KR100293522B1 - Optical Pickup Apparatus - Google Patents

Abstract

The present invention relates to an optical pickup apparatus which eliminates offset components of tracking error signals caused by uneven pit depth or by moving an objective lens from an optical axis center.

The optical pickup apparatus of the present invention comprises a light source for generating a light beam having a predetermined polarization direction, an objective lens for connecting the light beam to a recording medium, and astigmatism for adjusting the astigmatism direction of the light beam with respect to the polarization direction in a specific direction. An optical system and a light detecting element for converting the light beam of the reflected light reflected from the recording medium into an electrical signal.

By such a configuration, the optical pickup apparatus according to the present invention optically removes the DC offset included in the tracking error signal by the movement of the objective lens by adjusting the direction of astigmatism in a specific direction with respect to the polarization direction of the incident beam, thereby tracking. Allow the servo to operate stably.

Description

Optical Pickup Apparatus

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical disk drive apparatus, and more particularly, to an optical pickup apparatus for removing offset components of a tracking error signal caused by an uneven pit depth or movement of an objective lens from an optical axis center.

In recent years, high-density discs, such as DVD, whose recording density is superior to CD, have been developed. In parallel with this, high speed drive devices are being developed that can drive a disc at a high speed to increase a transfer speed when recording / reproducing. The high speed optical disc drive device requires a more precise servo technology to secure reliability in recording / reproducing.

The optical disc drive device is provided with an optical pickup for optically accessing the disc during recording / reproducing. In the recording mode, the optical pickup irradiates a recording film of the disk with high power light to form a pit, which is a minimum unit cell of information. In the reproduction mode, the optical pickup reads the pit information by irradiating light of low power to the recording film of the disk and photoelectrically converting the light reflected from the disk by the light receiving system. In order for recording / reproducing to be performed correctly, an accurate tracking servo must be prioritized so that the beam spot irradiated and collected from the optical pickup can be accessed by accurately following the pit train. The tracking servo is made possible by driving an actuator on which an objective lens is mounted in accordance with a tracking error signal detected by signal processing a detection signal of a light receiving system divided in various directions with respect to the disc.

FIG. 1 is a diagram illustrating a detection principle of a tracking error signal using a phase difference detection (hereinafter referred to as "DPD") method. When the beam spot 2 passes through the pit 1 constituting the track, the intensity pattern of the beam spot 4 of the reflected light formed on the four-segment photodetector 3 changes in time. When the beam spot 2 passes through the center of the pit 1, that is, the center of the track, as shown in FIG. 1A, the pattern of the beam spot 4 on the photodetector 3 changes in contrast to the left and right. 1b and 1c, when the beam spot 2 is biased to the left or the right of the center of the pit 1, the pattern of the beam spot 4 on the photodetector 3 rotates in a specific direction. As shown in FIG. 1B, when the beam spot 2 passes to the left of the center of the pit 1, the pattern of the beam spot 4 on the photodetector 3 rotates clockwise. In contrast, when the beam spot 2 passes to the right rather than the center of the pit 1, the pattern of the beam spot 4 on the photodetector 3 rotates counterclockwise. This rotational change of the pattern is detected as the beam spot 4 on the photodetector 3 leaves the pit 1, so that the contrast of the pattern becomes clear. The DPD is a method of detecting a tracking error signal by using the change of the pattern. The DPD compares the phases of two signals obtained from the sum of the diagonals of the four-segment photodetector 3, and compares the beam spots 2 from the preceding or late amounts of the phases. You can see the degree of deviation from the center of the track.

In general, the diffraction pattern on the quadrant photodetector 3 is (A + B) light-receiving divided in the radial direction of the disc when the depth of the pit 1 is 1/4 λ when the wavelength of the light beam is λ. It becomes the same in the area and the light receiving area of (C + D). Therefore, even when the objective lens moves and the beam spot 4 on the photodetector 3 moves, the light receiving area of (A + C) and (B + D) when the beam spot 4 is at the track center is The output signal phase difference becomes zero.

On the other hand, when the depth of the pit 1 is different from 1/4?, A level difference appears in the light receiving region of (A + B) and the light receiving region of (C + D). If the reflected light on the photodetector 3 does not move, there is no level difference between the light receiving areas of (A + C) and (B + D) and the tracking error signal is not zero. However, when the objective lens is moved, an asymmetry phenomenon occurs between the light receiving areas of (A + C) and (B + D), and thus, a phase difference occurs, thereby causing a DC offset in the tracking error signal.

As described above, the case in which the DC error component is included in the tracking error signal often occurs in random access for jumping a plurality of tracks. In detail, jumping the track is done by the sled servo. When the optical pickup is moved to a specific track by the sled servo, the sled servo drives a sled motor to transfer the optical pickup to the target track in the inner or outer circumferential direction of the disk. Here, the tracking servo is turned off in the operation section of the sled motor so that the actuator is in the turn-off state. When the optical pickup is transported by the sled motor, the object is accelerated and decelerated at the start or the stop, respectively, so that the objective lens is biased in the opposite direction to the transport direction of the optical pickup by inertia. This inertia causes the objective lens to vibrate for some time even after the sled motor is turned off to stop the optical pickup. While the objective lens is biased or oscillated from the optical axis center, the DC error component is included in the tracking error signal detected when the tracking servo is turned on.

When the DC error component is included in the tracking error signal, as shown in FIG. 2, the ground potential GND of the tracking error signal becomes higher or lower than the ground potential GND in normal state to level shift the tracking error signal. Referring to FIG. 2, when the tracking error signal includes a DC offset of ΔA _GND level, the ground potential GND is increased by a DC offset, so that the quadrant photodetector may follow the center of the pit 1 even when the beam spot follows the center of the pit 1. The phase difference occurs in each of the output signals of (A + C) and (B + D) detected in (3). This causes the tracking error signal to have a value corresponding to the phase difference, which causes the objective lens to move even though the beam spot normally follows the track center. In Fig. 2, Δ A _GND / A _pp is called a tracking error signal balance (%), and the smaller the value of the tracking error signal balance is, the less offset is included in the tracking error signal.

In order to correct the offset component included in the tracking error signal in this way, the tracking offset correction circuit is provided in the optical disc drive device.

In the configuration of FIG. 3, the conventional tracking offset correction circuit includes first to fourth amplifiers 12 ₁ to 12 ₄ connected to each of the detection pieces A, B, C, and D of the quadrature photodetector 3. And first and second delayers 14 and 16 for delaying the output signals of the first and third amplifiers 12 ₁ and 12 ₃ , respectively, and the output signal and the second amplifier of the first delayer 14. A first adder amplifier 18 for adding the output signal of 12 ₂ , and a second adder amplifier for adding the output signal of the second delayer 16 and the output signal of the fourth amplifier 12 ₄ ( 20, the phase comparator 22 for comparing the phases of the output signals of the first and second adder amplifiers 18 and 20, and the output signal of the phase comparator 22 by low pass filtering to perform a phase difference tracking error signal ( Digital signal processor for adjusting phase delay values of the low pass filter (hereinafter referred to as " LPF ") 24 and the first and second delayers 14 and 16, which generate " DPD TE " (Digltal Sign al Processor: hereafter referred to as " DSP " The DPD detects the DPD TE by comparing the phases of the output signals of the photodetectors arranged diagonally with respect to the track direction in the four-segment photodetectors A, B, C, and D detected by the photodetector 3. Done. When the objective lens deviates from the center of the optical axis during random access, the beam spot 4 of the reflected light collected on the four-segment photodetector 3 is moved to include a DC offset voltage proportional to its magnitude. This DC offset should be corrected in the normal recording / reproducing mode because it causes an unstable tracking servo during recording / reproducing. The DSP 26 detects a DC offset at the DPD TE and adjusts the phase delay values of the first and second phase retarders 14, 16 so that the offset can be compensated. The first and second phase retarders 14 and 16 then delay the A photodetection signal or the B photodetection signal of the quadrature photodetector 3 which is input to itself under the control of the DSP 26. The sum of the signals of the diagonal signal is adjusted.

However, this offset correction method has a problem in that it limits the overall system design because it is necessary to secure a margin and gain for offset correction in the offset correction circuit by correcting the offset in a circuit.

Accordingly, an object of the present invention is to provide an optical pickup apparatus for optically removing a DC offset component included in a tracking error signal.

1 is a diagram showing a detection principle of a tracking error signal using a phase difference detection method.

2 is a diagram showing the change of the ground potential level when the DC error component is included in the tracking error signal.

3 is a tracking offset correction circuit diagram of a phase difference detection method.

4 is a view showing an optical pickup apparatus according to an embodiment of the present invention.

5 is a diagram showing polarization characteristics of a light beam generated from the laser diode shown in FIG.

FIG. 6 is a view showing polarization directions and astigmatism directions of the incident beam in the optical pickup device shown in FIG. 4. FIG.

7A and 7B are characteristic diagrams showing a tracking error signal balance that ignites according to a boiling point angle when an incident beam has a polarization direction of a P wave in the optical pickup device shown in FIG.

8A and 8B are characteristic diagrams showing a tracking error signal balance that varies according to a boiling point angle when an incident beam has a polarization direction of an S wave in the optical pickup device shown in FIG.

* Explanation of symbols for main parts of the drawings

1: Pit 2: Beam spot on recording medium

3: 4-split photodetector 4: Beam spot on photodetector

12 ₁ to 12 ₄ : amplifier 14, 16: delay

18, 20: add amplifier 22: phase comparator

24 low pass filter 26 phase comparator

41, 42: laser diode 43: prism

43a: 45 ° slope 44: collimation lens

45: total reflection mirror 46: objective lens

47: half mirror 48: boiling point plate

100: disk

In order to achieve the above object, the optical pickup apparatus of the present invention specifies a light source for generating a light beam having a predetermined polarization direction, an objective lens for connecting the light beam to a recording medium, and an astigmatism direction of the light beam with respect to the polarization direction. A non-pointing optical system for adjusting in the direction, and a light detecting element for converting the light beam of the reflected light reflected from the recording medium into an electrical signal.

Other objects and features of the present invention in addition to the above objects 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 with reference to FIGS. 4 to 8.

4 is a view showing an optical pickup apparatus according to an embodiment of the present invention.

In the configuration of FIG. 4, the optical pickup apparatus according to the present invention includes first and second laser diodes 41 and 42 used as light sources, and an objective lens 46 for focusing the light beam incident on the disk 100 on the disk 100. ), A four-segment photodetector 49 for converting the light beam reflected from the disk 100 into an electrical signal, and the light beam irradiated from the laser diodes 41 and 42 toward the objective lens 46, Half mirror 47 for transmitting the light beam reflected from the disk 100 toward the quadrant photodetector 49, prism 43 and collimating lens provided between the laser diodes 41, 42 and the half mirror 47 (44), a total reflection mirror (45) provided between the half mirror (47) and the objective lens (46), and a boiling point plate (48) provided between the half mirror (47) and the photodetector (49). The first and second laser diodes 41 and 42 generate light having different wavelengths so as to correspond to different types of disks 100. For example, the first laser diode 41 generates light of 780 nm to correspond to the CD series, and the second laser diode 42 generates light of 650 nm to correspond to the DVD series. Light generated by the laser diodes 41 and 42 has a unique polarization characteristic. The polarization of the light generated from the laser diodes 41 and 42 is divided into P and S waves which are orthogonal to each other according to the polarization direction. The light beams generated from the laser diodes 41 and 42 are converted into parallel light by the collimating lens 44 via the prism 43 and are incident on the half mirror 47. The prism 43 transmits the light beam incident from the first laser diode 41 toward the collimation lens 44, and reflects the light beam incident from the second laser diode 42 toward the collimation lens 44. The prism 43 has a 45 ° inclined surface 43a coated with a wavelength selective material for selectively transmitting or reflecting incident light according to the wavelength range. The first laser diode 41 and the second laser diode 42 generate a 780 nm P-wave polarization beam and a 650 nm S-wave polarization beam, respectively, and the 45 ° inclined plane 43a transmits the 780 nm wavelength and the 650 nm wavelength. Assuming reflection, a 780 nm P-wave polarization beam incident from the first laser diode 41 toward the prism 43 passes and travels toward the collimating lens 44. On the other hand, the 650 nm S-wave polarization beam incident from the second laser diode 42 toward the prism 43 is reflected on the 45 ° inclined surface 43a and travels toward the collimating lens 44. The parallel light beam transmitted through the collimating lens 44 is reflected toward the objective lens 46 by the half mirror 47 and the total reflection mirror 45. The objective lens 46 connects the light beam incident upon it to the recording film of the disc 100 in the form of a micro beam spot. Beam spots focused on the disk 100 are reflected to travel the optical path. The reflected light beam sequentially passes through the half mirror 47 and the boiling point plate 48 and is focused on the quadrant photodetector 49. The quadrant photodetector 49 has a configuration substantially the same as that of the photodetector 3 shown in FIGS. 1 and 3.

In the present invention, the focusing method uses astigmatism. As described above, the light beams generated by the laser diodes 41 and 42 are incident on the optical system with P waves or S waves. According to the polarization direction, the spot shape on the disc 100 has different directivity as shown in FIG. 5. Referring to FIG. 5, incident light is connected to the disk 100 by the objective lens 46 while oscillating in a plane parallel to the pit 1 arranged in the P paraplane tangential direction (T). Incident light is connected to the disk 100 by the objective lens 46 while oscillating in a plane parallel to the radial direction R perpendicular to the S paraplane pit 1. As such, the beam spot having a direction with respect to the pit 1 rotates in the form of the beam spot formed on the quadrant photodetector 49 according to the defocus between the disk 100 and the objective lens 46. Will change. In the present invention, the astigmatism component of the beam spot is adjusted according to the polarization direction of the incident beam to adjust the direction of astigmatism in a specific direction with respect to the polarization direction of the incident beam. In this case, the DC offset component included in the tracking error signal is eliminated by maintaining the symmetrical shape by circularizing (directionally) the. Such beam spots do not rotate on the quadrant photodetector 49 even when the distance between the disc 100 and the objective lens 46 is closer or closer than the normal focal length.

The relationship between the polarization direction of incident light and the direction of astigmatism is shown in FIG. 6.

Referring to FIG. 6, the P wave is parallel to the pit 1 direction, that is, the tangential direction T, and the boiling point at this time is parallel to the radial direction R. As shown in FIG. The S wave is parallel to the radial direction R, and the boiling point at this time becomes the tangential direction T. At this time, the light distribution is parallel with the direction of astigmatism.

In order to reduce the direction of astigmatism, the direction of astigmatism is set in the optical system by setting the astigmatism directions of the collimating lens 44 and the objective lens 46 in the same direction. By adjusting according to the polarization direction, the direction of astigmatism can be adjusted in the vertical direction with respect to the polarization direction.

7 and 8 show the experimental data of the tracking error signal balance and the direction of astigmatism with respect to the polarization direction of the incident beam in actual optical pickup by varying the astigmatism angle with respect to the polarization direction of the incident beam under ideal conditions, and then measuring The displayed tracking error signal balance data. 7 and 8, the horizontal axis represents the amount of movement of the objective lens 46 in the radial direction, and the vertical axis represents the change of the tracking error signal balance according to the change in the amount of movement of the objective lens. And the angle of astigmatism is based on the radial direction of the disk (100).

FIG. 7A is experimental data of a tracking error signal balance that varies according to a boiling point angle when an incident beam has a polarization direction of a P wave under ideal conditions. 7B shows tracking error signal balance data measured by assembling after adjusting the direction of astigmatism at various angles based on the polarization direction when the incident beam has the polarization direction of the P wave in the actual optical pickup. FIG. 8A is experimental data of a tracking error signal balance that varies according to a boiling point angle when an incident beam has a polarization direction of an S wave under ideal conditions. 7B shows tracking error signal balance data measured by assembling after adjusting the direction of astigmatism at various angles based on the polarization direction when the incident beam has the S-wave polarization direction in the actual optical pickup.

As can be seen in FIGS. 7 and 8, the tracking error signal balance (%) is minimized as the direction of astigmatism is closer to the perpendicular to the polarization direction.

As described above, the optical pickup apparatus according to the present invention optically removes the DC offset included in the tracking error signal by the movement of the objective lens by adjusting the direction of the astigmatism in a specific direction with respect to the polarization direction of the incident beam to track the tracking. Allow the servo to operate stably. Furthermore, the optical pickup device according to the present invention can remove the tracking offset correction circuit part in the system by optically removing the DC offset component included in the tracking error signal by the movement of the objective lens. margin and gain margin can be secured.

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)

A light source for generating a light beam having a predetermined polarization direction, An objective lens for connecting the light beam to a recording medium; An astigmatism optical system for adjusting the astigmatism direction of the light beam with respect to the polarization direction in a specific direction; And a light detecting element for converting the light beam of the reflected light reflected from the recording medium into an electrical signal. The method of claim 1, And the astigmatism optical system adjusts the astigmatism direction of the light beam perpendicular to the polarization direction. The method of claim 1, And a light beam guide optical system for guiding the light beam generated from the light source toward the objective lens and guiding the light beam of the reflected light reflected from the recording medium toward the light detection element. The method of claim 1, The astigmatism optical system has astigmatism adjusted in the same direction as the astigmatism direction of the objective lens and has a collimation lens for converting the light beam generated from the light source into parallel light. The method according to claim 1 or 4, The light beam guide optical system includes a prism member for guiding the light beam generated from the light source toward the collimating lens; And a half mirror member for reflecting the light beam transmitted through the collimating lens toward the objective lens and for reflecting the light beam of the reflected light reflected from the recording medium toward the light detecting element. The method of claim 1, And a light beam generated from the light source has one polarization direction among P waves and S waves. The method of claim 1, And the light source further comprises at least two light source modules to respectively irradiate at least two or more wavelengths corresponding to the heterogeneous recording media.