JPS6371939A - Optical head - Google Patents

Abstract

PURPOSE:To perform detection of out-of-focus with the higher stability and reliability by installing a light extracting member in a manner as to satisfy specific conditions. CONSTITUTION:The light extracting member is so installed as to satisfy x<=f0+ f<2>0/{2deltatf+(F-f0)}-L<2>/(f0-H) where the focal length of a 1st condenser means is designated as f0, the distance from the principal point of the 1st condenser means to the condensing point as F, the distance from the principal point of the 1st condenser means to the light extracting member as (x), the distance from the principal point of the 1st condenser means to the principal point of the 2nd condenser means as L, the distance from the principal point of the 1st condenser means to the light extracting member as H, and the permissible deviation of the recording layer of an information memory medium from the focal point of the light from the 1st condenser means as deltatf with the optical head to be used for a recording and reproducing device, etc. The out-of-focus is thereby stably detected with the high reliability.

Description

DETAILED DESCRIPTION OF THE INVENTION [Purpose of the Invention (Field of Industrial Application) The present invention is applicable to, for example, a CD (compact disc) for DAD, a video disc, an image file, a still image file,
The present invention relates to an optical head used in a reproducing or recording/reproducing device capable of at least reading information by irradiating an information storage medium such as a COM (computer output memory) with focused light.

(Prior Art) Recently, as shown in FIG. , a light extraction member (light-shielding plate such as knife etching) that extracts light asymmetrically with respect to this optical axis, a photodetector having a lens e1 and two photodetection cells f and g, and a spot at the photodetector. Instead of detecting defocus by size, it is possible to detect defocus by the movement of beam spot i on the photodetector (in the direction of arrow j), thereby making it less susceptible to the effects of diffraction. It's arrived.

As shown by the solid line in FIG. 2, when the focus is on, the value is "0", and when the objective lens b' and the information forming layer a' approach each other, a beam spot is formed on the upper photodetection cell g. 1 hits and a negative signal is generated, and the objective lens b'
and the information forming layer a' are so far apart that the beam spot 1 hits the lower photodetection cell and a positive signal is obtained.

However, as shown by the two-dot chain line in FIG. There is a serious problem in that a negative signal is output as if the objective lens b' and the information forming layer a' were too close to each other, as shown by the broken line in FIG.

Therefore, it is necessary to be able to correct even a relatively large amount of defocus, since it is not desirable in terms of characteristics if the image is reversed even if the image is slightly out of focus.

Furthermore, not only the above-mentioned optical system but also optical systems such as those shown in FIGS. 3 and 4 in which an aperture, slit, prism, mirror, photodetector, etc. are used instead of knife etching also have the problem of the slender pattern.

Note that d in FIG. 3 is an aperture, and d' is a transparent portion. Further, d3 in FIG. 4 is a pipe rhythm.

(Problems to be Solved by the Invention) The present invention has been made based on the above circumstances, and its purpose is to provide an optical system that enables defocus detection to be performed more stably and with high reliability. The purpose is to provide the head.

[Structure of the Invention] (Means and Effects for Solving the Problems) The present invention provides a first condensing means for condensing light onto an information storage medium having a recording layer storing information, and an information storage medium. Reflected by the recording layer L,
a second condensing means for condensing the light that has passed through the first condensing means; a light detection means for detecting the light condensed by the second condensing means; and a second condensing means. A light extraction member is provided between the light detection means and extracts light that is asymmetrical with respect to the optical axis. The distance from the point to the condensing point is F1 The distance from the principal point of the first condensing means to the light extraction member is x, From the principal point of the first condensing means to the principal point of the second condensing means , the distance from the principal point of the second condensing means to the light extracting member is H1, the allowable deviation amount of the recording layer of the information storage medium from the focus of the light from the first condensing means is δ0. It is characterized in that the light extraction member is installed so as to satisfy the condition of L/(fo-H), and defocus detection can be performed stably and with high reliability. It is something.

(Example) Hereinafter, an example of the present invention will be described with reference to the drawings. FIG. 5 schematically shows an information recording and reproducing apparatus. In the figure, 2 is an optical disk as an information storage medium, and this optical disk 2 has a pair of disk-shaped transparent plates 4 and 6 arranged between inner and outer spacers 8, A light reflecting layer 12.14 as an information recording layer is formed on the inner surface of each of the transparent plates 4.6 using a steam tube.

A tracking guide 16 (see FIG. 6) is formed helically on each of the light reflecting layers 12 and 14, and information is recorded in the form of pits on this tracking guide 16. . A hole is made in the center of the optical disc 2, and the optical disc 2 is placed on a turntable (not shown).
When the turntable is placed, the center spindle 20 of the turntable is inserted into the hole of the optical disk 2, so that the centers of rotation of the turntable and the optical disk 2 are aligned. A chuck device 22 is further attached to the center spindle 20 of the turntable, and the optical disc 2 is fixed on the turntable by this chuck device 22. The turntable is rotatably supported by a support base (not shown) and driven by a drive motor 2.
4, it is rotated at a constant speed.

Further, 26 is an optical head, which is provided so as to be movable in the radial direction of the optical disk 2 by a linear actuator 28 or a rotary arm, and within this optical head 26 is a laser device 30 that generates a laser beam. It is provided. When writing information onto the optical disk 2, a laser beam whose light intensity is modulated according to the information to be written is generated from the laser device 30, and when reading information from the optical disk 2, a constant light intensity is generated. A laser beam having the following values is generated from the laser device 30.

A laser beam generated by the laser device 30 is diverged by a concave lens 32 , converted into a parallel beam by a convex lens 34 , and directed to a fa light beam splitter 36 . The parallel laser beam reflected by the polarizing beam splitter 36 passes through the quarter-wave plate 38 and enters the objective lens 40, and is focused toward the light reflective layer 14 of the optical disc 2 by the objective lens 4o.

The objective lens 4o is supported by a voice coil 42 so as to be movable in its front axis direction, and when the objective lens 40 is positioned at a predetermined position, the beam waist of the focused laser beam emitted from the objective lens 40 is is light reflective layer 1
4 and a minimum beam spot is formed on the surface of the light reflective layer 14. In this state, the objective lens 40 can be kept in focus and information can be written and read. When writing information, pits are formed in the tracking guide (pre-groove) 16 on the light reflective layer 14 by a laser beam modulated with optical intensity, and when reading information, a constant optical intensity is applied. The laser beam formed on the tracking kite 16 is modulated in light intensity and reflected.

The diverging laser beam reflected from the light reflection layer 14 of the optical disk 2 is converted into a parallel beam by the objective lens 40 when focused, passes through the quarter-wave plate 38 again, and enters the surface optical beam splitter 36. be returned. As the laser beam goes back and forth through the quarter-wave plate 38, the plane of polarization of the laser beam is rotated by 90 degrees L compared to when it is reflected by the polarizing beam splitter 36, and the plane of polarization is rotated by this 90 degrees. The laser beam is polarized by polarizing beam splitter 3
6 and passes through the polarizing beam splitter 36. The laser beam that has passed through the polarizing beam splitter is divided into two systems by a half mirror block 44, one of which is irradiated by a convex lens 46 onto a first photodetector 48 consisting of a detection element 48A148B.

The first signal detected by the first photodetector 48 includes information recorded on the optical disc 2 and is transmitted to the signal processing device 50.
The signal is sent to the digital signal generator, where it is converted into digital data, and output as a dragging signal 50A and a total signal 50B. From the other laser beam separated by the half-mirror pro-I 44, only the component that passes through a region separated from the optical axis 53 is extracted by a light shielding plate (light extracting member) 52, and passes through the projection lens 54. The light is reflected by the rear mirror 56 and enters the second photodetector 58 .

Here, the light shielding plate 52 may be formed of any one of a prism, an aperture slit, a knife edge, and the like. Further, the half mirror block 44, the light shielding plate 52, and the projection lens 54 are in close contact with each other. Second photodetector 5
The signal detected at 8 is processed by a focus signal generator 60, and the focus signal generated by the focus signal generator 60 is applied to a voice coil drive circuit 62. The voice coil drive circuit 62 drives the voice coil 42 according to the focus signal and controls the objective lens 4.
0 in focus.

Note that in order to accurately trace the trunking guide 16 that is not formed on the light reflective layer 14 of the optical disc 2,
The signal from the second photodetector 18 may be processed to actuate the linear actuator 28, and the objective lens 40 may be moved laterally or a galvano mirror (not shown) may be actuated. Also good.

Figure 6 shows a simplified optical system for detecting the time of focus shown in Figure 5.To further explain, the trajectory of the laser beam related to focus detection is shown in Figure 7 (A). (B)(
It can be drawn as shown in c). That is, when the objective lens 40 is in focus, the beam waist is projected onto the light reflective layer 14, and the minimum beam spot, that is, the beam waist subo-soto 64, is projected onto the light reflective layer 14. It is formed. Normally, the laser beam that enters the objective lens 40 from the laser device 30 is a parallel beam of light, so the beam waist is formed on the focal point of the objective lens 40. However, if the laser beam incident on the objective lens 40 from the laser device 30 is slightly diverging or converging, the beam waist is formed near the focal point of the objective lens -1O. In the optical system shown in FIG. 5 [2I, FIG. 6 and FIG. They are arranged on the imaging plane of I-Toro 4. Therefore,
When focused, an image of the beam waist spot 64 is formed at the center of the light receiving surface of the photodetector 58. That is, as shown in FIG. 7(a), the beam waist spot 6
4 is formed on the light reflection layer 14 , and the laser beam reflected by the light reflection layer 14 is converted into a parallel beam by the objective lens 40 and directed toward the light shielding plate 52 . Light shielding plate 5
2, only the light component passing through the area diverged from the optical axis 53 is extracted, focused by the projection lens 54, and minimized on the photodetector 58, and a beam waist spot image is formed thereon. When the zero-order objective lens 40 approaches the light reflection layer 14, the beam waist is such that the laser beam approaches the light reflection layer 14 as shown in FIG.
It is caused by reflection. In other words, the beam waist is
Light reflection occurs between the objective lens 40 and the T lens 14. In such an out-of-focus state, the beam waist usually occurs within the focal length of the objective lens 40, so the beam waist
As is clear from the assumption that the waist functions as a light spot, the laser beam reflected by the light reflecting layer 14 and emitted from the objective lens 40 is converted by the objective lens 40 into a diverging laser beam. Since the laser beam components that have passed through the light shielding plate 52 are similarly divergent, even if these laser beam components are focused by the projection lens 54, they are not focused to the minimum on the light receiving surface of the photodetector 58, and are not detected by the light. The light will be focused towards a point that is much further away than the vessel 58.

Therefore, the laser beam component is projected upward in the figure from the center of the light-receiving surface of the photodetector 58, and a pattern larger than the beam spot image is formed on the light-receiving surface. Further, when the objective lens 110 is separated from the light reflective layer 14 as shown in FIG. 7(c), the laser beam is reflected by the reflective layer 1=1 after forming a beam waist. In such an out-of-focus state, the beam waist is usually outside the focal length of the objective lens 40 and is formed between the objective lens 40 and the reflective layer IL1, so that the beam waist is directed from the objective lens 40 toward the light shielding plate 52. The reflected laser beam will be convergent. Therefore, the laser beam component that has passed through the light shielding plate 52 is further converged by the projection renderer 4, and after forming a convergence point, is projected onto the light receiving surface of the photodetector 58. As a result, a pattern larger than the image of the beam waist spot is formed on the light receiving surface of the photodetector 58 from the center upward and downward in the figure.

The above-described change in the trajectory of the laser, that is, the change in the trajectory of the light beam, can be explained from the perspective of geometrical optics as follows, and the value h3 at which the laser beam component is directed on the photodetector 58 can be determined. The geometrical optical imaging system of the objective lens 40 can be expressed as shown in FIG. Here, fo is the focal length of the objective lens 40, and δ is the moving distance of the objective lens 40, that is, the light reflective layer 1 of the optical disc 2, from the in-focus state to the out-of-focus state. The ray locus shown by a solid line in FIG. 8 is emitted from the beam waist, is on the main surface of the objective lens 40, and is distanced from the optical axis 53. Passing through points spaced apart by L, it shows what is focused. At the time of focus shown in FIG. 7(a), it is clearly δ-0, and at the time of out-of-focus shown in FIG. 7(b), the optical disc 2 is close to the objective lens 4o by a distance δ L.
Since the beam waist is formed by being reflected by the light reflecting layer 14, the beam waist is twice as close to the objective lens 40 as the beam waist. (If close,
δ is O. ) Also, in the out-of-focus state shown in FIG. 7(c), the optical disk 2 is separated from the objective lens 40 by a distance δ, and the laser beam is reflected from the light reflection layer 14 after forming a beam waist. Therefore, it is the same as that a beam waist is formed substantially behind the light reflecting layer 14, and the beam waist is 2δ apart from the objective lens 4.
It will be separated from 0. At the time of focusing, if the beam waist is formed at the focal position of the objective lens 40, when the optical disk 2 moves by δ, the beam waist is formed at the focal position of the objective lens 40, as shown in FIG. The distance between the principal surfaces is (r.

−2δ). If we consider He 1 and Waist as a light point, the angle β in Figure 8 is. and β1 are as follows (1
) and (2).

Also, according to the lens imaging formula, β1=βo+h0/f0 O 2°. 27°, -(2) FIG. 9 shows the trajectory of light rays in the optical system of the projection renderer 4, assuming that the projection lens 54 consists of a pair of combined lenses 541.542.

Here, the lenses 54-1 and 54-2 each have a focal length f
,, f2 is L, a light shielding plate 52 is arranged at a position spaced apart from the main surface of the objective lens 4o by a distance, and a light shielding plate 52 is arranged at a position spaced apart from the main surface of the objective lens 40 by a distance L from the main surface of the lens 5tl-1. is arranged, and furthermore, from the main surface of this lens 54-1, H
It is assumed that the main surfaces of the lenses 54-2 are arranged with a distance of . The light ray locus shown by a solid line in the figure shows the light transmission surface of the light shielding plate 52 that is focused by the objective lens 40 and is spaced apart from the optical axis 53 by y.

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

y=ho−aβ1 Here, if F (δ)=(fo+fo/2δ), then equation (3) is expressed by the following equation.

y-ho (1aF(δ)) −
(4) Therefore, b -... (5) '1-aF(δ) Also, the position h1 from the optical axis 53 where the light ray passes on the main surface of the lens 54-1 is expressed by equation (6). .

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

...(7) Similarly, the optical axis 5 of the light ray passing on the main surface of the lens 54-2
3 is represented by position h2 from above.

h2-hl-HI3...(8) In the optical system shown in FIG. 8, it is assumed that the beam waist is formed at the focal point of the objective lens 40, but if a diverging or converging laser beam When the beam is incident on the objective lens 10, the beam waist is formed at position b and 1δ from the focal point. Therefore, the entire optical system can be regarded as one composite lens and can be set as L, 2δ-2δ'-b.

Next, FIG. 10 shows the relationship between the defocus detection signal Y and the defocus IX, and FIG. 11 shows the relationship between the defocus amount The relationship between the sum Z of light amounts is shown.

As can be seen from this figure, the objective lens 40 and the light reflecting layer 1
If the distance between
If the objective lens 40 is above the center line and is further away than δtf (the allowable deviation amount of the light reflection layer 14 from the focal point of the light emitted from the objective lens 40) in FIG. The same positive signal as in the state where the light reflection N (information forming layer) 14 is too close is output. At this time the 10th
The different characteristics of the 38 classes are shown as curves in Figures and Figure 11. That is, when the end of the light shielding plate 2 (the end face of the knife edge or the end around the aperture or slit) is located on the center of the optical axis of the optical system, it exhibits the characteristic of curve A. In addition, if the end of the light shielding plate 52 is off the center of the optical axis 53 and the light passing through the center of the optical axis is not blocked by the light shielding plate 52 and can pass through as it is, then the curve B and the optical axis 53 In the case where the light passing through the center of the curve is blocked by the light shielding plate 52 and cannot reach the second photodetector 58, the characteristics of the curve C are shown.

Furthermore, except for the vicinity of the in-focus position, the absolute value of the defocus detection signal (that is, in the Y direction) of the curve in FIG. 10 is approximately equal to the graph in FIG. 11. Also the first
In FIG. 1, in the vicinity of the focal point position, a large amount of the beam spot on the second photodetector 58 falls within the leading edge sensing area that exists between the photodetection cells. Therefore, the amount of photocurrent flowing decreases, and the total amount Z of detected light amounts decreases.

Next, as shown in FIG. 12, a laser beam 53 consisting of a light shielding plate (knife edge) 52' is emitted from the rear principal point of the objective lens 40.
The distance to the member from which a part is extracted is determined by the distance of 1 m between X and L, X and δ, and f.

When the focus is out of focus by δ1, the laser beam is focused on the optical axis at 152 degrees above the knife.

In the case of FIG. 12, the knife edge 52'' is placed at the position of the projection lens 54-2 in FIG.
) For the equation, set h2-Ola-0 and L-'-H=x.In addition, recording is performed by changing the state, such as making a hole in the reflective layer 14 of the recording medium. In this case, if defocus occurs and the spot on the reflective layer 111 becomes larger, recording becomes difficult. The spot size aj on the reflective layer 14 at the time of focusing is a, Q = 0.82λ/NA...
(27), (where L and λ are the laser wavelengths): A is the numerical aperture. ). In addition, the intensity distribution at this time is similar to a Gaussian distribution, and the radius of the ring zone where the intensity at the beam waist is 1/'e of the center intensity is ω
. When it is closed, the radius ω (Z
′), that is, the radius ω(Z′) on the reflective layer 14 when the focus is out of focus by Z′, the spot center intensity at this time decreases to . If the lowest central intensity that can be recorded is I nin, now λ = 0.83 μm, NA = 0.6, ll1li
If n = 0.7, = 0.81 μm λ = 0.83 μm * NA = 0.5. ＋l
When m1n = 0.7, the allowable defocus amount is 0.
It is about 5 to 2.0 JJ, m.

According to this, the allowable defocus amount is roughly 2.0 μm. Therefore, δtr needs to be 2.0 μm or more. Therefore, in the case of the optical system shown in FIG. 12, X≦fo
ifo y'2δtf' L2/(fl-H Irigumi L,
δtr needs to be 2.0 μm.

The distance from the front principal point of the objective lens 40 to the focal point in the direction of the information storage medium is large. Instead, F=f.

In the case of an optical system with =b, 2δ1f in equation (a) is replaced by 2δ1f.
,, +b = 2δtf-i-1-fo) enters, and X≦f
o-! -fo /2δtf+ (F-fO') -L2
/(fl-H), where δ and f are 2.0 μm. now(
In F-. >=b<o, since it must be true that x>O, the value of δ1f becomes large.

Only in this case, 2δu= (F−fo) is −! -2,0
It is necessary that the thickness be larger than μm. In other words, the light reflected by the information storage medium M (or layer) 14 is reflected L,
The light that passes through the objective lens 40 has a characteristic of being spectrally divergent in focus, so it will not be condensed at the light shielding plate 52 unless it is greatly out of focus. In this case, objective lens 4
The focus is blurred L until the light passing through 0 becomes parallel, and the defocus detection signal does not invert until it deviates by 2.0 μm, so 2δtf”(F−fO)≧2.0 μm.

In Fig. 12, a knife etch 32'' is used as a member for extracting part of the laser beam 4, but other materials such as an aperture, prism, mirror, photodetector, ground glass, lens, light path shielding material, etc. may be used. However, the present invention is also applicable to the case where they are used.

Furthermore, in FIG. 12, the photodetector 58 is placed at the focal point (imaging point) in the direction of the photodetector 58 at the time of focusing, but the present invention can be similarly applied to an optical system shifted from that position. Ru.

2. Effects of the Invention As described above, the present invention provides outstanding effects such as being able to detect out-of-focus more stably and with high reliability.

[Brief explanation of the drawing]

Figure 1 is an explanatory diagram showing the locus of the laser beam when the first conventional optical system is in focus and out of focus, and Figure 2 is the amount of light detected as the beam spot moves on the optical detector. FIG. 3 is an explanatory diagram showing the trajectory of the laser beam when the second conventional optical system is in focus and out of focus, and FIG. 4 is an explanatory diagram showing the trajectory of the laser beam when the second conventional optical system is in focus. Explanatory diagram showing the locus of the laser beam when it is focused and when it is out of focus, Fig. 5~
FIG. 11 is a diagram for explaining the present invention, FIG. 5 is a block diagram showing an information recording/reproducing device, FIG. 6 is a diagram showing the optical system shown in FIG. 5, and FIG. 7 is a focusing diagram. Figure 8 is a diagram for analyzing the trajectory of the laser beam passing through the objective lens shown in Figure 6. Figure 10 is a diagram showing the relationship between the amount of defocus and the defocus detection signal, and Figure 11 shows the amount of defocus and the amount detected by the defocus detection detector. FIG. 12 shows an example of the present invention, and shows the relationship between the amount of deviation of the recording layer from the focal point of the light emitted from the condensing means and the amount of light emitted from the condensing means. It is a figure for analyzing the relationship with the distance to an extraction member. 40... Focusing means (objective lens), 52... Light extracting member (light shielding plate), 58... Second photodetector. Applicant's Representative Patent Attorney Takehiko Suzue Figure 1 Figure 2 Figure 3 Figure 4 (A) (B) (C) Figure 7 Figure 8 Figure 9

Claims (3)

[Claims] (1) a first condensing means that condenses light onto an information storage medium having a recording layer storing information, and light that is reflected by the recording layer of the information storage medium and passes through the first condensing means; a second condensing means for condensing light; a light detection means for detecting the light condensed by the second condensing means; and between the second condensing means and the light detection means. and a light extraction member for extracting light that is asymmetric with respect to the optical axis, the focal length of the first focusing means being f_0, and the distance from the principal point of the first focusing means to the focusing point. F is the distance from the principal point of the first condensing means to the light extraction member, x is the distance from the principal point of the first condensing means to the principal point of the second condensing means. L is the distance from the principal point of the second condensing means to the light extraction member, H is the allowable distance from the focal point of the light from the first condensing means on the recording layer of the information storage medium. The amount of deviation is δ
When, x≦f_0+f_0/{2δ_t_f+(F−f_0)
}-L^2/(f_0-H) An optical head characterized in that the light extraction member is installed so as to satisfy the following condition. (2) When F−f_0≧0, 2δ_t_f=2.0μ
The optical head according to claim 1, wherein m is defined as m. (3) When F−f_0<0, 2δ_t_f+(F−f
The optical head according to claim 1, wherein _0)=2.0 μm.