JPH045968B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for separating and extracting a plurality of light beams forming a slight angle to each other, and particularly to an optical recording method for video tapes, digital audio discs, etc.

For example, information writing or reading apparatuses have been known for writing or reading information through an objective lens or focusing a reading light spot onto an information track recorded in a spiral or concentric form on a recording medium. For example, there is a recording medium called a video disk. On this video disk, video signals and audio signals encoded in information tracks are recorded as optical information such as optical transmission characteristics, reflection characteristics, phase characteristics, etc. 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 write information onto information tracks with such narrow width and pitch, and to accurately read the original information from such information tracks, it is necessary to ensure that the writing or reading light spot is always focused on the information recording surface. It is necessary to perform focusing control to control the information, and tracking control to control the writing or reading light spot so that it always scans over a predetermined information track without deviating from it, and these controls are generally performed approximately parallel to each other. This is done using two or three laser beams. In this case, it is necessary to simultaneously irradiate these beams onto the recording medium, separate the transmitted light or reflected light, and perform detection using a plurality of separate photodetectors.

As such optical path separation means for a plurality of light beams, there are known methods such as increasing the magnification of the optical system and spatially separating the light beams, and making the wavelengths of each light beam different and separating them using a dichroic mirror. ing.

However, the former requires a large optical path length and the optical system becomes large, and the latter requires light sources with sufficiently different wavelengths, making it difficult to use semiconductor lasers or the like.

In the optical recording method of the present invention, two beams are focused by an objective lens on a recording medium on which information is recorded in a track shape, and the reflected light is guided to a photodetector to detect a control signal. A slight angle is formed between the optical axes of the two light beams reflected from the light beam, and the light velocity of one of these light beams is incident on the interface between the high refractive index medium and the low refractive index medium at an angle of approximately the critical angle, The other light beam is made incident on the boundary surface at an angle equal to or less than the critical angle, the one light beam is totally reflected on the boundary surface, and the other light beam is transmitted through the boundary surface to separate the light beams, and the The present invention is characterized in that a focusing control signal is obtained by detecting the light intensity distribution of one of the light beams, and a tracking control signal is obtained by detecting the light intensity of the other light beam.

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

FIG. 1 is a diagram showing the configuration of an example of an optical information recording device equipped with a beam separating means according to the present invention.

In FIG. 1, 1 is a laser light source, which has two light emitting points A and B. In this example, the beam from the light emitting point A is used as the main beam for recording (indicated by a solid line), and the beam from the light emitting point B is used as the sub beam for tracking (indicated by a broken line).

The means for obtaining the main beam and sub-beam is not limited to a laser light source with two light emitting points; it is sufficient that the optical axes of each are spatially separated and tilted to each other.
For example, by separating one beam into two beams using the polarization characteristics of a crystal, grating, etc., or by dividing one beam into a reflected beam and a transmitted beam using a prism etc., one beam can be converted into a main beam. You can also get a secondary beam.

The two beams emitted from the laser light source 1 enter the collimator lens 2, and each beam becomes a parallel light beam, and the optical axes of the two beams are tilted to each other. This is because the center ray (indicated by a chain line) of the main beam is incident on the center of the collimator lens 2, whereas the center ray (indicated by a chain double dot line) of the sub beam is shifted from the center of the collimator lens 2. When the focal length f of the collimator lens is 9 mm and the distance between the light emitting points A and B is 125 μm, the inclination of the optical axes of both output beams is about 48'.

The main and sub beams of the laser light source 1, which are made into parallel beams by the collimator lens 2, are configured to have S polarization characteristics with respect to the reflective surface of the polarizing prism 3, so both beams are reflected by the polarizing prism 3. All of the light is reflected by the surface and enters the 1/4 wavelength plate 4, and both light beams (converted to circularly polarized light) that exit this light enter the objective lens 5.

In this example, the objective lens 5 is placed at the focal point of the collimator lens 2 so that the main beam and the sub beam completely overlap at the objective lens, but the beam diameters of both beams are either larger or smaller to some extent than the aperture of the objective lens. In this case, the two beams may not completely overlap, that is, the objective lens may be placed at a distance from the focal length.

Each beam exiting the objective lens 5 is directed to the disk 6
The image is focused on the recording surface 7 of the image. That is, the main beam is parallel to the optical axis of the objective lens 5, so it forms a spot on the optical axis, and the sub beam is tilted with respect to the optical axis of the objective lens 5, so it forms a spot at a position offset from the optical axis. . For example, if the optical axes of both beams are tilted approximately 48' as in the example above, and the focal length of the objective lens is 4.3 mm, the distance between the two spots is 60 μ.
It will be about m.

Both light beams reflected by the recording surface 7 pass through the objective lens 5 and then enter the 1/4 wavelength plate 4. The light that has passed through the quarter-wave plate 4 has its polarization direction changed by 90 degrees from that at the time of incidence.
In other words, since it becomes P-polarized light, it completely passes through the polarizing prism 3 and enters the detection prism 8.

In the present invention, the reflective surfaces of the detection prism 8, that is, the interface surfaces 9-1 and 9-2 between the high refractive index medium and the low refractive index medium, are set at substantially critical angles or larger than the critical angle with respect to the optical axis of the main beam. The angle is set to be less than or equal to the critical angle with respect to the optical axis of the sub beam. In this way, as is clear from FIG. 2 which shows the relationship between the incident angle and the reflectance of such reflective surfaces, the approximately critical angle or For the main beam incident at an angle greater than that, almost all of the rays are reflected on the reflecting surface 9
1 and 9-2 and travels laterally to the left, the reflection surface 9 is slightly tilted with respect to the main beam.
-1 and 9-2 at an angle less than the critical angle, almost all of the rays are transmitted through these reflecting surfaces and proceed diagonally upward to the left, so the main beam and the sub-beam make a slight angle to each other. The optical paths of the beams can be largely separated and extracted, and detected by photodetectors 10 and 11, respectively. Furthermore, Fig. 2 shows the incident angle vs. reflectance characteristics of the reflecting surface for P-polarized light and S-polarized light when the refractive index of the detection prism 8 is 1.50.
R _P and R _S (reflectance for unpolarized light is the intermediate value of these (R _P + R _S )/
2), and in this example, since it is P-polarized light, the separation efficiency is good. For example, as described above, when the sub beam is incident at an angle of 48' with respect to the main beam that is incident at right angles to the detection prism 8, the angle of inclination of both when incident on the reflecting surfaces 9-1 and 9-2 is the angle of inclination of the detection prism 8. The refractive index of
When it is 1.5, it is about 32', and in this case, the transmittance per one reflection is about 69% for the P polarized light in this example, as is clear from Figure 3, so the total transmission amount is 96' with three reflections. %. In other words, only a small amount of the sub beam of about 4% is incident on the main beam photodetector 10. In addition, in order to simplify the drawings, FIGS. 1 and 2 illustrate the case where the refractive index of the detection prism 8 is n=√2, that is, the critical angle is about 45°.

If it is desired to reduce the amount of light of the sub-beam that enters the main beam photodetector 10, it is sufficient to weaken the emission intensity of the sub-beam relative to the main beam.

In this example, the sub-beam transmitted light flux a ₁ generated by the first reflection and the third transmitted light flux a ₃ are detected by the sub-beam photodetector 11 . However, the transmitted light flux _a2 of the sub-beam generated by the second reflection may be detected, or two sub-beam photodetectors may be provided to detect all three transmitted light fluxes.

By using a detection prism set at a critical angle in this way, it is possible to sensitively detect a slight angular deviation between the main beam and the sub beam, and to spatially separate the two beams. On the other hand, spatial separation can be achieved at the focal plane by using a lens with a sufficiently long focal length compared to the focal length of the objective lens, but in this case, the disadvantage is that the optical system becomes large. be.

The main beam and sub beams separated in this way are controlled so that the optical spot is always focused on the information recording surface, and focusing control is performed so that the optical spot always scans on a predetermined spiral or concentric track without deviating from it. This control is used for tracking control, and these controls will be explained below.

First, focusing control is performed using the main beam. In the focused state, the main beam that passes through the objective lens 5 and prism 3 and enters the reflective surfaces 9-1 and 9-2 is a parallel beam, so approximately all the rays are reflected three times on the reflective surfaces 9-1 and 9-2. The light is totally reflected and enters the photodetector 10. When the disk 6 deviates from the focused state in the direction a or b, the main beams incident on the reflecting surfaces 9-1 and 9-2 become a bundle of rays with a tilt component, and in the figure, the main beams incident on the detection prism 8 The incident angle of the left half of the rays from the center ray or the right half of the rays from the center ray becomes smaller than the critical angle, and only the right half of the rays or the left half of the rays are totally reflected and enter the photodetector 10.
The photodetector 10 has light-receiving areas 10A and 10B divided into two with the center ray of the main beam as the border, and has reflective surfaces 9-1 and 9- that change as described above depending on whether or not they are in focus. The focus error signal is obtained by detecting the light amount distribution of the reflected light of the main beam from 2.

Therefore, by detecting the difference in output between the light receiving areas 10A and 10B, 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 indicated by arrow Y can be performed. Moreover, in the focused state, there is almost no transmitted component on the reflective surface 9,
The loss of light quantity is extremely small, and if the focus is out of focus, the light beam on either side of the central ray will be totally reflected, and the reflection intensity of the light beam on the other side will be extremely reduced. The difference in light amount between 10A and 10B becomes significant. Therefore, focus detection can be performed with sufficient accuracy.

Next, tracking control is performed using the sub beam, which will be explained with reference to FIGS. 3 and 4.

Figure 3 shows the positional relationship between the recording main beam and the tracking sub-beam spots (indicated by M and S, respectively) that are irradiated onto the recording surface of the disk by the optical system in Figure 1, and the information recorded by the main beam. The main beam is turned on and off according to information signals such as encoded video signals, audio signals, and data signals to be recorded, forming pits on the recording surface, and the sub beams form pits on the recording surface. Its center is located at the edge of the recorded track. Since the positional relationship between the main beam and the sub beam is constant, if the position of the sub beam is controlled so that its center is always located at the edge of the adjacent recorded track, the newly recorded pit will always be located at the edge of the adjacent track. are recorded at regular intervals.

Figure 4 shows the relationship between the radial positions of the sub beam and the main beam and the light quantities of the sub beam and main beam reflected from the recording surface, where P _S is the position of the sub beam whose center is located at the edge of the pit, and P _M indicates the position of the main beam located at the center of the pit. As is clear from this, when the sub-beam is displaced in the disk radial direction centering on the edge of the pit, the amount of return light of the sub-beam suddenly changes around the amount of light (V _O ) when the center of the sub-beam is located at the edge of the pit. The first
A signal is obtained from the sub-beam photodetector 11 shown in the figure that changes in both directions around V _O in accordance with the deviation of the pit of the sub-beam from the edge. Therefore, by comparing this signal with the reference value corresponding to V _O , a signal representing the magnitude and direction of the deviation from the edge of the pit of the sub beam, that is, the edge of the recording track, that is, a tracking error signal, can be obtained. As a result, the objective lens 5 in FIG. ₁ is moved in the disk radial direction as indicated by the symbol If the tracking is controlled so that the main beam is positioned at the adjacent recorded track, the main beam is always maintained at a constant distance from the adjacent recorded track. Incidentally, if a tracking track is previously provided on the disk surface, similar tracking control can be performed by positioning the sub-beam at the edge of this groove.

FIG. 5 shows a modification of the optical system in FIG.
Elements corresponding to those in the figures are designated by the same reference numerals. In this example, a convex lens 2' is used in place of the collimator lens 2 shown in FIG.
A concave lens 12 is disposed between the detection prism 8 and the polarizing prism 3 so that the main and sub-beams returning through the polarization prism 3 are made into parallel light fluxes and are made to enter the detection prism 8. In this way, the cross-sectional area of the incident light beam on the detection prism can be reduced, so the detection prism can be made smaller, which is particularly advantageous when the detection prism has a large refractive index. 1, there is an advantage that the cross-sectional area of the incident light beam on the objective lens can be made larger than in the case of FIG. 1. On the other hand, the main and sub beams returning from the disk through the objective lens 5 are treated as diverging beams, and an appropriate lens is placed between the detection prism and the polarizing prism 3 to convert these beams into parallel beams. It is also possible to make it incident on .

Although specific examples of the present invention have been described above,
The present invention is not limited thereto, and numerous modifications and changes are possible. For example, in the example above, 2
Two reflective surfaces are used to separate two nearly parallel beams of light by three reflections, but the number of reflections is arbitrary. Of course, you can also do so. Further, in the above example, two beams are separated, but according to the present invention, three or more beams can also be separated. For example, in a known three-beam reading device, three beams are separated. To do this, direct the first beam to the first reflecting surface at an angle less than the critical angle,
The second and third beams are made incident at an angle approximately equal to or greater than the critical angle, and the second and third beams are totally reflected by this reflecting surface.
The second beam of the beams is then incident on the second reflecting surface at an angle less than or equal to the critical angle, and the third beam is incident on the reflecting surface at an angle substantially greater than the critical angle. It is clear that it can be separated sequentially. Further, in the above example, the refractive index of the detection prism was set to 1.50, but any refractive index can be used as long as the reflective surface of the detection prism is set near the critical angle. Furthermore, although polarized light is used in the above example, the present invention can be effectively applied even when non-polarized light is used. Furthermore, it goes without saying that the present invention can be applied to various other devices that use a plurality of light beams that are substantially parallel to each other.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the configuration of an example of a recording device equipped with a beam separating means according to the present invention, FIG. 2 is a diagram showing the incident angle versus reflectance characteristics of a beam separating reflective surface, and FIGS. 3 and 4 1 is a diagram explaining the principle of tracking of the apparatus shown in FIG. 1, and FIG. 5 is a diagram showing the configuration of a modification of the recording apparatus shown in FIG. 1. 1... Light source, A, B... Luminous point, 2... Collimator lens, 3... Polarizing prism, 4... 1/4 wavelength plate, 5... Objective lens, 6... Disk, 7...
...Recording surface, 8...Detection prism, 9-1, 9-2
... Reflection surface, 10 ... Main beam photodetector, 11
...Sub-beam photodetector, 2'...Convex lens, 1
2...Concave lens.

Claims (1)

[Claims] 1. Information is recorded on a recording medium in the form of a track. 2.
When the beam of a book is focused by an objective lens and the reflected light is guided to a photodetector to detect a control signal, a slight angle is formed between the optical axes of the two light beams reflected from the recording medium. One of the light beams is made incident on the interface between a high refractive index medium and a low refractive index medium at an angle approximately equal to the critical angle, and the other light beam is made incident on the interface at an angle less than or equal to the critical angle. A focusing control signal is generated by totally reflecting the luminous flux on the boundary surface and transmitting the other luminous flux through the boundary surface to separate the luminous fluxes, and detecting the light intensity distribution of the one luminous flux. An optical recording method characterized in that a tracking control signal is obtained by detecting the light intensity of a luminous flux.