JPS6087438A - Tracking error detector - Google Patents

Abstract

PURPOSE:To stabilize the actuation of a tracking error detector by using a diffraction grating to produce four secondary spots on a disk. CONSTITUTION:A grating pattern of a diffraction grating 23 is formed with continuous grids 24 of parallelograms. The grid 24 has about 3:1 or 3:2 correspondence between its longer side (m) and shorter side (n), and the vertical theta angle of the grid 24 is sufficiently small. A level difference (d) exists between grids adjacent to each other. Thus the grating 23 has a projected/recessed surface. The light is transmitted through such grating 23 to produce simultaneously the diffracted beams from both sides (m) and (n). The difference (d) is common with both sides (m) and (n) and therefore the intensity is equal among four primary beams produced by the grating 23. This grating 23 can perform the same actuation as two conventional diffraction gratings.

Description

Detailed Description of the Invention [Field of Application of the Invention] The present invention provides a tracking error detection device for detecting whether light from a light source is correctly focused on an information track of an optical disc in a recordable optical disc device. It is related to.

[Background of the invention]

In read-only optical disk devices, the main detection methods for detecting the inside of the tracking of an optical head include the twin beam method, the Guzziple method, the wobbling method, and the heterodyne method.

Among these, the twin beam method will be explained.

Figures 1 and 2 are explanatory diagrams for explaining the operation of the twin beam method, respectively. Figure 1 is a partially enlarged view showing the surface of the optical disk, and Figure 2 is a schematic diagram showing the detection unit. .

In FIGS. 1 and 2, 1 indicates recording pits 2a and 2b.
, 2c are laser spots, and 3a are respectively laser spots.

3b and 3C are reflected lights, 4a, 4b, and 4c are photodiodes, respectively, 5 is a tracking differential signal, and 6
is a differential amplifier, and 7 is a reproduced RF signal.゛In the twin beam method, the laser beam is divided into six parts by a diffraction grating, and the laser beam is divided into six parts by a diffraction grating, and the laser beams are divided into six parts so that the bit string 1 on the optical disk is arranged as shown in Fig. 1 (laser spora) 2a, 2b, 2c
, and the reflected light 5a, 5b, as shown in FIG.
5c into six 71) diodes 4a, 4b, 4
Each leads to 0. The tracking error signal 5 is extracted as a difference signal from the photodiode 42.4C using the differential amplifier 6, and the reproduced RF signal 7 is extracted from the photodiode 4b.
from the output of

Although this method has a more complex optical system than other detection methods, the electrical circuit is simple, and pits and groups (
Since the tracking error signal can be stably detected regardless of the shape of the guide groove, it is used in playback-only video discs and DAD players. - However, when this detection method is used in an optical disk device that can perform both recording and playback, stable operation cannot be achieved due to the following reasons. FIG. In the figure, 9 is a group (guide groove).

That is, as shown in FIG. 5, a group (guide groove) 9 is formed in advance in the recordable light <X direction,
When recording, information is recorded by forming a recording pictogram 1 using the main slot 2b. At this time, assuming that the optical disc advances in the direction of arrow A, the sub spot 2a is located on group 9 immediately before recording, and the sub spot 2C is located on pit 1 immediately after recording, so the former is modulated only by group 9. However, the latter is also modulated by the formed pit 1, so even if a tracking error signal is obtained by detecting and comparing each modulated reflected beam, it may not be possible to obtain a normal signal. This is because it cannot.

In order to solve this problem, a tracking error detection method using two diffraction gratings simultaneously has been proposed.

FIG. 4 is a schematic diagram showing a conventional tracking error detection device.

In FIG. 4, 8 is an optical disk, 10 is a semiconductor laser, 11 is a laser beam, 12 is a collimating lens, 13 is a parallel beam, and 1! , 15 is a diffraction grating, 16 is a polarizing beam splitter, 17 is a quarter wavelength plate, 18 is an objective lens, 19 is a beam spot, 20 is reflected light, 21 is a detection lens, and 22 is a detection section.

As shown in FIG. 4, a laser beam 11 emitted from a semiconductor laser 10 becomes a parallel beam 13 by a collimating lens 12, is divided into three by a diffraction grating 14, and is further divided into six by a diffraction grating 15.
It is divided into 9 parts in total. However, the diffraction grating 14.
15, the intensity of the 1st-order diffracted light is several percent of the 0th-order diffracted light.
Since it is designed so that the ±1st-order light divided by the diffraction grating 14 is further divided by the diffraction grating 15
When the secondary light is formed, the light intensity of these four laser beams becomes very small and can be ignored.In the end, the parallel light 13 is divided into 5 parts by the diffraction grating 14 and 15. .

Next, these split light beams are divided into polarizing beam splitters 16, 1/4 wavelength plates 17, and 17. The beam spot 19 is irradiated onto the optical disk 8 through the objective lens 18 .

FIG. 5 is an explanatory diagram showing the beam spot arrangement on the optical disk. In the figure, 19a to 19e are beam spots, respectively.

Beam spots 198 to 19e are located on the optical disc 8, respectively.
Group 9 is irradiated as shown in FIG.

Next, as shown in FIG. 4, the beam spots 19 irradiated onto the optical disk 8 are each reflected and become reflected light 20, which is reflected by the objective lens 18. i/4 wavelength plate 17. Polarizing beam splitter 16. Through the detection lenses 21,
The light enters the detection unit 22 .

FIG. 6 is a schematic diagram showing the arrangement of reflected light in the detection section. In the figure, 20'a to 20e are reflected lights, respectively.
22a to 22e are photodiodes, respectively.

As shown in FIG. 6, each of the reflected lights 20a to 20e has a
photodiodes 22a to 22e. The tracking error signal 5 is extracted as a difference signal between the photodiodes 22a and 22b using a differential amplifier 6, and the reproduced F signal 7 is extracted from the output of the photodiode 22c. In this case, as shown in FIG.
9b are both located on the gluker immediately before recording, so they are only modulated by group 9, and there is no difference between them due to modulation, so that no errors occur in the tracking error signal 5. As described above, the apparatus shown in FIG. 4 can perform stable operation.

However, in such a conventional device, two diffraction gratings are required, and the group 9 on the optical disk is
In order to irradiate the five laser spots 19a, 19b, 19c, 19d, and 19e, the angles of the two diffraction gratings must be adjusted, and the adjustment work is complicated and difficult. There were some drawbacks.

[Purpose of the invention]

The purpose of the present invention is to eliminate the above-mentioned drawbacks of the prior art, and to
An object of the present invention is to provide a tracking error detection device that can generate four sub-spots on a disk using only , diffraction gratings and operate stably.

[Summary of the invention]

In order to achieve the above object, the present invention uses a concave-convex diffraction grating made up of a series of parallelogram grids, so that the light from the optical head passes through the diffraction grating, resulting in four Sub-beams are generated, and these four sub-beams are unevenly projected on both sides of the information trunk of the optical disk, and the two sub-beams located in front or behind the main beam for recording information are projected onto the information track. Two modulated reflected beams obtained by modulation are detected by two detectors, and the detection results are compared to detect the tracking error of the original head.

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 7 is a perspective view showing a grating pattern of a diffraction grating used in the present invention.

The Kono lattice pattern is composed of a series of parallelogram grids 24. The ratio of the long side m to the short side n in this parallelogram grid 24 is approximately 3:1 or 3:2. Further, the apex angle θ of the grid 24 is a sufficiently small value. Also, there is a step d between adjacent grids,
The diffraction grating 23 has a concavo-convex shape.

By passing light through such a diffraction grating 23, it is possible to simultaneously generate diffracted light by the long side m and diffracted light by the short side n. Also, the step d is on the long side.

Since the short sides are also common, the intensities of the four primary lights generated by this diffraction grating 23 are all the same.

FIG. 8 is a schematic diagram showing an embodiment of the present invention.

As shown in FIG. 8, in this embodiment, the diffraction grating 26 shown in FIG. 7 is used, and the two diffraction gratings i4. It operates in the same way as the i5.

To explain this example more specifically using numerical values, the wavelength of the laser beam is λ-[33Q1m.

Refractive index of the diffraction grating n = 1.5.0-order diffraction light intensity.

Ratio of the light amount 11 of the first-order diffracted light to the ratio I, /Io=0.0
5, the step difference d is 0.18 μm.

Also, spot 19a and spot 19 shown in Figure 5
C distance x-15μm, spot 19b and spot 19
c interval y = 10 μm, spot 19a and subpot)
For the deviation δ of group 9 of 19b = 0.66 μm (about 1/4 of the rack pitch of 2.5 μm), the apex angle θ
is 0.050 radian (= 2.9°).

Also, the focal length fI of the objective lens 18 = 3.611IL
, the long side m and short side n of the grid 24 are medium 6.0
It becomes 4.01 El in the proverb door.

Further, the distance u between the photodiode 22a and the photodiode 22c shown in FIG. 6 is 6ooμm.

For the distance v=4QQμm between the photodiode 22b and the photodiode 22C, the focus x of the detection lens 21
it! is 1401111 in t, = tl”-<=tl-> x y.

In this embodiment, the ratio of the long side m to the short side n of the grid 24 is about 6:2, but it may be about 3:1. in this way,
By setting the ratio of the long side m to the short side to be about 3:1 or about 3:2, unnecessary second-order diffracted light can be separated from first-order diffracted light.

In other words, as shown in FIG. 6, unnecessary second-order diffracted light 2]f20
g is located outside the photodiodes 22a and 22e, and is separated from the first-order diffracted lights 20a and 20e.

〔Effect of the invention〕

According to the present invention, the same effect as the conventional one can be obtained with only one diffraction grating, so the number of parts can be reduced accordingly. Furthermore, the arrangement of the laser spot for each group on the optical disk can be set by the grating pattern of the diffraction grating, so there is an effect that the angle adjustment of two diffraction gratings, which was conventionally required, is no longer necessary.

[Brief explanation of the drawing]

Figure 1 is a partially enlarged view showing the surface of an optical disc in the twin beam method, Figure 2 is a schematic diagram showing the detection section in the twin beam method, and Figure 3 is a diagram showing the surface of an optical disc in the twin beam method. Fig. 4 is a schematic diagram showing a conventional tracking error detection device, Fig. 5 is an explanatory drawing showing the beam spot arrangement on the optical disk, and Fig. 6 shows the reflected light at the detection section. FIG. 7 is a perspective view showing a grating pattern of a diffraction grating used in the present invention, and FIG. 8 is a schematic diagram showing an embodiment of the present invention. Explanation of symbols 1...recording pit, 5...tracking error signal, 8...optical disc, 9...group, 10...
Semiconductor laser, 1B...Objective lens, 19a to 19e
...Spot, 208-20e...Reflected light, 22a
~22e...Photodiode, 23...Diffraction grating, 24...Grid. Fig. 2 Fig. 3 Fig. 4 Fig. S Fig. 6 D entrance

Claims (1)

[Claims] 1) A concave-convex diffraction grating made up of a series of parallelogram grids, and at least four sub-beams generated when the light from the optical helad passes through the diffraction grating, on both sides of the information track of the optical disk. Two detection methods for detecting two modulated reflected beams obtained by modulating the two sub beams on the information track, which are projected in a biased manner and are located in front or behind the main beam for recording information. 1. A tracking error detection device, characterized in that the tracking error of the optical head is detected by comparing detection results obtained by two detectors.