical or electrical disturbances, and the objective lens may collide with the information storage medium or move far away from it.

11) If the light reflecting layer or information recording layer is missing over a wide area in a part of the information storage medium, or if part of the optical path is completely blocked for some reason, the laser beam will be irradiated onto the defocus detector. Because of this, defocus detection is not performed and the objective lens begins to run out of control.

111) Depending on the optical system, the out-of-focus detection signal may become small when the focus is significantly out of focus from the in-focus position, and in this case, out-of-focus detection is not performed even though the focus is out of focus. It turns out.

For this reason, in addition to the feedback circuit for defocus correction, a means to detect whether the focus is near the in-focus position or if the focus is greatly out of focus is required, but there is currently no technology available for this. The rest is unknown.

[Purpose of the invention]

The present invention has been made based on the above-mentioned circumstances, and an object of the present invention is to move an objective lens close to an in-focus position with respect to an information storage medium to automatically start defocus correction near the in-focus position. The entire optical head is capable of detecting when the amount of defocus has become extremely large, and detecting when the amount of defocus has become extremely large. We are busy providing.

[Summary of the invention]

The present invention is capable of reading at least information from an information storage medium using focused light, and provides a condensing means for condensing light onto the information storage medium, and a light condensed by the condensing means. is reflected on the information storage medium and has passed through the condensing means again. A source extracting member extracts the nine asymmetrically with respect to its original axis, and this source extracting member detects the extracted nine and at least A photodetector for detecting defocus, and a control means for controlling the light converging means to approach and separate from the information storage medium based on the detection result of the photodetector so as to bring the light condensing means at least close to a focused position. This is because the detector is constructed in such a way that when the amount of focus is very large, the light on the detector extends far beyond the light sensing area, and the sum of the detection signals becomes sufficiently small.

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to the drawings. 1 in Figure 1 is a laser device (semiconductor laser),
The emitted light is converted into a parallel beam by a collimator lens 2 and then heads toward a polarizing beam slitter 3. Furthermore, the parallel laser beam reflected by the polarizing beam splitter 3
The beam passes through a 174-wavelength plate 4 and enters an objective lens 5, and is focused by this objective lens 5 toward a light reflecting layer (or information recording layer) 7 of a disk (information storage medium) 6. Ru. The objective lens 5 is supported by a voice coil 8 so as to be movable in the direction of its original axis.
is positioned at a predetermined position, the beam waist of the focused laser beam is projected onto the surface of the light reflection layer 7 through the objective lens 5, and the minimum beam spot is projected onto the surface of the light reflection layer 70. It is formed. In this state, the objective lens 5 can be kept in focus and information can be written and read. information'! When writing, a bit is formed on the tracking guide 9 on the light reflective layer 7 by a laser beam modulated in light intensity.When reading out information, a laser beam with a constant light intensity is
The light intensity is modulated by the bits formed on the tracking guide 9 and reflected.

The diverging Radhe beam reflected from the light reflection layer 7 of the original disk 6 is converted into a parallel beam by the objective lens 5 when focused, passes through the quarter-wave plate 4 again, and enters the polarizing beam splitter 3. be returned. Laser beam is 1/4
By making the entire round trip of the wave plate 4, the polarization plane of the Rade beam is rotated by 90 degrees compared to when it is reflected by the polarizing beam splitter 3, and the Rade beam whose polarization plane has been rotated by this 90 degrees becomes a polarized beam. - It is not reflected by the splitter 3 and passes through this polarizing beam splitter 3. The Radhe beam that has passed through the polarized beam snoritter 3 is divided into two systems by a half mirror IO, and one of the systems (track deviation detection system) is irradiated onto the first photodetector 12 by a convex lens IJK.

The first signal detected by the first photodetector 12 includes information recorded on the optical disc 6, and is sent to a signal processing device and converted into digital data. From the other (focal detection system) radar beam divided by the half mirror 10, only the component passing through a region separated from the optical axis 14 is extracted by a light shielding plate (original extraction member) 13. After passing through the projection lens 15 , the light is incident on the second photodetector 16 . Here, the light shielding plate 13 may be composed of a prism, an aperture, a slit, a knife, an edge, or the like. The signal detected by the second photodetector 16 is processed by an IA focus signal generator (not shown),
A focus signal generated from this focus signal generator is applied to the key chair coil drive circuit 17. The voice coil drive circuit 17 drives the voice coil 8 according to the focus signal to maintain the objective lens 5 in a focused state. Note that in order to accurately trace the tracking guide 9 formed on the light reflective layer z of the optical disc 6, the linear actuator may be activated by processing the signal from the second photodetector 16. Alternatively, the objective lens may be moved laterally, or a galvano mirror (not shown) may be operated.

The optical system for detecting the time of focus shown in Fig. 1 is simplified as shown in Fig. 2, and the trajectory of the laser beam related to focus detection is shown in Figs. It can be drawn as shown in c). When the objective lens 5 is in focus, the beam waist is projected onto the light reflective layer 7, and the minimum beam
A spot, ie a beam waist spot 18, is formed on the light reflective layer 7. Normally, the radar beam that enters the objective lens 5 from the radar device 1 is a parallel beam of light, so the beam waist is formed by the focal point of the objective lens 5. However, if the laser beam incident on the objective lens 5 from the laser device 1 is slightly divergent or convergent, the beam waist will be formed near the focal point of the objective lens 5. In the optical systems shown in FIGS. 1, 2, and 3 (a), (o), and (c), the light-receiving surface of the photodetector 16 is located at the focal point of the beam waist spot 18 in the focused state. arranged on the image plane.

Therefore, when focusing, the beam waist spot 18
An image is formed at the center of the light-receiving surface of the photodetector 16. In other words, as shown in FIG.
l-18 is formed on the original reflective layer z, and this light reflective layer 7
The laser beam reflected by the laser beam is converted into a parallel beam by the objective lens 5 and directed toward the light shielding plate 13 . Light shielding plate 1
3, only the light component passing through the area separated from the optical axis 14 is extracted, focused by the projection lens 15, and narrowed down to the minimum on the photodetector 16, so that the beam waist spot image is formed on top. Next, when the objective lens 5 approaches the light-reflecting layer 7, a beam waist is generated as the Rede beam is reflected by the light-reflecting layer 2, as shown in FIG. 3(b). That is, the beam waist occurs between the objective lens 5 and the light reflecting layer 7. In such an out-of-focus state, the beam waist usually occurs within the focal length of the objective lens 5, so assuming that the beam waist functions as a light spot, it is clear that the beam is reflected by the light reflecting layer 7. The LED beam emitted from the objective lens 5 is converted into a diverging laser beam by the objective lens 5. The laser beam component that has completely passed through the light shielding plate 52 is also diverging, so even if this laser beam component is focused by the projection lens 15, it is not focused to the minimum on the light receiving surface of the photodetector 16 and is not detected. Vessel 16 will also be focused towards a distant point. Therefore, the laser beam component is projected upward in the figure from the center of the light-receiving surface of the photodetector, and a pattern larger than the beam spot image is formed on the light-receiving surface. Further, when the objective lens 5 is separated from the light reflective layer 7 as shown in FIG. 3(c), the laser beam is reflected by the reflective layer 2 after forming a beam waist. When out of focus like this,
Normally, the beam waist is outside the focal length of the objective lens 5 and is formed between the objective lens 5 and the reflective layer 7, so that the reflected radar beam directed from the objective lens 5 toward the light shielding plate 13 has a convergent diameter. That will happen. Therefore, the light shielding plate 13
'<The passed Rade beam component is further converged by the projection lens 15, and the rear photodetector 16 forms a convergence point.
is projected onto the light-receiving surface of. As a result, a pattern larger than the image of the beam waist spot is formed on the light receiving surface of the photodetector 16 from the center upward and downward in the figure.

Next is the optical system mentioned above? The operating control circuit will be described with reference to FIG. The photoelectric signals obtained from each photodetection cell 16-1. After being amplified by the preamplifiers 21 and 22 of the detection photodetection cell, it is subjected to subtraction processing and addition processing by subtraction circuits 23 and 24 and addition circuits 25 and 26.
Input to pu2y. The CPU 27 drives the voice coil 8 (by the defocus correction drive circuit 17) and monitors the outputs of the subtraction circuits 23, 24 and addition circuits 25, 26 while moving the objective lens 5fc. When the CPU 27 determines from the monitor signal that the objective lens 5 is at the optimal position, it issues a command to the switching circuit 28 to connect the return loop for defocusing. A waveform correction circuit 29 for frequency characteristic improvement, phase compensation, etc. is included in the middle of the return loop. Even if the return roots are connected, the subtraction circuit 23,
Adding circuit 25゜26 output t is monitored and when the amount of focus reaches a certain level, the switching circuit 28 is turned off)
, and find the optimum position of the objective lens 5. Note that 30 is a waveform correction circuit, and 31 is a driver circuit for correcting a displacement.

In addition, under certain conditions as described above, the CPU 27 automatically starts defocus correction and determines whether the focus is correct or not. The values of the optical parameters are determined so that abnormalities can be easily handled when the 1-quantity force S becomes extremely large. In other words, the value of the optical nosolameter is determined so that when the amount of defocus becomes extremely large, the sum of the amount of laser light irradiated onto the photodetection cells J6-J and 26-2 on the photodetector 16 becomes small. ing. If the amount of defocus becomes very large, the output voltage of the adder circuit 25 will decrease, so the CPU 27 determines a certain reference value and sets the output voltage of the adder circuit 25 to be lower than that value. If the amount of defocus is low, it is determined that the amount of defocus is very large, and when the amount of defocus becomes very large, the total amount of radar light irradiated onto the photodetection cell 16-1, 16-2 is reduced. Photodetector 1
Self-detection of laser spot on 6 6-1 t1
It is configured to protrude largely from 6-2.

This will be explained in detail below. First, considering the outputs of the addition circuit 25 and the subtraction circuit 23 when the focus crab f is aligned,
Defocus detection signal Y for defocus amount X (subtraction circuit 2
3 outputs) is shown in FIG. 4, and FIG. 5 shows the relationship between the sum 2 (output of the adder circuit 25) of the amount of light detected by the defocus detector with respect to the defocus iX. Focus position (X
= 0) The rise of the defocus detection signal with respect to the amount of defocus X in the vicinity is steeper than that of other defocus detection methods, and when the amount of defocus is At the point where δ2. or δpn
It gets saturated at that point. However, in FIGS. 4 and 5, the defocus detection signal Y or the total saturation 2 of the amount of detected light when saturated in the characteristic A is normalized to l#. Furthermore, when the focus is out of focus, the beam spot on the photodetector expands δ. , or δ. The beam spot protrudes from the photodetector at point n. Therefore, δof ”is δ
. . If the focus becomes further unfocused, the sum 2 of the defocus detection signal Y and the amount of detected light decreases by the amount that the beam spot protrudes from the photodetector. However, if the distance between the objective lens 16 and the light reflection layer 7 of the original disk 6 is too far, the beam spot will be located on the center line p on the photodetector 16, as shown in FIG. δ1 in Figure 4. However, when the objective lens 5 is far away, the same glass signal is produced as if the objective lens 5 and the light reflection layer 7 of the optical head 6 were too close together. At this time, the curves in Figures 4 and 5 have three different characteristics. When the end face of the light shielding plate 13 is located on the center of the optical axis of the optical system (that is, when the minimum distance from the optical axis 14 to the light shielding plate 13t = 0), the characteristic A is exhibited. Also, if the end face of the light shielding plate 13 is offset from the center of the optical axis 14 (Dk'===O), the light passing through the center of the optical axis 14 can pass through without being blocked by the light shielding plate 13. In this case, the light passing through the mouth and the center of the optical axis 14 is blocked by the light shielding plate 13, and the light passes through the photodetector 1.
If you cannot reach up to 6, you will have the respective characteristics of C. Also, except for the vicinity of the in-focus position,
The absolute value of the defocus detection signal (that is, in the Y direction) in the graph is almost equal to the graphs A and C in FIG. In addition, in FIG. 5, near the focused position, the beam spot on the photodetector 16 falls within the light-insensitive area that exists between the photodetection cells 16-1 and 16-2. Since the amount of light stored is large, the amount of photocurrent flowing is small, and the total amount 2 of the detected light amount is small.

Next, optical characteristics for each optical parameter are calculated using the ray tracing method.

The change in the trajectory of the laser described above, that is, the change in the trajectory of the light beam, is as follows:
The value hsk at which the laser beam component is deflected onto the photodetector 16 can be determined using geometrical optics as described below. The geometric optical imaging system of the objective lens 5 can be expressed as shown in FIG. Here, fo is the focal length of the objective lens 5, and δ is the total moving distance of the objective lens 5, that is, the light reflective layer 7 of the optical disc 6, from the in-focus state to the out-of-focus state, and in FIG. The ray trajectory shown by the solid line is emitted from the beam waist and passes through the objective lens 5.
The light beam passes through a point on the main surface of the optical axis 14 and is spaced apart by a distance h0 from the optical axis 14, and is focused. Figure 3 (a)
At the time of focusing shown in , it is clear that δ=0,
In the out-of-focus state shown in FIG. 3(b), the optical disc 6 approaches the objective lens 5 by a distance δ, and the beam waist is
Since the beam is formed by being reflected by the light reflecting layer 7, the beam waist is twice as close to the objective lens 5. (If they are close, δ<0.) Also, the third
At the time of out-of-focus as shown in FIG. This is similar to the case where a beam-waist is formed behind the reflective layer 7, and the beam-waist is 2.
It is separated from the objective lens by δ. At the time of focusing, if the beam waist is formed at the focal position of the objective lens 5, when the optical disk 6 moves by δ, the beam waist and the objective lens 5 will change as shown in FIG. The distance between the principal surfaces of is expressed as (fo+2δ).

If the beam waist is regarded as the total light spot, angles β0 and β1 in FIG. 7 are expressed by the following equations (1) and (2).

O −−1=−(−βo)kiβ.・・・・・・・・・
(1)fo+2δ Also, according to the lens imaging formula, β12βo+hθ/f.

O fo + fa / 2δ −°−°°−(2) 8th
The figure shows all the ray trajectories in the optical system of the projection lens 15, and the projection lens 15 is a pair of combined lenses 1.5-1.
, 15-2.

Here, the lenses 15-1.15-;l have focal lengths f1 and fxk, respectively, and a light-shielding plate 13 is arranged at a position spaced apart from the main surface of the objective lens 5 by a distance. The main surface of the lens 15-1 is arranged at a distance of 100 cm from the main surface of the lens 15-1, and the main surface of the telelens 15-2 is further spaced from the main surface of the lens 15-1 by a distance of t. It is assumed that 16 light receiving surfaces are arranged.

The light ray locus shown by a solid line in the figure shows the light transmission surface of the light shielding plate 13 that is focused by the objective lens 5 and is spaced apart from the optical axis 14 by y.

The distance y is expressed by the following equation (3).

y = b (1-aβl where F(δ)"'(fo+f'/2δ)
-1, equation (3) is expressed by the following equation.

y=ho(1aF(δ))...---
-・------(4) Therefore, G..." i -a F(a)...
・・・・・・・・・・・・(5) Also, the light ray is a lens,
The position h1 from the optical axis 14 passing on the main surface of 15-1 is
It is expressed by equation (6).

h1=y-(L a)β1 If angle β2 is found in the same way as equation (2), angle β2 is (
7) It is expressed by the formula.

Similarly, the optical axis 1 of the light ray passing on the main surface of the lens 15-2
4. The position h2 and the incident angle β3 from above are detected by the photodetector 16.
The position h3 from the optical axis 14 of incidence on the light-receiving surface of
The amount of deviation is expressed by equations (8) to (10), respectively.

h, = h1-4/lz - (: H+L (1-H/fx) ) XF (δ)
・・・・・・・・・・・・(8) and ha = bz (L-H)β3 As already mentioned, the optical system shown in FIGS. 7 and 8 has a focused point, that is, δ= 0, the light beam on the detector 16 is h
Since it is focused at 3=O, under this condition, F(0)=O, and the α expression is expressed by the following expression.

In addition, since only the light beams passing through the optical axis 14 are extracted by the light shielding plate 13, it is difficult to see what is going on.

It is. Therefore, if we simplify the formula using this formula, we can obtain formula (g) or formula 64.

Or, if δ is sufficiently small (δ<fo2), (a
Since fo ) < fo /2δ, the beam waist formed on the light receiving surface of the photodetector 16 with respect to the beam waist on the light reflective layer 7 of the optical disk 6 at the time of focusing (δ = 0) The lateral magnification m of the image is
It is expressed by the following formula 09.

m=-β0/β3 (inverted image) Here, β0 at δ=0 is, so if fzk is eliminated from α4 formula in formula (6), from this formula,
If we find the complete solution for fl, in the actual optical system, fl
-H, so if we consider the case in which an erect image is formed (m
=β0/β3), fl is expressed by the following formula.

Therefore, if we calculate the total f2 from equation (6), we can substitute it into the Kanshiki7q] equation and express it by hsk horizontal magnification m, then we get (
Equations (c) and (c) are obtained.

However, (a-fo)<fo2/2δIf the projection lens 15 is a single lens in the optical system shown in FIG.
Since 2=ω, fx=and m=ft/fo, and in the optical system shown in FIG. 7, it is assumed that the beam waist is formed at the focal point of the objective lens 5. When a diverging or converging laser beam is incident on the objective lens 5, the beam waist is formed offset by b from the focal point, as shown in FIG.

In this case, the optical system after the laser beam irradiated onto the optical disk 6 is reflected from the disk surface through the optical path as shown in the figure is connected to the objective lens 5 and the detection system lens (projection lens) 15.
are all treated as one composite lens. The focal length of this composite lens is 'frf', the distance from the front focal position of the composite lens to the optical disc 6 (radar source focused light) when it is in focus is C, and the disc 6 is the focal point by δ. The optical path of the reflected light when it deviates from
If all the detection system lenses 15 are considered as one composite lens, from FIG. Also, C+2δ β3=βo+h"/7"-β.-(βo,=-)
C+2δ m--7-β.

If the second photodetector 16 is placed at the imaging point when the focal point of the laser beam and the position of the optical disk 6 match (when δ=O), when δ=0, h3=0. Therefore, C =壬f”7m However, m is always a positive number, and when it is 10m, it represents an inverted image, and when it is -m, it represents an erect image.

Furthermore, if the distance from the rear principal point of the composite lens to the second photodetector 16 is k A 1 , then h3=i)''−A
=-β(f''+c-1-2δ)=-βo((1壬1/m
)f”+2δ)-f”=β. ((壬Al/m f”+ )+(At/f
"-1) x 2 δl ...... (c) For any β0, at δ = 0, from h3 = 0, kx = f" (1,000 m) ......・
Substituting equation (2) into equation (c) gives 1-4-2mδ×β0...
. . .) If the focal position of the objective lens 5 and the laser condensing point are shifted by b as shown in Fig. 9, then (1
) ~ In the equation, each equation for ray tracing holds true as is by converting 2 sum to 2 δ plus. Therefore, from equation (5), and by substituting equation () into equation (]), substituting equation (C) into equation (C), we get: Here, if a=00, and if fo+b>2δ, then , becomes. , Now, the radius k A of the aperture (exit pupil) of the objective lens 5,
The total distance Fst from the principal point of the objective lens 5 on the side closer to the optical disk (information storage medium) 6 to the focal point (beam waist position) in the direction of the optical disk 6 is the photodetector 16 at the focused point as described above. The above imaging lateral magnification is assumed to be im. When there is no light blocking object in the optical path, the optical disc 6 and objective lens 5
If the distance of is shifted by δ from the focused point, the elongated spot on the photodetector 16 becomes a circle with radius R, but R is
c) In the formula, a"'01j'o + b-F,
b3 = R, R = l oblique h x 2 δ1 (given by 3 arms).If the working distance of the objective lens 5 is iWD, if the objective lens 5 approaches by the force, it will hit the optical disk 6.

Therefore, in order to prevent the objective lens 5 and the optical disc 6 from being damaged when the detection sensitivity varies, the system is configured to detect that the focus is greatly out of focus. In addition, since there are variations in electrical detection sensitivity and variations in the light reflectance of the information storage medium itself for the detection signal, the threshold level for detection is the one that is least susceptible to fluctuations in signal strength and errors caused by noise. The amount of the output voltage of the adder circuit 25 is set to half the maximum value so that it is difficult to detect. In other words, when the objective lens 5 approaches the optical disk 6 by WD/2 from the focal point position, the laser spot radius on the photodetector 16 changes from the 0-bu type, but at this time, light is detected as the intensity distribution within the spot is uniform. The area of the laser spot that protrudes from the photodetection cell 16-1゜16-2 (sensing area) is larger than the total area of the laser spot on the photodetection cell 16-1゜x6-2 (sensing area) on the device 16. The photodetector 16
It is necessary to decide the size of.

FIG. 11 is a diagram showing the entire relationship between the photodetector 16 and the beam spot shape. The objective lens 5 moves from the focal point position to W.
When approaching the optical disk 6 by D/2, the product Sin of the beam spot 32 irradiated onto the light detector 16 and the beam spot 32 irradiated onto the part protruding from the photodetector 1G are calculated. Sinri 5 outs in 5 outs
It must be.

FIG. 11 (a) where the light shielding plate 13 is not present after the light reflection layer 7 of the optical disc 6 is reflected until it reaches the photodetector 16.
In this case, the following relationship holds between the radius ra of the photodetector 16 and the beam spot radius R: πr, 2<<, n2 -2 . By the way, the light reflecting layer 7 and the photodetector 1
When the light shielding plate 13 enters the optical path between the beam spots 32 and 6, the shadow of the light shielding plate 13 appears on the beam spot 32.
Sout, in Figure 11 (c), Singo Sout't
, in Figure 11) fat in (S out , 1st
In Figure 1 (E), 5in=O (5out % O). Therefore, no matter what state the light shielding plate 13 is in, in order to satisfy 5out>Sin when δ=-WD/2, the size of the photodetector 16 must be adjusted so that it completely fits inside the circle with the calculated radius rb. All you have to do is decide. First, the 11th
The total area S of the beam spot 32 in the state shown in figure (b),
Find in+〇 out 〇From this, the area of the shaded part in Figure 1.1 is = rB n + R” 5in
−””. Therefore, in order to obtain 5 in = 5 out K, we need to solve the following. But this equation is too complicated. Therefore, the conditions of the present invention are slightly narrowed so that when δ=−pool force, S out )S ”≧Sin.

Beam spot 32' is modeled as a regular square inscribed in beam spot 32 with radius R.
Calculate as. The situation is shown in Figure 13 (A) to (E).
Shown below. The length of one side of a regular quadrangle inscribed in the actual beam spot 32 is k t .

Then, 5t = 2R, then sito = J"'i R song (b). In Figure 13 (b), 5ffut = Sjn"
R division to x (z + rb) == 2πr, '2πrb-4o
The solution rb of the equation rbt/2=0 is ・rb) Entering the 7 conditions of O.

By substituting Equation (B) and Equation (31) into Equation (C), the size of the light sensing area of the photodetector 16 is determined so that it falls within a circle with radius r given by Equation 6η.

In the above, when the objective lens 5 approaches the light reflection layer 7 of the optical disk 6 by WD/2 from the focused position, it is determined that the amount of defocus has become extremely large. However, suppose that for some reason the objective lens 5 begins to run out of control and moves forcefully toward the optical disk 6. Even if an abnormality is detected for the first time when the deviation is WD/2, and control money is sent to stop the runaway, the runaway will be stopped due to the inertia of the objective lens 5, and it will start to return to its original state only when the objective lens 5 is on the optical disk 6. It's going to be close. In addition, since the optical disk 6 rotates with considerable surface runout and deflection, it is always moving toward or away from the frame of the optical head at a slow speed. Therefore, if the timing between the two is bad, there is a risk that the objective lens 5 and the optical disk 6 will collide even if an abnormality is detected. Therefore, in view of a large margin, it is safer to perform detection at a location where the objective lens 5 approaches the focused position by WD/4. Further, the detection signal level was also set to half of the maximum output voltage of the adder circuit 25 in the above case. However, as mentioned above, in the vicinity of the focal point position in FIG. Since the amount of light entering and deflecting is large, the amount of photocurrent flowing is small.The sum of the detected light amounts 2 (output voltage of the adder circuit 25) is small, so if the detection signal level is too high, even if the photocurrent is at the in-focus position. Regardless, there is a risk of erroneous detection as if the focus is greatly out of focus. Further, the light reflectance of the light reflective layer 7 of the optical disc 6 also varies widely.

Furthermore, although we have treated the spot as having a uniform intensity distribution at this size, some optical systems have a high intensity density near the center of the spot, and in this case, the light detection cell 16
Even when the total area of the laser spot 32 on -1.16-2 is equal to the area of the laser spot 32 protruding from the photodetection cell 16-1.16-2, the output voltage of the adder circuit 25 is close to the maximum state. It may indicate a value. From the above, in order for the device to operate with a sufficient margin, only 1/4 of the total area of the laser spot 32 is irradiated onto the photodetection cells 16-1 and 16-2 before the focus is reached. It is preferable to design the image so that it is detected as being significantly blurred. Expressing this numerically, when δ=-WD/4, 5out>3Sin. δ=
- Ru at the time of WD/4 (from formula 3, rb when S out = 38 inches is 4πrb-to
From rb-4o2/2-0, substituting 0' into equation (B).The above explanation was given only for the case where the objective lens 5 approaches the light reflective layer 7 of the optical disc 6, but on the contrary, when the objective lens 5 approaches the light reflective layer 7 of the optical disc 6, It would be even more preferable if an abnormality could be detected even when the defocus buds become large. In addition, if the object is far away, the objective lens 5
There is no absolute standard value that states that all abnormality detection must be performed because there is no problem that the optical disk 6 may be hit by the optical disc 6. Therefore, geometrically, when the distance from the principal point of the objective lens 5 on the side closer to the optical disk 6 to the focal point (beam waist position) in the direction of the optical disk 6 is iF, when the distance from the focal point position is F/10, the light Detection cell 16-1
．． The area of the laser spot 32 protruding from the photodetection cell 16-2 is set to be three times or more larger than the area of the laser spot 32 on the photodetection cell 16-1 (16-2). That is, (substituting δ=+F/10 k into equation 3'4, and substituting equation 01 into equation (to) gives the following.

In summary: 1) Since an abnormality can be automatically detected when the focus is greatly out of focus, it is possible to prevent the objective lens 5 from hitting the optical disk (information storage medium) 6 and damaging both of them.

ii) Since defocus correction can be started after the objective lens 5 approaches the in-focus position, defocus correction can be started very stably.

l11) Abnormalities such as large defocusing can be detected by the amount of detection light irradiated to the photodetector 16, so when a light source such as a semiconductor laser does not light up or when the light convergence point on the optical disk 6 is absent from the light reflective layer 7. When approaching a place where the light is
□It is possible to prevent the objective lens 5 from running out of control.

iV) As is clear from FIGS. 4 and 5, when the focus is significantly out of focus, the amount of the defocus detection signal (output of the subtraction circuit 23 in FIG. 4) becomes small is always detected by the defocus detection detector 16. If the sum of the detected light amounts (the output of the adding circuit 25 in FIG. 4) is also small, an abnormality is detected. Therefore, it will not happen that the objective lens 5 does not move because the defocus detection signal is small even though the object is largely out of focus.

In addition, in the above embodiment, as shown in FIG.
Although the optical system using the light shielding plate 13 has been explained,
As shown in FIG. 12(a)(b) and FIG. 13(a)(b), it is also applicable to optical systems using slits 13-1 and holes xs-22 as light shielding plates 13. Applicable. In this case, the shape of the beam spot 32 on the photodetector 16 when the focus in each optical system is greatly out of focus is as shown in FIGS. Beam spot 32 irradiated onto photodetector 16
0 area Si・n and the area 5out'i of the beam spot 32 irradiated on the part protruding from the photodetector 16, δ
= -Vl/D/2, Sln≦Sout, δ=-W
The size of the photodetector 16 may be limited so as to satisfy the following conditions: 5out>3Sin when D/4, 5out>3Sjn when F/4o. Note that 32 in FIGS. 12(B) and 13(B) is a beam spot on the photodetector 16 when the slit 13-1 and the aperture 13-2 are not provided.

Up to this point, the second photodetector 16 is located at the position of the imaging point with respect to the light reflection layer 7 of the optical disc (information storage medium) 6 at the time of focusing.
The following describes a case where the second photodetector 16 is placed at a position shifted from the imaging point for the light reflective layer 7 of the optical disc 6 at the time of focusing. First, the formula representing its optical behavior is derived as follows.

The entire optical system is considered as one composite lens, and when one of the focal points coincides with the position of the light reflection layer 7 of the optical disc 6, the light is reflected from the rear principal point of the composite lens in the direction further away. If the distance to the other focal point (the imaging point for the aforementioned focal point) is 'fi = Ao, then the formula is Ao = f'' (1 + m)... Here, m is the lateral magnification, and fl is the focal length of the composite lens.If the photodetector 16 is placed at a position closer to the composite lens by Δ than this position, the composite lens The distance Al from the rear principal point to the photodetector 16 is A1=Ao-Δ=f"(1+m)-Δ...
・Given by (to). Substituting equation (6) into equation (c), h3=h"-A, β3 =-β(,f"10+2δ)+A・×εshiヱμβ・f"=βo (f"4:f ”/m 2δ = β・(±−1(±”+7+) ”■) Here, (
g) Substituting the sum total, it is now assumed that the light reflected from the optical disk 6 reaches the photodetector 16 in a somewhat focused state after passing through the objective lens 5. In this case, a point where the light is focused is created at a location other than where the photodetector 16 is present. Then, the distance from the position of this condensing point to the photodetector 16 at the time of focusing is Δ, and the above condensing point is established by the imaging relationship of one composite lens with respect to the light reflection layer 7 of the optical disc 6. The focal length of this composite lens when considered as k f'', and the imaging magnification (lateral magnification) of the image point created by this apparent composite lens when the focus is focused.
krn, radius A1 of the aperture (exit pupil) of the objective lens 5
The distance iF from the principal point of the objective lens 5 closer to the optical disk 6 to the focal point (beam waist position) in the direction of the optical disk 6 is determined. When there is no light shielding plate in the middle of the optical path, if the distance between the optical disk 6 and the objective lens 5 deviates from the focused point by δ, the beam spot 32 on the photodetector 16 becomes a circle with radius r, but the radius R In the set, a
=0°f o + b ”F, h3=R, y=A is replaced with the following equation.

Below, when δ=-WD/2, Sin is 5 out, δ is W
When D/4, So ut 〉3 Sin sδ=F/
10, the size of the photodetector 16 under each condition of 5 out> 3 sin is determined in the same manner as above.

First, in the case of δ=-WD/2, R becomes from the equation. Then, by substituting the equations (b) and (9) into the equation (g), when δ=-WD/4, R is 5o from equation v14.
When ut=36in, rb is 4xr -trb-t2/2=O. Substituting the formula (to) into this equation, then when δ=+F/10, substituting into the ←Φ equation, we get R F/10=l(+Δ/m×(±m+Δ/f4) bow)×
娃t・・・・・・・・・・・・(SQ(51) is substituted into the formula No. 9, it becomes ・・・・・・・・・・・・(5′).

〔Effect of the invention〕

As explained above, according to the present invention, in an apparatus capable of reading at least information from an information storage medium using focused light, there is provided a light condensing means for concentrating light onto the information storage medium; ) The focused light is reflected on the information storage medium, and the light that has completely passed through the focusing means is extracted asymmetrically with respect to the optical axis. a photodetector that detects the light and at least detects out-of-focus; and based on the detection result of the photodetector, controls the light condensing means to approach and separate from the information storage medium so as to bring it closer to at least a focused position. The control means is configured such that when the amount of defocus is very large, a large amount of light emanates from the photosensitive area on the detector, and the sum of the detection signals becomes sufficiently small. When the objective lens approaches the in-focus position and automatically starts defocus correction, it can be detected that the objective lens is close to the in-focus position, and abnormalities in the defocus can be corrected, thereby reducing the amount of defocus. It has excellent effects, such as being able to detect when it becomes very large, stably and reliably all electrically.

[Brief explanation of the drawing]

The drawings show one embodiment of the present invention. Fig. 1 is a configuration diagram showing an optical head and a control circuit, Fig. 2 is a diagram showing a defocus detection system, and Fig. 433 (A) ←) (C) An explanatory diagram showing the trajectory of the laser beam when in focus and out of focus,
FIG. 4 is a diagram showing the defocus detection signal with respect to the amount of defocus, FIG. 5 is a diagram showing the amount of light detected by the defocus detector with respect to the amount of defocus, and FIG. 6 is a diagram showing the amount of light detected by the defocus detector with respect to the amount of defocus. Figure 7 shows a state in which the objective lens and optical disk are further apart from each other, Figure 7 is a diagram for analyzing the entire trajectory of a ray passing through the objective lens, and Figure 8 is a diagram for analyzing the trajectory of a ray passing through the entire projection lens. Figure 9 shows an optical system in which the focal position of the objective lens and the focal point of the laser beam are misaligned. The diagram shown in Figure 10 is a diagram for analyzing the trajectory of the light ray when the objective lens and projection lens are considered as one threat lens, Figure 11 (A).
〜0 is a diagram showing the relationship between the photodetector and the beam spot shape, FIG. 12 is a diagram for performing definite integral of a circle,
Figures 3 (a) to (e) show Al? - It is a figure showing the case where a charger is used. 6... Information storage medium (optical disk), 5... Objective lens, 13... Light extraction member (light shielding plate: Naifenode)
, JJ-1...Light extraction member (socket, to), 13-2.
...Light extraction member (aperture), )'6...Second
photodetector, 7... light reflective layer (information recording layer). Applicant's agent Patent attorney Takehiko Suzue No. 11O Figure 2 Figure 11. Figure ('') (B) (C) Figure 12 Figure 13 (A) (B) (C) (
D) (Soup,) Fig. 14 (A) (B) Fig. 15 (A) 52... -20″

Claims (9)

[Claims] (1) In a device capable of reading at least information from an information storage medium using a focusing block C7, a focusing means for focusing light onto the information storage medium, and a light condensed by the focusing means can be used to collect information. A light extraction member that extracts the light reflected on the storage medium and completely passed through the condensing means again asymmetrically with respect to its optical axis; and - the light extracted by this light extraction member is detected and at least defocused. It is fully equipped with a photodetector for performing detection, and a control means for controlling the focusing means to approach and separate from the information storage medium so that the focusing means approaches at least the focal point position based on the detection result of the optical detector, and the focusing means The amount of blur is very thick 1
The optical head is characterized in that the source on the detector is configured such that the source largely protrudes from the original sensing area on the detector, and the sum of the detection signals becomes sufficiently small. (2) The photodetector detects the light when the working distance of the light collecting means is WD, and the light collecting means approaches the light reflecting layer or the information recording layer of the information storage medium by a distance from the focal point position. 2. The optical head according to claim 1, wherein the area of the light protruding from the light sensing area of the device is larger than the area of the light on the sensing area. (3) When the light collecting means approaches the light reflecting layer or the information recording layer of the information storage medium by a distance T from the combination position, the photodetector detects
The optical head according to any one of claims 1 to 2, wherein the area of the light protruding from the photosensitive area of the photodetector is three times or more larger than the area of the light on the sensing area. . (4) The photodetector has a total distance F from the principal point of the light collecting means on the side closer to the information storage medium to the light collecting point in the direction of the information storage medium.
When the light focusing means moves away from the focused position by π with respect to the light reflecting layer or the information recording layer of the information storage medium, the area of the light protruding from the light sensing area of the photodetector is the light sensing area. The optical spatula according to claim 1, wherein the area of the light above is three times or more larger.
Do. (5) the control means monitors the sum of the signals from the photodetectors;
Claims 1 to 2. The defocus correction circuit is configured to be disconnected when the defocus correction circuit becomes less than a reference value.
The optical head according to any one of Section 3.4. (6) The photodetector is arranged at at least one of the imaging positions with respect to the light reflection layer or the information recording layer of the information storage medium at the time of focusing, and the radius of the opening of the light condensing means is set to A1 imaging magnification f m, the distance from the principal point of the light collecting means on the side closer to the information storage medium to the light collecting point is 'kF, and the light sensing area of the photodetector is the radius .=H-E□×h, ,
The optical head according to any one of claims 1, 2, and 5, wherein the optical head is configured so that the circle activation (F4) of vir> is also reduced. (7) The photodetector is arranged at at least one of the image formation positions with respect to the light reflection layer or the information recording layer of the information storage medium at the time of focusing, and is configured to be located at the position of the light reflection layer or the information recording layer of the information storage medium. The optical head according to any one of Items 1, 2, 3, 5, and 6. (8) The photodetector is arranged at at least one of the imaging positions for the light reflection layer or the information recording layer of the information storage medium at the time of focusing, and is configured to emit light therefrom. Optical head according to any of paragraphs 4.5.6 to 4.5.6 0 (9) The photodetector is arranged slightly shifted from the imaging position relative to the light reflection layer or information recording layer of the information storage medium at the time of focusing, and When it is assumed that the relationship between the focal point on the layer and the point where the light is reflected from there and refocused near the photodetector is established by the imaging relationship of one composite lens, When the focal length k f * is the distance from the position of the condensing point near the photodetector to the photodetector at the time of focusing, the circle of light sensitivity of the photodetector becomes smaller. The optical head according to any one of claims 1 and 5, wherein the photodetector is configured such that the optical head 0 00 is configured such that the optical head 0 00 is configured such that the optical head 0 00 is configured such that the optical head 0 00 is configured such that the optical head 0 00 is configured such that the optical head 0 00 is configured such that the optical head 0 00 is configured such that the optical head 0 00 The optical head according to any one of claims 1, 5, and 10, wherein the optical head is arranged so as to be slightly shifted from each other so that the light sensing circle becomes small. α ■ A patent in which the photodetector is arranged slightly shifted from the imaging position relative to the light reflection layer or information recording layer of the information storage medium at the time of focusing, and the original sensing radius is also small. An optical head according to any one of claims 1 to 5.