KR20000016993A - Optical Pick-up Apparatus - Google Patents

Abstract

PURPOSE: Apparatus that picks up light is provided to overcome the weakness of the conventional picking up apparatus that has very complex structure and need excessive cost because open-hole type aligner and twin lens type apparatus need additional parts. CONSTITUTION: The apparatus that picks up light comprises: hologram unit that transforms from light beam that is received through reflecting light to electric signal, second hologram unit that generates light beam and transform from light beam that reflect at compact disc to electric signal, objective lens that concentrates the incoming light beam, beam splitter that reflect or pass away selectively the light beam rom the first hologram unit and the second hologram unit, collimator lens that maintain parallel light beam and that is located between the beam splitter and objective lens, laser diode that makes the light beam, and detecting device that diffracts the light beam from digital versatile disc, amplifier that transforms from current signal to voltage signal and that amplifies the signal.

Description

Optical Pick-up Apparatus

The present invention relates to an optical pickup apparatus capable of supporting a plurality of types of optical recording media having different recording formats such as substrate thickness.

In general, an optical recording and reproducing apparatus for driving a disk-type medium such as a compact disc (CD) or the like, which is widely known as a recording medium using light, irradiates a recording surface of a disk with a laser beam while rotating the disk. Record or play back. To this end, the optical recording and reproducing apparatus includes an optical pickup for irradiating a laser beam generated from a semiconductor laser as a light source to the recording surface of the optical disc using optical system elements such as an objective lens.

Recently, a DVD (Digital Versatile Disc) capable of recording a larger amount of information than a conventional CD has been commercialized. In general, a DVD is designed in consideration of a light source that generates light beams having a wavelength different from that of the CD. In this case, the wavelength and the numerical aperture of the light beam are related to the size of the beam spot and are adopted in consideration of the size of the beam spot. Then, the beam spot is selected in terms of minimizing the crosstalk effect between signal tracks formed on the optical disk recording surface. Accordingly, since the DVD has an increased recording density than the CD, the track pitch is smaller, and therefore the beam spot size must be smaller than the CD beam spot size. In this case, in order to reduce the size of the beam spot, a method using a shorter wavelength and an increased numerical aperture may be considered. This is because the size of the beam spot has a relationship inversely proportional to the numerical aperture NA, while the size of the beam spot is proportional to the wavelength of the light beam as shown in Equation 1 below.

Where d is the size of the beam spot, k is a constant, λ is the wavelength of the light beam, and NA is the numerical aperture of the objective lens. In Equation 1, it can be seen that the DVD adopts a short wavelength light beam and a large numerical aperture in order to obtain a beam spot having a smaller size than the CD. In fact, the optical pickup accessing the CD uses an optical beam having a wavelength λ of 780 nm and an objective lens having a numerical aperture NA of 0.45, while the optical pickup accessing a DVD has an optical beam and a numerical aperture having a wavelength λ of 650 nm. An objective lens having a (NA) of 0.6 is used. In addition, since the characteristics of the light beam become sensitive to the thickness of the disk as the numerical aperture of the light beam increases, the DVD is set to have a depth of the recording surface, that is, a thickness of the light transmitting layer smaller than that of the CD. In other words, when an optical lens having a numerical aperture of 0.6 is transmitted through an optically transparent layer of the same thickness as the CD, the optical component of the DVD transmits the thickness because the noise component is increased due to the increase of optical aberration so that data cannot be recorded or reproduced properly. It is set smaller than the thickness of the CD light transmitting layer. In fact, the light transmissive layer of the CD has a thickness of 1.2 mm, while the light transmissive layer of the DVD has a thickness of 0.6 mm, which is half thereof.

The optical pickup for accessing such CD and DVDs must have two light sources for generating light beams of different wavelengths and two objective lenses having different numerical apertures. However, when two light sources and an objective lens are respectively provided in the optical pickup, the size of the optical pickup is not only large but also the structure is complicated and the manufacturing cost is increased. In order to solve this problem, studies on optical pickups that can access CDs and DVDs by using one light source and appropriately adjusting the numerical aperture of the objective lens according to the corresponding disks have been conducted.

For example, Japanese Patent Application Laid-open No. Hei 9-185839 discloses an optical pickup that can access two types of optical discs having different thicknesses of the light transmitting layer by adjusting the numerical aperture of the objective lens using a liquid crystal shutter and a polarizing filter. have. This optical pickup is to change the polarization characteristics of the light beam generated from the light source by the liquid crystal shutter is turned on (on) / off (off) depending on whether the voltage is applied, and according to the polarization characteristics of the light beam changed by the liquid crystal shutter in the polarization filter By selectively blocking part of the light beam, the numerical aperture of the objective lens is adjusted in two modes. In addition, Japanese Patent Application Laid-Open No. 9-198704 discloses two types of optical discs by providing two objective lenses in one lens support by a twin-lens method and switching the positions of the objective lenses according to the rotation of the lens support. An optical pickup that can access is disclosed.

However, the optical pickup device of the numerical aperture control method or the twin lens method has a problem in that the component is complicated and the manufacturing cost is increased. Further, in the optical pickup device, since the numerical aperture adjusting means is assembled to the actuator together with the objective lens or the twin lens is assembled, the load of the actuator movable portion is increased to decrease the sensitivity thereof. Such a decrease in sensitivity of the actuator movable portion acts as a barrier to higher speed of optical pickup.

Accordingly, it is an object of the present invention to provide an optical pickup apparatus capable of recording / reproducing a plurality of types of optical discs having different recording formats such as substrate thickness without the need for a separate numerical aperture adjusting means.

1 is a graph showing a change in aberration for each magnification of an objective lens;

2 is a graph showing a change in light spot size for each magnification of an objective lens;

3 is a graph showing a relationship between an optical spot size and a reproduction signal size according to scaling.

4 is a graph showing the relationship between the light spot size and the crosstalk component size according to the scaling.

5 is a graph showing the relationship between the light spot size and the jitter amount according to magnification.

Fig. 6A shows the jitter amount and defocus relationship of the reproduction signal using each of the optical spot by the CD optical system and the optical spot by the optical system which causes aberration to the CD light, and Fig. 6B shows the jitter amount of the reproduction signal and Fig. 6C is a graph showing the relationship between the jitter amount and the radial tilt amount of the reproduction signal.

7 is a diagram illustrating an optical pickup apparatus according to a first exemplary embodiment of the present invention.

8 is a view showing a detailed configuration of the hologram unit shown in FIG.

FIG. 9 is a diagram showing a playing principle of a CD in FIG. 8; FIG.

10 is a view showing an optical pickup device according to a second embodiment of the present invention.

<Brief description of symbols for the main parts of the drawings>

10: first hologram unit 11: light source

12 beam splitter 13 light detector

14: collimating lens 15: hologram

16: objective lens 18A: DVD

18B: CD 20: second hologram unit

22: first light source 24, 28: half mirror

26 second light source 30 first photodetector

32: second photodetector

In order to achieve the above object, the optical pickup device according to the present invention comprises at least two or more light sources for generating light of different wavelengths; A condensing optical system for condensing light from the light sources on the optical record carrier; A light receiving optical system for receiving light reflected from the optical record carrier; The condensing optical system is configured such that a predetermined aberration is generated in an optical spot formed on the recording medium with respect to one of the light sources.

Other objects and advantages of the present invention in addition to the above object will become apparent from the description of the preferred embodiment of the present invention with reference to the accompanying drawings.

Before describing the embodiments of the present invention, the background on which the present invention appears will be described. The present invention adds another light source for a disk to the optical system, which is suitably configured to access any one of a plurality of disks having different substrate thicknesses and recording formats, so that the light emitting points thereof can be changed, In addition to varying the distance according to the magnification of the objective lens, it was derived from experiments to measure the size (ie diameter) of light spots formed on the disk. In order to carry out this experiment, the present inventors installed the light source for CD to be movable in an optical system that is configured to be suitable for accessing a DVD. Then, the present inventors measured the change state of the aberration and the change state of the diameter of the light spot formed on the CD while adjusting the distance between the light emitting point of the CD light source and the disk, that is, the magnification of the objective lens. As a result, the aberration characteristics of the CD light according to the magnification as shown in FIG. 1 and the spot size characteristics of the CD light beam changed according to the magnification as shown in FIG. The following description is taken as an example of using an objective lens having a focal length of 3.37 mm with respect to the wavelength for DVD (650 nm). Referring to FIG. 1, it can be seen that the aberration has a minimum value of 0.02 when the light emitting point of the CD light source is separated from the disk by a distance corresponding to a magnification of 0.181460. As the distance between the light emitting point of the CD light source and the disk becomes smaller than the distance corresponding to the magnification of 0.181460, the aberration of the light for the CD increases. Further, the aberration of the CD light becomes large even when the distance between the light emitting point of the CD light source and the disc becomes larger than a distance corresponding to a magnification of 0.181460. On the other hand, Fig. 2 illustrates that the optical spot having the smallest diameter of 1.48 mu m is formed on the disk when the light emitting point of the CD light source is separated from the disk by a distance corresponding to a magnification of 0.176258. The size of the light spot becomes larger as the distance between the light emitting point of the CD light source and the disk becomes smaller or larger than a distance corresponding to a magnification of 0.176258. In addition, the optical spot is used by the CD optical system when the distance between the light emitting point of the CD light source and the disk has a distance in a range corresponding to a magnification of 0.164 to 0.167 or a range corresponding to a magnification of 0.193 to 0.198. It has a size of 1.640 µm to 1.720 µm which is the same as the light spot formed on the phase. As a result, FIGS. 1 and 2 provide a CD light source in the optical system for DVD so that aberration occurs in the CD light (that is, by adjusting the distance between the light emitting point of the CD light source and the disc added to the optical system for DVD). It shows that a light spot of the same size as the light spot formed by the optical system for the CD can be obtained.

In addition, the present inventors change the magnification, that is, gradually change the size of the light spot as shown in Fig. 2, while the state of change of the reproduced signal size picked up by the light spot, the state of change of crosstalk component and the amount of jitter change. Was measured. Accordingly, the characteristics of the reproduction signal according to the size of the optical spot as shown in FIG. 3, the characteristics of the crosstalk component varying according to the size of the optical spot as shown in FIG. A characteristic of the quantity was obtained. Referring to FIG. 3, when the light spot has a size of 1.640 μm to 1.720 μm by adjusting the distance between the light emitting point of the CD light source and the disc added to the optical system for DVD, that is, formed on the disc by the optical system for CD. In the case of having the same size as the light spot, it can be seen that the size of the reproduction signal picked up by the light spot is the largest. When the pit having the size of 3T and 11T is reproduced on the disc by the optical spot having the size range, the reproduction signal of the maximum size can be obtained. 4 and 5 show that the size and jitter amount of the crosstalk component included in the reproduction signal detected by the light spot having the above-mentioned range are small, and there is no practical problem. As a result, FIGS. 3 to 5 provide a CD light source in the optical system for DVD so that aberration occurs in the CD light (that is, by adjusting the distance between the light emitting point of the CD light source and the disc added to the optical system for DVD). When the CD is reproduced with the obtained light spot, it is shown that the characteristics of the reproduction signal and the crosstalk and jitter amount characteristics similar to those of the optical spot formed by the optical system for CD can be obtained.

Further, FIGS. 6A to 6C show a reproduction signal picked up by an optical spot equivalent to an optical spot formed by the CD optical system by adjusting the distance between the light emitting point of the CD light source and the disk added to the optical system for DVD (hereinafter, The characteristics of the reproduction signal of the present invention and the reproduction signal picked up by the optical spot obtained by the optical system for CD (hereinafter referred to as a conventional reproduction signal) are compared and shown. FIG. 6A shows the jitter amount and defocus relationship of the reproduction signal, FIG. 6B shows the relationship between the jitter amount and the tangential tilt amount of the reproduction signal, and FIG. 6C shows the jitter amount and the radial tilt of the reproduction signal. Tilt) amount is shown. Referring to FIG. 6A, it can be seen that when the defocus characteristic of the reproduction signal 1 and the conventional reproduction signal 2 of the present invention are compared, the optimum focus point is moved but the defocus characteristic is similar. 6B and 6C, it can be seen that the reproduction signals 3 and 5 of the present invention and the conventional reproduction signals 4 and 6 have similar tilt window characteristics with respect to the tangential direction and the radial direction. As a result, FIGS. 6A to 6C show that the CD light source is installed in the optical system for DVD so that aberration occurs in the CD light (that is, by adjusting the distance between the light emitting point of the CD light source and the disc added to the optical system for DVD). When the CD is reproduced with the obtained light spot, it shows that it has similar defocus and tilt window characteristics as the light spot formed by the optical system for the CD.

As described above, in the present invention, the CD light source is additionally installed in the optical system for the DVD so that aberration occurs in the light for the CD. Thus, the DVD and the CD can be accessed interchangeably without the need for a separate numerical aperture adjusting means. This simplifies the configuration of the optical pickup that can be accessed by making DVD and CD compatible. Such effects of the present invention will be apparent from the detailed description of the embodiments of the present invention. Hereinafter, preferred embodiments of the present invention will be described in detail.

7 shows an optical pickup apparatus according to a first embodiment of the present invention. The optical pickup device of FIG. 7 generates a first light beam having a first wavelength (about 650 nm) and a first hologram unit 10 for converting a light beam reflected from and received by the DVD 18A into an electrical signal. The second hologram unit 20 generates a light beam of two wavelengths (about 780 nm) and converts the light beam reflected from the CD 18B into an electrical signal, and the incident light beam is converted into a CD 18B or DVD 18A. An objective lens (16) for focusing on the recording surface of the &lt; RTI ID = 0.0 &gt;), a beam splitter (12) for selectively transmitting / reflecting light beams from the first and second hologram units (10, 20), &lt; / RTI &gt; A collimator lens 14 positioned between the splitter 12 and the objective lens 16 and traveling in parallel with the light beam emitted from the first hologram unit 10 is provided. As illustrated in FIG. 8, the first hologram unit 10 diffracts a laser diode 11 for generating a light beam, a light detector 13 for detecting a reflected light beam, and converting the light beam into an electrical signal. It is provided with a hologram 15 to advance to the photodetector (13). The laser diode 11 generates a light beam of the first wavelength. The hologram 15 transmits the light beam generated by the laser diode 11 as it is, while diffracting the reflected light beam from the DVD 18A to advance to the photodetector 13. The photo detector 13 detects the reflected light beam and converts it into an electrical signal, that is, a current signal. In addition, the photodetector 13 further includes a current / voltage conversion circuit and an amplifier circuit to convert the current signal into a voltage signal and then amplify and output the converted voltage signal. This current / voltage conversion circuit and the amplification circuit can be separated separately and included in the external circuit of the pickup. The beam splitter 12 transmits the light beam incident from the first hologram unit 10 as it is and proceeds toward the collimation lens 14, and also transmits the reflected light beam from the DVD 18A as it is to transmit the first hologram unit 10. To proceed. The collimating lens 14 causes the divergent light beam from the beam splitter 12 to proceed to the parallel light beam. The objective lens 16 focuses the light beam incident from the collimation lens 14 onto the recording surface of the DVD 18A, and also causes the light beam reflected on the recording surface of the DVD 18A toward the collimation lens 14. Here, the collimation lens 14 and the objective lens 16 are optimized for the aberration correction state of the lens including the wavelength of use, the numerical aperture, and the thickness of the disk so that a light spot of a suitable size can be formed on the recording surface of the DVD 18A. Thus, the aberration-free optical system is configured.

On the other hand, the second hologram unit 20 has the same configuration as the first hologram unit 10. However, the laser diode of the second hologram unit 20 generates a light beam having a wavelength of 780 nm suitable for the CD 18B. As described above, the second hologram unit 20 for generating the CD light beam is arranged to generate a predetermined spherical aberration. In this case, the second hologram unit 20 may be installed so that the light emitting point of the laser diode is located closer or farther than the focal position of the collimating lens 14. Preferably, the second hologram unit 20 may be installed so that the light emitting point of the laser diode is located closer to the focal position of the collimation lens 14. In this case, the light beam generated by the laser diode of the second hologram unit 20 is incident to the objective lens 16 in the form of some divergent light through the collimating lens 14 as shown in FIG. 9. The light incident on the objective lens 16 through the collimation lens 14 in the form of some divergent light is refracted to the same as the light emitted from the puncturing point P shown in FIG. 9 and incident on the objective lens 16. Characteristically, the light is collected on the recording surface of the CD 18B. In other words, the distance between the light emitting point of the laser diode of the second hologram unit 20 and the CD 18B is shorter than the distance from the scratch point P corresponding to the magnification of the objective lens to the CD 18B. This is because the aberration of the CD light beam output from the laser diode of the second hologram unit 20 is compensated by the collimating lens 14. Accordingly, in order to obtain a light beam spot having a size as mentioned in FIG. 2, the objective lens 16 and the collimation lens (depending on the distance between the emission point of the laser diode of the second hologram unit 20 and the objective lens 16) 14) and spacing should be adjusted accordingly. As a result, the CD light beam has spherical aberration caused by the distance between the light emitting point and the disc set via the optical system for DVD having a predetermined spherical aberration with respect to the wavelength, and the objective lens 16 and the collimating lens according to the distance. By adjusting the interval (14), the recording surface of the CD 18B is condensed to a desired light spot size. The beam splitter 12 reflects the light beam incident from the second hologram unit 20 to travel toward the collimation lens 14, and reflects the reflected light beam from the CD 18B to the second hologram unit 20. Let it go

10 shows an optical pickup apparatus according to a second embodiment of the present invention. In the optical pickup device of FIG. 10, the DVD light source 22 and the photodetector 30, the CD light source 26 and the photodetector 32 are separately arranged and reflected light beams, in contrast to the optical pickup device of FIG. 7. The half mirrors 24 and 28 for advancing to the photodetectors 30 and 32 are further configured, and otherwise have the same components. As described above, the CD light source 26 is disposed at a position where a predetermined spherical aberration is generated. The optical pickup apparatus accesses the DVD 18A using the aberration-free optical system, and accesses the CD 18B using the CD light source arranged so that a predetermined spherical aberration occurs in the DVD optical system.

On the other hand, when the size of the optical spot is adjusted using the spherical aberration as described above, the influence of the spherical aberration also spreads to the photodetector, thereby increasing the defocus when the focus error signal is detected. However, the increase in defocus due to the influence of spherical aberration can be corrected by adjusting the position of the sensor lens (not shown) or adjusting the position of the CD light detecting unit 32. In addition, when the light source and the light detecting unit are constituted in one package as in the hologram unit 20 shown in FIG. 6, mutual compensation is performed by generating defocus in the reverse direction when assembling the hologram unit 20 for a CD. By the method, defocus due to the influence of spherical aberration can be corrected. Accordingly, in the present invention, the optical pickup can perform a servo control by detecting a stable servo signal without the influence of spherical aberration when recording / reproducing a CD.

The objective lens is usually designed as a perfect lens in the design stage, but in the manufacturing process, spherical errors, aspherical errors, thickness errors, and refractive index errors of the material accumulate, resulting in astigmatism, coma, and spherical aberration. Therefore, it is preferable to simultaneously utilize a method of adjusting the distance from the light source light emitting point of the CD optical system to the light spot forming point on the disk together with the remaining spherical aberration of the objective lens itself. In this case, the residual spherical aberration of the objective lens is minimized as much as possible, and then the desired light spot size is adjusted by adjusting the distance between the light emitting point of the light source and the spot formation point on the disc in consideration of the minimized remaining spherical aberration. You can get it.

As described above, the optical pickup apparatus according to the present invention can access DVD and CD in a compatible manner using a CD light source installed in the optical system for DVD so that aberration occurs in the optical system for CD and optical for CD. do. Accordingly, since the optical pickup apparatus according to the present invention does not need a separate numerical aperture adjusting means for compatibility of DVD and CD, the configuration can be simplified and the manufacturing cost can be reduced as compared to the conventional DVD / CD combined optical pickup. .

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)

An optical pickup apparatus for recording / reproducing optical recording media having different recording formats, such as a substrate thickness, At least two light sources for generating light of different wavelengths; A condensing optical system for condensing light from the light sources onto the optical record carrier; A light receiving optical system for receiving light reflected from the optical record carrier; And the condensing optical system is configured such that a predetermined aberration occurs in an optical spot formed on the recording medium with respect to one of the light sources. The method of claim 1, And the light spot has a larger spot diameter than the light spot formed by the anhydrous system. The method of claim 1, And the distance between the collimation lens and the objective lens included in the condensing optical system is appropriately adjusted to form the light spot. The method of claim 1, And an emission point of the light source is located within a focal point of the collimating lens included in the condensing optical system.