JP2737194B2 - Optical recording medium signal detection method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for detecting a tracking error signal or a focus error signal of an optical recording medium.

SUMMARY OF THE INVENTION The present invention relates to a method for detecting a servo control signal of an optical recording medium, wherein the optical recording medium is irradiated with a laser beam, and the laser beam reflected from the optical recording medium is divided into two using a beam splitter. The first divided laser light is detected by the first photodetector via the first cylindrical lens, and the other divided laser light is orthogonal to the direction of the generatrix of the first cylindrical lens. Detected by a second photodetector through a second cylindrical lens having the direction of the generatrix and operated by using the second photodetector, the eccentricity, skew, and birefringence of an optical recording medium, particularly, an optical disk are caused. Detection of tracking error signal and focus error signal, which has high coupling efficiency and is not affected by eccentricity of the optical recording medium, because offset and groove crossing noise can be removed. Is made possible.

[Conventional technology]

As a tracking error signal detection method for an optical recording medium, for example, an optical disk, a so-called push-pull method and a differential push-pull method are widely adopted. However, in the push-pull method, if the optical disk has eccentricity or skew (inclination), an offset occurs in the tracking error signal because the spot of the laser beam shifts on the photodetector. In the differential push-pull method, three spots (one main spot and two side spots) of a laser beam are formed by a diffraction grating, and both size spots are separated from the main spots on the optical disc by two times the track pitch in the radial direction. The offsets can be removed by detecting the push-pull signal and taking the differential between the main and side spots. However, the differential push-pull method has lower utilization efficiency (coupling efficiency) of laser light than the above-described push-pull method.

Next, as a method for detecting a focus error signal of an optical disc, a so-called astigmatism method is widely adopted.
However, in the astigmatism method, when crossing a groove (guide groove) provided in advance on an optical disk, so-called groove crossing noise is mixed in a focus error signal, and thus there is a problem in seeking. When a signal (magneto-optical signal) of a magneto-optical disk is simultaneously detected by one optical system, an offset occurs in the focus error signal due to birefringence. As a method of removing the cross-groove noise, a method of inserting a spatial filter (a slit having a predetermined shape) for cutting the cross-groove noise into an optical path is used. However, if the magneto-optical signal is 1
If detection is performed simultaneously by two optical systems, the amount of light incident on the photodetector decreases, and the S / N ratio deteriorates.

In addition, a method using an astigmatism method for detecting a focus error signal of an optical disk and a differential push-pull method for detecting a tracking error signal has been widely adopted. This method is an effective method with no offset of the focus error signal due to birefringence and no offset of the tracking error signal due to eccentricity or skew. However, since three spots are used as described above, the coupling efficiency deteriorates.

Incidentally, the focus pull-in range by the current focus servo control is generally ± 5 to ± 10 μm. Therefore, the optical disk is shaken at a large acceleration, and the distance between the optical disk and the objective lens is within ± 5 of the above range.
If it exceeds ± 10 μm, the focus will be lost and the system will be down. On the other hand, when the focus pull-in range is widened, the focus detection sensitivity decreases.

[Problems to be solved by the invention]

Accordingly, the present invention has been made in view of the above-described conventional circumstances, and provides a method of detecting a servo signal having a high coupling efficiency and not being affected by eccentricity, skew, and birefringence of an optical recording medium, particularly an optical disk, and vibration. A strong focus error signal detection method is provided.

[Means for solving the problem]

The signal detection method of the optical recording medium according to the present invention, in order to solve the above problems, irradiating the optical recording medium with a laser beam,
The laser light reflected from the optical recording medium is split into two using a beam splitter, and one of the split laser lights is detected by a first photodetector via a first cylindrical lens, and The other laser beam is passed through a second cylindrical lens having a direction of a generatrix perpendicular to the direction of the generatrix of the first cylindrical lens.
, A push-pull signal is obtained from the output signals from the first and second photodetectors, and the differential output is output as a tracking error signal. The focus error signal is obtained from the output signals of the first and second photodetectors by the aberration method, and the differential output is output as a focus error signal.

(Operation)

In the signal detection method for an optical recording medium according to the present invention, the signal of the optical recording medium is detected by the first and second photodetectors via the first and second cylindrical lenses whose generatrix directions are orthogonal to each other. By performing calculations using them, the eccentricity of optical recording media, especially optical discs, skew, offset and groove crossing noise due to birefringence can be removed,
Servo signals and magneto-optical signals which have high coupling efficiency and are not affected by the eccentricity of the optical disk can be detected.

Further, there are two optical systems for detecting the signal,
Since the focal lengths of the respective optical systems can be different, so-called focus pull-in ranges can be set to two types.

〔Example〕

An embodiment of a method for detecting a servo signal or a magneto-optical signal of an optical recording medium, for example, a magneto-optical disk according to the present invention will be described with reference to the drawings.

FIG. 1 is a configuration diagram of a so-called optical pickup used in a method for detecting a servo signal or a magneto-optical signal of a magneto-optical disk. In this figure, a laser beam from a laser light source 11 is converted into a parallel beam through a collimator lens 12, and then passes through a beam splitter 13 and an objective lens 14 movable in the optical axis direction and the radial direction of the magneto-optical disk. Irradiates the magneto-optical disk 30.

Next, the laser light reflected by the magneto-optical disk 30 is again incident on the beam splitter 13 via the objective lens 14, and the direction of the optical axis is changed by 90 degrees. The laser light whose optical axis direction has been changed is rotated by 45 degrees in the polarization plane by the half-wave plate 15, and is incident on the polarization beam splitter 17 via the converging lens 16. In the polarization beam splitter 17, the light is split into polarization components having polarization planes orthogonal to each other. The polarization component having one polarization plane transmitted through the polarization beam splitter 17 passes through a cylindrical lens 18 in which the direction of the generatrix of the cylindrical lens is set at 45 degrees with respect to the track direction of the magneto-optical disk as described later. And 4
The light is incident on the divided photodetector 19 composed of, for example, a PIN photodiode (detector). On the other hand, after the polarization component having another polarization plane reflected by the polarization beam splitter 17 is changed in the optical axis direction by 90 degrees through the mirror 20, the direction of the generatrix of the cylindrical lens is changed to the angle of the generatrix of the cylindrical lens 18. Through a cylindrical lens 21 set to be orthogonal to the direction, the light is incident on a photodetector 22 divided into four parts, for example, constituted by a PIN photodiode (detector). The servo signal and the magneto-optical signal described above are obtained from the output signal of each detector.

Next, in FIG. 2, the intensity distribution of the spot of the laser beam reflected by the magneto-optical disk 30, the direction of the generatrix of the cylindrical lenses 18, 21 and the laser beam on each of the detectors of the photodetectors 19, 22 are shown. The relationship between spot intensity distributions will be described. 2A shows the intensity distribution of the spot 33 of the laser beam reflected by the magneto-optical disk 30. The hatched portions and the mesh portions in the figure interfere with the grooves 32 provided on the magneto-optical disk 30 in advance. Occurs and the amount of reflected light decreases. Direction of generatrix L ₂ of the bus L ₁ and the cylindrical lens 21 of the cylindrical lens 18 is inclined by 45 degrees in opposite directions with respect to the direction of the track as shown in FIG.

In FIG. 2, b and c show the intensity distribution of the laser light spots 34 and 35 on the detectors of the photodetectors 19 and 22, respectively.
The hatched portion and the mesh portion in FIG. 2A correspond to the hatched portion and the mesh portion in b and c in FIG. 2, respectively, and indicate that the image is darkened by the interference. In addition, a, b in FIG.
And c correspond to the alphabets O, P, Q and R, respectively. That is, the laser beam spot intensity distribution pattern of the second drawing b as axis generating line L ₁ of the cylindrical lens 18, in which the laser light intensity distribution pattern in Fig. 2 a was a half turn. The laser beam spot intensity distribution pattern of the second figure c is the axial generatrix L ₂ of the cylindrical lens 21, in which the laser light intensity distribution pattern in Fig. 2 a was a half turn.

2, A ₁ , B ₁ , C ₁ and D ₁ in FIG. 2 represent the powers detected by the four-divided detectors 19A, 19B, 19C and 19D of the photodetector 19, respectively. A ₂ , B ₂ , C ₂ and
D ₂ are divided into four detectors 22A of the photodetector 22, 22B, 22
C and 22D represent the power detected respectively. The relationship between the powers detected by these detectors is A ₁ = C ₂ , B ₁ = D ₂ ,
C ₁ = A ₂ and D ₁ = B ₂ .

In the above case, the tracking error signal is
And (22) and (22) by obtaining each push-pull signal and taking the differential thereof, [(A ₁ + D ₁ ) − (B ₁ + C ₁ )] − [(A ₂ + D ₂ ) − (B ₂ +
C ₂ )]. Here, when the spot of the laser beam moves in the inner circumferential direction of the magneto-optical disk, the shaded portion becomes darker than the mesh portion. That is, (A ₁ + D ₁ ) <(B ₁ + C ₁ ), (A ₂ + D ₂ )> (B ₂ + C ₂ ), and the value of the tracking error signal becomes negative.
Conversely, when the spot of the laser beam moves toward the outer periphery of the magneto-optical disk, the mesh portion becomes darker than the hatched portion, contrary to the above. That is, (A ₁ + D ₁ )> (B ₁ + C ₁ ), (A ₂ + D ₂ ) <(B ₂ + C ₂ ), and the value of the tracking error signal is positive.
That is, the objective lens 14 shown in FIG. 1 can be moved in the radial direction of the magneto-optical disk 30 (tracking servo control) so that accurate tracking can be performed so that the value of the tracking error signal is always zero. Become.

Next, the focus error signal is obtained by using the output signals of the photodetectors 19 and 22 to determine the respective focus error signals by the astigmatism method and taking the difference between them, thereby obtaining [(A ₁ + C ₁ ) − (B _{1 + D 1)] - [} (A 2 + C 2) - (B 2 +
D ₂ )].

Here, the focus error signal will be described with reference to FIGS. 3A to 7A show the intensity distribution of the spot 34 of the laser beam on the detector of the photodetector 19, and FIGS. 3B to 7B show the intensity distribution of the spot 34 on the detector of the photodetector 22. The intensity distribution of the spot 35 of the laser light is shown, and the hatched portion and the mesh portion in the figure indicate that the light amount is small.

FIG. 3 shows a state in which the laser beam is in focus. The shapes of the laser beam spots 34 and 35 are circular, and the relationship between the powers of the detectors is A ₁ = D ₁ , B ₁ = C ₁ , A ₂ = D ₂ , B ₂ = C ₂ . Therefore, the focus error signal [(A ₁ + C ₁ ) − (B ₁ + D ₁ )] obtained from the output signal of the photodetector 19 and the focus error signal [(A ₂ + C ₂ ) obtained from the output signal of the photodetector 22 ) − (B ₂ + D ₂ )] becomes zero, and the value of the focus error signal also becomes zero.

FIG. 4 shows a state in which the objective lens is close to the magneto-optical disk, and the shape of the spot 34 of the laser beam is
The shape of the spot 35 of the laser light is an elliptical shape rising to the right, and the relationship between the powers of the detectors is A ₁ > D ₁ , B ₁ <C ₁ , A ₂ <D ₂ , B ₂ > C ₂ . Therefore, the value of the focus error signal [(A ₁ + C ₁ ) − (B ₁ + D ₁ )] obtained from the output signal of the photodetector 19 becomes positive, and the focus error signal [ The value of (A ₂ + C ₂ ) − (B ₂ + D ₂ )] becomes negative, and the focus error signal becomes a positive value.

FIG. 5 shows a state where the objective lens is far from the magneto-optical disk, and the shape of the spot 34 of the laser light is
The shape of the laser beam spot 35 is an elliptical shape rising to the left, and the relationship between the powers of the detectors is A ₁ <D ₁ , B ₁ > C ₁ , A ₂ > D ₂ , B ₂ <C ₂ . Therefore, the value of the focus error signal [(A ₁ + C ₁ ) − (B ₁ + D ₁ )] obtained from the output signal of the photodetector 19 becomes negative, and the focus error signal [ (A ₂ + C ₂ ) − (B ₂ + D ₂ )] is positive, and the focus error signal is negative.

FIG. 6 shows a case where the laser light spots 34 and 35 are shifted due to the eccentricity and skew of the magneto-optical disk described above, and shows a state in which the laser light is in focus. The shapes of the laser light spots 34 and 35 are circular. , Which is shifted upward. The relationship between the powers of the detectors is A ₁ = D ₁ , B ₁ = C ₁ , A ₂ = D ₂ , and B ₂ = C ₂ . Therefore, the focus error signal [(A ₁ + C ₁ ) − (B ₁ + D ₁ )] obtained from the output signal of the photodetector 19 and the focus error signal [(A ₂ + C ₂ ) obtained from the output signal of the photodetector 22 ) − (B ₂ + D ₂ )] becomes zero, and the value of the focus error signal also becomes zero.

FIG. 7 is a diagram showing the influence of birefringence when there is birefringence. The effect of this birefringence is an in-phase component for the photodetector 19 and the photodetector 22. Therefore, as described above, the focus error signals can be obtained by obtaining the respective focus error signals from the output signals of the photodetectors 19 and 22, and can be removed by taking the difference between them.

That is, it is possible to move the objective lens 14 in FIG. 1 in the optical axis direction (focus servo control) so that accurate focusing is performed so that the value of the focus error signal is always zero. Further, even if there is eccentricity, skew, or birefringence of the magneto-optical disk, the offset of the focus error signal due to these factors is in phase with the photodetectors 19 and 22, so that the photodetectors 19 and
The above-mentioned offset can be removed by obtaining each focus error signal from the 22 output signals and taking the differential.

In FIG. 2, the magneto-optical signal is obtained by (A ₁ + B ₁ + C ₁ + D ₁ ) − (A ₂ + B ₂ + C ₂ + D ₂ ).

The so-called pull-in signal is obtained by (A ₁ + B ₁ + C ₁ + D ₁ ) + (A ₂ + B ₂ + C ₂ + D ₂ ).

In the above embodiment, the focal lengths of the cylindrical lenses 18 and 21 in FIG. 1 are substantially equal. However, the focal lengths of the cylindrical lenses 18 and 21 are F ₁ and F ₂ (for example, F ₁ <F ₂ ). The focus range that can be detected by the photodetector 19 is, for example, ± 40 μm, and the focus range that can be detected by the photodetector 22 is, for example, ± 5 μm. FIGS. 8 and 9 show the shapes of spots of laser light on the detectors of the photodetectors 19 and 22 in this case.

8 and 9, a in FIG. 8 and FIG. 9 shows the shape of the spot 34 of the laser beam on the detector of the photodetector 19, and b in FIG. 8 and FIG. The shape of the spot 35 of the laser beam on the 22 detector is shown.

FIG. 8 shows a state where the distance between the magneto-optical disk and the objective lens is short. In this case, since the range of focus that can be detected by the photodetector 19 is as wide as ± 40 μm, the spot of laser light
34 has a shape that is not so deformed from a circle,
The focus error signal [(A ₁
+ C ₁ ) − (B ₁ + D ₁ )] is a small value. However, the above distance is within a range that can be detected by the photodetector 22, and the spot 35 of the laser beam has an elliptical shape rising to the right. Therefore, the value of the focus error signal [(A ₂ + C ₂ ) − (B ₂ + D ₂ )] obtained from the output signal of the photodetector 22 is a large enough value for performing the focus servo control.

FIG. 9 shows a state in which the distance between the magneto-optical disk and the objective lens is long and exceeds the range in which the photodetector 22 can detect the focus ± 5 μm. In this case, the spot 35 of the laser light has an elliptical shape that is larger than the area of the detector of the photodetector 22, and the focus error signal obtained from the output signal of the photodetector 22 cannot be used. However, the above distance is within the range of focus that can be detected by the photodetector 19, and the spot 34 of the laser beam has an elliptical shape rising to the left. Therefore, a focus error signal [(A ₁ + C ₁ ) − obtained from the output signal of the photodetector 19.
(B ₁ + D ₁ )] is a large enough value for performing the focus servo control.

FIG. 10 shows a focus error signal obtained from the output signals of the photodetectors 19 and 22 in the above case and a focus error signal (composite focus error signal) obtained by combining them. In this figure, a in FIG. 10 is a photodetector.
A focus error signal obtained from the output signal of 22 is shown, b in FIG. 10 shows a focus error signal obtained from the output signal of the photodetector 19, and c in FIG. 10 combines the two focus error signals. 5 shows a composite focus error signal obtained by the above. Here, it is assumed that focus servo control of the objective lens is performed so that the composite focus error signal is always zero. Alternatively, the focus servo control is performed by switching between the two focus error signals obtained from the output signals of the photodetectors 19 and 22.
That is, when the focus is greatly deviated (± 5 ± 40
μm), focus servo control (rough / focus servo control) is performed using a focus error signal obtained from the output signal of the photodetector 19, and if the focus is not significantly shifted (± 5 μm or less), the photodetector Focus servo control (just / focus servo control) is performed using a focus error signal obtained from the output signal of No. 22 to expand the range in which the focus servo control is possible and maintain high focus detection sensitivity. As a result, it is possible to avoid the system down due to the above-mentioned out of focus. In FIG. 10, the dashed line is out of a predetermined focus detectable range, and the spot of the laser beam is too wide, so that the photodetector 1
This shows how the amount of light incident on 9, 22 decreases and the focus error signal decreases.

Next, switching of the focus error signal will be described with reference to FIG. FIG. 11A shows a pull-in signal obtained from the output signal of the photodetector 22, and FIG.
Indicates a pull-in signal obtained from the output signal of the photodetector 19, and FIG. 11c indicates a signal obtained by synthesizing the two pull-in signals (synthesized pull-in signal). The synthesis pull signals two sets the threshold level TH ₁ and TH ₂ in the range for rough focus servo control described above and TH ₁ to TH _2, the range for the just focus servo control TH ₂
As described above, the focus error signal is switched by detecting in which range the value of the combined pull-in signal exists. The above two threshold levels TH ₁ and TH ₂ is instead of setting the composite pull-in signal may be set to each of the pull-in signal obtained from the output signal of the photodetector 19 and 22.

The display of the threshold level TH ₁ and TH ₂ various, it is also possible to use the status display of the optical disk apparatus. That is, the value of the composite pull-in signal is when the TH ₂ or more is a just-focus servo control state, TH
₁ when in the range of to TH ₂ is that it is a rough focus servo control state, when it becomes time to TH ₁ or less to indicate that the defocus has occurred can utilize these threshold levels . In addition, when the focus is out of focus, the alarm can be displayed as an emergency alarm or the like depending on the degree of the out of focus.

Note that, in FIG.
Although it is arranged between the wave plate 15 and the polarizing beam splitter 17, a converging lens is used instead.
The same effect can be obtained by disposing the lens between the mirror 20 and the cylindrical lens 18 and between the mirror 20 and the cylindrical lens 21. In this case, the convergent lens and the cylindrical lens may be adjusted as a unitary lens on the optical axis.

〔The invention's effect〕

As described above, according to the signal detection method of the present invention, it is possible to detect a tracking error signal or a focus error signal having high coupling efficiency, no groove crossing noise, and no offset due to birefringence.

In the method of the present invention, the tracking error signal and the focus error signal are obtained by taking the differential output of the output signals from the first and second photodetectors, so that the common mode noise is efficiently canceled. As a result, gains of the tracking error signal and the focus error signal can be increased, and accurate tracking control and focus control can be performed.

Further, in the method of the present invention, two focus pull-in ranges can be provided by using two astigmatism optical systems as separate systems and changing the focal length of a cylindrical lens used for the two systems. . As a result, for example, by increasing one focus pull-in range to ± 50 to ± 100 μm, there is no out-of-focus even with a sudden defocus exceeding the other focus pull-in range, for example, ± 5 μm, and it is resistant to vibration. The system can provide. Further, a circuit for adjusting the gain in the conventional differential push-pull method becomes unnecessary, and the optical system can be made simple.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an optical pickup according to an embodiment of the present invention, FIGS. 2 to 7 are diagrams showing an intensity distribution of laser light, and FIGS. 8 and 9 are spot shapes of the laser light. FIG. 10 is a diagram showing characteristics of a focus error signal, and FIG.
FIG. 11 shows the characteristics of the pull-in signal. 11 Laser light source 17 Polarizing beam splitter 18 Cylindrical lens 19 Photodetector 21 Cylindrical lens 22 Photodetector

Claims (2)

(57) [Claims] An optical recording medium is irradiated with a laser beam, a laser beam reflected from the optical recording medium is divided into two using a beam splitter, and one of the divided laser beams is passed through a first cylindrical lens. Through the first photodetector, and the other of the divided laser beams is transmitted through the second cylindrical lens having a bus line direction orthogonal to the bus line direction of the first cylindrical lens. A signal detection of an optical recording medium which is detected by a photodetector, obtains respective push-pull signals from output signals from the first and second photodetectors, and outputs a differential output thereof as a tracking error signal. Method. 2. An optical recording medium is irradiated with a laser beam, a laser beam reflected from the optical recording medium is divided into two by a beam splitter, and one of the divided laser beams is passed through a first cylindrical lens. Through the first photodetector, and the other of the divided laser beams is transmitted through the second cylindrical lens having the direction of the generatrix perpendicular to the direction of the generatrix of the first cylindrical lens. Optical recording in which a focus error signal is detected from the output signals of the first and second photodetectors by an astigmatism method, detected by a photodetector, and a differential output thereof is output as a focus error signal. A method for detecting a signal of a medium.