JPS6132226A - Optical displacement detecting device - Google Patents

Abstract

PURPOSE:To increase a spot of light and detect the displacement of an object to be detected accurately by separating diffracted light and direct transmitted light from each other during optical displacement detection by utilizing the difference in angle of incidence on a hologram between the 1st light and the 2nd light. CONSTITUTION:Light beams 2a-2c which are converged by a lens 5 and reflected by a beam splitter 4 are mode incident on a recording medium 7 such as a photographic dry plate on which a hologram is recorded and are made incident on a photodetector 8 through this recording medium 7. The hologram on the recording medium 7 consists of the 1st and the 2nd holograms which are formed by double exposure. The photodetector 8 is arranged nearby convergence points of spots 21 and 22 of the 1st and the 2nd holograms formed by the converged light beams so that the spots 21 and 22 are detected by photodetection parts 8a and 8b, and 8c and 8d. Consequently, the displacement of the recording surface of an optical disk 6 in an optical axis direction and its direction are detected and a focus adjustment is made.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an optical displacement detection device for optically detecting displacement of an object to be detected both in the optical axis direction and in the direction perpendicular to the optical axis. It is.

Background Art and Problems Therein, for example, an optical disc player requires a focus servo and a tracking servo to maintain a predetermined positional relationship between the optical disc and the optical system. To do this, it is necessary to detect the focus error, that is, the displacement of the recording surface of the optical disc from a predetermined position in the direction of the optical axis, and the tracking error, that is, the displacement of the recording surface of the optical disc from a predetermined position in the direction perpendicular to the optical axis. There is a need to.

One method for detecting focus errors and tracking errors is to form 0th-order diffracted light and 11th-order diffracted light using a diffraction grating, detect the signal and focus error using the 0th-order diffracted light, and detect the 11th-order diffracted light. A three-spot method for detecting trunking errors using diffracted light is conventionally known.

In this three-spot method, the 0th-order and 11th-order diffracted lights are detected separately, but the spots of these lights are only slightly separated from each other. Therefore, unless the plurality of photodetectors are positioned extremely accurately, the displacement of the object to be detected cannot be detected with high accuracy.

OBJECTS OF THE INVENTION The present invention solves the above-mentioned problems. Even if i number of photodetectors are not positioned very precisely, optical displacement can accurately detect the displacement of +J objects in both the optical axis direction and the direction perpendicular to the optical axis. The purpose is to provide a detection device (W).

Summary of the Invention The present invention provides a recording medium on which a hologram forming a predetermined angle with respect to the surface is recorded, and an object to be detected that is incident on the recording medium at an angle substantially corresponding to the angle and is diffracted by the hologram. a first photodetector that detects first light from the object; and a second light from the object that is incident on the recording medium at an angle that does not correspond to the angle and directly passes through the hologram. and a second photodetector for
The detection output of the first photodetector detects the displacement of the object in the optical axis direction, and the detection output of the second photodetector detects the displacement of the object in the direction perpendicular to the optical axis. The present invention relates to an optical displacement detection device that detects the displacement of the object.

Embodiment Hereinafter, an embodiment of the present invention applied to an optical disc player will be described with reference to FIGS. 1 to 9.

FIG. 1 shows the configuration of this embodiment. Coherent light 2 emitted from a semiconductor laser 1 such as a laser diode is divided by a diffraction grating 3 into first light 2a, which is 0th-order diffracted light, and second light 2b, 2C, which is ±1st-order diffracted light. separated into These lights 2a, 2b, and 2C pass through the beam brick 4 and are focused by the lens 5 onto the recording surface of the optical disc 6.

Light 2a, 2b, 2C reflected by the recording surface of the optical disc 6
enters the lens 5 again. Light 2a converged by this lens 5 and reflected by the beam brick 4,
2b and 2C enter a recording medium 7 such as a photographic plate on which a hologram is recorded, and enter a photodetector 8 via this recording medium 7.

The expansion and flexibility and area of the photodetector 8 can be varied depending on the placement position, but in this embodiment, as shown in FIG. 2, a disk and an annular plate circumscribing the disk are used. Six photodetectors 8a to 8f divided into four and two equal parts in the circumferential direction, respectively.
It consists of

The hologram on the recording medium 7 consists of double exposed first and second holograms. The first hologram is recorded by interference between the lights 11a and 11b as shown in FIG. 3A, and the second hologram 1 is recorded by the fourth hologram.
As shown in Figure A, this is recorded by interference between lights 12a and 12b.

The light Za is located at the convergence point F (lens 5) of the light 2a shown in FIG.
In FIG.
, 2C to the incident surface and the opposite surface of the beam block 1
This is the light that enters through 3. The light 11b is transmitted through the lens 14
, is light that enters the recording medium 7 via the sheet-like aperture 15 and the beam splitter 13.

The diaphragm 15 is provided with an elliptical aperture 15a as shown in FIG.
The top spot is oval shaped. Therefore, light 11a and ll
The first hologram recorded by interference with the aperture 15a also has an elliptical shape similar to the aperture 15a.

The light 12a is reflected by the recording surface of the optical disk 6 located at a distance of 2-Δ2 from the lens 5 and enters the recording medium 7 at an angle θ2 corresponding to the incident angle of the light 2a, which is the angle of incidence of the recording medium 7 in FIG. This light is incident through the beam splitter 13 onto a surface opposite to the incident surface of the lights 2a, 2b, and 2c. As is clear from the lens imaging formula, the angle θ2 is smaller than the above angle θ1. Light 12b, Ren 14
, is the light that enters the recording medium 7 via the sheet-like aperture 16 and the beam splitter 13.

As shown in FIG. 4B, the diaphragm 16 is provided with an elliptical aperture 16a that is long in the direction perpendicular to the aperture 15a, so that the spot on the recording medium 7-A caused by the light 12b is elliptical. be. Therefore, the second hologram recorded by interference between the lights 12a and 12b also has an elliptical shape similar to the aperture 16a.

Holograms are classified into several types depending on their recording method, and in this embodiment, a phase hologram is used.

A phase hologram diffracts reproduced light by phase modulation, and in this embodiment, the light is converged by this diffraction.

In the phase hologram, it is also possible to make the reproduction light utilization rate 100% by selecting the intensity and exposure time of the lights 11a, llb, 12a, and 12b, which are the recording lights for the recording medium 7.

As described above, a photographic plate or the like is normally used as the recording medium 7, but in the case of a phase hologram, the material of the medium is appropriately selected depending on the above-mentioned utilization rate. Gelatin is used as a medium in order to obtain a higher utilization rate than silver salts used in ordinary photographic plates.

When gelatin is used as a medium, depending on the material of the photosensitive material, the recording lights 11a, 11b, 12a, 1
2b and the reproduction light 2a are normally of different wavelengths. For example, while the wavelength of the light 2a is in the band around 0.8 μm, the light 11a, llb, 12a
, 12b is a shorter wavelength band than this band.

When light with different wavelengths is incident on a hologram,
Even if the incident angle is the same, the output angle is different, so in order to obtain the same convergence point, it is necessary to set different incident angles for light having different wavelengths.

By the way, taking orthogonal coordinates in which the center of the recording medium 7 is the origin and the direction perpendicular to the recording medium 7 is the Z-axis direction, the light 11b or 12b travels backwards from the recording medium 7 only in the air and converges. Let the coordinates of the point be (X+, V+, 2
1), the coordinates of the light source of light 11a or 12a are (p+
, Q+, r+), and the coordinates of the point where the light 2a in FIG. 1 is converged by the hologram on the recording medium 7 are (XS',
(pz, Qz
r rz ), and the recording light beams 11a, ll
λ1 is the wavelength of 12a, 12b, and 2a is the reproduction light.
When the wavelength of is λ2, and the enlargement magnification of the hologram after recording is m, the hologram imaging formula is -
1−−−−−−−−−−−−−−−−− ■−−−−−
−−−−−−−−−−−−−−■−−−−−−−−−−
−−−−−−−−−■

If the hologram on the recording medium 7 is an on-axis hologram as shown in FIGS. 3 and 4, the point where the light 11b or 12b travels backwards from the recording medium 7 only through the air and converges. The position, that is, the positional relationship of the photodetector 8 in FIG. 1 with respect to the recording medium 7 can be determined only from equation 0.

The recording medium 7 receives the lights 11a, 11b and 1 as described above.
A hologram, which is a diffraction grating, is recorded by 2a and 12b, and the intensity of the diffracted light changes depending on the incident angle of the light to the diffraction grating, and the highest diffraction efficiency is obtained when the Bragg angle condition is met.

In other words, as shown in FIG. 1, when the light 2a reflected by the recording surface of the optical disk 6 enters the recording medium 7, the light 2a passes through the first hologram and becomes the light 11a. The light traveling in the opposite direction is separated into light that is converged by this first hologram and has an elliptical cross section that travels in the opposite direction to the light 11b, and as for the second hologram, the light simply passes through this second hologram. The light beam 12a and the light beam 12b are converged by the second hologram and have an elliptical cross section, which travels in the opposite direction.

Regarding the first hologram, the recording surface of the optical disk 6 is located at a distance of 2 from the lens 5, and the light 2a is emitted at an angle θ1.
When the light enters the recording medium 7, the ratio of convergent light to transmitted light is the highest due to the angle selectivity of the hologram.

On the other hand, for the second hologram, when the recording surface of the optical disk 5 is located at a distance of 2-Δ2 from the lens 4 and the light 2a is incident on the recording medium 7 at an angle θ2,
The ratio of convergent light to transmitted light is highest.

As shown in FIG. 2, a spot 21 of convergent light from the first hologram is detected by the photodetectors 8a and 8b, and a spot 22 of convergent light from the second hologram is detected by the photodetectors 8C and 8d. The photodetector 8 is placed near the point of convergence of the spots 21 and 22 so that the spots 21 and 22 can be converged. Therefore, outputs shown by curve S in FIG. 5 are obtained from the photodetectors 8a and 8b, and outputs shown by curve S2 in FIG. 5 are obtained from the photodetectors 8c and 8d.

Note that a mirror or the like may be used instead of the beam splitter 13 during hologram recording.

In this embodiment as described above, the displacement of the recording surface of the optical disk 6 in the optical axis direction can be detected by the output shown by the curve S1 in FIG. 5, and by the output shown by the curve S2. It is possible to detect the direction of this displacement. Therefore, focus adjustment can be performed based on these results.

Note that the change in the intensity of the light 2a due to the skew of the optical disc 6 is caused by the output of the photodetector 8a and the output of the photodetector 8b canceling each other out at a certain rate, and the output of the photodetector 8c and the output of the photodetector 8d canceling each other out. Since they cancel each other out at a certain rate, there is no problem.

By the way, on the recording surface of the optical disc 6, as shown in FIG.
, 2c are formed.

Figures 6 to 9 show these three spots) 23a.

The positional relationship between 23b and 23C and the recording track 24 on the optical disk 6 is shown.

The light 2a reflected from the center spot 23a enters the recording medium 7 as light for signal detection and focus error detection as described above, but the light 2a reflected from the center spot 23a enters the recording medium 7 as light for signal detection and focus error detection as described above.
2b and 2c reflected from the recording medium 7 also enter the recording medium 7.

However, since the spots 23b and 23c are separated from the spot 23a, the lights 2b and 2c are reflected at the angles θ1 and θ.
2 and enters the hologram at a different angle. spot 2
3b and 23c are on the conjugate point of lens 5.
Spot 2 so that the angle to the
If 3b and 23C are separated from spot 23a,
The lights 2b and 2c are not diffracted by the hologram on the recording medium 7, but simply pass through this hologram.

As a result, the respective spots of the lights 2b and 2C are formed over substantially the entire photodetector 8, so that these spots substantially overlap each other on the photodetector 8.

Therefore, as shown in FIG. 6, spots 23a, 23b, 2
3c is located on the recording track 24, no difference signal is output from the photodetectors 8e and 8f. On the other hand, as shown in FIG. 7, spots 23a, 23b, 2
3c is displaced from the recording trough 24, a difference signal is output from the photodetectors 8e and 8f. In other words,
This difference signal becomes a tracking error signal, and trunking adjustment can be performed based on this 1-tracking error signal.

By the way, when the diffraction grating 3 rotates about the optical axis, the rows of spots 23a, 23b, and 23c are inclined with respect to the recording track 24, as shown in FIG. However, even in this case, if the center spot 23a is located on the recording track 24, the spots 23b and 230 have opposite brightness and darkness patterns, so the spots of the lights 2b and 2C overlap on the photodetector 8. As a result, the photodetectors 8e and 8f do not output a difference signal indicating a tracking error.

Then, as shown in FIG. 9, a spot 23a.

If the rows 23b and 23C remain inclined with respect to the recording track 24 and the central spot 23a is displaced from the recording track 24, a difference signal indicating a tracking error is output from the photodetectors 8e and 8f. . In other words, even if the angle of the diffraction grating 3 is not very precisely adjusted around the optical axis, tracking errors can be detected with high accuracy.

Further, the tracking error signal as described above is generated not only when the rows of spots 23a, 23b, and 23C are displaced in a direction perpendicular to the recording track 24 as shown in FIGS. 7 and 9, but also when the optical It can also be obtained when the entire system rotates.

The lights 2b and 2C for detecting a tracking error also enter the light detection units 2a to 2d for detecting a focus error. However, the spots 21 and 22 of the light 2a and the light detection unit 2
If a to 2d are made sufficiently small, the leakage between the focus error signal and the trunking error signal will be reduced, and the tracking error signal can be further increased.

Furthermore, if the hologram recorded on the recording medium 7 is an off-axis hologram, leakage between the focus error signal and the tracking error signal can be further reduced.

Although the embodiments in which the present invention is applied to an optical disc player have been described above, it goes without saying that the present invention can be applied to other devices besides optical disc players.

Effects of the Invention As described above, according to the optical displacement detection device of the present invention, the first light and the second light are separated from each other as diffracted light and directly transmitted light due to the difference in the angle of incidence on the hologram. Therefore, the spot of these lights can be made large, and even if the plurality of photodetectors are not positioned very accurately, the displacement of the object to be detected can be detected with high accuracy.

[Brief explanation of drawings]

FIG. 1 is a schematic side view showing one embodiment of the present invention, FIG.
The figure is a schematic plan view showing the photodetector used in the embodiment shown in Fig. 1, and Figs. 3 and 4 are hologram recordings used in the embodiment shown in Fig. 1. A schematic side view showing the method, FIG. 5 is a graph showing the output of the photodetector shown in FIG. 2, and FIGS. 6 to 9 show the relationship between the recording track of the optical disc and the light spot. FIG. In addition, in the symbols used in the drawings, 2a--1st light 2b,
2c--------Second light 6--------
−−−−−−−−−Optical disc 7−−−−−−−−−
----------1 Recording medium 8---------------------
---One photodetector 8a to 8d---One photodetector 8e, 8f---One photodetector 11a, 1
．． 1b-light 12a, 12b-light.

Claims (1)

[Claims] A recording medium on which a hologram forming a predetermined angle with respect to the surface is recorded, and a first light from an object to be detected that is incident on the recording medium at an angle substantially corresponding to the angle and diffracted by the hologram. a first photodetector for detecting; a second photodetector for detecting second light from the object to be detected that is incident on the recording medium at an angle that does not correspond to the angle and directly passes through the hologram; The detection output of the first photodetector is used to detect the displacement of the object in the optical axis direction, and the detection output of the second photodetector is used to detect the displacement of the object in the direction perpendicular to the optical axis. An optical displacement detection device configured to detect displacement of the object to be detected.