JPS63222341A - Optical pickup - Google Patents

Abstract

PURPOSE:To decrease the crosstalk noises of adjacent tracks on a disk information surface by increasing the grating depth of a diffraction grating from the central part toward the peripheral part along the grating. CONSTITUTION:The grating part 13b of the diffraction grating 13 which diffracts the light emitted from a semiconductor laser 1 and splits the light to a main beam and sub-beam is formed that the grating depth increases from the central part toward the peripheral part along the grating. The quantity of the light of the main beam is thereby decreased from the central part toward the peripheral part and a filter effect provided to the main beam without losing the quantity of the light and generating aberration by an optical filter. The crosstalk noises of the adjacent tracks on the information surface of the disk are thus decreased.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an optical pickup that optically reads information by projecting a light spot onto an information track on which high-density signals are recorded on a disk.

2. Description of the Related Art In recording and reproducing apparatuses that handle analog signals such as video discs, optical filters are generally used for the purpose of reducing crosstalk from adjacent tracks.

Figure 7 shows a conventional general 3-beam optical system.
1 is a semiconductor laser, 2 is a diffraction grating, 3 is an optical filter, 4 is a half mirror, 16 is an objective lens, 6 is a disk, 7 is a concave lens, 8 is a cylindrical lens, and 9 is a light receiving element.

The optical filter 3 has a characteristic of reducing the amount of light from the center to the periphery in the direction perpendicular to the track as shown in FIG. It has the effect of effectively reducing the radiation angle in the direction.

Figure 9 shows the relationship between the information tracks 10, 11 &, 11b in a cross-section in the direction perpendicular to the tracks of the disk and the light intensity distribution 12 on the information surface, and the figure (&) shows the relationship between the optical filter. When not used, the same figure (b) shows the case where an optical filter is used.

As can be seen from the figure, when no optical filter is used, the sidelobes of the light irradiated onto the disc fall directly on adjacent tracks, which causes crosstalk from adjacent tracks. Filters can be used to reduce this sidelobe and reduce crosstalk from adjacent tracks.

Problems to be Solved by the Invention However, the above-mentioned optical filters require advanced technology for manufacturing and are expensive, and also have problems such as increased aberrations due to the increase in the number of optical components and loss of light amount due to the filter. there were.

SUMMARY OF THE INVENTION In view of the above problems, it is an object of the present invention to provide an inexpensive optical pickup that reduces aberrations and light loss due to an optical system.

Means for Solving the Problems The present invention solves this problem without using the optical filter by increasing the grating depth of a diffraction grating that diffracts light from a light source from the center to the periphery along the grating. It has the same effect.

Effect: By using the diffraction grating having the above structure, the present invention can reduce crosstalk noise between adjacent trunks on the disk information surface without using an optical filter, and can also reduce the crosstalk noise of adjacent trunks on the disc information surface. In comparison, an inexpensive and high-performance optical pickup can be realized without causing aberrations or light loss.

Embodiment FIG. 1 is a block diagram of an optical pickup according to an embodiment of the present invention, and parts that are the same as those in the conventional example are given the same reference numerals and explanations thereof will be omitted.

In FIG. 1, 13 is a diffraction grating having the shape shown in FIG. 2, and is composed of a substrate portion 13a and a grating portion 13b. It has a shape that changes as follows.

Generally, the relationship between the diffraction grating and the intensity distribution of diffracted light can be determined by Fourier transformation. FIG. 3 shows a general diffraction grating expressed as a function f(x). The Fourier transform of this diffraction grating f' (x) is given by the following equation.

Note that W represents the width of the lattice portion, and P represents the pitch of each lattice portion.

In this case, φ is the phase difference (optical path difference) caused by the diffraction grating.
and 2 π t φ”' (n-1) λ (λ: wavelength, t: grating depth, n: refractive index).

Taking the numerical values, the electrolytic amplitude Fn of the n-order light is obtained from equation (1).

Therefore, the electrolytic intensity 11Col of the main beam (0th order light) after passing through the diffraction grating is expressed by the following equation.

11! :o l =P2+(2W2-2WP)(←biteφ)・・・・・・・・・(3) Kokote 2 W2-2WP
is negative, so therefore.

As t increases, the value of μsφ approaches 1, and the field strength increases to 1.
Eol can be made smaller.

The present invention utilizes the above-mentioned characteristics to change the grating depth t of the diffraction grating so that it becomes deeper from the center to the periphery as shown in FIG. As you go to the main beam (
This provides a filtering effect that reduces the electrolytic strength (1Xol) of the zero-order light (0-order light) and reduces the amount of light of the main beam.

For example, the thickness is 33oO at the center and about 6400 at the outer periphery.
The diffraction grating made of human MgF2 film has a wavelength of 78
When uniform light of 0 nm and NA 0.09 is incident, the main beam after passing through the diffraction grating can have a Gaussian distribution with a full width at half maximum of about 9 degrees.

Also, at this time, the electrolytic intensity of the sub-beam (primary light) is , and as shown in Fig. 4, the amount of light in the peripheral area increases compared to when it passes through a grid with a uniform depth (14a).
14b) The tracking error detection sensitivity can be increased.

As described above, according to this embodiment, it is possible to provide a filter effect to the main beam without causing light loss or aberration due to optical filters, reduce crosstalk from adjacent tracks, and reduce tracking Error detection sensitivity can also be increased.

Further, in this embodiment, the grating portion 13b and the substrate portion 131L of the diffraction grating 13 are made of separate members, but the same effect can be obtained even if they are made of the same member as shown in FIG.
It is clear that the same effect can be obtained even if the thickness of the substrate portion 13a is changed to make the grating depth deeper in the peripheral portion as shown in FIG.

Effects of the Invention As described above, the present invention has the advantage that the grating depth of the diffraction grating provided in the optical path until the light emitted from the light source is focused on the disk is adjusted to the central part in the direction perpendicular to the disk information track. By increasing the depth from the center to the periphery, it creates a filter effect that reduces the light intensity of the main beam from the center to the periphery without using the conventional optical filter, which reduces costs. At the same time, it is possible to eliminate the disadvantages of increased aberrations and loss of light amount due to the use of optical filters, and at the same time, it is possible to increase tracking error detection sensitivity.

[Brief explanation of the drawing]

Fig. 1 is a side view showing the configuration of an optical system of an optical pickup in an embodiment of the present invention, Fig. 2 is a perspective view of an embodiment of the diffraction grating of the present invention, and Fig. 3 is a function of the diffraction grating. 4 is an intensity distribution diagram of sub-beams by the diffraction grating of the embodiment, FIG. 6 is a perspective view showing another embodiment of the diffraction grating of the present invention, and FIG. FIG. 8 is a side view showing the configuration of a general optical system, FIG. 8 is a characteristic diagram of an optical filter in a conventional example, and FIG. 9 is a graph showing differences in light intensity distribution on the disk information surface due to filter effects. 1... Semiconductor laser, 13... Diffraction grating of the present invention, 4... Half mirror %6...
...Objective lens, 6...Disc, 7...
...Concave lens, 8...Cylindrical lens,
9... Light receiving element. Name of agent: Patent attorney Toshio Nakao and 1 other person No. 1
figure ! - Manual measurement - f-4 - Half mirror - 5-j + object lens Fig. 2 Fig. 3 Fig. 5 Fig. 6 Fig. 7 Fig. 8 Fig. 9 (α) (b)

Claims (1)

[Claims] A diffraction grating that diffracts the light emitted from the light source and divides it into a main beam (0-order light) and a sub-beam (1st-order light), an objective lens that focuses the light onto a disk, and reflected light that is modulated by the disk. and a light-receiving element for receiving light, wherein the grating depth of the diffraction grating is increased along the grating from the center to the periphery.