KR100260705B1 - Optical pickup compatible with optical recording media - Google Patents

Abstract

PURPOSE: An optical pickup compatible for an optical recording medium is provided to exchange a disk of a CD system in a disk reproducing process by adjusting a distance between a collimating lens and a laser diode. CONSTITUTION: A laser diode(41) outputting a light having 650nm wave length for recording and reproducing a signal with respect to a DVD(47). A laser diode outputting a light having 780nm wave length for recording and reproducing a signal with respect to a disk(50) of CD system. A light detecting unit detects a light reflected from an information recording surface of the disk(50) of CD system. A hologram module(42) comprises the laser diode and the light detecting unit. The light having 650nm wave length is reflected on 'A' surface of a prism(43) and is penetrated on 'B' surface and outputs toward a mirror(44). The mirror(44) having a coating characteristic for totally reflecting a receiving light makes a reflected light be inputted to the object lens(47). A collimating lens(45) makes the inputted light in parallel, and the parallel light is focused by the object lens and forms a light spot on an information recording surface of a DVD(47). The formed light spot is reflected on an information recording surface of a DVD(47) and returned to the prism(43). A portion of the returned light penetrates the 'A' and 'B' surfaces and outputs toward an astigmatism lens(48). The astigmatism lens(48) focuses the light to a light detecting unit(49) for detecting a focus error of the inputted light. The light detecting unit(49) detects optical information from the inputted light.

Description

Optical pickup compatible with optical record carriers

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to an optical pickup for a digital video disc (DVD), which is compatible with a CD-based disc. It is about.

Optical pickup is a device that records and reproduces information such as video, sound, or data with high density. The recording medium of optical pickup is composed of disk, card, or tape, but the disk type is mainstream. Recently, products in the field of optical disc devices have been developed from LD, CD to DVD. Such an optical disk is composed of a plastic or glass medium of constant thickness in the axial direction in which light is incident and an information recording surface on which information is recorded.

To date, high density optical disc systems have increased the numerical aperture of the objective lens in order to increase the recording density, and use short wavelength light sources of 635 nm or 650 nm. By using this short wavelength light source, it has been developed to enable CD playback as well as recording and playback of DVDs of different thicknesses.

However, this system must use light of 780 nm wavelength for compatibility with the recordable CD-R, the latest form of CD. This is due to the recording characteristics of the CD-R recording medium. Thus, making it possible to use both light of 780 nm wavelength and light of 650 nm wavelength in one optical pickup, that is, using two laser diodes, has emerged as a very important technology for DVD and CD-R compatibility.

However, a system using two laser diodes uses only one objective lens to simplify the construction of the optical pickup. As a result, the aperture of the objective lens in the configuration of the optical system for CD playback uses 0.6, which is larger than 0.45 used in the conventional CD series, and thus the tilting and optical axis shift characteristics of the objective lens become very poor. As an example of this, the relationship between the amount of coma aberration according to the objective lens diameter is shown when the CD having a thickness of 1.2 mm is tilted 1 degree in FIG. 1. The coma aberration for the tilt of the disk increases in proportion to the third power of the objective aperture. Therefore, the smaller the objective lens aperture, the smaller the amount of coma aberration and the smaller the performance deterioration. Therefore, in the case of a general CD player, the objective lens aperture is limited to a value of 0.45 to 0.5. Thus, a system using two laser diodes is used by inserting a variable stop by dielectric coating on an optical path to reduce the optical system aperture of the CD to about 0.45.

As an example of a system using two laser diodes and a variable aperture, a conventional optical pickup compatible with DVD and CD-R will be described with reference to FIG.

2 shows the structure of a conventional optical pickup having two laser diodes as a light source for a DVD and a CD-R, and using a single objective lens and a variable aperture. The optical pickup of FIG. 2 uses light of 635 nm wavelength for DVD reproduction and light of 780 nm wavelength for recording and reproduction of CD-R.

Light having a wavelength of 635 nm emitted from the first laser diode 11 is incident on the first collimating lens 12. This ray is shown by the solid line. The first collimating lens 12 makes incident light into parallel rays. The light passing through the first collimating lens 12 is reflected by the beam splitter 13 and then proceeds to the interference filter type prism 14.

Light of 780 nm wavelength emitted from the second laser diode 21 passes through the second collimation lens 22, the light splitter 23, and the condenser lens 24, and then proceeds to the interference filter type prism 14. This ray is shown by the dotted line. Here, light of 780 nm wavelength is converged in the interference filter prism 14, and the optical system having such a structure is called a finite optical system.

The interference filter-type prism 14 battles the light of the 635 nm wavelength reflected by the light splitter 13 and totally reflects the light of the 780 nm wavelength focused on the condenser lens 24. As a result, the light from the first laser diode 11 is incident on the wave plate 15 in a form parallel to the first collimator lens 12. The light from the second laser diode 21 is incident on the wave plate 15 in the form of diverging by the condenser lens 24 and the interference filter type prism 14. The light transmitted through the wave plate 15 passes through the thin film type aperture 16 and then enters the objective lens 17.

The objective lens 17 is designed to focus on the information recording surface of the DVD 18 having a thickness of 0.6 mm, and focuses the light of the 635 nm wavelength emitted from the first laser diode 11 on the information recording surface of the DVD 18. To bear. Therefore, the light reflected from the information recording surface of the DVD 18 contains the information recorded on the information recording surface. The reflected light is incident on the photodetector 19 which penetrates the light splitter 13 and detects optical information.

When the above-mentioned finite optical system is not applied, the 780 nm wavelength light emitted from the second laser diode 21 is used for the CD-R disc 25 having a thickness of 1.2 mm using the objective lens 17 described above. If the information recording surface is focused, spherical aberration occurs due to the difference in the thickness of the DVD 18 and the thickness of the CD-R disc 25. This spherical aberration is caused by the information recording surface of the CD-R disc 25 farther from the information recording surface of the DVD 18 on the optical axis with respect to the objective lens 17. In order to reduce such spherical aberration, a configuration of a finite optical system using the condenser lens 24 is required. By the use of the variable stop 16, which will be described later in conjunction with FIG. 3, the light having a wavelength of 780 nm forms an optical spot of a size optimized for the information recording surface of the CD-R disc 25. FIG. Then, the light of 780 nm wavelength reflected from the CD-R disc 25 is reflected by the light splitter 23 to be detected by the photodetector 26.

As shown in FIG. 3, the variable stop 16 shown in FIG. 2 has a thin film structure that can selectively transmit light contained in an area of 0.6 or less NA corresponding to the diameter of the objective lens 17. FIG. Have That is, the variable stop 16 is divided into a region 1 that transmits light of both 635 nm wavelength and 780 nm wavelength and a region 2 that totally reflects light of 780 nm wavelength and transmits light of both 635 nm wavelength and 780 nm wavelength around the numerical aperture 0.45 with respect to the optical axis. . Here, region 1 is an area with a numerical aperture of 0.45 or less and region 2 is an area outside the region 1, which is formed by coating a dielectric thin film. The region 1 is composed of a quartz (SiO ₂ ) thin film to eliminate optical aberration caused by the dielectric thin film coated region 2.

By the use of such a variable stop 16, light having a wavelength of 780 nm passing through area 1 with a numerical aperture of 0.45 or less forms an optical spot suitable for the CD-R disc 25 on its information recording surface. Therefore, the optical pickup is compatible with an optimized optical spot when the optical recording medium is changed from the DVD 18 to the CD-R disc 25.

However, the optical pickup described above with reference to FIG. 2 should configure a finite optical system for light of 780 nm wavelength in order to remove spherical numbers generated during DVD and CD-R compatibility. Therefore, there is a problem that the assembly of optical parts is difficult due to the complicated optical system configuration. In addition, the optical path difference between the light passing through the area 1 having a numerical aperture of 0.45 or less and the area 2 having a numerical aperture of 0.45 or more by the dielectric thin film, which is an optical thin film formed in the area 2 having a numerical aperture of 0.45 or more, of the variable aperture 16. difference), so to eliminate this, the formation of a special optical thin film in area 1 is required. Thus, although the quartz coating was formed in the region 1 and the multilayer thin film was formed in the region 2, the manufacturing process was complicated and the thickness of the thin film had to be adjusted with the precision of nm unit, which was not suitable for mass production.

Accordingly, in order to solve such a problem, the present invention is to provide an optical pickup for DVD that is compatible with a CD-type disc during disc playback by adjusting the distance between the collimator lens and the laser diode.

1 is a view showing the relationship between the objective lens aperture and the amount of coma aberration.

2 is a view showing the configuration of a conventional optical pickup.

3 is a view showing the structure of a variable stop.

4 is a diagram showing the configuration of an optical pickup for DVD that is compatible with a CD-based disk according to the present invention.

* Explanation of symbols for main parts of the drawings

41: laser diode 42: hologram module

43: Prism 44: Mirror

45: collimating lens 46: objective lens

47: DVD 48: astigmatism lens

49: photodetector 50: CD disc

A feature of the present invention for achieving the above object is that in the optical pickup that is compatible with at least two kinds of optical recording media that use different wavelengths of light for recording and reproducing information, A first laser diode that is emitted, a second laser diode that emits light having a relatively long wavelength, a prism that separates and transmits or reflects incident light and reflected light, and reflects light incident from a prism to an objective lens, A mirror for reflecting light incident from the light beam toward the prism, a collimating lens for collimating light incident from the mirror, an objective lens for focusing light incident from the collimating lens on the information recording surface of the optical recording medium, and an incident light from the prism Astigmatism and astigmatism lenses for detecting focus errors with relatively short wavelengths of light A distance between the collimator lens and the second laser diode including a photodetector for detecting optical information from light incident from the lens and a second photodetector for detecting optical information from relatively long wavelength light incident from the prism. The optical pickup is compatible with optical recording media.

Hereinafter, exemplary embodiments of the present invention will be described with reference to the accompanying drawings.

4 is a diagram showing the configuration of an optical pickup for DVD that is compatible with a CD-based disk according to an embodiment of the present invention. The optical pickup of FIG. 4 has a laser diode 41 which emits light of 650 nm wavelength for recording and reproducing a signal to the DVD 47. FIG. In addition, the optical pickup detects the light reflected by the laser diode emitting 780 nm wavelength and the information recording surface of the CD series disk 50 for recording and reproducing the signal on the CD series disk 50. The photodetector has a hologram module 42 integrated therein. This hologram module 42 is for simplifying assembly and adjustment of the optical system.

Light of 650 nm wavelength emitted from the laser diode 41 is reflected on the A side of the prism 43, transmitted on the B side, and directed to the mirror 44. The mirror 44 has a coating property so as to totally reflect all of the received light so that the reflected light is incident on the objective lens 47. Thus, light of 650 nm wavelength directed to the mirror 44 is totally reflected to the collimating lens 45. The collimation lens 45 makes incident light parallel, and the parallel light is collected by the objective lens 46 to form a light spot on the information recording surface of the DVD 47. The formed light spot is reflected on the information recording surface of the DVD 47 and returns to the prism 43 again. A part of the light returned to the prism 43 passes through the A and B planes and goes to the astigmatism lens 48. The astigmatism lens 48 causes light to enter the photodetector 49 so as to detect a focus error of incident light. Then, the photodetector 49 detects optical information from the incident light.

On the other hand, the light of 780nm wavelength emitted from the hologram module 42 is reflected from the B surface of the prism 43 to the mirror 44. Light incident on the mirror 44 is also reflected to form an optical spot on the information recording surface of the CD-based disk 50 through the collimating lens 45 and the objective lens 45. The formed light spot is reflected on the information recording surface of the CD-based disk 50 and returns to the prism 43 again. The light returned to the prism 43 is reflected by the B surface and is incident on the photodetector of the hologram module 42. The photodetector of the hologram module 42 detects optical information from the incident light.

The optical pickup of FIG. 4 having two laser diodes 41 and 42 performs a signal reproducing operation for the DVD 47 and the CD-based disk 50 using one objective lens 46. FIG. The objective lens 46 is a lens designed for an infinite optical system, and exhibits optimal performance for a disc when the incident beam is parallel light. Thus, this objective lens 46 is optimized for the thin DVD 47. In the case of recording and reproducing a disc having the same thickness as that of the CD-based disc 50, the objective lens 46 exhibits optimum performance when the beam incident on the objective lens 46 is adjusted to a slight diverging beam. The degree of divergence or parallelism of the light incident on the objective lens 46 is controlled by the distance change of the hologram module 42 and the collimation lens 45 with respect to the light of 780 nm wavelength. This reduces the spherical aberration due to the disc thickness difference. That is, as the distance between the collimator lens 45 and the hologram module 42 is changed, the size of the light spots mounted on the disks 47 and 50 changes. This means that the effective aperture of the objective lens 46 changes. In Table 1 below, the optimization distance that minimizes the light spots focused on the disks 47 and 50, that is, between the hologram module 42 and the collimation lens 45, is minimized. Based on the distance of 14.6mm, the effective aperture change and the size of the light spot condensed on the disks 47 and 50 are shown.

As shown in Table 1, as the distance change amount, that is, the distance between the hologram module 42 and the collimation lens 45 increases, the effective aperture decreases linearly, and the size of the light spot focused on the disks 47 and 50 is reduced. Increase linearly. In addition, side lobes generated in light spots condensed by the distance change are reduced by using an objective lens of an annular shield. Thus, performance degradation due to side lobes can be reduced.

As described above, by adjusting the distance between the collimating lens and the hologram module, the spherical aberration due to the difference in the thickness of the disk is reduced, and the CD-based disk due to the objective lens large diameter generated by using an existing single objective lens for DVD. It is possible to reduce the deterioration of optical performance at

Claims (5)

An optical pickup compatible with at least two types of optical recording media using different wavelengths of light for recording and reproducing information, the optical pickup comprising: a first laser diode emitting light having a relatively short wavelength; A second laser diode emitting light of a relatively long wavelength; A prism separating and transmitting or reflecting the incident light and the reflected light; A mirror configured to reflect light incident from the prism to the objective lens and reflect light incident from the objective lens to the prism; A collimating lens for collimating light incident from the mirror; An objective lens for condensing light incident from the collimating lens on an information recording surface of the optical recording medium; An astigmatism lens for detecting a focus error for light of a relatively short wavelength incident from the prism; A first light detector for detecting optical information from light incident from the astigmatism lens; And a second photodetector for detecting optical information from light of a relatively long wavelength incident from the prism, the optical pickup being compatible with optical recording media by adjusting the distance between the collimating lens and the second laser diode. 2. The first surface of claim 1, wherein the prism reflects light incident from the first laser diode and transmits light having a relatively short wavelength reflected from the mirror, and a relative reflected light from the first surface. A second surface for transmitting light having a short wavelength and light having a relatively short wavelength reflected from the mirror, and reflecting light incident from the second laser diode and light having a relatively long wavelength reflected from the mirror; Optical pickup, characterized in that consisting of. The optical pickup of claim 1, wherein the objective lens is an annular shield lens. The optical pickup of claim 1, wherein the second laser diode and the second photodetector are integrated to form one hologram module. The optical pickup of claim 1, wherein the future has a coating property to reflect incident light.

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