JPS6224443A - Focus detector - Google Patents

Abstract

PURPOSE:To constitute a small optical system by providing a detecting prism having a single reflecting surface set so as to become approximately a critical angle with respect to the optical axis of a reflected light beam. CONSTITUTION:The reflecting surface 11 is set to a critical angle or slightly smaller than said angle with respect to an incident rays of light (parallel flux) in the focused state. When a disk 6 is displaced in directions (a) and (b) to deviate from the focused state, the light and dark states against a paper surface including a central rays of light are reverse each other with a vertical surface as a boundary, because reflecting intensity on the reflecting surface 11 is sharply changed due to a slight change in the incident angle. On the other hand, rays of light are reflected uniformly in the focused state, and therefore such light and dark never appears. A photodetector 12 detects the luminous energy distribution of the reflected rays of light from the reflecting surface 11 and is constructed with two light receiving areas 12A and 12B available from splitting an area with a central rays of light (optical axis) as a boundary.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an apparatus for reading information by focusing a reading light spot through an objective lens onto an information trunk recorded in a spiral or concentric manner on a recording medium, for example.

The above-mentioned information reading device is conventionally known, and examples of recording media having information tracks include those called video discs.

On this video disc, video signals and audio signals encoded in information tracks are recorded as optical information such as optical transmission characteristics, reflection characteristics, and phase characteristics. Information recorded on a video disc is recorded on a video disc by rotating it at high speed, focusing the laser light emitted from a laser light source onto the information trunk through an objective lens, and detecting the transmitted or reflected light modulated by the information track. and reading it. 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 an information trunk with such a narrow width and pitch, the objective lens must always be in focus on the video disc surface, and the diameter of the light spot on the disc surface must be adjusted. needs to be made smaller. For this reason, in such an optical reading device, a force sowing 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 front 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. Light emitted from a laser light source 1 (linearly polarized in the plane of the paper) is made into parallel light by a collimating lens 2, and passes through a polarizing prism 3 having a polarizing film, an A wavelength plate 4, and an objective lens 5, and contains an information trunk. It is focused onto the disk 6. This light beam is reflected by the information track having a concave and convex bit shape, and enters the polarizing prism 3 via the objective lens 5 and the A-wave plate 4. The reflected light incident on the polarizing prism 3 passes through the A wavelength plate 4
Since the light is polarized in a direction perpendicular to the plane of the paper due to the action of , 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 condensing lens 7 and the cylindrical lens 8 will change to a state in which it is correctly focused on the information track if the position of the disk 6 is shifted up or down. Deforms in a direction perpendicular to the (focus position) as a boundary. Conventionally, this shape change is detected by, for example, a quarter-divided photodetector (not shown) to obtain a focus error signal, and this signal is used to perform force focusing control.

However, the above-described conventional focus detection method requires an optical path length for focusing after reflection from 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 placed accurately in the direction of the two wheels, that is, the direction of the optical axis and the direction in a plane orthogonal thereto, there is a drawback that 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. Furthermore, as a conventional technology, "Fokussi
erungin einer Anor-dnung
zum Au5lesen eines op
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igers ” Neves aus der
TechnikNr, 6 vom15. Deze
m'ber 1977, pages 1-2, which describes a forcing detection device using a right-angle prism. Such a device has the disadvantage that the device becomes large and the optical path becomes complicated.

It is an object of the present invention to eliminate the above-mentioned drawbacks, to provide a focus detection device that can have a compact optical system, facilitate the placement of an optical detector for obtaining a focus error signal, and always accurately detect the focus state. It is in the offering.

The focus detection device of the present invention includes an objective lens that focuses light emitted from a light source and irradiates the object to be irradiated, and a light beam that shares a small portion of the reflected light between the objective lens and a photodetector. a detection prism having a single reflecting surface set to have a y-critical angle with respect to the optical axis of the reflected light or one ray in the incident reflected light beam; The present invention is characterized in that the photodetector is configured to obtain a focus error signal of the objective lens with respect to the irradiated object based on changes in the light intensity distribution of the light beam modulated by the reflective surface of the detection prism.

The present invention will be described in detail below with reference to the drawings.

FIG. 2 is a diagram showing the configuration of essential parts of an example of an optical reading device implementing the focus detection device according to the present invention. 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 1o, and the light beam reflected by the reflecting surface 11 is received by the detector 12. The reflective surface 11 is set so as to form a critical angle or a little smaller than a critical angle with respect to an incident light beam (parallel light beam) in a focused state. If the critical angle is set exactly, in the focused state, all the rays reflected by the polarizing prism 3 will be totally reflected by the reflecting surface 11 (actually, since the condition of the reflecting surface is not perfect, it will be slightly shifted in the n direction shown in the figure). When the disk 6 is shifted 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 all''alt with respect to the reflecting surface 11. When the disc 6 deviates from the focused state in the b direction, the incident light beam on the reflective surface 11 becomes 1) il
The result is a beam of light having an inclination component represented by ~bi. That is, when the disk 6 is out of focus, the incident light rays on the reflective surface 11 change continuously around the critical angle, except for the central ray (dotted chain line) on the optical axis. Therefore, disk 6
is displaced in directions a and b and out of focus, the intensity of reflection at the reflecting surface 11 changes rapidly with a slight change in the angle of incidence near the critical angle, as shown in FIG. The brightness and darkness are reversed across a plane perpendicular to the plane containing the paper. On the other hand, in the focused state, the light is totally reflected at -1, so such brightness and darkness do not appear. 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. Two light receiving areas 1
It consists of 2A and 12B. Note that FIG. 3 shows the reflection intensities Rp and Rs of P-polarized light and S-polarized light, respectively, when the refractive index of the detection prism 10 is 1.50. The reflection intensity for unpolarized light is
The intermediate value between these is (Rp +Rs)/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 rays, starting with the outermost incident ray ai+. The angle of incidence of is smaller than the critical angle. Therefore, in this portion, there is transmitted light, and a bundle of light rays including the outermost transmitted light rays aL+ to n is transmitted. The intensity of the reflected ray bundle including the outermost reflected ray a- to the center ray is weakened by the transmitted amount. Reflective surface 11
Of the light incident on , the light flux above the center ray in the figure is the outermost incident ray a,! The angle of incidence of all incident rays, starting with , is larger than the 9n boundary 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 the outermost reflected light vAart to the center ray. Therefore, in this case, in the light amount distribution on the photodetector 12, the light receiving area 12A becomes dark and the light receiving area 12B becomes bright. If the reflective surface 11 of the detection prism 10 is set at a perfect critical angle,
As can be seen from Figure 3, it remains bright and does not change, but if it is set slightly smaller than the critical angle, the light receiving area 12
B changes brightly. However, as shown in Figure 3, Rp
, Rs theoretically becomes infinite at the critical angle, so the sensitivity near focus is highest when set completely at the critical angle. Furthermore, if the angle of field is set to be larger than the 9p field angle, a zone will occur where the amount of light 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 α■ area 12 of the photodetector 12
The brightness relationship of A and 12B is reversed. In this case, the reflected light and the transmitted light on the reflecting surface 11 are each denoted by the symbol brl.
+tlr! and brZ.

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

Therefore, by detecting the difference between the outputs of the respective 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 based on this signal, the objective lens 5 is moved along the optical axis. Focus lens control for controlling movement in the direction can be performed, and a signal corresponding to a pit recorded on the disc 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. 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.

For example, as the objective lens 5, NA=0.5, f,
If a three-fin type is used, the change in the angle of incidence on the reflecting surface 11 when the disc 6 changes by 1 μm is determined by the light ray with the largest change (the outermost one), assuming that the refractive index of the detection prism 10 is 1.50. The angle is approximately 0.015° with respect to the light ray), and the amount of light changes sufficiently.

In addition, when the disk 6 is shifted by about 0.2 degrees in the a direction, the focal position is f=3 m, and from the objective lens 5 to the disk 6
Since the position is 19.5n away from the photodetector 12, the diameter of the beam irradiated onto the photodetector 12 becomes large. On the other hand, if the focus position is shifted by about 0.2N in the b direction, the focal position will be 25.5n away from the objective lens 5 on the opposite side of the disk 6, and the photodetector 12
must be placed closer than this. However, under this condition (the condition that the photodetector 12 is located at a position of 25.5 fins from the objective lens 5), the disk 6 is 0 in the b direction.
．． Even if the distance is two cranes or more, the objective lens 5 is controlled further away from the disk 6, so collision with the disk 6 can be avoided without any modification.

In addition, in Fig. 2, the refractive index of the detection prism lO is shown.

As v7, the direction of the reflected light beam is changed by 90' with respect to the incident light beam, but if a material with a refractive index larger than this is used, the change in direction can be made smaller than 90°.

FIG. 4 is a diagram showing the configuration of main parts of another example of an optical reading device implementing the focus detection device according to the present invention. In this example, a part of the light beam reflected by the polarizing prism 3 is made incident on the detection prism 1o, and reflected light and transmitted light are made to exist at a predetermined ratio at the time of focusing on the reflecting surface 11. The light is received by photodetectors 15 and 16, and the other configuration is the same as that in FIG. 2. For this reason, the reflecting surface 11 is installed so as to form a critical angle or a slightly smaller angle with respect to one ray 0 in the reflected light beam. In this way, when the disk 6 is displaced in the a direction and the b direction and is out of focus, a corresponding difference in output will appear on the photodetectors 15 and 16, so as in FIG.
A focus error signal representing the amount and direction of deviation can be obtained from the amount and polarity of this output difference.

In this case, at the time of focusing, the photodetector 15.
Since it is sufficient that the amount of light incident on the disk 16 is a predetermined ratio, the reflected light beam from the disk 6 does not necessarily have to be a parallel light beam, but may be a convergent light beam or a diverging light beam. Also,
The pit-corresponding signal can be obtained by detecting the sum of the outputs of the photodetectors 15 and 16 or by detecting the portion of the light beam that does not enter the detection prism 10 out of the reflected light from the polarizing prism 3.

According to the present invention described above, a minute spot is formed like the conventional focus detection device using a cylindrical lens described above,
There is no need for the troublesome adjustment of aligning the center of the four-split photodetector, and since there is no need to narrow down the beam, a large luminous flux can be input to the photodetector, making optical adjustment extremely easy. It becomes easier. Moreover, since there is no need for adjustment in two axes, the photodetector and the detection prism can be mechanically positioned and attached as a unit by rotation adjustment within the plane of the paper. even smaller subo/
), the entire device can be constructed to be lightweight and compact.Therefore, the movement of the objective lens in the direction of the optical axis and in the direction orthogonal to this and orthogonal to the information track is controlled. It is also possible to incorporate the entire optical system into the dimensional driver. Additionally, when downsizing an optical system, the number of lenses in the objective lens (the figure shows one lens, but in reality it has multiple lenses) is generally reduced, and only spherical aberration is considered. ing. In such cases, it is required not to use off-axis light rays, and it is necessary to make parallel light beams incident on the objective lens. Therefore, as in the present invention, if focusing is detected in the state of parallel light beams, it is extremely effective in downsizing the optical system.

This also applies when an aspherical lens or the like is used as the objective lens. Furthermore, in the case of an optical reading device for a video disc in which a spiral or concentric information trunk is formed on the recording medium as shown in the above-described embodiment, the optical system must be arranged so that the bit moves in the direction of the plane of the paper. For example, the focus error signal is not affected by changes in the light amount distribution when crossing the track.

It should be noted that the present invention is not limited only to the above-mentioned example, and can be modified or changed in many ways.
In the figure, the S-polarized light is incident on the reflective surface 11 of the detection prism 10, but as shown in FIG.
8, the light flux incident on the detection prism 10 can be made into P-polarized light. In this way, as is clear from FIG. 3, the change in reflection intensity becomes steep near the 9R field angle, so the detection sensitivity can be further increased. Furthermore, in order to make the P-polarized light incident on the detection prism 10 in this way, the relationship between the polarization prism 3 and the detection prism 10 can be adjusted by 90 degrees from the state shown in FIG. The polarizing prism 3 may be rotated in the direction shown in FIG. Furthermore, in order to increase the detection sensitivity, as shown in Figure 7, a long detection prism 1 is used.
It is also possible to increase the sensitivity by the number of reflections by causing multiple reflections within 0'. Furthermore, as shown in FIG. 8, the relationship between the polarizing prism 3 and the detection prism 10 may be reversed.

Furthermore, the configurations shown in FIGS. 5 to 8 can be effectively applied to the focus detection device shown in FIG. 4.

Furthermore, in the above example, the refractive index of the detection prism was exemplified as a for convenience, but since this can be set near the critical angle, any refractive index can be used as long as that condition is met. Furthermore, although the above-described example uses polarized light, the present invention can be effectively applied even when non-polarized light is used. Furthermore, in FIG. 4, the reflective surface 11 only needs to form a critical angle with respect to one of the reflected light rays incident on the detection prism 10, or slightly smaller than this, so that the incident light ray is The light beam is not limited to a parallel light beam, but may be a converging or diverging light beam.Furthermore, although the polarizing prism 3 is used in each of the above examples, a half mirror may also be used. Furthermore, the present invention can be applied to focus detection of various optical devices in addition to the above-mentioned optical reading device for video discs.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the configuration of an optical reading device applied to a conventional focus detection device, and FIG. 2 is a diagram showing the configuration of an example of the main part of an optical reading device implementing the focus detection device according to the present invention. 3 is a line diagram showing an example of the reflection intensity near the critical angle, and FIG. 4 is a line diagram showing the configuration of the main part of another example of an optical reading device implementing the focus detection device according to the present invention. 5, 6, 7, and 8 are diagrams each showing the configuration of a main part of still another example of an optical reading device implementing the focus detection device according to the present invention. 1-.-Laser beam, 2-.-Collimating lens,
3- Polarizing prism, 4-'A wavelength plate, 5--Objective lens, 6-... Disc, 10.10'...
・Detection prism, 11-...-Reflection surface, 12.15.16
-・Photodetector, 18-・-90゛ rotor. Fig. 1 b Fig. 4 Fig. 5

Claims (1)

[Claims] an objective lens that focuses the light emitted from the light source and irradiates the object to be irradiated; and an objective lens arranged so that a small portion of the reflected light is incident between the objective lens and the photodetector, and a detection prism having a single reflective surface set at a substantially critical angle with respect to the optical axis of the reflected light or one ray in the incident reflected light beam, and the detection prism is modulated by the reflective surface of the detection prism. A focus detection device characterized in that the focus detection device is configured to use the photodetector to obtain a focus error signal of the objective lens with respect to the irradiated object based on a change in the light intensity distribution of the received light beam.