KR19990011285A - Optical pickup device for high density optical discs - Google Patents

Abstract

The present invention relates to an optical pickup apparatus for a high density optical disc, which is suitable for reading out a path of an optical disc recorded at a high density with a minimum of crosstalk between tracks and pit interference by a peripheral beam.

The optical pickup apparatus for a high density optical disc of the present invention limits the size of the photodetector to a certain size.

Therefore, according to the present invention, there is an effect that noise due to crosstalk is reduced.

Description

Optical pickup device for high density optical discs

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical pickup apparatus, and more particularly, to an optical pickup apparatus for a high density optical disc suitable for reading information of an optical disc recorded at a high density with a minimum of crosstalk between tracks and interference between the pit by a peripheral beam. .

Recently, optical discs have been replaced by conventional compact discs (CDs) in response to the demand for higher capacity.

Digital Buster Tiles (DVDs) have emerged. Accordingly, development of an optical pickup capable of reproducing information from such a high-density optical disc is continuing. The conventional optical pickup reduces the diameter of the light beam irradiated onto the recording surface of the optical disc by increasing the numerical aperture of the objective lens for condensing the light beam in order to read information from the optical disc recorded at a high density by reducing the pit and track intervals. By the way, the numerical aperture of this objective lens cannot be enlarged by the summation and must be limited to a certain limit or less.

Hereinafter, a conventional optical pickup apparatus will be described in detail with reference to FIGS. 1 to 3.

Fig. 1 shows a conventional optical pickup device, which includes a laser diode (hereinafter referred to as LD) 1 for generating a light beam and an objective lens for focusing the light beam on the recording surface of the optical disc 5; (4), a photo detector for detecting the light beam reflected from the optical disc 5 (hereinafter referred to as PD), 7 and the light beam from the LD 1 to reflect to the optical disc 5 to proceed. A reflection mirror 3 for passing the light beam reflected from the optical disk 5 toward the PD 7, and a light beam from the LD 1 divided into three by being located between the LD 1 and the reflection mirror 3; A diffraction grating 2 and an actuator 8 for driving the objective lens 4 up, down, left, and right are provided.

Usually, the optical pickup reflects a light beam on the optical disc 5 and reads out the information. For this purpose, the light beam generated in the LD 1 of the optical pickup is dispersed by the diffraction grating 2 into the main beam and two peripheral beams.

The light beam diffracted by the diffraction grating 2 is reflected from the reflecting mirror 3 toward the objective lens 4, and the pit 6 continuously recorded on the track of the information recording surface of the optical disc 5 by the objective lens 4 Condensed).

The light beam focused on the pit 6 of the optical disk 5 is reflected on the pit 6, and the reflected light beam is focused on the PD 7 by passing through the objective lens 4 and the reflecting mirror 3.

The PD 7 converts the collected reflected light beam into an electrical signal to obtain an RF signal and simultaneously detects an error signal for focusing and tracking servo.

A focus servo 9, a tracking servo 9, and an RF signal unit 11 are connected to the PD 7 of the optical pickup in parallel.

In detail, the PD 7 is composed of three PDs for detecting three light beams divided by the diffraction grating 2 as shown in FIG.

Here, the first PD 7A detects a zeroth order beam, that is, a main beam having a high intensity, among the three reflected light beams, and detects an RF signal and a focus error signal from the main beam.

In detail, the first PD 7A is divided into four parts such as a, b, c, and d. The RF signal section 11 adds and amplifies the sum signal from the a and c regions and the sum signal from the b and d regions of the first PD 7A to detect the RF signal.

The focus servo 9 adjusts the actuator 8 up and down by the difference signal between the sum signal from a and c and the sum signal from b and d of the first PD 7A to control the focus of the light beam to be constant. .

The second and third PDs 7B and 7C then detect two +/- primary beams, i.e., peripheral beams, which are used for the tracking servo. The tracking servo 10 adjusts the actuator 8 to the left and right by the sum signal of the electrical signals from the second PD 7B and the third PD 7C to control the light beam to track the correct information track.

On the other hand, in the conventional optical pickup, the numerical aperture of the objective lens is increased to reduce the size of the light beam irradiated onto the optical disk, as shown in FIG.

Referring to FIG. 3 in detail, the distribution of light beams from LD (1) of a general optical perkup is a Gaussian function so that the vertical and horizontal divergence angles are different depending on the size of the emission cross section. Field is oval as shown.

Therefore, the optical pickup adjusts the numerical aperture of the objective lens 4 to make the shape of the light beam focused on the optical disk circular and to reduce the size of the light beam.

This is expressed as an equation.

However, NA is the numerical aperture of the objective lens 4, n is the refractive index of the medium, and θ is the angle at which the light beam of the LD 1 is condensed on the disk 5 by the objective lens 4.

Then, the size of the light beam formed on the disk 5 is expressed as follows.

Where _o is the magnitude of the light beam formed on the optical disk 5, k is an integer,? Is the wavelength of the light beam, and NA is the numerical aperture of the objective lens.

In this case, the value of k is about 1.3 when the light beam is Gaussian and about 0.96 when the light beam is constant.

Accordingly, in order to reduce the size ω _o of the light beam formed on the optical disc 5, a circular portion which is a part of an elliptical center portion in the form of a far field of the light beam of the LD 1 by a beam cutting part (not shown). Only light beams with a uniform distribution are used.

By the way, when the constant light beam is focused on the optical disc 6 by the objective lens 4, the light beam on the optical disc is as shown in Fig. 3, and the light beam has a main beam and a high intensity. Is distributed in small peripheral beams.

Therefore, in order to read the information of the optical disk, when the constant light beam is focused on the pit of the optical disk 5 by the objective lens 4, the peripheral pit is read by the peripheral beam, so that the reproduction signal generates noise.

In detail, since the track pitch between the tracks of the optical disc is reduced for higher density, signals between the tracks are mixed during information reproduction, thereby causing cross talk. In addition, for the sake of densification, the length between the pits is shortened to cause interference between the pits, thereby increasing the time jitter, thereby increasing the bit error ratio of the reproduction signal and affecting the tilt of the disc. Worse

Accordingly, an object of the present invention is to provide a high-density optical pickup apparatus capable of minimizing crosstalk between tracks and interference between pits by surrounding beams.

Another object of the present invention is to provide an optical pickup apparatus suitable for reading out information of a high density optical disk by reducing bit errors and minimizing the influence of the tilt of the optical disk.

1 shows a conventional optical pickup apparatus;

FIG. 2 is a diagram showing the detailed configuration of the photodetector PD shown in FIG.

3 is a view showing an optical pickup apparatus for a high density optical disc according to the prior art.

4 is a view showing distribution characteristics of a light beam focused on an optical disk;

5 is a diagram showing an optical pickup device for a high density optical disc according to the present invention;

6 is a characteristic diagram showing the relationship between the size of the photodetector and the crosstalk;

* Explanation of symbols for main parts of the drawings

1: LD (Laser Diode) 2: Grating

3: reflector 4: objective lens

5: optical disc 6: feet

7,20: PD (Photo Diode) 8: Actuator

9: focusing servo 10: tracking servo

11: RF signal part

In order to achieve the above object, the high-density optical pickup apparatus according to the present invention is a light source for generating a light beam, an objective lens for condensing the light beam from the light source on the optical disk, and has a limited size within a certain range reflected from the optical disk And a photodetector for converting the light beam into an electrical signal, a reflector disposed between the light source and the photodetector and the objective lens to determine the path of the light beam, and a diffraction grating separating the light beam from the light source. .

Other objects and features of the present invention other than the above object will become apparent from the description of the embodiments with reference to the accompanying drawings.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to FIGS. 1, 2, and 5.

1 is a view showing the configuration of a conventional optical pickup device, Figure 2 is a view showing a detailed configuration of the PD (7) in the configuration of the optical pickup of Figure 1 detailed description of the operation of each configuration will be omitted do.

5 is a view showing an optical pickup apparatus according to the present invention.

The light beam generated in the LD 1 of FIG. 1 is dispersed by the diffraction grating 2 into a main beam and two peripheral beams. The light beam diffracted by the diffraction grating 2 is reflected from the reflecting mirror 3 toward the objective lens 4, and the mitts 6A and 6B continuously recorded on the information track of the optical disc 5 by the objective lens 4. 6C). At this time, the main beam among the three light beams is focused on the pit 6A to be read, and the two peripheral beams are focused on the peripheral feet 6B and 6C to form an image on the optical disc 5 in the form as shown in FIG. do. The light beam reflected by the pit 6A, 6B, 6C of the optical disc 5 is transmitted through the objective lens 4 and the reflecting mirror 4 to be focused on the PD 7.

The PD 7 converts the collected reflected light beam into an electrical signal to obtain an RF signal and simultaneously detects an error signal for focusing and tracking servo.

At this time, the reflected light beam formed on the PD 20 shown in FIG. 5 (a) is mixed with the reflected light beams of the main beam and the peripheral beam to have a shape as shown in FIG. 5 (c). Therefore, in the present invention, the size of the PD 20 is limited so that the signals b and c which are noise components are not detected among the reflected optical signals shown in FIG.

First, the size of the PD 20 for detecting the RF signal is determined by the following principle.

When the light beam reflected from the optical disc 5 is imaged on the PD 20, the magnification of the objective lens 4 is M, and the distance between the information feet 6A and 6B on the optical disc 5, i.e., between the track pitches. If the distance is D, the distance between a and b on the PD 20 is d.

At this time, if the size of the PD 20 is large enough, the reflected optical signals of a, b, and c are read out, and the signals b and c act as noise. If the size of the PD 20 is too small, the signal gain on the PD is reduced. There is a problem of becoming smaller.

Therefore, in consideration of this, the size of the PD 20 to be limited in the present invention may be determined within the following range.

Figure 6 shows the relationship between the crosstalk and the size of the PD adjusted according to the present invention.

For example, when information is reproduced by using an optical pickup having a magnification M of the objective lens 6 from a high density optical disk having a track pitch D of 0.9 µm, an incidence of crosstalk in accordance with the size of the PD is determined. It is shown for each slope (0 degrees to 0.3 degrees).

At this time, FIG. 6 shows that the PD having a size of between 5.4 μm and 10.8 μm is reduced by cross talk when information is reproduced by Equation 4 above.

In addition, although the reproduction signal was sensitive to the inclination of the optical disc 5, when the size of the PD 20 is adjusted within the above range, the sensitivity of the disc is inferior to the inclination of the disc as shown in FIG. You are less affected by it.

As described above, according to the high-density optical pickup apparatus of the present invention, the noise due to crosstalk is reduced by limiting the size of the PD for detecting the reproduction signal within a predetermined range. In addition, when the high-density optical pickup apparatus of the present invention is used for information recording, the distance between the pit and the track can be reduced, which can also contribute to the high density of the optical disc.

Those skilled in the art will appreciate that various changes and modifications can be made without departing from the technical spirit of the present invention. Therefore, the technical scope of the present invention should not be limited to the contents described in the detailed description of the specification but should be defined by the claims.

Claims (4)

A light source for generating a light beam, an objective lens for receiving the light beam from the light source to the optical disk, a photodetector for converting a light beam reflected from the optical disk with a limited size within a predetermined range into an electrical signal, and the light source and the photodetector And a diffraction grating for separating the light beams from the optical window, and a reflecting plate positioned between the objective lenses to determine the path of the light beams. The optical pickup apparatus of claim 1, wherein the size of the photodetector is limited by the distance between the magnification of the objective lens and the track pitch of the optical disk when the reflected light beam is formed on the photodetector. The method of claim 1, wherein the limiting range of the photodetector size is (Where D is the distance between the track pitches of the optical disc, M is the magnification of the objective lens when the reflected light beam is formed in the photodetector, and d is the size of the photodetector) An optical pickup device for a high density optical disc, characterized in that. The optical pickup apparatus of claim 1, wherein the size of the photodetector for detecting a reproduction signal is limited by the photodetector.