JPS647407B2 - - 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 method for detecting defocus and an apparatus for implementing such a method.

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 features 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 method in a conventional optical reading device. Light emitted from a laser light source 1 (linearly polarized in the plane of the paper) is converted into parallel light by a collimating 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. It is focused onto a disk 6 containing information tracks. 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 quarter-wave plate 4, this light is reflected by the polarizing prism 3. The light beam reflected by the polarizing prism 3 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 by, for example, a quarter-divided photodetector (not shown) to obtain a focus error signal, and focusing control is performed using this signal.

However, the conventional focus detection method described above requires a long optical path length to focus after reflection by the polarizing prism 3, which has the disadvantage 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.

The applicant has proposed a focus detection method and device that eliminates the above-mentioned drawbacks, allows a compact optical system, allows easy placement of an optical detector for obtaining a focus error signal, and allows accurate detection of the in-focus state at all times. Already proposed.

The focus detection method developed by the applicant focuses the light emitted from the light source onto the irradiated object using an objective lens, and applies a small portion of the reflected light to approximately one ray in the reflected light beam. Changes in the light intensity distribution of the reflected light reflected by the reflective surface of this optical member when the light is incident on an optical member having a reflective surface set to have a critical angle, or the difference between the reflected light reflected by the reflective surface and the refracted light A focus error signal of the objective lens with respect to the object to be irradiated is obtained by detecting a change in the amount of light with respect to the transmitted light using a divided photodetector or two photodetectors.

FIG. 2 is a diagram showing the configuration of essential parts of an example of an optical reading device that implements the focus detection method developed by the applicant. The optical reading device shown in this example has the same configuration as the optical reading device shown in FIG. 1 until the reflected light from the disk 6 is reflected by the polarizing prism 3. Represents a member. In this example, the light beam reflected by the polarizing prism 3 is incident on the detection prism 10, and the light beam reflected by the reflective surface 11 is detected by the photodetector 12.
It receives light. The reflective surface 11 is set to form a substantially critical angle with respect to an incident light beam (parallel light beam) in a focused state. 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 an inclination component shown by the outermost rays a _i1 to a _i2 with respect to the reflecting surface 11. Further, when the disk 6 deviates from the focused state in the direction b, the light beams incident on the reflective surface 11 become a bundle of light rays having tilt components with the outermost light rays indicated by b _i1 to b _i2 .
That is, if the disk 6 deviates from the focused state,
The light rays 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 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. 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 amount distribution of the reflected light from the reflecting surface 11,
As shown in the plan view in Fig. 2, there are two light-receiving areas 12A and 12B divided into two with the central ray (optical axis) as the border.
It consists of: 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 _t1
A bundle of rays including from n to n is transmitted. The intensity of the reflected ray bundle including the outermost reflected ray a _r1 to the center ray is weakened by the amount of light transmitted. 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 _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 a _r2 to the center ray. Therefore, in this case, the photodetector 1
The light intensity distribution on 2 is such that the light receiving area 12A becomes dark,
The light receiving area 12B remains bright and does not change.

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 areas 12A and 12B of the photodetector 12 The relationship between brightness and darkness is reversed, and the light-receiving area 12A remains bright and unchanged, but the light-receiving area 12B becomes dark. In this case, the reflected light and the transmitted light on the reflecting surface 11 are denoted by b r1 and b _r1 , respectively.
Denoted by b _r2 and b _t2 .

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

Therefore, by detecting the output difference between the light receiving areas 12A and 12B with the differential amplifier 33, a focus error signal representing the amount and direction of deviation can be obtained from the amount and polarity, and based on this signal, It is possible to perform focusing control to control the movement of the objective lens 5 in the optical axis direction, and also to control the movement of the objective lens 5 in the light receiving area.
By calculating the sum of the outputs of 12A and 12B in an adder 14, an information signal corresponding to a pit recorded on the disk 6 can be detected. 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.

In the example shown in FIG. 2, the light reflected by the reflective surface 11 is received by the photodetector 12 having two divided light receiving areas 12A and 12B. The light is received by a photodetector and changes in the light intensity distribution are detected, and the reflected light and transmitted light are
A focus error signal can also be obtained by receiving light with two photodetectors and detecting changes in the amount of light.

Although such a focus detection method has various advantages as described above, it is possible to set the detection prism 10 so that its reflective surface 11 is aligned with a predetermined light beam in the reflected light beam from the disk 6, for example, the central light beam on the optical axis. In addition, it is necessary to arrange the photodetector 12 so that it receives light separately on both sides of the central ray, and also to receive the same amount of light when in focus. Therefore, there is a problem that optical adjustment becomes difficult. Furthermore, since a bisected photodetector or two separate photodetectors is used, the size of the detection prism 10 increases and a plurality of detection prisms are required, resulting in a complicated configuration. In particular, if a large-sized detection prism is used, a focus error signal will be mixed into the information signal, resulting in a problem that the S/N ratio of the information signal will deteriorate.

As a technique similar to the above, there is Japanese Patent Application Laid-open No. 130102/1983. In this FiG.7, the reflected light beam from the recording medium is divided into two light beams, a focus detection prism called the so-called Foucault method is placed in one light beam, and a photodetector for detecting information signals is placed in the other light beam. However, such a focus detection device has the disadvantage that the optical path length is long. Also Tokko Akira
FIG. 1 of Publication No. 51-20888 describes a technique in which a focus detection member using a pinhole is arranged in a similar optical path configuration. However, such a focus detection device has the drawback that detection accuracy cannot be obtained.

An object of the present invention is to provide a focus detection device that eliminates the various drawbacks mentioned above.

The information reading device of the present invention focuses light emitted from a light source onto an information recording medium using an objective lens,
In a focus detection device that obtains a focus error signal and an information signal by detecting the reflected light flux, the reflected light flux from the recording medium is divided into two light fluxes, and at least one light beam in one of the light fluxes is divided into two. A prism with a surface set at an approximately critical angle is arranged, and reflected light from the prism is guided to at least two photodetectors to obtain a focus error signal, and information is read in the other light beam. The device is characterized in that a photodetector is arranged to obtain an information signal.

Hereinafter, the present invention will be explained in detail with reference to the drawings.

FIG. 4 is a diagram showing the configuration of an example of an apparatus for implementing the focus detection method of the present invention. Laser light source 21
A polarized light beam (P-polarized light) emitted from the polarization lens 22 becomes a parallel beam of light and enters the objective lens 25 via a polarizing prism 23 and a quarter-wave plate 24 . The objective lens 25 focuses this parallel light beam and projects a spot light onto the disk 26. The reflected light from the disk 26 is passed through the objective lens 25.
The light is focused and passed through a 1/4 wavelength plate 24. Therefore, this light beam becomes S-polarized light, and polarizing prism 2
It is reflected at 3. A portion of the light beam reflected by the polarizing prism 23 enters the detection prism 27. The reflective surface 27a of the detection prism 27 is set to be smaller than a critical angle with respect to the incident light beam, and
In particular, in this example, the amount of reflected light reflected by the reflecting surface 27a and the amount of transmitted light refracted at the time of focusing are set to be approximately equal. As described above with reference to FIG. 2, when focusing, a parallel beam of light is emitted from the objective lens 25, and the light incident on the reflective surface 27a is reflected and refracted by this surface and transmitted. In this example, the reflective surface 27
A photodetector 28 receives the reflected light at point a. When the disk 26 is displaced in the direction a from this focused state, the angle of incidence of the light beam incident on the reflective surface 27a approaches the critical angle, so that the amount of light incident on the photodetector 28 becomes brighter. On the other hand, when the disk 26 is displaced in the b direction, the angle of incidence of the light rays incident on the reflective surface 27a becomes even smaller than the critical angle, and the amount of light that is refracted and transmitted by the reflective surface 27a increases, so that the light rays are detected by the photodetector 28. The amount of light incident on becomes darker. Therefore, the photodetector 28
The output changes as shown in FIG.

As shown in FIG. 4, among the light beams reflected by the polarizing prism 23, the light beams that do not enter the detection prism 27 are received by the photodetector 30. This photodetector 30
An information signal can be obtained by amplifying the output with the amplifier 31. Since the light beam incident on the photodetector 30 does not pass through the detection prism 27, there is no change in the amount of light even if there is a defocus, so an information signal with a good S/N ratio can be obtained.

On the other hand, the output of the photodetector 28 is filtered through the reduction filter 33.
The high frequency components are removed through the filter and supplied to one input terminal of the differential amplifier 34. The other input terminal of this differential amplifier 34 is supplied with a reference voltage from a reference voltage source 35.
Apply Vr. As shown in FIG. 5, this reference voltage Vr has a value approximately equal to the output voltage obtained from the reduction filter 33 during focusing. A desired focus error signal representing the direction and amount of deviation from the focus position can be obtained. That is, when the disc 26 is in the focused position, the output of the differential amplifier 34 is "0", and when the disc 26 shifts from the focused position in the direction a, the output of the differential amplifier 34 becomes positive according to the shift. Conversely, when the disk 26 shifts in the b direction, the potential becomes negative in accordance with the shift. Therefore, if focusing control is performed to move the objective lens 25 in the optical axis direction based on the output of the differential amplifier 34 so that the output becomes "0",
The objective lens 25 can always be kept in focus with respect to the disk 26.

FIG. 6 is a diagram showing the configuration of another example of an apparatus for implementing the focus detection method of the present invention. In this example, almost all of the return light from the disk 26 that is emitted from the polarizing prism 23 is made to enter the detection prism 37. The reflective surface 37a of the detection prism 37 is set to be smaller than the critical angle with respect to the incident light beam as described above, but in this example, the reflective surface 37a
A total reflection mirror 38 is provided except for a part of the total reflection mirror 38, and the return light from the disk 26, which is totally reflected by the total reflection mirror 38, is incident on the photodetector 30 for detecting the information signal. reflective surface 37
The return light from the disk 26, which is refracted and transmitted by a, is received by a focus detection photodetector 39, and the outputs of these photodetectors 30 and 39 are processed in the same manner as described above to generate an information signal and a focus error signal. get. Note that the amount of light incident on the photodetector 39 in this case is different from that of the disk 2 because the light transmitted through the reflective surface 37a is received.
If 6 is shifted in the direction a, it becomes smaller, and if it shifts in the direction b, it becomes larger. Therefore, since the output of the photodetector 39 is opposite to that shown in FIG. 5 as shown in FIG. 7, the focus error signal obtained from the differential amplifier 34 is also The polarity is opposite to that in the previous case.

FIG. 8 shows a photodetector 28 or 39 for focus detection.
12 shows the configuration of another example of a focus error detection circuit that processes the output of and obtains a focus error signal. The output of the photodetector 28 is supplied via a reduction filter 33 to the positive input terminals of a differentiating circuit 41, a gate circuit 42, and a differential amplifier 34. The differentiating circuit 41 is the photodetector 2
8 is detected, and its output is smoothed through a detection circuit 43 and a smoothing circuit 44 and supplied to one input terminal of a differential amplifier 45.
A predetermined reference voltage is applied from a reference voltage source 46 to the other input terminal of the differential amplifier 45, and the difference therebetween is detected. Now, assuming that the output shown in FIG. 5 is supplied from the photodetector 28, the disc 26 is in the direction a from the focusing position,
When the incident light beam on the reflecting surface 27a of the differential amplifier 7 deviates to be larger than the critical angle, the output of the differentiating circuit 41 becomes zero, and this output is supplied to one input terminal of the differential amplifier 45. In this example, the reference voltage value applied to the other input terminal by the reference voltage source 46 is set to approximately zero so that the output of the differential amplifier 45 is approximately zero at this time. When the output of the differential amplifier 45 is zero, the signal causes the gate circuit 42 to
is opened and the photodetector 28 passes through the reduction filter 33.
The output of the differential amplifier 47 is supplied to the positive input terminal of the differential amplifier 47. A predetermined maximum value (V _nax in FIG. 5) obtained from the photodetector 28 stored in the reference value memory 48 is applied to the negative input terminal of the differential amplifier 47, and the difference between these values is added to the adder 49. Supplied to one input terminal. The other input terminal of this adder 49 receives the predetermined maximum value from the reference threshold value memory 50.
Focusing threshold voltage corresponding to V _nax (in Fig. 5)
Vr) and add these to the threshold memory 5.
Supply to 1. This threshold memory 51 is controlled to be written by the output of the differential amplifier 45 when the output is approximately zero, that is, in synchronization with the opening of the gate circuit 42, and the written output of the differential amplifier 49 is controlled. is supplied to the negative input terminal of the differential amplifier 34 as a focusing threshold. In such a configuration, the reduction filter 33
That is, since the output of the photodetector 28 is always compared with a predetermined reference voltage Vr, for example, the output light intensity of the light source 21 may fluctuate, the reflected light intensity of the disk 26 may fluctuate, or the photodetector 28 Even if the output signal of Vr fluctuates due to the influence of drift or the like, the reference voltage Vr will follow and fluctuate, so it is possible to always obtain an accurate focus error signal.

In addition, in Fig. 8, the difference between the set maximum value and the detected actual maximum value is calculated, and this difference is added to the set reference threshold value to correct the focusing threshold. It is also possible to divide the value by a predetermined partial pressure ratio (for example, 1/2) and use it as the focusing threshold. Further, the focusing threshold value can be corrected by, for example, providing a switching circuit between the reduction filter 33 and the differentiating circuit 41, and instantaneously operating this circuit, for example, every rotation of the disk to supply the output of the reducing filter 33 to the differentiating circuit 41. It is also possible to make corrections. Furthermore, in FIG. 8, when the output of the differential amplifier 45 is almost zero, the gate circuit 42 is opened and writing to the threshold memory 51 is controlled, but the output of the differential amplifier 45 is inverted and when the output is high. It can also be configured to perform the above-mentioned necessary control when the level is reached. Further, the correction value of the focus threshold value can also be calculated digitally. Furthermore, the focus error detection circuit as shown in FIG. 8 can be similarly applied to the focus detection device shown in FIG.

Note that the present invention is not limited to the above-mentioned example, and can be modified or changed in many ways. For example, the focusing range can be set by giving a range to the threshold value. Further, in the above-described example, the detection prisms 27 and 37 have one reflective surface 27a and 37.
a is formed, but it is also possible to form two opposing reflecting surfaces so that the incident light beam is reflected multiple times. In this way, detection sensitivity can be further improved. In addition, the reflective surfaces 27a, 37
A may be formed of a plate-shaped optical member without a prism. Furthermore, in the above example, the S-polarized light is incident on the detection prisms 27 and 37, but the polarization prism 23 and the detection prisms 27 and 37
A 90° rotating element may be placed between the light source 21 and the detection prisms 27 and 3 relative to the polarizing prism 23.
7 is reversed so that S-polarized light is emitted from the light source 21, and the P-polarized light is emitted from the detection prisms 27 and 37.
It is also possible to make polarized light incident. In this case, as is clear from FIG. 3, since the reflection intensity characteristics of P-polarized light have better sensitivity than S-polarized light, the focus error can be detected with higher sensitivity. Further, in the above example, the information signal is obtained from the photodetector 30, but this photodetector 30 is divided into appropriate light receiving areas according to the depth of the pit of the disk 26, and the returned light from the disk 26 is Therefore, if the image formed on the pupil plane of the objective lens 25 is formed on the photodetector 30, a tracking error signal representing the relative positional deviation of the objective lens 25 with respect to the information track can be detected at the same time as the information signal. be able to. Furthermore, in the above-mentioned example, the return light from the disk 26 that is emitted from the polarizing prism 23 becomes a parallel light beam during focusing, but the return light from the disk 26 that is emitted from the polarizing prism 23 during focusing is The present invention can be effectively applied to both diffused light and focused light. Furthermore, a half mirror can be used instead of the polarizing prism 23. In addition, in order to prevent noise due to unnecessary reflection of light within the detection prism, when receiving the reflected light from the reflection surface, cover the detection prism's surfaces other than the entrance surface, reflection surface, and output surface with a light absorber. Furthermore, when receiving refracted and transmitted light, noise can be reduced by covering the surfaces of the detection prism other than the incident surface and reflective surface with a light absorber. Further, the present invention can be effectively applied to the focus detection of various optical devices such as recording devices or magneto-optical disk recording and reproducing devices, in addition to the above-mentioned optical disk reproducing device.

As explained in detail above, in the present invention,
Since it is sufficient to set the reflective surface of the optical member that receives the return light from the irradiated object to be smaller than the critical angle with respect to the incident light beam, manufacturing of the optical member and overall optical adjustment are facilitated, and the present invention The purpose of this can be effectively achieved.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the configuration of an optical reading device that applies a conventional focus detection method, and Fig. 2 shows the configuration of an example of the main part of an optical reading device that implements the focus detection method developed by the applicant. Figure 3 is a diagram showing an example of reflection intensity near the critical angle, Figure 4 is a diagram showing an example of reflection intensity near the critical angle.
The figure is a diagram showing the configuration of the essential parts of an example of an optical reading device that implements the focus detection method according to the present invention, FIG. 5 is a signal waveform diagram for explaining its operation, and FIG. 6 is a diagram according to the present invention. A diagram showing the configuration of the main parts of another example of an optical reading device that implements the focus detection method, FIG. 7 is a signal waveform diagram for explaining the operation, and FIG. 8 is a diagram showing the focus error detection circuit and other components. FIG. 21...Laser light source, 22...Collimating lens, 23...Polarizing prism, 24...1/4 wavelength plate,
25...Objective lens, 26...Disc, 27,
37...Detection prism, 27a, 37a...Reflection surface, 28, 30, 39...Photodetector, 33...Reduction filter, 34...Differential amplifier, 35...Reference voltage source, 38...Total reflection mirror.

Claims (1)

[Scope of Claims] 1. An information reading device that focuses light emitted from a light source onto an information recording medium using an objective lens, and obtains a focus error signal and an information signal by detecting the reflected light beam, comprising: The reflected light beam from the recording medium is divided into two light beams, a prism having a surface set to form an approximately critical angle with respect to at least one ray in the light beam is placed in one of the light beams, and the beam is separated from the prism. Information characterized in that the reflected light is guided to at least two photodetectors to obtain a focus error signal, and a photodetector for information reading is placed in the other light beam to obtain an information signal. reading device.