JPH0323977B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical information recording/reproducing device such as an optical disk, and more particularly to an optical track tracking device using diffracted light from an information track.

As an information track tracking device used in an optical information recording/reproducing device such as an optical disk, the depth of the track groove or pit is from λ/4 to the wavelength λ of the light source (e.g. He-Ne laser, semiconductor laser). There is known a device that detects a tracking control signal and performs track tracking by utilizing the fact that the edges of the track groove cause asymmetry in the diffracted light when the track groove deviates. In this device, two photodetectors arranged parallel to the track receive the diffracted light beam from the track groove, and a tracking control signal is detected by taking the difference in the outputs of these two photodetectors. However, since the detector is located in the far-field region of the aperture lens, when the optical spot is moved for tracking control, the diffracted light beam on the photodetector surface is also deflected, and this deflection is This appears as a deviation component in the tracking control signal. This results in the disadvantage that normal track tracking cannot be performed.

Such a device is shown in Japanese Patent Application Laid-Open No. 49-60702, and will be explained with reference to FIGS. 1 and 2.

FIG. 1 is a diagram showing a schematic configuration of a track tracking device using diffracted light. The light beam emitted from the laser 1 becomes parallel light by the lens 2, and the light beam is transformed into a parallel beam by the prism 3.
The light then passes through the focusing lens 4 and is focused on the surface of the disk 5 into a spot of approximately 1 μmφ. An uneven information track 51 is provided on the surface of the disk 5. The light spot 6 is diffracted at the edge of the track 51, and this diffracted reflected light passes through the focusing lens 4 and the prism 3, and is received by two photodetectors 7a and 7b placed on either side of the track. Since the diffraction component of the reflected light causes an imbalance in the amount of light on both photodetectors depending on the positional relationship between the light spot and the track, the relative position of the light spot and the track can be determined by detecting the output difference between the two detectors. Can be detected. The two outputs from the photodetectors 7a and 7b are differentiated by a differential amplifier 8, and this output is led to a tracking control circuit 9, which controls the actuator 10 attached to the aperture lens 4, thereby controlling the lens 4. Track tracking is performed by making slight movements. However, when the focusing lens 4 moves for tracking, the diffracted light beam also moves on the detector surface as the optical axis moves. That is,
When the spot is moved by a distance d on the disk surface, the optical axis of the reflected light beam will move by 2d on the photodetector surface. For this reason, the signals obtained differentially include a tracking control signal indicating the deviation δ between the center of the optical spot and the center of the track, and a diffracted light beam that passes on the surface of the photodetector 7 due to the movement of the lens 4 by d. Deviation components caused by parallel movement will be superimposed.

The magnitude of this deviation component largely depends on the configuration of the optical system, the size of the photodetector, and its arrangement.

The present invention has been made in view of the above points, and is an optical track tracking device that can reduce the deviation component caused by the deflection of the diffracted light beam on the photodetector surface, thereby achieving more accurate track tracking. The purpose is to provide

First, the principle of the present invention will be explained. Figure 2a,
b and c are diagrams showing the positional relationship between the spot narrowed down by the focusing lens and the track, and Fig. 2a shows the case where the center of the spot 6 is at the center of the track 51 (this state is referred to as P _C) . ), Fig. 2b shows that when the center of the spot 6 is shifted to one side from the center of the track 51 (this state is designated as P _L ), the second
Figure c shows a case (P _R ) in which the center of the spot 6 is shifted from the center of the track 51 in the opposite direction to that in Figure 2b. FIG. 3 is a diagram showing the light amount distribution of diffracted light on the photodetector surface in each of the states P _C , P _L , and _PR . In the figure, point O is the center of the diffracted light beam when the lens movement is 0, the horizontal axis indicates the position on the photodetector surface, and the track extends perpendicular to the plane of the paper. ), and the vertical axis shows the amount of light at each position. Further, A represents the diameter of the diffracted light beam on the photodetector surface. If the center of the spot is in the center of the track, i.e.
In the P _C state, the 0th-order light component of the diffracted light appears at the center, showing a nearly symmetrical light intensity distribution. On the other hand, P _R ,
When the center of the spot deviates from the center of the track as in P _L , the light intensity distribution exhibits asymmetry. Therefore, as shown in Figures 7a and 7b, by arranging two photodetectors symmetrically with respect to the track direction so as to sandwich the track, and taking the difference signal of their outputs, the track deviation information of the spot can be obtained. Tracking control can be performed using the information.
Note that the size of the zero-order light component appearing at the center may vary depending on the depth of the track (or pit).

Figure 4 shows the state of P _C in Figure 3 (solid line) and
A state in which the aperture lens 4 is moved by a distance d by the actuator 10 in the state of P _C (P' _C , indicated by a broken line) is shown. When the lens 4 moves by d, the optical axis of the diffracted light beam moves by 2d on the surface of the photodetector. In such a state, even though there is no positional deviation between the track center and the spot, if two photodetectors are very close to each other as shown in 7a and 7b, the differential signal will be Using each of the shaded portions I ₁ to I ₃ in the figure, (I ₂ +I ₃ )−I ₁ is obtained, and a false differential signal is generated. This acts as a deviation signal and interferes with the normal operation of tracking control. In Figure 4, there is no track deviation.
The case of P _C is shown, but there is a track deviation.
Similar deviation signals are superimposed on P _L and P _R as well.

In order to reduce this deviation signal, the present invention
As shown in 7a' and 7b' in the figure, it is characterized in that two photodetectors are placed apart by w. By providing such a dead zone with a width w, the differential signal becomes approximately _I3 , and the deviation signal can be reduced.

The width w of the dead zone is smaller than the diameter A of the diffracted light beam on the photodetector surface, and the moving _distance of the optical axis of the diffracted light beam on the detector surface (2d _n ×2), that is, in the range of A>w>4d _n . That is, two photodetectors are arranged symmetrically with respect to the direction of the track so as to sandwich the track in the shaded area in FIG. In addition,
In FIG. 5, it is assumed that the tracks extend in the vertical direction of the drawing. In the above equation, d _n generally corresponds to the maximum eccentricity of the disk in the case of an optical disk device. The larger w is within the above range, the more 0
Although the effect of secondary light and cutting increases, if it is too large, the tracking information also decreases, so in practice, the optimum value is selected based on the balance between the two.

Figure 6c shows a case where the diameter A of the diffracted light beam on the detector surface is 5mm, a photodetector with a size of 1.0 x 2.5mm is used, and the dead zone width is 0mm (Figure 6a) and 1.5mm.
(FIG. 6b) shows the results obtained by computer simulation of the deviation signal generated by lens movement (decentering).

Furthermore, Fig. 6c shows the results when the track pitch is 1.6 μm, the track depth is λ/6, the wavelength λ of the light source is 0.8 μm, the aperture of the aperture lens is 5 mm, and the numerical aperture of the aperture lens is 0.5. be.

In the figure, the curve α indicated by the dotted line is
The curve β shown by the solid line is shown in Fig. 6b.
The case is shown below. According to Fig. 6c, the dead band width w is
It can be seen that by setting 1.5 mm, the deviation signal generated when the lens movement amount is ±200 μm is reduced to about 1/2 compared to the case without a dead zone.

d _n , which corresponds to the amount of disk eccentricity, can be calculated by focusing a laser beam on a rotating disk to a spot diameter about the same as the disk's track pitch, receiving the reflected light with a photodetector, and passing the output through a low-pass filter. , a signal representing the state in which the spot traverses the tracks during the rotation of the disk is obtained, and from this signal it can be measured by multiplying the number of tracks crossed by the laser beam during one rotation of the disk by the track pitch.

FIG. 7a is a diagram showing an embodiment of the photodetector of the present invention used in the optical track tracking device of FIG. 1. The photodetector shown in FIG. 7a is an example in which the lens 4 has an aperture of 5 mm, a numerical aperture of 0.5, and a focal length of 4.3 mm in FIG. 1. In FIG. 7a, 7a' and 7b' are the light receiving surfaces of the photodetector. In this example, the maximum eccentricity of the disk is 0.2mm.
and the dead zone width w is 0.9 mm. In FIG. 7a, the track extends in the vertical direction of the drawing, and the light receiving surfaces 7a' and 7b' are arranged symmetrically with respect to the direction of the track. That is, the light-receiving surfaces 7a' and 7b' are arranged so that their axes of line symmetry _O.sub.X are parallel to the direction of the track. The solid line β' in FIG. 7b shows the converted deviation of the deviation signal caused by the lens movement distance when the photodetector shown in FIG. 7a is used. Note that the dotted line α' in FIG. 7b shows the characteristics when the dead zone width w is 0.06 mm in a photodetector of the same size as in FIG. 7a. According to this, when the lens moving distance is 200 μm, the deviation signal is reduced to less than half by setting the dead zone width w to be at least four times the lens moving distance.

In the embodiment shown in FIG. 7a, the photodetector is placed only inside the diffracted light beam, but the present invention is not limited thereto. For example, as shown in Figure 8a, the dead zone width w is 4 times the maximum lens movement distance d _n
The photodetector may be made larger than the diffracted light beam as long as it is more than twice as large. Furthermore, the shape of the photodetector does not necessarily have to be rectangular. For example, it can also be shaped as shown in FIG. 8b.

In FIGS. 8a and 8b, the tracks extend in the vertical direction of the drawing, and are arranged so that the axes of line symmetry _OX of the light-receiving surfaces 7a' and 7b' are substantially parallel to the direction of the tracks. As described above, the detector used in the optical track tracking device of the present invention focuses on the center and is larger than four times the maximum movement distance d _n of the lens.
and has a dead zone with a width w smaller than the diameter A of the diffracted light beam on the photodetector surface, and consists of at least two light receiving parts arranged symmetrically on both sides with respect to the dead zone, and the dead zone is parallel to the direction of the track. All you have to do is arrange it so that it becomes .

In the above description, the light beams incident on the photodetectors 7a and 7b are parallel light beams, but the present invention is not limited to parallel light beams. For example, as shown in FIG. 9a or b, the photodetector 7
Convex lens 11 or concave lens 1 in front of a, 7b
2 may be used to converge or diverge the diffracted light beam incident on the photodetector, thereby reducing or expanding the diameter of the diffracted light beam. The dead zone width w in this case is specified as follows from geometrical optics proportional calculation. That is, the focal length of lens 11 or 12 is f, and the principal focus of each lens is 13,1
4 and the distance from each principal focus to the photodetector is h, then the dead zone width w of the photodetector 7 may be set as A>w>4d _n ·h/f. Here, d _n is the maximum moving distance of the lens 4, A is the diameter of the diffracted beam on the photodetector surface, and if the incident beam of the lens 11 or 12 is a parallel beam of diameter B, then A= B×h/f.

As described above, according to the present invention, in a track tracking device such as an optical disk, tracking information is not included.
In addition, by cutting off the zero-order light that adversely affects tracking, the tracking deviation signal can be reduced.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a schematic configuration of an optical track tracking device according to the conventional art and the present invention, and FIG. 2 a, b,
c is a diagram showing the positional relationship between the light spot and the track, FIG. 3 is a diagram showing the light intensity distribution on the photodetector surface, and FIG. 4 is a diagram for explaining the principle of the present invention.
Fig. 5 is a diagram showing the area where the photodetector used in the device of the present invention is arranged, Fig. 6 a, b, and c are diagrams for explaining the effects of the present invention, and Fig. 7 a is a diagram showing the area where the photodetector used in the device of the present invention is arranged. FIG. 7b is a diagram showing the relationship between the moving distance of the focusing lens and the deflection converted into a deviation signal, and FIGS. FIGS. 9a and 9b are diagrams showing other embodiments of the present invention, respectively.

Claims (1)

[Scope of Claims] 1. A light source, a first optical means that narrows down a light beam from the light source and guides it as a convergence spot to a track provided on a predetermined recording surface via an optical system, and detects a diffracted light beam from the track. a photodetector comprising two light receiving sections; a second optical means for guiding the diffracted light flux to the photodetector via the focusing optical system; and a means for detecting a deviation between the convergence spot and the track, and a means for correcting the deviation between the convergence spot and the track by moving the narrowing optical system according to the output of the detection means. In the tracking device, the two light receiving parts are arranged symmetrically with respect to the track direction, the diameter being smaller than the diameter of the diffracted light beam on the photodetector surface, and the optical axis of the diffracted light beam moving on the photodetector surface. An optical track tracking device characterized in that the optical track tracking devices are arranged separated by a distance greater than the distance. 2. In the optical track tracking device according to claim 1, the second optical means guides the diffracted light beam to the photodetector as a parallel light beam, and the light receiving section optically detects the diffracted light beam. When the diameter on the instrument surface is A, and the maximum movement distance of the aperture optical system is dm, the optical system is characterized in that they are separated by a distance w that satisfies the inequality A>w>4dm. Truck tracking device. 3. In the optical track tracking device according to claim 1, the second optical means guides the diffracted light beam to the photodetector via a converging optical system, and the light receiving section guides the diffracted light beam to the photodetector. The diameter on the photodetector surface is A, the maximum moving distance of the narrowing optical system is dm, the focal length of the converging optical system is f, and the distance from the focal point of the converging optical system to the photodetector is h. An optical track tracking device characterized in that the optical track tracking devices are arranged separated by a distance w that satisfies the inequality A>w>4dm·h/f. 4. In the optical track tracking device according to claim 1, the second optical means guides the diffracted light beam to the photodetector via a diverging optical system, and the light receiving section guides the diffracted light beam to the photodetector. When the diameter on the detector surface is A, the maximum movement distance of the narrowing optical system is dm, the focal length of the diverging optical system is f, and the distance from the focal point of the diverging optical system to the photodetector is h. , an optical track tracking device characterized in that the optical track tracking devices are arranged separated by a distance w that satisfies the inequality A>w>4dm·h/f.