JPS6010424A - Optical head - Google Patents

Abstract

PURPOSE:To stabilize the detection of an out-of-focus state by providing a photodetector, which is arranged in a position deviated from the image formation position for the in-focus state and has plural detecting cells, and a means which moves a spot in accordance with the deviation of a storage medium and placing one detecting cell on the optical axis at least. CONSTITUTION:The beam of an out-of-focus state detecting system which is divided by a half mirror 10 is taken out asymmetrically with respect to an optical axis 14 by a light shielding plate 13 and is made incident on a photodetector 16 having light detecting cells 16-1 and 16-2. An area 21 of the detector 16 has not an optical detection sensitivity practically. The light receiving face of the photodetector 16 is deviated from the image formation position in the in-focus state. The position of a center point 21' of the area 21 is deviated from an intersection 14' between the optical axis 14 and the detector 16. A width 21'' is so narrowed that the point 14' is placed on the outside of the area 21. In the out-of-focus state, a spot 22 is deviated upward from the center of the light receiving face and is larger than that in the in-focus state. A reflected beam from an object lens 5 is convergent. In this case, the spot 22 is converged once in a position deviated from the point 21'; and when the out-of-focus degree is increased, it is inverted downward. Thus, the out-of-focus state is detected stably.

Description

[Detailed Description of the Invention] [Technical Field of the Invention] 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 that can at least read information by irradiating an information storage medium such as a COM (computer output memory) with focused light.

[Technical background of the invention and its problems]

In the above-mentioned Yuzu optical head, a beam waist (the narrowest part) of the focused laser beam is formed at the information formation layer (recording layer or light reflection layer) of an information storage medium such as an optical disk or video disk. It is required that

For this reason, the defocus of the laser beam on the information forming layer is usually detected, and the objective lens is moved and focused based on this detection signal.

By the way, recently, as a method of detecting the defocus of laser light on the information forming layer, there is a method as shown in Figure 1 (a), (b), (c) or Figure 2 (a), (b), e) In the middle of the reflected optical path C of the light beam reflected from the information forming layer a and passed through the objective lens b, there is a light extraction member d or an anomaly e which is a light extraction member that extracts the light asymmetrically with respect to the optical axis. A method is proposed in which a photodetector 1 having a lens f1 and two photodetection cells g and h is provided, and defocus is detected as the movement of the beam spot j (in the direction of the arrow) on the photodetector amount. It was done. That is, in FIG. 1(A) or FIG. 2(A), when the laser beam C is focused on the information forming layer a, the reflected light passing through the objective lens b is parallel to the optical axis. Nashi, Naifetsuji d or that one? - Light extracted by e (beam spot)
is 1f (7) photodetection cell g and second 5'c, detection cell/I
/h. However, in Figure 1 (b) or Figure 2
As shown in figure (b), if the information forming layer a and the objective lens b are too close to each other and cannot be focused, the reflected light passing through the objective lens b will spread outward to gradually move away from the optical axis. C,
The light (beam suno J pit) that has passed through the naifetsu d or aperture 〇 falls on the first detection cell g located above. Conversely, as shown in FIG. 1 (C) or FIG. 2 (C), if the information forming layer 8 and the objective lens b are too far apart and cannot be brought into focus, the information passing through the objective lens b. The reflected light is focused inward so as to gradually approach the optical axis, and the light (beam spot) that has passed through the knife error) d or the anno (char e) is focused on the second detection cell h located below. It will be a hit.

The optical system described above has the following problems because the photodetector 1 is placed at the position of the condensing point on the side of the in-focus dawn photodetector i.

1) When the focused light is focused by the objective lens b and passes over the bits as signal information in the information formation layer a of the information storage medium, the light is focused at the focusing point on the photodetector i side. The image formation/deturn of the bit appears. The imaging pattern of this bit becomes a disturbance noise signal when viewed from the entire defocus detection system.

11) When recording or reproducing information on the information forming layer a of an information storage medium, light is generally focused on a light reflecting layer through a substrate made of a transparent material. Therefore,
The objective lens b is designed to condense light through a transparent substrate. By the way, if large warpage or waviness occurs in the information storage medium a and the substrate is tilted, aberrations such as coma aberration occur. Therefore, at the time of focusing, an image formation pattern including coma aberration appears on the photodetector l. When the objective lens system has little aberration, a substantially circular pattern appears on the photodetector i, and the focal point is stably determined as a detection system.

If warpage or ridges occur in the information storage medium, an image formation pattern containing aberrations of the objective lens system such as coma aberration appears on the photodetector 1. Then, even though the light is in focus, the amount of light irradiated onto the photodetection cells g and h becomes unbalanced, resulting in false detection as if defocus had occurred.

2) Near the focal point, the spot has a certain size due to the influence of light diffraction (wave nature of light). Therefore, if the photodetector is placed at the focal point on the photodetector side at the time of the in-focus point, the out-of-focus detection sensitivity near the in-focus point will be much smaller than the value calculated using geometric optics. ing.

1) The spot size at the focal point is very small, and in the case of an optical system in which a photodetector is placed there, even if the photodetector i shifts slightly due to temperature changes, for example, the spot will remain in focus. Despite this, it is falsely detected as if the object is out of focus, causing the object to become out of focus.

■) Large aberrations in the detection system lens itself (e.g. spherical aberration)
If there is a difference between the Gaussian image plane and the position of the smallest random circle, the defocus detection characteristics near the in-focus position will deteriorate and the detection sensitivity will also increase.

Recently, in order to solve the above problem, an optical system has been considered in which the photodetector is placed at a position shifted from the imaging position relative to the recording layer or light reflective layer of the information storage medium at the time of focusing. The beam spot on the photodetector is condensed at the point where the beam spot is out of focus, but if the beam spot enters the gap region between the photodetection cells at this time, no out-of-focus detection signal can be obtained. In other words, the detection optical system at the time of focus is in the state shown in Fig. 3 (A1), and the beam spot j on the photodetector l at this time is as shown in Fig. 5 (A). %, if the light 111X of the detection optical system passes through the gap region k of the photodetector l (the position of the point where the optical axis X of the detection optical system intersects the photodetector l) As shown in Fig. 3 (Rohe ball), when the focal point is filled in the direction in which the information storage medium a and the objective lens h approach, the beam spot on the photodetector quantity is j has a spread of 9, as shown in Figure 1 (λ to book).

At this time, since the amount of light irradiated onto the photodetection cell g is greater than the amount of light irradiated onto the photodetection cell h, a defocus detection signal can be reliably obtained. However, as shown in FIG. 3, when the focus is loosened in the opposite direction, the beam spot j gradually becomes smaller and converges on a single point on the photodetector i. However, since the laser beam C is condensed on the optical axis X of the detection optical system, it is condensed in the gap region on the photodetector i as shown in FIG. 3 (bar xXS).

Therefore, as described above, almost no defocus detection signal is obtained, or the signal becomes unstable.

[Purpose of the invention]

The present invention has been made based on the above circumstances, and its object is to provide an optical method that allows defocus detection to be carried out more stably and with high reliability.

[Summary of the invention]

The present invention provides a condensing means for condensing irradiation light irradiated onto an information storage medium and for retaking reflected light reflected by the condensed irradiation light on the information storage medium; The information storage medium is located at a position shifted from the imaging position on the information storage medium at the time when the focal point of the irradiation light collected by the light collection means and the information storage medium coincide, and the light is focused by the light collection means. a photodetector having a plurality of detection cells for detecting the focused reflected light; and a photodetector having a plurality of detection cells for detecting the focused reflected light; means for moving the beam spot of the reflected light on the detector, the photodetector comprising at least one
The present invention is characterized in that the detection cells are arranged so as to be located on the optical axis of the reflected light.

[Embodiments of the invention]

A laser beam generated from a laser device 1 is collimated by a collimator lens 2 and directed toward a polarizing beam slitter 3. The parallel laser beam reflected by the polarized beam snoritter 3 passes through the 1/4 wavelength plate 4 and enters the objective lens 5. layer) 7. The objective lens 5 is supported movably in the optical axis direction by a voice coil 8, and when the objective lens 5 is positioned at a predetermined position, the objective lens 5
The beam waist of the focused laser beam emitted from the information forming layer 7 is projected onto the surface of the information forming layer 7, and a minimum beam spot 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, a laser beam whose optical intensity is modulated is focused on the tracking guide (pre-group) 9 on the information forming layer 7, and when reading information, a laser beam with a constant optical intensity is focused on the tracking guide (pre-group) 9 on the information forming layer 7. , the light intensity is modulated by the bits formed on the tracking guide 9 and reflected.

The diverging laser beam reflected from the information forming layer 7 of the information storage medium 6 is converted into a parallel beam by the objective lens 5 when focused, passes through the quarter-wave plate 4 again, and enters the polarized beam splitter 3. be returned. The laser beam is 1/
By reciprocating through the four-wavelength plate 4, the laser beam is
The plane of polarization is rotated by 90 degrees compared to when it is reflected by the polarizing beam splitter 3, and the laser beam whose plane of polarization has been rotated by this 90 degrees is not reflected by the polarizing beam splitter 3.
It will pass through this polarizing beam splitter 3. The laser beam that has passed through the polarizing beam sinter 3 is divided into two systems by a half mirror block 10 , one of which (track deviation detection system) is irradiated onto a first photodetector 12 by a convex lens 11 .

The first signal detected by the first photodetector 12 includes information recorded on the information storage medium 6, and is sent to a signal processing device and converted into digital data. The other laser beam (focal tz detection system) separated by the half mirror block 10 is extracted asymmetrically with respect to the optical axis 14 by a light shielding plate (light extracting means) 13, and is directed to the projection lens 1.
5, the light enters the second photodetector 16, which includes a photodetection cell 16-1 and a second photodetection cell 16-2 of the cylinder 1. Here, the light shielding plate 13 may be formed of a prism, an aperture slit, a knife etching, or the like. The signal detected by the second photodetector 16 is
The focus signal is processed by a focus signal generator (not shown), and the focus signal generated from the focus signal generator is given to a voice coil drive circuit. The voice coil drive circuit drives the voice coil 8 according to the focus signal,
The objective lens 5 is maintained in focus. In addition,
In order to accurately trace the tracking guide 9 formed on the information formation layer 2 of the information storage medium 6, the signal from the second photodetector 16 may be processed to operate the linear actuator. , if the objective lens 5 is moved laterally, or if a galvano mirror (not shown) is activated, this will be more effective than a PIN photodiode.
It is structured as follows. That is, on the back side of one layer (also called n-layer) 17 with a low impurity concentration, there is an N layer (n+ layer (p+ layer) 19.
19 are provided. And these two layers 19.1
9 becomes the photodetection cell 16-1, 16-2.

In addition, there is a depletion layer 2 in the boundary region between the second layer 19 and the first layer 17.
0°20 is spreading. Therefore, when light enters from the surface, a hole-electron is generated in the photodetector 16.
air is generated. The middle region E of the depletion layers 20, 20 is hol
When an e-electron pair occurs, each
Layer 19.19.1 It is attracted to layer 17 and becomes a detection signal,
hole-elec-tr generated in the middle area of 1st layer 17
Most of the on-pairs recombine and disappear, and only a small number of carriers diffuse to reach the depletion layer 20.20 and become a detection signal. In addition, even if the light that reaches the lower part via the G part generates a hole-electron pair, many of the hole-electron pairs reach the depletion f@20, 20 due to diffusion, recombine with the instar that becomes the detection signal, and disappear. Resulting in. Therefore, two layers 19
The region G between the two layers 19 and 19 becomes a region with high photodetection sensitivity. Furthermore, if the distance between the two layers 19°19 (width of region G) is wide, the center of region G is irradiated with light and the generated hole
-Most of the electron pair is a depletion layer 20,
In this case, the region G can be called a dead region. In addition,
There is also a structure in which an A1 layer is provided in region G to shield light, and an N layer (8) is provided in region F.
In any case, the area G is still an insensitive area or an area with low detection sensitivity. Furthermore, it is not known whether the carriers generated in the region F will reach the left or right depletion layer 20.20. From the above, let's focus the light on the area G between the photodetection cells 16-1 and 16-2, and use the movement of the light in the area G to perform detection such as defocusing. If this is the case, there is a possibility that the reliability of the optical head itself will decrease.Therefore, the area G (hereinafter referred to as Gyazzo area 21) may be regarded as an area with almost no light detection sensitivity, or the area where most of the light is transmitted. While being irradiated with G, it is treated as having no detection accuracy or reliability.

Next, to explain the optical system for detecting when the focus is focused, in the optical system shown in the second figure, the light receiving surface of the second photodetector 16 is located at the beam waist spot in the focused state. They are arranged not at the image formation position, but in the vicinity thereof, that is, at positions slightly shifted from the image formation position, for example, toward the projection lens 15 side. In addition, the photodetector J6 is arranged so that the total sum of the laser beams irradiated to the photodetection cell 16-1 and the total sum of the laser beams irradiated to the photodetection cell 16-2 are equal at the time of focusing. The midpoint between the photodetection cells 16-1 and 16-2 (gap region 21
The position of the center point 21' is shifted from the position of the point 14' where the optical axis 14 of the detection optical system and the photodetector 16 intersect.

Further, the gap area 2 is adjusted so that the point 14' where the optical axis 14 of the detection optical system and the photodetector 16 intersect is always outside the gap area 21 (inside the second photodetection cell 16-2). The width 21'' is sufficiently narrow. Therefore, when focusing, the beam spot 22 is centered on the light receiving surface of the second photodetector 16, as shown in FIG. It is formed.

That is, a beam waist spot is formed on the information forming layer 7, and the laser beam reflected by the information forming layer 7 is converted into a parallel beam by the objective lens 5 and directed toward the light shielding plate 13. A light component is taken out asymmetrically with respect to the axis 14 and focused by the projection lens 15 (note that at this time, the laser beam is focused at a position far from the photodetector 16), and is directed to the second photodetector. 1
6, and a beam spot 22 is formed thereon as shown in FIG. Next, when the first objective lens 5 approaches the information forming layer 7, the beam waist is generated as the beam is reflected by the information forming layer 7, as shown in FIG. That is, the beam waist occurs between the objective lens 5 and the information forming layer 7. In such an out-of-focus state, the beam waist usually occurs within the focal length of the objective lens 5, so the beam waist is generated between the objective lens 5 and the information forming layer 7. Assuming that it functions as a light spot, the S laser beam reflected by the information forming layer 7 and emitted from the objective lens 5 is as follows.
It is converted into a diverging laser beam by an objective lens 5. Since the laser beam component that has passed through the light shielding plate 13 is also diverging, this radar beam component is focused toward a point rC that is further away from the focal point at the time of focusing. Ru. This Toki Beam Sujit 22 is 8+! i figure (testimonial λ
) As shown in FIG. Objective lens 5 as shown in
If the laser beam is separated from the information forming layer 7, the laser beam is reflected by the information forming layer 7 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 5 and is formed between the objective lens 5 and the information forming layer 7, so that the beam is directed toward the light shielding plate 13 from the objective lens 5. The reflected radar beam will have convergence. Therefore, the laser beam component that has passed through the light shielding plate 13 is further focused by the projection lens 15, and the second
is projected onto the light receiving surface of the photodetector 16. At this time, the beam spot 22 is located at the
It is once focused on a position slightly shifted from the center 21' of the gap region 2) between the second detecting element 16-1 and the second detecting element 16-2, that is, on the second detecting element nozzle 2, and as the focus progresses further, It is formed by inverting downward.

As described above, the beam spot 22 does not enter the gap region 21 no matter what the state of the focus is, and it is possible to continue to obtain a stable focus detection signal.

By the way, as mentioned above, the photodetector 16 is located at a position slightly shifted from the focal point formed on the photodetector 16 side when the focus is focused.
In the case of the optical system in which the tracking guide 90 is placed, the diffraction pattern of the tracking guide 90 always appears in the beam spot 22 on the photodetector 16. When the focused spot on the information forming layer 7 of the information storage medium 6 crosses the track, the diffraction/f-turn of the tracking guide 9 on the photodetector 16 moves, generating a defocusing pseudo signal, and the focus sensor changes the focus f. When connected, it disrupts focus.

So. A. Figure 4, Figure 7. 1. Direction when the track direction of the information storage medium 6 is projected onto the photodetector 16 via the optical system, and the photodetector when it is out of focus. 16 is preferably parallel to the moving direction of the upper beam spot 22. In this case, when the focused spot on the information forming layer 7 of the information storage medium 6 crosses the track, the diffraction nosoturn 23 of the tracking guide moves on the photodetector 16, but the diffraction pattern 23 moves when it crosses the track. direction 23'
The moving direction 22' of the beam spot 22 on the photodetector 16 during defocus does not affect the detection of the first defocus.

It also applies to academic fields. Here, 16' indicates a state in which the photodetector 16 is closer to the focal point, and 16'' indicates a state in which it is farther away from the focal point. Also, 24 is a mirror, 25 is a lens, and 26 is a pipe rhythm.This explanation In the above, we have explained the case where the tracking guide 9 has a continuous uneven shape in the direction of the track rack, but the information storage medium 6 in which discontinuous uneven shaped bits continue along the trunk can also be used in the trunk direction. The focal spot can be moved at high speed along the

Next, in FIG. 1S, if the objective lens 5 is far away from the information storage medium 6, the automatic focusing function will be damaged, but if the objective lens 5 is too close to the information storage medium 6, both may rub against each other and cause scratches on either side. Therefore, in order to prevent the objective lens 5 from getting too close to the focal point, it is desirable to obtain a large optical defocus detection signal even when the objective lens 5 approaches the in-focus position by a large distance.

Here, the change in the trajectory of the laser in FIG. 15, that is, the change in the trajectory of the light beam, will be explained in terms of geometrical optics, and the value h3 at which the laser beam component is polarized on the second photodetector 16 will be determined. In this case, first, Figure 4 (A) ←) (C)
As shown in FIG. 2, the second photodetector 16 is analyzed as being disposed on the imaging plane of the beam waist spot in the focused state. The geometrical optical imaging system of the objective lens 5 can be expressed as shown in the first diagram. Here, fO is 1, which is the focal length of the objective lens 5, and δ is the moving distance of the objective lens 5, that is, the 8° information forming layer 7 of the information storage medium 6, when in focus and out of focus. , the ray locus shown by the solid line in the first imperial map is emitted from the beam waist,
When the information storage medium 6 approaches the objective lens 5 by a distance δ, the beam waist is reflected by the information forming layer 7. Since the beam waist is formed by the beam waist, the beam waist is twice as close to the objective lens 5 as the beam waist. (If they are close, δ>0.) Also, Fig. 1S (c)
When the information storage medium 6 is out of focus as shown in FIG. This is similar to the case where a beam waist is formed behind the beam waist, and the beam waist is separated from the objective lens 5 by 2δ. At the time of focusing, if the beam waist is formed at the focal position of the objective lens 5, when the information storage medium 6 moves by δ, the first
As shown in the diagram, the distance between the beam waist and the main surface of the objective lens 5 is expressed as (fo+2δ). If we consider the beam waist as a light point, the angle β0 in Fig. 17
and βl are represented by the following formulas (1 and (2)).

Also, according to the image formation formula of the lens, FIG.
x, 15-2.

Here, the lenses 15-1 and 15-2 each have a focal length f1.
*f2, the light shielding plate 13 is arranged at a position spaced apart by a from the main surface of the objective lens 5, the main surface of the lens 15-1 is arranged at a position spaced apart by L from the main surface of the objective lens 5, Furthermore, it is assumed that the main surface of the lens 15-2 is arranged at a distance H from the main surface of the lens 15-1, and the light receiving surface of the second photodetector 16 is arranged at a distance t from the main surface of the lens 15-1. . The light ray locus shown by the solid line in the figure shows the light transmission surface of the light shielding plate 13 that is focused by the objective lens 5 and is spaced apart by Y from the optical axis.

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

Here, F(δ) = (f o + f o2/2δ)
-1, equation (3) is expressed by the following equation.

y;ho(1-aF(δ))...(4) Therefore, the position hl from the optical axis where the light ray passes over the principal surface of the lens 15-1 is expressed by equation (6).

If we calculate the angle β2 in the same way as in equation (2), the angle β2 becomes (7
) is expressed by the formula.

Similarly, the position h2 and the incident angle β3 from the optical axis of the light ray passing on the main surface of the lens 15-2 are detected by the second photodetector 1.
The position h3 from the optical axis of the light incident on the light-receiving surface of 6, that is, the amount of deviation, is calculated by formulas (8) to (1o), respectively. As mentioned above, when the system is focused, that is, when δ=0, the light beam is focused on the second photodetector 16 at h3=o, so under this condition, the light beam is focused at h3=o.・==0, formula (1o) is expressed by the following formula.

Furthermore, since only the light passing through the off-axis light is extracted by the light shielding plate 13, y'=0. Therefore, if the θ() formula is simplified using this formula, the 01 formula or the 64 formula can be obtained.

Or, if δ is sufficiently small (δ<fO2), (a f
o ) < fo2/2δ, so next, when focusing (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 on the information forming layer 7 of the information Me storage medium 6 is expressed by the following α0 formula.

m=-β0/β3 (inverted image) Here, fO at δ=0 is h.

βo=”’-- fOf.

Therefore, by eliminating f2 from the α4 equation in equation (2), we get from this equation,
If we solve for fl, in an actual optical system, fl-
Since it is considered that there is no force exerted on
m=βO/β3), fl is expressed by the following formula. Subtracting f2 from, Substituting equation (a) into equation 01 and expressing h3 by the horizontal magnification m, equations Q and (a) can be obtained.

my h33±-δ...) fO However, (a - fo ) < to /2δ First, if the projection lens 15 is a single lens in the optical system shown in the figure, f2 = ω theal color, fl = and m=ft/fo
So far, we have considered only the case where the light emitted from the semiconductor laser light source 1 passes through the collimator lens 2, enters the objective lens 5 as parallel light, and is focused at the focal position of the objective lens 5. was. However, as shown in FIG. 19, there are some optical heads in which the focal position of the objective lens 5 and the focal point of the laser beam are shifted by b (boo). The optical system after the laser beam irradiated is reflected from the surface of the information forming layer 7 is transferred to the objective lens 5 and the detection system lens (projection lens).
15 are all treated as one composite lens.

The focal length of this composite lens is fl, and the distance from the front focal position of the composite lens to the information forming layer (focal point of radar light) 2 when in focus is C. The optical path of the reflected light when the information storage medium 6 is shifted from the focal point by δ is (1)
Calculate in the same way as from formula to formula (a). If the objective lens 5 and the detection system lens 15 are all considered as one composite lens, as shown in Figure 20, h9 = -β6 When the photodetector 6 is placed at the imaging point when the positions match (when δ=0), δ
When h3=0, the imaging magnification m at this time becomes Tm=-β0/β3. Therefore, c = + f''7m However, m is always a positive number, and when '+m, it represents an inverted image.When -m, it represents an erect image.Also, the distance from the rear principal point of the composite lens to the photodetector 16 is A. Then, h3=h"-Aβ3=-β(I
+ 2δ) h3=0 at δ=0 for any β0
Okay, A=f” (1 m)... (C) Substituting the formula (C) into the formula (C), the focal position of the objective lens 5 and the convergence of the radar as shown in the 2' @ figure. If the point is shifted by b, the equations for ray tracing hold as they are by converting equations (1) to OI to 2δ+2δ+b.Therefore, from equation (5), it is still possible to convert equation (b) into equation (1). Substituting -7...(c) fo+(1-a/fo)(2δ+b) Substituting equation (c) into equation (c), especially when a = O, fo+b)>2δ Regarding the range, since equations up to Equation (31) are for the case where the photodetector 16 is placed at the position of the imaging point with respect to the information forming layer 7 of the information storage medium 6 at the time of focusing, next, In this section, an equation representing the optical behavior when the photodetector 16 is placed at a position shifted from the imaging point for the information forming layer 7 of the information storage medium 6 is derived.

The entire optical system is considered as one composite lens. At the time of in-focus, that is, when one of the focal points coincides with the position of the optical axis forming layer 7 of the information storage medium 6, the other focal point (or Let A be the distance from the above-mentioned focal point to the imaging point. As is clear from equation (c), Ao = f'' (1 king m) ·-(32).

Here, m is the lateral magnification and f* is the focal length of the composite lens. If the photodetector 16 is placed at a position that is closer to the composite lens by Δ than this position, the distance Nii A from the rear principal point of the composite lens to the photodetector 16 is A = A, -Δ = f"(1+m)-Δ ・(33) Substituting equation (33) into equation (24), each of the above = βo(f*l:f"/in 2δ+(f"(1+m
) − Δ] Substituting equation (c) here, Δ does not take a large value in equation (37), so Δ
< f *, and m ) 1 to increase the out-of-focus detection sensitivity.
Since there are many cases where , can be approximated as .

According to this equation (35), if the photodetector 16 is brought closer to the detection system lens 15 than the condensing point at the time of focusing (if Δ>0
) K is when the objective lens 5 approaches the information storage medium 6 (
When δ<0), 1h31 increases monotonically, and Ihal decreases once and then increases. The objective lens 5 moves away (
δ>0) Since the time increases quickly, the beam spot 22 on the photodetector 16 quickly protrudes from the photodetection cell 16-1, 16-2, and the optical defocus detection signal becomes small. On the other hand, in the case of Δ<0, in which the photodetector 16 is moved away from the detection system lens 15, the opposite is true. That is, when the objective lens 5 approaches the information storage medium 6 and δ<0, 1h31
once becomes 0, then becomes large, and when δ>0, 1h31 increases monotonically. Therefore, in this case, the beam spot 22 on the photodetector J6 is closer to the photodetector cell 16 than when δ<0 is farther away than when the beam spot 22 is closer to the photodetector cell 16.
~J, does not protrude outside of 16-2. Even if the objective lens 5 approaches the in-focus position with a large deviation, the optically out-of-focus detection signal remains large. The objective lens 5 moves away from the information storage medium 6 (
δ>0) and the beam spot 22 on the photodetector J6 can also be applied to the optical system shown in the photodetection cell 16-1, 16-2.

〔Effect of the invention〕

As explained above, according to the present invention, the irradiation light irradiated onto the information storage medium is condensed, and the reflected light reflected by the condensed irradiation light on the information storage medium is collected again. A light condensing means, and a position shifted from the imaging position on the information storage medium at the time when the focal point of the irradiation light condensed by the condensing means and the information storage medium coincide with each other. a photodetector having a plurality of detection cells arranged to detect the reflected light focused by the light focusing means; means for moving a beam spot of the reflected light on the photodetector in response to a shift of the information storage medium, the photodetector having at least one detection cell arranged along the optical axis of the reflected light. Since it is arranged so as to be located above, excellent effects such as being able to perform defocus detection more stably and with high reliability can be achieved.

[Brief explanation of drawings]

FIGS. 1(A), 1(B), and 1(C) are explanatory diagrams showing a first conventional optical system that detects defocus by moving the beam spot,
Figure 2 (a) (→ (c) is an explanatory diagram showing a second conventional optical system that similarly detects defocus by moving the beam spot, and Figure 3 (a), (b), and (c) are the same. A configuration diagram showing the beam science head, FIG. V is a sectional view showing the second photodetector,
Figure 1 (Ihe 1) is an explanatory diagram showing the locus of the beam in focus and out of focus;
Figure 7 and Figure Y' are diagrams for explaining the influence of the /1 f-turn on the diffraction of the tracking guide. Figure V shows the optical system for out-of-focus detection when the second photodetector is moved away from the focal point on the photodetector, and Figures 1e (a), (b), and (c) show the focal point. A diagram showing the trajectory of the laser beam when the second photodetector is placed in the first photodetector. The figure is a diagram for analyzing the trajectory of the ray passing through the objective lens, Figure 19 is a diagram for analyzing the trajectory of the ray passing through the projection lens, and Figure m20 is the focal position of the objective lens and the condensing point of the radar light. FIG. 21 is a diagram showing an optical system in which the rays are shifted from each other, and is a diagram for analyzing the trajectory of a light ray when the objective lens and the projection lens are considered as one composite lens. 6... Information storage medium, 5... Objective lens, 16...
- Second photodetector, 13... light extraction member (light shielding plate),
14... Optical axis, 16-1... First detection cell, 16
-2... Second detection cell, 21... Gap region. Applicant's agent Patent attorney Takehiko Suzue Figure 3 Figure 4 Figure 17 Figure 13 (B) (C) No. 18v! J et al. Figure 19

Claims (2)

[Claims] (1) A condensing means for condensing the irradiation light irradiated onto the information storage medium and retaking the reflected light reflected by the condensed irradiation light on the information storage medium, and this condensing means The irradiation light is placed at a position shifted from the imaging position on the information storage medium at the time when the focal point of the irradiation light and the information storage medium coincide with each other, and the irradiation light is focused by the light focusing means. a photodetector having a plurality of detection cells for detecting reflected light; and a photodetector having a plurality of detection cells for detecting the reflected light; means for moving a beam spot of the reflected light on a photodetector, and the photodetector is configured such that at least one detection cell is located on the optical axis of the reflected light. optical head. (2) The photodetector is configured such that the width of the gap area is sufficiently narrow so that the intersection between the photodetector and the optical axis of the reflected light is outside the gap area between the detection cells. optical head.