JPS63222333A - Detector for optical head tracking signal - Google Patents

Abstract

PURPOSE:To prevent the degradation of a tracking error signal for defocusing by inserting a mask provided with a pair of symmetrical slits to the front of a detector. CONSTITUTION:A mask 9 having two slits consisting of apertures which are parallel with the center line, where the 0th-order diffracted light and the + or -1st- order diffracted light of a light beam overlap, and are symmetrical with respect to this center line is inserted in front of a two-divided detector 6. The width of each aperture of the mask 9 is made wider than 1/10 of the width of overlapping between th e 0th-order diffracted light and the + or -1st-order diffracted light and is made narrower than 1/2 of said width. By this constitution, the vicinity of the part where the position difference between the 0th-order diffracted light and the + or -1st-order diffracted light is not changed is detected to detect the tracking error signal, thus preventing the degradation of the tracking error signal for defocusing.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an optical head for recording and reproducing information in an optical information processing device.

In an optical disk or an information recording/reproducing device using light, a light beam from a light source is focused into a light spot of about 1 μm, and the optical energy is used to record and reproduce information. This optical system device is called an optical head and is the most important element of an optical information processing device9, and it has been desired to make the device run more safely, be smaller, and be less expensive.

BACKGROUND OF THE INVENTION FIG. 4 shows a conventional optical head tracking method. This method is usually called a far-field tracking method or a push-pull tracking method. An incident beam 41, which is a substantially parallel light beam emitted from a light source, is reflected by a half mirror 42 and passes through an objective lens 43.
is transmitted and converged onto an information track 46 on a disk 44.

The light beam reflected on the disk undergoes diffraction by the information track and is split into 0th-order light and ±1st-order light, and then passes through the objective lens 3.
incident on . The light beam whose aperture is limited by the pupil of the objective lens 43 passes through the half mirror 42 and enters the two-split detector 46 . Each element of two-part detector 47.48
The detected optical signal is converted into a difference signal by a circuit (not shown) and becomes a tracking error signal. The tracking error signal changes depending on the position of the light spot focused on the information track. When the light spot is centered on the track and between the tracks, the detector 4
The diffraction pattern of the light incident on 7 and 48 becomes symmetrical, and the tracking error output becomes zero. When the light spot is located at the end of the track, a phase difference occurs in the light spot, causing asymmetry in the diffraction patterns on the detectors 47 and 48, and the tracking error signal becomes maximum. The sign of the tracking error signal is determined by the direction of the phase difference experienced by the light spot. The asymmetry rate changes depending on the phase difference experienced by the light spot, and is maximum when the phase difference is π/4.

Problems to be Solved by the Invention The tracking error detection method of far-field detection, which has been commonly used in the past, has the drawback of large S/N deterioration during defocusing.

An object of the present invention is to provide optical head tracking signal detection that eliminates this drawback.

Next, the principle used in the present invention will be explained.

When the phase depth of the track groove is about λ/8 and defocus Δ2 occurs, the phase difference Δθ between the zero-order diffracted light and the first-order diffracted light is... ......It is expressed as (1). Here, P is the track pitch λ and the wavelength used α is the angle toward the diffraction direction of the incident light beam.

From equation (1), it can be seen that the phase difference between the zero-order diffracted light and the first-order diffracted light changes depending on the location where the diffracted lights overlap when defocus occurs. Specifically, formula (1) is λ=800 nm.

For the case of an optical system with P=1.6 pm, NA==o, 5, Δz=o, ±'1.6. ±1.6
．． Figure 6 shows the calculations for the case of ±4.8 μm. Note that in FIG. 6, differences in light distribution are shown by differences in linear density. As shown in Figure 6, the defocus is 0 μm.
When , the distribution in the overlapping portion of the zero-order diffracted light and the first-order diffracted light is uniform and has a phase difference of i. On the other hand, when defocus occurs, the distribution on the center line where the zero-order light and the first-order light overlap does not change, but the phase difference changes in other places, so the light becomes brighter or darker.

As the amount of defocus increases, thin interference fringes appear. When the light spot crosses the track, the phase difference between the zero-order light, the +1st-order light, and the -1st-order light changes by π species. In the case of FIG. 5, it can be seen that when the defocus is 1.6 μm or more, the amount of light in the portion where the zero-order light and the first-order light overlap does not change significantly as an integral value. This is given as the focus depth d of the objective lens: λ' = 2NA" = '°6"0 °−°°−(2)
This corresponds to the limit value obtained from .

In the previous explanation, it became clear that the phase difference on the center line of the overlapping portion of the zero-order light and the first-order light does not change due to defocus. Therefore, if only the light on this center line is extracted, the term due to defocus becomes zero.

This is the case when the condition is λ in equation (1). That is, sin α=□, and if p λ = 800 nm and P = 1.6 μm, C1 = about 140. If a photodetector with a symmetrical slit or a slit-shaped photodetector is provided on a line centered on the point α=14°, the influence of defocus can be reduced.

Means for Solving the Problems The present invention provides an optical head tracking system comprising a pair of photodetectors having apertures symmetrical to the center line of the superimposed portion of the zero-order diffracted light and the ±1st-order diffracted light by the information track on the information carrier. It is a signal detection device.

Function As described above, by detecting the tracking error signal by detecting the vicinity of the portion where the phase difference between the zero-order diffracted light and the first-order diffracted light does not change, it is possible to prevent the tracking error signal from deteriorating due to defocus.

For example, when defocusing occurs due to impact, noise, etc., the tracking error signal may deteriorate in the conventional method and the track may deviate from the target track.

A similar problem also occurs when defocus occurs due to a tracking interference signal mixed into a focus signal. According to this method, the S/N of the tracking error signal
is less sensitive to defocus, and tracking does not occur even if some defocus occurs. Therefore, the reliability of the optical disk device can be instantly improved.

Embodiment FIG. 1 shows an embodiment of the present invention. The incident beam 1 is reflected by the half mirror 2, transmitted through the objective lens 3, reflected by the disk 4, and together with the light beam diffracted by the information track 6, passes through the objective lens 3 again and enters the two-split detector 6. This process is exactly the same as the conventional example. A feature of the present invention is that a mask 9 having a slit consisting of two openings is inserted in front of the detector 6.

The slits 7 and 8 are formed parallel to the center line where the zero-order diffraction light and the first-order diffraction light of the light beam overlap, and are arranged so as to have openings that are symmetrical with respect to the center line.

In a second embodiment of the present invention, a slit-shaped detector is used instead of placing a mask with slits in front of the butyphthalate. FIG. 2 shows the shape of a detector according to the present invention. The detector is composed of 6 parts, and tracking error detection, which is the purpose of the present invention, is performed using the detector 43.4.
Take it from 4.

On the other hand, by summing the outputs 21 to 26 of each detector, it can be used for reading signals from an optical disc.

The basic concept of the present invention is to obtain a tracking error signal by installing a detector at a point where the phase difference between the diffracted light by the track and the zero-order light is not affected by defocus. From this point of view, the slit 7.8 shown in FIG. 1 or the slit-like detector portion 23.8 shown in FIG.
A narrow slit width of 24 improves kneading properties. However, if the slit width becomes narrower, the amount of incident light decreases, and the S/N of the tracking error signal decreases. Furthermore, if the slit width is narrow, it is difficult to accurately extract a signal from a portion where the phase difference does not change, as a mounting problem.

According to the inventor's experiments, h~341 in the overlapping portion of the zero-order diffracted light and the ±1st-order diffracted light! i! At this time, the S of the tracking error signal was good, and good tracking servo could be performed without significant signal deterioration due to defocus. However, if a stronger light beam than that used for normal optical disc playback is obtained, narrowing the slit width is advantageous for defocusing, and a signal with a good S/N ratio can be obtained even with a width of % or less. .

Effect of the invention FIG. 3 shows the effects of the present invention in comparison with a conventional example.

The optical system used was a wavelength of 830 nm and an objective lens of N.
Ao,s disks with a track pitch of 1.6 μm were used.

The tracking error signal level is standardized at zero defocus. Figure 3 shows the conventional example and the optical system of the conventional example.
The effect of improving the tracking error signal when using a mask having two slit openings is obvious. In this example, the slit width was OJm for the objective lens aperture of 4.6-.

When the present invention is used, the range in which the 4IF motion can be performed normally has been increased from about ±1.2 μm to approximately ±2 μm. The signals S and /N are also good, and it can be said that the effects of the present invention are great.

[Brief explanation of drawings]

Fig. 1 is a configuration diagram of an embodiment of the present invention, Fig. 2 is a plan view showing an example of the shape of a detector in the present invention, Fig. 3 is a comparison diagram of measurement data showing the effects of the conventional method and the present invention, and Fig. 4
This figure is an explanatory diagram of a conventional far-field tracking error signal detection method, and FIG. 6 is an explanatory diagram schematically showing a far-field pattern at the time of defocusing. 1... Incoming beam %2... Half mirror for beam splitter 13... Objective lens, 4... Disk, 6... Information track ,
6. Old... Two-part detector, 7, 8... Slit, 9... Mask ◎ Name of agent: Patent attorney
Toshi Nakao Male 1 person Figure 1 7 Sso p) tl) d Slit (2) Figure 2 ≠ Fusu-Izu (μriyo 4 Figure WXs diagram

Claims (2)

[Claims] (1) An optical system that receives a light beam emitted from a radiation source and focuses it onto an information carrier, and zero-order diffracted light due to an information track on the information carrier that is reflected or transmitted on the information carrier.
An optical head tracking signal detection device comprising a pair of photodetectors having apertures symmetrical to the center line of a superimposed portion of first-order diffracted light. (2) The width of the aperture of each photodetector is ±1 from the zero-order diffracted light.
The optical head tracking signal detection device according to claim 1, which is larger than 1/10 and smaller than 1/2 of the width of the overlapped portion with the second-order diffracted light.