KR20010055741A - Apparatus for Detecting Focus Error Using Astigmatism And Method Thereof - Google Patents

Abstract

PURPOSE: Detecting device and method for a focus error are provided to compensate a focus error noise signal generated in crossing tracks by a simple signal process using a tracking error signal. CONSTITUTION: In case of performing focus search, a switch is turned off. Thus, a tracking error signal does not operate as a noise signal to a focus error signal. In case of a precise focus, a focus servo and a tracking servo are turned on to perform focus and tracking servos while driving a disk. Herein, the switch is turned off for the tracking error signal not to operate as the noise signal. In case of an optical spot crossing tracks, the focus servo is on state while turning off the tracking servo. Moreover, the switch is turned on for compensating a focus error noise signal by using the tracking error signal.

Description

Apparatus for Detecting Focus Error Using Astigmatism And Method Thereof}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a recording / reproducing apparatus for an optical recording medium, and more particularly, to a focus error detecting apparatus and method capable of removing a crosstalk phenomenon in a focus error detecting method using astigmatism. .

The optical recording / reproducing apparatus includes an optical pickup for irradiating a light beam to a disk and converting the reflected light beam into an electrical signal, and a servo for performing control operations such as tracking and focusing of the optical pickup. Here, the tracking servo causes the objective lens to move in the left and right directions so that an optical spot of the optical pickup of the optical pickup follows the center of the signal track of the disc. The focusing servo causes the objective lens to be moved up and down so that an optical spot by the objective lens of the optical pickup falls within the depth of focus in the signal track of the disc. For this focusing servo, the optical recording / reproducing apparatus mainly adopts a focus error detection method using astigmatism. This is because the focus error detection method using the astigmatism method is easy to configure and adjust and has high detection sensitivity. However, when the focus servo using the astigmatism method is performed on a recording disc having mountain / goal tracks, the focus servo becomes unstable due to the crosstalk phenomenon in which the tracking error signal is introduced into the focus error signal. There is a problem. Hereinafter, a problem of the conventional focus error detection apparatus using astigmatism will be described with reference to the accompanying drawings.

Referring to FIG. 1, a focus error detection apparatus using a conventional astigmatism method is illustrated. The focus error detection apparatus of FIG. 1 transmits a light source 2 for generating a light beam, an objective lens 8 for focusing the light beam on the optical disk 10, an incident light beam to the optical disk 10, and reflects the reflected light beam. A beam splitter 6 for making a light beam, a photodetector 14 for detecting a reflected light beam from the optical disk 10, and a collimating lens for advancing the light beam from the light source 2 in parallel. 4 and an optical pickup device comprising a sensor lens 12 that focuses the light beam from the beam splitter 6 toward the photodetector 14 and generates astigmatism. In addition, the focus error detecting apparatus of FIG. 1 includes a signal detecting unit which detects the focus error signal FE by combining signals output from the photodetector 14 of the optical pickup apparatus.

In FIG. 1, the light source 2 generates a light beam. The collimating lens 4 causes the divergent light beam from the light source 2 to travel in parallel to the beam splitter 6. The beam splitter 6 transmits the incident light beam from the collimating lens 4 toward the objective lens 8 and reflects the reflected light beam from the objective lens 8 toward the sensor lens 12. The objective lens 8 causes the light beam from the beam splitter 6 to be focused onto the optical disc 10. As the sensor lens 12, a cylindrical lens is used to generate astigmatism to detect a focus error. The photodetector 14 is divided into four parts and constituted by the first to fourth photodetector pieces A, B, C, and D, and each of the first to fourth photodetector pieces A, B, C, and D is incident. An electrical signal is generated in proportion to the amount of light. Servos such as recording / reproducing, focusing, tracking, etc., are performed using electrical signals generated by the photodetector 14. In this case, the focus error signal is detected by the shape of the beam formed in the photodetector 14 between the first focus point and the second focus point formed by the sensor lens 12 generating astigmatism. In detail, the signal detector outputs the sum signal A + C of the signals output from the first and third photodetectors A and C in the diagonal direction and the second and fourth photodetectors B and D. The focus error signal FE is detected by differentially amplifying the sum signal B + D of the signals. The tracking error signal TE is a sum signal A + B of signals output from the first and second photodetection cells A and B and a signal output from the third and fourth photodetection cells C and D. The sum signal C + D is differentially amplified and detected.

However, the conventional method of detecting a focus error by astigmatism is applied to a recording optical disc having a track structure of hills and valleys, and when only focus servo is performed without tracking servo, that is, an optical spot such as a disc search tracks on a disc. A relatively large tracking error signal 18 is generated as shown in FIG. In general, an optical disc having tracks of hills and valleys detects a tracking error signal by a push-pull method or the like even in a state where information is not recorded, thereby enabling proper tracking servo. For this purpose, in the case of the recording optical disc 20 having the tracks 20A and 20B of the hills and valleys as shown in FIG. 2, the depth of the valleys 20A is increased so that the tracking error signal 18 is large. Consists of. Here, when the astigmatism method is used as the focus error detection method for focus control at the same time as the tracking error signal detection, if the light quantity difference occurs only in the direction in which the light spot crosses the track (that is, the radial direction), the focus error signal is There should be little change. However, even in the case of anhydrous aberration, the light quantity distribution on the photodetector produced by the astigmatism method due to the depth difference between the valleys and the mountains 20A and 20B is not only a direction crossing the track but also a direction perpendicular thereto (ie, a tangential direction). ), The tracking error signal TE is induced to the focus error detector, and crosstalk occurs. In other words, the tracking error signal TE detected when the light spot crosses the track is induced as a noise signal in the focus error signal so that the focus error noise signal FEN having a relatively large change as shown in FIG. Will occur. Considering aberrations and adjustment errors that remain substantially in the optical system, the focus error noise signal FEN becomes larger. Such crosstalk between servo signals has a problem in that the focus servo becomes unstable and degrades the performance of the recording / reproducing system.

In response to the problem of the focus error detection method using the astigmatism method, US Pat. No. 5610883 discloses a focus that compensates for a focus error noise signal detected by the astigmatism method using a tracking error signal when an optical spot crosses a track. An error detection method has been proposed. However, since a phase difference occurs between the tracking error signal TE and the focus error noise signal FEN detected when the light spot crosses the track, as shown in FIG. 2, the focus error signal is compensated by a simple signal combination. There is a problem that it is impossible and complicated circuit configuration and signal processing process is required.

Accordingly, it is an object of the present invention to provide a focus error detection apparatus and method which makes it possible to compensate for a focus error noise signal generated at track crossing by simple signal processing using a tracking error signal.

1 is a diagram illustrating a conventional focus error detection apparatus using astigmatism.

FIG. 2 is a graph showing a focus error noise signal and a tracking error signal generated in the focus error detection device of FIG. 1 when traversing a track of a recordable optical disc; FIG.

3 is a diagram illustrating a focus error detection apparatus using astigmatism according to an exemplary embodiment of the present invention.

4 is a view showing in detail the configuration of the light receiving unit shown in FIG.

FIG. 5 is a graph showing a focus error noise signal and a tracking error signal generated in the focus error detection device of FIG. 3 when traversing a track of a recordable optical disc; FIG.

FIG. 6 is a view illustrating another embodiment of the light receiver shown in FIG. 3. FIG.

FIG. 7 is a view illustrating still another embodiment of the light receiver shown in FIG. 3; FIG.

8 is a graph showing a comparison of a focus error noise signal generated in the conventional and the error detection apparatus of the present invention.

9 is a diagram illustrating the switch control method illustrated in FIG. 3 step by step in a focus error detection method according to an exemplary embodiment of the present invention.

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

2, 22: light source 4, 24, 76: collimation lens

6, 26: beam splitter 8, 32: objective lens

10, 36: optical disk 12, 42: sensor lens

14, 44: photodetector 20: recordable optical disk

28 beam shaping means 30 polarization converting means

34: actuator 38: condenser

40, 74: beam splitting means 46: light receiving portion

48, 50, 54, 56, 64: addition amplifier 52, 58, 60: differential amplifier

65: signal detector 66: spherical lens

68: cylindrical lens 78, 80: boiling point plate

In order to achieve the above object, the focus error detection apparatus using the astigmatism method according to the present invention comprises a light collecting system for generating a light beam to focus on the optical disk, and splits the light beam reflected from the optical disk in the tangent direction and generates astigmatism A light receiving system for receiving and receiving light, a signal detecting means for detecting a focus error signal and a tracking error signal using an output signal of the light receiving system, and a focus error signal using a tracking error signal selectively according to the driving state of the disc. And focus error correction means.

A focus error detection method using astigmatism according to the present invention includes a photoelectric conversion step of focusing an optical beam on an optical disk and dividing the reflected optical beam in a specific direction to cause a 180 degree phase difference between the focus error signal and the tracking error signal, and driving the optical disk. And selectively correcting the focus error signal using the tracking error signal according to a state.

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.

3 illustrates a focus error detection apparatus using astigmatism according to a first embodiment of the present invention. The focus error detection apparatus of FIG. 3 includes a light collecting system 38 for generating light beams and condensing them on the recording surface of the optical disk 36, and a light receiving system 46 for detecting the reflected light beam reflected from the recording surface of the optical disk 36; And a signal detection and correction unit 65 for detecting the focus error signal by detecting the focus and tracking error signals using the output signals of the light receiver 46.

In FIG. 3, the light collecting system 38 includes a collimating lens 24, a beam splitter 28, a beam shaping means 28, and a polarization converting means arranged side by side on an optical path between the light source 22 and the optical disk 36. 30, the objective lens 32 is configured. The collimating lens 24 causes the divergent light beam generated by the light source 22 to travel in parallel to the beam splitter 26. The beam splitter 26 causes the incident light beam from the collimating lens 24 to be directed toward the beam shaping means 28, and the reflected light beam reflected by the optical disk 36 toward the light receiving system 46. The beam shaping means 28 shapes the light beams from the beam splitter 26, and the polarization converting means (for example, the phase plate) 30 converts the polarization components of the shaped light beams. The objective lens 32 installed on the bobbin of the actuator 34 and driven to enable focus control and tracking control focuses the light beam from the polarization converting means 30 on the recording surface of the optical disc 36. The light beam focused on the recording surface of the optical disc 36 is modulated and reflected by a pit or mark formed on the optical disc 36, and then passes through the objective lens 34, the polarization converting means 30, and the beam shaping means 28. The light is incident on the splitter 26, reflected by the beam splitter 26, and incident on the light receiving system 46.

The photometer 46 includes a beam splitting means 40, a sensor lens 42, and a photodetector 44. The beam splitting means 40 splits the reflected light beam from the beam splitter 26 in the tangential direction on the optical disc 36 so that only the upper beam (or the lower beam) is incident on the photodetector 44. This is to prevent the influence of light quantity difference in the tangential cell direction on the photodetector 44 in detecting the focus error component. The sensor lens 42 focuses the light beam incident from the beam splitter 40 on the photodetector 44 by generating astigmatism. In this case, the photodetector 44 is positioned between the first focus point and the second focus point formed by the sensor lens 42. The beam splitting means 40 is positioned at the front end of the sensor lens 42 that generates astigmatism, or close to the sensor lens 42 that generates astigmatism when positioned at the rear end. The four-segment photodetector 44 detects the light beam received through the sensor lens 42 and converts the light beam into an electrical signal.

The signal detection and correction unit 65 connects the first to fifth adder amplifiers 48, 50, 52, 54, and 64, the first to third differential amplifiers 52, 58, and 60, and the switch 62. It is set as a configuration. The first addition amplifier 48 adds and outputs the output signals of the first and second photodetector pieces A and B, and the second addition amplifier 50 outputs the third and fourth photodetector pieces C,. Add and amplify the output signal of D) and output it. The first differential amplifier 52 differentially amplifies the output signals of the first and second add amplifiers 48 and 50 to generate a tracking error signal TE. The third addition amplifier 54 adds and amplifies the output signals of the first and third photodetection cells A and C. The fourth add amplifier 56 adds and outputs the output signals of the second and fourth photodetection cells B and D. The second differential amplifier 58 differentially amplifies the output signals of the first and second add amplifiers 48 and 50 to generate a tracking error signal TE. The third differential amplifier 60 differentially amplifies the output signals of the third and fourth add amplifiers 54 and 56 to generate a focus error noise signal FEN. The switch 62 selectively switches the tracking error signal TE from the second differential amplifier 58 according to the driving state of the disk. In other words, the switch 62 is turned on only when the tracking error signal is included as a noise component of the focus error signal, that is, when crossing the track, and outputs the tracking error signal TE. The fifth add amplifier 64 adds the tracking error signal TE input through the switch 62 and the focusing error noise signal FEN from the third differential amplifier 60 to add and amplify the focus. An error signal FE is generated.

Referring to FIG. 4, there is shown a first embodiment of the light receiving system shown in FIG. 3. In FIG. 4, the beam dividing means 40 is divided in the tangential direction so that the upper part transmits the upper beam of the incident light beam as it is, while the lower part has a diffraction surface 40A so that the lower beam is diffracted. The sensor lens 42 includes a cylindrical lens 66 and a spherical lens 68. The cylindrical lens 66 serves to generate astigmatism and rotate the incident light in the tangential / radial direction. The spherical lens 68 is formed into a spherical surface to serve to relay incident light so that a desired light spot is formed in the photodetector 44. As the sensor lens 42, a toric lens having different curvatures in the horizontal and vertical directions may be used instead of the cylindrical lens 66 and the spherical lens 68. The photodetector 44 is configured by four divided first photodetection cells 44A and second and third photodetector cells 44B and 44C disposed on the left and right sides thereof. The upper beam split by the beam splitting means 40 is incident on the first photodetecting cell 44A. In other words, the upper beam divided in the tangential direction by the beam dividing means 40 is rotated by the cylindrical lens 66 so that the first and fourth photodetecting pieces A of the first photodetecting cell 44A can be rotated. , D) is incident only to the second and third photodetection pieces (B, C). In this case, the focus error noise signal FEN and the tracking error signal TE detected using the signal detector 65 shown in FIG. 3 may be represented by Equation 1 below.

FEN = (B + D)-(A + C), TE = (A + B)-(C + D)

Here, A to D represent the output signals of each of the first to fourth light detecting pieces A, B, C, and D constituting the first light detecting cell 44A. Since B = 0 and D = 0 by the beam splitter 40 described above in Equation 1, the focus error noise signal FEN and the tracking error signal TE can be expressed as Equation 2 below.

FEN = D-A, TE = A-D

In Equation 2, the focusing error noise signal FEN and the tracking error signal TE always have the same magnitude while having opposite signs. In other words, the focusing error noise signal FEN and the tracking error signal TE detected by the signal detector 65 always have a 180 degree phase difference as shown in FIG. 5. Accordingly, by adding the tracking error signal TE to the focusing error noise signal FEN by using the tracking error signal TE as a correction signal at the time of traversing the track of the mountain / gol disc, it is possible to remove the noise component flowing into the focusing error signal FE. . As a result, the focusing error signal FE detected at the time of track traversal is almost unchanged as shown in FIG. In this case, the gain of the third differential amplifier 58 shown in FIG. 3 is appropriately adjusted so that the tracking error signal TE is equal in magnitude to the focusing error noise signal FEN.

Meanwhile, in FIG. 4, the lower beam divided by the beam splitter 40 is diffracted and incident on each of the second and third photodetection cells 44B and 44C. In this case, the high frequency signal RF may be represented as a sum signal A + B + C + D + E + F of signals output from the first to third photodetection cells 44A, 44B, and 44C. It is possible to prevent the magnitude of the high frequency signal RF from being lowered due to the loss of light amount by the means 40.

However, when the decrease in the size of the high frequency signal (RF) due to the amount of light loss is not a problem, as shown in FIG. 6, the light splitter 74 simply takes the form of blocking the upper beam or the lower beam, and the photodetector 44. Is to include only the four divided photodetection cells. In FIG. 6, the spherical lens 68 and the cylindrical lens 66 constituting the sensor lens 42 have the same role as those of FIG. The beam dividing means 74 is disposed between the spherical lens 68 and the cylindrical lens 66 because the beam dividing means 74 is optimally placed in front of the cylindrical lens 66 in any case.

Referring to FIG. 7, another embodiment of the light receiver 46 illustrated in FIG. 3 is illustrated. In the light receiving unit 46 of FIG. 7, the collimating lens 76 is used instead of the spherical lens 68 of FIG. 3, and the first and second boiling point plates 78, instead of the cylindrical lens 66 for generating astigmatism, are used. 80). Then, the light splitting means 40 is positioned between the first and second boiling point plates 78 and 80. The collimating lens 76 serves to condense the incident light beam on the photodetector 44 like the spherical lens 68 described above. The first and second boiling point plates 78 and 80 are to generate astigmatism and have an inclined surface with respect to incident light. Here, a half mirror may be used as the first boiling point plate 78. The beam splitting means 40 is positioned between the first boiling point plate 78 which generates astigmatism but does not rotate in the tangential / radial direction, and the second boiling point plate 80 which causes the astigmatism direction to rotate. Let's do it.

As described above, in the optical pickup apparatus of the present invention, when the optical spot traverses the track on the recordable optical disc by using the beam splitting means 40 divided in the tangential direction, the detected focus error noise signal FEN is a tracking error signal. Even when there is a (TE) signal and residual aberration and adjustment error, there is always a phase difference of 180 degrees. Accordingly, the corrected focus error signal FE can be detected by adding the tracking error signal TE as a correction signal to the focus error noise signal FEN. As a result, as shown in the graph shown in Fig. 8, a focus error noise signal 16 having a relatively large change occurs in a conventional optical pickup apparatus when traversing a track of a recordable optical disk, while in the optical pickup apparatus of the present invention, the change almost occurs. The focus error signal 82 without is generated.

8 illustrates a method of controlling the switch 62 shown in FIG. 3 in the focus error detection method in the optical pickup apparatus of the present invention. First, when the focus search is performed in step 2 (S2), the switch 62 is turned off. This is because the tracking error signal TE acts as a noise signal to the focus error signal FE when the switch 62 is in the ON state during the focus search, so that the focus cannot be properly applied. When the focus is properly set in step 4 (S4), the process proceeds to step 6 (S6) to turn on the focus servo and the tracking servo to perform the focus and tracking servo together with the disk drive. In this case, when the switch 62 is in the on state, the tracking error signal TE acts as a noise signal to the focus error signal FE, so that the switch 62 is turned off. Then, in step 8 (S8), when the light spot traverses the track for the disc search, that is, when the focus servo is on and the tracking servo is off, the tracking error signal TE is turned on by turning on the switch 62. To compensate for the focus error noise signal FEN.

As described above, in the focus error detection apparatus and method using astigmatism according to the present invention, the tracking error signal TE and the focusing error noise signal FEN have a 180 degree phase difference by using beam splitting means. Accordingly, according to the focus error detecting apparatus and method using astigmatism according to the present invention, the tracking error signal TE and the focusing error noise signal FEN are added to the focusing error signal by simply adding the tracking error signal TE and the focusing error noise signal FEN during the track crossing of the recording type optical disc. Noise components can be removed.

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 (7)

In the focus error detection apparatus using astigmatism, A light collecting system for generating light beams and condensing them on an optical disk; A light receiving system for dividing the light beam reflected from the optical disk in a tangent direction and generating light by generating astigmatism; Signal detecting means for detecting a focus error signal and a tracking error signal by using the output signal of the light receiver; And focus error correction means for selectively correcting the focus error signal by using the tracking error signal in accordance with the drive state of the disc. The method of claim 1, The light receiver is A photodetector for detecting the received light beam; Beam dividing means for dividing the reflected light beam from the optical disk in the tangent direction and transmitting only the light beam of one side as it is; And astigmatism generating means for condensing the reflected light beam with the photodetector such that astigmatism is generated. The method of claim 2, The beam splitting means is a focus error detection device using astigmatism, characterized in that for blocking or diffracting the light beam of the other side. The method of claim 1, The focus error detecting means And a focus error signal is corrected by adding the tracking error signal and the focus error signal when traversing a signal track of a recordable optical disc. In the focus error detection method using astigmatism, A photoelectric conversion step of condensing the light beam on the optical disc to divide the reflected light beam in a specific direction so that the focus error signal and the tracking error signal have a 180 degree phase difference; And correcting the focus error signal using the tracking error signal selectively according to the driving state of the optical disc. The method of claim 5, In the photoelectric conversion step, a focus error detection method using astigmatism, characterized in that for receiving only the light beam of one side of the light beam divided in the tangent direction photoelectric conversion. The method of claim 5, In correcting the focus error signal And a focus aberration detection method for correcting the focus error signal by adding the tracking error signal and the focus error signal when crossing a signal track of a recordable optical disc.