JPH0685232B2 - Optical head - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical head for focusing a light beam for writing or reading information on an information recording medium such as an optical disk.

[Technical background of the invention] A reproduction-only information storage medium such as a DAD CD (compact disc) or video disc, an image file / still image file COM (computer output memory), and the like.
Optical recording is performed on an information storage medium in which information is recorded by changing the state of the recording layer by irradiation of light and information is reproduced from the recording layer, and an information erasable medium (hereinafter, simply referred to as an optical disk) from which information can be erased. An information recording / reproducing apparatus capable of writing and reading information has been developed. In these information recording / reproducing apparatuses, it is required that the light beam focused by the objective lens is always focused on the optical disc during either writing or reading. That is, it is required that the minimum beam spot is formed on the optical disc. Therefore, the optical head includes a focus detection device that detects the focus state of the light beam and performs focus control.

The following device has been developed as one of focus detection devices.

As shown in FIGS. 1 (a), (b) and (c), the light beam reflected from the recording layer or the light reflection layer (hereinafter referred to as the light reflection layer) a and passed through the objective lens b is in the middle of the reflection optical path c. , A light extraction means (knife wedge) d for extracting asymmetrically with respect to this optical axis, a lens e, and a photodetector h having two photodetection cells f and g are focused on the photodetector h side at the time of focusing. It is considered that the defocus is detected as the movement of the beam spot i on the photodetector h (the direction of arrow j) on the photodetector h.

[Problems of Background Art] However, in the conventional device, when the photodetector h is arranged at the position of the condensing point on the photodetector h side at the time of focusing, there are the following problems.

i) At the time of focusing, pits as signal information in the recording layer of the information storage medium or the light reflection layer a are focused by the objective lens b on the fine irregularities on the surface of the recording layer of the information recording medium or the light reflection layer a. When the focused light thus passed through is diffracted, a diffraction pattern appears in the spot at the focal point on the photodetector h side, as shown in FIG. Strictly speaking, this diffraction pattern generally does not have a symmetrical shape, and becomes a disturbance noise signal when viewed from the entire defocus detection system. Therefore, due to the influence of this diffraction pattern, the focus may be erroneously detected even when it is in focus.

ii) When information is recorded on or reproduced from an information storage medium, it is usually focused on a recording layer or a light reflecting layer a through a substrate made of a transparent material. Therefore, the objective lens b is designed to collect light through a transparent substrate. By the way, when a large warp or waviness occurs in the information storage medium and the substrate tilts, aberrations such as coma occur. Therefore, an image formation pattern including aberrations such as coma appears on the photodetector h at the time of focusing. That is,
When there is little aberration in the objective lens system, an imaging pattern i appears on the photodetector h, as shown in FIG.
Operates (detects) stably as a defocus detection system. However, when a warp or undulation occurs in the information storage medium, as shown in FIG. 2C, an image formation pattern i including an aberration of the objective lens system such as a coma aberration appears on the photodetector h.
Then, the light detection cells h-1, h-
The light amounts of the light radiated to 2 are unbalanced and are erroneously detected as if defocus occurs.

iii) The spot has a specific size in the vicinity of the converging point due to the influence of light diffraction (light wave nature). Therefore, when the photodetector h is arranged at the position of the focal point on the photodetector h side at the time of focusing, the defocus detection sensitivity in the vicinity of the focusing point becomes dull than the value calculated geometrically. (Details will be described later).

iv) In the case of an optical system in which the spot size at the converging point is very small and the photodetector h is placed there, it is still in focus even if the photodetector h slightly shifts due to temperature change or the like. Nevertheless, it is erroneously detected as if defocus occurs and the focus is blurred.

v) When the detection system lens itself has a large aberration (for example, spherical aberration), the position of the smallest random circle deviates from the Gaussian image plane, and the defocus detection characteristic near the in-focus position deteriorates. It becomes dull.

In consideration of the above problems, and for the purpose of increasing the detection output of the photodetector, the far field for the light reflection layer a is
It is considered to arrange the photodetector at the position where the d pattern is generated. However, at this time, there are the following problems.

i) In many cases, a spiral groove is formed in advance in the information storage medium as a tracking guide for recording / reproducing information in order to increase the amount of information stored. In such a case, when the focused laser light spot crosses the groove, it is affected by the diffraction pattern of the groove on the photodetector as shown in FIG.

The present invention has been made based on the above circumstances, and reduces the effects of lens aberrations and pits by blurring the image formation pattern on the recording layer or the light reflection layer of the information storage medium on the photodetector during focusing. Moreover, it is possible to improve the positional deviation margin of the photodetector and the detection sensitivity to defocus, and the photodetector is not displaced until it is affected by the diffraction pattern generated by the fine irregularities on the recording layer or the light reflecting layer. Accordingly, it is an object of the present invention to provide an optical head capable of detecting defocusing more stably and reliably.

[Summary of the Invention] The present invention irradiates an information storage medium having a light source unit that emits light in order to achieve the above object, and a recording layer in which information generated by focusing the light emitted from the light source unit is stored. First to do
Light collecting means, second light collecting means for collecting light from the recording layer of the information storage medium, and light detecting means for detecting light collected by the second light collecting means, An optical head having means for moving a beam spot of light on the light detecting means in response to a deviation from a position where the first light converging means is in focus with respect to a recording layer of an information storage medium, When the first light condensing unit is at the in-focus position with respect to the recording layer of the information storage medium, from the light condensing point of the light formed by the second light condensing unit, (1 / π) (f / a) ²
λ (where f: focal length of the second light converging means, a: front principal point of the second light converging means at the time of focusing (principal point on the side close to the first light condensing means) Radius of beam spot of light, λ: wavelength of light generated from the light source unit), and (3.9d ₀ ) ² / λ (where d ₀ : central maximum intensity in the light intensity distribution of light is 1) The light detecting means is arranged at a position closer than 1 / e ² ).

Embodiments of the Invention Hereinafter, embodiments of the optical head of the present invention will be described with reference to FIGS. 4 to 11.

The change of the trajectory of the laser in the focus detection device of the optical head, that is, the change of the ray trajectory is geometrically explained as shown in FIGS. 4 and 5, and the laser beam component is detected by the photodetector.
The value h ₃ that is deflected on 58 can be determined. The geometrical optical imaging system of the objective lens 40 can be represented as shown in FIG. Here, f ₀ is the focal length of the objective lens 40, and δ is the moving distance of the objective lens 40, that is, the light reflection layer 14 of the optical disc 2 from the time of focusing to the time of non-focusing. In FIG. 3, the ray trace indicated by the solid line indicates that which is emitted from the beam waist, passes through a point on the main surface of the objective lens 40, which is separated from the optical axis 53 by a distance h ₀ , and is focused. When the focus shown in FIG. 1 (a) is apparent, δ = 0, and when the non-focus shown in FIG. 1 (b), the optical disc 2 is close to the objective lens 40 by the distance δ, Since the beam waist is formed by being reflected by the light reflecting layer 14, the beam waist is
It will be closer to the objective lens 40 by a factor of 2. (In case of close proximity, δ> 0.) In addition, when the optical system 2 is out of focus as shown in FIG.
Is separated from the objective lens 40 to form a beam waist, and then the laser beam is reflected from the light reflecting layer 14, which is substantially the same as forming a beam waist behind the light reflecting layer 14. Beam waist is 2δ
It will be separated from the objective lens only. When in focus,
If the beam waist is formed at the focal position of the objective lens 40, when the optical disc 2 moves by δ,
Beam waist and objective lens as shown in FIG.
The distance between the 40 principal surfaces is represented by (f ₀ + 2δ). If we regard the beam waist as a light spot, the angle β in Fig. 4
₀ and β ₁ are represented by the following equations (1) and (2).

Also, from the imaging formula of the lens, Therefore FIG. 5 shows a ray trace in the optical system of the projection lens 54, in which the projection lens 54 is a pair of combination lenses 54-1 and 54-2.
It is treated as consisting of.

Here, the lenses 54-1 and 54-2 have focal lengths f ₁ and f ₂ , respectively, and the light shielding plate 52 is arranged at a position separated from the main surface of the objective lens 40 by a, and The main surface of the lens 54-1 is arranged at a position separated from the surface by L, and the main surface of the lens 54-2 is separated from the main surface of the lens 54-1 by H and the main surface of the lens 54-2 is separated by l again. It is assumed that the light receiving surface of detector 58 is aligned. A ray trace shown by a solid line in the drawing shows a light beam which is converged by the objective lens 40 and which is a light transmitting surface of the light shielding plate 52 and is separated from the optical axis 53 by y.

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

Here, if F (δ) = (f ₀ + f ₀ / 2δ) ⁻¹ , then (3)
The formula is represented by the following formula.

y = h ₀ (1-aF (δ)) (4) Further, the position h ₁ from the optical axis 53 where the light ray passes on the main surface of the lens 54-1 is expressed by the equation (6).

(2) be determined likewise the angle beta ₂ and wherein the angle beta ₂ is
It is expressed by equation (7).

Similarly, the position h _{2 of the} ray passing through the principal surface of the lens 54-2 from the optical axis 53 and the position h ₂ of the incident angle β _{3 of the} ray incident on the light receiving surface of the photodetector 58 from the optical axis 53. ₃ That is, the deviation amounts are expressed by the equations (8) to (10), respectively.

As described above, the optical system shown in FIGS. 4 and 5 is in focus, that is, at δ = 0, the light beam on the detector 58 is h ₃ =
Since it is focused on 0, under these conditions,
F (0) = 0, and the equation (10) is represented by the following equation.

Further, since only the light passing through the light beam outside the optical axis 53 is taken out by the light shielding plate 52, y ≠ 0. Therefore, By simplifying equation (10) with this equation, equation (13) or (1
Equation 4) is obtained.

When δ is sufficiently small (δ << f ₀ ² ), (a−f ₀ )
《F ₀ ² / 2δ), Next, at the time of focusing (δ = 0), the lateral magnification m of the beam waist image formed on the light receiving surface of the photodetector 58 with respect to the beam waist on the light reflection layer 14 of the optical disc 2 is as follows ( It is expressed by equation 16).

m = −β ₀ / β ₃ (inverted image) where β _{0 at} δ = ₀ is Therefore, By eliminating f ₂ from equation (16) in equation (12), If we find the solution for f ₁ from this equation, Since it is considered that f ₁ = H is not set in an actual optical system, Considering the case where an erect image is formed in the same way, (m
= Β ₀ / β ₃ ), f ₁ is represented by the following equation.

Therefore, When f ₂ is calculated from the equation (12), By substituting equation (20) into equation (13) and expressing h ₃ by the lateral magnification m, equations (21) and (22) are obtained.

However, if the projection lens 54 is a single lens in the optical system of (a−f ₀ ) << f ₀ ² / 2δ, then f ₂ = ∞, so f ₁ = 1 and m = f ₁ / f ₀ , Further, in the optical system shown in FIG. 4, it is assumed that the beam waist is formed at the focal point of the objective lens 40, but when a divergent or converging laser beam enters the objective lens 40, The beam waist is formed by deviating from the focus by b. Therefore, if the entire optical system is regarded as one compound lens and 2δ = 2δ ′ + b is set and the same calculation is performed,
The deviation amount h ₃ is obtained.

Here, when a = 0, If f ₀ + b >> 2δ, Becomes

The above equations are all geometrically-optically calculated for the trajectory of the light beam, but in terms of wave optics, they always have a certain size on the light receiving surface of the photodetector 58 due to the diffraction phenomenon. This magnitude is calculated as follows.

Assuming that a parallel laser with a uniform numerical aperture is incident on a lens with a numerical aperture (NA) of NA and ideally no aberration, the spot size a1 at the point after passing through the lens is , Where the wavelength of the laser is λ, it is generally given by the following equation.

Here, al represents the diameter of a ring having an intensity of 1 / e ^{2 when} the intensity at the center of the spot is the unit, that is, 1 as shown in FIG. If this beam spot corresponds to the beam waist formed by the objective lens 40, the size of the image of the beam waist formed on the light receiving surface of the photodetector 58 during focusing is ad
Is represented by the lateral magnification m of the lens system 40, 54.

Next, the spot when the light shielding plate 52 is arranged in the optical path is changed as follows. First, when the light shielding plate 52 is not arranged in the optical path as shown in FIG. 7 (a), a parallel light flux having a uniform intensity distribution, that is, rec (x / a) is incident on the projection lens 54. If it is incident, Fourier transform is performed on the light receiving surface of the photodetector 54.
A pattern having an amplitude of a sinc (a ξ) is formed.
That is, F {rect (x / a)} = a sinc (aξ) intensity I ₀ is It is represented by.

In such a state, it is assumed that the light shielding plate 52 is positioned in the optical path and half of the light flux is shielded as shown in FIG. At this time, the Fourier distribution is similarly applied to the light receiving surface of the photodetector 58 to obtain the amplitude distribution.

The intensity distribution Ik is It is represented by.

Comparing equations (29) and (30), the parallel light flux is shielded.
If you cut it in half at 52, you can see that the center position of the spot does not change and the spot diameter doubles. From these results, it is estimated that in the one-dimensional model, the spot diameter spreads to 1 / R times, where R is the proportion of light that is transmitted without being blocked by the light shielding plate 52. Therefore, in the one-dimensional model, if the light shielding plate 52 is inserted in the optical path and the light is transmitted by the ratio R in the optical path, the photodetector is used.
On the light receiving surface of 54, a spread spot is formed having a diameter indicated by ak below in a direction parallel to the insertion direction of the light shielding plate 52.

In these optical systems, from the objective lens 40 to the projection lens
The light flux directed to 54 does not actually have a uniform light intensity distribution as shown in FIGS. 7 (a) and 7 (b), but has a Gaussian distribution. Therefore, the light receiving surface of the photodetector 58 is irradiated with the light flux having the distribution shown in FIG. In FIG. 8, the trajectory of the light flux shown by the broken line I shows the case where the light shielding plate 52 is not inserted in the optical path at all, and the trajectory of the light flux shown by the solid line II shows the case where the light shielding plate 52 is inserted in the optical path. Shows. In the in-focus state, it can be considered that the light receiving surface of the photodetector 58 is located at the position X ₀ , and when defocus occurs, the light receiving surface of the photodetector 58 can be considered. Can be considered equivalent to being located at position X ₁ or X ₂ .

The present invention is slightly different from the position of the focal point formed on the photodetector 58 side at the time of focusing when the focal point focused by the objective lens 40 and the position of the light reflection layer (recording layer) 14 of the information storage medium match. The feature is that the photodetector is placed at a position deviated from.

The proper shift amount when the photodetector 58 is shifted will be considered below.

For "i) the pit pattern appears on the photodetector 58" and "ii) the aberration pattern of the objective lens 40 such as coma aberration due to the tilt of the substrate appears on the photodetector 58". May shift the photodetector 58 and blur the image formation pattern on the photodetector 58. Depth of focus Z of imaging system
Is that when circular uniform light is incident on the detection system lens, , And when Gaussian light is incident on the detection system lens, It is known to be given as. However, here'f;
The focal length when the detection system lens or a group is considered to be a synthetic lens. a; Radius of incident light at the front principal point (principal point on the side close to the objective lens) of the detection system lens (or lens group) at the time of focusing. λ; wavelength of light used ”. It seems that it is sufficient to shift the depth by more than the depth of focus Z. The intensity distribution of the light incident on the actual detection system lens differs depending on the entire optical system, It seems that it is necessary to shift the above.

Next, of the above-mentioned problems, “iii) reduction of defocus detection sensitivity due to light diffraction” will be described in detail. When a slight defocus occurs from the in-focus position, the radius on the photodetector is given by geometrical optics as given by equation (26). The semi-circle of the circle is originally detected, and the defocused portion is detected by detecting the part that has become the semi-circle by one of the photodetector cells. Here, y is the radius of the opening of the objective lens. However, due to the effect of light diffraction even when focused That is, the spot size spreads, and the detection sensitivity for defocusing becomes poor near the in-focus position. The physical meaning of this phenomenon is explained in FIG. As shown in FIG. 8, it can be seen that the detection sensitivity of the photodetector 58 is actually lower than the value expected from geometrical optics when the focus is slightly out of focus. That is, in the vicinity of the in-focus state, it is equivalent to that the light receiving surface of the photodetector 58 is displaced between the positions X ₁ and X ₂ when defocus occurs, but in this range, it is slightly Only the projection position of the light flux II is changed, and the deviation value is extremely small compared to outside the range of the positions X ₁ to X ₂ . Further, in this range, the spread of the light flux I is also smaller than that outside the range. In addition, the pattern formed on the light receiving surface of the photodetector 58 is elongated in the direction in which the light shielding plate 52 is inserted, as shown in the locus of the light beams I and II shown in FIG. The displacement is caused in accordance with the shift δ and spreads in the direction perpendicular to the insertion direction.

Now, let us consider how much the photodetector 58 should be displaced from the focal point to show purely geometrical optical behavior without being disturbed by wave nature. Normally, the intensity distribution of light at the focal point differs depending on the optical system used, but consider an optical system with a Gaussian distribution. Assuming that the beam spot radius at the condensing point (beam waist) is W ₀ and the used wavelength is λ, the beam spot radius W (Z) at a position apart from the condensing point (beam waist) by Z is Given in. Where Z is large to some extent Can be approximated by The first term of this equation represents the geometrical optical behavior, and the second term represents the spread of the spot due to the wave nature of light. Therefore, when the value of the first term of this equation is about four times or more of the value of the second term, it is considered that the geometrical optical behavior is exhibited. Therefore As for the condition of Z ² 2 (πW ₀ ² / λ) ² Is obtained.

Examine and see the magnitude relationship between and. When parallel light of uniform distribution is incident on the detection system lens at the time of focusing, the diameter d ₀ of the spot at the focal point is when the convergence angle is 2θ. Given in. When θ is sufficiently small, θ ≈ a / f Becomes

Next, with reference to FIG. 9, an information recording / reproducing apparatus to which the optical head of the present invention is applied will be schematically described. The optical disc 2 includes a pair of disk-shaped transparent plates 4 and 6 and inner and outer spacers 8 and 1.
Formed by laminating through 0, its transparent plate 4,6
A recording layer or a light reflecting layer (hereinafter referred to as a light reflecting layer) 12 and 14 is formed on each inner surface of each by vapor deposition. A tracking guide 16 is helically formed on each of the light reflecting layers 12 and 14, and information is recorded on the tracking guide 16 in the form of pits. A hole is bored in the center of the optical disc 2 so that when the optical disc 2 is placed on the turntable 18, the center of the turntable 18
The spindle 20 is inserted into the hole of the optical disc 2 so that the turntable 18 and the optical disc 2 are rotated at the same center of rotation. A chuck device 22 is further mounted on the center / sbindle 20 of the turntable 18, and the optical disk 2 is fixed on the turntable 18 by the chuck device 22. The turntable 18 is rotatably supported by a support (not shown), and is rotated at a constant speed by a drive motor 24.

An optical head 26 is provided so as to be movable in the radial direction of the optical disk 2 by a linear actuator 28 or a rotating arm, and a laser device 30 for generating a laser beam is provided in the optical head 26. When writing information on the optical disc 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 disc 2,
A laser beam having a constant light intensity is generated from the laser device 30. The laser beam generated from the laser device 30 is diverged by the concave lens 32, converted into a parallel light flux by the convex lens 34, and directed to the polarization beam splitter 36. The parallel laser beam reflected by the polarization beam splitter 36 passes through the quarter-wave plate 38 and is incident on the objective lens 40, which is focused by the objective lens 40 toward the light reflection layer 14 of the optical disc 2. The objective lens 40 is movably supported by the voice coil 42 in the optical 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. Are projected onto the surface of the light reflecting layer 14 and a minimum beam spot is formed on the surface of the light reflecting layer 14. In this state, the objective lens 40 is kept in focus, and writing and reading of information becomes possible.
When writing information, pits are formed in the tracking guide 16 on the light reflection layer 14 by the light intensity-modulated laser beam, and when reading information, the laser beam having a constant light intensity is The light intensity is modulated by the bits formed in the guide 16 and reflected.

The divergent laser beam reflected from the light reflection layer 14 of the optical disc 2 is converted into a parallel light flux by the objective lens 40 at the time of focusing, passes through the 1/4 wavelength plate 38 again, and is returned to the polarization beam splitter 36. Be done. By reciprocating the laser beam through the quarter-wave plate 38, the laser beam has a polarization plane of 90 degrees as compared with when it is reflected by the polarization beam splitter 36.
The laser beam which is rotated by 90 degrees and whose plane of polarization is rotated by 90 degrees is not reflected by the polarization beam splitter 36, but passes through this polarization beam splitter 36. The laser beam that has passed through the polarization beam splitter is split into two systems by the half mirror 44, and one of them is irradiated onto the first photodetector 48 by the convex lens 46. The first signal detected by the first photodetector 48, including the information recorded on the optical disc 2, is sent to the signal processing device and converted into digital data. In the other laser beam divided by the half mirror 44, only a component passing through a region separated from the optical axis 53 is taken out by a light shielding plate 52 serving as a light extracting means, and after passing through a projection lens 54, a mirror 56. The reflected light is incident on the second photodetector 58. Here, the light shield plate 52 is a prism, aperture,
It may be constituted by either a slit or a knife edge. The signal detected by the second photodetector 58 is processed by the focus signal generator 60, and the focus signal generated by this focus signal generator 60 is given to the voice coil drive circuit 62. The voice coil drive circuit 62 is
The voice coil 42 is driven according to the focus signal to keep the objective lens 40 in focus. When the tracking guide 16 formed on the light reflection layer 14 of the optical disc 2 is accurately traced, the second photodetector is used.
The signal from 48 may be processed to actuate the linear actuator 28, the objective lens 40 may be moved laterally, or a galvano mirror (not shown) may be actuated.

The optical system for detecting the in-focus time shown in FIG. 9 is shown in a simplified manner as shown in FIG. 10, and the locus of the laser beam for the in-focus detection is shown in FIGS. It can be drawn as shown in (c). When the objective lens 40 is in focus,
A beam waist 64 is projected onto the light-reflecting layer 14 and a minimum beam spot, or beam waist spot, is formed on the light-reflecting layer 14. Normally, the laser incident on the objective lens 40 from the laser device 30 is a parallel light flux,
The beam waist is formed on the focal point of the objective lens 40. However, when the laser 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 40. In the optical system shown in FIG. 9, FIG. 10 and FIG. 11 (A) to FIG. 11 (C), the light receiving surface of the photodetector 58 is the beam waist spot 65 of the beam waist spot 65 in the focused state. They are arranged on the image plane. Therefore, when focusing
A beam waist spot 65 is formed at the center of the light receiving surface of the photodetector 58. That is, as shown in FIG. 11 (a), a beam waist spot is formed on the light reflection layer 14,
The laser beam reflected by the light reflection layer 14 is converted into a parallel light flux by the objective lens 40 and is directed to the light shielding plate 52. Only the light component passing through the area separated from the optical axis 53 is taken out by the light shielding plate 52, focused by the projection lens 54, narrowed down to the minimum on the photodetector 58, and the beam waist
The spot 65 is formed on it. Next, when the objective lens 40 approaches the light reflection layer 14, the beam waist 64
Occurs when the laser beam is reflected by the light reflecting layer 14 as shown in FIG. That is, beam waist 64
Occurs between the objective lens 40 and the light reflection layer 14. Since the beam waist 64 normally occurs within the focal length of the objective lens 40 in such an out-of-focus state, it is clear that it is assumed that the beam waist 64 functions as a light spot. The laser beam reflected by the objective lens 40 and emitted from the objective lens 40 is converted into a divergent laser beam by the objective lens 40. Since the laser beam component that has passed through the shading plate 52 is also divergent, this laser
Even if the beam component is focused by the projection lens 54, it is not focused to the minimum on the light receiving surface of the photodetector 58, but is focused toward a point farther than the photodetector 58. Therefore, the photodetector
The laser beam component is projected upward from the center of the light receiving surface of 58, and a beam spot with a pattern larger than the beam waist spot 65 is projected on the light receiving surface.
66 is formed. Further, when the objective lens 40 is separated from the light reflecting layer 14 as shown in FIG. 11C, the laser is reflected by the reflecting layer 14 after forming the beam waist 64. In such an out-of-focus state, the normal beam waist 64 is formed outside the focal length of the objective lens 40 and between the objective lens 40 and the reflective layer 14, and therefore the objective lens 40
The reflected laser beam from the light shield plate 52 to the light shield plate 52 has a converging property. Therefore, the laser that has passed through the shading plate 52
The beam component is further converged by the projection lens 54, forms a convergence point, and is then projected onto the light receiving surface of the photodetector 58.
As a result, a beam spot with a pattern larger than the beam waist spot 65 is formed on the light receiving surface of the photodetector 58.
67 is formed from the center downward in the figure. In addition, the photodetector 58 'shown in FIGS. 11 (a) to 11 (c) is in a state closer to the condensing point, and the photodetector 58 "is in a state farther from the converging point. The pattern of the laser beam on the light receiving surface of the detector 58 'is also shown.

The gist of the present invention is to dispose the photodetector so as to be displaced from the image forming point at the time of focusing, and it does not matter which side it is originally displaced. However, it is mechanically somewhat compact if it is moved toward the detection system lens.

The content of the present invention is similar to an optical system capable of detecting a defocus with one photodetector by shifting the photodetector largely from the focal point at the time of focusing and simultaneously detecting a track shift. appear. By the way, with respect to the track deviation detection in this method, a track deviation is detected by electrically observing a diffraction pattern of light reflected from a light reflection layer (recording layer) of an information storage medium having fine irregularities on the surface. The so-called "Push-Pull method" is used as a means. However, in the case of the Push-Pull method, when the photodetector is arranged at the position where the far field pattern is generated with respect to the light reflecting layer having minute irregularities on the surface, a large track deviation detection signal is obtained, and in contrast to this, It is known that when a photodetector is arranged at a position where an image pattern is generated, almost no track deviation detection signal can be obtained.
Therefore, in this optical system, it should be a photodetector.
It is desirable to place it near where the far field pattern can be obtained. However, as described above, the phenomenon that the disturbance of the pattern on the far field surface, which occurs when the laser spot crosses the tracking guide having the continuous uneven shape, enters the defocus detection as a disturbance and adversely affects this optical It also appears in the system. Therefore, in the present invention, it is necessary not to place the photodetector on the far field surface. It has been confirmed from many experiments that an error signal that has a great influence on defocus detection is not generated even when a tracking guide having an uneven shape is traversed on the image plane of the light reflection layer of the information storage medium at the time of focusing. It is Fra field to the Fraunhoffer
At least Fresnel if it corresponds to the area
The photodetector must be placed closer to the image formation point (focusing point on the photodetector side at the time of focusing) than the boundary point between the region and the Fraunhoffer region. If the spot size d ₀ at the condensing point of the photodetector is the width (diameter) at the place where it becomes 1 / e ² when the central maximum intensity is 1 in the light intensity distribution in FIG. The beam spot has a spread of about 3.9d ₀ (the target of 3.9 times is the range up to the dark stripe outside the secondary maximum of the Airy pattern). Therefore, Fraunh for patterns with such a spread
The boundary between the offer area and the Fresnel area is approximately (3.9d ₀ ) ² / λ
Therefore, at the time of focusing, it can be said that the distance from the condensing point in the photodetector direction to the photodetector 58 should be at least not less than (3.9d ₀ ) ² / λ. Further, as described above, d ₀ theoretically corresponds to ad in Eq. (28) and ak in Eq. (31).

Then, i) The image formation pattern of the signal information pits in the recording layer or the light reflection layer 14 of the information storage medium does not appear clearly on the photodetector and becomes a disturbance noise signal.

ii) Even if the information storage medium is warped and the substrate is tilted, defocusing due to the aberration of the objective lens 40 is unlikely to occur.

iii) The detection sensitivity to defocus is approximately equal to that obtained by geometrical optics without blurring near the in-focus position.

iv) Defocus is relatively unlikely to occur due to displacement of the photodetector.

v) A lens having a relatively large aberration can be used as the detection system lens, and even if such a lens is used, the optical characteristics are not significantly deteriorated.

In addition, FIG. 12 shows, as another embodiment, the locus of the laser beam at the time of focusing and non-focusing of the optical system using the aperture as the light shielding plate 52.

[Effects of the Invention] As described above, according to the present invention, by defocusing the image formation pattern on the recording layer or the light reflection layer of the information storage medium during light detection at the time of focusing, the influence of lens aberration and pits can be reduced. It is possible to improve the detection margin for the position deviation of the photodetector and defocus, and to adjust the photodetector until it is affected by the diffraction pattern generated by the fine irregularities on the recording layer or the light reflection layer. By not shifting, defocus detection can be performed more stably and reliably.

[Brief description of drawings]

FIGS. 1 (a), (b), and (c) are explanatory views showing the basic principle of detecting defocus by moving the beam spot.
Figures (a), (b), and (c) show the state of the beam spot on the photodetector, FIG. 3 shows the state of the beam spot on the photodetector, and FIG. 4 shows the objective lens. FIG. 5 is a diagram for analyzing the trajectory of a ray passing therethrough, FIG. 5 is a diagram for analyzing the trajectory of a ray passing through the projection lens shown in FIG. 4, and FIG. 6 is a beam waist formed by the objective lens. 7 (a) and 7 (b) are views for explaining the difference in light intensity distribution on the light receiving surface of the photodetector, and FIG. 7 (a) shows FIG. 7 (b) shows the case where the light shielding plate is not inserted in the optical path, and FIG. 8 shows the case where the light shielding plate is inserted in the optical path, and FIG. 8 shows the laser when the light shielding plate is not inserted in the optical path and when the light shielding plate is inserted. FIG. 9 is a diagram showing a beam trajectory, and FIG. 9 is an information record provided with an optical head according to an embodiment of the present invention. Block diagram of a reproducing apparatus, FIG. 10 shows the optical system shown in FIG. 9, FIG. 11 (a) to
(C) is an explanatory view showing the trajectory of the laser beam during focusing and non-focusing, and FIGS. 12 (A), (B) and (C) are during focusing of an optical system using an aperture as a light shielding plate. It is explanatory drawing which shows the locus | trajectory of a laser beam at the time of non-focusing. 2 ... Information storage medium, 12, 14 ... Recording layer or light reflection layer, 40 ... Objective lens, 52 ... Light extraction means, 58 ... Photodetector.

Claims (2)

[Claims] 1. A first light condensing means for irradiating an information storage medium having a light source section for generating light, and converging the light generated from the light source section for recording information therein. Second condensing means for condensing light from the recording layer of the storage medium,
Corresponding to the deviation from the position where the light detecting means for detecting the light condensed by the second condensing means and the first condensing means with respect to the recording layer of the information storage medium are in focus. An optical head having means for moving a beam spot of light on the light detecting means, the first light collecting means is located at a focus position with respect to a recording layer of the information storage medium, and (1 / π) (f / a) ² λ from the light condensing point formed by the second condensing means (where f: the focal length of the second condensing means, a: the focal point at the time of focusing) Radius of the beam spot of light at the principal point on the front side of the second condensing means (principal point on the side closer to the first condensing means, λ: wavelength of light generated from the light source unit) At (3.9d ₀ ) ² / λ
(However, the optical detecting means is arranged at a position closer than d ₀ : diameter at which 1 / e ² is obtained when the central maximum intensity is 1 in the light quantity distribution of light) . 2. The light detecting means detects the light formed by the second light collecting means when the first light collecting means is in a focused position with respect to the recording layer of the information storage medium. From the focus point (However, W ₀ : radius of beam spot at converging point, λ:
The optical head according to claim 1, wherein the optical head is arranged at a position separated by at least a wavelength of light generated from the light source unit).