JPS62172539A - Tracking control method for optical information reader - Google Patents

Abstract

PURPOSE:To attain miniaturization and simplification of the titled reader by locating a recording face to an afocal position, leading the reflected light to a photodetector so as to detect a tracking error from the output of each detection element thereby applying the far field method. CONSTITUTION:The light beam reflected in a recording face 10a becomes a linearly polarized S light through an image forming lens 22 and a lambda/4 plate 20, separated from a P polarization incident light beam at a beam splitter 18, the beam diameter is stopped by a beam reducer 24, a part is given to a 4-split photodetector 28 and the rest is led to a 4-split photodetector 38. The focus error obtained by the detector 38 is fed back to a focus control system of a lens 22 and the light beam focus is controlled to a position deviated from the face 10a. Further, one split part of the detector 28 is arranged in parallel with the running direction of a track image and an RF signal representing the presence of a contrast pit is obtained by the sum of detection elements clipping the split part and the tracking error is obtained by the difference of the detector elements (c, d).

Description

Detailed Description of the Invention (Industrial Application Field) The present invention reflects or transmits a light beam on the recording surface of an optical recording medium that does not have a tracking guide groove, and the reflected or transmitted light reduces tracking errors. The present invention relates to a tracking control method in an optical information reading device that detects.

(Technical Background of the Invention) Optical disks have conventionally been used as one of the recording media for recording and reproducing high-density and large-capacity information. For example, there is an optical disk with a guide groove in which a track is formed by arranging a large number of bits each having a concave portion having a width of 0.6 to 1.0 mm and a length of 1 to 3 JLm in a circular or spiral shape.

In this case, the focus of the light beam is controlled so that it is always focused on the recording surface of the optical disc. Further, tracking control is performed such that the deviation of the light beam from the track (referred to as tracking error) is constantly detected based on changes in the reflection characteristics of the light beam due to the pits, and the light beam is irradiated onto the track.

Various methods have been proposed for this tracking control, but in order to miniaturize the optical system and simplify the configuration,
Far field is most suitable.

This far-field method utilizes the phenomenon that when a light beam is diffracted by a pit, an asymmetric light intensity distribution of reflected light occurs in the far-field region in response to tracking error. The tracking error is determined by detecting changes in the light intensity distribution. This far field method includes a push-pull method and a heterodyne method.

On the other hand, some optical disks do not have tracking guide grooves and have pits formed in a dark and light pattern, but such dark and light pits do not cause asymmetry in the light intensity distribution in the far field region, and the push as described above does not occur. Far field methods such as the pull method could not be applied. For this reason, a complicated tracking control method had to be adopted, and it was difficult to miniaturize the optical system. In addition, in the case of pits having such a dark and light pattern, it has been considered to provide guide grooves on the recording surface separately from the light and shade pits in order to make the above-mentioned far field method applicable.

However, in this case, a problem arises in that the number of manufacturing steps for the optical disc increases.

(Objective of the Invention) The present invention was made in view of the above circumstances, and it is possible to apply the far field method to an optical recording medium that does not have a tracking guide groove, and to miniaturize and simplify the optical system. It is an object of the present invention to provide a tracking control method for an optical information reading device.

The present invention also provides a tracking control method for an optical information reading device that enables the far-field method to be applied not only to optical discs but also to various forms of optical recording media such as cards, sheets, tapes, and drums without tracking guide grooves. The purpose is to provide

Furthermore, the present invention provides tracking control for an optical information reading device that enables the far-field method to be applied not only to optical recording media in which information is read using reflected light, but also to optical recording media in which information is read using transmitted light. The purpose is to provide a method.

(Structure of the Invention) According to the present invention, the object is to guide a light beam to an optical recording medium without a tracking guide groove, and to generate a tracking error from reflected light or transmitted light of the light beam by the recording surface of the optical recording medium. In this method, the recording surface is positioned at an out-of-focus position deviated by a predetermined distance from the focal point of the light beam, while the focus control is performed to position the recording surface at an out-of-focus position that is deviated by a predetermined distance from the focus of the light beam. The method is characterized in that the light or transmitted light is guided to a photodetector provided in a far field region and divided into two in a direction parallel to the track, and a tracking error is detected from the output of each detection element of this photodetector. This is achieved by a tracking control method for an optical information reading device.

That is, as a result of various experiments, the inventor discovered that, for an optical recording medium such as an optical disk without a guide groove in which pits are formed in a dark and dark pattern, a light beam is focused at a position slightly deviated from the recording surface. It was discovered that an asymmetry in the distribution of the amount of reflected light or transmitted light occurs in the far-field region, and based on this discovery, tracking control using the far-field method became possible. Here, the amount of deviation of the focal point of the light beam from the recording surface is determined by using a semiconductor laser with a wavelength of about 0.83 gm as the light source and a focal pressure of 9.
When using an imaging lens of about 3.6 mm, (0,4
It is desirable to set it to approximately 0.6) m.

(Example) FIG. 1 is a conceptual diagram showing an example of an optical system for explaining the method of the present invention. In this figure, reference numeral 10 denotes an optical disk as an optical recording medium without a tracking guide groove, and on its lower recording surface 10a, pits consisting of a light and shade pattern are formed concentrically or spirally. A track is formed. This optical disk 10 is fixed to a clamp 12 and rotated together with the clamp 12 by a servo motor (not shown).

14 is a semiconductor laser, for example, a wavelength of 0.83 pm
outputs a laser beam with P linear polarization of approximately This P-polarized laser light passes through a collimator lens 16, a polarizing beam splitter 18, and a reflecting prism 19, becomes clockwise circularly polarized light by an input quarter plate 20, and is further guided to a recording surface 10a through an imaging lens 22. The imaging lens 22 is controlled by 7 degrees to focus on a position 0.4 to 0.8 gm, preferably 0.5 gm away from or near the recording surface 10a.

The light beam hits the recording surface 10a and is reflected, and returns to the imaging lens 22 as counterclockwise circularly polarized light. This reflected light further passes through the input quarter plate 20 and becomes S-polarized linearly polarized light. This S-polarized reflected light is separated from the P-polarized incident light beam by a polarizing beam splitter 18 and sent to a beam reducer 2.
4, and the beam diameter is narrowed down. A part of this narrowed reflected light is guided to a four-split photodetector 28 by a half prism 26, and the remaining reflected light passes through a reflecting prism 30, two orthogonal cylindrical lenses 32, 34, and a reflecting prism 36. The light is guided to a split photodetector 38. The two cylindrical lenses 32 and 34 have different focal lengths, and the sum of the outputs of the two photodetecting elements at diagonal positions of the 4-split photodetector 38 is calculated, and the focus error (FER) is determined by the difference between these two sums. demand. That is, the focus error is determined by the astigmatism method, the focus error is fed back to the focus control system of the imaging lens 22, and the focus of the light beam is approximately 0.5I from the recording surface 10a.
It is controlled to a position deviated by Lm.

The four-division photodetector 28 is arranged so that one of the divided parts is parallel to the running direction of the track image, and the sum of the pair of detection elements a and b sandwiching the divided part parallel to the track image is An RF multiplied signal indicating the presence or absence of dark and light pits is obtained. Further, the tracking error (TER) is calculated from the difference between the other pair of detection elements c and d. That is, in this embodiment, the tracking error is determined by the push-pull method. The substrate 40 supporting the imaging lens 22 or the entire optical system is moved in a direction perpendicular to the track by a tracking control system so that this tracking error is minimized.

FIG. 2 is a diagram showing the results of a calculation analysis of the phenomenon of the method of the present invention. In this figure, the horizontal axis represents the amount of deviation χ between the light beam irradiated onto the recording surface 10a and the track, and the vertical axis represents the amount of deviation χ between the light beam irradiated onto the recording surface 10a and the track. The tracking error (TER) detected by elements c and d of the detector 28 is shown. The wavelength of the light source is 0.831.
A semiconductor laser 4 of Lm is used, the bit width is IJLm, the reflectance of the pit part of the recording surface 10a is 0%, and the reflectance of the part other than the pit is 100%, and the imaging lens 22 has a focal path @ f = 3. It is assumed that Ei m m is used.

When determining the amount of deviation δ of the focal point of the light beam from the recording surface by the imaging lens 22, the depth of focus of the imaging lens 22 should be taken into account, and the (A position δ should be within the depth of focus). In other words, as is clear from the explanatory diagram in FIG.
This is because there is no problem in detecting the F-fold signal focus error. For example, the aperture ratio of the lens 22 N A = 0
．． 47, the depth of focus d is d=Shiri/2 (NA
) 2 =±1.9p-m. Therefore, in this case, the deviation amount δ (= 0.5g
m) is sufficiently close to the focal point compared to the depth of focus d, and its influence on imaging performance can be ignored.

In Figure 2, A is this deviation amount δ=±0.5 JLm
B and C show the cases of ±0.8 and ±0.4±gm, respectively. For reference, the calculation results in the case of having a guide groove with a depth of 1/8 are shown in D.

The above results are obtained when an optical disc with dark and light pits is used as an optical disc without a guide groove. However, the present invention is applicable to various forms of optical recording media, such as amplitude-type optical discs that detect the presence or absence of pits based on not only density pits but also amplitude changes in reflected light, and cards, sheets, tapes, and drums that do not have tracking guide grooves. The present invention also includes such applications.

Further, in the above embodiments, tracking control is performed by the push-pull method, but the present invention encompasses other methods such as the heterodyne method as long as they are included in the far-field method.

Furthermore, the present invention includes not only a device that reads information using reflected light but also a device that reads information using transmitted light.

(Effects of the Invention) As described above, the present invention performs focus control so that the recording surface of an optical recording medium is positioned at a predetermined distance deviation from the focal point of an imaging lens, and at the same time controls reflected light or transmitted light in the far field region. Tracking errors are detected based on changes in the light intensity distribution. Therefore, the far field method can be applied to an optical recording medium that does not have a tracking guide groove, and control methods that simplify the optical system, such as the push-pull method and the heterogain method, can be used. Therefore, the device can be made smaller and simpler.

[Brief explanation of drawings]

FIG. 1 is a conceptual diagram showing an example of an optical system for explaining the method of the present invention, FIG. 2 is a diagram showing experimental results using this example, and FIG. 3 is an explanatory diagram of the depth of focus. DESCRIPTION OF SYMBOLS 10... Optical disc, 10a... Recording surface, 14... Semiconductor laser, 22... Imaging lens, 28.38... 4-split photodetector, TER... Tracking error.

Claims (2)

[Claims] (1) A light beam is guided to an optical recording medium without a tracking guide groove, a tracking error is detected from the reflected light or transmitted light of the light beam by the recording surface of this optical recording medium, and this tracking error is minimized. In the method of performing tracking control in a direction in which the recording surface is focused so as to be positioned at an out-of-focus position deviated by a predetermined distance from the focus of the light beam, the reflected light or the transmitted light is provided in a far field region. A tracking control method for an optical information reading device, characterized in that the tracking error is detected from the output of each detection element of the photodetector, and the tracking error is detected from the output of each detection element of the photodetector. (2) The amount of deviation of the focal point of the light beam from the recording surface is (0
．． 4. The tracking control method for an optical information reading device according to claim 1, wherein the tracking control method is set within a range of 4 to 0.6) μm.