JPH07153104A - Optical pickup and optical disk device - Google Patents

Abstract

PURPOSE:To reduce the effect of a tangential tilt and to improve the degree of a total allowance by imparting previously aberrations corresponding to the tilt of a tangential direction to plural beams and irradiating a disk with these beams and a 0-order diffracted light beam. CONSTITUTION:A diffraction element 16 is made to be a grating pattern whose interval is widen and whose curvature is increased, for example, from a center toward an edge part. Thus, + or -1st-order diffracted light beams having coma aberrations are obtained. The grating pattern is determined by imparting a diffraction direction, the dimension of the coma aberration and the light quantity ratio of diffracted light beams. Since conjugate diffracted light beams with a +1st-order light beam and a -1st-order light beam are generated, the pattern becomes symmetric with respect to the spot of + or -1st-order diffracted light beams converged on a disk 22. Further, the spot of the 0-order diffracted light beam becomes an aberration-free spot because it is not subjected to a phase-shifting by the diffraction element 16.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical pickup and an optical disc device, and more specifically to an improvement of the optical pickup and the optical disc device suitable for an optical disc having a relatively high recording density.

[0002]

2. Description of the Related Art The recording capacity of an optical disk (hereinafter simply referred to as "disk") is determined by the pitch of recording tracks and the wavelength of recording laser light, and the limit value of each of these values is the laser used for reading information. It is almost defined by the size (diameter) D of the light spot. The size D of the laser beam spot is determined by the wavelength λ of the laser beam and the numerical aperture NA of the objective lens of the reproduction optical pickup, and is represented by the following equation (1). D ≒ λ / NA ……………………………… (1)

Considering a conventional optical disc, for example, a CD (compact disc), λ = 0.78 μm, NA =
0.45 is a typical value. These values are described in (1) above.
Substituting into the equation, the spot size D becomes about 1.7 μm. From this value, in the CD system, the track pitch is 1.6 μm and the shortest recording wavelength of the laser light is 1.7.
μm.

Under these conditions, in order to increase the capacity of recorded information on the disc, that is, to improve the recording density, it is necessary to shorten the laser light wavelength λ or increase the numerical aperture NA of the objective lens. Either method is required. However, it is extremely difficult to shorten the wavelength of the semiconductor laser, and it is considered that the one having λ = 0.67 μm is the practical limit for the time being. Then, the recording density can be improved by increasing the numerical aperture NA of the objective lens.

However, when the numerical aperture NA is increased, there arises a problem that an allowable range (system margin) when the pickup system is deviated from an ideal state becomes very narrow. Of the various factors that determine the margin of this system, the one that most affects performance is the tilt of the disc. When tilt occurs, so-called coma aberration occurs, but according to the third-order aberration theory, this coma aberration increases in proportion to the cube of the numerical aperture NA. Therefore, when the numerical aperture NA is increased, the tilt allowance of the system is narrowed and the stability is extremely deteriorated.

For example, in the case of the above CD, if the NA is 0.45 to 0.6, and the laser light wavelength λ is the same, the recording capacity is (0.6 / 0.45) ² =
Although it increases 1.78 times, the amount of aberration generated by the same amount of tilt becomes 2.37 times. On the CD,
A disc tilt of 0.6 degrees is allowed, but if the numerical aperture NA is 0.6, the allowable range is only about 0.2 degrees. The accuracy with respect to such tilt is C
Satisfaction with a plastic molded disk having excellent mass productivity such as D is extremely difficult. For this reason, a method for properly correcting the tilt is essential from the viewpoint of improving the recording density.

By the way, the tilt of the disc can be considered by distinguishing it from the tangential direction and the radial direction. The tangential direction is a tangential direction to the disc, and is a direction along a pit row formed in a concentric circle shape or a spiral shape. The radial method is in the radial direction with respect to the disc and in the direction orthogonal to the pit row. From the perspective of the entire system consisting of both the disc and the drive, the tilt of the disc
It must be less than or equal to the value determined by the margins of the optical system and signal system.

As a conventional tilt correction technique, a radial tilt servo used in an LD is known.
This is a method of detecting the tilt of the disk in the radial direction by a sensor and mechanically tilting the entire pickup by an amount commensurate with the tilt to correct the tilt. Further, Japanese Patent Laid-Open No. 1-171159 discloses a disc player in which the transfer function of the noise reduction circuit is changed according to the tilt of the disc instead of mechanically tilting the pickup.

[0009]

Here, comparing the effects of tilt in a disc that handles digital signals, such as a CD, it can be seen that tilt occurs more in the tangential direction than in the radial direction. , Known to have a more serious impact on the system. That is, in a system handling a digital signal, a large intersymbol interference occurs due to the disturbance of the spot due to the tangential tilt, which immediately deteriorates the quality of the reproduced signal and raises the read error rate.

Next, the factors causing the tilt of the disk as described above will be considered. The main cause of tilt is disk deformation. Of these, the main cause of tilt in the radial direction is disc droop from the inside to the outside. This is a low frequency component and can be corrected by a conventional method described later. On the other hand, the tilt in the tangential direction generally has a high component equal to or higher than the rotation frequency of the disc. Therefore, remarkably high frequency response performance is required for the correction.

In order to establish the system without performing the tilt correction in the tangential direction, it is necessary to extremely suppress the deformation of the disc including the change in the environment of use of the plastic disc, and the system is established. Is accompanied by difficulties.

In the above-described LD that handles analog signals, crosstalk from adjacent tracks seriously affects the quality of a reproduced image due to coma aberration generated by tilt in the radial direction. Therefore, it is only necessary to improve the tilt in the radial direction. I am getting good results. In the LD system, since the numerical aperture NA of the objective lens is low and the signal is FM-modulated, the tangential tilt does not pose a problem. Further, since the entire pickup is mechanically tilted, it is not suitable for high-speed response. However, since the inclination of the disk in the radial direction is mostly a gentle warp of the surface from the inner circumference to the outer circumference of the disk, high-speed response is not required, and the tilt correction can be satisfactorily performed by the conventional method.

In the radial component of tilt, there is a component higher than the disc rotation frequency similar to the frequency component of tilt in the tangential direction. However, the influence of the high frequency component in the radial direction is relatively small as compared with the high frequency component in the tangential direction. Therefore, if the tilt in the tangential direction is properly corrected and the margin of the system is improved, it is possible to sufficiently cope with the situation.

As described above, in order to achieve high density recording information on the disc, it is necessary to satisfactorily correct the tilt in the tangential direction having a high frequency. However, the conventional method can speed up the tilt correction. There is an inconvenience that we cannot handle it.

The present invention focuses on these points,
It is an object of the present invention to provide an optical pickup and an optical disc device that can improve the margin in a good manner in response to tilt in the tangential direction with a simple device configuration.

[0016]

In order to achieve the above-mentioned object, the optical pickup according to the present invention allows some of a plurality of beams irradiated in the tangential direction of a track of an optical disc to be tangentially directed to the optical disc. It is characterized in that it is provided with an aberration imparting means for giving an aberration corresponding to the tilt in advance, and a signal detecting means for detecting each beam obtained from the optical disk and converting the signal.

An optical disk apparatus according to the present invention includes the optical pickup, tilt detecting means for detecting a tilt in at least a tangential direction at a beam irradiation position of the optical disk, and the signal based on the tilt information detected by the tilt detecting means. Among the plurality of beam detection signals output from the detection means, there is provided signal selection means for selecting a detection signal of a beam having an aberration opposite to the aberration caused by the tangential tilt of the optical disc.

Another invention of the optical disk device is that the optical disk device is provided with a signal synchronizing means for adjusting the output signal of the signal detecting means by synchronizing the time corresponding to the interval of each beam spot. Characterize.

[0019]

According to the present invention, aberrations corresponding to tilt in the tangential direction are given to a plurality of beams in advance. For example, coma aberration is added to the ± 1st order diffracted light by the diffraction element. Then, these and the 0th-order diffracted light are applied to the disk. When the disc has a tilt in the tangential direction, coma aberration occurs in the incident beam. However, if the beam in which this aberration and the aberration given in advance are canceled is selected, the tangential tilt is achieved even though the configuration is simple. It is possible to reduce the influence of and improve the margin.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of an optical pickup and an optical disk device according to the present invention will be described in detail below with reference to the accompanying drawings. FIG. 1 shows the configuration of the optical pickup according to this embodiment. This embodiment has basically the same structure as a three-beam type pickup used in a CD system or the like. The main difference from the CD system is diffractive optics.
The diffractive element of this embodiment has a function of giving coma to ± 1st order diffracted light.

In the figure, a collimator lens 14 is arranged on the output side of the semiconductor laser element 12 of the optical pickup 10, and a laser beam output side of this collimator lens 14 is provided with a polarization beam splitter 18 via a diffraction element 16. Is provided. The disc 22 is located on the laser beam transmitting side of the polarization beam splitter 18 via the objective lens 20.
A photodetector 26 is provided on the laser light reflection side of 8 through a detection lens 24.

Of these, the semiconductor laser device 12 is
It is for outputting a laser beam for recording or reproducing information on the disk 22. Collimator lens 1
Reference numeral 4 is for collimating the incident laser light and outputting it.

For example, as shown in FIG. 2, the diffractive element 16 has a grating pattern in which the distance increases from the center toward the end and the curvature increases. As a result, ± first-order diffracted light with coma can be obtained. The grating pattern is determined by giving the diffraction direction, the magnitude of coma and the diffracted light quantity ratio. Since the + 1st-order light and the -1st-order light generate a conjugate diffracted light, the influence of the coma aberration appearing on the spot of the ± 1st-order diffracted light focused on the disk 22 is affected by the central 0th-order diffracted light spot. Be symmetrical.
Since the 0th-order diffracted light spot is not subjected to phase modulation by the diffractive element 16, it becomes a spot with no aberration.

In the present embodiment, as shown in FIG. 3, the spots SA of the −1st order diffracted light, the 0th order diffracted light, and the + 1st order diffracted light,
SB and SC are formed side by side in the pit row direction. Since the disk is rotating in the direction of arrow FA, the spot SC of the + 1st order diffracted light is at the top. Further, the intensity distributions of these spots SA to SC in the pit row direction are as shown in FIGS. 4 (A) to 4 (C), respectively. The portions indicated by arrows FB and FC are flare portions corresponding to coma aberration, which are symmetrical with respect to the central spot SB.

Returning to FIG. 1, the polarization beam splitter 18
Is a laser beam incident from the side of the diffraction element 16 on the disc 2
The laser light is transmitted to the second side and reflected to the photodetector 26 side from the laser light reflected from the disk 22. The objective lens 20 is for focusing the incident laser light on a predetermined surface of the disk 22. Detection lens 24
Is for collecting the laser light reflected and output from the polarization beam splitter 18 on the photodetector 26. In the present embodiment, the −1st order diffracted light, the 0th order diffracted light, and the + 1st order diffracted light are respectively focused on the detectors 26A, 26B, and 26C. The detector 26B is divided into four in order to perform tracking servo and focus servo.

Next, FIG. 5 shows the optical pickup 1 described above.
An embodiment of the optical disk device 30 using 0 is shown. In the figure, the optical pickup 10 is installed on the radial tilt mechanism 32. Further, among the photodetectors 26 of the optical pickup 10, detectors 26A and 26B are provided.
Is connected to the switching input side of the signal switching device 38 via the delay circuits 34 and 36, respectively, and the detector 26C
The output side of is directly connected to the switching input side of the signal switch 38.

On the other hand, the tilt of the disk 22 rotatably driven by the spindle motor 40 is detected by the tilt sensor 42, and the output side of the tilt sensor 42 is connected to the tilt detection circuit 44.
The output side of the tilt detection circuit 44 for the radial tilt signal is connected to the radial tilt drive circuit 46, and the output side of the tangential tilt signal is connected to the control input side of the signal switch 38. The output side of the signal switch 38 is connected to the signal processing circuit 48.

Of the above parts, the radial tilt mechanism 3
Reference numeral 2 is for swinging the optical pickup 10 in accordance with the radial tilt of the disc 22 as shown by an arrow FD, and its drive is performed based on a drive signal supplied from the radial tilt drive circuit 46. Has become. The delay circuits 34 and 36 are for giving a predetermined delay to the reproduced signals obtained by the beam spots SB and SC, and when the signals are switched by the signal switch 38, the signals are well continuous before and after the switching. It is provided to do.

Specifically, the delay circuit 34 has a delay amount corresponding to the beam spots SA and SB, and the delay circuit 36 has a delay amount corresponding to the beam spots SA and SC. For example, assuming that the intervals of the beam spots SA to SC are equal, the delay circuit 36 is set to have a delay amount that is twice that of the delay circuit 34. Incidentally, in a disc in which the linear velocity of the disc changes, for example, in the inner and outer circumferences, this delay amount is variable according to the spot irradiation position.

The signal switch 38 is for switching the input based on the tangential tilt signal. The tilt sensor 42 can obtain the tilt information in the radial direction and the tangential direction of the disk 22 by projecting the sensor beam light on the disk 22 and detecting the reflected light by the four-division light receiving element, for example. Has become. This tilt sensor 42 is used for the disc 2
In order to obtain the tilt information at the beam incident position of the second beam, it moves in conjunction with the optical pickup 10. The tilt detection circuit 44 can detect the degree of tilt of the disk 22 in both radial and tangential directions based on the output signal of the tilt sensor 42.

Next, the operation of this embodiment will be described. The disk 22 is rotated by the spindle motor 40. On the other hand, in the optical pickup 10, the laser light output from the semiconductor laser element 10 is collimated by the collimator lens 14 and then enters the diffraction element 16. Diffractive element 16
In the above, three beams are formed from the incident laser light, and these are incident on the disc 22 via the polarization beam splitter 18 and the objective lens 20, and beam spots SA to SC are formed on the disc 22.

The reflected light of these beams by the disk 22 is transmitted through the objective lens 20 to the polarization beam splitter 1.
The light that has entered the detector 8 and is reflected here enters the detectors 26A to 26C of the photodetector 26 through the detection lens 24. In the detectors 26A to 26C, each reflected beam is converted into an electric signal. The signals thus obtained by the beam spots SA to SC are transmitted to the delay circuit 3
After synchronization processing by a time delay corresponding to the spot interval by 4, 36, it is sent to the signal switch 38.

On the other hand, the tilt of the beam incident position of the disk 22 is detected by the tilt sensor 42, and if the tilt exists, the tilt detection circuit 44 outputs a radial tilt signal or a tangential tilt signal.

Operation for Radial Tilt First, the tilt correction operation in the radial direction will be described. When the tilt in the radial direction is present, the tilt detection circuit 44 outputs a radial tilt signal, which is supplied to the radial tilt drive circuit 46. The radial tilt drive circuit 46 drives the radial tilt mechanism 32 based on the input tilt signal. Then, the entire optical pickup 10 is tilted in the arrow FD direction according to the degree of radial tilt, and is automatically adjusted in the optimum direction. As a result, the radial tilt is corrected. The tilt correction in the radial direction is performed at a frequency sufficiently lower than the rotation speed of the disk 22.

Operation for Tangential Tilt Next, the operation for the tangential tilt will be described. If there is a tangential tilt,
A tangential tilt signal is output from the tilt detection circuit 44 and supplied to the signal switch 38. Signal switch 3
In 8, the input channel is switched according to the detected tangential tilt amount and direction.

More specifically, the three beam spots SA to S obtained by the diffraction element 16 of the optical pickup 10 will be described.
C has the intensity distribution shown in FIG. 4, and the coma aberration is 2
The directions are opposite at the two spots SA and SC. On the other hand, when the disk 22 is tilted, the arrows FE and F in FIG.
As indicated by F, the direction of coma produced in the beam spot is reversed depending on the direction of inclination. The tilt degree in the figure is
In the present embodiment, the figure (A) is negative, the figure (B) is no tilt, and the figure (C) is positive, depending on how to take it.

Therefore, by the disk tilt, as shown in FIG.
The coma aberration can be canceled by combining the coma aberrations shown in (A) and (C) with the beam spot in which the opposite coma aberration occurs. For example, when the coma aberration shown in FIG. 6A occurs and the signal is read by the beam spot SA shown in FIG. 4A, the coma aberration portions indicated by the arrows FB and FE are generated. As a result, the influence of the tangential tilt can be reduced as a result.

Further, in the case of the tilt state in which the coma aberration shown in FIG. 6C occurs, if the signal is read by the beam spot SC shown in FIG.
The coma aberration portions indicated by C and FF are canceled, and the influence of the tangential tilt can be similarly reduced. Further, if there is no tangential tilt shown in FIG. 6B, the beam spot S shown in FIG.
The signal may be read at B.

FIG. 7 shows the intensity distributions of the three beam spots SA to SC when the disc 22 is tilted in the tilt angle = positive direction. Since the tilt angle is positive, the coma aberration shown in FIG. 6C is generated in each of the beam spots SA to SC. Here, the coma amount is as shown in FIG.
It is assumed that (C) and FIG. First, with respect to the beam spot SA, the coma aberration of FIG. 6 (C) is doubled as it is superimposed on FIG. 4 (A), and a large disturbance occurs as shown in FIG. 7 (A).

The beam spot SB is shown in FIG.
The coma aberration shown in FIG. 6C is generated in the state without the coma aberration shown in FIG. 6B, as shown in FIG. 7B, and the effect of the tangential tilt appears. However, with respect to the beam spot SC, the coma aberration of FIG. 4C and the coma aberration of FIG. 6C are canceled, and an aberration-free spot is realized as shown in FIG. 7C. Will be. Similarly, when the tilt angle is negative, the beam spot SA realizes a non-aberration spot.

FIG. 8 shows the relationship between the error rate and the tilt angle of the signals reproduced by the beam spots SA to SC as described above. As shown in the figure, when the tangential tilt angle is in the range of -0.6 to -0.2 degrees, the influence of tilt can be satisfactorily reduced by using the spot SA of the -1st order diffracted light. When the tangential tilt angle is in the range of -0.2 to +0.2 degrees, the effect of tilt can be reduced well by using the spot SB of the 0th-order diffracted light. Tangential tilt angle is +0.2 to +0.6
In the range of degrees, if the spot SC of the + 1st-order diffracted light is used, the influence of tilt can be satisfactorily reduced.

In the signal switch 38 of FIG. 5, the beam selection as described above is performed based on the tangential tilt signal supplied from the tilt detection circuit 44. That is, the tilt angle can be obtained from the input tangential tilt signal by utilizing the relationship between the tangential tilt signal and the tilt angle of the disc 22 which are obtained in advance. Then, the signal of the beam spot with the lowest error rate is selected from the value of this tilt angle with reference to FIG. At this time,
Since the input signals are synchronized by the action of the delay circuits 34 and 36, even if the signal switching device 38 switches the signals, the signals will continue satisfactorily.

The signal output from the signal switch 38 is supplied to the signal processing circuit 48, where necessary processing is performed. For example, when the disc 22 is a disc for music, a reproduction process of an audio signal is performed.

As described above, according to the present embodiment, the aberration given to the diffracted light is set as the amount of coma aberration generated at twice the allowable value of the tilt angle in the tangential direction. Tilt allowance can be obtained. In other words, if the diffracted light has a coma aberration corresponding to a tilt of 2θ (θ is a tilt angle allowance), there will be no aberration at the tilt angle 2θ. ± 1 for 0th-order diffracted light having such an aberration
The second diffracted light can obtain a good reproduction signal in the range of ± θ centering on 2θ. Therefore, the tilt allowable angle is ± 3θ as a whole.

As described above, the laser light wavelength 670n
The tilt allowable angle of a system using a pickup with m and numerical aperture NA = 0.6 is a very strict value of ± 0.2 degrees, but by applying this embodiment, it becomes ± 0.6 degrees.
It becomes possible to obtain the tilt margin equivalent to that of the CD system.

<Other Embodiments> The present invention is not limited to the above embodiments, and includes, for example, the following. (1) As the diffractive element, various elements such as a diffraction grating and a holographic element that have the above-described functions are included.
A beam having coma may be obtained by any method.

Various methods are known as methods for manufacturing the diffraction element. For example, a necessary lattice shape is obtained by numerical calculation, and based on the result, a lattice pattern is obtained by an electron beam drawing method similarly to the mask pattern formation used in the LSI manufacturing technology. Then, a mold is made by plating using this lattice pattern, and the replicas are mass-produced. As another method of obtaining a mask, a lattice pattern may be transferred to a resist applied on glass by a method similar to the LSI manufacturing technique, the glass may be etched to a predetermined depth, and then the resist may be removed. .

(2) In the above embodiment, the signals obtained by the respective beams are synchronized by using the delay circuit, but a memory, for example, may be used instead.
That is, the three reproduction signals of the beam spots SA to SC are respectively stored in the memory, and the signals are sequentially taken out from the memory to achieve synchronization. The minimum memory capacity is the amount corresponding to the reproduction time corresponding to the spot interval for the last spot. Even with this method, signals can be switched smoothly.

(3) The tilt sensor used in the LD may be used. Further, in the above embodiment, 1
Although the tilt information in two directions, radial and tangential, is obtained by one sensor, two tilt sensors for obtaining information in one direction may be provided. (4) The arrangement order of the diffracted light, the numerical values of the beam spot number, the numerical aperture, the tilt angle, the positive / negative of the tilt angle, and the like shown in the above embodiments are examples, and may be appropriately set as necessary.

[0050]

As described above, the optical pickup and the optical disk device according to the present invention have the following effects. (1) Since the aberration is given to the beam in advance to cancel the aberration caused by the tilt of the disc in the tangential direction, it is possible to reduce the influence of the tilt in the tangential direction and perform good signal detection. it can.

(2) Since the aberration is given to the beam in advance to cancel the aberration caused by the tilt of the disc in the tangential direction, a margin can be favorably dealt with by the tilt of the tangential direction with a simple device configuration. It is possible to improve the degree. (3) Furthermore, since the detection signals of a plurality of beams are synchronized, even if the beam detection output is switched, a good continuous signal can be obtained.

[Brief description of drawings]

FIG. 1 is a perspective view showing an embodiment of an optical pickup according to the present invention.

FIG. 2 is an explanatory diagram showing an outline of a pattern of the diffraction element of the example.

FIG. 3 is an explanatory diagram showing a positional relationship of beam spots on a bit string.

FIG. 4 is a graph showing the intensity distribution of each beam spot.

FIG. 5 is a block diagram showing an embodiment of an optical disc device.

FIG. 6 is a graph showing the relationship between tilt angle and coma aberration.

FIG. 7 is a graph showing the relationship between coma aberration and each beam spot in the example.

FIG. 8 is a graph showing a relationship between a tilt angle and an error rate at each beam spot.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 ... Optical pickup 12 ... Semiconductor laser element 16 ... Diffraction element (aberration giving means) 22 ... Disk 26 ... Photodetector (signal detection means) 30 ... Optical disk device 32 ... Radial tilt mechanism 34, 36 ... Delay circuit (signal synchronization means) ) 38 ... Signal switcher (signal selection means) 40 ... Spindle motor 42 ... Tilt sensor (tilt detection means) 44 ... Tilt detection circuit (tilt detection means) 46 ... Radial tilt drive circuit 48 ... Signal processing circuit SA ...- 1 Next time Beam spot of broken light SB ... Beam spot of 0th order diffracted light SC ... Beam spot of + 1st order diffracted light

Claims (3)

[Claims] 1. An optical pickup for irradiating a plurality of beams in a tangential direction of a track of an optical disc, wherein some of the plurality of beams are given an aberration which gives an aberration corresponding to a tilt of the optical disc in the tangential direction in advance. An optical pickup comprising means and signal detection means for detecting and converting each beam obtained from the optical disk. 2. An optical disc apparatus equipped with the optical pickup according to claim 1, further comprising: tilt detecting means for detecting a tilt in at least a tangential direction at a beam irradiation position of the optical disc,
Based on the tilt information detected by this, a detection signal of a beam having an aberration reverse to the aberration caused by the tangential tilt of the optical disc is selected from the detection signals of the plurality of beams output from the signal detecting means. An optical disk device comprising a signal selecting means. 3. The optical disk device according to claim 2, further comprising a signal synchronizing means for synchronizing the output signal of the signal detecting means by adjusting the time corresponding to the interval of each beam spot. Optical disk device.