JPH06119647A - Method for detecting errors in light beam spot - Google Patents

Abstract

PURPOSE:To reduce the number of optical parts and to simplify adjustment in assembly by commonly using a split beam photodetector with plural light receiving areas for the detection of tracking errors and focusing errors as a photoelectric conversion means for sub-beams CONSTITUTION:The detection of tracking errors is performed by a three beam method. That is, a laser beam from a semiconductor laser 21 is divided into a main beam B1 and sub-beams B2, B3, and an light beam spot is each formed on a magneto-optical disk 10. With a light quantity reflected from pre-grooves 15 and recording tracks 16 being different, and with the relative relation of the reflected light quantity of the beams B2, B3 being varied at spot positions, tracking errors are detected. Simultaneously, the beams B2, B3 that are made incident on the edges of the grooves 15 are each separated into zero-order and + or -1st-order diffracted light beams, and made incident on bisecting photodetectors 32, 33 as plural resultant beams. Consequently, in a focused state, bright and dark patterns due to interference appear on the light receiving area of each detector. Thus, the detection of focusing errors is performed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical spot error detecting method for tracking and focusing of an optical disk device.

The optical disc device has a larger storage capacity per one optical disc as a recording medium than the magnetic disc device, and since the optical discs can be exchanged freely like the flexible disc, it is used as an external storage device of a computer. It is spreading rapidly. Along with this, in order to reduce the size, weight and cost of the optical disk device, it is desired to simplify the configuration of each part including the optical head.

[0003]

2. Description of the Related Art In an optical disk apparatus, optical system servo control for forming an optimum light spot on a recording track of an optical disk, that is, tracking for positioning the light spot in the radial direction of the optical disk and focusing for positioning in the optical axis direction are performed. It is essential.

Generally, in tracking, as a method of detecting a track error of a light spot, a three-beam method,
Alternatively, the push-pull method or the like is used. At that time, in the read-only type optical disc device, the pit row corresponding to the recording information is used as the track position information, and in the write-once and rewritable type optical disc devices, the pre-groove (guide groove) provided in the optical disc in advance is the track position information. It is considered as information.

On the other hand, in focusing, as a method for detecting a focus error of a light spot, an astigmatism method or a wedge prism method, which is a type of Foucault method, is used.

[0006]

However, the astigmatism method requires a cylindrical lens or a cylindrical lens.
Further, the wedge prism method has a smaller light amount loss than the knife edge method of the same kind, but requires a wedge prism (roof-shaped lens).

That is, conventionally, when the light spot is three-dimensionally positioned, it is necessary to provide a special optical lens for detecting a focus error and perform a complicated position adjustment in addition to an optical component for detecting a track error. It was

Therefore, in the optical disk device using the conventional error detection method, it is difficult to reduce the size and weight of the optical head, and there are many parts and adjustment points, which is disadvantageous in terms of assembly cost.

In view of the above problems, it is an object of the present invention to reduce the number of optical components required for detecting a tracking error and a focus error, to reduce the size and weight of an optical head and to simplify assembly and adjustment.

[0010]

In order to solve the above-mentioned problems, a detection method according to the present invention uses an optical disk device having an optical disk 10 having a pre-groove 15 as a recording medium, as shown in FIGS. 1 and 3. 1, the track error of the light spot 41 is detected by the three-beam method, the optical depth of the pre-groove 15 is set to 1/4 wavelength, and at least one sub-beam B2 is detected.
The divided photodetector 32 having a plurality of light receiving surfaces 321 and 322 is used for the photoelectric conversion of, and the difference in the amount of incident light on each of the light receiving surfaces 321 and 322 due to the diffraction of the sub-beam B2 that irradiates the edge of the pregroove 15 is determined. The light spot 41
Is configured to be detected as a focus error.

[0011]

The focus error of the light spot 41 is detected by the push-pull method known as a track error detection method. That is, in the detection of the track error by the push-pull method, the distribution of the reflected light from the optical disk 10 changes according to the positional relationship between the detection light spot and the pre-groove 15, and becomes asymmetric when the track error occurs. Therefore, the reflected light is made incident on the two-divided photodetector, and the difference between the photoelectric conversion signals corresponding to the respective light receiving surfaces is obtained as an error signal. At this time, if the optical depth of the pre-groove 15 is ¼ wavelength, the 0th-order diffracted light and the ± 1st-order diffracted light generated by the pre-groove 15 at the time of focusing (at the time of just focusing) It is known that the phase difference becomes π (180 °) and the reflected light distribution becomes symmetrical regardless of the positional relationship between the detection light spot and the pre-groove 15. Therefore, by setting the optical depth of the pre-groove 15 to 1/4 wavelength, it is possible to detect the focus error using the split photodetector that can determine the symmetry of the reflected light distribution.

To detect such a focus error,
The sub-beam B2 for detecting a track error by the three-beam method is used, and the split photodetector 32 having a plurality of light-receiving surfaces 321 and 322 is commonly used for detecting a track error and a focus error as photoelectric conversion means of the sub-beam B2. To be done.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a schematic diagram showing the structure of the essential part of a magneto-optical disk device 1 to which the error detecting method of the present invention is applied.

The magneto-optical disk device 1 has a magneto-optical disk 10 which is a rewritable recording medium, and an optical head 20 for recording / reproducing information using a laser beam.

The magneto-optical disk 10 includes a substrate 11 made of, for example, polycarbonate, a recording magnetic layer (not shown),
And a protective layer 13 and the like, and has a pre-groove 15 for servo control of the optical head 20 on the inner surface side of the substrate 11. In the magneto-optical disk 10, the pre-groove 1
The portion between 5 becomes the recording track 16.

The pre-grooves 15 are spirally provided in a plan view at a pitch of 1.6 μm and are used not only for tracking but also for focusing. Therefore, the optical depth of the pre-groove 15 is set to 1/4 wavelength. That is, when the wavelength of the laser beam emitted from the optical head 20 is λ and the refractive index of the substrate 11 is n, the depth dimension of the pre-groove 15 is approximately λ / 4n.

On the other hand, the optical head 20 comprises the semiconductor laser 2
1, collimator lens 22, beam shaping prism 23,
Diffraction grating 24, beam splitter 25, objective lens 2
6, 1/2 wavelength plate 27, condenser lens 28, polarization beam splitter 29, photodetectors 30, 31, 32, 33 and the like.

When reading (reproducing) the recorded information utilizing the magnetic Kerr effect, the amount of rotation of the polarization plane corresponding to the recorded information is determined by separating the detection light by using the ½ wavelength plate 27 and the polarization beam splitter 29. It is converted into the data signal RF by a method of detecting a difference between respective light amounts, that is, a so-called differential detection method which is advantageous in terms of S / N ratio. for that reason,
As one differential input of the differential amplifier 51, the sum signal of the photoelectric conversion outputs of the three photodetectors 31, 32, 33 used for servo control as described later is added, and the other differential input is used as the photodetector. 30 photoelectric conversion outputs are applied.

Now, in the optical head 20 of the present embodiment, recording / recording is performed by minute movement of the objective lens 26 in two directions.
Tracking and focusing at the time of reproduction are performed, and when detecting a track error and a focus error for that purpose, the above-described plurality of optical components are completely shared.

The method for detecting the track error and the focus error will be described in detail below. Track error detection is performed by the three-beam method. That is, the laser beam emitted from the semiconductor laser 21 is transmitted by the collimator lens 2
2 and the beam shaping prism 23 to form a parallel and circular beam, which is incident on the diffraction grating 24, and the main beam B
It is divided into one and two sub-beams B2 and B3. These three beams B1, B2, B3 are
After passing through 5, the light is condensed by the objective lens 26 to form a light spot having a diameter of, for example, about 1 μm on the magneto-optical disk 10.

FIG. 2 is a plan view showing the positional relationship between the light spot and the pre-groove. In the figure, the light spot 4
1 corresponds to the main beam B1, and the light spots 42, 43
Correspond to sub-beams B2 and B3, respectively.

As shown in FIG. 2, light spots 42, 43
Are formed at positions symmetrical with respect to the light spot 41, and in the on-track state where the light spot 41 is located at the center of the recording track 16, the light spots 42, 43 are formed.
The center of each of these is located directly above the edge of the pre-groove 15 (the boundary between the pre-groove 15 and the recording track 16).

Three beams B reflected on the inner surface of the substrate 11
1, B2, B3 enter the polarization beam splitter 29 via the objective lens 26 and the beam splitter 25,
The light amount of one polarization component of each is detected by the photodetectors 31, 32, and 33 whose positions have been adjusted appropriately.

As shown in FIG. 3, in the present embodiment, in order to detect a tracking error and a focus error by the sub-beams B2 and B3, a pair of light-receiving surfaces 321, serving as a photodetector 32 that performs photoelectric conversion of the sub-beam B2. A two-part photodetector with 322 is used. Further, similarly, as the photodetector 33 responsible for photoelectric conversion of the sub beam B3,
A two-division photodetector having a pair of light receiving surfaces 331 and 332 is used.

The photoelectric conversion outputs corresponding to the light receiving surfaces 321 and 322 and the photoelectric conversion outputs corresponding to the light receiving surfaces 331 and 332 are combined by adders 52 and 53, respectively. Then, a differential signal between the outputs of the adders 52 and 53 is generated by the differential amplifier 54 and sent to the drive control system of the objective lens 26 as a track error signal ST. That is, the track error is detected by utilizing the fact that the amount of reflected light differs between the pre-groove 15 and the recording track 16 and the relative relationship between the amounts of reflected light of the sub-beams B2 and B3 changes according to the position of the light spot 41. .

In parallel with the detection of such a track error, the magneto-optical disk device 1 detects a focus error by the push-pull method. As described above, the sub beams B2 and B3 incident on the edge of the pre-groove 15 are
Each of them is mainly separated into 0th order light and ± 1st order diffracted light, and enters the photodetectors 32 and 33 in the form of a combined beam of a plurality of diffracted lights. Therefore, depending on the focus state, the light receiving surface 32
1, 322 and the light receiving surfaces 331, 332 have bright and dark patterns due to interference.

FIG. 4 is a plan view showing an example of the light and dark patterns appearing on the light receiving surfaces of the photodetectors 32 and 33. In the figure,
The shaded area is the dark area (the area where the amount of received light is small). In the on-track state, when the objective lens 26 is on the side closer to the magneto-optical disk 10 with respect to the focus position, as shown in FIG. 4A, the photodetector 32 is on the right side of the drawing (on the light receiving surface 322 side). ) Has a dark area, and conversely the photodetector 33
In the figure, a dark portion appears on the left side (light receiving surface 331 side) of the figure. Further, when the objective lens 26 is on the side far from the magneto-optical disk 10 with respect to the in-focus position, as shown in FIG. In the detector 33, a dark portion appears on the right side of the figure.

On the other hand, when the objective lens 26 is in the in-focus position, as shown in FIG.
Regardless of the positional relationship between 2, 43 and the pre-groove 15,
The brightness becomes uniform in the photodetectors 32 and 33. This means that the optical depth of the pre-groove 15 is 1 as described above.
This is a phenomenon caused by setting / 4 wavelength.

That is, focusing on the photodetector 32, for example, the presence or absence of a focus error can be determined by the presence or absence of light and dark, and the direction of the focus error can be determined from the positional relationship between light and dark. Then, when out of focus, the photodetector 32 and the photodetector 3
Since the bright and dark patterns are symmetrical with respect to 3 and 3, the highly sensitive focus error detection by the differential detection method can be performed based on the outputs of the photodetector 32 and the photodetector 33.

Therefore, as shown in FIG. 3, the differential amplifier 55 obtains a signal indicating the difference in the amount of incident light on the light receiving surfaces 321, 322, and similarly, the differential amplifier 56 similarly obtains the amount of incident light on the light receiving surfaces 331, 332. A signal indicating the difference between Then, the difference signal between these two signals is generated by the differential amplifier 57 and sent to the drive control system of the objective lens 26 as the focus error signal SF.

According to the above-described embodiment, a special optical component for detecting a focus error is provided only by changing the photodetectors 32 and 33 among the optical components necessary for detecting the track error to the two-division type. It is possible to detect the track error and the focus error with high accuracy, and to reduce the size and weight of the optical head 20.

According to the above-described embodiment, the two-part split photodetectors 32 and 33 are used for photoelectric conversion of the two sub-beams B2 and B3, respectively, so that the focus error signal SF is obtained in the so-called differential detection form. Therefore, the reliability of focus error detection can be improved.

According to the above-described embodiment, the influence of birefringence is small as compared with the case where the astigmatism method is used for detecting the focus error. Therefore, the material of the substrate of the magneto-optical disk 10 is limited to glass having a small birefringence. Instead, a resin substrate such as polycarbonate (PC) which is lighter and cheaper than a glass substrate can be used as the substrate of the magneto-optical disk 10.

In the above-described embodiment, the example of application to the magneto-optical disk device 1 which is a rewritable optical disk device is shown, but the present invention can also be applied to a write-once optical disk device.

[0035]

According to the present invention, it is possible to reduce the number of optical components required for detecting a tracking error and a focus error, and to reduce the size and weight of an optical head and simplify assembly and adjustment.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a configuration of a main part of a magneto-optical disk device to which an error detection method of the present invention is applied.

FIG. 2 is a plan view showing a positional relationship between a light spot and a pre-groove.

FIG. 3 is a block diagram showing an example of a configuration of an error detection circuit according to the present invention.

FIG. 4 is a plan view showing an example of a light-dark pattern appearing on a light-receiving surface of a photodetector.

[Explanation of symbols]

 1 Magneto-optical disk device (optical disk device) 10 Optical disk 15 Pre-groove 41 Optical spot B2 Sub beam 32 Photodetector (divided photodetector) 321, 322 Light receiving surface

Claims (1)

[Claims] 1. An optical disk device (1) having an optical disk (10) having a pre-groove (15) as a recording medium, wherein a track error of a light spot (41) is detected by a three-beam method, and the pre-groove is detected at that time. The optical depth of the groove (15) is set to 1/4 wavelength, and at the same time, at least one of the sub-beams (B2) has a plurality of light receiving surfaces (321) (322) for photoelectric conversion. 32) by using the sub-beam (B2) for irradiating the edge of the pre-groove (15), the light-receiving surface (32).
1) The difference in the amount of incident light of (322) is taken as the light spot (4
A method of detecting an error of a light spot, which is characterized in that it is detected as a focus error of 1).