JPH1116190A - Objective lens and optical head using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To accurately reproduce a CD and a DVD without losing a light quantity by adding a ring zonal phase shifter for reducing the aberration of convergent light spots having respective wavelengths together to an objective lens to be integrated as one body at the time of converging these laser beams having two wavelengths on the optical disks having their different substrates in the thickness. SOLUTION: This objective lens 1 for a DVD adds a doughnut-like ring zonal phase shifter area 101 to a normal DVD objective lens. This ring zonal phase shifter area 101 can mount a thin film or the lens can be directly worked in such a form from the first. Since the normal DVD lens is designed so as to be stigmatic when a substrate is 0.6 mm in the thickness, when the DVD is reproduced by a laser beam of a wavelength 650 nm, aberration to be added by this phase shifter is diminished as much as possible. On the other hand, when a CD having the substrate having 1.2 mm thickness is reproduced by a laser beam of a wavelength 780 nm, spherical aberration generated by the substrate thickness having 0.6 mm error is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical disk apparatus for optically reproducing information from an optical recording medium, and more particularly to an optical head for reproducing signals from optical disks having different substrate thicknesses by using light sources having different optical wavelengths. It relates to an objective lens used for it.

[0002]

2. Description of the Related Art Optical disks have been making remarkable progress as large-capacity interchangeable information recording media in recent years. For this reason, recording / reproducing methods, recording densities, and disk sizes are diversified, and it is becoming difficult to ensure their compatibility. In particular, the most widely used CD (Compact Disc) is a recordable CD that is compatible with the CD.
DR (Compact Disk-Recordable) is also incidentally popularized. In developing new optical disks, there is a great demand for compatibility with these CDs and CD-Rs. As a next-generation high-density ROM following these, recently DVD (Digital
Video Disk) has been released. Here, in order to improve the recording density, the numerical aperture of the objective lens (Numerical Aperture: N
A) is improved from 0.45 of the conventional CD to 0.6. When the wavelength of the laser light source used is λ, the size of the condensed spot on the optical disk is proportional to λ / NA. Therefore, the shorter the wavelength and the larger the NA, the smaller the light spot. If the light spot is small, high-density information pits can be reproduced with good quality, so that the recording density of the optical disk can be improved. Therefore, the DVD uses a semiconductor laser wavelength of 780 nm to 650 nm for CD. On the other hand, an increase in NA sharply increases coma aberration generated when the disk is tilted, and rather deteriorates the light spot, so that it cannot be performed unnecessarily. Therefore, the substrate thickness of DVD is from 1.2 mm to 0.6 m of CD.
m to suppress the coma due to the disk tilt as NA increases. However, if the thickness of the substrate is changed from that of a CD, a spherical aberration is generated when the CD is reproduced by an objective lens dedicated to DVD, and the light spot is blurred. This is because an objective lens for an optical disc is designed in advance so as to have a spherical aberration that compensates for a specific substrate thickness.

A conventional means for solving this problem is disclosed in, for example, Optical Review, Vol. 1, No. 1, 1994, 27-2.
Page 9 (Optical Review, Vol.1, No. 1 (1994) pp.27-2
9.) Here, a hologram is formed on the surface of the 0.6 mm objective lens, and the diffracted light is used to form a CD.
Is reproduced, and the DVD is reproduced by the transmitted light. Here, the hologram pattern is designed in advance so as to compensate for the spherical aberration that occurs when reproducing a CD. However, since a hologram is used in this case, a light spot for DVD is generated even when reproducing a CD, and a light spot for CD is generated when reproducing a DVD. Also, the light reflected by the disk is diffracted again. Due to these, there is a disadvantage that a loss of light quantity cannot be avoided.

A second conventional example is described in Mitsubishi Electric News Release, Development No. 9507 (June 21, 1995). In this method, both an objective lens for 0.6 mm and an objective lens for 1.2 mm are mounted on an optical head, and the two lenses are switched and used by a movable actuator. However, in this case, since two lenses are switched, there are problems such as an increase in cost due to the use of two lenses, reproducibility of lens positions, and deterioration of response characteristics due to a large and heavy actuator.

A third conventional example is Nikkei Electronics 19
No. 654, Jan. 29, 1996, pp. 15-16. Here, a limiting aperture made of liquid crystal is provided, and C
In reproducing D, the NA is reduced to 0.35 to reduce the aberration. However, since a semiconductor laser having a wavelength of 635 nm is used for both the CD and DVD,
Can be reduced to this point, but there is a drawback that this method cannot be used at the time of reproducing a CD-R in which the reflectance is significantly reduced with light having a wavelength shorter than 780 nm.

On the other hand, the present inventors have filed Japanese Patent Application No.
-342203, DVD and C at 650 nm wavelength
In order to make both D compatible, an invention is proposed in which the optimized substrate thickness is changed inside and outside the objective lens. However, when reproducing a CD at a wavelength of 780 nm, it is necessary to set the NA of this division to at least NA 0.45 or more. In this case, there is a disadvantage that aberrations when reproducing a DVD become extremely large. .

In view of this, an object of the present invention is to reproduce a CD having a substrate thickness of 1.2 mm with light of a wavelength of 780 nm accurately and inexpensively without loss of light amount, and to reproduce a CD of a substrate thickness of 0.6 mm with light of a wavelength of 650 nm. To play the DVD.

[0008]

In order to solve the above-mentioned problems, according to the present invention, when converging laser beams of two wavelengths onto optical discs having different substrate thicknesses, aberrations of condensed spots of respective wavelengths are obtained. Is added integrally with the objective lens.

Alternatively, an annular phase shifter for reducing the aberration of the condensed spot of one wavelength at least without changing the aberration of the condensed spot of one wavelength is added integrally with the objective lens.

[0010] Alternatively, an annular phase shifter for reducing both aberrations of a focused spot of laser light of two wavelengths is provided on a lens having different substrate thicknesses for focusing light without aberration on the inside and outside of the objective lens. To be added as one.

Alternatively, as an optical head, a semiconductor laser of two wavelengths, branching means for branching reflected light from the optical disk from an optical path from the semiconductor laser to the optical disk, and control of a condensing spot position based on the branched light. The above-described objective lens is used for condensing light of each wavelength on an optical disk having a different substrate thickness by an optical head comprising at least a detecting means for detecting a signal and a reproduction signal.

Alternatively, two semiconductor lasers having different wavelengths, an objective lens for condensing light of each wavelength on an optical disc having a different substrate thickness, and an optical path from the semiconductor laser to the optical disc for reflecting light from the optical disc. And a branching means for branching from the light source, and an optical head comprising at least a detecting means for detecting a focused spot position control signal and a reproduction signal from the reflected light branched by the branched means. An annular phase shifter that reduces both aberrations is added.

Alternatively, two semiconductor lasers having different wavelengths, branching means for branching reflected light from the optical disk from the optical path from the semiconductor laser to the optical disk, and a focused spot position based on the reflected light branched by the branching means In an optical head comprising at least a detection means for detecting a control signal and a reproduction signal, light of each wavelength is condensed on an optical disc having a different substrate thickness. And an annular phase shifter for reducing the aberration of the condensed spot of each wavelength.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a basic image diagram of an objective lens according to the present invention. Objective lens 1 for DVD according to the present invention
In a conventional DVD objective lens, a donut-shaped annular phase shift region 101 is added. The annular phase shift region 101 may be loaded with a thin film, or the lens may be directly processed into such a shape from the beginning. A normal DVD lens is designed to have no aberration when the substrate thickness is 0.6 mm.
When reproducing D, the aberration applied by the phase shifter is made as small as possible. On the other hand, when reproducing a CD having a substrate thickness of 1.2 mm with a laser beam having a wavelength of 780 nm, the spherical aberration generated due to a substrate thickness error of 0.6 mm is reduced.

Hereinafter, a description will be given of how the aberration is qualitatively reduced. FIG. 2 shows a schematic diagram of the wavefront shape of the spherical aberration when the focal position is optimized. Here, the horizontal axis is the radius coordinate of the pupil of the objective lens, and the vertical axis is the amount of wavefront aberration. Due to the difference in the substrate thickness between the CD and the DVD, the light spot when reproducing the CD with the DVD-only lens has a wavefront shape roughly represented by such a quartic function. On the other hand, FIG. 3 shows a schematic diagram of the wavefront shape when the phase shift is performed in an annular shape. It can be seen that the maximum value of the aberration is reduced by the phase shift.

On the other hand, using this lens, DVD
When reproducing DVD, the aberration of the DVD must not be large. One method for achieving this is to use the difference between the wavelength for reproducing a CD and the wavelength for reproducing a DVD so that the phase is shifted only in the CD and not in the DVD. To do so, set the CD playback wavelength to λ1, DVD
Assuming that the reproduction wavelength is λ2 and the phase difference generated during CD reproduction is φ,

[0018]

(Equation 1)

The integers m and n may be selected so as to satisfy the above. If there is no appropriate m or n, the phase shift may be performed as shown in FIG. In this case, the same wavefront shape as that of FIG. 3 can be realized by adding a phase shift of −φ to the region other than the annular region. So in this case,

[0020]

(Equation 2)

It suffices if the following condition is satisfied. Thus, for example, if λ1 is 780 nm and λ2 is 650 nm, the phase difference φ in each case is as shown in FIG. By selecting the phase difference in this way, it is possible to reduce spherical aberration during CD reproduction without affecting the wavefront of DVD at all. The name of the inverted orbicular zone phase shifter is a name in consideration of a case where a phase shift is realized by a phase delay such as when a film having a refractive index larger than that of air is added. When the phase shift can be realized by the phase advance such as the lens scratching, the annular zone may be directly scratched. These are equivalent in both cases, but hereinafter, including this case, will be referred to as an inverted orbicular zone phase shifter.

Hereinafter, the shape of the annular phase shifter and the optimization of the phase difference will be described. As an evaluation index of the light spot, there is a Strehl intensity which is a center intensity of a light spot having an aberration standardized by a center intensity of an aberration-free spot,
In this case, there is no difference in NA when there is a limiting aperture.
Therefore, the ratio of the spot center intensity to the total amount of light incident on the pupil of the objective lens is used as a new evaluation index even when there is a limiting aperture. When this is used, for example, N
The larger the A, the smaller the spot diameter and the larger the center intensity, the larger the evaluation index. This metric is

[0023]

(Equation 3)

As can be seen from the above, it is proportional to the product of the Strehl strength and the square of the limiting aperture radius R standardized by the total aperture radius of the lens. Hereinafter, a value obtained by multiplying the streak strength by the square of the standardized limiting opening radius is defined as η. Since a normal CD pickup has a wavelength of 780 nm and an objective lens NA of 0.45, if there is no aberration with respect to the objective lens NA of DVD of 0.6, η = 1 × (0.45 / 0.6) ＾ 2 = 0 .
56, η = 0.45 in the Strehl strength lower limit value 0.8 based on Marechal's criterion. The fourth order spherical aberration is caused by the substrate thickness error.

[0025]

(Equation 4)

The sixth order spherical aberration is

[0027]

(Equation 5)

Given by However, n in these equations represents a refractive index. Using these, the aberration added with an annular phase shifter that delays the phase from radius R1 to R2 by φ is

[0029]

(Equation 6)

It can be expressed as follows. The streak strength is

[0031]

(Equation 7)

R1, R2, φ that maximize η, NA of the limiting aperture, W20, W
Find 00. Actually, using mathematical formula processing software, W2
0 and W00 were obtained analytically, and R1, R2, φ, and the NA of the limiting aperture were numerically obtained. As a result, the inner diameter of the phase shifter is NA 0.20, the outer diameter is NA 0.42, and the NA of the limiting aperture is NA.
Is 0.46, and the phase difference is 0.265λ (λ = 780n
In the case of m), it was found that η = 0.48 was the maximum and exceeded η = 0.45 according to Marechal's criterion. On the other hand, when the phase shifter was not used and the optimization was performed only with the limiting aperture, the maximum was η = 0.34 at NA 0.39. In other words, when converted to NA of 0.45, the streak strength is 0.1.
This corresponds to an improvement from 61 to 0.86. The aberration generated at the time of DVD reproduction with respect to this phase difference was 0.033λ (λ = 650 nm) in RMS wavefront aberration. This is almost equivalent to the processing accuracy of the lens, and it is considered that no problem actually occurs.

When this optimum phase difference of 0.265λ is compared with the above-mentioned phase difference which does not affect the DVD, the closest phase difference is the reverse orbicular zone phase mask when m = 2 and n = 2, or m = 4, 0.333 of the annular phase mask when n = 3
It can be seen that it is λ. However, as m increases, the film or phase difference of the lens that causes a phase difference increases, and the phase difference shift increases when a wavelength shift occurs in the semiconductor laser. Therefore, the reverse annular phase mask is more preferable here. . When the shape of the phase shifter when fixed to a phase difference that does not affect the DVD is obtained, the inner diameter is NA 0.20,
When the outer diameter is NA 0.44 and the limiting aperture NA is 0.48, η
= 0.47 was the maximum. This is almost equal to the above-mentioned optimal phase difference.

The effect of reducing aberration when this annular phase shifter was applied to a DVD-only lens was confirmed by ray tracing. FIG. 6 shows the specifications and the surface shape of the lens used for ray tracing. Here, R of the surface shape is a central radius of curvature, k is a conic constant, A4, A6, A8, and A10 are quaternary, 6 respectively.
Next, the 8th and 10th order aspherical coefficients. The surface shape using these

[0035]

(Equation 8)

Can be expressed as follows. Here, a shape symmetric with respect to the optical axis is assumed, and r is a radial coordinate. When collimated light having a wavelength of 780 nm is condensed through a CD substrate thickness of 1.2 mm using this lens, the numerical aperture NA = 0.4
RMS (Root Mean Squar) in the range of 5
e) The wavefront aberration was 0.1279λ at the best image point. On the other hand, when the above-mentioned annular phase plate is added,
The RMS wavefront aberration was reduced to 0.07366λ. However, since the ray tracing software used at this time could not handle a phase difference of 1λ or more, the thickness of the phase plate was 0.3333λ corresponding to m = 1 and n = 1. Therefore DVD
Is necessarily 0, and the spot aberration at a wavelength of 650 nm through a DVD substrate thickness of 0.6 mm is 0.001λ or less.

Although the above description has been made on the premise that a limiting opening is used, this does not necessarily mean that an actual opening is required. Actually, it is considered that this is almost equivalent to designating an evaluation range of a pupil when obtaining an optimum focus position using the RMS wavefront aberration as an evaluation function. Assuming that the defocus is adjusted so that the RMS wavefront aberration is reduced as much as possible within the range of the limiting aperture, the light outside the range of the limiting aperture naturally has a large aberration and a large wavefront inclination. Thus, the light rays in such an area intersect the focal plane at a position far from the focal point. Therefore, it is almost equivalent to the absence of such a light beam with respect to the focused spot.

When only the annular phase shifter is used, the spot performance is improved, but the NA is 0.45.
If it is equivalent to 0.86 in terms of Strehl strength, there is a possibility that spot deterioration due to displacement of optical components, disc tilt, defocus, etc. is not necessarily sufficient.
Therefore, in combination with this, the substrate thickness to be optimized inside and outside the lens is changed. Hereinafter, this is referred to as a split lens. This was invented by the inventors as a method of making both DVD and CD compatible at a wavelength of 650 nm (Japanese Patent Application No. 7-342203). However, when reproducing a CD at a wavelength of 780 nm, the NA of this division must be at least NA0. In this case, there is a disadvantage that aberrations when reproducing a DVD become extremely large. Therefore, when the phase shifter and the split lens are combined and the phase shifter shape, phase difference, inner and outer split radius, and inner substrate thickness are simultaneously optimized, the wavelength 78 generated by the split lens is obtained.
Aberrations when reproducing CDs at 0 nm and D at a wavelength of 650 nm
It has been found that there is a solution that reduces both aberrations during VD reproduction and further improves the spot performance of the CD. This will be described below.

The wavefront aberration when the split lens and the phase shifter are combined is

[0040]

(Equation 9)

Can be expressed as follows. Here, R1 is the inner diameter of the annular phase shifter, R2 is the division radius, R3 is the outer diameter of the annular phase shifter, and R4 is the limiting aperture radius. The thickness of the disk substrate for achieving aberration-free at the division radius is different, and D
This is between 0.6 mm and 1.2 mm due to optimization, 0.6 mm for VD and inside. Accordingly, the aberration coefficients W60 and W40 of the spherical aberration are displayed differently with the suffixes 1 and 2 accordingly. The defocuses W201 and W202 are determined from the amount of spherical aberration so as to minimize the RMS wavefront aberration inside and outside the division, and the constant term W
001 and W002 are determined such that the average values of the wavefront aberrations inside and outside the division are the same, and the entire RMS wavefront aberration is optimized. The difference between W201 and W202 and between W001 and W002 is determined by the difference in the thickness of the corresponding substrate inside and outside the lens.
W201 and W001 were also obtained by analytically obtaining the RMS under the condition of the given phase shifter under the condition of the given phase shifter using mathematical processing software. Further, for the given inner corresponding substrate thickness and given division radius R2, R1, R3, R4,
The condition for maximizing η was obtained by numerically changing the phase difference.
FIG. 7 shows the result. Here, the horizontal axis is the division radius of the division lens, and the vertical axis is η, and the calculation results under the optimum conditions are plotted while changing the central substrate thickness. Also the CD in the graph
And the lower limit level equivalent to a Strehl strength of 0.8,
The above-mentioned optimal phase shifter and fixed phase shifter are indicated by broken lines. Since these do not use a split lens, they cannot be plotted on this graph as points. On the other hand, FIG. 8 shows the RMS wavefront aberration generated in DVD reproduction at this time.
As can be seen by comparing FIGS. 7 and 8, the closer the central substrate thickness is to 1.2 mm, the higher the performance of the CD and the greater the aberration of the DVD. Therefore, which of these points is to be adopted as the optimum point depends on the distribution of various margins of the system.
However, for example, the center substrate thickness is 0.76 mm,
At A0.45, CD performance η = 0.526 (CD Strehl intensity conversion of 0.94) and RMS wavefront aberration of DVD of about 0.0299λ are considered to be almost acceptable. At this time, if there is no phase shifter,
The RMS wavefront aberration of DVD was 0.031λ, and that of CD was η = 0.414. Therefore CD, D
It can be seen that the aberration is reduced by the phase shifter for both VD. Also at this point, the maximum value of CD performance,
The minimum values of the DVD aberration coincide. At this time, the phase difference of the annular phase shifter is 0.2985λ (λ = 780 nm),
The inner diameter was 0.2145 NA, and the outer diameter was 0.45 NA, which coincided with the NA at the division boundary.

FIG. 9 is a schematic diagram of a split lens provided with an inverted orbicular zone phase mask. Since the inverted annular mask is built into the lens, the zone of the annular phase mask is recessed. At this time, the step due to the split lens is also shown on the surface of the disk having a relatively gentle curvature, but it is also possible to design this only on the image side.

FIG. 10 shows the value of the CD reproduction spot performance η due to the shift of the CD reproduction wavelength. The range of the horizontal axis is ± 20 nm
Actually, the wavelength range that shifts due to temperature change is ± 1
It is considered to be about 0 nm. Within this range, the degradation is about η = 0.52 at a wavelength shift of −10 nm from η = 0.53, and the change is about 0.93 to 0.92 when converted to a Strehl intensity at NA of 0.45. Absent. In the figure, the optimum annular phase shifter and the fixed annular phase shifter described above are also shown.

FIG. 11 shows the RMS wavefront aberration with respect to the wavelength shift at the time of DVD reproduction at the wavelength of 650 nm. When the split lens and the optimal phase shifter are combined, the aberration is 0.03.
It increases from 0λ to 0.036λ at a wavelength shift of −10 nm. This is also considered sufficiently within the allowable range. In the figure, the optimum annular phase shifter and the fixed annular phase shifter described above are also shown. For the fixed phase shifter, a phase difference is selected so that no aberration occurs in the DVD. Therefore, the aberration is 0 at a wavelength shift of 0. The phase difference of only the optimal phase shifter deviates from the phase difference that causes no aberration in the DVD, so that the wavefront aberration linearly changes toward the wavelength shift amount that becomes the phase difference.

FIG. 12 shows the wavefront aberration shape at the time of CD reproduction at a wavelength of 780 nm. NA of each limiting aperture
Since the defocus is optimized in the range, and the horizontal axis is indicated by the radial coordinates of the pupil over the entire aperture of NA 0.6, the peripheral portion has a very large aberration. In this case, the vertical axis is ± 0.
Since the image is folded and displayed within the range of 5λ, the periphery appears to vibrate sharply. In these, the aberration is suppressed at a wider NA than in the case where optimization is performed only with the limiting aperture. In addition, the rise of the wavefront outside the range of the limiting aperture NA is also steep, and it is expected that the effect of the limiting aperture due to the large aberration will be more remarkable.

FIG. 13 shows wavefront aberrations during DVD reproduction at a wavelength of 650 nm. The wavefront aberration is completely zero when only the limiting aperture in FIG. 12 is used and when only the fixed phase shifter is used.
Therefore, here, the case where the split lens and the optimum phase shifter are combined and the case where only the optimum phase shifter is used are displayed. Since the aberration is not zero even at the outermost peripheral portion where no aberration occurs, it is understood that a slight defocus is caused as a whole. This is because, when the phase difference generated by the phase shifter is considered as an aberration, slightly defocusing reduces the entire RMS wavefront aberration. In any case, the value on the vertical axis of the graph is considerably small, and the peculiarity of the wavefront shape is suppressed to the RMS wavefront aberration that does not substantially affect the wavefront shape.

FIG. 14 shows the calculation results of the spot shape.
The horizontal axis of the graph indicates exponential versus peak intensity of the spot.
The full width of the spot of (-2) times the intensity, the vertical axis is the value obtained by normalizing the intensity of the side lobe with the central intensity. Therefore, it is desirable that both the spot and side lobe are small,
It can be said that the closer the plot point is to the lower left of the graph, the higher the resolution of the spot. Here, a symmetrical Gaussian distribution is assumed as the intensity distribution of the pupil of the objective lens, and the ratio of the lens aperture to the width of an intensity range of exp (-2) times the intensity of the center of the Gaussian distribution in the pupil is 0. 1. The intensity of the peripheral portion with respect to the intensity of the central portion of the lens is 0.9.
It is a calculation result when it is 8. The white circles in the figure are CDs having no aberration, and the closer to this, the same level of reproduction performance as CDs can be expected. The black square indicates the case where only the limiting aperture is used for a normal DVD lens. When the limiting aperture is actually inserted, when the limiting aperture is taken directly at the focal position, when the limiting aperture is taken, the spot center intensity is reduced. There are three plot points when the focal position is shifted so as to be the maximum. In each case, the spot resolution is inferior to that of the aberration-free CD. The black triangles indicate the case where only the optimum annular phase shifter is inserted, and there are three plot points in the same manner. Although the spot diameter is considerably improved as compared with the limiting aperture alone, the side lobe is considerably larger without the limiting aperture. The white squares represent the case where the split lens and the optimum annular zone phase shifter are combined. Similarly, there are three plot points, and it can be seen that these three points are quite close. In other words, in this case, there is almost no change whether or not there is a limiting aperture, and the aberration of light outside the range of the virtual limiting aperture sharply increases, so that spot formation is not substantially affected. You can see that.
In this case, the light spot has a slightly smaller spot diameter and a slightly larger side lobe than the CD no aberration.
The reason why the value of η, which is an evaluation index of spot performance, was almost equal to or slightly inferior to that of CD probably offset the effect of not reducing the side lobe by reducing the spot diameter. It is presumed that the situation has been reached. On the other hand, the calculation result of the spot when reproducing a DVD is plotted in the lower left of the graph by a white triangle and a diamond. The diamonds represent spots for reproducing a DVD with no aberration, and the triangles represent the case where the optimum division lens and the optimum phase shifter are combined. DVDs have almost the same spot shape.

FIG. 15 shows an embodiment of the optical head. The light from the semiconductor laser 4 is converted into parallel light by the collimator lens 5 and the elliptical beam is converted into a circular beam by the beam forming prisms 61 and 62. If the efficiency of the optical system is sufficiently high or the track pitch of the disk is wider than the distance between the main lobe of the light spot on the disk and the first dark line, removing the beam shaping prism can reduce the number of parts and reduce adjacent track crosstalk. In some cases, it may be advantageous. Further, this light passes through the beam splitter 71, is further reflected by the rising mirror 8, and is focused on the optical disk by the objective lens 3 according to the present invention mounted on the two-dimensional actuator 9. The optical disk may be a CD or a DVD. The two-dimensional actuator 9 moves in the disk radial direction according to the tracking error signal, positions the light spot on the track, moves in the optical axis direction according to the focus error signal, and positions the focus position on the disk. The reflected light is reflected by the beam splitter 71 again via the objective lens 3 and the rising mirror 8, and is guided to the detection optical system. The light transmitted through the beam splitter 72 is condensed by the condensing lens 111 and is incident on the beam splitter 73. Here, the transmitted light passes through the cylindrical lens 12 and enters the four-divided photodetector 13. The differential signal between the sum signals of the diagonal components of the split detector is output by the differential amplifier 141 to be a defocus signal. On the other hand, the light reflected by the beam splitter 73 is incident on the split photodetector 15, and the differential signal of each output is output by the differential amplifier 142 to obtain a tracking error signal. The light reflected by the beam splitter 72 is condensed on the photodetector 16 by the condensing lens 112 and the signal subjected to the photoelectric conversion is converted to an amplifier 17.
And a reproduced signal is obtained. The reproduction signal may be detected from the sum signal of the outputs of the servo signal detection detector. In this case, the signal detected up to the signal band may be band-limited by a low-pass filter or the like to detect the servo signal. The servo detection optical system is an example, and other methods can be used.

Although the embodiment in which the annular phase shifter is built in the objective lens has been described above, FIG.
This is an embodiment in which an objective lens 18 dedicated to VD and an independent annular phase shifter 19 are integrated into a hybrid and mounted on a two-dimensional actuator. Here, it is assumed that only a portion corresponding to the optical system from the rising mirror to the disk in FIG. 15 is replaced, and only that portion is shown.

[0050]

By optimally combining an annular phase shifter or an objective lens having an aberration-free substrate thickness inside and outside the lens, a DVD having a substrate thickness of 0.6 mm can be converted to a laser beam having a wavelength of 650 nm by a laser beam having a wavelength of 650 nm. A CD having a substrate thickness of 1.2 mm can be reproduced by a single lens with a laser beam without requiring a limiting aperture, and a small and inexpensive optical head can be provided.

[Brief description of the drawings]

FIG. 1 is a basic image diagram of an objective lens according to the present invention.

FIG. 2 shows a spherical aberration wavefront shape.

FIG. 3 is a wavefront aberration shape obtained by an annular phase shifter.

FIG. 4 is a wavefront aberration shape by an inverted orbicular zone phase shifter.

FIG. 5 shows a phase shift amount of a CD under a condition that does not affect a DVD.

FIG. 6 shows a DVD lens specification used for ray tracing.

FIG. 7 is a CD reproduction spot performance when a split lens and a phase shifter are combined.

FIG. 8 shows RMS wavefront aberration generated in DVD reproduction.

FIG. 9 is a schematic view of the shape of a divided lens in which an optimum inverted annular phase shifter is built.

FIG. 10 shows a change in CD reproduction spot performance due to a shift in CD reproduction wavelength.

FIG. 11 shows RMS wavefront aberration with respect to a wavelength shift during DVD reproduction.

FIG. 12 is a wavefront aberration shape during CD reproduction.

FIG. 13 is a wavefront aberration profile during DVD reproduction.

FIG. 14 is a calculation result of a spot shape.

FIG. 15 is an embodiment of an optical head.

FIG. 16 shows an embodiment in which the objective lens and the annular phase shifter are integrated in a hybrid.

[Explanation of symbols]

1} objective lens with an annular phase shifter, 101 ° annular phase shift area, 2 ° spherical aberration wavefront, 102 ° inverted annular phase shift area, 3 ′ inverted annular phase shifter integrated split lens, 4 ‥‥ semiconductor laser, 5 ‥‥ collimating lens, 61, 62 ‥‥ beam shaping prism, 71, 72,
73 ‥‥ beam splitter, 8 ‥‥ rising mirror, 9
{2D actuator, 10} Optical disk, 11
1, 112 condensing lens, 12 cylindrical lens, 13 quadrant detector, 141, 142
Differential amplifier, 15 ‥‥ split detector, 16 ‥‥ detector, 17 ‥‥ amplifier, 18 ‥‥ DVD objective lens, 19 対 物 ring phase shifter.

Claims (6)

[Claims] An objective lens for converging laser light of two wavelengths at different wavelengths through substrates having different thicknesses, wherein the objective lens has an annular shape for reducing both aberrations of converging spots of respective wavelengths. An objective lens, wherein a phase shifter is added integrally with the objective lens. 2. An objective lens for converging laser light of two wavelengths on substrates having different thicknesses at respective wavelengths, without changing at least aberration of a condensed spot of one wavelength. An objective lens, wherein an annular phase shifter for reducing aberration of a condensed spot of another wavelength is added integrally with the objective lens. 3. An objective lens for converging laser light of two wavelengths at different wavelengths through substrates having different thicknesses, wherein the objective lens condenses the laser light inside and outside the objective lens without aberration. An objective lens characterized in that an annular phase shifter for reducing aberrations of condensed spots of respective wavelengths is added to lenses having different thicknesses integrally with the lenses. 4. The objective lens according to claim 1, wherein the two semiconductor lasers having different wavelengths, and the light having the respective wavelengths are focused on optical disks having different substrate thicknesses, and reflected light from the optical disk. An optical head comprising at least a branching means for branching from a light path from the semiconductor laser to the optical disk, and a detecting means for detecting a focused spot position control signal and a reproduction signal from the reflected light branched by the branching means. . 5. An optical system comprising: two semiconductor lasers having different wavelengths; an objective lens for condensing light of each wavelength on an optical disk having a different substrate thickness; and an annular shape for reducing both aberrations of condensed spots of each wavelength. Phase shifter, branching means for branching the reflected light from the optical disk from the optical path from the semiconductor laser to the optical disk, and detecting the focused spot position control signal and the reproduction signal from the reflected light branched by the branching means. An optical head comprising at least the detecting means. 6. An optical system comprising: two semiconductor lasers having different wavelengths; and an objective lens having different substrate thicknesses for condensing light of the respective wavelengths on an optical disk having different substrate thicknesses without aberration between inside and outside. A ring-shaped phase shifter for reducing the aberrations of the condensed spots of the respective wavelengths, a branching unit for branching the reflected light from the optical disk from the optical path from the semiconductor laser to the optical disk, and a branching unit for branching the light. An optical head comprising at least detecting means for detecting a condensed spot position control signal and a reproduction signal from reflected light.