JPH03192303A - Optical element - Google Patents

Abstract

PURPOSE:To separate light beams which correspond to an ordinary light beam and an extraordinary light beam surely and spatially, and to obtain a stable reproduction signal by combining a 1st prism member which is made of an optical anisotropic medium with a 2nd prism member which is made of an isotropic medium having an isotropic refractive index. CONSTITUTION:A detecting optical element 14 is formed by joining the prism member 4 made of the optical anisotropic medium and the prism member 6 made of the optical isotropic medium with each other. A light beam which is passed through a light reflecting and transmitting layer 10 passes through a mask layer 8 and is guided to the prism 4. At this time, the light is refracted by a 1st border surface 14A, made incident on the prism 6, and refracted by a 2nd border surface 14B. One of the prisms 4 and 6 is made of the optical anisotropic medium, so when the light beam is refracted by the 1st and 2nd border surfaces 14A and 14B, the light beam is separated by the ordinary light beam 22A and extraordinary light beam 22B, which are guided in different directions, so that a detector 24 detects these ordinary and extraordinary light beams which are passed through the projection surface of the prism 6.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention relates to an optical element that separates a light beam, and in particular to an information device that reproduces information from an information storage medium using magneto-optical effect. Regarding the optics of storage/reproduction devices and the improvement of optical elements at the throat.

(Prior Art) Various types of information storage and reproducing devices for reproducing information from a magneto-optical storage medium using the magneto-optical effect are known, and various proposals have been made.

For example, JP-A-57-169H4 (υ5P448280
3) Or, JP 56-57013 (USP 43582)
00) discloses a defocus detection optical system for an information storage/reproduction device. In the optical system disclosed in this publication, a parallel plate or wedge prism with a half mirror formed on one surface is placed in the light source optical path, and the light beam reflected by the half mirror is directed toward the information storage medium. An optical system is disclosed in which a light beam reflected from an information storage medium is transmitted through a parallel flat plate or a wedge prism, and furthermore, this transmitted light beam is given astigmatism and detected by a photodetector. There is. Also, JP-A-58-171739, JP-A-59
-77649 or υ5P477144, a pair of birefringent prisms are joined so that their optical axes are orthogonal to each other, and the light beam is passed through the birefringent prisms, and the two mutually orthogonal polarized components of the light beam travel. An optical system is disclosed that separates and detects the light by changing its direction, and detects the difference in the detected light intensity as a reproduction signal.

(Problem to be solved by the invention) JP 57-169934 (USP 4482803) or JP 56-57013 (USP 4358200)
The magneto-optical information reproducing device disclosed in 2003 can downsize its defocus detection optical system, but the focus detection optical system is separate from the focus detection optical system.
It is required to provide an optical system for reproducing magneto-optical information.

Therefore, there is a problem that the optical system of the information reproducing device becomes complicated and the device itself becomes large. On the other hand,
8-171739, JP 59-77649 or U
The magneto-optical information reproducing device disclosed in SP477144 is
Only an optical system for information reproduction is disclosed, but no equivalent disclosure is made regarding defocus and tracking detection optical systems. When incorporating a defocus and tracking detection optical system into such an information reproducing device, even if an optical system using a birefringent prism is adopted as the reproducing optical system, the optical system becomes complicated and the device itself becomes large. There is a problem of

As described above, the conventional information reproducing apparatus using the magneto-optical effect has a problem in that the optical system becomes complicated and the apparatus itself becomes large.

An object of the present invention is to provide a miniaturized and simplified optical element that can miniaturize an information reproducing apparatus itself that utilizes the magneto-optical effect.

[Structure of the Invention] (Means for Solving the Problems) According to the present invention, a first medium is made of an optically anisotropic medium having a different refractive index depending on the vibration direction of light passing through the medium; a first prism member having a surface and a second surface inclined with respect to the first surface; a second prism member made of an isotropic medium having an isotropic index of refraction; It has a third surface joined to the surface and a fourth surface inclined with respect to the third surface, and the angle formed by the first surface and the fourth surface is the angle between the first surface and the second surface. An optical element characterized by comprising a second prism member whose angle is smaller than the angle formed by the second prism member with the surface is provided.

(Function) According to the optical element of the present invention, a light beam incident on the optical element is reliably separated into light beams corresponding to an ordinary ray and an extraordinary ray within an optically anisotropic medium. The separated light beam is such that the angle formed between the first surface of the first prism and the fourth surface of the second prism is the angle between the first surface of the first prism and the second surface of the first prism. This ensures that the light beams corresponding to the ordinary and extraordinary rays passing through this optical element are spatially separated outside the optical element and are detected separately by the detector. , a stable reproduction signal is generated with the signal from the detector. Moreover, since optical elements are simplified and miniaturized, (
1) The weight of the entire optical system is reduced, the optical head can be moved quickly, and information access speed is improved. (2)
Since the optical system is simplified, it becomes easy to assemble and adjust the optical system, and the cost of the apparatus itself can also be reduced. (3) Furthermore, since the number of optical parts can be reduced, the cost of the device can be kept low. In addition, since focus detection and track detection are performed using a light beam corresponding to an ordinary ray with small aberrations, the focusing means can be maintained in a focused state and a focused track state with sufficient sensitivity and reliably. .

(Embodiment) FIG. 1 schematically shows an optical system of an information reproducing apparatus using the magneto-optical effect according to an embodiment of the present invention. This optical system uses a semiconductor laser 2 as a light source that generates a substantially linearly polarized light beam, or a semiconductor laser 2 that generates a light beam and a polarizer (see Figure 1) that converts the light beam from the semiconductor laser 2 into a linearly polarized light beam. A light source device consisting of a combination of (not shown) is used. In FIG. 1, assuming that the tracking guide formed on the magneto-optical storage medium in which information is stored, that is, the optical disk 12 for magneto-optical storage, extends in the X direction, the light beam from this light source is The polarized light is generated so that its plane of polarization is inclined with respect to the X-axis or the Z-axis and forms a predetermined angle with respect to these axes, and is directed toward the detection optical element 14 .

This detection optical element 14 has a first and second interface 14A.
, 14B, one of which is made of an optically anisotropic medium and the other of which is made of optical glass or transparent plastic (acrylic, polycarbonate, A
A prism member 4.6 made of an optically isotropic medium made of BS resin, etc. is joined via the second interface 14B,
The first boundary surface 14A of the prism member 4, which faces the boundary surface 14B, has an optical axis passing through the intersection as shown in FIG. A mask layer 8 is formed on the light-transmitting layer as shown by diagonal lines, and a light-reflecting-transmitting layer 10 having a specific reflectance and transmittance is formed on the mask layer 8.
is formed.

The mask layer 8 and the light transmission reflection layer 10 may be laminated on the surface of the prism member 4 by vacuum deposition or sputtering, or the light transmission reflection layer 10 may be formed on one surface and the light transmission reflection layer 10 may be formed on the other surface. a parallel plate on which a mask layer 8 is formed;
Alternatively, a parallel plate having the light transmitting and reflecting layer 10 and the mask layer 8 formed on one surface may be bonded to the surface of the prism member 4. Furthermore, the mask layer 8 may be formed on the first parallel flat plate, and the transmissive reflective layer 10 may be formed on the second parallel flat plate, and these may be bonded together and further bonded to the surface of the prism member 4. As an optically anisotropic medium, the optical alignment can be adjusted relatively easily, and the prism member 4.6
A uniaxial crystal is preferable, considering that the characteristics do not change easily even if the medium is tilted with respect to the optical axis.Moreover, this medium is
A positive crystal having a larger refractive index for extraordinary rays than for ordinary rays is more preferable. As such a positive crystal,
Rutile or quartz, which can be produced artificially, can be used.

A part of the diverging light beam incident on such a detection optical element 14 is reflected by the light reflection/transmission layer 10,
The remaining components are transmitted through the detection element 14. Detection element 14
The light beam introduced into the interior passes through the light reflection/transmission layer 10, the mask layer 8, and the prism member 4, and the center portion where the light intensity is high is incident on the photodetector 16 provided on the side surface of the prism member 4. , the light intensity is detected. The detection signal from this photodetector 16 is transmitted to the drive circuit of the semiconductor laser 2 (
(not shown), and the light beam generated from the semiconductor laser 2 is controlled in accordance with this detection signal. That is, in the drive circuit, the detection signal is compared with the reference signal voltage, a drive current corresponding to the difference signal voltage is supplied to the semiconductor laser 2, and the semiconductor laser 2 is stably driven.

The diverging light beam component reflected by the light-reflecting and transmitting layer 10 is focused by an objective lens 18 towards a tracking guide (not shown) on the information storage medium 12 . When the objective lens 18 is placed in focus, the beam waist of the focused light beam is projected onto the reflective surface of the information storage medium 12, and the minimum beam spot is located on the information storage medium 12.
It is formed on the reflective surface of No. 2. When the objective lens 18 moves slightly closer to the information storage medium 12 along its front axis from the focused state, or when the objective lens 18 moves away from the information storage medium 12 and is out of focus, the beam waist of the convergent light beam becomes A spot larger than the minimum beam spot is formed on the reflective surface of the optical disk 12 without being projected onto the reflective surface of the storage medium 12 . The diverging light beam returned from the information storage medium 12 is converted into a convergent light beam by the objective lens 18 and is again directed via the objective lens 18 to the detection optical element 14, which reflects and transmits the light. layer 10
A part of it is reflected towards the semiconductor laser 2,
The remaining portion is moved into it.

The light beam that has passed through the light reflection/transmission 11110 is partially blocked by the mask layer 8, passes through the mask layer 8, and is guided to the prism 4. At this time, the light is refracted at the first boundary surface 14A, further incident on the prism 6, and further refracted at the second boundary surface 14B.

Since one of the prisms 4.6 is made of an optically anisotropic medium, the light beam is directed to the first or second interface 14A.
, 14B, the ordinary ray 22A and the extraordinary ray 22
These ordinary rays and extraordinary rays, which are separated into B and directed in different directions and passed through the exit surface of the prism 6, are directed to different detection areas of the photodetector 2o and detected by this detector 20. In this example, the ordinary ray 22
The defocus signal and the tracking signal are generated from the signal processing circuit 24 using A, and the detected ordinary ray 22
The difference in light intensity between the light beam A and the extraordinary light beam 22B is generated from the signal processing circuit 24 as a reproduction signal. The voice coil 19 is driven in response to the defocus signal, and the objective lens 18 or the entire optical system is driven in the optical axis direction to maintain the objective lens 18 in focus. The entire system is driven in a direction across the tracks of the information storage medium 12, the tracks are tracked by the focused light beam, and the objective lens 18 is maintained in the aligned track state. A detection signal generated from the detection area of the detector 16 while the objective lens 18 is maintained in the focused state and the focused track state is processed by the signal processing circuit 18 and recorded on the information recording medium 12. The reproduced signal that is converted into a reproduced signal corresponding to the information and generated by the signal processing circuit 24 is displayed as reproduced information on an external display device (not shown) or the like.

The refractive index for ordinary rays in an optically anisotropic medium is always constant and does not depend on the angle of the passing ray.

On the other hand, since the refractive index for the extraordinary ray 22B changes depending on the angle of the ray passing through the optically anisotropic medium, a larger aberration occurs than for the ordinary ray 22A, and the detector 20
A large aberration will appear in the beam spot formed above. In a defocus detection system that detects changes in the shape of a beam spot formed on the photodetector 20, if coma aberration affects changes in the shape of the beam spot beyond a permissible range, the defocus detection characteristics will deteriorate. Ru. Therefore, in the optical systems shown in FIG. 9, FIG. 11, FIG. 24, and FIG. It uses side light. Furthermore, when a positive crystal is used, the refractive index of the extraordinary ray 22B is larger than that of the ordinary ray 22A, and when a negative crystal is used, the refractive index of the extraordinary ray 22B is larger than that of the ordinary ray 22A.
The refractive index of the extraordinary ray 22A is larger than that of the extraordinary ray 22A.

The larger the internal refractive index of the two light beams, the larger the refractive power, and the larger the coma aberration becomes.If the value of the coma aberration increases to a certain extent, and if it exceeds a permissible range, the defocus detection characteristics will deteriorate. For this reason, as mentioned above, as the material of the optically anisotropic medium, the extraordinary ray 2'A is higher than the ordinary ray 22'A.
It is preferable to use a positive crystal with a large refractive index relative to 2B, and an ordinary ray with a small refractive index is used for defocus and tracking detection or defocus detection.

Prism member 4 of the detection optical element 14 shown in FIG.
6, a convergent light beam is incident from the objective lens 18 as already mentioned, but this convergent light beam is
As shown in FIG.
It is focused toward a focusing line defined by Pc. (3rd
In FIG. A and FIG. 3B, for convenience of explanation, the prism member 4.6 is equivalently replaced with one prism body 114. Furthermore, as described above, it is preferable to detect defocus and tracking using ordinary rays, so the drawing focuses only on ordinary rays. ) In reality, the rays La, Lb, L constituting a focused light beam
c are respectively incident on different incident points on the first interface 14A of the detection optical element 14 and refracted, and then refracted again at different incident points on the second interface 14B of the detection optical element 14, The light passes through this and is emitted from the emission surface 14C. This means that in FIG. 3B, which shows the prism body 114 equivalent to the prism member 4.6 of the detection optical element 14 from the side, the light rays La, L, b, and Le are different from each other on the surface 114A of the prism body 114. Incident points Ra, Rb, R
c, passes through the prism body 114, is refracted at the surface 114C, and is directed toward the detector 24. As is clear from FIGS. 3A and 3B, in the prism body 114, the light rays La, Lb, Lc pass through optical paths having physical distances fiA, B, C, that is, different optical path lengths to the exit surface. It reaches 114C.

This means that as shown in FIG. 3A, the rays La and Lb
, Lc are different points of incidence Ra, R along the optical axis
Since it is incident on the surface 114A at b and Re, the surface 1
When being refracted by 14A, it is separated into light rays La, Lb, and Lc that pass through different optical paths in plan view. Therefore, the light rays 1. a
, Lb, Lc are different focal points Pa, Pb, Pc
focused on.

As described with reference to FIGS. 3A and 3B, the convergent light beam is given an aberration close to coma by the detection optical element 14. Therefore, the detection surface of the photodetector 24 is at a point Da that is substantially perpendicular to the optical axis 0 as shown in FIG. 3B,
When placed on the plane defined by Db and Da, light beam spots Sa, Sb, and Sc as shown in FIGS. 4A, 4B, and 4C are formed on the detection surface of the photodetector 24. Ru. Here, the detection surface of the photodetector 24 passes through its center, and the optical axis O and the focal points Pa, Pb, Pc
It is divided into upper and lower regions by a dividing line 26 perpendicular to the dividing line 26, and divided into detection regions 24a to 24d having the same area by a dividing line 27 perpendicular to the dividing line 26. In such an arrangement of the detector 24,
When the objective lens 18 is in focus, the center line of the detection surface of the photodetector 24 is aligned with the focal point pb, and a focal point Pa is formed slightly in front of the detection surface of the photodetector 24. 2
When the focal point Pc is formed slightly behind the detection surface of the photodetector 24, a point-like spot corresponding to the focal point pb is formed on the center line of the detection surface of the photodetector 24. Also, the focal point Pa
Since a diverging light beam is incident on the areas 24a and 24b above the detection surface of the photodetector 24, a beam segment spot 5a-L is formed, and a convergent light beam heading toward a convergence point Pc is photodetected. Since the regions 24 c and 24 d below the detection surface of the device 16 are irradiated, a beam segment spot 5 a - 2 is formed. 2nd light in the middle of the optical path
A mask layer 8 is provided in which regions corresponding to the first and third quadrants divided by mutually orthogonal dividing lines as shown in the figure are formed of a light-transmitting layer as indicated by diagonal lines, and therefore, Since a portion of the light beam is blocked by the mask layer 8, a beam spot Sa having a shape corresponding to a half of a figure 8 as a whole is formed on the detection surface of the photodetector 24. On the other hand, when the objective lens 18 approaches the information storage medium 12 and is in an out-of-focus state, the convergence of the light beam from the objective lens 18 is weakened, so that the convergence point Pa is As it approaches the detection surface of the photodetector 24, the focal point pb is shifted to the rear of the detection surface of the photodetector 24, and the focal point Pc becomes further away from the detection surface of the photodetector 24. Therefore, as shown in FIG. 4B, the photodetector 24
The beam segment spots 5b-t formed in the areas 24a and 24b above the detection surface of the photodetector 24 are formed smaller than the beam segment spot 5a-1 at the time of focusing, and the area 24C924d below the detection surface of the photodetector 24 is formed smaller than the beam segment spot 5a-1 at the time of focusing. The beam segment sub-section 5b-2 formed is larger than the beam segment sub-section 5a-1 at the time of focusing. Furthermore, when the objective lens 18 is in an out-of-focus state where it is far away from the information storage medium 12, the light beam from the objective lens 18 has a strong convergence, so that the convergence point P
a becomes further away from the detection surface of the photodetector 24, the focal point pb is shifted to the front of the detection surface of the photodetector 24, and the focal point Pc approaches the detection surface of the photodetector 24. Therefore, as shown in FIG. 4C, the beam segment spot 5c-1 formed in the areas 24a and 24b above the detection surface of the photodetector 24 is formed larger than the beam segment spot 5a-1 when focused. The beam segment spot 5e-2 formed in the region 24C124d below the detection surface of the photodetector 24 is formed larger than the beam segment spot 5a=2 when focused.

As shown in FIG.
4d is connected to the first adder 44 and its detection area 2
Detection signals from 4a and 24d are added by a first adder 44. Furthermore, the detection areas 24b, 24 of the photodetector 24
C is connected to the second adder 46 and its detection area 24
The detection signals from b and 24C are added by a second adder 46. The summed signals from the first and second adders 44, 46 are input to a differential amplifier 48, and the difference therebetween is amplified and generated as a focus control signal.

As is clear from the explanation of FIG. 4A, at the time of focusing, the detection signals from the detection areas 24a and 24b are equal to each other,
Furthermore, the detection signals from the detection areas 24c and 24d are equal to each other. Therefore, during focusing, the operational amplifier 48 generates a zero-level focus control signal indicating focusing. On the other hand, when the objective lens 18 approaches the information storage medium 12 and is in an out-of-focus state, the first The detection areas 24a and 24d are
The focus control signal is smaller than the first sum signal obtained by adding the detection signals from the first adder 44 by the first adder 44, and the operational amplifier 48 generates a focus control signal of, for example, a negative level. When the objective lens 18 is in an out-of-focus state away from the information storage medium 12, the detection area 2411. 24
The first sum signal obtained by adding the detection signals from the detection regions 24b and 24c by the first adder 44 becomes larger than the second sum signal obtained by adding the detection signals from the detection regions 24b and 24c by the second adder 46. For example, a negative level focus control signal is generated from the operational amplifier 48.

As shown in FIG. 5, when the light beam is diffracted by the tracks of the information recording medium 12, a diffraction pattern 42 is generated as a dark part in the beam spot Sa formed on the detection surface of the photodetector 24. In order to detect this diffraction pattern 42, as shown in FIG.
, 24b are connected to a third adder 36, and the detection signals from the detection areas 24a24b are added by the third adder 36. Furthermore, the detection areas 24c, 24 of the photodetector 24
d is connected to the fourth adder 38 and its detection area 24
The detection signals from c and 24d are added by a fourth adder 38. The third and fourth summed signals from the third and fourth adders 36, 38 are input to a differential amplifier 40, where the difference is amplified and generated as a track control signal.

In a track-aligned state where the tracks of the information storage medium 12 are accurately tracked by the light beam, a diffraction pattern is generated symmetrically with respect to the dividing line 26, as shown in FIG. Third and fourth summation signals of equal level are therefore generated from the third and fourth adders 36,38, and the operational amplifier 40
A zero level track control signal is generated.

On the other hand, in the valve stand track state in which the light beam is slightly deviated from the center of the track of the information storage medium 12, the diffraction pattern 42 is detected by the photodetector 24 in the light beam spot Sa.
detection area 24a, 24b or detection area 24C32
shifted slightly to either side of 4d. Therefore, from the third and fourth adders 36.38 the third
and a fourth addition signal are generated, and the operational amplifier 40 generates a track control signal of a plus or minus level.

Detection areas 24.24b, 24c of the photodetector 24.

24d is connected to an adder 50 as shown in FIG. On the information storage medium 12, the light beam focused on the information storage medium 12 generates the information recorded on the track of the optical storage medium 12, that is, the light beam incident on the information storage medium 12 according to the direction of the magnetic field of the magnetic domain. The plane of polarization is slightly rotated. The light beam whose polarization plane has been rotated is transmitted to the detection optical element 1
4, the light beam is separated into an ordinary ray and an extraordinary ray according to the rotation of the plane of polarization, that is, according to the recorded information. The ordinary ray is
As already explained, the detection area 24a324 of the detector 24
b, 24c, and 24d, and the extraordinary rays are detected by the detector 2.
The light is incident on the detection area 24e of No. 4. In order to obtain the difference between the two light beams, the sum signal from the adder 50 and the signal from the detection area 24e are input to the comparator 62. Therefore, comparator 62
A reproduced signal is generated as a difference signal.

In the defocus detection method described above, it is preferable to detect defocus using ordinary rays, especially for the following reasons. In the detector 24 as shown in FIG. 5, the spot size of the beam spot in the short axis direction (X direction) generated during focusing has a great influence on the focus detection sensitivity. Therefore, in the optical system shown in FIG. 1, it is preferable that the aberration in the X direction is small. It is known that the refractive index of an extraordinary ray passing through an optically anisotropic medium changes depending on the direction in which it passes, and the change in the value corresponds to the change in the direction across the index ellipsoid. Therefore,
When detecting defocus using extraordinary rays, aberrations are imparted to the extraordinary rays, causing erroneous detection of defocus. On the other hand, with ordinary rays, the refractive index is always constant within the optically anisotropic medium, so no aberration is imparted, and defocus can be detected accurately.

For the same reason, it is preferable to perform tracking detection using ordinary light. That is, when the beam spot size in the long axis direction (Y direction) of the light beam spot on the detector 24 is small, if the aberration is too large, the light rays will cross in the middle of the optical path, and the tracking guide will be detected by the so-called bush-pull method. I become unable to do so. In track deviation detection using the bush-pull method, the light intensity pattern of the light beam reflected from the information storage medium 12 and passed through the objective lens 18, that is, the intensity change of the diffraction pattern from the tracking guide, is detected in two light detection regions. . Normally, a detection lens is used to reduce the spot size and project the light beam onto a small photodetector 24, but this causes large aberrations in the extraordinary rays and causes the objective lens 1
When the light beam that has passed through the photodetector 24 crosses in the middle of the optical path and enters the detection area on the opposite side of the detector 24, the intensity distribution of the pattern on the photodetector 24 becomes the light intensity distribution immediately after passing through the objective lens 18. Unlike this, the characteristics of the track deviation detection signal deteriorate. Therefore, the beam spot size in the long axis direction (Y direction) of the light pattern on the photodetector 24 is small, and light is detected due to the effect of the refractive index change depending on the traveling direction of the extraordinary ray passing through the optically anisotropic medium. If the intensity distribution of the pattern on the device 24 is significantly different from that immediately after passing through the objective lens 18, it is preferable to perform focus detection and track deviation detection using only ordinary rays.

In the photodetector probe 14 shown in FIG. 1, a prism 4. made of an optically anisotropic medium and an optically isotropic medium.
6 is used, but it is not preferred to make both prisms 4.6 of an optically anisotropic medium. That is, if both prisms 4.6 are made of an optically anisotropic medium and arranged so that their optical axes are perpendicular to each other, a large separation angle can be given to the extraordinary ray and the ordinary ray. However, when the ordinary rays and extraordinary rays separated within a prism made of an optically anisotropic medium enter an optical element made of another optically anisotropic medium, the arrangement of that optical element changes. This is because the ordinary ray acts as an extraordinary ray, and the above-mentioned aberration is imparted to the ray to be detected, causing erroneous defocus detection.

Therefore, the detection optical element 14 in the present invention is made by bonding an optically anisotropic medium and an optically isotropic medium such as optical glass or transparent plastic made of acrylic, polycarbonate, ABS resin, etc. The feature is that it enables the existence of light rays whose refractive index does not change depending on the direction in which they travel, such as ordinary rays.

The spot size of the light pattern on the photodetector in the long axis direction (Y direction) is large, and the intensity distribution of the pattern on the detector 24 of the extraordinary ray does not change much compared to immediately after passing through the objective lens 18. If not, defocus may be detected using the ordinary ray, and track deviation may be detected using the extraordinary ray.

In such an embodiment, as shown in FIG. 6, the area for detecting extraordinary rays is separated into two, and this detection area
-1 and 24e-2 are connected via a preamplifier to a comparator 48 that generates a track deviation signal, respectively, and are also connected to an adder 70. Detection regions 24a, 24d are directly connected to each other and to a comparator 40 via a preamplifier, and detection regions 24b, 24c are likewise directly connected to each other and connected to a comparator 40 via a preamplifier. . The circuit shown in FIG. 9 generates a defocus signal, a track deviation signal, an information reproduction signal, and a preformat signal through operations similar to those of the circuit shown in FIG. 5, and therefore a description thereof will be omitted.

According to the circuit configuration shown in FIG. 9, the circuit can be simplified and the number of parts can be reduced compared to the circuit shown in FIG. 5.

The detection optical element 14 shown in FIG. 1 will be considered. In the detection optical element 14, the light beam from the information storage medium 12 is separated into an ordinary ray and an extraordinary ray in order to reproduce information.
The difference in refractive index of the medium in A and 14B is utilized.

Therefore, the larger the angle formed by the first and second boundary surfaces 14A and 14B, the more reliably the light beam can be separated into the ordinary ray and the extraordinary ray. As shown in FIG. 7, when a light ray passes through a wedge-shaped prism 70, the deflection angle of the emitted light ray relative to the incident light ray, that is, the deflection angle δm of the light ray is θ1−θ1°, θ
It is known that it becomes the minimum when r-θr'. Therefore, as shown in FIG. 8, the deviation angle δ■ can be increased as the inclination angle of the exit surface is made larger than the inclination angle of the entrance surface with respect to the plane perpendicular to the optical axis. Therefore, if the optical system shown in FIG. 9 is replaced with the optical system shown in FIG. 1, it is possible to obtain a larger separation angle between the ordinary ray and the extraordinary ray. That is, in FIG. 9, an optically anisotropic medium is provided between a first boundary surface 14A and a second boundary surface 14B, and is perpendicular to the optical axis with respect to the laser beam passing therethrough. The second boundary surface 14B is arranged so that the second inclination angle with respect to the music to be played is large, and the second boundary surface 14B is arranged so that the second inclination angle with respect to the plane perpendicular to the optical axis is larger than the inclination angle of tJJl. The first boundary surface 14A is inclined in the same direction as the boundary surface. More preferably, the separation angle between the ordinary ray and the extraordinary ray is determined to be large in this way. In addition, in this defocus detection method, in order to obtain desired defocus detection characteristics, the spot size in the long axis direction (Y direction) and short axis direction (X direction) of the light pattern on the photodetector is determined at the time of focusing. . Therefore, the angle between the entrance surface (14A) and the exit surface (14C) of the detection optical element 14 with respect to the optical axis is determined. When the first boundary surface 14A is used as a reference, the inclination angle of the second boundary surface 14B is determined to be large in order to increase the separation angle between the ordinary ray and the extraordinary ray as described above, but compared to this inclination angle, the inclination angle is relatively large. third boundary surface 14
The inclination angle of C becomes smaller.

The optically anisotropic medium constituting any of the prism members 4.6 has an optical axis (defined as the long sleeve of the refractive index ellipsoid of the optically anisotropic medium) as follows. Preferably, the arrangement is such that one of the following conditions is satisfied.

(1) The optical axis of the optically anisotropic medium is arranged at an angle close to a right angle to the traveling direction of ordinary rays passing through the optically anisotropic medium. Specifically, the direction or plane in which this optical axis is arranged corresponds to the X-axis direction or the Y-Z plane in the optical system shown in FIG. is defined as the direction perpendicular to the traveling direction of the ordinary ray passing through the ray, and by such an arrangement, the refractive index nc of the extraordinary ray with respect to the medium can be maximized, and the refractive index no of the ordinary ray with respect to the medium can be maximized. The difference can be maximized. Therefore, even if the angle that the first boundary surface 14A forms with the second boundary surface 14B is relatively small, the separation angle between the ordinary ray and the extraordinary ray can be made sufficiently large. By making the angle formed between the first boundary surface 14A and the second boundary surface 14B relatively small, the overall thickness of the detection optical element 14 can be reduced, and as a result, light is detected during focusing. The amount of comatic aberration occurring in the light beam/<turn formed on the device 24 can be sufficiently reduced,
It is possible to reduce the deterioration of the focus +r t characteristics caused by this coma aberration. Furthermore, since the optical path length within the detection optical furniture can be kept short, the entire optical system can be downsized.

2) The optical axis of the optically anisotropic medium is the first interface 14A.
Or the second boundary surface 14B or the third boundary surface 14
placed parallel to C. When manufacturing any of the prism members 4.6 with an optically anisotropic medium, the first, second and third interfaces 14A, 14B, 14C are polished, but the optical axis is the first interface. By setting it parallel to the surface 14A, the second boundary surface, and the third boundary surface, processing of the optically anisotropic medium is facilitated, its manufacturability is improved, and the cost as a single component is reduced. can be done. Two methods of setting the optical axis parallel to the second boundary surface 14B include a method of setting the optical axis in the X-axis direction, and a method of setting the optical axis in a direction tilted at 45 degrees with respect to the Y-axis within the Y-Z plane. There is.

(3) The optical axis of the optically anisotropic medium is arranged at an angle of 45 degrees with respect to the S-polarized light component or the P-polarized light component of the light beam incident on the light transmission and reflection layer 10. Specifically, this optical axis is set in a direction inclined at 45 degrees with respect to the X-axis or the Y-axis within the X-Y plane. The laser beam generated from the semiconductor laser 2 has a substantially elliptical cross-sectional shape, and the focused spot formed on the information storage medium 12 is also formed in an elliptical shape having a major axis and a minor axis. Generally, in order to improve the recording density on the information storage medium 12, it may be necessary to set the long axis direction of the focused spot perpendicular to the direction in which the tracking guide of the information storage medium 12 extends. In the semiconductor laser 2, the direction of the polarization plane of the generated laser beam is generally parallel to the extending direction of the active layer. Therefore, the polarization plane of the laser beam reflected from the information storage medium 12 is It becomes parallel to the S-polarized light component or the P-polarized light component of the light beam incident on the light beam 10. In order to separate the light beam reflected from the information storage medium 12 into ordinary rays and extraordinary rays in the anisotropic storage medium 14, the optical axis is arranged at an angle of 45 degrees with respect to the light transmitting and reflecting film 10. required to be present.

From the conditions (1) and (2), the optical axis of the medium is parallel to the S polarization direction of the light beam incident on the first boundary surface 14A, that is, in the optical system of FIG. 1, the optical axis of the medium is parallel to the X axis. It is preferable that the condition (3) is satisfied.

In the embodiments shown in FIGS. 1 and 9, the optical axis is defined parallel to the Y-2, where the direction of the component (X-axis direction) matches the direction of the electric displacement De of the extraordinary ray with respect to the optically anisotropic medium, and the direction of the P component (Y-axis direction) and the electric displacement Do of the ordinary ray are the same. defined within a plane.

In order for the ordinary ray and the extraordinary ray to be detected as having the same intensity by the photodetector 24 at the time of focusing, the polarization plane of the laser beam is tilted with respect to the X axis or the Y axis, and the semiconductor laser 2
It is required that a laser beam be generated from. Here, the S component (amplitude component of the electric field) and P component (amplitude component of the electric field) of the light beam generated from the semiconductor laser 2 and before reaching the light transmitting and reflecting layer 10 are respectively I se l
ml and Ipe'"1, and the reflection coefficient (amplitude reflectance) and transmission coefficient (amplitude transmittance) of S-polarized light and P-polarized light on the light transmitting and reflecting surface 10 are respectively R5+RP+"r
s+T, the light intensity reflectance of the information storage medium 12 is r2, the light intensity reduction rate of the light beam when passing back and forth through the objective lens 18 is g2, and S-polarized light and P-polarized light generated during reflection and transmission in the light reflection-transmission layer 10. If the amount of phase shift between the components is δ, when the recording layer of the information storage medium 12 is not magnetized or when a nonmagnetic film is used as the recording layer, the information is reflected from the information storage medium 12 and the object lens 18 ,
The prism 4 passes through the light transmission reflection layer 10 and the mask layer 8.
．． Amplitudes I of the ordinary and extraordinary rays of the light beam guided into the optically anisotropic medium of 6. , I[! is expressed by the following formula.

I E -T5 l *reR5e l
s* 61 (wl+J) 10=TP 1*r conflict Rp
Ip me'-'Therefore, the light intensity Il of the ordinary and extraordinary rays: 12. l Io 12 is represented by the following formula.

III l 2 mTs”e 12 a r
2 dispute R, 2 # 1.21 (, l 2 m7,
2a 12 e r2 aRP2* Ip2 As mentioned above, in the differential detection method that utilizes the magneto-optical effect and generates the difference in light intensity between the ordinary ray and the extraordinary ray as a reproduction signal, the light intensity 1. , IO satisfies the condition of llI −1°, the C/N ratio can be maximized. Furthermore, it has been experimentally confirmed that a relatively good C/N ratio can be obtained by this differential detection method when the ratio of 1g12/11o12 is in the range of 1/2 to 2.0. Furthermore, experimentally, l
IE 12/l Io 12 ratio from 174 to 4.0
It has been confirmed that signal detection is possible using this differential detection method even in the range of .

Therefore, the ratio Is 12/l Io 12-TS2R321s2/T
The following inequality holds true as P2R and 2Ip2.

1/4 ≦Ts 2 R52I s 2 /Tp 2
Rp 21p 2 <4. O...(1) Preferably,
The following inequality holds.

1/2 ≦TS”R821S2/ Tp2Rp21p2 <2.0 (2) The optical characteristics of the light reflective/transmissive layer 10 and the polarization plane of the laser beam generated from the semiconductor laser 2 are adjusted to satisfy these conditions. The direction of (1
) and (2), the polarization plane I3 of the laser beam reflected from the information storage medium 12 and entering any prism member 4.8 made of an optically anisotropic medium.
This means that the angle θ formed by the direction of the electric displacement I0 of the ordinary ray and the direction of the electric displacement I0 of the ordinary ray satisfies the following inequality as shown in FIG.

j a n ”'J 1/4 < θ < t a
n-'J4 or j a n-'J l/2 <
θ <tan-1v'2 In the differential exposure method, it is preferable that the light intensity changes Δ1lol" and ΔIIEI2 for the ordinary ray and the extraordinary ray on the information storage medium 12 are approximately equal. Here, (1) and In equation (2), Ts TP and R5 TP hold, and I
If set to sta I, Δ1lol": Δ11E
+2 is established. Light intensity change ΔIro 12 ,
In the case where ΔIIH+2 is made almost equal, if the values of Ts and T are significantly different, 1, or if the values of R5 and R are significantly different, the values of ■ and ■ are changed to T,
It is necessary to make a large difference in relation to the values of , ``r'' or ``R'', but when designing an optical system with different values of 15 and 1. If the adjustment in step 2 is slightly incorrect, the polarization plane of the laser beam will be rotated,
There is a possibility that the values of Is and IP may deviate from the set values and the above-mentioned conditions may not be satisfied. On the other hand, the light transmitting reflective layer 10
satisfies Ts TP and R, 2R, and l5
When set to g1p, even if the adjustment of the semiconductor laser 2 is slightly out of order and the polarization plane of the laser beam is rotated,
Deterioration of the reproduced signal can be kept to a minimum.

As described above, the non-polarized beam splitting layer is used as the light reflection/transmission layer 10, and by setting the plane of polarization of the light beam from the semiconductor laser 2 so that the ordinary ray and the extraordinary ray are equal when they are detected. It is possible to stabilize the reproducing circuit, and to set a large tolerance when adjusting the sieving of the optical system.

In the optical system shown in FIG. 1 or FIG. 9, a non-polarized beam split layer is used as the light transmitting and reflecting layer 10, but the laser beam generated from the semiconductor laser 2 or The polarization plane of the laser beam generated from the semiconductor laser 2 and immediately after passing through the polarizer (not shown) is the light transmission/reflection plane shown in FIG. 1 or FIG. 9 around the optical axis of the optical system. rotated by 45 degrees with respect to the inclination direction of the entrance surface of the layer 10,
It is tilted at 45 degrees with respect to the direction of the electric displacement DO of the ordinary ray or the electric displacement De of the extraordinary ray in the optically anisotropic medium.

The light transmitting and reflecting layer 10 under the conditions already explained is T S
Although it is possible to set = 1 so that 2 T p and Rs ” Rl are satisfied, strictly T s =
It is difficult to design such that T p and U Rs -RP are satisfied.

In the optical systems shown in Figures 1 and 9, one of the first and second prisms 4.6 is made of an optically anisotropic medium, and the other is made of an optically isotropic medium. Although it is made,
This is based on the following reasons. When a parallel light beam that passes through a wedge-shaped prism made of an optically anisotropic medium is focused by a condensing lens and incident on a photodetector, Ordinary rays and extraordinary rays can be reliably separated using only a wedge prism.

However, if only a convergent light beam, or in some cases a diverging light beam, is incident on a photodetector through a wedge prism made of an optically anisotropic medium, In this case, each beam spot formed by the ordinary ray and the extraordinary ray becomes too long, and there is a possibility that they may overlap each other. Prisms made of optically isotropic media are arranged so that the beam spots do not overlap with each other. When two pieces of a wedge prism made of an optically anisotropic medium and a wedge prism made of an optically isotropic medium are joined together, it looks like a thick, almost parallel glass plate when viewed from the outside. Therefore, when a condensing light beam is passed through the photodetector, the spot size cannot be widened to a large extent. On the other hand, the Taber angle of a wedge prism made of an optically anisotropic medium is large, so the separation angle between the ordinary and extraordinary rays is wide. A short beam spot of an appropriate size is formed by a prism, the beam spots are reliably separated on the detector, and defocus is reliably detected by detecting one of the beam spots. When the leading beam is incident on a parallel plate arranged at an angle, the incident light beam and the outgoing light beam maintain a parallel relationship.Therefore, as shown in FIG. 1 or FIG. detected element 14
When the first boundary surface 14A and the third boundary surface 14C are arranged in parallel, the photodetector 24 at the time of focusing
A relatively small beam spot is formed above. On the other hand, the second boundary surface 14B and the third boundary surface 14C
In order to widen the angle in the traveling direction between the ordinary ray and the extraordinary ray while keeping the rays substantially parallel to each other, the first and second boundary surfaces 14 are
A.

If the angle formed by 14B is increased, the beam spot formed by both light beams will be formed longer, and there is a possibility that they will overlap each other. For this reason, the angle formed by the second boundary surface is set larger than the angle formed by the photobleeding 14C with respect to the first boundary surface 14A, and the angle between the first boundary surface 14A as a light incidence surface and the light The injection bleeding 14C is arranged so as to be almost parallel to each other, and the first boundary surface 14A and the second boundary surface 1
A sufficient angle is given to the surfaces of both 4B and 4B to constitute a separating optical system, so that the light beam is reliably separated into an ordinary ray and an extraordinary ray, and the beam spot formed by both rays is made small. There is.

In the optical system shown in FIGS. 1 and 2, the first
Although it is assumed that the prism 4 is made of an optically anisotropic medium and the second prism 6 is made of an optically isotropic medium, it is clear that the first prism 4 is made of an optically anisotropic medium. The second prism 6 may be made of an optically anisotropic medium, and the second prism 6 may be made of an optically anisotropic medium. Furthermore, in the optical system shown in FIGS. 1 and 2, an example is shown in which a pair of prisms are combined, but the detection optical element 14 is not limited to a pair of prisms, but can also be an optical anisotropic element. It may also be a laminated structure in which a plurality of prisms made of an optically isotropic medium and an optically isotropic medium are combined. The prisms 4.6 are glued together,
For example, the surfaces may be joined and fixed using an ultraviolet curable adhesive, or the surfaces may only be in contact with each other and the periphery may be fixed with adhesive or a mechanical member such as a screw. Also good.

In the detection optical element 14 shown in FIGS. 1 and 9, the photodetector 16 has a PIN structure in which a silicon chip is placed in a package, and this is fixed to the side surface of the prism member 4. However, this is not limited to the first
As shown in FIG. 1, the photodetector 16 may be provided from the side surface of the prism member 4 to the surface of the prism member 6. In addition, in FIG. 11, the reference numeral 72 indicates the detection optical element 14.
This figure shows a coma correction plate that corrects coma aberration that may be imparted to ordinary rays by. When the detector 16 is arranged in this way, it is preferably formed as a membrane structure. That is, a transparent conductive layer film such as a Nesa film is formed as a base on the surface to be formed, and a P-N junction structure is formed using amorphous silicon on this to form a photodetector. Also, as this structure, T
A photoconductive film made of an organic photoconductive film such as e, Cds, or copper phthalocinine may be formed in a laminated structure sandwiched between a transparent conductive film such as a Nesa film and a conductive film such as A1 or Cu. . Regardless of its structure, the photodetector 16 generally has reflective properties, such that a portion of the incident light beam is reflected by the photodetector 16.
This reflected light beam is reflected by the semiconductor laser 2 or
There is a risk that the light may enter the photodetector 24 and the operation of the semiconductor laser 2 may become unstable, or that noise may be mixed into the signal generated from the photodetector 24. In this way, the photodetector 1
In order to suppress the reflected light beam from the prism 4.6 from being directed toward the photodetector 24 or the semiconductor laser 2, the surface of the prism 4.6 on which the photodetector 16 is provided is formed into a diffuse reflection surface, that is, a rough surface. It is preferable that Alternatively, the optical path of the reflected beam from the photodetector 16 is made different from the optical path of the laser beam from the semiconductor laser 2 toward the photodetector 16, and the optical path from the information storage medium 12 to the photodetector 24 is made different. That is, it is preferable that the optical system is configured so that the optical paths of the ordinary ray and the extraordinary ray within the detection optical element 14 are different. For example, by arranging the surface of the detector 16 substantially parallel to the optical axis passing through the objective lens 18 and the detection optical element 14, the reflected light beam reflected from the photodetector 16 is transmitted from the semiconductor laser 2 to the photodetector 16. The directed laser beam and the ordinary and extraordinary rays in the detection optical element 14 will be directed in a different direction.

The mask layer 8 of the detection optical element 14 includes a light-blocking area made of a light-absorbing material in order to prevent the light rays reflected from the light-blocking area from forming an unnecessary focused spot on the information storage medium as stray light. It is preferable to have. Even if this light-shielding area has a slight reflective property and slightly reflects the incident light that enters this area, the light intensity of the reflected light ray is kept to 1/1o or less of the light intensity of the incident light ray. It should be done. Therefore, this light-shielding region is required to be made of a light-absorbing material with a light reflectance of 10% or less. Further, in the case where this light-shielding area is made of a light-absorbing member, and this light-absorbing member has a property of slightly transmitting light, the light transmittance of this light-absorbing member is 20% or less. It has been experimentally determined that this is preferable. This is because the light beam transmitted through the light absorbing member is detected by the detector 24 and the defocus detection sensitivity is degraded, but this degradation in detection sensitivity is allowed up to about 80%. This mask layer 8 is made, for example, by a lift-off method. That is, the first interface 14
A photoresist layer is formed on A, a mask pattern is placed thereon, and a negative pattern is formed by exposure and development. After a light absorption layer is formed on this negative pattern, the photoresist layer is removed using a solvent to form a desired mask layer 8. In the mask layer 8 having such a structure, the area other than the light-shielding area is defined as a light-transmitting area, and in this area, the boundary surface 14A of the direct node 1 is exposed, and light is reflected on the exposed light-transmitting area. A transparent layer, that is, a non-polarization splitting layer 1o is formed.

In the optical systems shown in FIGS. 1 and 9, it has been explained that the optically anisotropic medium is made of a material such as quartz crystal or rutile, but these materials such as quartz crystal or rutile are A laser beam that passes through an optically anisotropic medium has a chromatic dispersion of 6, a positive Abbe number, and a property that the refractive index of the passing light ray increases as the wavelength of the passing light ray becomes shorter. Even if the wavelength of not only ordinary rays but also extraordinary rays is slightly changed, the traveling direction of the rays after refraction is changed. Therefore, the center of the optical pattern on the photodetector 24 shifts due to wavelength fluctuations caused by the output chemotaxis of the laser beam generated from the semiconductor laser 2, which adversely affects detection of defocus and track deviation. There is a possibility. According to this invention, in the detection optical element 14 having a structure in which an optically anisotropic medium and an optically isotropic medium are joined, the separation angle between the ordinary ray and the extraordinary ray is set to be large as described above. At the same time, the spot size in the elongated direction on the photodetector 14 is reduced to reduce the two spot sizes to prevent overlap between the two spots and also perform color correction. That is, when the wavelength of the laser beam changes, the direction of deflection in the traveling direction that the laser beam receives after passing through the optically anisotropic medium and the direction of deflection that the laser beam receives after passing through the optically isotropic medium become opposite to each other. In this configuration, changes in the traveling direction of the laser beam after passing through the optical detection optical element 14 are offset against variations in the wavelength of the laser beam. Rutile or
Like crystal, general optical glass also has a positive Abbe number (the shorter the wavelength, the higher the refractive index). By reversing , the above-mentioned countervailing effect can be obtained. That is, in FIG. 1 or FIG. 9,
The wedge angle formed by the first boundary surface 14A and the second boundary surface 14B on both sides of the optically anisotropic medium, and the wedge formed by the second boundary surface 14B and the third boundary surface 14C on both sides of the optically isotropic medium. This is achieved by reversing the direction of the corners. When quartz is selected as the optically anisotropic medium, the chromatic dispersion of the quartz is
Because it is relatively small, it is suitable for BK7 series, BK series, PK series, FK series, PSK series, SK series, BaK series, K series, LaK series, SSK series, Ba
LF-based, KF-based, LaK-based, and La5K-based materials are considered suitable as materials for optically isotropic media. On the other hand, rutile has a large refractive index and large refractive index fluctuations due to wavelength changes, so the Atsube number is small. Also suitable for SF series, L
cF system, Ba5F system, BaF system, F system, LF system, LLF
A transparent plastic material may be suitable.

In the embodiment described above, a non-polarizing beam splitter layer is used as the light reflective/transmissive layer 50, and the optical axis of the optically anisotropic medium is in the S polarization direction (as shown in FIG. 1) with respect to the first boundary surface 14A. The optical system is arranged so that the plane of polarization of the laser beam generated from the semiconductor laser 2 is tilted by 45 degrees with respect to the X axis and the Z axis, respectively. A laser beam generated from a semiconductor laser generally has an elliptical beam cross section, and its plane of polarization is generally parallel to the short axis of the ellipse. In the optical system shown in FIG. 1, the two orthogonal boundaries of the mask layer 8 are defined parallel to the X-axis and the Y-axis, respectively, so that they pass through the objective lens reflected from the information storage medium 12. The cross-section of the light beam after this has an elliptical intensity distribution, the long sleeve of which is tilted at 45 degrees with respect to the two border lines of the mask layer.

In the embodiment of the present invention, the long axis of this elliptical intensity distribution is arranged to cross the light transmitting region of the mask layer 8, as shown in FIG. As a result, the intensity of the light beam passing through the mask layer 8 is increased compared to the case where the long sleeve direction is arranged so as to cross the light absorption region, and the C/N ratio of the reproduced signal can be improved.

Further, in the embodiments described above, a non-polarizing beam splitter layer is used as the light reflective/transmissive layer, but in another embodiment of the present invention, this light reflective/transmissive layer has polarization characteristics and is used for magneto-optical detection. An optical system that improves the C/N ratio of the signal may be used. In other embodiments, the polarization characteristics of the light reflective/transmissive layer include a reflectance of S-polarized light of 20% or less (preferably 0%) and a transmittance of 80% or more (preferably 100%).
), and the reflectance of P-polarized light is 90% from 5096, and
The transmittance is set from 50% to 10%, and the polarization direction of the laser beam immediately after being generated from the semiconductor laser element is aligned with the Z-axis direction. Another method is to increase the reflectance of P-polarized light to 20
% (preferably approximately 0%), and transmittance is 80% or more (preferably approximately 0%).
Preferably, there is a method in which the reflectance of S-polarized light is set from 50% to 90% and the transmittance is set from 50% to 10% at 100%), and the polarization direction of the laser light immediately generated from the semiconductor laser 2 is adjusted to the X-axis direction. . In any of the embodiments, in this case, the direction of the optical axis of the optically anisotropic medium when viewed from the Z-axis direction and projected onto the X-Y plane is inclined at 45 degrees with respect to the X-axis and Y-axis. ing. That is, the method of determining the optical axis of the optically anisotropic medium is determined by (2) and (2) of the methods described above.
3), the direction of the optical axis is parallel to the first boundary surface, and the direction when projected onto the X-Y plane when viewed from the Z-axis direction is 45 degrees with respect to the X-axis and Y-axis. It is tilted.

The present invention is not limited to the defocus detection systems shown in FIGS. 1 and 9, but may be applied to conventionally known defocus detection optical systems. FIG. 11 shows an embodiment in which the astigmatism method is adopted as the defocus detection optical system. In this embodiment, two optically anisotropic media 4.6 are superimposed to generate astigmatism in the light beam passing through the optically anisotropic media 4.6 (astigmatism method is used). (When used, the two are not a combination of an optically anisotropic medium and an optically isotropic medium, but are both optically anisotropic media, and are combined so that their optical axes are perpendicular to each other.) Signal detection For this purpose, the laser beam is divided into two, an ordinary ray and an extraordinary ray. In this optical system, coma aberration occurs simultaneously with astigmatism, but this coma aberration is removed by the light beam passing through an inclined parallel flat plate 37 for correcting coma aberration. At the time of focusing, a pattern as shown in FIG. 13 is formed on the photodetector 24.

This photodetector 24 has detection areas 24a, 24b, and 24c divided by dividing lines that are orthogonal to each other.

24d, and the image 42 of the tracking guide on the information storage medium appears on the detector 24 at 0°T'' along one dividing line.
It is formed in the direction shown. In this photodetector 24, as already known, detection areas 24a and 24b
, 24 c, 24 d, and 24 e respectively^,
13. C, D, and E, the focus detection signal is (A
+C)-(I3+D), the track deviation detection signal is
From (A+13) - (COD), a reproduced signal using the magneto-optical effect is given by (A+B+C+D)-1E.

Furthermore, FIG. 14 shows an embodiment in which the Knifezi method is adopted as the defocus detection method. In this embodiment, unlike the embodiment shown in FIG. 1, the boundary surface of ff1l and the third boundary surface are formed substantially parallel to each other. As a result, at the time of focusing, a light pattern with a small width is formed on the photodetector 24, and the defocus detection sensitivity by the Knifezi method is improved. A mask layer 8 is formed in the second interface between the prism member 4 made of optically isotropic medium and the prism member 6 made of optically anisotropic medium, thereby providing a detection Manufacturability of the optical element 14 can be further improved. As shown in FIG. 15, the mask layer consists of a light transmitting area 8A and a light absorbing area 8B, and detects areas 24a, 24b, 24c, and 24d of the photodetector 24 during focusing.
A light pattern having the shape shown in FIG. 16 is formed thereon.

The output signals from the detection area of the photodetector 24 are A, B,
Assuming C, D, and E, the focus detection signal is (^+B)-(
COD), the track deviation detection signal is (^+C')
-(BAD) and signal detection using the magneto-optical effect can be obtained from (^+B+C+D)-E.

In the optical system described above, the detection optical element 1 is used as a method of splitting the laser beam for signal detection using the magneto-optical effect.
Although the light beam is passed through the interior of Figures 17 and 18,
As shown in the figure and FIG. 19, a reflective optical element may be used. That is, the laser beam is totally reflected on one end face of the prism member 6 made of an optically anisotropic medium.

Since the prism member 6 is formed into a model shape,
The laser beam is separated into an ordinary ray and an extraordinary ray by utilizing refraction at the time of incidence and exit at the end face joined to a prism 4 made of an optically isotropic medium placed on the surface facing the reflective surface. Ru.

As shown in FIGS. 20 to 24, by placing an optical path converting prism 80 made of an optical isotropic medium, for example, optical glass such as BI3, in the middle of the optical path and bending the optical path. The height of the entire optical system can be reduced. As shown in FIGS. 20 and 21, the optical system can be further miniaturized by joining and integrating the optical path converting prism 80 with the detection optical element 14. In the embodiment shown in FIG.
is not provided at the joint between the optical path conversion prism 80 and the detection optical element 14, but is formed at the joint between the prisms 4 and 6. In the embodiment shown in FIGS. 22 and 23, a non-polarizing beam splitter layer 10 is formed between two optically isotropic media, one of which serves as an optical path converting prism. In this embodiment, defocus is detected by an astigmatism method using a converging lens 79 and a cylindrical lens 81 without the mask layer 8.

In the optical system described above, the semiconductor laser 2 as a light source, the objective lens 18, the photodetector 24, and the detection optical element 14 are arranged separately, but as shown in FIGS. 19 and 24, The condensing means 18 and the semiconductor laser 2 may be arranged close to each other and configured as one body, thereby achieving miniaturization of the optical system. FIG. 25 shows an assembled view in which the semiconductor laser 2 and the condensing means 18 are integrated, and FIG. 26 shows an exploded perspective view thereof. Supporting means for semiconductor laser 2, condensing means] Supporting means for semiconductor laser 2 and semiconductor laser 2
The heat sink is formed of a support member 94 made of a metal member with good thermal conductivity, such as iron-based, aluminum-based, or inmber with low thermal expansion, in order to prevent deterioration of the semiconductor laser 2 due to oxidation or moisture absorption. covered with hermetic sealing glass 96.98. The surface 91 on which the chip of the semiconductor laser 2 is mounted is formed as a diagonally inclined surface so that the laser beam reflected from the light reflection/transmission layer 10 coincides with the optical axis of the condensing means. The assembly order is such that the semiconductor laser chip 2 is first mounted on the surface 91, the hermetic sealing glass plates 96 and 98 are placed on both sides of the support member 94 for hermetic sealing, and finally the light condensing member, i.e., is mounted on the support member 94. , the objective lens 18 is tilted. Such a structure allows the device to be made more compact.

[Effects of the Invention] According to the optical element of the present invention, a light beam incident on the optical element is reliably separated into light beams corresponding to an ordinary ray and an extraordinary ray within an optically anisotropic medium. The separated light beam is such that the angle formed between the first surface of the first prism and the fourth surface of the second prism is the angle between the first surface of the first prism and the second surface of the first prism. This ensures that the light beams corresponding to the ordinary and extraordinary rays passing through this optical element are spatially separated outside the optical element and are detected separately by the detector. , a stable reproduction signal is generated with the signal from the detector. Moreover, according to the present invention, (1) the number of optical components is reduced, which reduces productivity and cost. (2
) The entire optical system is smaller and lighter, and high-speed access can be achieved. (3) A medium with optical anisotropy and an optically isotropic medium are joined together, and one boundary surface of the optically anisotropic medium is set to have a large inclination. By configuring the detection optical element by setting the inclination of the interface between the orthotropic media small, the separation angle between the ordinary and extraordinary rays can be increased, and the spot size on the photodetector can be reduced. Furthermore, chromatic aberration caused by the optically anisotropic medium can also be reduced.

[Brief explanation of drawings]

FIG. 1 schematically shows an optical system of an information reproducing device using magneto-optical effect according to an embodiment of the present invention, and FIG.
FIGS. 3A and 3B are a perspective view showing the detection optical element shown in FIG. , 4B and 4C are plan views showing the light beam spot patterns produced on the detection surface of the detector shown in FIG. 1 in focused and unfocused states, respectively.
The figure is a block diagram showing a circuit example of the signal processing circuit shown in FIG. 1, and FIGS. 7 and 8 are explanatory diagrams explaining the function of the prism of the detection optical element shown in FIG. 2. , No. 9
The figure schematically shows an optical system of an information reproducing apparatus using magneto-optical effect according to another embodiment of the present invention, and FIG.
Figure 2 is a vector diagram showing the relationship between ordinary rays and extraordinary rays in the detected optical element, Figures 11 to 2
FIG. 6 schematically shows an optical system and its detector of an information reproducing apparatus using magneto-optical effect according to another embodiment of the present invention. 2... Semiconductor laser, 4, 6... Prism member, 8
...Mask layer, 10...Light transmission reflection layer, 12...
Information storage medium, 14.24...detection optical element, 16
...Detection optical element, 18...Objective lens.

Claims (1)

[Claims] Made of an optically anisotropic medium having a different refractive index depending on the direction of vibration of light passing through the medium, a first surface and an inclined surface with respect to the first surface. a first prism member having a second surface; a third surface made of an isotropic medium having an isotropic refractive index and joined to the second surface of the first prism member; a second prism member having a fourth surface tilted relative to An optical element comprising: