JP2006018969A - Optical pickup, its assembling method, and its inspection method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical pickup in which deterioration of a signal by influence of wave front aberration and astigmatism, and a good multi-level signal is obtained stably. <P>SOLUTION: This is an optical pickup performing one or more of recording, reproducing, and erasing of multi-level information for an information recording medium by level variation of strength of reflected light. The optical pickup is provided with an objective lens for converging light on the information recording medium, and has such constitution that when astigmatism of light fluxes before incidence on the objective lens of the optical pickup is assumed to W<SB>AS</SB>[λrms], and when astigmatism when parallel light fluxes having ideal wave front are transmitted through the objective lens single body and a substrate of the information recording medium is assumed to W<SB>OL</SB>[λrms], the light fluxes before incident on the objective lens having W<SB>AS</SB>and the objective lens single body having W<SB>OL</SB>and the light fluxes transmitted through the substrate of the information recording medium are constituted of an optical system satisfying a numerical formula. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

The present invention relates to an optical pickup, an assembly method thereof, and an inspection method thereof, and more particularly to an optical pickup that suppresses signal deterioration due to the effects of wavefront aberration and astigmatism and stably obtains a good multilevel signal.
The present invention also relates to an optical pickup assembly method and an optical pickup inspection method capable of simply assembling and inspecting the optical pickup.

<Background-densification>
In recent years, optical recording media such as CDs and DVDs are becoming popular as means for storing information. Further, there is a strong demand for further improvement in recording density and increase in capacity. Of the methods for increasing the recording density in this optical recording medium, the optical pickup is formed on the optical recording medium by means of shortening the wavelength of the light source, increasing the numerical aperture (NA) of the objective lens, or the like. The spot diameter can be reduced.

<Multi-value recording>
In addition, multi-level recording is one of the methods for achieving high density information and high speed transfer other than optical pickup. The recording method using phase change is generally binary recording in which information is recorded by the presence or absence of a recording mark. Multi-value recording is a method of recording a plurality of information in one recording unit (cell). Multi-level recording is performed by recording one mark in one cell and switching the mark length in multiple stages in the tangential direction.

That is, the multi-value information is reproduced by changing the ratio between the amorphous mark in one cell and the base of the crystalline state and detecting the change in the reflected light intensity. When such a multi-value recording method is used, it is possible to achieve higher density and higher speed transfer than binary recording.
The cell length of multilevel recording is about the shortest mark length of a binary mark. Therefore, since one cell is divided into, for example, eight mark lengths, the multi-value shortest mark is very small. Therefore, good spot performance is required.

<Wavefront aberration>
Wavefront aberration can be cited as an optical factor that degrades spot performance. However, since the value of wavefront aberration is inversely proportional to the wavelength, the allowable optical component accuracy and the margin of assembly error are reduced as the wavelength is shortened. In the case of multi-value recording, recording / reproduction is performed using a smaller mark than the binary mark. Therefore, a highly accurate optical pickup optical system is required by shortening the wavelength, increasing the NA, and increasing the number of signals.

  In an optical pickup, main wavefront aberrations that cause signal deterioration are spherical aberration, coma aberration, and astigmatism. Of these, astigmatism is a phenomenon in which the focal length of a light beam differs between the vertical direction and the horizontal direction of a plane perpendicular to the traveling direction of the light beam. The light fluxes at two different points on the principal ray passing through the center of the light flux are approximately line segments, which are called focal lines. The focal lines are perpendicular to each other, and there is a point where the luminous flux is circular between the focal lines. In general, astigmatism is known to occur when transmitting through a cylindrical convex lens or when refracting through a glass surface in a diverging light path and transmitting.

  The main causes of astigmatism, which is a problem with optical pickups, are the astigmatism of semiconductor lasers that are light sources, the accuracy of optical components, and assembly errors. Even within optical pickups that have the same optical system The direction and magnitude of each astigmatism will be different.

Conventionally, several proposals for correcting astigmatism have been made.
There is a proposal that minimizes signal degradation due to astigmatism by adjusting the rotation direction of an objective lens molded using a mold so that astigmatism is minimized (for example, Patent Document 1).

  There is also a proposal for correcting astigmatism of different directions and sizes using liquid crystals. That is, it is a proposal in which a phase difference based on a difference in refractive index is given by liquid crystal so as to cancel generated astigmatism (for example, Patent Document 2 and Patent Document 3).

Further, there is a proposal in which astigmatism is corrected by arranging a parallel plate obliquely in the diverging light path and rotating and adjusting the parallel plate so as to cancel astigmatism (for example, Patent Document 4).
Japanese Patent Publication No. 6-48543 Japanese Unexamined Patent Publication No. 2000-40249 JP 2000-67453 A JP 2000-222767

  By the way, in order to provide an optical pickup capable of recording and reproducing multi-value information, a highly accurate optical system is required. As a method of assembling and inspecting a high-precision optical system, there is a method of measuring the wavefront aberration and astigmatism of the focused spot of the optical system with an interferometer or the like. This method has an advantage that aberration can be reduced by assembling the optical component or the objective lens even when there is aberration in each optical component.

  However, since the above-described aberration reduction method is performed by adjusting the relative inclination and decentration of the light beam and the objective lens, the adjustment amount and each aberration are not independent, and multiple adjustments may be necessary. There is a problem that the converged state is not the best position and the minimum aberration cannot be reached. Even if the minimum aberration can be reached after multiple adjustments, if the assembled state has eccentricity or inclination that deviates significantly from the design center, the margin for actual drive operation will be reduced. There is also the inconvenience of becoming smaller.

  Further, at the production site, the wavefront is measured by using an interferometer as a means for inspecting the focusing spot performance of the pickup. An objective lens for an interferometer is required to measure the wavefront of the pickup condensing spot. This objective lens is difficult to manufacture and expensive because it requires extremely high surface accuracy and the wavefront aberration to be suppressed within a certain range so that the wavefront to be measured can be accurately measured. .

  Furthermore, since the structure and adjustment of the interferometer are complicated, precise adjustment is required and measurement error increases. Therefore, it can be said that it is an unsuitable inspection method at the production site where the pickup is assembled.

The problems (problems) of the prior art will be described in detail.
<Signal degradation due to astigmatism>
Among the wavefront aberration components, astigmatism in the 45 ° direction with respect to the radial direction of the information recording medium has a greater effect on spot performance degradation than astigmatism, coma and spherical aberration in the 0 ° direction. There is a problem of giving.

The following description will be made using an MTF (Modulation Transfer Function) generally used as a parameter for spot image evaluation.
FIG. 13 shows MTF calculation results when a single component aberration such as astigmatism in the 0 ° direction, astigmatism in the 45 ° direction, spherical aberration, and coma aberration is given to the spot with respect to the radial direction of the disk. FIG. The wavelength is 400 nm, and the NA of the objective lens is 0.65.

MTF indicates a spatial frequency value of 1800 lp / mm corresponding to a mark of 0.275 μm as the cell length of multilevel recording. Compared to 0 ° astigmatism, spherical aberration, and coma with respect to the radial direction of the information recording medium, astigmatism in the 45 ° direction significantly reduces MTF and causes spot performance degradation. .
As described above, astigmatism in the 45 ° direction among wavefront aberrations has a problem that it has a large influence on spot performance deterioration compared to other types of aberration components.

<Spot astigmatism measurement is difficult>
There is a problem that it is difficult to measure the wavefront of the focused spot during the assembly and inspection process of the pickup.
<Multi-value recording>
In the case of multilevel recording, there is a problem that a smaller mark has to be recorded and reproduced as compared with conventional binary recording.

The present invention has been made to solve the above-described problems, and an object of the present invention is to provide an optical pickup that suppresses signal deterioration due to the effects of wavefront aberration and astigmatism and stably obtains a good multilevel signal.
It is another object of the present invention to provide an optical pickup assembly method and an optical pickup inspection method capable of simply assembling and inspecting the optical pickup.

In order to achieve this object, the invention according to claim 1 is an optical pickup that performs one or more of recording / reproducing / erasing of multi-value information on an information recording medium by changing a level of reflected light intensity.
An objective lens for focusing on the information recording medium,
The astigmatism of the light beam before incidence of the objective lens of the optical pickup is W _AS [λrms], and the astigmatism when the parallel light beam having an ideal wavefront is transmitted through the objective lens alone and the substrate of the information recording medium. When W _OL [λrms],
The light beam before the objective lens incident with the W _AS,
The objective lens alone having the W _OL and the light flux that passes through the substrate of the information recording medium,
The optical system that satisfies Equation 1 is used.

In this way, since it is constituted by an optical system and the objective lens alone before the objective lens incident with W _AS and W _OL satisfying Equation 1, even when the astigmatism of different directions has occurred, A good multilevel signal quality can always be obtained stably.

The invention according to claim 2 is the optical pickup according to claim 1,
The wavefront aberration of the light beam before entering the objective lens of the optical pickup is TW [λrms], and the wavefront aberration that the parallel light beam having an ideal wavefront has when transmitted through the objective lens alone and the substrate of the information recording medium is TW _OL [ λrms]
The luminous flux before incidence of the objective lens having TW,
The objective lens alone having the TW _OL and the light flux that passes through the substrate of the information recording medium,
It is configured as an optical system that satisfies Equation 2.

In this way, since the optical system before the entrance of the objective lens having TW and TW _OL satisfying Equation 2 and the objective lens alone are configured, it is possible to always stably obtain a good multilevel signal quality.

According to a third aspect of the present invention, in the optical pickup according to the first aspect,
The optical pickup includes a beam shaping optical system,
The beam shaping direction of the beam shaping optical system is different from the radial direction of the information recording medium by 45 °.

  In this way, since the beam shaping direction and the radial direction of the information recording medium are configured differently, astigmatism in the 45 ° direction can be corrected without using an astigmatism correction element, An increase in the number of parts and an increase in cost can be suppressed, and good multilevel signal quality can be stably provided.

The invention according to claim 4 is the optical pickup according to claims 1 to 3,
The rotational position around the optical axis of the objective lens is set so that the astigmatism of the focused spot is minimized or less than the predetermined value 0.025.

  In this way, by adjusting the direction of rotation of the objective lens, the astigmatism of the light beam before entering the objective lens is corrected by the astigmatism of the objective lens. Point aberration can be corrected. Therefore, an increase in the number of parts and an increase in cost can be suppressed, and good multilevel signal quality can be stably provided.

The invention according to claim 5 is the optical pickup according to claims 1 to 4,
The optical pickup includes a means for correcting at least one of spherical aberration, coma aberration, and astigmatism.

  In this way, since a means for correcting at least one of spherical aberration, coma and astigmatism is provided, a margin for optical components and assembly accuracy can be widened, and a good multi-value signal can be obtained. Quality can be provided stably.

According to a sixth aspect of the present invention, there is provided an optical information processing apparatus for performing one or more of recording / reproducing / erasing of multi-value information by irradiating a recording surface of an information recording medium with a light beam.
An optical information processing apparatus has the optical pickup according to any one of claims 1 to 5.

  According to this configuration, since the optical pickup according to any one of claims 1 to 5 is mounted, an optical information processing apparatus capable of stably obtaining good multilevel signal quality can be provided.

The invention described in claim 7 is an inspection method or an assembling method of the optical pickup according to claims 1 to 5 and the optical information processing apparatus according to claim 6,
By detecting the wavefront aberration of the light beam transmitted through the objective lens alone and the substrate of the information recording medium, detecting the wavefront aberration of the light beam before incidence on the objective lens, and determining whether or not Expression 1 or Expression 2 is satisfied, This is a method for inspecting or assembling an optical pickup.

The invention described in claim 8 is the optical pickup inspection method or the optical pickup assembly method described in claim 7,
Optical pickup inspection method or optical pickup combination using an interferometer for detecting wavefront aberration of an incident lens incident light beam and an interferometer for detecting wavefront aberration of a light beam transmitted through the objective lens and the substrate of the information recording medium There is a method of attaching.

  According to the inventions of claims 7 and 8, the optical pickup and the information recording apparatus according to claims 1 to 6 separately inspect the wavefront of the light beam before incidence on the objective lens and the transmitted wavefront of the objective lens alone. Aberration measurement during the manufacturing process of the pickup can be easily performed with a simple configuration. Furthermore, the objective lens can be assembled without greatly deviating from the design center, and the margin during operation of the information recording / reproducing apparatus can be widened.

According to the present invention, since it is constituted by an optical system and the objective lens alone before the objective lens incident with W _AS and W _OL satisfying Equation 1, if the astigmatism of different directions has occurred However, it is always possible to stably obtain a good multilevel signal quality.

According to the invention of claim 2, since it is constituted by the optical system before the entrance of the objective lens having TW and TW _OL satisfying Formula 2 and the objective lens alone, it is possible to always obtain a good multi-level signal quality stably. it can.

  According to the invention of claim 3, since the beam shaping direction is different from the radial direction of the information recording medium, astigmatism in the 45 ° direction can be corrected without using an astigmatism correction element. Therefore, the increase in the number of parts and the cost can be suppressed, and good multilevel signal quality can be stably provided.

  According to the invention of claim 4, by adjusting the rotation direction of the objective lens, the astigmatism of the light beam before entering the objective lens is corrected by the astigmatism of the objective lens. Astigmatism can be corrected. Therefore, an increase in the number of parts and an increase in cost can be suppressed, and good multilevel signal quality can be stably provided.

  According to the invention of claim 5, since the means for correcting at least one of spherical aberration, coma aberration, and astigmatism is provided, a margin for optical components and assembly accuracy can be widened, and the device is excellent. Multi-level signal quality can be provided stably.

  According to the sixth aspect of the invention, since the optical pickup according to the first to fifth aspects is mounted, an optical information processing apparatus capable of stably obtaining good multilevel signal quality can be provided. .

  According to the inventions of claims 7 and 8, the optical pickup and the information recording apparatus according to claims 1 to 6 separately inspect the wavefront of the light beam before incidence on the objective lens and the transmitted wavefront of the objective lens alone. Aberration measurement during the manufacturing process of the pickup can be easily performed with a simple configuration. Furthermore, the objective lens can be assembled without greatly deviating from the design center, and the margin during operation of the information recording / reproducing apparatus can be widened.

Hereinafter, the present invention will be described based on the illustrated embodiments.
[First Embodiment]
<Optical system>
FIG. 1 is a configuration diagram of a “schematic optical system of a first optical pickup” of the present embodiment.

  As shown in FIG. 1, a schematic illumination optical system SK1 of the first optical pickup includes a semiconductor laser 1, a collimating lens 2, a beam shaping optical system 3, a diffraction grating 4, a chromatic aberration correcting element 5, and a λ / A four-plate 6, an objective lens 7, a polarization beam splitter 9, a detection lens 11, a cylindrical lens 12, and a photodetector 13 are provided. Reference numeral 8 denotes an optical recording medium (information recording medium).

  The light emitted from the semiconductor laser 1 is made into substantially parallel light by the collimating lens 2. The collimating lens 2 is preferably corrected for chromatic aberration. The light emitted from the collimating lens 2 enters the beam shaping optical system 3. The beam shaping optical system 3 is arranged with a pair of prisms and the like decentered, and does not act on the light flux in the direction perpendicular to the paper surface, but expands the light flux in the paper surface direction.

  The position of the collimating lens 2 is adjusted so that the emitted light of the beam shaping optical system 3 relaxes the astigmatic difference. The beam-shaped light enters the polarization beam splitter 9 and the chromatic aberration correction element 5. The chromatic aberration correction element 5 has a function of correcting the movement of the condensing position of the objective lens 7 due to wavelength variation. Then, by focusing through the objective lens 7, recorded information is formed and reproduced on the optical recording medium 8.

  The reflected light from the optical recording medium 8 passes through the objective lens 7 and the λ / 4 plate 6, is then separated from the incident light by the polarization beam splitter 9, is deflected, and is divided into four-divided light receiving elements by the detection lens 11 and the cylindrical lens 12. (Photodetector) 13 is guided to detect a reproduction signal, a focus error signal, a track error signal, and the like. The recording / reproducing wavelength is 400 nm, the numerical aperture NA of the objective lens is 0.65, and the spot diameter on the optical information medium is about 0.54 to 0.55 μm.

<Optical recording medium>
The optical recording medium 8 of the present embodiment is a phase change type capable of recording in a blue region. The substrate is made of polycarbonate having a diameter of 120 mm and a thickness of 0.6 mm, and a groove is formed on the surface of the substrate by injection molding. The track pitch is 0.46 μm. On this substrate, a dielectric film, a phase change recording film made of Ge—Sb—Te, a dielectric film, and a reflective film were sequentially laminated to produce an optical recording medium 8.

<Multi-level playback signal>
As shown in FIG. 2, the reproduction of the multi-value data is performed by sampling the reproduction signal at a predetermined frequency (for example, the center position of the recording cell) and discriminating the multi-value data from the sampled signal level. Items for evaluating the signal of this data are RF amplitude and σ / DR. Here, σ is a value obtained by averaging deviations in reflected light intensity for each multi-value data calculated from the reproduction result, and DR is a difference between the maximum reflected light intensity and the saturated reflected light intensity.

<Relationship between astigmatism and playback signal σ / DR>
It has also been required from experiments that astigmatism in the direction of 45 ° with respect to the radial direction of the optical disk has a significant effect on signal degradation.
An example is shown in FIGS. 3 is a diagram showing the relationship between spot astigmatism and σ / DR, and FIG. 4 is a diagram showing the relationship between spot wavefront aberration and σ / DR. The direction of the astigmatism focal line is 45 ° with respect to the radial / tangential direction. The multi-value cell length is 0.32 μm, and the spot diameter is approximately equal to about 0.54 μm.

FIG. 5 shows the spot intensity distribution in dB when there is no astigmatism and when the astigmatism in the 45 ° direction is 0.05 λrms. FIG. 5 shows that the spot diameters are the same regardless of the magnitude of astigmatism in the 45 ° direction, so that the spot performance does not appear to deteriorate at first glance. However, as can be seen from FIG. 3 and FIG. The effect of directional astigmatism on signal degradation is very large.
Therefore, in order to obtain a good multilevel signal, not only the value of the wavefront aberration but also its component is important.

<Aberration value limitation>
Therefore, the limit value of aberration for obtaining a stable and good multilevel reproduction signal is defined by wavefront aberration and astigmatism in the 45 ° direction. The astigmatism value is 0.025λ rms or less where σ / DR is less than or equal to the allowable value from FIG. Similarly, the wavefront aberration value at this time is 0.05 λrms from FIG. This allowable value is a σ / DR value (1.8%) at which the error rate after correction by the equalizer is generally less than 10 ⁻³ , which can be reproduced. As described above, when the astigmatism of the spot is 0.025 λrms or less and the wavefront aberration is 0.05 λrms or less, a good multilevel signal can always be obtained.

<Inspection method>
An inspection method for satisfying the aberration value of the focused spot to be less than 0.05λrms wavefront aberration and 0.025λrms astigmatism is shown. As described above, the wavefront measurement of the focused spot of the optical pickup is complicated in configuration and adjustment, easily causes measurement errors, and is not suitable for the manufacturing site.

On the other hand, the wavefront measurement of a parallel light beam such as a light beam before incidence on the objective lens does not require the use of an objective lens for the interferometer, and the configuration and adjustment of the interferometer are very easy, and the measurement error is small. Further, the transmission wavefront measurement of the objective lens alone and the substrate of the information recording medium (optical recording medium) is inspected at the time of manufacturing the objective lens, and an objective lens within a certain specification value is manufactured.
Therefore, the pick-up inspection or assembly may be performed by determining whether or not the light beam before incidence on the objective lens satisfies the value obtained by subtracting the specified aberration value of the objective lens alone from the condensing spot aberration limit value.

In other words, the wavefront aberration of the converging spot, the transmission wavefront aberration of the objective lens and the information recording medium TW _OL, astigmatism W _OL, the wavefront aberration of the objective lens incident light beam TW, and astigmatism W _AS At this time, the pickup may be inspected or assembled by determining whether or not the following expressions 1 and 2 are satisfied. When Expressions 1 and 2 are not satisfied, it is determined as defective and reassembly is performed or parts are replaced.

The wavefront aberration measurement method will be described below.
FIG. 6 is a block diagram of a measurement system (wavefront measurement evaluation system) that inspects the wavefront of the optical pickup before the incident light beam incident on the objective lens.
This wavefront measurement evaluation system is a system that measures a substantially parallel light beam by a Mach-Cender interferometer, and can measure the blue laser diode 1A of the pickup as a light source of the interferometer (wavefront measuring device) 20.

The light beam before incidence on the objective lens of the optical pickup is made incident on the interferometer 20.
This parallel light is amplitude-divided by a half mirror 21, and one part is taken out by a pinhole 22 and used for an ideal reference wavefront. The other is the wavefront to be detected. Interference fringes 23 between the reference wavefront and the test wavefront of the pickup are captured in an image sensor 24 such as a CCD camera. The phase is obtained from the intensity distribution of the interference fringes 23, and TW and _WAS are calculated using Zernike polynomials for the wavefront aberration.

FIG. 7 is a diagram showing a measurement arrangement for inspecting the transmitted wavefront of the objective lens and the substrate. In the case of an infinite objective lens, a plane wave emitted from the interferometer is incident on the lens 31 to be examined, and the light beam that has passed through the lens 31 is folded back by the reference spherical surface 32. Interference fringes 23A with the light beam folded from the reference plane 33 are taken into the image sensor 24A, and the transmitted wavefront aberrations TW _OL and W _OL are measured. Reference numeral 34 denotes a diverging lens.

  As shown in FIG. 8, in the multilayer optical information medium 8A, it is necessary to satisfy the above-described Equations 1 and 2 on each of a plurality of recording surfaces. 8Aa is an intermediate layer.

By adopting such an inspection or assembly process, it is not necessary to measure the complicated wavefront of the focused spot twice, that is, the inspection process of the objective lens alone and the pickup manufacturing process.
In other words, at the time of pick-up inspection or assembling for determining whether a good spot performance, measured only TW and W _AS pickup parallel beam, it is sufficient to determine whether it satisfies the Equations 1 and 2, Assembly, adjustment, and inspection are facilitated, and an optical pickup that stably obtains a good signal even with astigmatism in any direction can be provided.

  In addition, by separately inspecting the objective lens and the other optical system, the objective lens can be assembled without greatly deviating from the design center, and the margin during operation of the information recording / reproducing apparatus can be widened. .

[Second Embodiment]
In the first embodiment, correction of at least one of astigmatism, spherical aberration, and coma caused by using a light source or an optical component with poor wavefront accuracy or an assembly error so as to be equal to or less than an aberration limit value. Introduce means.

FIG. 9 is a configuration diagram of a “second optical pickup schematic optical system SK2” that is a typical configuration example. In the following description of each embodiment, parts that have already been described are denoted by the same reference numerals, and redundant description will be omitted.
As the astigmatism aberration correcting element 5A, a liquid crystal is used. Further, astigmatism can be corrected by inserting the parallel plate 41 in the diverging optical path and tilting it with respect to the optical axis.

  Examples of coma aberration correcting means include means using liquid crystal or correcting by tilt of an objective lens mounted on a four-axis actuator. Examples of the spherical aberration correcting means for correcting the spherical aberration occurring due to the substrate thickness error or multilayers include a liquid crystal and an expander. Even when recording or reproducing a disk or a multilayer disk having an error in the substrate thickness, a stable and good signal can be obtained by setting the spherical aberration to a minimum or satisfying the mathematical expression 2.

[Third Embodiment]
In this embodiment, astigmatism is corrected by adjusting the rotation direction of the objective lens around the optical axis. The wavefront measuring means shown in the first embodiment can measure not only the magnitude of astigmatism but also the direction. The astigmatism of the spot can be suppressed by shifting the astigmatism generation direction of the light beam before incidence of the objective lens and the generation direction of the transmission light beam of the objective lens and assembling the objective lens to the optical pickup.

FIG. 10 shows the relationship between σ / DR and the relative angle between the astigmatism direction of the objective lens and the astigmatism direction of the light beam before entering the objective lens. The astigmatism of the objective lens is 0.018 λrms, and the astigmatism of the incident light beam is 0.01 λrms.
This astigmatism correction method is different from the decentration and tilt adjustment of the objective lens for adjusting the rotation direction with respect to the optical axis, and is independent of coma aberration and spherical aberration. Astigmatism can be corrected without shifting from the center of the assembly position.

  In addition, there is an advantage that aberration can be reduced by assembling an objective lens whose generation direction is shifted without performing wavefront measurement of the focused spot.

[Fourth Embodiment]
This embodiment is a case of an optical system in which the beam shaping direction and the radial direction of the information recording medium are 45 ° in the beam shaping optical system.
The beam shaping optical system can control astigmatism in the beam shaping direction and the non-beam shaping direction by adjusting the focal direction of the collimating lens. This is because the focal position of the square of the beam shaping magnification m in the non-shaping direction varies in the beam shaping direction with respect to the collimating lens moving distance. The present embodiment utilizes such a principle.

  By adjusting the collimating lens, astigmatism in the beam shaping direction can be reduced, so that the astigmatism in the 45 ° direction with respect to the remaining beam shaping direction can be reduced in the information recording medium (optical recording medium). An optical system such as “schematic optical system SK3 of the third optical pickup” shown in FIG.

  The deflection mirror 42 is tilted so that the x axis is 45 ° with respect to the rotation center 43 and the z axis is 22.5 ° with the rotation center 43 as the center. By adopting such a configuration, astigmatism in the 45 ° direction, in which signal deterioration is large due to the collimating lens adjustment in the beam shaping optical system 3, can be reduced without using an astigmatism correction element, and a good multilevel signal can be obtained. Obtainable.

[Fifth Embodiment]
The optical reproducing apparatus and optical recording / reproducing apparatus provided with the above-described optical pickup will be described with reference to FIG. 12, which is a schematic configuration diagram of the optical recording / reproducing apparatus.

  As shown in FIG. 12, the optical recording / reproducing apparatus 50 is roughly constituted by a spindle motor 51, a feed motor 52, an optical pickup apparatus 53, and the like, which are controlled by a system controller 54 that controls the entire optical recording / reproducing apparatus. . The movement of the optical pickup device 53 in the tracking direction is performed by a control drive unit including a guide mechanism (not shown) and a feed motor 52 including a linear motor or the like.

  For example, when reproducing the optical recording medium 8 chucked by the spindle motor 51, a control signal from the system controller 54 is supplied to the servo control circuit 55 and the modem circuit 56. In the servo control circuit 55 to which the control signal is supplied, the spindle motor 51 is rotated at the rotation speed set to the focusing pull-in state and the feed motor 52 is driven, so that the optical pickup device 53 is moved to, for example, the inner circumference of the optical recording medium 8. Move to the side. In the optical pickup device 53 moved to the inner peripheral side of the optical recording medium 8, focusing servo is applied by a focus search operation, and tracking servo is applied later.

  The focusing error signal, the tracking error signal, the position information of where the optical recording medium 8 is read out, and the like detected by the photodetector constituting the optical pickup device 53 are supplied to the modulation / demodulation circuit 56. Of these, the focusing error signal and the tracking error signal are filtered and supplied to the servo control circuit 55 via the system controller 54 as a focusing control signal and a tracking control signal.

  The servo control circuit 55 drives, for example, a focusing coil of a biaxial actuator that constitutes the optical pickup device 53 by the focusing control signal, and drives a tracking coil of the biaxial actuator that constitutes the optical pickup device 53 by the tracking control signal.

  The low frequency component of the tracking control signal is supplied to the servo control circuit 55 via the system controller 54 and drives the feed motor 52. As a result, feedback servo of focusing servo, tracking servo, and feed servo is performed.

  Further, the position information of where the optical recording medium 8 is read is processed by the modulation / demodulation circuit 56, supplied to the spindle motor 51 as a spindle control signal, and the reproduction position of the optical recording medium 8 chucked by the spindle motor 51. The motor is controlled and driven at a predetermined rotational speed according to the above, and actual reproduction is started from here. The reproduction data processed and demodulated by the modem circuit 56 is transmitted to the outside via the external circuit 57.

  For example, when recording external data supplied from the outside onto the optical recording medium 8 chucked by the spindle motor 51, the same process as the reproduction is performed until the feedback servo of the focusing servo, tracking servo and feed servo is applied. .

  From the system controller 54, a control signal indicating where the input data input via the external circuit 57 is recorded on the optical recording medium 8 is supplied to the servo control circuit 55 and the modem circuit 56. The servo control circuit 55 controls the spindle motor 51 to a predetermined rotational speed and drives the feed motor 52 to move the optical pickup device 53 to the information recording position.

  The input signal input to the modulation / demodulation circuit 56 via the external circuit 57 is modulated based on the recording format in the modulation / demodulation circuit 56 and supplied to the optical pickup device 53. In the optical pickup device 53, the modulation of the emitted light based on the modulation signal and the emitted light power based on the information recording position are controlled to irradiate the optical recording medium 8, and recording on the optical recording medium 8 is started.

  If the pickup of the present invention is configured in an optical pickup device 53 provided in a reproduction-only optical reproduction device and an optical recording / reproduction device capable of both recording and reproduction, multi-value information can be supplied stably.

1 is a configuration diagram of a schematic optical system of a first optical pickup according to a first embodiment of the present invention. It is a figure explaining the method to discriminate | determine multi-value data from the sampled signal level in the said 1st Embodiment. It is a figure which shows the relationship between the astigmatism of a spot and (sigma) / DR in the said 1st Embodiment. It is a figure which shows the relationship between the wavefront aberration of a spot, and (sigma) / DR in the said 1st Embodiment. It is a figure which shows spot intensity distribution by dB display in the case where there is no astigmatism and the astigmatism of a 45 degree direction is 0.05 (lambda) rms in the said 1st Embodiment. It is a figure which shows the measurement system (wavefront measurement evaluation system) which test | inspects the wave front before the objective lens incident light beam of the optical pick-up in the said 1st Embodiment. It is a figure which shows the measurement arrangement | positioning which test | inspects the transmitted wave front of an objective lens and a board | substrate in the 1st Embodiment. It is a figure explaining the case where the optical recording medium is a multilayer optical information medium in the first embodiment. It is a block diagram of the schematic optical system of the 2nd optical pick-up of 2nd Embodiment of this invention. It is a figure which shows the relative angle of the astigmatism direction of an objective lens in the 3rd Embodiment of this invention, the astigmatism direction of the light beam before objective lens incidence, and the relationship of (sigma) / DR. It is a figure which shows the schematic optical system of the 3rd optical pick-up of 4th Embodiment of this invention. It is a schematic block diagram of the optical recording / reproducing apparatus of 5th Embodiment of this invention. It is a figure which shows the calculation result of MTF when a single component aberration such as astigmatism in 0 ° direction, astigmatism in 45 ° direction, spherical aberration and coma aberration is given to the spot with respect to the radial direction of the disc. .

Explanation of symbols

SK1 to SK3 Schematic illumination optical system 1 of first to third optical pickups 1 Semiconductor laser 1A Blue laser diode 2 Collimator lens 3 Beam shaping optical system 4 Diffraction grating 5 Chromatic aberration correction element 5A Astigmatism aberration correction element 6 λ / 4 Plate 7 Objective lens 8 Optical recording medium (information recording medium)
8A Multilayer optical information medium 8Aa Intermediate layer 9 Polarizing beam splitter 11 Detection lens 12 Cylindrical lens 13 Photo detector 20 Interferometer (wavefront measuring device)
21 Half mirror 22 Pinhole 23, 23A Interference fringes 24, 24A Image sensor 31 Test lens 32 Reference spherical surface 33 Reference plane 34 Divergent lens 41 Parallel plate 42 Deflection mirror 43 Center of rotation 50 Optical recording / reproducing device 51 Spindle motor 52 Feed motor 53 Optical pickup device 54 System controller 55 Servo control circuit 56 Modulation / demodulation circuit 57 External circuit

Claims (8)

In an optical pickup that performs one or more of recording / reproducing / erasing multi-value information on an information recording medium by changing the level of reflected light intensity,
An objective lens for focusing on the information recording medium,
The astigmatism of the light beam before incidence of the objective lens of the optical pickup is W _AS [λrms], and the astigmatism when the parallel light beam having an ideal wavefront is transmitted through the objective lens alone and the substrate of the information recording medium. When W _OL [λrms],
The light beam before the objective lens incident with the W _AS,
The objective lens alone having the W _OL and the light flux that passes through the substrate of the information recording medium,
An optical pickup comprising an optical system that satisfies Formula 1.

The optical pickup according to claim 1,
The wavefront aberration of the light beam before entering the objective lens of the optical pickup is TW [λrms], and the wavefront aberration that the parallel light beam having an ideal wavefront has when transmitted through the objective lens alone and the substrate of the information recording medium is TW _OL [ λrms]
The luminous flux before incidence of the objective lens having TW,
The objective lens alone having the TW _OL and the light flux that passes through the substrate of the information recording medium,
An optical pickup comprising an optical system that satisfies Formula 2.

The optical pickup according to claim 1,
The optical pickup includes a beam shaping optical system,
An optical pickup characterized in that a beam shaping direction of the beam shaping optical system and a radial direction of the information recording medium are different by 45 °. In the optical pickup according to claims 1 to 3,
An optical pickup characterized in that a rotational position around the optical axis of the objective lens is set so that an astigmatism of a condensing spot is minimized or less than the predetermined value 0.025. In the optical pickup according to claims 1 to 4,
The optical pickup is provided with means for correcting at least one of spherical aberration, coma aberration, and astigmatism. In an optical information processing apparatus that performs one or more of recording / reproducing / erasing multi-value information by irradiating a recording surface of an information recording medium with a light beam,
An optical information processing apparatus comprising the optical pickup according to claim 1. A method for inspecting or assembling the optical pickup according to any one of claims 1 to 5 and the optical information processing device according to claim 6,
By detecting the wavefront aberration of the light beam transmitted through the objective lens alone and the substrate of the information recording medium, detecting the wavefront aberration of the light beam before incidence on the objective lens, and determining whether or not Expression 1 or Expression 2 is satisfied, An inspection method for an optical pickup or an assembly method for an optical pickup, characterized by inspecting or assembling the optical pickup. An inspection method for an optical pickup or an assembly method for an optical pickup according to claim 7,
An inspection method for an optical pickup, comprising: an interferometer that detects wavefront aberration of an incident light beam incident on an objective lens; and an interferometer that detects wavefront aberration of a light beam transmitted through the objective lens and the substrate of the information recording medium. Or how to assemble an optical pickup.

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