Optical head for adjusting a positional relation between the information medium and the optical head

An optical head apparatus consists of a semiconductor laser for radiating a light beam, a first collimator lens for collimating the light beam, a wedge-like prism for reshaping the light beam, a beam splitter for transmitting the light beam in an outgoing path and splitting the light beam in an incoming path, an objective lens for converging the light beam at an information medium in which information is stored, a plane type of hologram lens integrally formed with the objective lens for excessively correcting chromatic aberration of the objective lens to cancel out chromatic aberration of the collimator lens, a second collimator lens for converging the light beam split, a photo detector for detecting intensity of the light beam converged to obtain an information signal and servo signals, and an actuating unit for slightly moving the objective lens and the hologram lens according to the servo signals. A wavelength of the light beam is lengthened as intensity of the light beam is increased. Even though an astigmatic difference occurs in the semiconductor laser, the change of a focal length of the collimator lens cancels out the change of the astigmatic difference to prevent the occurrence of astigmatic aberration.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an image-formation optical system in which 
a light beam is optically converged at an information medium such as an 
optical medium or a magneto-optical medium like an optical disk or an 
optical card, an optical head apparatus in which information is written in 
the information medium with the image-formation optical system and the 
written information is read out or erased, and an optical information 
apparatus in which a positional relation between the information medium 
and the optical head apparatus is precisely adjusted. Also, the present 
invention relates to an information writing and reading method for 
optically writing information in the information medium with the 
image-formation optical system and reading the information with the 
optical head apparatus. 
2. Description of the Related Art 
An optical memory technique has been put to practical use to manufacture an 
optical disk in which a pit pattern indicating information is formed. The 
optical disk is utilized as a high density and large capacity of 
information medium. For example, the optical disk is utilized for a 
digital audio disk, a video disk, a document file disk, and a data file 
disk. To store information in the optical disk and to read the information 
from the optical disk, a light beam radiated from a light source is 
minutely narrowed in diameter in an image-formation optical system, and 
the light beam minutely narrowed is radiated to the optical disk through 
the image-formation optical system. Therefore, the light beam is required 
to be reliably controlled in the image-formation optical system with high 
accuracy. 
The image-formation optical system is utilized for an optical head 
apparatus in which a detector is additionally provided to detect intensity 
of the light beam reflected by the optical disk. Also, the optical head 
apparatus is utilized for an optical information apparatus in which a 
control section is additionally provided to adjust a positional relation 
between the optical disk and the optical head apparatus. Fundamental 
functions of the optical information apparatus are classified into 
converging performance for minutely narrowing a light beam to form a 
diffraction-limited micro-spot of the light beam radiated on an optical 
disk, focus control in a focus servo system, tracking control in a 
tracking serve system, and detection of pit signals (or information 
signals) obtained by radiating the light beam on a pit pattern of the 
optical disk. 
The image-formation optical system is composed of a light source for 
radiating a light beam, and a group of lenses including an objective lens 
for converging the light beam at an optical disk and directing the light 
beam reflected by the optical disk to an information signal detecting 
system. The information signal detecting system is provided with optical 
elements for dividing the light beam into signal light beams indicating 
various signals such as an information signal, a photo detector for 
detecting intensities of the signal light beams, and an actuating unit for 
moving the objective lens of the image-formation optical system. The 
optical head apparatus is composed of the image-formation optical system 
and the information signal detecting system. The optical information 
apparatus is composed of the optical head apparatus and a control section 
for controlling the position of the objective lens of the image-formation 
optical system under control of a focus servo system and a tracking servo 
system. A gas laser is initially utilized as the light source, and a 
semiconductor laser has been recently utilized as the light source because 
the semiconductor laser can be manufactured at a moderate cost in a small 
size. 
However, in cases where the semiconductor laser is utilized as the light 
source, a driving current supplied to the semiconductor laser changes each 
time a writing operation and a reading operation are exchanged for each 
other. Therefore, a refractive index of semiconductor laser material 
changes in dependence on the driving current. As a result, a wavelength of 
the light beam changes each time the writing operation and the reading 
operation are exchanged for each other. In this case, a refractive index 
of the objective lens for the light beam changes in dependence on the 
change of the wavelength of the light beam. Therefore, a light spot of the 
light beam converged at the optical disk is in a defocus condition until 
focus control in a focus servo system follows up the change of the 
wavelength of the light beam. That is, there is a drawback that the 
exchange of the reading and writing operations cannot be quickly 
performed. 
To reduce the change of the refractive index of the objective lens, a first 
trial in which the objective lens is made of material having a low 
wavelength-dispersion characteristic has been proposed. However, because 
the material having the low wavelength-dispersion characteristic has a low 
refractive index, a curvature of the objective lens is required to be 
enlarged. Therefore, it is difficult to make the objective lens having a 
large numerical aperture (NA) and the low wavelength-dispersion 
characteristic. Also, a second trial in which a combination lens formed by 
combining a plurality of lenses having various wavelength-dispersion 
characteristics is utilized for the image-formation optical system has 
been proposed. 
2.1. First Previously Proposed Art 
FIG. 1 is a constitutional view of a conventional optical head apparatus. 
As shown in FIG. 1, a light beam B1 linearly polarized is radiated from a 
semiconductor laser 11 in a conventional optical head apparatus 10. The 
light beam B1 is collimated by a combination lens 12, and the cross 
section of the light beam B1 is reshaped in circular shape by a wedge-like 
prism 13. Thereafter, the light beam B1 transmits through a beam splitter 
14 and is converged by an objective lens 15 at an information medium in an 
outgoing light path. In this case, the position of the objective lens 15 
is adjusted with an actuating unit 17 to focus the objective lens 15 on 
the information medium 16. Therefore, a light spot Ls is formed on the 
information medium 16. The light beam B1 reflected by the information 
medium 16 transmits through the objective lens 15 in an incoming light 
path, and a major part of the light beam B1 is reflected by the beam 
splitter 14. Thereafter, the light beam B1 is converged by a collimator 
lens 18, and a wavefront of the light beam B1 is changed in a servo signal 
detecting unit 19 to obtain a focus error signal and a tracking error 
signal. Thereafter, the intensity of the light beam B1 is detected in a 
photo detector 20. Therefore, the focus and tracking error signals and an 
information signal is obtained by calculating the intensity of the light 
beam B1 detected, and the actuating unit 17 is moved in dependence on the 
focus and tracking error signals to adjust the position of the objective 
lens 
In this case, to quickly move the objective lens 15, the objective lens 15 
is required to be lightweight. Therefore, a combined lens composed of a 
plurality of refracting lenses cannot be utilized for the objective lens 
15. As a result, chromatic aberration of the objective lens 15 necessarily 
exists because of the change of the refractive index in the objective lens 
15 for the beam light B1 of which the wavelength changes each time the 
reading and writing operations are exchanged for each other. To reduce 
adverse influence of the chromatic aberration of the objective lens 15, 
chromatic aberration of the combination lens 12 is excessively corrected 
to cancel out the chromatic aberration of the objective lens 15. That is, 
a focal length of the objective lens 15 is lengthened as the wavelength of 
the light beam B1 becomes longer because of the increase of the driving 
current supplied to the semiconductor laser 11. In contrast, a focal 
length of the combination lens 12 is shortened as the wavelength of the 
light beam B1 becomes longer. 
2.2. Second Previously Proposed Art 
Next, an example of achromatization performed in a single lens is shown in 
FIG. 2. The lens utilized for the achromatization has not been applied to 
any optical head apparatus. 
FIG. 2 is a cross-sectional view of a conventional achromatic lens in which 
a hologram lens is combined. 
As shown in FIG. 2, a conventional achromatic lens 21 consists of a 
diffraction grating type of hologram lens 22 and a refracting lens 23. 
Where a symbol f.sub.Ho denotes a focal length of the hologram lens 22 for 
a light beam L.sub.B having a wavelength .lambda..sub.o and a symbol 
f.sub.H1 denotes a focal length of the hologram lens 22 for another light 
beam L.sub.B having a wavelength .lambda..sub.1, an equation (1) is 
satisfied. 
EQU f.sub.H1 =f.sub.Ho .times..lambda..sub.o /.lambda..sub.1 ( 1) 
The focal length f.sub.H of the hologram lens 22 is shortened as the 
wavelength .lambda. of the light beam L.sub.B becomes longer. Also, where 
a symbol n(.lambda.) denotes a refractive index of the refracting lens 23 
for the light beam L.sub.B having the wavelength .lambda. and a symbol 
f.sub.D (.lambda.) denotes a focal length of the refracting lens 23 for 
the light beam L.sub.B having the wavelength .lambda., an equation (2) is 
satisfied. 
EQU f.sub.D (.lambda..sub.1)=f.sub.D 
(.lambda..sub.o).times.(n(.lambda..sub.o)-1)/(n(.lambda..sub.1)-1) (2) 
The focal length f.sub.D (.lambda.) of the refracting lens 23 is lengthened 
as the wavelength .lambda. of the light beam L.sub.B becomes longer. That 
is, the dependence of the focal length f.sub.D (.lambda.) on the 
wavelength .lambda. of the refracting lens 23 is opposite to that in the 
hologram lens 22. Therefore, a condition that a combination lens of the 
lenses 22, 23 functions as the achromatic lens 21 is formulated by an 
equation (3). 
##EQU1## 
Accordingly, because the dependence of the focal length f.sub.D (.lambda.) 
on the wavelength .lambda. of the refracting lens 23 is opposite to that 
in the hologram lens 22, the achromatic lens 21 can be formed by the 
combination of the lenses 22, 23. Therefore, curvature of the achromatic 
lens 21 can be small. Also, because the hologram lens 22 is a plane type 
of element, the lightweight achromatic lens 21 can be made in large scale 
manufacture. The conventional achromatic lens has been proposed in a first 
literature (D. Faklis and M. Morris, Photonics Spectra (1991), November 
p.205 & December p.131), a second literature (M. A. Gan et al., S.P.I.E. 
(1991), Vol.1507, p.116), and a third literature (P. Twardowski and P. 
Meirueis, S.P.I.E. (1991), Vol.1507, p.55). 
Also, as is described in the first literature, the hologram lens 22 is 
manufactured according to a manufacturing method shown in FIGS. 3A to 3F. 
FIGS. 3A to 3F are respectively a cross-sectional view showing a 
manufacturing method of the hologram lens 22. 
As shown in FIG. 3A, a hologram substrate 24 is coated with a resist 25, 
and the resist 25 is covered with a first patterned photomask 26. 
Thereafter, the resist 25 is exposed to ultraviolet radiation to transfer 
a first pattern to the resist 25. After the photomask 26 is taken off, the 
resist 25 exposed is developed to pattern the resist 25 with the first 
pattern as shown in FIG. 3B. After development, the hologram substrate 24 
exposed is etched with an etchant at a depth H1 in a first etching 
process, and the resist 25 is stripped as shown in FIG. 3C. 
Thereafter, the hologram substrate 24 etched is again coated with a resist 
27, and the resist 27 is covered with a second patterned photo mask 28 as 
shown in FIG. 3D. Thereafter, the resist 27 is exposed to ultraviolet 
radiation to transfer a second pattern to the resist 27. After the 
photomask 28 is taken off, the resist 27 exposed is developed to pattern 
the resist 27 with the second pattern as shown in FIG. 3E. After 
development, the hologram substrate 24 exposed is again etched with an 
etchant at a depth H2 in a second etching process, and the resist 27 is 
stripped as shown in FIG. 3F. 
Accordingly, the hologram lens 22 of which the surface is blazed and formed 
in echelon shape as a multilevel hologram can be manufactured by repeating 
a lithography process and an etching process. 
Also, another manufacturing method of a hologram lens has been proposed in 
a fourth literature (K. Goto et al., J.J.A.P. (1987), Vol. 26, Supplement 
26-4). 
FIG. 4 shows an original form of a hologram substrate patterned with a 
cutting tool. 
As shown in FIG. 4, a hologram lens 29 can be manufactured with a cutting 
tool 30 of a super precision CNC lathe. 
In the fourth literature, a combination lens of a spherical lens and the 
hologram lens 29 is applied to an optical head apparatus to reduce 
aberration such as off-axis aberration according to an aspherical lens 
effect. Therefore, chromatic aberration of the combination lens is not 
corrected. That is, when the wavelength of a light beam radiated from a 
semiconductor laser fluctuates, the focal point of the combination lens is 
moved. 
2.3. Third Previously Proposed Art 
A prior art laid open to public inspection under Provisional Publication 
No. 155514/91 (H3-155514) and under Provisional Publication No. 155515/91 
(H3-155515) is cited as a third previously proposed art. 
FIG. 5 is a cross-sectional view of a conventional optically converging 
system consisting of an objective lens and a chromatic aberration 
correction element. 
As shown in FIG. 5, a conventional optically converging system 31 consists 
of an objective lens 32 focused on an information medium 33 and an 
chromatic aberration correction lens 34 arranged at a light source side. 
The objective lens 32 is allowed to be exchanged for a hologram lens. The 
chromatic aberration correction lens 34 is a combination lens of a 
positive lens 35 (or a convex lens) and a negative lens 36 (or a concave 
lens). Though the chromatic aberration correction lens 34 has no lens 
function, chromatic aberration of the objective lens 32 is corrected with 
the chromatic aberration correction lens 34 because a 
wavelength-dispersion coefficient of the positive lens 35 differs from 
that of the negative lens 36. 
Therefore, the chromatic aberration correction lens 34 functions in the 
same manner as the combination lens 12. 
2.4. Problems to be Solved by the Invention 
In cases where a light beam radiated from a semiconductor laser is 
converged by a lens to form an image, the lens generally has astigmatic 
aberration because an astigmatic difference necessarily occurs in an 
active layer of the semiconductor laser. The reason that the astigmatic 
difference occurs in the active layer is described with reference to FIGS. 
6 and 7. 
As shown in FIG. 6, where a longitudinal direction of an active layer 37 in 
an end facet 11a of the semiconductor laser 11 is defined as a horizontal 
direction, an outgoing radiation point Pr.sub.H in the horizontal 
direction is positioned within the active layer 37 by a length .delta.. In 
contrast, another outgoing radiation point Pry in a vertical direction 
perpendicular to the horizontal direction is positioned at the end facet 
11a. Therefore, the astigmatic difference occurs in the active layer 37 so 
that the astigmatic aberration occurs in the conventional optical head 
apparatus 10. 
To remove the astigmatic aberration, a collimator lens 38 composed of the 
combination lens 12 and the wedge-like prism 13 in the first previously 
proposed art is moved in the outgoing light path direction. In detail, 
because an elliptic cross section of the light beam B1 is corrected in 
circular shape with the wedge-like prism 13, a focal length f.sub.cv of 
the collimator lens 38 in the vertical direction differ from another focal 
length f.sub.CH of the collimator lens 38 in the horizontal direction, as 
shown in FIGS. 7(a), 7(b). A light beam path in the vertical direction 
from the outgoing radiation point Pr.sub.v to the information medium 16 is 
shown in FIG. 7(a), and a light beam path in the horizontal direction from 
the outgoing radiation point Pr.sub.H to the information medium 16 is 
shown in FIG. 7(b). Symbols shown in FIGS. 7(a), 7(b) denote as follows. 
f.sub.o : a focal length of the objective lens 15; 
f.sub.c : a focal length of the combined lens 12, and a focal length of 
first collimator lens 44 in embodiments according to the present 
invention; 
f.sub.cv : an equivalent focal length of the collimator lens 38 in the 
vertical direction (=f.sub.c); 
f.sub.CH : an equivalent focal length of the collimator lens 38 in the 
horizontal direction (=f.sub.c .times..gamma.); 
.gamma.: an elliptic beam correction coefficient in the wedge-like prism 
13, .gamma.&gt;1; 
.delta.: the astigmatic difference; 
.delta..sub.v : a difference between a focal point Fc of the collimator 
lens 38 and an objective point (or the outgoing radiation point Pr.sub.v) 
in the vertical direction; .delta..sub.H : a difference between the focal 
point Fc of the collimator lens 38 and an objective point (or the outgoing 
radiation point Pr.sub.H) in the horizontal direction; 
.epsilon..sub.v : a difference between a focal point Fo of the objective 
lens 15 and an image point Pi.sub.v in the vertical direction; and 
.epsilon..sub.H : a difference between the focal point Fo of the objective 
lens 15 and an image point Pi.sub.H in the horizontal direction. 
To set a relationship .epsilon..sub.v =.epsilon..sub.H, the collimator lens 
38 is moved towards the objective lens 15 to increase the difference 
.delta..sub.v (.delta..sub.v &gt;0) so that the focal point Fc positioned at 
the end facet 11a of the semiconductor laser 11 is also moved towards the 
objective lens 15. Therefore, the image points Pi.sub.v, Pi.sub.H in the 
horizontal and vertical directions are moved towards the objective lens 
15. 
Because a relation in longitudinal magnification is satisfied, equations 
(4), (5) are obtained. 
##EQU2## 
To remove the astigmatic aberration occurring in an image which reflects on 
the information medium 16, the relationship .epsilon..sub.v 
=.epsilon..sub.H is required. Therefore, an equation (6) is obtained by 
use of the equations (4), (5). 
EQU .delta..sub.v .times..gamma..sup.2 =.delta..sub.H ( 6) 
Therefore, in cases where the collimator lens 38 is moved towards the 
objective lens 15 on condition that the equation (6) is satisfied, the 
astigmatic aberration is removed. 
However, even though the position of the collimator lens 38 is adjusted to 
satisfy the equation (6) in the reading operation in which intensity of 
the light beam B1 is low because the driving current supplied to the 
semiconductor laser 11 is low, the astigmatic aberration occurs in an 
image reflecting on the Information medium 16 in the writing operation in 
which intensity of the light beam B1 is high because the driving current 
supplied to the semiconductor laser 11 is high. 
In detail, as the intensity of the light beam B1 is increased, the 
wavelength of the light beam B1 is lengthened. Therefore, the focal length 
f.sub.c of the collimator lens 38 is shortened because the chromatic 
aberration of the combination lens 12 is excessively corrected to cancel 
out the chromatic aberration of the objective lens 15. As a result, the 
difference .delta..sub.v is increased to maintain a summed value 
.delta..sub.v +f.sub.c of the difference .delta..sub.v and the focal 
length f.sub.c, and the difference .delta..sub.H is increased to maintain 
a summed value .delta..sub.H +f.sub.c of the difference .delta..sub.v and 
the focal length f.sub.c. Accordingly, a ratio .delta..sub.H 
/.delta..sub.v becomes smaller than a value .gamma..sup.2 in the writing 
operation. In addition, as the intensity of the light beam B1 is 
increased, the astigmatic difference .delta. is decreased, as is well 
known. Therefore, the ratio .delta..sub.H /.delta..sub.v is moreover 
decreased in the writing operation. 
Accordingly, in cases where the reading operation and the writing operation 
are exchanged for each other in the conventional optical head apparatus 
10, the collimator lens 38 cannot be moved on condition that the equation 
(6) is satisfied in the writing operation. Also, in cases where the 
position of the collimator lens 38 is adjusted to satisfy the equation (6) 
in the writing operation, the ratio .delta..sub.H /.delta..sub.v becomes 
larger than the value .gamma..sup.2. Therefore, the collimator lens 38 
cannot be moved to satisfy the equation (6) in the reading operation. As a 
result, there is a drawback that the astigmatic aberration necessarily 
occurs on the information medium 16. 
Next, various drawbacks in the conventional achromatic lens 21 of the 
second previously proposed art are described. 
The conventional achromatic lens 21 has no chromatic aberration. However, 
in cases where the lens 21 is applied to an image-formed optical system or 
an optical head apparatus, many drawbacks occur as follows. 
1. To apply the achromatic lens 21 to the optical head apparatus 10, the 
chromatic aberration of each of lenses utilized in the optical system is 
required to be corrected. Therefore, degree of freedom in design is 
considerably decreased. Specifically,. though the astigmatic difference 
.delta. varies in dependence on the change in the output intensity of the 
semiconductor laser 11, the lenses cannot be designed so as to compensate 
the variation of the astigmatic difference .delta.. Therefore, the 
astigmatic aberration necessarily occurs in the light spot Ls on the 
information medium 16, so that an information signal obtained in the 
apparatus 10 deteriorates. 
2. To apply the achromatic lens 21 to the optical head apparatus 10, three 
hologram lenses 22 are required to be utilized in the collimator lenses 
18, 38 and the objective lens 15 for the purpose of the correction of the 
chromatic aberration of the lenses 15, 18, 38. Therefore, many number of 
hologram lenses 22 are provided in the apparatus 10, so that the apparatus 
10 cannot be manufactured at a moderate cost. 
3. A diffraction efficiency of the hologram lens 22 varies in dependence on 
the wavelength of light transmitting through the hologram lens 22. Also, a 
design method for appropriately setting the diffraction efficiency in an 
optical head apparatus has never been proposed in the prior art. 
Therefore, in cases where the achromatic lens 21 is applied to the optical 
head apparatus, the diffraction efficiency of the hologram lens 22 becomes 
lowered in the reading operation. In this case, even though first-order 
diffraction light is mainly generated in the hologram lens 22 to obtain an 
information signal, other light such as zero-order diffraction light (or 
transmitting light), minus first-order diffraction light, and second-order 
diffraction light is undesirably generated in the hologram lens 22. The 
undesired other light becomes stray light so that the undesired other 
light functions as noise. Therefore, a signal-noise ratio (S/N ratio) 
considerably deteriorates. 
Next, various drawbacks in the conventional optical system 31 of the third 
previously proposed art are described. 
1. Because the chromatic aberration correction lens 34 is formed by 
combining the positive lens 35 and the negative lens 36, manufacturing 
costs such as material costs, production costs, combination costs 
including adjustment costs of the positive lens 35 and the negative lens 
36, and attachment costs are required. Therefore, the conventional optical 
system 31 cannot be applied to an image-formation optical system or an 
optical head apparatus at a moderate cost. 
2. Because the chromatic aberration correction lens 34 is formed by 
combining the positive lens 35 and the negative lens 36, the lens 34 
becomes heavy and large. Therefore, in cases where the lens 34 is utilized 
in an image-formation optical system or an optical head apparatus, the 
system or the apparatus becomes heavy and large. 
3. Because the chromatic aberration correction lens 34 is heavy and because 
the objective lens 32 is required to be slightly moved at high speed under 
control of a focus servo system and a tracking servo system, the lens 34 
cannot be integrally formed with the objective lens 32 on condition that 
the objective lens 32 and the lens 34 are slightly moved at high speed. 
Therefore, a positional relation between the lenses 32, 34 changes when 
the objective lens 32 is moved. Therefore, even though the chromatic 
aberration of the objective lens 32 is always corrected by the chromatic 
aberration correction lens 34 regardless of the change in the positional 
relation, other aberration such as the astigmatic aberration are required 
to be independently corrected. In this case, degree of freedom in the 
design of lenses such as the objective lens 32 becomes low. As a result, 
aspherical lenses are required in an image-formation optical system or an 
optical head apparatus. Accordingly, the design and manufacturing of the 
system or the apparatus becomes difficult, and the system or the apparatus 
cannot be manufactured at a moderate cost. 
4. The chromatic aberration correction lens 34 is heavy. Also, the 
objective lens 32 is slightly moved at high speed under control of a focus 
servo system and a tracking servo system. Therefore, the lens 34 cannot be 
integrally formed with the objective lens 32. In this case, a holding 
element for holding the lens 34 is required independently of another 
holding element for holding the objective lens 32. Accordingly, the 
conventional optical system 31 cannot be applied to an image-formation 
optical system or an optical head apparatus at a moderate cost, and the 
system or the apparatus becomes large. 
SUMMARY OF THE INVENTION 
A first object of the present invention is to provide, with due 
consideration to the drawbacks of such a conventional optical system and a 
conventional optical head apparatus, an image-formation optical system in 
which a light beam is converged at an information medium to read or write 
information from/in the information medium while preventing the movement 
of a focal point of an objective lens and the occurrence of astigmatic 
aberration even though the wavelength of light radiated from a 
semiconductor laser changes and the astigmatic difference changes. 
A second object of the present invention is to provide an optical head 
apparatus in which information stored in an information medium is read 
while preventing the movement of a focal point of an objective lens and 
the occurrence of astigmatic aberration by use of the image-formation 
optical system. 
A third object of the present invention is to provide an optical 
information apparatus in which the position of an objective lens is 
precisely adjusted by use of the optical head apparatus while the 
occurrence of astigmatic aberration are prevented. 
A fourth object of the present invention is to provide an information 
reading or writing method for optically reading or writing information 
from/in an information medium by use of the image-formation optical 
system. Also, the fourth object is to provide an information reading 
method for optically reading information from an information medium by use 
of the optical head apparatus. 
The first object is achieved by the provision of an image-formation optical 
system for converging a light beam at an information medium to read or 
write information from/in the information medium, comprising: 
a semiconductor laser for radiating a light beam, a wavelength of the light 
beam being lengthened as intensity of the light beam is increased; 
a first convex lens for refracting the light beam radiated from the 
semiconductor laser, a focal length of the first convex lens being 
lengthened as the wavelength of the light beam becomes longer; 
a refraction type of objective lens for converting the light beam refracted 
by the first convex lens at the information medium to read or write 
information from/in the information medium, a focal length of the 
objective lens being lengthened as the wavelength of the light beam 
becomes longer; and 
a chromatic aberration correcting element for excessively correcting 
chromatic aberration of the objective lens to cancel out chromatic 
aberration of the first convex lens. 
In the above configuration, a light beam having a wavelength is radiated 
from the semiconductor laser. The light beam is refracted by the first 
convex lens to, for example, collimate the light beam. Thereafter, the 
light beam transmits through the chromatic aberration correcting element 
and is converged at the information medium to read or write information 
from/in the information medium. 
In this case, intensity of the light beam in a writing operation is greatly 
larger than intensity of the light beam in a reading operation. Therefore, 
a wavelength of the light beam in the writing operation becomes larger 
than another wavelength of the light beam in the reading information. 
Therefore, a focal length of the first convex lens in the writing 
operation is larger than another focal length of the first convex type of 
lens in the reading operation. Also, a focal length of the objective lens 
in the writing operation is larger than another focal length of the 
objective lens in the reading operation. Therefore, unless chromatic 
aberration of both the objective lens and the first convex lens is 
corrected by the chromatic aberration correcting element, the chromatic 
aberration occurs in an image reflecting on the information medium. In the 
present invention, the chromatic aberration of both the objective lens and 
the first convex lens is completely canceled out by the chromatic 
aberration correcting element. Accordingly, no chromatic aberration occurs 
in the image-formation optical system. 
In addition, even though no chromatic aberration occurs, astigmatic 
aberration occurs in a conventional optical system because an astigmatic 
difference .delta. inevitably occurs in a semiconductor laser. To remove 
the astigmatic aberration in the present invention, an equation 
.delta..sub.v .times..gamma..sup.2 =.delta..sub.H is required to be 
satisfied. Here the symbol .gamma. denotes an elliptic beam correction 
coefficient which is constant (.gamma.&gt;1) even though the wavelength of 
the light beam changes, the symbol .delta..sub.v denotes a difference from 
a focal point Fc of the first convex lens to an outgoing radiation point 
of the light beam in a vertical direction, and the symbol .delta..sub.H 
denotes a difference from the focal point Fc of the first convex lens to 
an outgoing radiation point in a horizontal direction. In the present 
invention, as the wavelength of the light beam is increased, the 
differences .delta..sub.v, .delta..sub.H are respectively decreased 
because the focal length of the first convex lens is lengthened. 
Therefore, a ratio .delta..sub.H /.delta..sub.v is forced to be increased. 
In contrast, the astigmatic difference .delta. is decreased as the 
wavelength of the light beam is increased. Therefore, the differences 
.delta..sub.v, .delta..sub.H are respectively increased so that the ratio 
.delta..sub.H /.delta..sub.v is forced to be decreased. Accordingly, the 
ratio .delta..sub.H /.delta..sub.v can be set to a constant value even 
though the wavelength of the light beam changes. That is, the equation 
.delta..sub.v .times..gamma..sup.2 =.delta..sub.H can be always satisfied 
even though the reading and writing operations are exchanged for each 
other. As a result, the astigmatic aberration can be reliably removed in 
the image-formation optical system according to the present invention. 
It is preferred that the chromatic aberration correcting element is a plane 
type of hologram lens functioning as a diffraction optical element, and 
the hologram lens is integrally formed with the objective lens. 
In the above configuration, the hologram lens is moved with the objective 
lens. Therefore, it is not required to independently prevent other 
aberration such as a coma aberration from occurring on the information 
medium 42. In other words, all types of aberration can be prevented as a 
whole. Accordingly, degree of freedom in the design of the objective lens 
is increased, so that the image-formation optical system can be 
arbitrarily designed to prevent all types of aberration. 
Also, it is preferred that the image-formation optical system additionally 
include a light beam reshaping element for reshaping an elliptic cross 
section of the light beam refracted by the first convex lens into an 
circular cross section, a combined focal length of both the light beam 
reshaping element and the first convex lens in a major axis direction of 
the elliptic cross section differing from a combined focal length of those 
in a minor axis direction of the elliptic cross section to form an image 
on the information medium without astigmatic aberration even though an 
astigmatic difference occurs in the semiconductor laser, and the light 
beam reshaped by the light beam reshaping element transmitting through the 
chromatic aberration correcting element and the objective lens. 
In the above configuration, the major axis direction of the elliptic cross 
section agrees with the vertical direction, and the minor axis direction 
of the elliptic cross section agrees with the horizontal direction. 
Therefore, a combined focal length f.sub.cv in the vertical direction 
becomes shorter than another combined focal length f.sub.cH in the 
horizontal direction. In this case, even though the astigmatic difference 
.delta. occurs in the semiconductor laser, a difference .epsilon..sub.v 
between a focal point Fo of the objective lens and an image point Pi.sub.v 
in the vertical direction agrees with another difference .epsilon..sub.H 
between the focal point Fo and an image point Pi.sub.H in the horizontal 
direction. 
Accordingly, the occurrence of the astigmatic aberration can be prevented. 
Also, it is preferred that the chromatic aberration correcting element 
comprise a polarizing anisotropic hologram lens for diffracting the light 
beam which is linearly polarized in an X direction in an outgoing light 
path, and a quarter-wave plate for converting the light beam diffracted by 
the polarizing anisotropic hologram lens into a circularly-polarized-light 
beam in the outgoing light path and again converting the 
circularly-polarized-light beam of which a rotating direction is reversed 
on the information medium into a linearly-polarized-light beam polarized 
in a Y direction perpendicular to the X-direction in an incoming light 
path, the linearly-polarized-light beam polarized in the Y direction by 
the quarter-wave plate transmitting through the polarizing anisotropic 
hologram lens without being diffracted. 
In the above configuration, because the light beam in the outgoing light 
path is diffracted in the polarizing anisotropic hologram lens and because 
the light beam in the incoming light path is not diffracted in the 
polarizing anisotropic hologram lens, the incoming light path differs from 
the outgoing light path. Therefore, even though the light beam reflected 
by the information medium is fed back to the semiconductor laser, the 
light beam does not couple to an active layer of the semiconductor laser. 
Accordingly, noise caused by the light beam fed back to the semiconductor 
laser can be prevented. 
The second object is achieved by the provision of an optical head apparatus 
for reading information from an information medium, comprising: 
a semiconductor laser for radiating a light beam, a wavelength of the light 
beam being lengthened as intensity of the light beam is increased; 
a first convex lens for refracting the light beam radiated from the 
semiconductor laser, a focal length of the first convex lens being 
lengthened as the wavelength of the light beam becomes longer; 
a refraction type of objective lens for converting the light beam refracted 
by the first convex lens at the information medium to read information 
from the information medium, a focal length of the objective lens being 
lengthened as the wavelength of the light beam becomes longer; 
a chromatic aberration correcting element moved with the objective lens for 
excessively correcting chromatic aberration of the objective lens to 
cancel out chromatic aberration of the first convex lens; 
a second convex lens for converging the light beam which is reflected by 
the information medium and again transmits through the objective lens and 
the chromatic aberration correcting element, a focal length of the second 
convex lens being lengthened as the wavelength of the light beam becomes 
longer, and chromatic aberration of the second convex lens being corrected 
by the chromatic aberration correcting element; and 
a photo detector for detecting intensity of the light beam converged by the 
second convex lens to obtain an information signal indicating the 
information stored in the information medium. 
In the above configuration, the chromatic aberration of the objective lens 
and the first and second convex lenses is corrected by the chromatic 
aberration correcting element. Also, an image is formed on the information 
medium without the the chromatic aberration or the astigmatic aberration 
because the image-formation optical system described above is utilized in 
the optical head apparatus. 
Accordingly, the information stored in the information medium can be read 
with high accuracy. 
It is preferred that the chromatic aberration correcting element be a plane 
type of hologram lens functioning as a diffraction optical element and a 
plurality of relieves be concentrically arranged on a surface of the 
hologram lens to form a hologram pattern, a height H of the reliefs being 
determined to maximize a diffraction efficiency of the hologram lens in a 
reading operation in which a wavelength .lambda..sub.R of the light beam 
is shorter than that in a writing operation. In the above configuration, 
because the height H of the relieves is determined to maximize a 
diffraction efficiency of the hologram lens in a reading operation, the 
occurrence of unnecessary light such as zero-order diffracted light (or 
transmitting light) can be prevented to the utmost. Accordingly, in cases 
where the height H of the reliefs is, for example, set to satisfy an 
equation H=.lambda..sub.R /(n(.lambda..sub.R)-1) where the symbol 
n(.lambda..sub.R) denotes a refractive index of the hologram lens, noise 
including the information signal can be reduced. 
Also, it is preferred that the optical head apparatus additionally include: 
a servo signal generating optical element partitioned into a plurality of 
diffracted light generation regions for converting the light beam 
converged by the second convex lens into one or more servo signal light 
beams, the servo signal light beams being detected by the photo detector 
to generate one or more servo signals; and 
an actuating unit for moving both the objective lens and the chromatic 
aberration correcting element under control of the servo signals generated 
in the photo detector. 
In the above configuration, servo signals such as a focus error signal and 
a tracking error signal are generated in the photo detector because the 
light beam is divided and converted in the diffracted light generation 
regions into the servo signal light beams. Therefore, the position of both 
the objective lens and the chromatic aberration correcting element moved 
together by the actuating unit can be adjusted under control of the servo 
signals to decrease focus and tracking errors. 
The third object is achieved by the provision of an optical information 
apparatus for reading or writing information from/in an information 
medium, comprising: 
an information medium driving mechanism for rotating the information 
medium; 
an external electric source for supplying driving power to the information 
driving mechanism; 
an optical head apparatus comprising 
a semiconductor laser for radiating a light beam, a wavelength of the light 
beam being lengthened as intensity of the light beam is increased, 
a first convex lens for refracting the light beam radiated from the 
semiconductor laser, a focal length of the convex type of lens being 
lengthened as the wavelength of the light beam becomes longer, 
a refraction type of objective lens for converting the light beam refracted 
by the first convex lens at the information medium to read or write 
information from/in the information medium, a focal length of the 
objective lens being lengthened as the wavelength of the light beam 
becomes longer, 
a chromatic aberration correcting element moved with the objective lens for 
excessively correcting chromatic aberration of the objective lens to 
cancel out chromatic aberration of the first convex lens, 
a second convex lens for converging the light beam which is reflected by 
the information medium and again transmits through the objective lens and 
the chromatic aberration correcting element, a focal length of the second 
convex lens being lengthened as the wavelength of the light beam becomes 
longer, and chromatic aberration of the second convex lens being corrected 
by the chromatic aberration correcting element, 
a servo signal generating element for dividing the light beam converged by 
the second convex lens into an information signal light beam and servo 
signal beams such as a focus error signal light beam and a tracking error 
signal light beam, 
a photo detector for detecting intensities of the light beams obtained in 
the servo signal generating element to obtain an information signal 
indicating the information stored in the information medium and servo 
signals such as a focus error signal and a tracking error signal, and 
an actuating unit for moving both the objective lens and the chromatic 
aberration correcting element under control of the servo signals generated 
in the photo detector; 
an optical head driving apparatus for roughly positioning the objective 
lens of the optical head apparatus on a desired pit of the information 
medium; and 
a control circuit for generating a control signal according to the servo 
signals obtained in the photo detector and precisely positioning the 
objective lens of the optical head apparatus on the desired pit of the 
information medium under control of the control signal. 
In the above configuration, the information medium is rotated by the 
information medium driving mechanism, and the objective lens of the 
optical head apparatus is roughly positioned by the optical head driving 
apparatus. Thereafter, servo signals such as a focus error signal and a 
tracking error signal is obtained in the optical head apparatus, and a 
control signal is generated in the control circuit according to the servo 
signals. The objective lens is precisely positioned under control of the 
control signal. 
Accordingly, because the optical head apparatus is provided in the optical 
information apparatus, information can be reliably read or stored from/in 
the information medium without the chromatic aberration or the astigmatic 
aberration. 
The fourth object is achieved by the provision of a method for converging a 
light beam at an information medium to optically read or write information 
from/in the information medium, comprising the steps of: 
collimating a light beam radiated from a semiconductor laser in a 
collimator lens, a wavelength of the light beam being lengthened as 
intensity of the light beam being increased, and a focal length of the 
collimator lens being lengthened as the wavelength of the light beam 
becomes longer; 
converging the light beam collimated by the collimator lens at the 
information medium by use of an objective lens to read or write 
information from/in the information medium, a focal length of the 
objective lens being lengthened as the wavelength of the light beam 
becomes longer; and 
excessively correcting chromatic aberration of the objective lens by use of 
a hologram lens to cancel out chromatic aberration of the collimator lens, 
a focal length of the hologram lens being shortened as the wavelength of 
the light beam becomes longer, a combined focal length of the collimator 
and objective lenses and the hologram lens being constant even though the 
wavelength of the light beam changes, and the hologram lens being moved 
with the objective lens. 
In the above steps, a light beam radiated from a semiconductor laser is 
collimated and converged at the information medium. In this case, the 
chromatic aberration of the collimator lens and the objective lens is 
corrected by the hologram lens. Therefore, an image can be clearly formed 
on the information medium without the chromatic aberration. 
Also, an astigmatic difference occurs in the semiconductor laser. The 
astigmatic difference changes in dependence on the wavelength of the light 
beam. The change of the focal length of the collimator lens cancels out 
the change of the astigmatic difference. Therefore, the occurrence of 
astigmatic aberration can be prevented. 
In addition, because the hologram lens is moved with the objective lens, 
other types of aberration are not required to be independently removed. 
Also, the fourth object is achieved by the provision of a method for 
optically reading information from an information medium, comprising the 
steps of: 
collimating a light beam radiated from a semiconductor laser in a 
collimator lens, a wavelength of the light beam being lengthened as 
intensity of the light beam being increased, and a focal length of the 
collimator lens being lengthened as the wavelength of the light beam 
becomes longer; 
converging the light beam collimated by the collimator lens at the 
information medium by use of an objective lens to read information stored 
in the information medium, a focal length of the objective lens being 
lengthened as the wavelength of the light beam becomes longer; 
excessively correcting chromatic aberration of the objective lens by use of 
a hologram lens to cancel out chromatic aberration of the collimator lens, 
a focal length of the hologram lens being shortened as the wavelength of 
the light beam becomes longer, a combined focal length of the collimator 
and objective lenses and the hologram lens being constant even though the 
wavelength of the light beam changes, and the hologram lens being moved 
with the objective lens; 
transmitting the light beam through the objective lens and the hologram 
lens; 
converging the light beam which is reflected by the information medium by 
use of a refractive lens, a focal length of the refractive lens being 
lengthened as the wavelength of the light beam becomes longer, and 
chromatic aberration of the refractive lens being corrected by the 
hologram lens; and 
detecting intensity of the light beam converged by the refractive lens to 
obtain an information signal indicating the information stored in the 
information medium. 
In the above steps, a light beam radiated from a semiconductor laser is 
collimated and converged at the information medium to read information 
stored in the information medium. In this case, the chromatic aberration 
of the collimator lens and the objective lens is corrected by the hologram 
lens in an outgoing light path. Therefore, an image can be clearly formed 
on the information medium without the chromatic aberration. Thereafter, 
the light beam is reflected by the information medium and again transmits 
through the objective lens and the hologram lens. Thereafter, the light 
beam is converged at a photo detector to obtain an information signal. In 
this case, the chromatic aberration of the objective lens and the 
refractive lens is corrected by the hologram lens in an incoming light 
path. Therefore, the information signal can be clearly obtained. 
Also, an astigmatic difference occurs in the semiconductor laser. The 
astigmatic difference changes in dependence on the wavelength of the light 
beam. The change of the focal length of the collimator lens cancels out 
the change of the astigmatic difference. Therefore, the occurrence of 
astigmatic aberration can be prevented. 
In addition, because the hologram lens is moved with the objective lens, 
other types of aberration are not required to be independently removed.

DETAIL DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Preferred embodiments of an image-formation optical system, an optical head 
apparatus, an optical information apparatus, and an information storing 
and reading method according to the present invention are described with 
reference to drawings. 
FIG. 8 is a constitutional view of an optical head apparatus according to a 
first embodiment of the present invention. 
As shown in FIG. 8, an optical head apparatus 41 for storing and reading 
information in/from an information medium 42 such as an optical disk, 
comprises an image-formation optical system 43 for optically converging a 
light beam at the information medium 42, and an information detecting 
system 44 for detecting the information read by the image-formation 
optical system 43. The image-formation optical system 43 comprises a 
semiconductor laser 45 for radiating a light beam B2 of which a cross 
section is in elliptic shape, a first collimator lens 46 for almost 
collimating the light beam B2, a wedge-like prism 47 for reshaping the 
light beam B2, a beam splitter 48 for transmitting through the light beam 
B2 reshaped in an outgoing light path and splitting the light beam B2 
reflected on the information medium 42 in an incoming light path, a 
hologram lens 49 for correcting chromatic aberration in the 
image-formation optical system 43 to prevent the chromatic aberration from 
occurring on the information medium 42, and an objective lens 50 
integrally formed with the hologram lens 49 for converging the light beam 
B2 at a pit of the information medium 42 to form a light spot Ls on the 
information medium 42. 
The hologram lens 49 is a phase type diffraction optical element and a 
plane type optical element, and a plurality of blazed reliefs are 
concentrically arranged on a surface of the hologram lens 49 to form a 
hologram pattern. Therefore, the phase of the light beam B2 is changed in 
the hologram lens 49 to diffract the light beam b2 in a particular 
direction. Also, the hologram lens 49 functions as a convex lens to 
refract the light beam B2. The center of the hologram pattern and the 
center of the objective lens 50 are respectively positioned on a central 
line of the outgoing light path to decrease off-axis aberration such as 
astigmatic aberration and coma aberration. 
A focal length of the objective lens 50 is lengthened as the wavelength of 
the light beam B2 becomes longer. Also, a focal length f.sub.c of the 
first collimator lens 46 is lengthened as the wavelength of the light beam 
B2 becomes longer. In contrast, as the wavelength of the light beam B2 
becomes longer, a focal length of the hologram lens 49 is shortened to 
cancel out the increase of the focal lengths of both the first collimator 
lens 46 and the objective lens 50. Therefore, a combined focal length of 
both the hologram lens 49 and the objective lens 50 is shortened as the 
wavelength of the light beam B2 becomes longer, and a combined focal 
length of the lenses 46, 49 and 50 is constant even though the wavelength 
of the light beam B2 changes. 
In the above configuration, the light beam B2 radiated from the 
semiconductor laser 43 is almost collimated by the first collimator lens 
46. Thereafter, the cross section of the light beam B2 having an elliptic 
cross section is circularly reshaped by the wedge-like prism 47. That is, 
an intensity distribution of the light beam B2 reshaped in an X-axis 
direction perpendicular to the outgoing light path becomes the same as 
that in a Y-axis direction perpendicular to both the outgoing light path 
and the X-axis direction. This reshaping is called an elliptic beam 
correction or a light beam reshaping. Thereafter, the light beam B2 
transmits through the beam splitter 48 and is diffracted by the hologram 
lens 49. Thereafter, the light beam B2 is converged at the information 
medium 42 by the objective lens 50 to write or read information in/from 
the information medium 42. 
In this case, the chromatic aberration of the objective lens 50 is 
excessively corrected by the hologram lens 49 because a combined focal 
length of both the hologram lens 49 and the objective lens 50 is shortened 
as the wavelength of the light beam B2 becomes longer. Therefore, though 
the focal length f.sub.c of the first collimator lens 46 is lengthened as 
the wavelength of the light beam B2 becomes longer, the excessive 
correction of the hologram lens 49 cancels out the chromatic aberration of 
the first collimator lens 46. 
Accordingly, the chromatic aberration in the image-formation optical system 
43 can be canceled out because the hologram lens 49 is provided in the 
system 43. Also, even though the wavelength of the light beam B2 radiated 
from the semiconductor laser 43 changes, a combined focal length of the 
lenses 46, 49, and 50 becomes constant. Therefore, a small light spot Ls 
of the light beam B2 can be always formed on the information medium 42 
without occurring the chromatic aberration. 
Also, because only the hologram lens 49 is additionally provided in the 
system 43 and because the hologram lens 49 made of the plane type optical 
element is light in weight, the image-formation optical system 43 can be 
manufactured in lightweight and small size. 
In addition, in cases where the wavelength of the light beam B2 becomes 
longer because of the increase of output intensity of the light beam B2 
radiated from the semiconductor laser 43, the focal length f.sub.c of the 
first collimator lens 46 is lengthened. In this case, the difference 
.delta..sub.v is decreased to maintain a summed value .delta..sub.v 
+f.sub.c of the difference .delta..sub.v and the focal length f.sub.c, and 
the difference .delta..sub.H is decreased to maintain a summed value 
.delta..sub.H +f.sub.c of the difference .delta..sub.H and the focal 
length f.sub.c. Therefore, the ratio .delta..sub.H /.delta..sub.v is 
forced to be increased according to an external effect. In contrast, as 
the output intensity of the light beam B2 is increased, the astigmatic 
difference .delta. is decreased, as is well known. Therefore, the ratio 
.delta..sub.H /.delta..sub.v is forced to be decreased according to an 
internal effect. As a result, the increase of the ratio .delta..sub.H 
/.delta..sub.v according to the external effect is canceled out by the 
decrease of the ratio .delta..sub.H /.delta..sub.v according to the 
internal effect. 
Accordingly, even though the wavelength of the light beam B2 radiated from 
the semiconductor laser 43 changes, the equation (6) can be stably 
satisfied. In other words, no astigmatic aberration occurs on the 
information medium 42. 
Also, because the hologram lens 49 functions as the convex lens, the 
curvature of the objective lens S0 can be small. Also, lens material 
having a comparatively large dispersion value .DELTA.n/(n-1) (for example, 
Abbe number (n-1)/.DELTA.n is less than 60) can be utilized for the 
objective lens 50. Here the symbol n is a refractive index of the lens 
material, and the symbol .DELTA.n is a change rate of the refractive index 
with respect to a wavelength of light. In this case, because a refractive 
index of the objective lens 50 can be comparatively large, the curvature 
of the objective lens 50 can be moreover small. Accordingly, the objective 
lens can be easily produced, and the image-formation optical system 43 can 
be manufactured at a moderate cost. 
The information detecting system 44 comprises a second collimator lens 51 
for converging light beam B3 which is generated by reflecting the light 
beam B2 by the information medium 42 and is split by the beam splitter 48, 
a wavefront converting element 52 for converting wavefront of the light 
beam B3 to divide the light beam B3 into focus light indicating a focus 
error signal, tracking light indicating a tracking error signal, and 
information light indicating an information signal, a photo detector 53 
for detecting the intensity of the focus light, the intensity of the 
tracking light, and the intensity of the information light to obtain the 
information signal and servo signals such as the focus error signal and 
the tracking error signal, and an actuating unit 54 for slightly moving 
the hologram lens 49 and the objective lens 50 integrally formed with each 
other at high speed according to the focus error signal and the tracking 
error signal obtained in the photo detector 53. 
A focal length of the second collimator lens 51 is lengthened as the 
wavelength of the light beam B3 (or the wavelength of the light beam B2) 
becomes longer. The wavefront converting element 52 functions as a servo 
signal light generating element in which servo signal light such as the 
focus light and the tracking light is generated. 
In the above configuration, after the light beam B2 is converged at the 
information medium 42, a light beam B3 is reflected by the information 
medium 42. Thereafter, the light beam B3 is returned through the same 
incoming light path as the outgoing light path and is split by the beam 
splitter 48. Thereafter, the light beam B3 is converged by the second 
collimator lens 51, and the wavefront of the light beam B3 is converted by 
the wavefront converting element 52. Therefore, the light beam B3 is 
divided into focus light, tracking light, and information light. 
Thereafter, the intensity of the light beam B3 divided is detected by the 
photo detector 53. In the photo detector 53, the intensity of the focus 
light, the intensity of the tracking light, and the intensity of the 
information light are independently detected so that a focus error signal, 
a tracking error signal, and an information signal are obtained. 
Thereafter, the hologram lens 49 and the objective lens 50 integrally 
formed with each other are slightly moved at high speed by the actuating 
unit 54 according to the focus error signal and the tracking error signal. 
Therefore, the information signal can be obtained with high accuracy. 
In this case, because the chromatic aberration of the objective lens 50 is 
excessively corrected by the hologram lens 49, the excessive correction of 
the hologram lens 49 cancels out the chromatic aberration of the second 
collimator lens 51. Therefore, the occurrence of an offset in the focus 
error signal can be prevented. 
The weight of the hologram lens 49 becomes light because the hologram lens 
49 is the plane type optical element. For example, the hologram lens 49 is 
less than several tens mg in weight. Also, because the hologram lens 49 
functions as the convex lens, the function as the convex lens required of 
the objective lens 50 is reduced. Therefore, the objective lens 50 can be 
light in weight. Accordingly, even though the hologram lens 49 is 
integrally formed with the objective lens 50, the hologram lens 49 and the 
objective lens 50 can be smoothly moved at high speed by the actuating 
unit 54. 
Also, because the hologram lens 49 is integrally formed with the objective 
lens 50, the optical head apparatus 41 can be manufactured at a moderate 
cost in a lightweight and small size. 
Also, because the hologram lens 49 can be integrally formed with the 
objective lens 50, it is not required to independently prevent other types 
of aberration such as the astigmatic aberration and a coma aberration from 
occurring on the information medium 42. In other words, all types of 
aberration of both the hologram lens 49 and the objective lens 50 can be 
prevented as a whole. Accordingly, degree of freedom in the design of the 
objective lens 50 is increased, so that the optical head apparatus 41 can 
be arbitrarily designed to prevent all types of aberration. 
Also, because the hologram lens 49 can be integrally formed with the 
objective lens 50, a holding element for holding the hologram lens 49 and 
a holding element for holding the objective lens 40 can be in common. 
Accordingly, the optical head apparatus 41 can be manufactured at a 
moderate cost in a lightweight and small size. 
As is described above, the variation of the focal length and the occurrence 
of the astigmatic aberration are prevented in the image-formation optical 
system 43. The reason is because the hologram lens 49 is arranged between 
the wedge-like prism 47 and the objective lens 50 to correct the chromatic 
aberration. 
Next, a second embodiment according to the present invention is described 
with reference to FIG. 9. 
FIG. 9 is a constitutional view of an optical head apparatus according to a 
second embodiment of the present invention. 
As shown in FIG. 9, an optical head apparatus 61 for storing and reading 
information in/from the information medium 42, comprises an 
image-formation optical system 62 for optically converging a light beam at 
the information medium 42, and the information detecting system 44. 
The image-formation optical system 62 comprises the semiconductor laser 45, 
the first collimator lens 46, a second hologram lens 63 integrally formed 
with the first collimator lens 46 for auxiliarily correcting the chromatic 
aberration in the image-formation optical system 62, the wedge-like prism 
47, the beam splitter 48, the hologram lens 49, and the objective lens 50. 
The second hologram lens 63 is a phase type diffraction optical element and 
a plane type optical element, and a plurality of reliefs are 
concentrically arranged on a surface of the second hologram lens 63 to 
form a hologram pattern. Therefore, the second hologram lens 63 functions 
as a convex lens to refract the light beam B2. The center of the hologram 
pattern and the center of the first collimator lens 46 are respectively 
positioned on a central line of the outgoing light path to decrease 
off-axis aberration such as astigmatic aberration and coma aberration. 
In the above configuration, there is a possibility that the chromatic 
aberration of the lenses 46, 50 is not sufficiently removed by use of the 
hologram lens 49 while satisfying the equation (5) because appropriate 
lens material of the collimator lens 46 is not found out. However, even 
though the chromatic aberration of the lenses 46, 50 cannot be 
sufficiently removed by use of the hologram lens 49 while satisfying the 
equation (5) to prevent the astigmatic aberration from occurring on the 
information medium 42, the chromatic aberration of the lenses 46, 50 can 
be sufficiently removed by use of the second hologram lens 63 while 
satisfying the equation (5). Therefore, in cases where there is no lens 
material of the first collimator lens 46 having a particular dispersion 
value which is required to remove the chromatic aberration of the lenses 
46, 50 while satisfying the equation (5), the second hologram lens 63 
auxiliarily functions to correct the chromatic aberration of the lenses 
46, 50 in the image-formation optical system 62. 
Accordingly, because the dispersion value of the first collimator lens 46 
is not limited, wavelength-dispersion characteristics in the 
image-formation optical system can be arbitrarily designed. Also, the 
chromatic aberration of the lenses 46, 50 can be completely removed, and 
the occurrence of the astigmatic aberration on the information medium 42 
can be completely prevented. Also, the lenses 46, 50 can be formed by 
utilizing a cheap lens material. 
Next, a third embodiment according to the present invention is described 
with reference to FIG. 10. FIG. 10 is a constitutional view of an optical 
head apparatus according to a third embodiment of the present invention. 
As shown in FIG. 10, an optical head apparatus 71 for storing and reading 
information in/from the information medium 42, comprises the 
image-formation optical system 43, and an information detecting system 72 
for detecting the information read by the image-formation optical system 
43. 
The information detecting system 72 comprises the second collimator lens 
51, a third hologram lens 73 integrally formed with the second collimator 
lens 51 for auxiliarily correcting the chromatic aberration in the 
information detecting system 72, the photo detector 53, and the actuating 
unit 54. 
The third hologram lens 73 is a phase type diffraction optical element and 
a plane type optical element, and a plurality of reliefs are 
concentrically arranged on a surface of the second hologram lens 73 to 
form a hologram pattern. Therefore, the third hologram lens 73 functions 
as a convex lens to refract the light beam B3. The center of the hologram 
pattern and the center of the second collimator lens 51 are respectively 
positioned on a central line of the incoming light path to decrease 
off-axis aberration such as astigmatic aberration and coma aberration. In 
addition, the third hologram lens 73 functions in the same manner as the 
wavefront converting element 52. 
In the above configuration, even though the chromatic aberration of the 
lenses 50, 51 cannot be sufficiently removed in the incoming light path by 
use of the hologram lens 49 while satisfying the equation (5) to prevent 
the astigmatic aberration from occurring on the information medium 42, the 
chromatic aberration of the lenses 50, 51 can be sufficiently removed by 
use of the third hologram lens 73 while satisfying the equation (5). 
Therefore, in cases where there is no lens material of the second 
collimator lens 51 having a particular dispersion value which is required 
to remove the chromatic aberration of the lenses 50, 51 while satisfying 
the equation (5), the third hologram lens 78 auxiliarily functions to 
correct the chromatic aberration of the lenses 50, 51 in the optical head 
apparatus 71. 
Accordingly, because the dispersion value of the second collimator lens S1 
is not limited, wavelength-dispersion characteristics in the optical head 
apparatus 71 can be arbitrarily designed. Also, the chromatic aberration 
of the lenses 50, 51 can be completely removed, and the occurrence of the 
astigmatic aberration on the photo detector 58 can be completely 
prevented. Also, the lenses 50, 51 can be formed by utilizing a cheap lens 
material. 
Next, an information reading method for optically reading information 
stored in the information medium 42 is described according to a fourth 
embodiment with reference to FIGS. 11 to 13. 
In the fourth embodiment, a spot size detection method is utilized to 
detect a focus error signal. The method is proposed in Japanese Patent 
Application No. 185722 of 1990. In short, in cases where the method is 
adopted, an allowable assembly error in an optical head apparatus can be 
remarkably enlarged, and the servo signal such as a focus error signal can 
be stably obtained to adjust the position of the objective lens 50 even 
though the wavelength of the light beam B2 varies. 
FIG. 11 shows a hologram pattern of a plane type of hologram lens 76 
according to the fourth embodiment. As shown in FIG. 11, the hologram lens 
76 is partitioned into a plurality of first diffraction regions 77 and a 
plurality of second diffraction regions 78 alternately arranged in a 
Y-direction. The first and second diffraction regions 77, 78 are provided 
to generate two beams of diffracted light utilized for the detection of a 
focus error. A first hologram pattern is formed on the first diffraction 
regions 77, and a second hologram pattern is formed on the second 
diffraction regions 78. The first and second hologram patterns of the 
first and second diffraction regions 77, 78 are formed by interference 
fringes which are produced by actually interfering two light beams 
according to a two-beam interferometric process. Or, the hologram patterns 
are formed by interference fringes which are produced according to a 
computer generated hologram method. 
FIG. 12 is a constitutional view of an optical head apparatus according to 
the fourth embodiment of the present invention, explanatorily showing two 
types of beams of diffracted light (spherical wave) generated by the 
hologram lens 76 and a design method of the hologram lens 76. 
As shown in FIG. 12, an optical head apparatus 79 for storing and reading 
information in/from the information medium 42, comprises the 
image-formation optical system 43, and an information detecting system 80 
for detecting the information read by the image-formation optical system 
43. The information detecting system 80 comprises the second collimator 
lens 51, the hologram lens 76 integrally formed with the second collimator 
lens 51 for converting a spherical wave of the light beam B3 into a 
plurality of spherical waves having various curvatures, the photo detector 
53, and the actuating unit 54. 
In the above configuration, a spherical wave of the light beam B3 is 
radiated to the first diffraction regions 77 and the second diffraction 
regions 78 of the hologram lens 76. Therefore, the light beam B3 
diffracted in the regions 77 is changed to a first spherical wave of 
first-order diffracted light B4, and the light beam B3 diffracted in the 
regions 78 is changed to a second spherical wave of first-order diffracted 
light B5. A first curvature of the first spherical wave B4 differs from a 
second curvature of the second spherical wave B5. That is, the first-order 
diffracted light B4 has a first focal point F1 in the front of the photo 
detector 53, and the first-order diffracted light B5 has a second focal 
point F2 in the rear of the photo detector 53. 
The photo detector 53 comprises a six-division photo detector 81 in which 
six detecting sections S10, S20, S30, S40, S50, and S60 are provided. The 
intensity of the first-order diffracted light B4 is detected by each of 
the detecting sections S10, S20, and S30 of the photo detector 81 and is 
changed to electric current signals SE1, SE2, and SE3 in an electric 
circuit 82. Also, the intensity of the first-order diffracted light B5 is 
detected by each of the detecting sections S40, S50, and S60 of the photo 
detector 81 and is changed to electric current signals SE4, SE5, and SE6 
in the electric circuit 82. 
FIGS. 13A and 13C respectively show two far field patterns of spherical 
waves radiated to a six-division photo detector of the photo detector 53 
on condition that the objective lens 50 is defocused on the information 
medium 42. FIG. 13B shows two far field patterns of spherical waves 
radiated to a six-division photo detector of the photo detector 53 on 
condition that the objective lens 50 is just focused on the information 
medium 42. 
As shown in FIGS. 13A to 13C, a first far field pattern of the first 
spherical wave B4 reflecting on the detecting sections S10, S20, and S30 
of the photo detector 81 is formed in the same manner as an inverted image 
of the first diffraction regions 77. Also, a second far field pattern of 
the second spherical wave B5 reflecting on the detecting sections S40, 
S50, and S60 of the photo detector 81 is formed in the same manner as an 
erecting image of the second diffraction regions 78. In cases where the 
light beam B2 is converged at the information medium 42 on condition that 
the objective lens 50 is defocused on the information medium 42, the first 
far field pattern shown at the left side of FIGS. 13A or 13C is formed on 
the photo detector 81, and the second far field pattern shown at the right 
side of FIGS. 13A or 13C is formed on the photo detector 81. In contrast, 
in cases where the light beam B2 is converged at the information medium 42 
on condition that the objective lens 50 is just focused on the information 
medium 42, the first far field pattern shown at the left side of FIG. 13B 
is formed on the photo detector 81, and the second far field pattern shown 
at the right side of FIG. 13B is formed on the photo detector 81. 
The intensity of the first spherical wave B4 is detected in the detecting 
sections S10, S20, and S30 of the photo detector 53 and is changed to 
electric current signals SE10, SE20, SE30 in the electric circuit 82. 
Also, the intensity of the second spherical wave B5 is detected in the 
detecting sections S40, S50, and S60 of the photo detector 53 and is 
changed to electric current signals SE40, SE50, SE60 in the electric 
circuit 82. Thereafter, a focus error signal S.sub.fe is obtained in the 
electric circuit 82 according to an equation (7) . 
EQU S.sub.fe =(SE10+SE30-SE20)-(SE40+SE60-SE50) (7) 
Thereafter, the position of the objective lens 50 is adjusted by slightly 
moving the objective lens 50 in a direction of the outgoing light path at 
high speed so as to minimize the absolute value of the focus error signal 
S.sub.fe. 
In this case, the first far field pattern of the first spherical wave B4 is 
divided into many pieces because the first hologram pattern formed on the 
first diffraction region 77 is divided into many pieces, and the second 
far field pattern is divided into many pieces because the second hologram 
pattern formed on the second diffraction region 78 is divided into many 
pieces. However, the division of each of the far field patterns does not 
influence on the focus error signal S.sub.fe. 
Also, because the first focal point F1 of the diffracted light B4 is 
positioned in the front of the photo detector 53 and because the second 
focal point F2 of the diffracted light B5 is positioned in the rear of the 
photo detector 53, both terms SE10+SE30-SE20 and SE40+SE60-SE50 in the 
equation (7) are inversely decreased or increased. Accordingly, even 
though two far field patterns of the first and second spherical waves B4, 
B5 reflect on the photo detector 81 as shown in FIGS. 13A or 13C, the 
focus error signal S.sub.fe can be easily approached zero, and two far 
field patterns of the first and second spherical waves B4, B5 shown in 
FIG. 18B can be obtained. 
In the spot size detection method, the curvatures of the spherical waves 
B4, B5 differ from each other to detect the focus error signal S.sub.fe. 
However, two beams of diffracted light B4, B5 radiated to the photo 
detector 81 is not limited to the spherical waves to detect the focus 
error signal S.sub.fe according to the spot size detection method. That 
is, because the change of the far field patterns in an X-direction is 
detected by the photo detector 81 according to the spot size detection 
method, it is required that a one-dimensional focal point of the 
diffracted light B4 in the X-direction (focal line in a Y-direction) is 
positioned in the front of the photo detector 81 and a one-dimensional 
focal point of the diffracted light B5 in the X-direction (focal line in 
the Y-direction) is positioned in the rear of the photo detector 81. 
Therefore, it is applicable that diffracted light including astigmatic 
aberration be radiated to the photo detector 81. 
An information signal S.sub.in is obtained in the electric circuit 82 by 
adding all of the electric current signals according to an equation (8). 
EQU S.sub.in =SE10+SE20+SE30+SE40+SE50+SE60 (8) 
Because the information medium 42 is rotated at high speed, a patterned pit 
radiated by the small light spot Ls of the light beam B2 is rapidly 
changed one after another, so that the intensity of the information signal 
S.sub.in is changed. Therefore, the information stored in the information 
medium 42 can be reproduced by the information signal S.sub.in. 
Next, the reason that the hologram lens 76 is partitioned into many first 
diffraction regions 77 and many second diffraction regions 78 is 
described. 
When the light beam B2 is reflected by the information medium 42, the light 
beam B2 is diffracted by a track pit formed on the information medium 42 
to form a diffraction pattern in the light beam B3. Therefore, the 
intensity distribution of the light beam B3 radiated on the hologram lens 
76 changes in dependence on the positional relation between the light spot 
Ls of the light beam B2 and the track pit. For example, in cases where an 
extending direction of the diffraction regions 77, 78 (or X-direction in 
FIG. 12) is the same as that of the track pit, the intensity of the light 
beam B3 radiated to the upper half diffraction regions 77, 78 (positioned 
in +Y direction) of the hologram lens 76 is increased (or decreased). In 
contrast, the intensity of the light beam B3 radiated to the lower half 
diffraction regions 77, 78 (positioned in -Y direction) of the hologram 
lens 76 is decreased (or increased). Therefore, in cases where asymmetry 
of the first diffraction regions 77 and asymmetry of the second 
diffraction regions 78 are large in the Y direction, an offset of the 
focus error signal S.sub.fe occurs when the focus error signal S.sub.fe is 
produced according to the spot size detection method. In other words, even 
though the objective lens 50 is moved according to the focus error signal 
S.sub.fe to focus on the information medium 42, the value of the focus 
error signal S.sub.fe does not become zero. 
To solve the above drawback, a large number of diffraction regions 77, 78 
are alternately arranged on the hologram lens 76 to lessen both the 
asymmetry of the first diffraction regions 77 and the asymmetry of the 
second diffraction regions 78 in the Y direction. For example, it is 
preferred that the number of the diffraction regions 77, 78 be in a range 
from several to several tens regions. Therefore, the occurrence of the 
offset in the focus error signal S.sub.fe can be prevented in the fourth 
embodiment of the present invention. 
Accordingly, a stable focus servo characteristic can be obtained by 
partitioning the hologram lens 76 into many diffraction fields 77, 78. 
Also, because the hologram lens 76 additionally functions as a convex lens, 
a wavefront changing element, and a light beam dividing element, the 
hologram lens 76 can function as a servo signal generating optical 
element. Therefore, the number of constitutional elements in the optical 
head apparatus 71 can be decreased. As a result, a lightweight optical 
head apparatus can be reliably manufactured at a moderate cost according 
to simplified manufacturing processes. 
Next, another information reading method for optically reading information 
stored in the information medium 42 with the optical head apparatus 71 is 
described according to a fifth embodiment with reference to FIGS. 14, 15. 
FIG. 14 shows a hologram pattern of a hologram lens 83 according to the 
fifth embodiment. As shown in FIG. 14, the hologram lens 83 is partitioned 
into the first diffraction regions 77, the second diffraction regions 78, 
a third diffraction region 84, and a fourth diffraction region 85. The 
third and fourth diffraction regions 84, 85 are provided to generate two 
beams of diffracted light utilized for the detection of a tracking error. 
FIG. 15 is a constitutional view of an optical head apparatus according to 
the fifth embodiment, explanatorily showing two beams of diffracted light 
(spherical wave) generated by the hologram lens 83 and a design method of 
the hologram lens 83. 
As shown in FIG. 15, an optical head apparatus 86 for storing and reading 
information in/from the information medium 42, comprises the 
image-formation optical system 43, and an information detecting system 87 
for detecting the information read by the image-formation optical system 
43. The information detecting system 87 comprises the second collimator 
lens 51, the hologram lens 83 integrally formed with the second collimator 
lens 51 for converting a spherical wave of the light beam B3 into a 
plurality of spherical waves having various curvatures, the photo detector 
53, and the actuating unit 54. 
In the above configuration, a spherical wave of the light beam B3 is 
radiated to the diffraction regions 77, 78, 84, and 85 of the hologram 
lens 83. Therefore, the light beam B3 diffracted in the third diffraction 
region 84 is changed to a third spherical wave of first-order diffracted 
light B6, and the light beam B3 diffracted in the fourth region 85 is 
changed to a fourth spherical wave of first-order diffracted light B7. 
Also, the first spherical wave of first-order diffracted light B4 and the 
second spherical wave of first-order diffracted light B5 are generated by 
the hologram lens 83. The first-order diffracted light B6 has a third 
focal point F3, and the first-order diffracted light B7 has a fourth focal 
point F4. 
The photo detector 53 comprises the six-division photo detector 81, a 
tracking error photo detector 88 in which two detecting sections S70 and 
S80 are provided. The intensity of the first-order diffracted light B6 is 
detected in the detecting section S70 of the photo detector 88 and is 
changed to an electric current signal SE70 in the electric circuit 82. 
Also, the intensity of the first-order diffracted light B7 is detected in 
the detecting section S70 of the photo detector 88 and is changed to an 
electric current signal SE80 in the electric circuit 82. Thereafter, a 
tracking error signal S.sub.te is calculated in the electric circuit 82 
according to an equation (9). 
EQU S.sub.te =SE70-SE80 (9) 
Therefore, the asymmetry of the intensity distribution of the light beam B3 
radiated on the hologram lens 83, which changes in dependence on the 
positional relation between the light spot Ls of the light beam B2 and the 
track pit, is indicated by the tracking error signal S.sub.te. 
Thereafter, the objective lens 50 is moved in a radial direction so as to 
reduce a tracking error indicated by the tracking error signal S.sub.te. 
The radial direction is defined as a direction perpendicular to both the 
outgoing light path and the track pit. Therefore, the light spot Ls of the 
light beam B2 on the information medium 42 can be formed in the middle of 
the track pit, so that the tracking error becomes zero. 
Also, two beams of diffracted light B4, B5 are detected in the photo 
detector 81, and the focus error signal S.sub.fe and the information 
signal S.sub.in are obtained in the same manner as in the fourth 
embodiment. Therefore, the objective lens 50 is focused on the information 
medium 42. 
Accordingly, focus and tracking servo characteristics can be stably 
obtained by partitioning the hologram lens 83 into many diffraction fields 
77, 78, 84, and 85. 
Also, because the hologram lens 83 additionally functions as a convex lens, 
a wavefront changing element, and a light beam dividing element, the 
hologram lens 83 can function as a servo signal generating optical 
element. Therefore, the number of constitutional elements in the optical 
head apparatus 86 can be decreased. As a result, a lightweight optical 
head apparatus can be reliably manufactured at a moderate cost according 
to simplified manufacturing processes. 
Next, control of the diffraction efficiency of the hologram lens 49 
utilized in the optical head apparatuses 41, 61, 71, 79, and 88 is 
described according to sixth embodiment. Here the diffraction efficiency 
is defined as a converting efficiency of the light beam B2 into 
first-order diffracted light in the hologram lens 49. 
The diffraction efficiency of the hologram lens 49 changes in dependence on 
the wavelength .lambda. of the light beam B2 radiated from the 
semiconductor laser 45. Also, the wavelength .lambda..sub.R of the light 
beam B2 in a reading operation differs from the wavelength .lambda..sub.w 
of the light beam B2 in a writing operation. That is, the wavelength 
.lambda..sub.w is several nm larger than the wavelength .lambda..sub.R. In 
addition, in cases where the diffraction efficiency of the hologram lens 
49 is lowered, unnecessary diffracted light such as zero-order diffracted 
light (or transmitting light), minus first-order diffracted light, and 
second-order diffracted light is generated in the hologram lens 49. 
Therefore, an appropriate design of the diffraction efficiency is required 
to efficiently store and read information in/from the information medium 
42 with high accuracy. 
The hologram lens 49 can be manufactured by repeating a lithography process 
and an etching process. Therefore, a plurality of relieves are 
concentrically arranged on the surface of the hologram lens 49 in echelon 
shape as a multilevel hologram, as shown in FIG. 16. Or, the hologram lens 
49 can be manufactured with the cutting tool 30 of a super precision CNC 
lathe shown in FIG. 4. In cases where the light beam B2 transmits through 
the hologram lens 49, phase difference occurs in the light beam B2 so that 
the light beam B2 is diffracted. The degree of the phase difference 
occurring in the light beam B2 depends on a height H of the reliefs, the 
wavelength .lambda. of the light beam B2, and a refractive index 
n(.lambda.) of the hologram lens 49. The diffraction efficiency relates to 
the degree of the phase difference. Therefore, an appropriate design in 
which the diffraction efficiency is maximized in both the reading and 
writing operations is impossible. 
In the present invention, the height H of the reliefs is determined to 
maximize the diffraction efficiency in the reading operation. The reason 
is as follows. In cases where the diffraction efficiency is lowered in the 
reading operation, the unnecessary diffracted light is converged at the 
information medium 42 and is reflected to the photo detector 53. The 
unnecessary diffracted light functions as noise in the information signal 
S.sub.in. Therefore, the decrease of the diffraction efficiency in the 
reading operation adversely influences on the operation in the optical 
head apparatuses 41, 61, 71, 79 and 86. In contrast, even though the 
diffraction efficiency is lowered in the writing operation to increase the 
unnecessary diffracted light, the intensity ratio of the unnecessary 
diffracted light to the first-order diffracted light (or necessary 
diffracted light) is no more than 1% because the difference between the 
wavelength .lambda..sub.R of the light beam B2 in the reading operation 
and the wavelength .lambda..sub.w of the light beam B2 in the writing 
operation is no more than several nm. Also, light having an intensity over 
a lower limit is required to write a piece of information in the 
information medium 42. Therefore, no piece of information is written in 
the information medium 42 by the unnecessary diffracted light in the 
writing operation. Accordingly, the decrease of the diffraction efficiency 
in the writing operation does not adversely influence on the operation in 
the optical head apparatuses 41, 61, 71, 79 and 86. 
The height H of the reliefs is formulated by an equation (10). 
EQU H=.lambda..sub.R /(n(.lambda..sub.R)-1) (10) 
Accordingly, the information signal S.sub.in does not deteriorate, and the 
noise included in the information signal S.sub.in can be lowered. Also, 
any piece of information can be written in the information medium 42 with 
high accuracy. 
Next, a seventh embodiment according to the present invention is described 
with reference to FIGS. 17, 18. 
FIG. 17 is a constitutional view of an optical head apparatus according to 
a seventh embodiment of the present invention. 
As shown in FIG. 17, an optical head apparatus 91 for storing and reading 
information in/from the information medium 42, comprises an 
image-formation optical system 92 for optically converging a light beam at 
the information medium 42, and the information detecting system 44. The 
image-formation optical system 92 comprises the semiconductor laser 45 for 
radiating a light beam B8 linearly polarized, the first collimator lens 
46, the wedge-like prism 47, the beam splitter 48, and a combined lens 93 
for converging the light beam B8 at the information medium 42. 
FIG. 18 is a cross-sectional view of the combined lens 93, explanatorily 
showing the light beam B8 diffracted in an outgoing light path and not 
diffracted in an incoming light path. 
As shown in FIG. 18, the combined lens 93 comprises a polarizing 
anisotropic hologram lens 94, a quarter-wave plate 95 of which one side is 
attached on a flat surface of the hologram lens 94, and an objective lens 
96 attached on another side of the plate 95. 
The hologram lens 94 is a plane type optical element, and a plurality of 
reliefs are concentrically arranged on a surface of the hologram lens 94 
to form a hologram pattern. The center of the hologram pattern and the 
center of the objective lens 96 are respectively positioned on a central 
line of the outgoing light path to decrease off-axis aberration such as 
astigmatic aberration and coma aberration. Therefore, the polarizing 
anisotropic hologram lens 94 functions as a hologram lens for first light 
linearly polarized in an X direction. That is, the first linearly 
polarized light is diffracted and refracted by the hologram lens 94. In 
contrast, the polarizing anisotropic hologram lens 94 functions as a 
transparent plate for second light linearly polarized in a Y direction 
perpendicular to the X direction. That is, the second linearly polarized 
light transmits through the hologram lens 94 without being diffracted. The 
function of the polarizing anisotropic hologram lens 94 was laid open to 
public inspection under Provisional Publication No. 189504/86 
(S61-189504). 
A focal length of the objective lens 96 is lengthened as the wavelength of 
the light beam B8 becomes longer. In contrast, as the wavelength of the 
light beam B8 becomes longer, a focal length of the hologram lens 94 is 
shortened to cancel out the increase of the focal lengths of both the 
first collimator lens 46 and the objective lens 96. Therefore, a combined 
focal length of both the hologram lens 94 and the objective lens 96 is 
shortened as the wavelength of the light beam B8 becomes longer, and a 
combined focal length of the lenses 46, 94 and 96 is constant even though 
the wavelength of the light beam B8 changes. 
In the above configuration, coherent light radiated from an active layer of 
the semiconductor laser 43 is linearly polarized, as is well known. 
Therefore, the semiconductor laser 45 is positioned to radiate light beam 
B8 linearly polarized in the X direction. Thereafter, the linearly 
polarized light beam B8 transmits through the first collimator lens 46, 
the wedge-like prism 47, and the beam splitter 48. Thereafter, the 
linearly polarized light beam B8 is diffracted and refracted by the 
polarizing anisotropic hologram lens 94 of the combined lens 93 in the 
outgoing light path. Thereafter, the linearly polarized light beam B8 is 
converted into light beam B9 circularly polarized (or circular polarized 
light B9) by the quarter-wave plate 95. Thereafter, the circular polarized 
light beam B9 is converged at the information medium 42 by the objective 
lens 96. When the light beam B9 is reflected by the information medium 42, 
a rotating direction of the circular polarized light B9 is reversed to 
produce circular polarized light beam B10. Thereafter, the circular 
polarized light beam B10 is converted into light beam B11 linearly 
polarized in the Y direction by the quarter-wave plate 95. Thereafter, 
because the light beam B11 is linearly polarized in the Y direction, the 
light beam B11 transmits through the polarizing anisotropic hologram lens 
94 without being diffracted in the incoming light path. Therefore, the 
incoming light path differs from the outgoing light path. Thereafter, a 
major part of the light Beam B11 is split by the beam splitter 48 and is 
detected by the photo detector 53. Therefore, the information signal 
S.sub.in, the tracking error signal S.sub.te and the focus error signal 
S.sub.fe are obtained. Also, a remaining part of the light beam B11 
transmits through the beam splitter 48, the wedge-like prism 47, and the 
first collimator lens 46 before the light beam B11 is fed back to the 
semiconductor laser 45. In this case, Because the incoming light path 
differs from the outgoing light path, the light Beam B11 fed back is not 
coupled to the active layer of the semiconductor laser 45. 
Accordingly, the wavelength .lambda. of the light beam B8 radiated from the 
semiconductor laser 45 is stabilized because a radiation mode change of 
the light beam B8 caused by light fed back to the active layer can be 
prevented. Therefore, noise included in the light beam B8 can be removed. 
Also, the chromatic aberration of the objective lens 96 is excessively 
corrected by the hologram lens 94 because a combined focal length of both 
the hologram lens 94 and the objective lens 96 is shortened as the 
wavelength of the light beam B8 becomes longer. Therefore, though the 
focal length f.sub.c of the first collimator lens 46 is lengthened as the 
wavelength of the light beam B8 becomes longer, the excessive correction 
of the hologram lens 94 cancels out the chromatic aberration of the first 
collimator lens 46. 
Accordingly, the chromatic aberration in the image-formation optical system 
92 can be canceled out because the combined lens 93 is provided in the 
system 92. Also, even though the wavelength of the light beam B8 radiated 
from the semiconductor laser 43 changes, a combined focal length of the 
lenses 46, 94, and 96 becomes constant. Therefore, a small light spot Ls 
of the light beam B8 can be always formed on the information medium 42 
without occurring the chromatic aberration. 
Also, because only a combined set of the hologram lens 94 and the objective 
lens 96 is additionally provided in the system 92 and because the hologram 
lens 94 made of the plane type optical element is light in weight, the 
image-formation optical system 92 can be manufactured in lightweight and 
small size, as compared with in the third previously proposed art. 
In addition, in cases where the wavelength of the light beam B8 becomes 
longer because of the increase of output intensity of the light beam B8 
radiated from the semiconductor laser 43, the focal length f.sub.c of the 
first collimator lens 46 is lengthened. In this case, the difference 
.delta..sub.v is decreased to cancel out the increase of the focal length 
f.sub.c, and the difference .delta..sub.H is decreased to cancel out the 
increase of the focal length f.sub.c. Therefore, the ratio .delta..sub.H 
/.delta..sub.v is forced to be increased according to an external effect. 
In contrast, as the output intensity of the light beam B8 is increased, 
the astigmatic difference .delta. is decreased. Therefore, the ratio 
.delta..sub.H /.delta..sub.v is forced to be decreased according to an 
internal effect. As a result, the increase of the ratio .delta..sub.H 
/.delta..sub.v according to the external effect is canceled out by the 
decrease of the ratio .delta..sub.H /.delta..sub.v according to the 
internal effect. 
Accordingly, even though the wavelength of the light beam B8 radiated from 
the semiconductor laser 43 changes, the equation (6) can be stably 
satisfied. In other words, no astigmatic aberration occurs on the 
information medium 42. 
Also, because the hologram lens 94 functions as the convex lens, the 
curvature of the objective lens 96 can be small. Also, lens material 
having a comparatively large dispersion value .DELTA.n/(n-1) (or Abbe 
number (n-1)/.DELTA.n is less than 60) can be utilized for the objective 
lens 96. In this case, because a refractive index of the objective lens 96 
can be comparatively large, the curvature of the objective lens 96 can be 
moreover small. Accordingly, the objective lens 96 can be easily produced, 
and the image-formation optical system 92 can be manufactured at a 
moderate cost. 
The weight of the hologram lens 94 becomes light because the hologram lens 
94 is the plane type optical element. Also, because the hologram lens 94 
functions as the convex lens, the function as the convex lens required of 
the objective lens 96 is reduced. Therefore, the objective lens 96 can be 
light in weight. Accordingly, even though the hologram lens 94 is 
integrally formed with the objective lens 96, the hologram lens 94 and the 
objective lens 96 can be smoothly moved at high speed by the actuating 
unit 54. 
Also, because the hologram lens 94 is integrally formed with the objective 
lens 96, the optical head apparatus 91 can be manufactured at a moderate 
cost in a lightweight and small size. 
Also, because the hologram lens 94 is integrally formed with the objective 
lens 96, it is not required to independently prevent other types of 
aberration such as the astigmatic aberration and a coma aberration from 
occurring on the information medium 42. In other words, all types of 
aberration of both the hologram lens 94 and the objective lens 96 can be 
prevented as a whole. Accordingly, degree of freedom in the design of the 
objective lens 96 is increased, so that the optical head apparatus 91 can 
be arbitrarily designed to prevent all types of aberration. 
Also, because the hologram lens 94 is integrally formed with the objective 
lens 96, a holding element for holding the hologram lens 94 and a holding 
element for holding the objective lens 40 can be in common. Accordingly, 
the optical head apparatus 91 can be manufactured at a moderate cost in a 
lightweight and small size. 
Next, an eighth embodiment according to the present invention is described 
with reference to FIGS. 19, 20. 
FIG. 19 is a constitutional view of an optical head apparatus according to 
an eighth embodiment of the present invention. 
As shown in FIG. 19, an optical head apparatus 97 for storing and reading 
information in/from the information medium 42, comprises an 
image-formation optical system 98 for optically converging a light beam at 
the information medium 42, and the information detecting system 44. The 
image-formation optical system 98 comprises the semiconductor laser 45, a 
combined collimator lens 99, the wedge-like prism 47, the beam splitter 
48, the hologram lens 49, and the objective lens 50. 
FIG. 20 is a cross-sectional view of the combined collimator lens 99, 
explanatorily showing the light beam B8 diffracted in an outgoing light 
path and not diffracted in an incoming light path. 
As shown in FIG. 20, the combined collimator lens 99 comprises the 
polarizing anisotropic hologram lens 94, the quarter-wave plate 95, and a 
collimator lens 100 attached on another side of the plate 95. 
A focal length of the collimator lens 100 is lengthened as the wavelength 
of the light beam B8 becomes longer. The chromatic aberration of the 
collimator lens 100 is auxiliarily corrected by the hologram lens 94. 
Therefore, a combined focal length of the lenses 50, 94 and 99 is constant 
even though the wavelength of the light beam B8 changes. 
In the above configuration, the light beam B8 linearly polarized in the X 
direction is radiated from the semiconductor laser 48. Thereafter, the 
light beam B8 is diffracted and refracted by the polarizing anisotropic 
hologram lens 94 of the combined lens 99 in the outgoing light path. 
Thereafter, the light beam B8 linearly polarized is converted into light 
beam B12 circularly polarized (or circular polarized light B12) by the 
quarter-wave plate 95. Thereafter, the light beam B12 is collimated by the 
collimator lens 100. Thereafter, the light beam B12 transmits through the 
wedge-like prism 47, the beam splitter 48, the hologram lens 49 before the 
light beam B12 is converged at the information medium 42 by the objective 
lens 96. When the light beam B12 is reflected by the information medium 
42, a rotating direction of the circular polarized light B12 is reversed 
to produce circular polarized light beam B13. Thereafter, the circular 
polarized light beam B13 transmits through the objective lens 50 and the 
hologram lens 49. Thereafter, a major part of the circular polarized light 
beam B13 is split by the beam splitter 48 and is detected by the photo 
detector 53. Therefore, the information signal S.sub.in, the tracking 
error signal S.sub.te and the focus error signal S.sub.fe are obtained. 
Also, a remaining part of the circular polarized light beam B13 transmits 
through the beam splitter 48 and the wedge-like prism 47. Thereafter, the 
circular polarized light beam B13 is converged by the collimator lens 100 
and is converted into light beam B14 linearly polarized in the Y direction 
perpendicular to the X direction by the quarter-wave plate 95. Thereafter, 
because the light beam B14 is linearly polarized in the Y direction, the 
light beam B14 transmits through the polarizing anisotropic hologram lens 
94 without being diffracted in the incoming light path. Therefore, the 
incoming light path differs from the outgoing light path. Thereafter, the 
light beam B14 is fed back to the semiconductor laser 45. In this case, 
because the incoming light path differs from the outgoing light path, the 
light beam B14 fed back is not coupled to an active layer of the 
semiconductor laser 45. 
Accordingly, the wavelength .lambda. of the light beam B8 radiated from the 
semiconductor laser 45 is stabilized because a radiation mode change of 
the light beam B8 caused by light fed back to the active layer can be 
prevented. Therefore, noise included in the light beam B8 can be removed. 
The weight of the hologram lens 94 becomes light because the hologram lens 
94 is the plane type optical element. Also, because the hologram lens 94 
functions as the convex lens, the function as the convex lens required of 
the collimator lens 100 is reduced. Therefore, the collimator lens 96 can 
be light in weight. Accordingly, the hologram lens 94 can be integrally 
formed with the collimator lens 100. Also, because the hologram lens 94 is 
integrally formed with the collimator lens 100, the optical head apparatus 
97 can be manufactured at a moderate cost in a lightweight and small size. 
Next, an optical information apparatus for optically reading or storing 
information from/in the storage medium with the optical head apparatus 41, 
61, 71, 79, 86, 91, or 97. 
FIG. 21 is a constitutional view of an optical information apparatus 
according to a ninth embodiment. 
As shown in FIG. 21, an optical information apparatus 111 comprises the 
optical head apparatus 41 (or 61, 71, 79, 86, 91, or 97), an information 
medium driving mechanism 112 for rotating the information medium 42, an 
optical head driving apparatus 113 for roughly positioning the objective 
lens 50 (or the combined lens 93) of the apparatus 41 on a track pit of 
the information medium 42, a control circuit 114 for generating a servo 
control signal according to a focus error signal S.sub.fe and a tracking 
error signal S.sub.tr generated in the apparatus 41, and an external 
electric source 115 for supplying electric current to the information 
medium driving mechanism 112 and the optical head driving apparatus 113. 
In the above configuration, the information medium 42 is initially rotated 
by the information medium driving mechanism 112. Thereafter, the apparatus 
41 is moved by the optical head driving apparatus 113 so that the 
objective lens 50 is roughly positioned on a track pit of the information 
medium 42. Thereafter, a focus error signal S.sub.fe and a tracking error 
signal S.sub.tr are generated in the apparatus 41, and a control signal is 
generated in the control circuit 114 according to the focus error signal 
S.sub.fe and the tracking error signal S.sub.tr. Thereafter, the objective 
lens 50 is precisely positioned on the track pit under the control of the 
servo control signal with the optical head driving apparatus 113. 
Therefore, information indicated by the track pit can be read. Also, a 
piece of information can be written in a track of the information medium 
42 in the same manner. 
Accordingly, because the optical head apparatus 41, 71, 79, 86, 91, or 97 
in which an information signal S.sub.in having a superior S/N ratio is 
obtained is utilized in the optical information apparatus 111, the 
information stored in the information medium 42 can be stably reproduced 
with high accuracy. 
Also, because the optical head apparatus 41, 71, 79, 86, 91, or 97 is 
lightweight in a small size, the optical information apparatus 111 can be 
lightweight in a small size, and access time can be shortened. 
Also, because the chromatic aberration in the optical head apparatus 41, 
71, 79, 86, 91, or 97 is suppressed, pieces of information can be stably 
stored in the information medium 42 in the optical information apparatus 
111 with high accuracy. In addition, the information stored in the 
information medium 42 can be stably reproduced at high S/N ratio. 
Also, because the astigmatic aberration caused by the astigmatic difference 
.delta. of the semiconductor laser 45 is suppressed in the optical head 
apparatus 41, 71, 79, 86, 91, or 97, pieces of information can be stably 
stored in the information medium 42 in the optical information apparatus 
111 with high accuracy. In addition, the information stored in the 
information medium 42 can be stably reproduced at high S/N ratio. 
Having illustrated and described the principles of our invention in a 
preferred embodiment thereof, it should be readily apparent to those 
skilled in the art that the invention can be modified in arrangement and 
detail without departing from such principles. We claim all modifications 
coming within the spirit and scope of the accompanying claims.