JPH0630160B2 - Optical head - Google Patents

Description

TECHNICAL FIELD OF THE INVENTION The present invention relates to an information storage medium such as a CD (compact disc) for DAD, a video disc, an image file, a still image file, and a COM (computer output memory). The present invention relates to an optical head used in a reproducing or recording / reproducing apparatus or the like capable of reading at least information by irradiating focused light on the optical head.

(Technical Background of the Invention and Problems Thereof) In the above-mentioned optical head, the beam waist of the focused laser light is formed on the information forming layer (recording layer or light reflecting layer) of the information storage medium such as an optical disc or a video disc. It is required that (the most narrowed portion) be formed. For this reason, usually, the defocus of the laser light on the information forming layer is detected, and the objective lens is moved by this detection signal to adjust the focus.

By the way, recently, as a method for detecting the defocusing of the laser beam on the information forming layer, FIG. 1 (a), (b), (c)
Or, as shown in FIGS. 2 (a), (b), and (c),
A light extraction member for extracting a laser beam asymmetrically with respect to this optical axis, for example, a knife edge d, in the middle of a reflection optical path c of a light beam reflected from the information forming layer a and passing through the objective lens b.
Alternatively, a lens f, such as an aperture e, and a photodetector i having two photodetecting cells g and h are provided, and defocusing is detected as the movement (arrow direction) of the beam spot i on the photodetector i. The proposed method is proposed. That is, in FIG. 1A or FIG. 2A, when the laser light c is focused on the information forming layer a, the reflected light passing through the objective lens b becomes parallel to the optical axis. The light (beam spot) extracted by the knife edge d or the aperture e is located at the boundary between the first photodetecting cell g and the second photodetecting cell h.

However, as shown in FIG. 1 (b) or FIG. 2 (b), when the information forming layer a and the objective lens b are too close to each other to be in focus, the reflected light passing through the objective lens b is light. The light (beam spot) that spreads outwardly gradually away from the axis and passes through the knife edge d or the aperture e impinges on the first detection cell s so as to be positioned above. On the contrary, as shown in FIG. 1C or FIG. 2C, when the information forming layer a and the objective lens b are too far apart from each other to be in focus, they pass through the objective lens b. The reflected light is focused inward so as to gradually approach the optical axis, and the light (beam spot) passing through the knife edge d or the aperture e hits the second detection cell h located below. Become.

The above optical system has the following problems because the photodetector i is arranged at the position of the condensing point on the photodetector i side during focusing.

I) When the focused light focused by the objective lens b passes over the pit as signal information in the information forming layer a of the information storage medium at the time of focusing, the pit of the pit of the pit of the photodetector i side is passed. An imaging pattern appears. The image formation pattern of this pit becomes a disturbance noise signal when viewed from the entire defocus detection system.

II) When information is recorded on or reproduced from the information forming layer a of the information storage medium, it is usually focused on the light reflecting layer through a substrate made of a transparent material. Therefore, the objective lens b is designed to collect light through a transparent substrate. However, when the substrate tilts due to a large warp or waviness generated in the information storage medium a, aberrations such as coma occur. Therefore, at the time of focusing, an imaging pattern including coma appears on the photodetector i. Then, even when the light is focused, the light amounts of the light irradiated to the light detection cells g and h are unbalanced and are erroneously detected as if the defocus occurs.

III) The spot has a specific size in the vicinity of the condensing point due to the influence of light diffraction (light wave nature). Therefore, when the photodetector is arranged at the position of the focal point on the photodetector side at the time of focusing, the defocus detection sensitivity in the vicinity of the focusing point becomes smaller than the value calculated geometrically.

IV) Since the spot size at the condensing point is very small, in the case of an optical system in which a photodetector is placed, even if the photodetector i slightly shifts due to temperature change, etc. Despite the existence, it is falsely 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, so the defocus detection characteristic near the in-focus position deteriorates, and the detection impression It becomes dull.

(Problems to be Solved by the Invention) In recent years, in order to solve the above problems I) to V),
An optical head in which the photodetector i is arranged at a position slightly shifted from the focal position of the objective lens b is proposed as shown in FIGS.

According to FIG. 3, the optical axis of the detection optical system is passed through the gap region k of the photodetector i. That is, the intersection of the optical axis of the detection optical system and the photodetector i is located within the gap region k of the photodetector i having low detection sensitivity. Here, at the time of focusing, the state of FIG. 3 (a) is provided, and the beam spot j on the photodetector i is formed as shown in FIG. 4 (a).

As is clear from FIG. 3B, when the storage medium a and the objective lens b are defocused in the direction of approaching each other, the area of the beam spot j on the photodetector i is equal to that of the storage medium a and the objective lens b. It increases as the distance to b decreases.

As is apparent from FIG. 3C, when the storage medium a and the objective lens b are defocused in the direction away from each other, the area of the beam spot j on the photodetector i is equal to that of the storage medium a and the objective. The distance is gradually reduced as the distance from the lens b increases, and the light is condensed at a certain point at a point having substantially no area.

In the optical head described above, a defocus detection signal that increases substantially linearly is obtained in the state of FIG. 3 (b), but in the state of FIG. 3 (c), the output from the photodetector i is substantially Sometimes becomes “0”. Therefore, when the distance between the storage medium a and the objective lens b is increased to an undesired level, there is a problem that focus control regarding the objective lens b cannot be performed.

(Object of the Invention) An object of the present invention is to provide an optical head that can reliably provide a defocus detection signal regardless of the state of laser light guided to a photodetector.

(Means for Solving the Problems) The present invention has been made based on the above-mentioned problems, and collects the irradiation light irradiated onto the information storage medium, and the condensed irradiation light stores the information. Condensing means for re-introducing the reflected light reflected on the medium, and image formation on the information storage medium at the time of focusing when the converging point of the irradiation light condensed by the condensing means coincides with the information storage medium A detection unit arranged at a position displaced from the position, having a plurality of detection regions for detecting the reflected light condensed by the condensing unit, and at least one detection region being located on the optical axis of the reflected light; Light extraction means for moving the beam spot of the reflected light focused on the detection means in response to the displacement of the information storage medium from the focus point of the irradiation light focused by the light focusing means And the above photodetector Provides an optical head characterized in that at least one detection region is arranged on the optical axis of the reflected light.

(Operation) According to the optical head of the present invention, the optical axis defined between the objective lens and the projection lens is passed through one of the detection regions of the photodetector having the first and second detection regions. It Further, the width of the gap defined between the first and second detection areas is defined to be about the same as the minimum circle of confusion of the laser light focused through the objective lens and the projection lens. . Further, the photodetector is arranged on the optical axis at a position different from the focal positions of the objective lens and the projection lens.

Therefore, when the objective lens is in focus, beam spots having substantially equal areas are formed in the first and second detection regions of the photodetector. On the other hand, when the objective lens approaches the storage medium, the area of the beam spot formed in the detection region not including the optical axis is increased with respect to the two detection regions. On the other hand, when the objective lens moves away from the storage medium, the area of the beam spot formed in the detection region not including the optical axis is gradually reduced, and the beam spot remains only in the detection region including the optical axis.

(Embodiment of the Invention) An embodiment of the present invention will be described below with reference to the drawings. In FIG. 5, the laser beam generated from the laser device 1 is converted into parallel light by the collimator lens 2 and guided to the polarization beam splitter 3. The parallel laser beam reflected by the polarization beam splitter 3 is
The light passes through the quarter-wave plate 4 and enters the objective lens 5, and is focused by the objective lens 5 toward the information forming layer (information storing layer or light reflecting layer) 7 of the information storing layer 6. The objective lens 5 is supported by a voice coil 8 so as to be movable in the optical axis direction. When the objective lens 5 is positioned at a predetermined position, the focused laser beam waist emitted from the objective lens 5 is transferred to the information forming layer 7. The minimum beam spot is projected on the surface and is formed on the surface of the information forming layer 7. In this state, the objective lens 5 can be kept in focus and information can be written and read. When writing information, pits are formed in the tracking guide (pre-group) 9 on the information forming layer 7 by the laser beam whose light intensity is modulated, and when reading information, the laser beam having a constant light intensity. Are modulated by the pits formed in the tracking guide 9 and reflected.

The divergent laser beam reflected from the information forming layer 7 of the information storage medium 6 is converted into a parallel light flux by the objective lens 5 at the time of focusing, passes through the 1/4 wavelength plate 4 again, and then enters the polarization beam splitter 3. Will be returned. By reciprocating the 1/4 wavelength plate 4 of the laser beam, the plane of polarization is rotated by 90 degrees as compared with the case of being reflected by the polarization beam sputter 3.
From this, the laser beam whose plane of polarization is rotated is not reflected by the polarization beam splitter 3 and passes through this polarization beam splitter 3. The laser beam that has passed through the polarization beam splitter 3 is divided into two systems by the half mirror block 10, one of which (track deviation detection system) is divided into a first photodetector 12 by a convex lens 11.
Is irradiated. The first signal detected by the first photodetector 12 includes the information recorded in the information storage medium 6,
It is sent to the signal processing device and converted into digital data.
The other laser beam (focus blur detection system) divided by the half mirror block 10 is asymmetrically extracted as an objective lens on the optical axis 14 by the light shielding plate (light extraction means) 13, and passes through the projection lens 15. After that, the light is incident on the second photodetector 16 including the first photodetection cell 16-1 and the second photodetection cell 16-2. Here, the light shielding plate 13
May be any of prisms, aperture slits, knife edges and the like. Second photodetector 16
The signal detected in (1) is processed by a focus signal generator (not shown), and the focus signal generated from this focus signal generator is given to the voice coil drive circuit.
The voice coil drive circuit drives the voice coil 8 according to the focus signal, and maintains the objective lens 5 in a focused state. When the tracking guide 9 formed on the information forming layer 7 of the information storage medium 6 is accurately traced, the signal from the second photodetector 16 may be processed to operate the linear actuator. Alternatively, the objective lens 5 may be moved in the lateral direction, or a galvanometer mirror (not shown) may be operated.

The relationship between the focal lengths of the objective lens 5 and the projection lens 15 and the second photodetector 16 in the optical head shown in FIG. 5 will be described below in detail with reference to FIGS. 6 and 7. . According to FIGS. 6 and 7, the second photodetector 16
The light receiving surface of is arranged not in the image forming position of the beam waist spot in the focused state but in the vicinity thereof, that is, in a position slightly deviated from the image forming position toward the projection lens 15 side, for example. Further, at the time of focusing, the light detection cell 16-
The photodetector 16 is arranged so that the total sum of the laser light radiated to 1 and the total sum of the laser light radiated to the photodetection cell 16-2 are equal. From this, the light detection cell 1
The position of the intermediate point (center point of the gap region) 21 'between 6-1 and 16-2 is displaced from the position of the point 14' where the optical axis 14 of the detection optical system and the photodetector 16 intersect. Further, a point 14 'where the optical axis 14 of the detection optical system and the photodetector 16 intersect
Is always in the gap region 21 (second photodetector cell 16-2
The width 21 ″ of the gap region 21 is sufficiently narrowed so as to exist in the cell 16-1 and the cell 16-.
The width (gap) 21 ″ of the gap 21 between the two is preferably defined to be approximately the same as the circle of least confusion of the laser light focused through the objective lens 5 and the projection lens 15. At the time of focusing, as shown in Fig. 6 (a), the beam spot 22 is formed at the center of the light receiving surface of the second photodetector 16. That is, the beam waist spot is formed on the information forming layer 7, The laser beam reflected by the information forming layer 7 is converted into a parallel light flux by the objective lens 5 and directed to the light shield plate 13. The light shield plate 13 forms the optical axis 1
The laser light extracted asymmetrically with respect to 4 is focused by the projection lens 15 and projected onto the second photodetector 16. Here, the beam spot 22 as shown in FIG. 7A is formed. At this time, the light amounts applied to the first detection element 16-1 and the second detection element 16-2 are equal. Next, when the objective lens 5 approaches the information forming layer 7, the beam waist is generated when the laser beam is reflected on the information forming layer 7 as shown in FIG. That is, the beam waist is generated between the objective lens 5 and the information forming layer 7. In such an out-of-focus state, the beam waist is reflected by the information forming layer 7 as is apparent from the assumption that the beam waist normally functions as a light spot within the focal length of the objective lens 5, and is reflected from the objective lens 5. The emitted laser beam is converted into a divergent laser beam by the objective lens 5. Since the laser beam component that has passed through the light shielding plate 13 is also divergent, this laser beam component is focused toward a point farther than the focusing point at the time of focusing. At this time, the beam spot 22
Is displaced upward from the center of the light receiving surface of the photodetector 16 as shown in FIG. 7B, and is formed larger than the beam spot 22 at the time of focusing. Further, as shown in FIG. 6 (C), the objective lens 5 has the information forming layer 7
The laser is reflected by the information forming layer 7 after forming the beam waist when the laser beam is separated from. In such an out-of-focus state, since the normal beam waist is formed outside the focal length of the objective lens 5 and between the objective lens 5 and the information forming layer 7, the reflected laser beam from the objective lens 5 to the light shielding plate 13 is reflected. The beam will be convergent. Therefore,
The laser beam component that has passed through the light shielding plate 13 is further focused by the projection lens 15 and projected onto the light receiving surface of the second photodetector 16. At this time, the beam spot 22 is
As shown in FIG. 7C, the first and second detection elements 16-
1, 16-2, a position slightly deviated from the center 21 'of the gap region 21, that is, the second detection element 16-2.
The light beam is focused once on the upper side, and is inverted downward when the defocusing further proceeds.

Next, referring to FIGS. 8 to 13, depending on the position of the beam spot of the reflected light irradiated on the second photodetector 16 of the optical head shown in FIG. The principle by which the focus / non-focus of the medium 6 can be detected will be described in detail. Beam spot 2 formed on the second photodetector 16
2 and the relationship between the change in the size of the beam spot 22 and the change in the distance between the objective lens 5 and the storage medium 6 are reflected by the storage medium 6, and the objective lens 5 and the projection lens 15 are sequentially arranged. It is estimated by obtaining the amount h ₃ of the beam spot of the reflected light that has passed through the photodetector 16 being deflected. The amount h ₃ by which the beam spot of the reflected light is deflected
Can be obtained by geometrically explaining the change in the ray trajectory of the reflected light traveling from the storage medium 6 to the photodetector 16. Incidentally, FIG. 8 is an optical path characteristic diagram schematically showing a ray trace (optical path) of the optical head shown in FIG.
Further, in tracing the ray trajectory, in the optical head shown in FIG. 5, the position where the second photodetector 16 is arranged is different from the focal positions of the objective lens 5 and the projection lens 15. First, the ninth
Using FIG. 11 to FIG. 11, assuming that the photodetector 16 is placed at the focal position of the lenses 5 and 15, the deflection amount h
Ask for ₃ . Subsequently, using FIG. 12 and FIG. 13,
The deflection amount h ₃ obtained from FIGS. 9 to 11 will be verified with the position of the photodetector 16 moved along the optical axis 14.

As shown in FIGS. 9 (a), (b), and (c),
It is assumed that the second photodetector 16 is arranged on the image plane of the beam waist spot in the focused state.
The geometrical optical image forming system of the objective lens 5 can be represented as shown in FIG. Here, f ₀ indicates the focal length of the objective lens 5, and δ indicates the moving distance of the objective lens 5 or the information forming layer 7 of the information storage medium 6 from the time of focusing to the time of defocusing. To do. A ray axis trace shown by a solid line in FIG. 10 is emitted from the beam waist, passes through a point on the main surface of the objective lens 5 and is separated from the optical axis by a distance h ₀ , and is focused. Is shown.

At the time of focusing shown in FIG. 9 (a), as is clear,
δ = 0.

On the other hand, in the non-focus state shown in FIG. 9B, the information storage medium 6 is close to the objective lens 5 by the distance δ, and the beam waist is formed by being reflected by the information forming layer 7.
The beam waist is twice as close to the objective lens 5 (when it is close, δ> 0).

Further, in the non-focus state shown in FIG. 9C, the information storage medium 6 is separated from the objective lens 5 by the distance δ, and after forming the beam waist, the laser beam has a beam waist behind the information forming layer 7. As formed, the beam waist will be separated from the objective lens 5 by 2δ.

By the way, when focusing, the beam waist is the objective lens 5.
If it is formed at the focal position of
When only moved, as shown in FIG. 10, the distance between the beam waist and the main surface of the objective lens 5 is (f ₀ +
2δ). Here, if the beam waist is regarded as a light spot, the angles β ₀ and β ₁ in FIG. 10 are expressed by the following equations (1) and (2). That is, β ₀ is The imaging formula of the lens is Therefore, equation (1) is Is shown.

However, FIG. 9 shows a ray axis trace in the optical system of the projection lens 15, and the projection lens 15 is treated as a pair of combination lenses 15-1 and 15-2.

Here, the lenses 15-1 and 15-2 have focal lengths f ₁ and f ₂ , respectively, and the light shielding plate 13 is arranged at a position separated from the main surface of the objective lens 5 by a, and the objective lens 5 The main surface of the lens 15-1 is disposed at a position separated from the main surface by L, and further separated from the main surface of the lens 15-1 by H and the main surface of the lens 15-2 is separated by 1 again. It is assumed that the light receiving surface of the second photodetector 16 is arranged. A ray axis trace indicated by a solid line in the drawing shows a light transmitting surface of the light shielding plate 13 which is focused by the objective lens 5 and is separated from the optical axis by y (image height). Where the distance y is It is represented by. Here, if F is the distance when the focal length f ₀ changes by the distance δ, then F (δ) = (f ₀ + f ₀ ² / 2δ) ⁻¹ , and therefore the equation (3) is y = It can be transformed into h ₀ [1-aF (δ)] (4). Therefore, h ₀ = y / [1-aF (δ)] (5) is obtained.

Further, the position h ₁ from the optical axis where the light ray passes on the main surface of the lens 15-1 is It is represented by.

In this context (6), by obtaining the angle beta ₂ in the same manner as (2), the angle beta ₂ is It is represented by.

Similarly, the position h ₂ of the light ray passing through the main surface of the lens 15-2 from the optical axis and the incident angle β _{3 of the} light ray on the light receiving surface of the second photodetector 16 are similarly incident on the optical axis. Position h from
₃ (that is, the amount of deviation) is as well as, It is represented by.

As described above, the optical system shown in FIGS. 10 and 11 is in focus, that is, at δ = 0, the light beam is focused on the second photodetector 16 at h ₃ = 0. Therefore, under this condition, F (0) = 0, and equation (10) becomes It is represented by.

Further, since only the light ray outside the optical axis is extracted by the light shielding plate 13, y ≠ 0. Therefore, equation (11) becomes Is rewritten as By simplifying (10) in this equation (12), Or Is obtained.

Here, when “δ << f ₀ ² ” is satisfied (δ is sufficiently small), (a−f ₀ ) << f ₀ ² / 2δ Is satisfied.

Next, when focused (δ = 0), the lateral magnification m of the beam waist image formed on the light receiving surface of the photodetector 16 with respect to the beam waist formed on the information forming layer 7 of the information storage medium 6 is , M = −β ₀ / β ₀ (inverted image). Here, since β _{0 at} δ = 0 is β ₀ = −h ₀ / f ₀ = −y / f ₀ , Is obtained.

Here, if f ₂ is eliminated from equation (16) using equation (12), Becomes If we solve for f _{1 in} this equation, Is expressed as On the other hand, in an actual optical system, it is considered that f ₁ = H is not set, Can be transformed.

Similarly, considering the case where an erect image is formed,
“M = β ₀ / β ₃ ” and f ₁ are It is represented by. This equation (18) is Therefore, if f2 is calculated using equation (12), Is obtained.

Here, by substituting the equation (20) into the equation (13) and expressing h ₃ by the lateral magnification m, as well as, {However, the above equations (21) and (22) are satisfied only in the range of (a−f ₀ ) << f ₀ ² / 2δ}.

Here, if the projection lens 15 in the optical system of FIG. 11 is a single lens, then f ₂ = ∞, and since f ₁ = 1 and m = f ₁ / f ₀ , Equation (22) is Is rewritten as

Up to this point, it is considered that the light emitted from the laser device 1 enters the objective lens 5 as parallel light via the collimator lens 2 and is condensed at the focal position of the objective lens 5.

However, in an actual optical head, as shown in FIG. 12, the focal position of the objective lens 5 and the focal point of the laser light are b (b ≠ b ≠
There are quite a few optical heads that are offset by 0).
A consideration is added to the optical behavior in this case.

According to FIG. 12, regarding the laser light irradiated onto the information storage medium 6, the optical system after being reflected from the surface of the information forming layer 7, that is, the objective lens 5 and the detection system lens (projection lens) 15 is shown. An example is shown in which it is handled as one synthetic lens. Let f ^{* be} the focal length of the synthetic lens, and c be the distance from the front focal position of the synthetic lens to the information forming layer (focus point of laser light) 7 when the focus is achieved. The optical path of the reflected light when the information storage medium 6 is deviated from the focal point by δ is obtained by the same method as the calculation from the equations (1) to (22).
Considering the objective lens 5 and the detection system lens 15 as one composite lens, from FIG. Is obtained.

Here, when solving for _{^{h *, h * = -β 0}} × (f * + c + 2δ) is derived. Therefore, β ₃ is Is prescribed.

Here, when the photodetector 6 is arranged at the image forming point when the focal point of the laser light and the position of the information forming layer 7 coincide (when δ = 0), h ₃ = 0 when δ = 0. , Imaging magnification m at this time
Is ± m = −β ₀ / β ₃ , so that m = ± f ^* / c. Therefore, c = ± f ^* / m; {however, m is always a positive number, and it is defined as an inverted image when + m and an upright image when −m}.

On the other hand, if the distance from the principal point on the rear side of the synthetic lens to the photodetector 16 is A, Is guided.

At this time, when δ = 0 for arbitrary β ₀ , h ₃
Since 0, A = f ^* (1 ± m) (25) is satisfied. Substituting equation (25) into equation (24), Is obtained.

Next, when the focal position of the objective lens 5 and the focal point of the laser are deviated by “b”, “2δ” is transformed into “2δ + b” in the equations (1) to (10), The ray tracing formulas hold as they are. Therefore, from equation (5), Is guided. By substituting equation (27) into equation (1), Is obtained. Here, by substituting equation (28) into equation (26), Becomes

In this case, if a = 0 holds, equation (29) becomes Will be transformed.

In particular, in the range of “f ₀ + b >> 2δ”, Can be written as

Since the expressions up to (31) are calculation expressions for the case where the photodetector 16 is arranged at the position of the image forming point of the information storage medium 6 with respect to the information storage medium 6 at the time of focusing, the information storage medium 6 at the time of focusing is used. The optical behavior when the photodetector 16 is arranged at a position deviated from the image forming point with respect to the information forming layer 7 will be considered. Here, the entire optical system is regarded as one compound lens.
At the time of focusing, that is, when one focal point coincides with the position of the information forming layer 7 of the information storage medium 6, from the rear principal point of the synthetic lens in the direction farther from it to the other focal point (above). The distance from the focal point to the image formation point) is A ₀ . Here, as is apparent from the equation (25), A ₀ = f ^* (1 ± m) ... (32) is obtained. Here, it is apparent from the other formulas already described that m is the lateral magnification and f ^* is the focal length of the compound lens.

On the other hand, compared to the position specified by this equation (32),
When the photodetector 16 is arranged at a position closer to the compound lens side by Δ, the distance A from the rear principal point of the compound lens to the photodetector 16 is A = A ₀ −Δ = f ^* ( 1 ± m) −Δ ... (33)

By substituting the equation (33) into the equation (24) and sequentially calculating the above equations to obtain the deviation amount h ₃ between the center of the beam spot projected on the photodetector 16 and the optical axis, h ₃ is Becomes Here, substituting equation (28), Is obtained.

However, in the equation (34), since Δ does not take a very large value, “Δ << f ^* ” is satisfied, and in order to increase the defocus detection sensitivity, it is defined as “m >> 1”. In many cases, equation (34) is Can be approximated by

From this equation (35), the photodetector 16 is brought closer to the detection system lens 15 than the focal point at the time of focusing (lens 15 is Δ> 0.
Objective lens 5
Is approaching the information storage medium 6 (δ <0), |
h ₃ | monotonically increases, and then | h ₃ | once decreases and then increases again.

Here, the change in the size of the beam spot 22 on the photodetector 16, that is, the increase in | h ₃ | is faster than the case where the objective lens 5 moves away from the information storage medium 6 (δ> 0). Therefore, the photodetection cells 16-1 and 16-2 are swiftly protruded, and the optical defocus detection signal becomes small.

On the other hand, when the photodetector 16 is placed away from the detection system lens 15 and at a position where Δ <0 is satisfied, the opposite behavior is exhibited. That is, the lens 15 has Δ <
When it is arranged at a position where 0 is satisfied, | h ₃ | in the state where the objective lens 5 approaches the storage medium 6 (δ <0).
Once increases to “0” (or a very small size) and then increases, and | h ₃ | monotonically increases when the objective lens 5 is away (δ> 0).

In this case, the change in the size of the beam spot 22 on the photodetector 16, that is, the change in the size of | h ₃ |, occurs when the objective lens 5 approaches the storage medium 6 (δ <0). , Does not extend outside the photodetection cells 16-1 and 16-2. In other words, the defocus detection signal is reliably provided even when the objective lens 5 is largely displaced from the in-focus position.

By the way, in the optical head shown in FIGS. 5 to 7 and FIGS. 8 to 13, the second photodetector 16 is used.
Is arranged at a position different from the focal position of the objective lens 5 and the projection lens 15 and slightly displaced to the projection lens 15 side. Therefore, the diffraction pattern of the tracking guide 9 on the storage medium 6 is also projected in the beam spot 22 on the photodetector 16. In this case, there is a problem that the diffraction pattern of the tracking guide 9 projected on the photodetector 16 moves due to the converging spot on the information forming layer 7 of the storage medium 6 crossing the track.
This may cause a malfunction in the focus control for moving the objective lens 5 to the in-focus position using the size of the beam spot 22 on the photodetector 16.

From this fact, as shown in FIG. 14 or FIG. 15, the direction of projection onto the photodetector 16 through the optical system and the beam spot 22 on the photodetector 16 at the time of defocusing
It is preferable to dispose the photodetector 16 so that the direction in which is moved and the direction of the track of the information storage medium 6 are parallel.

In this case, as is clear from FIG. 16, the information storage medium 6
When the focused spot on the information forming layer 7 crosses the track, the diffraction pattern 23 of the tracking guide 9 is moved on the photodetector 16 in the direction of arrow 23 '. On the contrary,
The beam spot 22 on the photodetector 16 at the time of defocusing is moved in the direction of arrow 22 '. Therefore, even when the diffraction pattern of the tracking guide 9 is projected in the beam spot 22 on the photodetector 16, it is possible to reliably detect defocus.

17 to 20 show another embodiment of the present invention. In the example shown in FIGS. 17 to 20,
The objective lens 5, the information forming layer 7 of the storage medium 6, the projection lens 15, and the photodetector 16 are virtually shown.
In each example, 24 is a mirror, 25 is a lens, and 26 is a biprism. It is easily understood that accurate defocus detection can be performed by the optical head shown in FIGS. 17 to 20 as described above.

(Effects) As described above, in the optical head of the present invention, the photodetector irradiated with the beam spot of the reflected light reflected from the storage medium has at least two detection cells. The optical axis of the lens for collecting the reflected light on the photodetector passes through either one of the two cells. On the other hand, the photodetector is arranged at a position slightly shifted to the lens side from the focal position of the lens.

Therefore, when the objective lens and the storage medium are in focus, beam spots having substantially equal areas are formed in the two cells of the photodetector. On the other hand, when the objective lens approaches the storage medium, the area of the beam spot formed in the cell where the optical axis does not pass is increased. On the other hand, when the objective lens moves away from the storage medium, the area of the beam spot formed in the cell that does not include the optical axis is gradually reduced, and the beam spot remains only in the cell that includes the optical axis.

Therefore, it is possible to provide the optical head that can reliably provide the defocus detection signal regardless of the state of the laser light guided to the photodetector.

[Brief description of drawings]

FIGS. 1 (a), (b) and (c) are explanatory views showing a first conventional optical system for detecting defocus by moving the beam spot, and FIGS. 2 (a), (b) and (c). ) Is an explanatory view showing a second conventional optical system for detecting defocus by moving the beam spot, as in FIG. 1, FIG. 3 (a),
Similar to FIG. 1, (b) and (c) are explanatory views showing a third conventional optical system for detecting defocus by moving the beam spot, and FIG. 4 is a photodetector of the optical system. 5 to 13 are schematic views showing the pattern of laser spots in FIG. 5, showing an embodiment of the present invention, and FIG. 5 is a constitutional view showing an optical head device according to an embodiment of the present invention. , (A), (b) and (c) of FIG. 6 are schematic optical path diagrams showing a state in which defocus is detected by movement of a beam spot in the optical head device shown in FIG. 5, FIG. (I),
FIGS. 8B and 8C are views showing the state of the laser beam spot during focusing and out of focus in the optical head device shown in FIGS. 5 and 6, and FIG.
FIG. 5 is a schematic diagram showing an optical system for defocus detection when the second photodetector is moved away from the condensing point in the optical head of FIG.
9 (a), (b) and (c) are diagrams showing the trajectory of the laser beam when the photodetector is arranged at the converging point, and FIG.
The figure is an optical path diagram for analyzing the trajectory of the light beam passing through the objective lens. Fig. 11 is the optical path diagram for analyzing the trajectory of the light beam passing through the projection lens. Fig. 12 is the focal position of the objective lens and the laser. An optical path diagram showing an optical system in which the focal point of light is deviated,
FIG. 13 is an optical path diagram for analyzing the trajectory of light rays when the objective lens and the projection lens are regarded as one composite lens,
14 and 15 are schematic diagrams for explaining the influence of the diffraction pattern of the tracking guide, and FIG. 16 is a schematic diagram showing the pattern of the laser spot on the photodetector.
17 to 20 are schematic optical path diagrams showing examples of other optical systems to which the present invention is applied. 6 ... Information storage medium, 5 ... Objective lens, 16 ... Second
Photodetector, 13 ... Light extraction member (shading plate), 14 ...
Optical axis, 16-1 ... First detection cell, 16-2 ... Second
Detection cell, 21 ... Gap region.

Claims (1)

[Claims] 1. Condensing means for condensing irradiation light irradiated toward an information storage medium, and for recollecting and condensing reflected light reflected by the condensed illumination light on the information storage medium. A light extraction means for extracting a part of the reflected light including light asymmetric with respect to the optical axis of the reflected light and along the optical axis from the reflected light taken in and condensed by the light collecting means; Of the information storage medium from the condensing point of the illumination light extracted by the light extraction means and condensed by the light condensing means. One of the plurality of detection regions is arranged at a position for detecting the light along the optical axis of the reflected light that moves on the detection region corresponding to the deviation, and the irradiation condensed by the condensing unit is arranged. A light condensing point and the information storage medium is the light condensing means. Detection means arranged at a position where all the reflected light is applied to a detection region at a position arranged on the optical axis of the reflected light when the light is separated from the condensing point of the irradiated light and separated from the condensing point. An optical head comprising: a moving unit that moves the light collecting unit with respect to the information storage medium based on detection of reflected light by the unit.