Optical head device capable of reducing the light power loss

An optical head for an optical disk drive which can reduce the light power loss due to the beam characteristics of a laser beam to improve the efficiency of use of laser power. The optical head includes a laser diode, an objective lens for focusing a laser beam emitted from the laser diode onto an optical disk, an optical signal detector for detecting an optical signal from a reflected beam reflected on the optical disk, an error signal detector for detecting a focusing error and a tracking error of the laser beam focused on the optical disk from the reflected beam, and a collimator lens for collimating the laser beam emitted from the laser diode. Further, a hologram-lens unit is located between the laser diode and the collimator lens. The hologram-lens unit includes a gradient index microlens formed on one surface of a transparent substrate and a hologram formed on the other surface of the transparent substrate. The hologram transmits the laser beam emitted from the laser diode, and diffracts the reflected beam toward the error signal detector.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an optical head (optical pickup) for an 
optical disk drive. 
2. Description of the Related Art 
An optical disk inclusive of a magneto-optical disk is in the limelight as 
a memory medium which has become the core in the rapid development of 
multimedia in recent years. Usually, the optical disk is accommodated in a 
cartridge for actual use. An optical disk cartridge is loaded into an 
optical disk drive to read/write data on the optical disk by an optical 
head. In general, a semiconductor laser (laser diode) is used as a light 
source in the optical disk drive. 
A recording medium such as an optical disk and a magneto-optical disk is 
replaced by another similar recording medium in use, and these recording 
media have warpage or undulation due to strain in molding. As a result, 
such a recording medium tends to have eccentricity and tilt. Accordingly, 
focusing error detection and tracking error detection must be accurately 
performed to read information recorded on the recording medium. 
A conventional optical head for an optical disk drive is configured by 
using many optical components including a plurality of lenses and a 
plurality of polarized beam splitters to detect information recorded on an 
optical disk and further detect a focusing error and a tracking error. As 
another conventional optical head for an optical disk drive, various 
configurations employing a hologram have been proposed. An example of such 
a conventional optical head employing a hologram will now be described 
with reference to FIG. 1. 
A laser beam 4 emitted from a semiconductor laser or laser diode 2 is 
transmitted by a hologram unit 6 including a transparent substrate 8 and a 
hologram 10 formed on the substrate 8, and next enters a collimator lens 
12, by which the laser beam 4 is converted into a collimated beam 14. The 
collimated beam 14 transmits through a polarized beam splitter 16 and 
enters an objective lens 18, by which the collimated beam 14 is focused on 
a magneto-optical disk 20. 
A reflected beam reflected on a recording surface of the magneto-optical 
disk 20 is reconverted into a collimated beam by the objective lens 18, 
and next enters the polarized beam splitter 16, by which the collimated 
beam is split into a reflected beam 22 and a transmitted beam 24. The beam 
22 reflected by the polarized beam splitter 16 is separated into a 
P-polarized light component and an S-polarized light component by a 
Wollaston prism 26, and next detected by a photodetector unit 30. The 
photodetector unit 30 includes a photodetector for detecting the 
P-polarized light component and a photodetector for detecting the 
S-polarized light component. Signals detected by the two photodetectors 
are subjected to differential detection well known in the art, thereby 
detecting a magneto-optical signal. 
The beam 24 transmitted by the polarized beam splitter 16 is condensed by 
the collimator lens 12 to enter the hologram 10, by which the incident 
beam 24 is diffracted toward a photodetector unit 32 and detected by the 
photodetector unit 32. The photodetector unit 32 includes a plurality of 
first photodetectors for detecting a focusing error signal and a plurality 
of second photodetectors for detecting a tracking error signal. Then, the 
focusing error signal and the tracking error signal are detected by the 
first photodetectors and the second photodetectors, respectively. 
In the optical head shown in FIG. 1, the effective use of laser power is of 
importance. Therefore, an antireflection film coating is formed on the 
surface of each optical component to optimize the transmittances and/or 
reflectivities of the hologram 10, the polarized beam splitter 16, etc. 
However, the loss of laser power is yet high in the above optical head. 
One of the causes of such high loss is the divergence of the laser beam 4 
emitted from the laser diode 2. That is, all proportions of the laser beam 
4 emitted from the laser diode 2 are not incident on the collimator lens 
12. Specifically, about 40% of the laser beam 4 does not enter the 
collimator lens 12 because of the divergence of the laser beam 4 as shown 
in FIG. 1. 
As shown in FIG. 2A, the collimator lens 12 is conventionally positioned 
with respect to the laser diode 2 so that about 40% of the laser beam 4 
emitted from the laser diode 2 does not enter the collimator lens 12. 
Although the light not entering the collimator lens 12 becomes loss, a 
large aperture in the objective lens 18 can be obtained to improve the 
focusing characteristics of the laser beam on the surface of the 
magneto-optical disk 20. 
FIG. 3 shows the relation between the beam shape (beam pattern) of the 
laser beam 4 emitted from the laser diode 2 and the collimator lens 12 in 
the case that emphasis is placed on the focusing characteristic of the 
laser beam. As apparent from FIG. 3, the laser beam 4 emitted from the 
laser diode is elliptical in cross section. In the case that emphasis is 
placed on incorporation of the laser beam 4 from the laser diode 2 into 
the collimator lens 12, the positional relation between the collimator 
lens 12 and the laser diode 2 is changed to a relation shown in FIG. 2B. 
That is, as shown in FIG. 2B, all proportions of the elliptical laser beam 
4 emitted from the laser diode 2 enter a collimator lens 12'. 
In the latter case shown in FIG. 2B, however, a component of the elliptical 
laser beam 4 along its minor-axis direction enters the objective lens 18 
with insufficient aperture of incident light on the objective lens 18. 
Accordingly, a beam spot formed on the magneto-optical disk 20 becomes 
large. Of the above two cases, the beam spot size on the recording medium 
is conventionally taken as a matter of high priority. Accordingly, the 
collimator lens 12 is positioned with respect to the laser diode 2 so that 
about 40% of the laser beam 4 does not enter the collimator lens 12 as 
shown in FIG. 2A. In this case, however, the focusing characteristics of 
the beam by the objective lens are degraded because of the astigmatic 
difference of the laser beam. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide an optical 
head for an optical disk drive which can reduce the light power loss due 
to the characteristics of the laser beam emitted from the laser diode, 
thereby efficiently using the laser power. 
In accordance with an aspect of the present invention, there is provided an 
optical head for an optical disk drive, for reading and writing 
information on an optical disk, the optical head comprising a laser diode 
for emitting a laser beam; an objective lens for focusing the laser beam 
onto the optical disk; an optical signal detector for detecting an optical 
signal from a reflected beam reflected on the optical disk; an error 
signal detector for detecting a focusing error and a tracking error of the 
laser beam focused on the optical disk, from the reflected beam; a first 
lens for collimating the laser beam emitted from the laser diode; a 
transparent plane substrate interposed between the laser diode and the 
first lens, the transparent plane substrate having a first surface opposed 
to the laser diode and a second surface opposed to the first lens; a 
second lens formed integrally with the transparent plane substrate on the 
first surface or in the vicinity of the first surface of the substrate; 
and a hologram formed integrally with the transparent plane substrate on 
the second surface, for transmitting the laser beam emitted from the laser 
diode and diffracting the reflected beam toward the error signal detector. 
Preferably, the second lens has an astigmatic difference compensating for 
the astigmatic difference of the laser beam emitted from the laser diode. 
For example, the second lens is formed in an elliptical shape, and is 
positioned so that the major-axis direction of the second lens coincides 
with the minor-axis direction of the laser beam. Further, the second lens 
is formed as a gradient index lens. In this case, the surface of the lens 
element can be maintained plane. As a result, a thin film such as a 
reflection preventing film coating can be easily added to the surface of 
the lens element, thereby improving the convenience in handling. 
In accordance with another aspect of the present invention, there is 
provided an optical head for an optical disk drive, for reading and 
writing information on an optical disk, the optical head comprising a 
laser diode for emitting a laser beam; an objective lens for focusing the 
laser beam onto the optical disk; an optical signal detector for detecting 
an optical signal from a reflected beam reflected on the optical disk; an 
error signal detector for detecting a focusing error and a tracking error 
of the laser beam focused on the optical disk, from the reflected beam; a 
first transparent plane substrate interposed between the laser diode and 
the objective lens, the first transparent plane substrate having a first 
surface opposed to the laser diode and a second surface opposite to the 
first surface; a hologram formed integrally with the first transparent 
plane substrate on the first surface, for transmitting the laser beam 
emitted from the laser diode and diffracting the reflected beam toward the 
error signal detector; a first lens formed integrally with the first 
transparent plane substrate on the second surface, for collimating the 
laser beam emitted from the laser diode; and a second lens interposed 
between the laser diode and the hologram. 
The above and other objects, features and advantages of the present 
invention and the manner of realizing them will become more apparent, and 
the invention itself will best be understood from a study of the following 
description and appended claims with reference to the attached drawings 
showing some preferred embodiments of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 4, there is shown a general configuration of an optical 
head according to a first preferred embodiment of the present invention. 
In the following description of some preferred embodiments of the present 
invention, substantially the same parts as those shown in FIG. 1 
representing the prior art will be denoted by the same reference numerals. 
Reference numeral 34 denotes a hologram-lens unit, which is composed of a 
transparent substrate 36 having a surface formed with a gradient index 
microlens 38 and a transparent substrate 40 having a surface formed with a 
hologram 42 as a diffraction optical element. The transparent substrates 
36 and 40 are bonded together by an optical adhesive. 
A laser beam 4 emitted from a laser diode 2 is refracted in a converging 
direction by the microlens 38, and next transmitted by the hologram 42 to 
enter a collimator lens 12. Since the beam 4 is refracted in the 
converging direction by the microlens 38, almost all components of the 
beam 4 emitted from the laser diode 2 can be coupled to the collimator 
lens 12. The laser beam 4 incident on the collimator lens 12 is converted 
into a collimated beam 14 by the collimator lens 12 to enter a polarized 
beam splitter 16. The laser beam 4 emitted from the laser diode 2 to the 
polarized beam splitter 16 is a P-polarized light beam. Therefore, in this 
preferred embodiment, the laser beam 4 of P-polarized light is transmitted 
by the polarized beam splitter 16 with a transmittance of about 95%, and 
next focused on a magneto-optical disk 20 by an objective lens 18. 
The laser beam 4 is reflected on a recording surface of the magneto-optical 
disk 20, and a resultant reflected beam includes an S-polarized light 
component due to Kerr effect according to information recorded on the 
magneto-optical disk 20. This reflected beam is reconverted into a 
collimated beam by the objective lens 18 to enter the polarized beam 
splitter 16. The collimated beam incident on the polarized beam splitter 
16 is split into a reflected beam 22 and a transmitted beam 24. The 
P-polarized light component in the reflected beam from the magneto-optical 
disk 20 is transmitted by the polarized beam splitter 16 with a 
transmittance of about 95%, and about 5% of the P-polarized light 
component is therefore reflected by the polarized beam splitter 16. On the 
other hand, the S-polarized light component in the reflected beam from the 
magneto-optical disk 20 is reflected by the polarized beam splitter 16 
with a reflectivity of about 85%. 
The beam 22 reflected by the polarized beam splitter 16 enters a Wollaston 
prism 26, by which the beam 22 is separated into a P-polarized light 
component and an S-polarized light component, and next focused to a 
photodetector unit 30 by a lens 28. The photodetector unit 30 includes a 
photodetector for detecting the P-polarized light component and a 
photodetector for detecting the S-polarized light component. Signals 
detected by the two photodetectors are subjected to differential detection 
by a method well known in the art, thereby detecting a magneto-optical 
signal. 
On the other hand, the beam 24 transmitted by the polarized beam splitter 
16 is condensed by the collimator lens 12 to enter the hologram 42, by 
which the beam 24 is diffracted to enter a photodetector unit 32. As shown 
in FIG. 5, the hologram 42 has four different hologram-pattern regions 
42a, 42b, 42c, and 42d. The beam 24 incident on the four regions 42a to 
42d of the hologram 42 is diffracted in four different directions. The 
transparent substrate 36 and the microlens 38 shown in FIG. 4 are not 
shown in FIG. 5. 
The photodetector unit 32 includes two photodetectors 32a and 32b for 
detecting a focusing error and two photodetectors 32c and 32d for 
detecting a tracking error. The photodetector 32a is divided into two 
regions by a division line 44, and the photodetector 32b is also similarly 
divided into two regions by a division line 46. An arrow 48 denotes a 
track direction of the magneto-optical disk 20. The regions 42a and 42b of 
the hologram 42 are used for detection of a focusing error signal (FES), 
and the regions 42c and 42d of the hologram 42 are used for detection of a 
tracking error signal (TES). The size of each of the regions 42a to 42d is 
determined according to distribution of light power to be required by a 
magneto-optical disk drive. The beams diffracted by the regions 42a and 
42b enter the photodetectors 32a and 32b for detecting the focusing error 
signal, respectively. On the other hand, the beams diffracted by the 
regions 42c and 42d enter the photodetectors 32c and 32d for detecting the 
tracking error signal. 
Letting A denote a power of light incident on the right region of the 
photodetector 32a with respect to the division line 44, B denote a power 
of light incident on the left region of the photodetector 32a with respect 
to the division line 44, C denote a power of light incident on the left 
region of the photodetector 32b with respect to the division line 46, and 
D denote a power of light incident on the right region of the 
photodetector 32b with respect to the division line 46, the focusing error 
signal (FES) can be detected in accordance with the following equation. 
EQU FES=(A+C)-(B+D) 
Letting E denote a power of light incident on the photodetector 32c and F 
denote a power of light incident on the photodetector 32d, the tracking 
error signal (TES) can be detected in accordance with the following 
equation. 
EQU TES=E-F 
The gradient index microlens 38 may be fabricated by various methods such 
as a plastic diffusion polymerization method, an ion exchange diffusion 
method, and an electrolytic ion migration method. In this preferred 
embodiment, the gradient index microlens 38 is fabricated by using the ion 
exchange diffusion method. The material of a substrate for the gradient 
index microlens 38 may be selected from glass, quartz, plastic, crystal 
containing a semiconductor, etc. In this preferred embodiment, soda lime 
glass is used as the material of the substrate. 
The ion exchange diffusion method as one of the fabrication methods for a 
gradient index microlens is a method including a step of diffusing atoms 
(ions) having a large electron polarizability to molecules in a substrate 
such as glass to thereby change the refractive index of a region of the 
substrate where the ions have been diffused, thus forming a lens. The ion 
concentration in the ion diffused region of the substrate varies in such a 
manner that the ion concentration is highest at a central portion of the 
ion diffused region where the diffusion is started, and gradually lowers 
toward a peripheral portion of the ion diffused region where the ions are 
radially diffused. The refractive index varies with the gradient of the 
ion concentration. Further, the ions are spherically diffused. Therefore, 
the ion diffused region functions as a lens. 
A method of mass-producing many lenses arranged two-dimensionally on a 
plane substrate by a planar technique will now be described with reference 
to FIG. 6(a) to FIG. 6(f). As shown in FIG. 6(a), a soda lime glass 
substrate 50 is first prepared. A Ti film 52 as a protective film is next 
formed on the glass substrate 50 as shown in FIG. 6(b), so as to pattern 
an ion diffused portion. A photoresist is next applied to the Ti film 52, 
and next exposed to light by using a mask having a given pattern. Then, 
developing and etching are carried out to pattern the Ti film 52 as shown 
in FIG. 6(c). 
The substrate 50 is next immersed into a molten salt containing ions 54 to 
be diffused, e.g., silver ions, to inject the ions 54 into the substrate 
50 as shown in FIG. 6(d). Diffusion of the ions 54 in the substrate 50 is 
carried out at high temperatures for tens of hours to form many gradient 
index microlenses 56 each having a refractive index higher than that of 
the substrate 50 in the vicinity of the surface of the substrate 50 as 
shown in FIG. 6(e). Then, the Ti film 52 is removed by surface polishing, 
and an antireflection film coating 58 is next formed on the surface of the 
substrate 50 so as to cover the gradient index microlenses 56 as shown in 
FIG. 6(f), thus completing a microlens array. 
The hologram 42 may be fabricated by various methods such as a planar 
technique, an etching method, e.g., reactive ion etching, a photopolymer 
method (2P method) using ultraviolet hardening resin, and an injection 
method using molten glass. A method of mass-producing many holograms on a 
plane substrate by a planar technique will now be described with reference 
to FIG. 7(a) to FIG. 7(f). As shown in FIG. 7(a), a quartz substrate 60 is 
first prepared, and as shown in FIG. 7(b), a Ti film 62 as a protective 
film is uniformly formed on the quartz substrate 60. 
Then, the Ti film 62 is patterned as shown in FIG. 7(c) by photolithography 
including resist coating, exposure, and developing steps, and by etching. 
Then, the substrate 60 is immersed into a molten salt containing ions 64 
to be diffused to inject the ions 64 into the substrate 60 as shown in 
FIG. 7(d). Diffusion of the ions 64 in the substrate 60 is carried out at 
high temperatures for a sufficient time to form many volume holograms 66 
at the ion diffused regions of the substrate 60 where the refractive index 
has been varied as shown in FIG. 7(e). Finally, the Ti film 62 is removed 
by surface etching, and a reflection preventing film coating 68 is formed 
on the substrate 60 so as to cover the volume holograms 66 as shown in 
FIG. 7(f), thus completing a volume hologram array. 
The microlens array and the volume hologram array thus manufactured are 
aligned so that each microlens element and the corresponding volume 
hologram element have the same optical axis, and next bonded together by 
using an optical adhesive. Finally, the substrates bonded together are cut 
into elements by a dicing saw. These elements are used as individual 
hologram-lens units. In fabricating the hologram elements by using a 
photopolymer method, the hologram elements can be formed on the lower 
surface of a substrate whose upper surface is formed with lens elements. 
Accordingly, this method is advantageous over the planar method because 
the step of bonding the two substrates required in the planar method can 
be omitted. Similarly, the hologram elements can be fabricated by an 
etching method in such a manner that the hologram elements are formed by 
etching the lower surface of a substrate whose upper surface is formed 
with lens elements. 
A process of fabricating holograms by using a photopolymer method will now 
be described with reference to FIG. 8(a) to FIG. 8(c). As shown in FIG. 
8(a), a stamper 74 formed with a plurality of hologram patterns 75 and a 
transparent substrate 70 formed with a plurality of microlens elements 72 
are first prepared. A photopolymer 76 is next applied to the stamper 74, 
and the transparent substrate 70 is laid on the stamper 74 through the 
layer of the resin 76 with the stamper 74 and the substrate 70 being 
aligned with each other. As shown in FIG. 8(b), ultraviolet rays are 
irradiated onto the transparent substrate 70 to thereby harden the 
ultraviolet hardening resin 76. After the resin 76 is sufficiently 
hardened, the substrate 70 is separated from the stamper 74 to thereby 
form holograms 78 on the lower surface of a hardened resin layer 76' as 
shown in FIG. 8(c). 
A process of fabricating holograms by using an etching method will now be 
described with reference to FIG. 9(a) to FIG. 9(d). As shown in FIG. 9(a), 
a Ti film 84 is formed on the upper surface of a transparent glass 
substrate 80 whose lower surface is formed with a plurality of microlens 
elements 82. Then, a photoresist 86 is applied to the Ti film 84. Then, 
the photoresist 86 is exposed to light with a mask having a desired 
pattern corresponding to the pattern of holograms to be formed, and next 
developed to be patterned into a desired shape as shown in FIG. 9(b). 
Then, the Ti film 84 is etched and the glass substrate 80 is further 
etched by reactive ion etching as shown in FIG. 9(c). Then, the Ti film 84 
is removed by etching to form a plurality of hologram elements 88 on the 
upper surface of the substrate 80 whose lower surface is formed with the 
plurality of microlens elements 82. 
As mentioned above, the beam emitted from the laser diode 2 has a 
substantially elliptical cross section. Therefore, it is preferable that 
the microlens 38 shown in FIG. 4 is formed in an elliptical shape and that 
the elliptical microlens 38 is positioned so that the major-axis direction 
of the elliptical beam 4 and the minor-axis direction of the elliptical 
microlens 38 coincide with each other. That is, as shown in FIG. 10, an 
elliptical lens 90 as the microlens 38 is positioned with respect to the 
elliptical beam 4. Such an elliptical microlens may be easily fabricated 
by forming each opening of the Ti film 52 in the step shown in FIG. 6(c) 
into an elliptical shape. The use of such an elliptical lens as the 
microlens 38 allows compensation for the astigmatic difference of the 
laser beam 4 emitted from the laser diode 2. 
That is, letting A denote the minor-axis direction of the elliptical lens 
90 and B denote the major-axis direction of the elliptical lens 90 as 
shown in FIG. 11, a component of light incident on the elliptical lens 90 
along the minor-axis direction A is focused at a near position with 
respect to the elliptical lens 90 (sagittal focal line 94), because a 
portion of the ellipse corresponding to the minor-axis direction A has a 
radius of curvature larger than that of a portion of the ellipse 
corresponding to the major-axis direction B. Conversely, a component of 
the incident light along the major-axis direction B is focused at a far 
position with respect to the elliptical lens 90 (meridional focal line 
96). Accordingly, an astigmatic difference occurs between the two focal 
lines 94 and 96. The astigmatic difference of the elliptical laser beam 4 
can be compensated by positioning the elliptical lens 90 so that the 
major-axis direction of the elliptical laser beam 4 and the minor-axis 
direction of the elliptical lens 90 coincide with each other as shown in 
FIG. 10. 
FIG. 12 shows the relation between the position of the elliptical lens 90 
with respect to the elliptical laser beam 4 and a beam shape 92 after 
transmission of the beam 4 through the lens 90. As apparent from FIG. 12, 
the ellipticity of the beam shape (beam pattern) 92 after transmission of 
the beam 4 through the elliptical lens 90 is relaxed. Referring to FIG. 
13, there is shown the relation between the beam shape 92 after 
transmission of the beam 4 through the elliptical lens 90 and the shape of 
the collimator lens 12. As compared with the shape of the beam 4 in the 
prior art shown in FIG. 3, it is readily understood that the ellipticity 
of the beam shape 92 according to this preferred embodiment is relaxed. 
Referring to FIG. 14, there is shown a part of an optical head according to 
a second preferred embodiment of the present invention. A hologram-lens 
unit 98 in this preferred embodiment is configured by forming a convex 
lens 102 on one surface of a plane glass substrate 100 and a hologram 104 
on the other surface of the substrate 100. The convex lens 102 is 
fabricated by substantially the same method as that shown in FIG. 6(a) to 
FIG. 6(f). However, the diffusion time in the step shown in FIG. 6(e) is 
made longer to thereby cause a change in volume after the ion exchange and 
semispherically expand each ion diffused region. Such a semispherical 
expansion is utilized as the convex lens 102. The hologram 104 is 
fabricated by a photopolymer method (2P method) using ultraviolet 
hardening resin as shown in FIG. 8(a) to FIG. 8(c). 
The convex lens 102 has a large numerical aperture (NA). Accordingly, the 
convex lens 102 can be made to serve also as a collimator lens. As a 
result, adjustment of the hologram 104 and the collimator lens 102 can be 
simplified. However, to prevent overlap of diffracted light from the 
hologram 104 on the convex lens 102, it is required to ensure a large 
angle of diffraction by the hologram 104 and also ensure a large thickness 
of the glass substrate 100. To this end, the photodetector unit 32 must be 
located farther from the laser diode 2. 
From the viewpoint of simplification of a fabrication process for the 
convex lens 102, it is advantageous to adopt the 2P method. If the convex 
lens 102 is fabricated by using the ion exchange diffusion method, tens of 
hours are required for fabrication of the convex lens 102. To the 
contrary, several minutes are merely required for hardening of the 
ultraviolet hardening resin in the 2P method, so that the 2P method is 
greatly advantageous for mass production. 
To fabricate the convex lens 102 by using the 2P method, a stamper as an 
original pattern is required. The stamper may be fabricated by various 
method such as (1) ion exchange diffusion, (2) photolithography using 
crystalline glass, and (3) mechanical cutting such as electrical discharge 
machining. In particular, the mechanical cutting is advantageous in 
accuracy because a microlens array having an arbitrary shape such as 
elliptical lenses, cylindrical lenses, or aspherical lenses can be 
accurately fabricated. Further, by setting the refractive index of the 
ultraviolet hardening resin used in the 2P method to substantially the 
same as the refractive index of the glass substrate as a base material, 
reflection loss at the interface between the resin layer and the glass 
substrate can be reduced to allow effective use of the laser beam. 
Referring to FIG. 15, there is shown a part of an optical head according to 
a third preferred embodiment of the present invention. In this preferred 
embodiment, the hologram-lens unit 98 of the second preferred embodiment 
shown in FIG. 14 is used, and a drive mechanism 106 is further provided to 
move the collimator lens 12 in the direction across the tracks of a 
recording medium, thereby slightly changing tilt of a beam spot on the 
recording medium in the direction across the tracks of the recording 
medium. The drive mechanism 106 is constructed of a voice coil motor or a 
piezoelectric element, for example. In the event that the recording medium 
is inclined, the collimator lens 12 is slightly moved in the direction 
across the tracks of the recording medium by the drive mechanism 106, 
thereby correcting the tilt of the recording medium. The convex lens 102 
is preferably formed as an elliptical lens. 
Referring to FIG. 16, there is shown a part of an optical head according to 
a fourth preferred embodiment of the present invention. In this preferred 
embodiment, a hologram 114 and a collimator lens 116 are integrated. That 
is, a gradient index microlens 110 is formed in one surface of a plane 
transparent substrate 108. The hologram 114 is formed on one surface of a 
plane transparent substrate 112, and the collimator lens 116 is formed on 
the other surface of the substrate 112 by the above-mentioned method. 
Accordingly, the optical system can be reduced in size without a large 
design change from the conventional optical system, and the need for 
adjustment of the hologram and the collimator lens can be eliminated. 
Further, light focusing characteristics can be improved by the gradient 
index microlens 110. 
According to the present invention as described above, the light power loss 
due to the shape of the laser beam emitted from the laser diode can be 
reduced. Accordingly, it is possible to provide an optical head for an 
optical disk drive which can improve the efficiency of use of laser power. 
Further, since the microlens element and the hologram element are 
integrated, the optical system can be easily adjusted in assembling the 
optical disk drive.