JPH0120498B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is an optical system that focuses three reading light spots on an information track recorded spirally or concentrically on a recording medium through an objective lens to obtain an information signal and a tracking error signal. This invention relates to an improvement of a digital information reading device.

As a prior art using three beams as described above, there is Japanese Patent Application Laid-open No. 147403/1983. The photodetector explained in Fig. 3 of the publication has a second photodetector (shown in Fig. 9b) for reading information signals placed in the center, and the tracking error is adjusted so as to sandwich this photodetector. First and third photodetectors (shown in figures 9a and 9c) are arranged for detecting the signal. However, the photodetector is not configured to detect focus error signals. This JP-A-52-
Publication No. 147403 explains that "focus adjustment uses, for example, aero floating (see column 4, lines 1 to 3 of the publication)," and does not explain anything about focus detection using a photodetector.

In addition, as a prior art related to a tracking error signal detection method using a three-beam method,
Publication No. 13123 also describes cylindrical lenses and 4
US Pat. No. 4,079,247 is a prior art technique for detecting a focus error signal using a combination of split light detection elements. However, both of the prior art techniques only explain the individual techniques of tracking error signal detection and focus error signal detection, and do not explain the technique of integrating both to detect two types of error signals with one device. Not yet.

The present invention has an optical system that can detect both the tracking error signal and the focus error signal using a three-beam system, and also incorporates an integrated photodetector element that is excellent in mass production and allows easy optical adjustment. The purpose is to provide a reading device.

The information reading device of the present invention makes a light beam emitted from a light source enter a light splitting element to generate a plurality of beams, and then uses an objective lens to generate three beam spots arranged approximately linearly on an information track of a recording medium. In an optical information reading device, the reflected light is guided to a photodetector to detect an information signal and a tracking error signal. and a second photodetecting surface and a third photodetecting surface disposed on both sides of the first photodetecting surface, at least four photodetecting surfaces,
The first, second, and third photodetecting surfaces are integrally fixed as one optical element, and the first, second, and third photodetecting surfaces are arranged on substantially the same plane. A body type photodetecting element is disposed in the reflected optical path from the recording medium, and in the optical path between the objective lens and the integrated photodetecting element, according to the displacement of the recording medium in a direction perpendicular to the surface of the recording medium. An optical element for focus detection that causes a change in the beam shape is arranged, and all of the three beams are made incident on the optical element for focus detection, and an intermediate of the three beams emitted from the optical element for focus detection is arranged. incident on the first photodetecting surface, and making the two beams on both sides incident on the second photodetecting surface and the third photodetecting surface, and
A focus error signal is obtained by detecting a change in the beam shape on the photodetection surface of the second photodetector.
A tracking error signal is obtained by comparing the amount of light on the third photodetecting surface and the third photodetecting surface.

An embodiment of the present invention will be described below based on the drawings.

FIG. 1 is a diagram for explaining the principle of one system of a focus detection device of an optical information reading device. Light emitted from a laser light source 1 (linearly polarized in the plane of the paper) is made into parallel light by a collimator lens 2, and then passes through a polarizing prism 3 having a polarizing film, a 1/4 wavelength plate 4, and an objective lens 5. The data are collected on a disk 6 containing information tracks. This light beam is reflected by an information track having a concave and convex bit shape and enters the polarizing prism 3 through the objective lens 5 and the quarter-wave plate 4. Since the reflected light beam incident on the polarizing prism 3 is polarized in a direction perpendicular to the plane of the paper due to the action of the quarter-wave plate 4, this light is reflected by the polarizing prism 3. This polarizing prism 3
The light beam reflected by the condenser lens 7 and the cylindrical lens 8 are converged. Here, since the cylindrical lens 8 has a converging effect in the uniaxial direction, the condenser lens 7
And the shape of the convergent beam by the cylindrical lens 8 is:
When the position of the disk 6 is shifted vertically, it deforms in a direction perpendicular to the state where it is properly focused on the information track (focus position). Conventionally, this shape change is detected by, for example, a quarter-divided photodetector (not shown) to obtain a focus error signal, and focusing control is performed using this signal.

FIG. 2 is a diagram showing the configuration of essential parts of an example of a focus detection device previously proposed by the applicant. In the device of this example, the reflected light from the disk 6 is passed through the polarizing prism 3.
Since the structure is the same as that of the apparatus shown in FIG. 1 up to the point where the light is reflected, the same reference numerals as those shown in FIG. 1 represent the same optical members. The parallel light beam reflected by the polarizing prism 3 is incident on the detection prism 10, and the light beam reflected by the reflecting surface 11 is received by the detector 12. The reflective surface 11 reflects the incident light beam (parallel light beam) in a focused state.
The angle is set so that the angle is approximately the critical angle. In this way, in the focused state, all the light reflected by the polarizing prism 3 is totally reflected by the reflecting surface 11 (actually, since the condition of the reflecting surface is not perfect, some light is transmitted in the n direction shown in the figure). ), when the disk 6 deviates from the focused state in the direction a, the light beam reflected by the polarizing prism 3 becomes a light beam having a maximum tilt component of a _i1 to a _i2 with respect to the reflecting surface 11. Furthermore, when the disk 6 deviates from the focused state in the direction b, the incident light rays on the reflecting surface 11 become a bundle of light rays having tilt components shown by b _i1 to b _i2 . That is, when the disk 6 is out of focus, the light beams incident on the reflective surface 11 change continuously around the critical angle, except for the central ray (dotted chain line) on the optical axis. Therefore, when the disk 6 is displaced in directions a and b and is out of focus, 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. Because the light changes, the light and dark states are reversed across the plane perpendicular to the plane of the paper containing the central ray. On the other hand,
In the focused state, such brightness and darkness do not appear because the light is totally reflected uniformly. The photodetector 12 detects the light intensity distribution of the reflected light from the reflecting surface 11, and is divided into two parts with the central ray (optical axis) as the border, as shown in the plan view in FIG. It is configured with two light receiving areas 12A and 12B. Note that FIG. 3 shows the reflection intensities R _p and R _s of P-polarized light and S-polarized light, respectively, when the refractive index of the detection prism 10 is 1.50.

Note that the reflection intensity for unpolarized light is between these two (R _p +R _s )/2.

In FIG. 2, when the disk 6 is displaced in the direction a, among the light incident on the reflective surface 11, the light beam below the center ray in the figure is composed of all the incident light beams starting from the outermost incident ray a _i1 . The angle of incidence of the ray will be smaller than the critical angle. Therefore, there is transmitted light in this part, and the outermost transmitted light ray a _i1
A bundle of rays including from n to n is transmitted. Only in this transmitted portion, the intensity of the reflected ray bundle including the outermost reflected ray a _i1 to the center ray is weakened. Reflective surface 1
Among the light beams incident on 1, the angles of incidence of all the light beams above the central ray in the figure, starting with the outermost incident ray a _i1 , are larger than the critical angle. Therefore, there is no transmitted light in this portion, and all incident light rays, including those from the outermost reflected ray a _i2 to the center ray, are included and reflected at high speed. Therefore, in this case, the photodetector 1
Regarding the light amount distribution on 2, the light receiving area 12A becomes dark and the light receiving area 15B becomes bright.

On the other hand, when the disk 6 is displaced in the b direction, the relationship of the inclination of the incident light beam to the reflective surface 11 is opposite to that in the a direction described above, and therefore The relationship between light and dark is reversed. The reflective surface and transmitted light on the reflective surface 11 in this case are indicated by symbols b _r1 , b _r2 and b _t2 , respectively.

Note that in the focused state, the light receiving area 1 of the photodetector 12
The amounts of light incident on 2A and 12B are equal.

Therefore, by detecting the difference in the outputs of the angular light-receiving areas 12A and 12B, a focus error signal representing the amount and direction of deviation can be obtained from the amount and polarity, and the objective lens 5 is adjusted based on this signal. Focusing control that controls movement in the optical axis direction can be performed, and the light receiving areas 12A, 12
The information signal recorded on the disk 6 can be detected from the sum of the B outputs. 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. Since the reflected light beam is reflected and the reflected intensity of the light beam on the other side is extremely reduced, the difference in light amount between the light receiving areas 12A and 12B becomes significant. Therefore, focus detection can be performed with sufficient accuracy.

The focus detection devices shown in FIGS. 1 and 2 do not have a function to detect a tracking error signal, which is necessary as a function of an optical information reading device.

When reading information from an information recording medium such as a video disk, as mentioned above, it is necessary to perform focusing control so that the light spot always converges on the information recording surface, and also to prevent the light spot from deviating from the predetermined information track. Tracking control is required to always scan over it.

FIG. 4 is a diagram showing the configuration of an example of an optical information reading device of the present invention, in which information signals,
Three signals can be detected: a focus error signal and a tracking error signal. Light emitted from a laser light source 21 is guided to relay lenses 22A and 22B, and a diffraction grating 23 is placed in a parallel optical path between these relay lenses 22A and 22B to create three beams. These beams are each made into parallel light beams by a collimator lens 24, a polarizing prism 25 having a polarizing film, a 1/4 wavelength plate 26, and an objective lens 2.
7 and converge on the disk 28 containing the information track. The positional relationship of the three beams on the disk 28 is, for example, as shown in FIG.
In the direction of the track 29, the spots 30B and 30C are arranged so as to be slightly shifted in opposite directions with the spot 30A at the center, and the middle spot 3
0A is used for focusing control and information reading, and spots 30B and 30C on both sides are used for tracking control. In FIG. 4, the reflected light flux of each beam converged on the disk 28 passes through an objective lens 27, a quarter-wave plate 26, and a polarizing prism 25, and enters a detection prism 31 similar to that shown in FIG. However, in this embodiment, when each beam is in focus on the disk 28, the reflected light flux of each beam that has passed through the objective lens 27 is made to become a substantially parallel light flux, and the parallel reflected light flux of each of these beams is A convex cylindrical lens 33 is disposed between the polarizing prism 25 and the detection prism 31, which has a reflective surface 32 set to be almost critical to the reflected light beam, and the track 29 is formed on the disk 28 in the width direction. The reflected light flux of each of these beams on the reflecting surface 32 is passed through a convex cylindrical lens 33.
A photodetector 34 is placed on the focal plane of the photodetector 34 to receive light. In this way, in the focused state, each beam is reflected after passing through the convex cylindrical lens 33, as shown in the perspective view of the convex cylindrical lens 33, the detection prism 31, and the photodetector 34 in FIG. The light beam is almost completely reflected by the reflecting surface 32 of the detection prism 31, and spots 35A, 35B and 35C of the reflected light beams of each beam incident on the photodetector 34 are formed by the convex cylindrical lens 3.
It is a plane perpendicular to the center ray of each reflected light beam at the focal position of No. 3, is symmetrical with respect to the straight line connecting the center lines of each reflected light beam, and extends parallel to the direction of the track 29. In addition, in FIGS. 5 and 6, spots 30A and 35A, 30B and 3
5B, 30C and 35C correspond to each other.
Further, when the objective lens 27 is out of focus on the disk 28 and the focal position of the objective lens 27 is positioned in front of or behind the disk 28, the spots 35A to 35C on the photodetector 34 will be moved in the same way as described in FIG. , the bright and dark states are reversed with respect to the straight line connecting the center rays of each reflected light beam. Furthermore,
Each beam spot 30A-3 on the disk 28
From the state where the 0C position is shown in Fig. 5, the disk 28
When the track 29 shifts left and right in the width direction due to movement of the track 29, the tracking control spots 30B, 30
A spot 35B on the photodetector 34 corresponding to C,
In 35C, the brightness and darkness are reversed. The photodetector 34 detects the light intensity distribution of each reflected light beam as described above, and as shown in FIG. The light is received in two light-receiving areas 34A ₁ and 34A ₂ divided into two with
The respective reflected luminous fluxes of the other two tracking control beams are divided into two areas 34A ₁ and 3
The light receiving areas 34B and 34C provided on both sides of _4A2 are configured to receive light, respectively.
As shown in FIG. 6, this photodetector 34 is arranged so that light receiving areas 34B and 34C for tracking error signal detection closely sandwich light receiving areas 34A ₁ and 34A ₂ for detecting information signals and focus error signals. has been done. Furthermore, these light receiving areas 34A ₁ , 34A ₂ ,
The detection surfaces 34B and 34C are arranged on the same plane, and these regions are integrally connected. As shown in FIGS. 7A to 7C, by supplying the outputs of the light receiving areas 34A ₁ and 34A ₂ to the adder 36, the information signals recorded on the track 29 can be detected from the outputs, and the information signals recorded on the tracks 29 can also be detected. By supplying the output to the differential amplifier 37, a focus error signal can be detected from the output. Furthermore, by supplying the outputs of the light receiving regions 34B and 34C to the differential amplifier 38, a tracking error signal can be detected from the outputs. In addition, in FIG. 7, A is located exactly on the track 29 of the focusing and reading beam spot 30A as shown in FIG. 5, and each beam is imaged on the disk 28. In B, the focal position of the objective lens 27 is located in front of the disk 28, and each spot on the disk 28 has changed in the width direction of the track 29 from the state shown in FIG. C shows the case where the focal point of the objective lens 27 is located ahead of the disk 28, and each spot on the disk 28 is in a direction opposite to that shown in FIG. 7B. This shows a case where the image is out of alignment.

As is clear from FIG. 7, in this embodiment, by detecting the difference between the outputs of the light receiving areas 34A ₁ and 34A ₂ of the photodetector 34 and the difference between the outputs of the light receiving areas 34B and 34C, the amount and Since it is possible to obtain a focus error signal and a tracking error signal that represent the amount and direction of deviation from polarity,
Based on these error signals, focusing control for controlling the movement of the objective lens 27 in the optical axis direction and tracking control for controlling the movement of the objective lens 27 in the width direction of the track 29 perpendicular to the optical axis can be performed accurately.

FIG. 8 is a diagram showing the configuration of still another example of the optical information reading device of the present invention. In this example, the collimator lens 24 is replaced by a polarizing prism 25 and an objective lens 27.
The convergent reflected light beam passing through the polarizing prism 25 is turned into a parallel light beam only in the direction of the incident surface on the reflective surface 32 of the detection prism 31 by the concave cylindrical lens 41. The difference from the above-described embodiment is that the light is made to diverge in this manner and then enter the photodetector 42 through the detection prism 31. The photodetector 42 is arranged at the focal position of the collimator lens 24 and at the focal position of the concave cylindrical lens 41. Therefore, similar to the embodiment shown in FIG.
Three beams are created using a relay lens and a diffraction grating, and these beams are guided through a polarizing prism 25 to a collimator lens 24, and a photodetector 4.
If 2 is configured in the same manner as shown in FIG. 6, an information signal, a focus error signal, and a tracking error signal can be detected using three beams. The light rays indicated by broken lines in FIG. 8 indicate the light rays on both outer sides in the direction perpendicular to the paper (width direction of the track).

FIG. 9 is a diagram showing the configuration of still another example of the optical information reading device of the present invention. In this example, the light emitted from the laser light source 21 is transferred to the collimator lens 2.
4. A parallel beam of light passes through the concave cylindrical lens 43, the polarizing prism 25 and the convex cylindrical lens 44, and this is 1/
The reflected light beam passes through the 4-wave plate 26 and the objective lens 27 and is converged onto the disk 28, and the reflected light beam passes through the objective lens 27, the 1/4-wave plate 26, the convex cylindrical lens 44, the polarizing prism 25, and the detection prism 31, and is sent to the photodetector 45. It was designed to lead to. The convex cylindrical lens 44 converges the reflected light beam only in the direction orthogonal to the incident surface on the opposite surface of the detection prism 31, as in the above-described embodiment.
The photodetector 45 is arranged on the focal plane of the convex cylindrical lens 44 or on the image plane of the pupil plane of the objective lens 27. In this embodiment, the laser light source 21 is a laser light source whose beam divergence angles are greatly different vertically and horizontally, such as a DH (double hetero) laser. If such a laser light source is used as is, most of the light will be eclipsed by the objective lens aperture, resulting in poor efficiency. Therefore, in this embodiment, the concave and convex cylindrical lenses 43 and 44 are
By shaping the beam and guiding almost all of the beam emitted from the laser light source 21 to the objective lens 27, the beam utilization efficiency is improved. In addition, the convex cylindrical lens 44 converges the reflected light beam only in the direction orthogonal to the plane of incidence on the reflection surface 32 of the detection prism 31, and the direction of the plane of incidence on the reflection surface 32 of the detection prism 31 is almost parallel to the polarization prism. 25 to the detection prism 31, the information signal,
Focus error signals and tracking error signals can be effectively detected.

As described above, the present invention allows three beams to be incident on an optical element for focus detection, such as the detection prism 31, which changes the beam shape according to the displacement of the recording medium in the direction perpendicular to the surface of the recording medium. , the three beams emitted from the above optical element are incident on the integrated photodetector, and the beam shape of the intermediate beam is detected and the light intensity of the beams on both sides are compared and detected, and the tracking error is detected by one photodetector. It is possible to provide a compact and low-cost device using a three-beam system that can detect signals and focus error signals simultaneously.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the configuration of an optical reading device that applies the focus detection method, FIG. 2 is a diagram showing the configuration of an example of the focus detection device previously proposed by the applicant, and FIG.
The figure is a diagram showing an example of reflection intensity near the critical angle.
FIG. 4 is a diagram showing the configuration of an example of the optical information reading device of the present invention, FIG. 5 is a plan view showing an example of the positional relationship of the three beam spots on the disk shown in FIG. 4, and FIG. is a convex cylindrical lens 33 shown in FIG.
A perspective view of the detection prism 31 and the photodetector 34, and FIGS. 7A, B, and C are diagrams showing how the information signal, focus error signal, and tracking error signal are detected in the embodiment shown in FIG. 4. . FIGS. 8 and 9 are diagrams each showing the configuration of still another example of the optical information reading device of the present invention. 21...Laser light source, 22A, 22B...Relay lens, 23...Diffraction grating, 24...Collimator lens, 25...Polarizing prism, 26...1/4
Wave plate, 27...Objective lens, 28...Disk, 29...Track, 30A, 30B, 30C
... Spot, 31 ... Detection prism, 32 ...
Reflective surface, 33... Convex cylindrical lens, 34... Photodetector, 34A ₁ , 34A ₂ , 34B, 34C... Light receiving area, 35A, 35B, 35C... Spot, 3
6... Adder, 41... Concave cylindrical lens, 42...
Photodetector, 43... Concave cylindrical lens, 44... Convex cylindrical lens, 45... Photodetector.

Claims (1)

[Scope of Claims] 1. After the light beam emitted from the light source is incident on a light splitting element to generate a plurality of beams, an objective lens is used to spot three beam spots arranged approximately linearly on the information track of the recording medium. In an optical information reading device, the reflected light is guided to a photodetector to detect an information signal and a tracking error signal. It is composed of at least four photodetecting surfaces, a second photodetecting surface and a third photodetecting surface disposed on both sides of the first photodetecting surface,
The first, second, and third photodetecting surfaces are integrally fixed as one optical element, and the first, second, and third photodetecting surfaces are arranged on substantially the same plane. A body type photodetecting element is disposed in the reflected optical path from the recording medium, and in the optical path between the objective lens and the integrated photodetecting element, according to the displacement of the recording medium in a direction perpendicular to the surface of the recording medium. An optical element for focus detection that causes a change in the beam shape is arranged, and all of the three beams are made incident on the optical element for focus detection, and an intermediate of the three beams emitted from the optical element for focus detection is arranged. incident on the first photodetecting surface, and making the two beams on both sides incident on the second photodetecting surface and the third photodetecting surface, and
A focus error signal is obtained by detecting a change in the beam shape on the photodetection surface of the second photodetector.
An optical information reading device characterized in that a tracking error signal is obtained by comparing and detecting the magnitude of the amount of light on the first and third photodetecting surfaces. 2. The optical information reading device according to claim 1, wherein the second photodetection surface and the third photodetection surface are both close to the first photodetection surface. .