JPS639306B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides an apparatus for reading information by focusing a reading light spot onto an information track recorded spirally or concentrically on a recording medium through an objective lens. The present invention relates to a focus detection device that detects out-of-focus.

The above-mentioned information reading device is conventionally known, and recording media having information tracks include, for example, what is called a video disc. On this video disk, video signals and audio signals encoded into information tracks are recorded as optical information such as optical transmission characteristics, reflection characteristics, and phase characteristics. Information recorded on a video disk is recorded by rotating the video disk at high speed and focusing laser light emitted from a laser light source onto an information track through an objective lens, and transmitting or reflecting light modulated by the information track. It is detected and read. One of the characteristics of such recording media is that the information recording density is very high, so that the width of each information track is very narrow and the spacing between successive information tracks is also very narrow. In order to accurately read the original information from such a narrow information track, the objective lens must always be in focus on the video disk surface.
It is necessary to reduce the diameter of the light spot on the disk surface. For this reason, in such an optical reading device, focusing control is performed in which the out-of-focus of the objective lens with respect to the disk surface is detected and the objective lens is displaced in the direction of its optical axis based on this out-of-focus signal.

FIG. 1 is a diagram for explaining a focus detection device in a conventional optical reading device. The light emitted from the laser light source 1 (linearly polarized in the plane of the paper) is made into parallel light by the collimating lens 2, and the information is transmitted through the polarizing prism 3 having a polarizing film, the 1/4 wavelength plate 4, and the objective lens 5. It focuses on the disk 6 containing the track. This light beam is reflected by an information track having an uneven pit shape, passes through an objective lens 5 and a quarter-wave plate 4, and then enters a polarizing prism 3. Since the reflected light incident on the polarizing prism 3 is polarized in a direction perpendicular to the plane of the paper due to the action of the 1/4 wavelength plate 4, this light is reflected by the light splitting surface 3a of the polarizing prism 3 provided with a polarizing film. Ru.
The light beam reflected by this light splitting surface 3a is focused by a condenser lens 7 and a cylindrical lens 8. Here, since the cylindrical lens 8 has a focusing effect only in one axis direction, the shape of the focused beam by the focusing lens 7 and the cylindrical lens 8 will change to a state where the beam is correctly focused on the information track ( deforms in a direction perpendicular to the focal point). Conventionally, this shape change is detected using, for example, a quarter-divided photodetector (not shown) to obtain a focus error signal.
Focusing control is performed using this signal.

However, in the conventional focus detection device described above, an optical path length is required to form an image of the light beam reflected by the light splitting surface 3a of the polarizing prism 3.
The disadvantage is that the optical system becomes large. Furthermore, since the photodetector that obtains the focus error signal needs to be accurately placed in two axes: the optical axis direction and the plane orthogonal to the optical axis direction, it is difficult to adjust its position. Furthermore, since the area in which an error signal can be obtained due to changes in the shape of the focused beam is narrow, there is a drawback that no signal can be obtained if the beam is too far away from the focused state.

In order to solve these drawbacks, the applicant previously proposed a device that instead of focusing the reflected light from the disk 6 on the light splitting surface 3a using the condensing lens 7 and the cylindrical lens 8, it focuses the reflected light on the light splitting surface 3a. An optical member is provided that has a reflecting surface set to receive the reflected light and to form an approximately critical angle with respect to light in a predetermined direction of the reflected light, and the reflected light is reflected by the reflecting surface of the optical member. The optical detector is configured to detect a change in the light amount distribution of the reflected light and/or a change in the light amount of the reflected light reflected by the reflective surface of the optical member and the refracted transmitted light.

This device will be explained with reference to FIG. For the sake of clarity, the same members as in FIG. 1 are given the same reference numerals.

In Fig. 2, 1 is a laser light source, 2 is a collimating lens that converts the light emitted from the laser light source 1 into a parallel beam, 3 is a polarizing prism having a light splitting surface 3a provided with a polarizing film, and 4 is 1/4 wave plate,
Reference numeral 5 denotes a condenser lens that focuses the light that has been made into a parallel beam by the collimating lens 2 onto the disk 6 . A detection prism 10 provided on the side of the polarizing prism 3 receives the reflected light from the disk 6 reflected by the light splitting surface 3a, and when the light to be focused on the disk 6 is in focus, it enters the detection prism 10. A reflecting surface 11 set at a critical angle or slightly smaller than a critical angle with respect to a light beam (parallel light beam)
has. 12 is a photodetector that receives the light beam reflected by the reflecting surface 11; This photodetector 12 is
It detects the light intensity distribution of the reflected light from the above-mentioned reflecting surface 11, and as shown in the plan view in Figure 2, it is divided into two parts with a plane perpendicular to the plane of the paper containing the central ray (optical axis) as the border. It is configured with two light receiving areas 12A and 12B.

Next, the operation of this device will be explained.

The light emitted from the laser light source 1 (linearly polarized in the plane of the paper) is made into parallel light by the collimating lens 2, and the polarizing prism 3, which has a polarizing film, is 1/4
It is focused via a wave plate 4 and an objective lens 5 onto a disk 6 containing an information track. This light beam is reflected by an information track having an uneven pit shape, passes through an objective lens 5 and a quarter-wave plate 4, and then enters a polarizing prism 3. Since the reflected light incident on the polarizing prism 3 is polarized in a direction perpendicular to the plane of the paper due to the action of the 1/4 wavelength plate 4, this light is reflected by the light splitting surface 3a of the polarizing prism 3 provided with a polarizing film. Ru. The light beam reflected by the light splitting surface 3a is incident on the detection prism 10, and the light beam reflected by the reflection surface 11 is received by the detector 12. Reflective surface 1
1 is set to be at or slightly smaller than the critical angle with respect to the incident ray (parallel light flux) in the focused state, so that all the incident rays in the focused state that are incident on the reflective surface 11 are directed to the reflective surface 11. Therefore, in the focused state, the amounts of light incident on the light receiving areas 12A and 12B are equal. When the disc 6 deviates from the focused state in the direction a in FIG. 2, the polarizing prism 3
The luminous flux reflected by the reflective surface 11 has the sign a _i1
The light rays become diverging as shown by .about.a _i2 , and when the disk 6 deviates from the focused state in the b direction, the incident light rays on the reflecting surface 11 have convergence ratios as shown by the symbols b _i1 -b _i2 . Since the reflective surface 11 is set at approximately a critical angle, when the disk 6 is displaced in the a and b directions and deviates from the focused state, the reflective surface 11 changes according to this displacement.
The incident ray to 1 is the central ray on the optical axis (dotted chain line)
It changes continuously around the critical angle except for . That is, when the disk 6 is displaced in the direction a, the light beam incident on the reflective surface 11 becomes diverging light.
The incident angles of all the incident rays, starting with the outermost incident ray a _i1 , are smaller than the critical angle for the rays that are incident on the lower part in the figure than the area where the central ray of 1 is incident. Therefore, transmitted light exists in this portion, and a bundle of light rays including the outermost transmitted light rays b _t1 to n is transmitted. As much as this has passed through,
The intensity of the reflected ray bundle including the outermost reflected ray b _r1 to the center ray is weakened. Also, reflective surface 1
For the light beams incident on the portion above the portion where the central ray 1 is incident in the figure, the incident angles of all the incident rays, starting with the outermost incident ray b _i2 , are larger than the critical angle. Therefore, there is no transmitted light in this part, and all the incident light rays are reflected as being included in the light flux including from the outermost reflected light ray b _r2 to the center ray. As a result, the reflected light beam reflected by the reflective surface 11 that enters the light receiving area 12A of the photodetector 12 is reduced by the amount of the transmitted light beam that is transmitted without being reflected by the reflective surface 11. This light receiving area 12
The rate at which the reflected light rays incident on A decreases increases as the focal point where the objective lens 5 condenses shifts in the direction a, and furthermore, the rate at which the reflected light rays incident on A is reduced increases as the focal point where the objective lens 5 condenses shifts in the direction a. The more incident light rays are, the more the reflected light rays decrease. In addition, for light rays that are incident on a position away from the center ray of the reflecting surface 11 in the upper part of the drawing, all the incident rays are reflected, including those from the outermost reflected ray a _r2 to the center ray. The amount of light that enters the light receiving area 12B is
is the same as the amount of light rays incident on .

On the other hand, when the disk 6 is displaced in the b direction, the incident light rays on the reflective surface 11 become convergent light, so the relationship between the inclinations of the incident light rays on the reflective surface 11 is opposite to the case where the inclination of the incident light rays on the reflective surface 11 shifts in the a direction. Become. That is,
When the disk 6 is displaced in the b direction, the light beam incident on the reflective surface 11 becomes a convergent light, so that the light beam incident on the lower part of the reflective surface 11 in the figure than the part where the central light beam enters is the outermost one. The angles of incidence of all the incident rays, starting with the incident ray b _i1 , are larger than the critical angle. Therefore, there is no transmitted light in this part, and all the incident light rays are reflected as being included in the light flux including from the outermost reflected light ray b _i1 to the center ray. In addition, the light flux that enters the upper part of the drawing from the part of the reflecting surface 11 where the central ray enters is:
The angle of incidence of all the incident rays starting with the outermost incident ray b _i2 is smaller than the critical angle. Therefore, transmitted light exists in this portion, and a bundle of light rays including the outermost transmitted light rays b _t2 to n is transmitted. The outermost reflected ray only reflects this transmitted amount.
The intensity of the reflected ray bundle including b _r2 to the central ray is weakened. As a result, the light receiving area 1 of the photodetector 12
The reflected light beam reflected by the reflective surface 11 that is incident on the reflective surface 2B is reduced by the amount of the transmitted light beam that is transmitted without being reflected by the reflective surface 11. The rate at which the reflected light rays incident on the light receiving area 12B decreases is determined by the objective lens 5.
The more the focal position at which the light is condensed shifts in the direction b, the more the reflected light rays decrease, and the more the reflected light rays are incident on a position farther upward in the figure than the part of the reflecting surface 11 where the central ray is incident, the more the reflected light rays decrease. In addition, for light rays that are incident on a position further away from the center ray of the reflective surface 11 toward the bottom of the drawing, all of the incident rays are reflected, including those from the outermost reflected ray b _r1 to the center ray. The amount of light that enters the light receiving area 12A is the same as the amount of light that enters the light receiving area 12A in the focused state.

Therefore, when the disk 6 is displaced in directions a and b and deviates from the focused state, the intensity of reflection at the reflecting surface 11 suddenly changes with a slight change in the angle of incidence near the critical angle, as shown in FIG. Since the light intensity changes, the light amount distribution of the reflected light on the reflective surface 11 changes accordingly,
The light and dark states are reversed with respect to the plane perpendicular to the plane of the paper containing the central ray, and in the focused state, the incident rays on the reflecting surface 11 are parallel rays, and all of the rays are reflected almost uniformly, as described above. The brightness and darkness do not appear.

In addition, in FIG. 3, the refractive index of the detection prism 10 is
1.50, and shows the reflection intensities R _P and R _S for P-polarized light and S-polarized light, respectively. With this configuration, in the focused state, the light receiving area 12
The amounts of light incident on A and 12B are equal,
When the disk 6 deviates from the in-focus position in the direction a, the light-receiving area 12A becomes dark and the light-receiving area 12B becomes bright; when the disk 6 deviates from the in-focus position in the direction b, the light-receiving area 12A becomes bright. , since the light receiving area 12B becomes dark, each light receiving area 12A, 1
By detecting the difference between the outputs of 2B and 2B, a focus error signal representing the amount and direction of deviation can be obtained from the amount and polarity, and a focus error signal that controls the movement of the objective lens 5 in the optical axis direction can be obtained based on this signal. sing control can be performed. Further, the pit corresponding signal recorded on the disk 6 can be detected from the sum of the outputs of the light receiving areas 12A and 12B.
Moreover, in the in-focus state, there is almost no transmitted component on the reflective surface 11, so the loss of light quantity is extremely small, and when the focus is out of focus, the light beam on either side of the center ray is completely absorbed. light receiving area 1 because the reflected intensity of the light beam on the other side is extremely reduced.
The difference in light amount between 2A and 12B becomes significant. Therefore, focus detection can be performed with sufficient accuracy.

Furthermore, according to the focus detection device previously proposed by the applicant shown in FIG. 2, a minute spot is formed like the conventional focus detection device using a cylindrical lens shown in FIG. There is no need for troublesome adjustment to align the center of the photodetector, and since there is no need to narrow down the beam, a large luminous flux can be incident on the photodetector, making optical adjustment extremely easy. Moreover, since there is no need for adjustment in two axes, the photodetector and the detection prism can be mechanically positioned, and then they can be mounted as a unit by rotation adjustment within the plane of the paper. Furthermore, since there is no need for an optical path length to form a minute spot,
The entire device can be constructed to be lightweight and compact, and therefore the entire optical system can be easily incorporated into a two-dimensional driver that controls the movement of the objective lens in the direction of the optical axis and in directions orthogonal to this and orthogonal to the information track. can be incorporated.

However, in general, such a focus detection device has a polarizing prism and a detection prism that are very small, each side being several millimeters, and these prisms have the drawback of being difficult and expensive to manufacture. In the above-mentioned focus detection device as well, the polarizing prism is constructed by gluing two blocks together at the light splitting surface, and the detection prism is provided separately, so that manufacturing difficulties are unavoidable.

It is an object of the present invention to prevent this drawback as much as possible, to take full advantage of the advantages of the focus detection device shown in FIG. 2, to provide a focus detection device that enables highly sensitive focus detection and is easy to manufacture and inexpensive.

To achieve this purpose, the focus detection device of the present invention includes a light source, an objective lens that focuses the light emitted from the light source and irradiates the object to be irradiated, and guides the light emitted from the light source to the objective lens. Also, a light splitting element that guides the reflected light of the light irradiated on the irradiated object returning through the objective lens in a direction other than the light source, and the light split by the light splitting element in a direction other than the light source. a detection prism which receives at least a part of the light beam and has a reflective surface set at a critical angle or slightly smaller than the critical angle with respect to the light beam in a predetermined direction in the incident light; and at least part of the light that passes through the detection prism. detecting changes in the amount of reflected light and/or refracted transmitted light reflected by the reflective surface of the detection prism to obtain a focus error signal of the objective lens with respect to the irradiated object. The focus detection device includes an output photodetector, characterized in that the light splitting element and the detection prism are molded into one body.

The present invention will be explained in detail below with reference to the drawings. Fourth
The figure is a diagram showing the configuration of an embodiment of the focus detection device of the present invention. For the sake of clarity, the same members as in FIG. 2 are given the same reference numerals.

This focus detection device is made by gluing together the block on the side facing the detection prism 10 of the two blocks forming the polarizing prism 3 of the focus detection device shown in FIG. 2, instead of the detection prism 10. One block 13 having the same outer shape as the
A block 14 having the same shape as the block on the side not facing the detection prism 10 of the focus detection device shown in FIG. is provided to form a light splitting surface 3a. The reflective surface 15 provided on the block 13 receives the reflected light beam from the disk 6 reflected by the light splitting surface 3a, and when the light to be focused on the disk 6 is in focus, the light beam reflected by the light splitting surface 3a ( It is set at a critical angle or slightly smaller than the critical angle with respect to the parallel light beam (parallel light beam), and has the same function as the focus detection device shown in FIG. The setting of the reflection angle of the reflection surface 15 of this block 13 will be explained with reference to FIGS. 5a and 5b.
5a and 5b show blocks 13 and 14 of the focus detection device of FIG. 4. FIG. In order to reflect the optical axis of the reflected light from the disk 6 shown by the broken line laterally at an angle of 90°, the light splitting surface 3a provided with a polarizing film is arranged at an angle of 45° with respect to this optical axis.
Furthermore, the incident surfaces of these blocks 13 and 14 on which the light rays enter are planes perpendicular to the optical axis of the incident light rays in order to avoid bending of the rays at these incident surfaces. Therefore, ∠CDE and ∠ECD in block 14
is 45°, ∠DEC is 90°, and ∠ of block 13 is
BCD is 45°. When the plane DE of block 14 and the plane AD of block 13 are located on the same plane as shown in FIG. 5a, ∠ADC becomes 135°,
If the critical angle of the reflecting surface 15 is θ°, ∠BAD is 90°−
θ°, and ∠CBA becomes 90° + θ°. However, since the reflection angle of the optical axis of the light beam reflected by the reflective surface 15 is 2θ°, in this case, if θ° is other than 45°, the optical axis of the reflected light from the disk 6 that is reflected by the reflective surface 15 is 2θ°. The surface AD is no longer orthogonal to the surface AD, and the optical axis of the emitted light at the surface AD is refracted. Therefore, when θ° is other than 45°, this refraction can be avoided and the surface AD
In order to make it perpendicular to the optical axis of the emitted light, the fifth
It is necessary to set ∠BAD to θ° as shown in Figure b. In this case, ∠ADC becomes 225°−2θ°. Here, if the refractive index of the material of the prism is n, then the relationship θ=sin ⁻¹ 1/n holds between the critical angle θ and the refractive index n. Let the length of side EC of block 14 be α,
Assuming that the length of the side BC of the block 13 is β, and that the reflected light from the disk 6 is incident on the left side of the diagram from a position γ apart from the point C on the side BC, then the length of the side BC is β must satisfy the following conditions.

β＞{1+1/tan(2sin ^-1 1/n)}α−{tan(
sin ^-1 1/n) + 1/tan (2sin ^-1 1/n)}γ If the length β of side BC is smaller than the above value, the light reflected by the reflecting surface 15 enters the light splitting surface 3a again. You end up doing it. In addition, if the incident position of the reflected light from the disk 6 also includes point C, γ=0
Then, β>{1+1/tan (2sin ^-1 1/n)}α. The focus detection device of the embodiment shown in FIGS. 4 and 5 has blocks 13 and 14 forming a prism having the functions of both a polarizing prism and a detection prism, and the other configuration is the same as that of FIG. It was configured in the same way as the focus detection device.

Next, the operation of this device will be explained. Similar to the device shown in FIG. 2, the light emitted from the laser light source 1 (linearly polarized in the plane of the paper) is made into parallel light by the collimator lens 2, and the block 14, the light splitting surface 3a,
Block 13, quarter wave plate 4 and objective lens 5
The image is then focused onto the disk 6 containing the information track. This light beam is reflected by an information track having an uneven pit shape and enters a block 13 via an objective lens 5 and a quarter-wave plate 4. Since the reflected light incident on the block 13 is polarized in the direction perpendicular to the plane of the paper due to the action of the quarter-wave plate 4,
This light is reflected by a light splitting surface 3a provided between blocks 13 and 14. The light beam reflected by the light splitting surface 3a is incident on a reflection surface 15 provided on the block 13, and the light beam reflected by the reflection surface 15 is received by the detector 12.

Since this focus detection device was configured in this way,
Focus detection is carried out according to principles similar to those described above for the apparatus of FIG. Therefore, in the focused state, the amount of light incident on the light receiving areas 12A and 12B is equal, and when the disc 6 deviates from the focused position in the direction a, the light receiving area 12A becomes dark, the light receiving area 12B becomes bright, and the disc 6 deviates from the in-focus position in the b direction, the light receiving area 12A
is bright and the light-receiving area 12B is dark, so by detecting the difference between the outputs of the light-receiving areas 12A and 12B, a focus error signal representing the amount and direction of deviation from the amount and polarity can be obtained. Focusing control for controlling the movement of the objective lens 5 in the optical axis direction can be performed based on the signal. Further, the pit corresponding signal recorded on the disk 6 can be detected from the sum of the outputs of the light receiving areas 12A and 12B.

The focus detection device of the present invention with such a configuration has the advantages described with respect to the focus detection device of FIG.
The structure of these prisms is simplified because they are made of individual blocks, which makes them easier to manufacture and cheaper, as well as having a joint surface and a separate surface between the polarizing prism and the detection prism. Therefore, there is no loss of light due to the adhesive layer on the bonded surface or the separated surface and dirt on these surfaces, and more accurate focus detection can be performed.

FIG. 6 is a diagram showing the configuration of another embodiment of the present invention. For simplicity, the same members as in FIG. 4 are given the same reference numerals.

In this focus detection device, a block 16 is provided in place of the block 15 in the embodiment shown in FIG. 4, and the block 16 and the block 14 are bonded together, and a polarizing film is provided between the bonded surfaces to form a light splitting surface 3a. This is what I did. The difference from the embodiment shown in FIG. 4 is that the block 16 is provided with two reflective surfaces 17 and 18. The reflective surface 17 provided on this block 16 receives the reflected light from the disk 6 that is reflected by the light splitting surface 3a, and when the light to be focused onto the disk 6 is in focus, it is reflected by the light splitting surface 3a. It is set to be at or slightly smaller than the critical angle with respect to the light beam (parallel light flux), and has the same effect as the reflecting surface 15 in the embodiment shown in FIG. Further, when the light focused on the disk 6 is in the focused state, the reflective surface 18 is reflected by the light splitting surface 3a, and the reflective surface 18
This angle is set to be at or slightly smaller than the critical angle with respect to the light ray (parallel light flux) reflected by the angle 5.

The reflective surface 17 of this block 16 and the reflective surface 1
The setting of the reflection angle of No. 8 will be explained with reference to FIG.
FIG. 7 is a diagram showing blocks 16 and 14 of the focus detection device of FIG. In order to reflect the optical axis of the reflected light from the disk 6 shown by the broken line laterally at an angle of 90°, the light splitting surface 3a provided with a polarizing film is arranged at an angle of 45° with respect to this optical axis. Further, the incident surfaces of these blocks 16 and 14 on which the light beams enter are planes perpendicular to the optical axis of the incident light beams in order to avoid bending of the light beams at these incident surfaces. Therefore, ∠CDE and ∠ECD of block 14 are 45°, ∠
DEC is 90° and ∠BCD of block 16 is 45°. Reflective surface 1 as described above with respect to FIG.
If the critical angle of 7 and 18 is θ, the reflection angle of the reflected light from disk 6 reflected by these reflecting surfaces 17 and 18 is 2θ°. Therefore, the reflecting surface 18 and the light splitting surface 3a are on the same plane only when θ° is 45°. In order to set the reflective surface 17 at a critical angle, ∠CBA needs to be 90° + θ°. Further, in order to set the reflective surface 18 at a critical angle, ∠FDC is set to 225°-θ. In order to make the surface AF perpendicular to the optical axis of the light emitted from this surface, ∠AFD
It is necessary to set θ to θ. In this case ∠BAF is
It becomes 180°−θ. As described above in FIG. 5, if the refractive index of the material of the prism is n, then the relationship θ=sin ^-1 1/n holds between the critical angle θ and the refractive index n. Let the length of side EC of block 14 be α,
Assuming that the length of the side BC of the block 16 is β, and that the reflected light from the disk 6 is incident on the left side of the diagram from a position γ apart from the point C on the side BC, then the length of the side BC is β must satisfy the following conditions.

β＞{1+1/tan(2sin ^-1 1/n)}α−{tan(
sin ^-1 1/n) + 1/tan (2sin ^-1 1/n)}γ If the length β of side BC is smaller than the above value, the light reflected by the reflecting surface 17 enters the light splitting surface 3a again. You end up doing it. In addition, if the incident position of the reflected light from the disk 6 also includes point C, γ=0
Then, β>{1+1/tan (2sin ^-1 1/n)}α. The photodetector 12 is arranged so that the light beam reflected by the reflective surface 18 enters the photodetector 12, and as shown in the side view in FIG. The two light receiving areas 12A and 12B are divided into two with a vertical plane as the boundary. The rest of the structure is the same as that of the focus detection device of the embodiment shown in FIG.

Next, the operation of this device will be explained. Laser light source 1
light emitted from (linearly polarized within the plane of the paper)
is made into parallel light by the collimating lens 2, passes through the block 14, the light splitting surface 3a, the block 16, the 1/4 wavelength plate 4, and the objective lens 5, and is focused onto the disk 6 containing the information track. This light beam is reflected by an information track with an uneven pit shape,
The light passes through the objective lens 5 and the quarter-wave plate 4 and enters the block 16. Since the reflected light incident on the block 16 is polarized in the direction perpendicular to the plane of the paper due to the action of the quarter-wave plate 4, this light is reflected on the block 16.
The light is reflected by the light splitting surface 3a provided between and 14. The light beam reflected by this light splitting surface 3a is incident on a reflecting surface 17 provided on the block 16, and this reflecting surface 1
The light beam reflected by 7 is incident on the reflective surface 18,
The light beam reflected by this reflecting surface 18 is detected by the detector 1.
Receives light at 2. The reflective surfaces 17 and 18 are set so as to form a critical angle or a little smaller than the critical angle with respect to the incident light beam (parallel light flux) in the focused state. Therefore, all the incident light beams in the focused state that enter the reflective surfaces 17 and 18 are reflected by the reflective surfaces 17 and 18, so that in the current focused state, the amounts of incident light to the light receiving areas 12A and 12B are equal.

As described above with respect to the focus detection device of FIG. 2, when the disk 6 deviates from the in-focus state in the direction a in FIG. When the state deviates from the state in the b direction, the incident light beam to the reflecting surface becomes convergent light. Since the reflective surfaces 17 and 18 are set at approximately the critical angle, when the disk 6 is displaced in directions a and b and deviates from the focused state, the reflective surfaces 1
The incident light rays to 7 and 18 change continuously around the critical angle except for the central ray (dotted chain line) on the optical axis. That is, when the disk 6 is displaced in the direction a,
The angle of incidence of all the incident rays of light rays that are incident on the lower part of the reflection surface 17 than the part on which the central ray is incident is smaller than the critical angle. Therefore,
Transmitted light exists in this part. This reflective surface 17
The intensity of the reflected light beam reflected by the reflecting surface 17 is weakened by the amount of light that passes through the reflective surface 17. This reflective surface 1
The reflected light rays are incident on the lower part of the reflective surface 18 than the part on which the central ray is incident, and the reflected light rays are incident on the lower part of the reflective surface 18 than the part on which the central ray is incident. When the incident light ray is smaller than the critical angle, the light ray incident on the reflective surface 18 is also smaller than the critical angle, so there is also transmitted light on the reflective surface 18, and the amount of light that passes through this reflective surface 18 is reflected. The intensity of the reflected ray bundle reflected at surface 18 is weakened. The reflected light rays enter the lower part of the reflection surface 18 than the part on which the central ray enters, and the reflected light rays enter the light receiving area 12B of the photodetector 12, but the light rays incident on this light receiving area 12B are Compared to the focused state, the amount of light is reduced by the amount of transmitted light that is transmitted without being reflected by the reflecting surfaces 17 and 18. In addition, since the incident angles of all the incident rays of light rays incident on the portions of the reflecting surfaces 17 and 18 above the portion where the central ray is incident are greater than the critical angle, there is no transmitted light in these portions, and the incident All the light rays including the central ray are reflected, and the light receiving area 12
incident on A. As a result, the amount of light rays incident on the light receiving area 12A is the same as the amount of light rays in the focused state.

On the other hand, when the disk 6 is displaced in the b direction, the incident light rays on the reflective surfaces 17 and 18 become convergent light, so the relationship between the inclinations of the incident light rays on the reflective surfaces 17 and 18 is shifted in the a direction as described above. It will be the opposite. That is, when the disk 6 is displaced in the b direction, the incident light beams on the reflective surfaces 17 and 18 become convergent beams, so that all the light beams incident on the lower side of the figure than the center line of the reflective surfaces 17 and 18 are incident on the center line of the reflective surfaces 17 and 18. The angle of incidence of the incident ray is greater than the critical angle. Therefore, no transmitted light exists in this portion, and all incident light rays, including the central ray, are reflected. Also,
The angles of incidence of all the incident rays that are incident on the portions of the reflecting surfaces 17 and 18 above the portion where the central ray is incident in the drawing are smaller than the critical angle. Therefore,
Transmitted light exists in this portion, and the intensity of the reflected light beam is weakened by the amount of transmitted light. As a result, the reflected light beam reflected by the reflective surface 18 that enters the light-receiving area 12A of the photodetector 12 is reduced by the amount of the transmitted light beam that is transmitted without being reflected by the reflective surfaces 17 and 18. In addition, all of the incident light rays that are incident on the upper side of the reflection surface 17, 18 are reflected from the part where the center ray is incident, so that the amount of light that is incident on the light receiving area 12B is in the in-focus state. The amount of light that enters the light-receiving area 12B is the same as the amount of light that enters the light-receiving area 12B when With this configuration, in the focused state, the amount of light incident on the light receiving areas 12A and 12B is equal to each other, and when the disk 6 deviates from the focused position in the direction a, the light receiving area 12B becomes dark.
If the light receiving area 12A becomes bright and the disc 6 deviates from the in-focus position in the b direction, the light receiving area 12A becomes brighter.
2B is bright and the light receiving area 12A is dark, so
By detecting the difference in the output of each light receiving area 12A, 12B, a focus error signal representing the amount and direction of deviation can be obtained from the amount and polarity.
Based on this signal, focusing control for controlling the movement of the objective lens 5 in the optical axis direction can be performed. Further, the pit corresponding signal recorded on the disk 6 can be detected from the sum of the outputs of the light receiving areas 12A and 12B. Moreover, since this device uses two reflective surfaces 17 and 18, only one reflective surface can be used.
Compared to the above-mentioned embodiment in which only one light receiving area is provided, the rate of decrease in the amount of light to one of the light receiving areas 12A and 12B that occurs when the focal position shifts in either direction a or b is doubled. Since the difference in the amount of light becomes significant, the focus detection device becomes more expensive, and focus detection can be performed more accurately.

The focus detection device of the present invention with such a configuration has the advantages described with respect to the focus detection device of FIG.
The structure of these prisms is simplified because they are made of individual blocks, which makes them easier to manufacture and cheaper, as well as having a joint surface and a separate surface between the polarizing prism and the detection prism. There is no loss of light due to the adhesive layer at the joint surface, no refraction of light due to separated surfaces, and no loss of light due to dirt on these surfaces, making it a focus detection device that can perform more accurate focus detection. be.

Note that the focus detection device of the present invention is not limited to the illustrated embodiment, and various modifications can be made within the scope of the claims. As an example, a half mirror may be used instead of the polarizing film used in the above embodiment. Furthermore, the blocks constituting these prisms may be made of any material as long as it has good light transmittance and a predetermined refractive index.For example, plastic may be used instead of glass. For example, the blocks constituting the prism can be created more easily.

[Brief explanation of the drawing]

1 and 2 are diagrams showing the configuration of a conventional focus detection device, FIG. 3 is a diagram showing an example of reflection intensity near the critical angle, and FIG. 4 is a diagram showing an example of the focus detection device of the present invention. Diagrams showing the configuration of an embodiment; FIGS. 5a and 5b are diagrams showing the configuration of blocks constituting the prism of the focus detection device in FIG. 4; FIG. 6 is a diagram showing another embodiment of the focus detection device of the present invention. Diagram showing the configuration of the example, No. 7
This figure is a diagram showing the structure of blocks constituting the prism of the focus detection device of FIG. 6. 1... Laser beam, 2... Collimating lens,
3...Polarizing prism, 3a...Light splitting surface, 4...
1/4 wavelength plate, 5...objective lens, 6...disk,
10...detection prism, 11...reflection surface, 12...
...Photodetector, 12A, 12B... Light receiving area, 1
3, 14, 16...Block, 15, 17, 18
...Reflecting surface, A, B, C, D, E, F... points.

Claims (1)

[Claims] 1. A light source, an objective lens that focuses the light emitted from the light source and irradiates it onto an irradiated object, and guides the light emitted from the light source to the objective lens and returns the light through the objective lens. A light splitting element that guides the reflected light of the light irradiated onto the irradiated object in a direction other than the light source; a detection prism having a reflective surface set at a critical angle or slightly smaller than a critical angle with respect to a ray of light in a predetermined direction; and a detection prism arranged to receive at least a portion of the light that has passed through the detection prism. and a photodetector that detects changes in the amount of reflected light and/or refracted transmitted light reflected by the reflective surface of the object lens and outputs a focus error signal of the objective lens with respect to the irradiated object. 2. A focus detection device according to claim 1, wherein the light splitting element and the detection prism are molded into one body.