Movable optical head integrally incorporated with objective lens and hologram element

Disclosed is a compact optical pick-up head apparatus suitable for mass-production, which comprises a light radiation source, an element composed of photodetector split into a plurality of regions and integrated on the same base, an objective lens for focusing a beam from the above source on an optical disk, a blazed hologram integrated to the support member of the lens, a means for driving the support member to which the objective lens and blazed hologram are integrated in response to an output as servo signal output from the photodetector which receives a diffracted beam produced in such a manner that a return beam reflected or diffracted by the optical disk is incident on the objective lens integrated with the above hologram. The optical pickup apparatus according to the present invention employs a region-split-type phase hologram in which a pair of Flesnel zone plate-like patterns, by which a plurality of diffraction wavefronts having a different focus and the same diffraction order are produced outside an optical-axis, are non-superimposedly formed. The above hologram can be blazed with a pinpoint accuracy by lithography using a few mask patterns synthesized by a computer, and further a more preferably blazing can be realized by the combination of the ion beam etching and the lithography. Accordingly, not only noise caused by an unnecessary diffracted beam component but also the offset of a servo signal can be restrained and the size of the objective lens can be reduced by the integration of the entire optical system, which greatly contributes to the superminiaturization of an optical head.

BACKGROUND OF THE INVENTION 
The present invention relates to an optical head apparatus for recording 
and reproducing optical data recorded on an optical memory medium such as 
an optical disk, optical card and the like, a magnet-optic recording 
medium and the like. 
An optical memory technology, which can develop a pit-shaped pattern in 
correspondence to signals in time series, has been practically used as a 
memory medium of high density and large capacity with the fields of 
application thereof extended from a digital audio disk to a data file. 
The optical memory technology starts from the development of a memory 
medium and is completed by the development of an optical head apparatus. 
In particular, the reduction in size and cost and the improvement of 
performance and reliability of the optical head apparatus is indispensable 
to the spread of the optical memory technology and an optical head 
apparatus having an integrated source and photodetecting system as well as 
integrated electronic circuits has been developed. 
FIGS. 1(a) and 1(b) show two examples of the optical system of a 
conventional integrated optical head apparatus. FIG. 1(c) is a detailed 
view of the optical system shown in FIG. 1(a), wherein the wavefront of 
divergent light from a source 100 using a semiconductor laser is divided 
into a main beam and two auxiliary beams at a first grating 101 and 
incident on a second grating 102 and a third grid 103 each having a 
semi-circular opening. A zero order diffracted wave component of the 
incident beam is focused on a memory medium 4 through a collimator 104 and 
an objective lens 105. Here, the distance d' between the spots of the main 
beam 107a and auxiliary beams 107b, 107c is designed to about 10 to 20 
micrometers. These beams are reflected or diffracted by the pit surface of 
the memory medium 4, passes again through the objective lens 105 and the 
collimator 104, diffracted by the second and third gratings each arranged 
as a partial pattern of a holographic element, and received by a 
photodetecting system 108 split into 5 regions, whereby a focus is 
detected by a Foucault method and a tracking and an RF signal are detected 
by a 3-beam method. Note that the gratings 101, 102, and 103, which are 
formed on the opposite surfaces of the holographic element 109 can split 
the light into a plurality of wavefronts, and can be easily copied. 
However, when a semiconductor laser is used as a source, in the optical 
head apparatus, the permissible fluctuation of a wavelength of the optical 
head apparatus is limited to about +10 nm even if other assembly errors 
are zero. Further, the tolerance of the relative positional error between 
the source 100 and the photodetecting system 108 is set to 
.DELTA.X.congruent.17 micrometers in the X-axial direction and 
.DELTA.Y.congruent.40 micrometers in the Y axial direction in the example 
shown in FIG. 1 for the reduction in size and the integration of the 
optical head apparatus, supposing that the objective lens has the focus 
length f=4.5 mm and the numerical aperture NA=0.45, and the collimate lens 
has the focus length f=21 mm and the numerical aperture N=0.11, and thus 
the assembly must be carried out with a pinpoint accuracy in a 
manufacturing process. In particular, in the case of an optical disk drive 
device provided with a recording/erasing system, it is difficult to ensure 
the reliability thereof in the circumference in which the device is used. 
This is because that a conventional Foucault method or astigmatic method 
is used as a focusing servo signal sensing system for the optical head 
apparatus, that is, this is because of the restriction resulting from the 
system (Foucault method) for forming a micro-spot (.about.10 micrometer 
dia.) on the surface of the photo-sensor or the system (astigmatism 
method) for sensing the balance of a distributed light quantity on the 
surface of a photo-sensor split into four sections. Whereas, there has 
been developed an optical head apparatus employing a spot size detection 
system (SSD) by which spots focused on two different focuses are 
differentially detected by linear photo-sensors interposed between the two 
focuses, as a focusing signal detection system which can allow a large 
amount of the relative positional error between a source 100 and a 
photodetecting system 3600 as shown in FIG. 1(b). (U.S. Pat. No. 
4,929,823). 
However, since this system obtains a tracking signal by differentially 
sensing a far field pattern of the tracking groove or pit in a memory 
medium 4, when a memory medium having only a pit train without having a 
groove, such as the so-called compact disk is reproduced, the tracking 
signal is unstably detected due to the offset of a servo signal caused by 
the inclination of the compact disk or the movement of an objective lens 
3. Further, there remains a problem to realize a hologram element which 
can be easily blazed for restraining an unnecessary diffracted beam 
component as a hologram head arrangement for realizing a SSD system. In 
the arrangement shown in FIG. 1(b), the pattern of a third grating has a 
grating-shape having pitches of a few micrometers and further the pitches 
are set to 1 micrometer or less at the portion of the gratings where they 
intersect each other in order to make a distance h between the third 
grating and a source 100 to be a few millimeters, and thus the blazing 
thereof is very difficult. 
Therefore, an object of the present invention is to provide an optical head 
apparatus wherein a large amount of the assembly error of a source and a 
photodetecting system is allowed when they are integrated, a stable servo 
signal with a restrained offset can be detected even from a memory medium 
having a shallow pit depth, and a diffraction optical element is provided 
which is easily blazed for restraining an unnecessary diffracted beam 
component. Further, another object of the present invention is to 
integrate an optical system as a whole to thereby further reduce the size 
of an optical head apparatus, lower the cost, and improve the 
mass-productivity and reliability thereof. 
Here, a definition should be made, that is, the term "blazing" gives a 
meaning of giving optical characteristics to an optical element such that 
photoenergy is concentrated to a specified diffraction component. 
SUMMARY OF THE INVENTION 
To this end according to the present invention, there is provided a movable 
optical pick-up head apparatus of integrated lens and hologram type, 
comprising: a light radiation source, an imaging optics means having an 
optical axis, a hologram element including a diffraction optical element 
on which at least a pair of different blazed patterns are formed in 
separate zones which are not overlapped with each other so that the 
diffraction element is adapted for producing a zero order diffraction 
component beam from a beam which is received from the light radiation 
source, the imaging optics means converging the zero order diffraction 
component beam into a spot on a memory medium, and receiving a reflected 
beam from the spot, and further the diffraction element is adapted for 
producing a pair of first order diffraction component beams having 
different focused points from the reflected beam from the spot on the 
memory medium by way of the imaging option means, the first order 
diffraction component beams being deflected from the optical axis of the 
imaging optics means, an optical detection means having a plurality of 
split zones, for differentially obtaining a focusing error signal and a 
tracking error signal, and an actuator means for driving the imaging 
optics means in accordance with the tracking error signal and the focusing 
error signal so as to allow the spot to precisely scan the memory medium.

EMBODIMENTS 
A first embodiment according to the present invention will be described 
below with reference to FIG. 11(a). More specifically, in FIG. 11(a), 
designated at 1 is a source using a semiconductor laser producing a 
coherent beam having a wavelength in a near infrared rays region (e.g., 
.lambda.=780 nm); designated at 2 is a diffraction optical element using a 
hologram element in which Fresnel zone plate-like patterns are formed 
being not overlapped with each other for producing a plurality of 
diffraction wavefronts having the same diffraction order and different 
focuses outside the optical-axis; designated at 3 is an image optics using 
an objective lens having a numerical aperture NA =about 0.45; designated 
at 4 is a memory medium such as a compact disk, optical disk having a 
tracking groove or the like; and designated at 12 and 13 are a pair of 
photodetecting systems having a plurality of split regions for 
respectively receiving the above diffraction wavefronts and differentially 
obtaining a focusing error signal and a tracking error signal. 
A beam radiated from the beam radiation point 5 of the source 1 onto the 
diffraction optical element 2, is converged to a micro-spot at the image 
optics 3, and is incident upon a focusing point 7 on the pit surface 6 of 
the memory medium 4. At this time, the diffraction optical element 2 only 
uses a zero order diffracted beam on a going path and does not use a 
little defocused beam 9a and the like of a +1 order diffracted beam. In 
addition, a beam on a return pass reflected or diffracted from the pit 
surface 6 is incident upon the diffraction optical element 2 through the 
image optics 3 and produces the diffraction wavefronts 10, 11 of a 
plurality of beams 15, 16 having a different focus outside of an optical 
axis by the +1 order diffracted beam of the diffraction optical element 2. 
These diffraction wavefronts are incident upon a pair of the 
photodetecting systems 12, 13 in the vicinity of the focus of the +1 order 
diffracted beam. 
Designated at 8 is the drive unit of an actuator (not shown) having a 
movable unit 14 for integrally driving the focusing optics 3 and the 
diffraction optical element 2; designated at 35 is a holding unit for 
holding the source 1 and the photodetecting systems 12, 13; and X, Y and Z 
designate directions. 
In FIG. 11(b), each of the photodetecting systems 12, 13 has a plurality of 
split regions defined by slits, having a width of w and a length h in the 
split line direction. Further, each photodetector uses a linear type 
silicon photodetector and fixed on the upper surface of the mount 17 of 
the source 1 and spaced apart from the beam radiation point 5 by a height 
g. Note that the height g can be arbitrarily set depending on the design 
of a Fresnel zone plate-like pattern. Designated at 610 to 612 are the 
output terminals of the respective split regions of the photodetecting 
system 12 and designated at 620 to 622 are the output terminals of the 
respective split regions of the photodetecting system 13. The diffraction 
wavefronts 10, 11 shown in FIG. 11(a) have focuses to which beams 
reflected or diffracted by the pit surface 6 are converged, the focuses 
being located at positions apart from the beam receiving surface 36 of 
each of the photodetecting systems 12, 13 in the thicknesswise direction 
thereof by distances .delta.1, .delta.2, and when the focusing optics 3 
focuses, the respective beams 15, 16 on the beam receiving surface 14 have 
the same beam size, but when the imaging optics 3 does not focuse the 
beam, the beam has a different size. This difference is detected as a 
differential signal after the photodetecting systems 12, 13 having carried 
out a photoelectric conversion and is fed back to the movable unit 14 of 
the actuator to thereby adjust the focus of the imaging optics 3. 
Further, the above mentioned diffraction optical element 2 forms a Fresnel 
zone plate-like pattern on the side thereof facing to the phase-type 
hologram imaging optics 3, and is fixed to the movable unit of the 
actuator together with the imaging optics 3. Before fixed thereto, the 
diffraction optical element 2 or the cylindrical unit 18 thereof is 
adjusted by being rotated about the beam radiation point 5 by a slight 
angle (arrows 19a, 19b) so that the positions of the beams 15, 16 are 
optimized with respect to the photodetecting systems 12, 13 shown in FIG. 
11(b) in such a manner as shown in the figure. l.sub.1 and l.sub.2 
represent distances between the photodetecting systems 12, 13 and the beam 
radiation point 5, and .theta. represents an angle at which a pair of the 
photodetecting systems 12, 13 are observed from the beam radiation point. 
FIG. 12(a) is a conceptual view of the diffraction optical element 2 
wherein two kinds of coarse grating regions A, B each having a width Pj 
(=P1.about.Pn) are alternately disposed. Each of grating-like regions A, B 
is non-continously provided with linear Fresnel zone plate-like patterns 
disposed like a reed screen (off-axis linear Fresnel zone plates) so that 
they intersect at an angle .theta., and thus form blazed phase gratings 
having a cross section with a depth e as shown in FIG. 12(b). Further, 
each width Pj of regions A, B shown in FIG. 12(a) is differently set to 
substantially restrain the formation of a primary order diffracted beam 
corresponding to a coarse grating cycle period. Also, as plainly seen in 
FIG. 12(a), the grating regions A, B are separated by boundaries extending 
in a first direction and are disposed alternately an a second direction 
different from the first direction. 
FIG. 14(a) shows a design principle of the linear Fresnel zone plate-like 
patterns. The linear Fresnel zone plate-like patterns 20, 21 have centers 
at the points O.sub.1, O.sub.2 on a center line and a cross-sectional 
phase structure as shown in FIG. 14(b), are given by the following 
relation which is similar to that known as a two-dimensional Fresnel ring 
band plate, and the pattern in the circle 22 surrounded by a dotted line 
of FIG. 14(a) is used. 
EQU r.sup.2 =2j.lambda.fz+j.sup.2 .lambda..sup.2 
where, .lambda. represents the main wavelength of the source l, j 
represents a sequential number of a grating counted from the center 
O.sub.1, O.sub.2 of each Fresnel zone plate-like pattern, and fz 
represents a focal length (provided by the diffraction wavefront in one 
dimensional direction under the action of convex and concave lenses). 
These patterns can achieve the object thereof by being recorded by a laser 
interference fringe or being formed by a computer which calculates the 
patterns. 
Further, the linear Fresnel zone plate-like patterns 20, 21 intersect to 
each other at an angle .theta.. However, they are not used in a 
superimposed arrangement as shown in FIG. 14(a), but as shown in FIG. 
12(a), the grating regions A correspond to that obtained by drawing the 
half of the Fresnel zone plate-like pattern 20 of FIG. 14(a), as observed 
through the reed screen having a width Pj, the grating regions B 
correspond to that obtained by drawing the half of the Fresnel zone 
plate-like pattern 21, and they are not overlapped with each other. Here, 
the distances L.sub.1, L.sub.2 between the center of the opening 22 and 
the axial centers O.sub.1, O.sub.2 of the linear zone pattern are given by 
the following equations, when the distances l.sub.1, l.sub.2 shown in 
FIGS. 11(a) and 11(b) and H (the distance between the diffraction optical 
element 2 and a detected surface 14) are given. 
EQU L.sub.1 .apprxeq.l.sub.1 .times.fz.sub.1 /H 
EQU L.sub.2 .apprxeq.l.sub.2 .times.fz.sub.2 /H 
where, fz.sub.1 and fz.sub.2 represent the focal distances of a pair of the 
Fresnel zone plate-like patterns 20, 21 and are designed, taking the value 
of g into consideration so as .delta..sub.1 =.delta..sub.2, but the detail 
thereof is not described here. 
When output signals from the output terminals 610 to 612 and 620 to 622 of 
the photodetecting systems 12, 13 are represented by S610 to S612, S620 to 
S622, a focusing error signal FE is given by: 
EQU FE=S610-S620 
Alternately, when a beam utilization efficiency is more improved, it is 
given by: 
EQU FE=[S610-(S611+S612)]-[S620-(S621+S622)] 
Further, in this embodiment, the tracking direction of the memory medium 4 
is caused to coincide with the direction Y of the split line of the 
photodetecting systems 12, 13, so that a tracking error signal by a 
push-pull system represented by the following equation can be obtained. 
EQU TE=(S611-S612)+(S621-S622) 
In addition, an RF signal is given by: 
EQU RF=S610+S620 
or 
EQU RF=S610+S611+S612+S620+S621+S622 
Then, the actuator is driven so that the obtained focusing error signal and 
tracking error signal become zero. 
Note that in FIG. 11(b), 31 designates the focused +1 order beam obtained 
in such a manner that the beam 9a of the +1 order beam on the going path 
is reflected on the pit surface 6 and then diffracted by the diffraction 
optical element 2 on the return path which however does not affect the 
photodetecting systems 12, 13; and 32 designates the focused zero order 
beam obtained in such a manner that the beam 9a of the +1 order beam on 
the going path is reflected on the pit surface 6 and then diffracted by 
the diffraction optical element 2 on the return path, which raises no 
problems at all. 
According to the embodiment, the arrangement is such that the beam coming 
from the micro spot after having been focused on the memory medium 4 
produces a pair of diffracted beams through the diffraction optical 
element 2 and the diffracted beams are received by each split region of a 
pair of the photodetecting systems 12, 13 so that a tracking error signal 
and focusing error signal are differentially obtained, and thus a large 
degree of the relative positional error between the source 1 and the 
photodetecting systems 12, 13 can be allowed. More specifically, since the 
focus detection by the SSD system is employed, a mechanical adjustment 
error can be allowed to 5 to 10 times as large as that of, for example, 
the astigmatic method (the tolerance in the case of the SSD method as 
compared with the astigmatic method was evaluated as the result of an 
experiment effected by the inventors). 
Further, since the diffraction optical element 2 is formed with a pair of 
the Fresnel zone plate-like patterns which are not superposed with each 
other, the occurrence of a beat beam which would be produced of they were 
superimposedly formed can be prevented and the Fresnel zone plate-like 
patterns can be easily blazed, resulting in that an unnecessary diffracted 
beam component can be restrained and the diffraction optical element 2 can 
be integrated in the vicinity of the imaging optics 3. Moreover, a pair of 
the diffracted beams are received by split regions of a pair of the 
optical instrument systems and differentially detected, tracking and 
focusing stable servo signals can be detected even if a memory medium 
having a shallow pit depth is used. 
Further, since the Fresnel zone plate-like pattern is a linear Fresnel zone 
plate-like pattern formed only in the one dimensional direction and it 
gives a beam having a cross section of a micro width on the detecting 
surface of the photodetector, the size in the pattern train direction of 
the linear Fresnel zone plate-like pattern can be shortened. More 
specifically, the size of the photodetecting systems 12, 13 can be set as 
h&lt;w, as shown FIG. 11(b). Therefore, the size of the mount 17 as the 
substrate on which the source 1 and the photodetecting systems 12, 13 are 
integrated can be reduced and the range of a response frequency can be 
easily increased, and thus, tracking and focusing stable servo signals can 
be detected, even with, for example, a record and erasing type optical 
disk and the like having a large fluctuation in wavelength of a source. 
Further, the linearization of the Fresnel zone plate-like pattern enables 
the same to be more easily blazed and the serve detection is more 
stabilized by integrally driving the optical element 2 and the imaging 
optics 3. 
Note that the detail of a two-dimensional Fresnel zone pattern will be 
apparent from, for example, Japanese Unexamined Patent. Nos. 62-251025 and 
62-251026. 
The source 1 may be a semi-coherent beam. 
Further, although the Fresnel zone plate-like pattern has a sawtooth-shaped 
cross section as shown in FIG. 12(b), it may be blazed stepwise, as shown 
in FIG. 14(c), to restrain the ratio of a -1 order diffracted beam to a +1 
order diffracted beam. As a design example of FIG. 14(c), level 
differences between 4 stages can be made equal to each other so as to 
realize such conditions as a depth e.apprxeq.870 nm, a refractive index of 
an element substrate n.apprxeq.1.455, a wavelength .lambda.=788 nm, a beam 
utilization efficiency in the going and return paths .eta..sub.0 
.times..eta..sub.-1 =10%, and .eta..sub.-1 /.eta..sub.+1 =7%, where 
.eta..sub.-1 represents the diffraction efficiency of the -1 order 
diffracted beam and .eta..sub.+1 represents the diffraction efficiency of 
the +1 order diffracted beam. 
FIGS. 15(a) and 15(b) show a second embodiment according to the present 
invention. More specifically, this optical head apparatus is arranged in 
the first embodiment such that a plurality of each pair of the Fresnel 
zone plate-like patterns of the diffraction optical element 2 are 
alternately disposed with the same width. Further, a pair of 
photodetecting systems 29, 30 are composed of focusing detection units 37, 
38 for detecting diffraction wavefronts 15, 16 produced by a pair of the 
Fresnel zone plate-like patterns and tracking detection units 39 to 42 for 
detecting diffraction wavefronts 25 to 28 which are produced in the 
disposing direction of the pairs of the Fresnel zone plate-like patterns 
which are alternately disposed with the same width p thereof. 
More specifically, when all of the widths Pj of the grating-like regions A, 
B shown in FIG. 12(a) are set to have the same value Pj.tbd.P, a new 
primary order diffracted beam corresponding to the grating cycle periods 
having a width P is produced as an auxiliary beam. The auxiliary beam made 
by a diffraction optical element 2 on a going path produces micro-spots at 
the focusing points 24a, 24b on a pit surface 6 and the reflected or 
diffracted beam thereof is diffracted by the diffraction optical element 
20 thereby to produce auxiliary beams 25, 26 and 27, 28 on a return path. 
Then, a focusing error signal can be detected by the main beam on the 
diffraction wavefronts 15, 16 and a tracking error signal of a three-beam 
method can be detected by the auxiliary beams 25 to 28. In FIG. 15(b), 
each of the photodetecting systems 29, 30 is split into three regions each 
having a rectangular opening and uses a differential type linear 
photodetector. The output terminals 81, 82 of the photodetectors 37, 38 
and the output terminals 81, 82 of the photodetectors 39 to 42 deliver 
output signals S81, S82, S91, and S92, and accordingly, a focusing error 
signal FE and a tracking error signal TE are given by: 
EQU FE=S81-S82 
EQU TE=S91-S92 
Further, the relationship between the tracking direction (pit train 
direction) X.sub.0 of the a memory medium 4 and the direction X in which 
the regions A, B of the diffraction optical element 2 are disposed is 
substantially parallel, and thus the position of the auxiliary beam is 
adjusted similarly to that in a tracking detection system of the so-called 
3-beam method. 
Note that designated at 33, 34 are the centers of +1 order diffracted beams 
obtained in such a manner that the +1 diffracted beams of the auxiliary 
beam, which are made by the diffraction optical element 2 on the going 
path and focused on the focusing points 24a, 24b, are reflected on the 
memory medium 4 and are then diffracted by the diffraction optical element 
2. 
According to the embodiment, the diffracted beams made by pairs of the 
Fresnel zone plate-like patterns disposed with the same width P are 
differentially detected by the tracking detection units 39 to 42 thereby 
to obtain the tracking error signal by the 3-beam method. In this case, it 
is sufficient that if the photodetecting systems 29, 30 can obtain a 
correct signal in the vicinity of the focus of the diffraction optical 
element 2, the distance md between the respective diffracted beams can be 
minimized, where m is designed to about 0.2 in terms of the coupling ratio 
of an imaging optics 3 and d is designed to about 10 to 15 micrometers in 
terms of the distance between the point 7 on the pit surface 6 and the 
focusing points 24a and 24b. Therefore, the 3-beam tracking system, which 
has been widely applied for, for example, the memory medium 4 such as a 
compact disk having a pit train depth of about .lambda./5 to a disk medium 
having a groove, can be further developed, so that the effect of an 
abnormal tracking error signal caused by flaws on a disk can be restrained 
without sacrificing other characteristics by designing the spot spacing of 
main and auxiliary beams to 12 micrometer or less. Further, the reduction 
in the beam spacing can increase the width P of the Fresnel zone 
plate-like pattern, and thus the diffraction optical element 2 can be 
easily made. 
In addition, since the focusing detection units 37, 38 and the tracking 
detection units 39 to 42 of the photodetecting systems 29, 30 are disposed 
in parallel with the tracking direction X.sub.0 of the memory medium 4, 
i.e., the split line direction Y of the diffraction optical element 29, 30 
is made orthogonal to the tracking direction X.sub.0, a change in a far 
field pattern, which is caused when the micro-spot having focused a light 
beam on the memory medium 4 goes across the tracking groove or pit train 
of the memory medium 4, does not affect the focusing error signal, and 
thus a stable servo signal can be detected. 
Further, although the diffraction optical element of a hologram element 
using the conventional 3-beam method needs to form gratings on the front 
and the back surfaces thereof in order to achieve a desired object, thus 
object can be achieved only by a relief pattern formed on one surface in 
this embodiment, and thus this embodiment is excellent in 
mass-productivity and reliability. In particular, when considering that 
the diffraction optical element 2 of the hologram element is characterized 
in the mass-production of copies of the relief patterns, it will be 
understood that the embodiment has a great effect. 
In addition to the above, the embodiment has functional effects similar to 
those of the first embodiment. 
Note, although not particularly shown, as another embodiment, the 
diffraction optical element 2 may be designed as a reflecting type phase 
grating and further the grating may be blazed to a sawtooth shape in 
accordance with the first and second embodiments. In addition, a 
technology of blazing the diffraction optical element 2 which is common to 
the respective embodiments can be realized, for example, in such a manner 
that after rectangular waveform gratings have been formed on a quartz 
substrate as a photoresist pattern, asymmetric grating are formed by 
oblique ion beam etching or electron beam drawing. In this case, one of 
the diffracted beam components (-1 order beam) produced in the diffraction 
on the return path from the Fresnel zone plate-like pattern is a weak beam 
and thus even if a portion thereof is mixed on the photodetecting systems 
12, 13, 29, 30 as a zero order beam on the return path, the intensity 
ratio thereof to the -1 order beam can be made to 15 to 20% or less and 
the effect thereof can be substantially ignored. 
Here, a consideration will be made to the problem of a noise component 
caused when a diffracted beam, which in not necessary on the going and 
return paths, is incident on the photodetector. Incidentally, the problem 
would be caused if a hologram element and an objective lens were 
integrally supported and accommodated in an actuator. 
FIG. 2(a) exemplifies a recent case of the optical system of an optical 
head apparatus simplified by using a hologram (e.g., Japanese Unexamined 
Patent No. 64-62838). 
In FIG. 2(a), 1 designates a light radiation source such as a semiconductor 
laser and the like. A radiated beam 110 (laser beam) coming from the 
source 1 passes through the hologram 101, is incident on an objective lens 
8, and focused on an optical memory medium 4. Here, consideration will be 
made not on a servo detection system employed by the hologram element but 
on an unnecessary diffracted beam. A light beam reflected on the optical 
memory medium 4 follows an original beam path in the reverse direction and 
is incident upon the hologram 101. A +1 order diffracted beam 121 produced 
from the hologram is incident on a photodetector 36. A servo signal and a 
data signal can be obtained by calculating an output from the 
photodetector 36. 
In the case wherein an objective lens 3 can be moved by an actuator 
independently from the hologram 101, when the objective lens 3 is moved by 
following a tracking and the like, the emitted beam 110 on the hologram 
101 is also moved. Therefore, the image of a +1 order diffracted beam on 
the photodetector 36 is also moved, whereby the servo signal is adversely 
affected, whereas, as shown in FIG. 6, since the hologram 101 and the 
objective lens 3 are disposed keeping a predetermined relative position 
with respect to the actuator 100, even if the objective lens 3 is moved 
with respect to the actuator 100 to effect a tracking control, the beam 
reflected on the optical memory medium 4 is little moved on the hologram 
101. Therefore, a signal obtained from the photodetector 36 is not 
deteriorated regardless of that the objective lens 3 is moved. 
With the optical system arranged as described above, however, since a 
diffracted beam is also produced from the hologram 101 on the beam path 
from the beam radiation source 1 to the optical memory medium 4 
(hereinafter, referred to as a going path), the diffracted beam is 
reflected on the optical memory medium 4 and focused on the photodetector 
36 by the objective lens 3. When a +1 order diffracted beam that is 
produced on the beam path from the memory medium 4 (hereinafter, referred 
to as a return path), which goes to the photodetector 36 after being 
reflected on the optical memory medium 4, focused by the objective lens 3 
and diffracted by the hologram 101, is used to obtain a signal, a -1 order 
beam produced on the going path is incident upon the same position as that 
of the +1 order diffracted beam on the return path on the photodetector 
36. FIG. 2(b) shows this state. 
The -1 order diffracted beam 121 on the going path, which has become the 
zero order diffracted beam 130 on the return path and the +1 order 
diffracted beam 121 on the return path of the zero order diffracted beam 
120 on the going path are reflected on different positions on the optical 
memory medium 4, and thus they naturally have different data. Therefore, 
this optical system having the integrated lens and hologram has a problem 
in that the quality of a servo signal and data signal is deteriorated by 
the -1 order beam 121 on the going path. 
To solve the above problem and prevent an emitted beam on the hologram from 
being moved by the beam of an objective lens and the quality of a signal 
from being deteriorated by a -1 order diffracted beam on a going path, the 
present invention comprises a light radiation source, the objective lens 
for receiving a radiated beam coming from the light radiation source and 
causing the same to be focused on an optical memory medium, a blazed 
hologram integrally supported by the support member of the objective lens, 
and a photodetector for producing an output b receiving a diffracted beam 
which is produced by the radiated beam reflected on the above optical 
memory medium and incident on the objective lens integrated with the above 
hologram, the output being produced in accordance with each diffracted 
beam or a beam quality obtained by splitting each diffracted beam into a 
plurality of diffracted beams. 
Since the integration of the blazed hologram with the objective lens in the 
present invention enables an emitted beam on a return path to be incident 
on a predetermined portion of the hologram regardless of that the 
objective lens is moved because it follows a tracking, the diffracted beam 
produced from the hologram does not move on the photodetector. Further, 
since the hologram is blazed, i.e., since the cross section of the 
hologram is made asymmetric, the diffraction efficiency of the -1 order 
diffracted beam is smaller than that of a +1 order diffracted beam, and 
thus a servo signal or data signal is less deteriorated by the -1 order 
diffracted beam on the going path. Therefore, the servo and data signals 
can be stably read. 
An embodiment of the present invention will be described below in this 
regard with reference to drawings. FIG. 6(a) is a diagram of the 
conceptual arrangement according to the present invention, which shows a 
simplified arrangement of FIG. 11(a). Designated at 1 is a light radiation 
source such as a semiconductor laser or the like. A radiated beam 110 
(laser beam) coming from the source passes through a hologram 2, is 
incident on an objective lens 3, and focused on a optical memory medium 4. 
The beam reflected on the optical memory medium 4 follows an original beam 
path and is incident on the hologram 2. A +1 order diffracted beam 121 
produced from the hologram 2 is incident on a photodetector 36. A servo 
signal and a data signal can be obtained by calculating an output from the 
photodetector 36. 
As described above, the present invention is characterized in that the 
hologram 2 is blazed and integrated with the objective lens 3. The effect 
that the diffraction efficiency of a -1 order diffracted beam is made 
smaller than that of a +1 order diffracted beam can be obtained by the 
blazing, which is described, for example, in "Blazing of Micro Fresnel 
Lens Made by Electron Beam Drawing" by Fujita, Nishihara, Koyama, 
Technical Report of The Institute of Electronics, Information and 
Communication Engineers, Vol. 82, No. 47, pp 49-55 (OQE 82-25), 1982 and 
the like. Although this paper uses blazing to improve the focusing 
characteristics of a Fresnel lens, the present invention uses the blazing 
to restrain the beam quantity of a -1 order diffracted beam on a going 
path produced from a hologram, wherein the blazing controls the beam 
quantity of the diffracted beam component of a zero order beam, a +1 order 
diffracted beam and the like. 
FIG. 6(b) shows an example of a blazed hologram 200 which is embodied as an 
analog type hologram having a sawtooth-shaped cross section. In FIG. 6(b), 
assuming that the difference between the crest and valley of a relief is 
d, the refractive index of a transparent substrate constituting the 
hologram 200 is n, the refractive index of the periphery of the hologram 
200 is n0, and the wavelength of a light radiation source 1 is .lambda., a 
phase modulation degree is determined by: 
EQU .phi.=2.pi..multidot.d(n-n0)/.lambda. 
FIG. 7 shows a graph of the relationship between .phi. and a diffraction 
efficiency. 
As shown in the above paper, it is sufficient to satisfy .phi.=2.pi. to 
maximize the diffraction efficiency of the +1 order diffracted beam, 
whereas in the present invention, since a +1 order diffracted beam 131 on 
a return path, which has been a zero diffracted beam 120 reflected on a 
optical memory medium 4 and then diffracted by a blazed hologram, is used 
to detect a signal, the diffraction efficiency (transmittance) of the zero 
order diffracted beam 120 on a going path must be also large. Furthermore, 
the diffraction efficiency of the -1 order diffracted beam must be smaller 
than that of the +1 order diffracted beam. Thus, when a sawtooth-shaped 
blazing is effected as shown, for example, in FIG. 6(b), the above object 
can be achieved by setting .phi. between .pi. to 2.pi.. The principle of 
design will be described below. 
FIGS. 4(a) to 4(d) and FIGS. 5(a) to 5(f) exemplify conventional methods of 
producing well-known blazed hologram. The blazed hologram is made in such 
a manner that, for example, as shown in FIGS. 4(a) to 4(d), a photoresist 
3002 is coated on the surface of a substrate 300 shown in FIG. 4(a), the 
photoresist is exposed after being covered with a chrome mask 3003 and 
then developed to make a masking pattern 3051, and further an ion beam 302 
is obliquely irradiated to the surface of the substrate 300 shown in FIG. 
4(b) to obtain a blazed hologram 2001 as shown in FIG. 4(c). FIG. 5(d) is 
an enlarged view of a main part of FIG. 5(c), wherein inclined surfaces A, 
B are asymmetrically formed. 
Furthermore, when etching is repeated a plurality of times as shown in 
FIGS. 5(a) to 5(f), i.e., when a resist 3200 and a mask 3004 are formed 
and exposure in FIG. 5(a) and development in FIG. 5(b) are carried out 
after the process in FIG. 5(c) has been carried out to form a masking 
pattern 3500, and then etching is carried out in this state, a blazed 
hologram 3082 having the surface thereof near to a sawtooth shape can be 
obtained, as shown in FIG. 5(f). 
However, the cross section obtained by the method shown in FIGS. 4(a) to 
4(d) is disadvantageous as shown in an enlarged view of FIG. 4(d). More 
specifically, a surface A is not vertically formed but is inclined. 
Further, an inclined surface B has a concave shape. Thus, the conventional 
method has a problem in that since this method only provides a cross 
section which is different from the accurate sawtooth-shaped cross section 
shown in FIG. 6(b), the intensity ratio of a conjugate diffracted beam 
cannot be sufficiently obtained. Further, since the blazed hologram 3082 
of the conventional example shown in FIGS. 5(a) to 5(f) has a stepwise 
cross section, it is also a little difficult to increase the intensity 
ratio of the conjugate diffracted beam. Further, since etching is carried 
out a plurality of times in the method shown in FIGS. 5(a) to 5(f), a 
pattern alignment needs a very high accuracy of a few percents of grating 
pitches (e.g., about 2 micrometers when a grating has pitches of 10 
micrometers) and thus a problem arises in that the manufacture thereof is 
more or less difficult. 
In view of the fore-going, another object of the present invention is to 
provide a method of easily manufacturing a blazed hologram having a cross 
section near to an ideal sawtooth shape with a pinpoint accuracy. Further, 
an object of the present invention is to provide an optical head apparatus 
capable of detecting a signal having a good S/N ratio. 
To achieve the above object, according to the present invention, a 
rectangular cross section is formed by etching, further a masking pattern 
is formed on a surface, and an ion beam is obliquely irradiated to form a 
blazed hologram. Further, an optical head apparatus is made using this 
hologram. More specifically, a method according to the present invention 
comprises the steps of selectively forming a first masking pattern on a 
substrate and etching a portion of the substrate using the first masking 
pattern as a mask; and removing the first masking pattern, forming again a 
second masking pattern on the substrate, irradiating an ion beam to the 
surface of the substrate in a direction different from a right angle 
thereto to thereby selectively etching a portion of the substrate, and 
forming a sawtooth-shaped surface on the surface thereof. Further, the 
present invention provides an optical head apparatus using the thus formed 
hologram. 
According to the present invention, since the rectangular cross section is 
formed by etching and further an ion beam etching is obliquely carried out 
after the completion of the masking pattern, the accuracy of a pattern 
alignment is eased, and further since an amount of etching effected by the 
obliquely irradiated ion beam is small, a cross section nearer to a 
sawtooth shape can be obtained and thus a difference of the diffraction 
efficiency of a conjugate diffracted beam can be increased. Then, the 
present invention can realize an optical head apparatus capable of 
detecting a signal having a good S/N ratio. 
According to the present invention, as shown in FIGS. 19(a) to 19(g), after 
the completion of the exposure shown in FIG. 19(a) by a beam l.sub.1 and 
the development shown in FIG. 19(b), a rectangular cross section 3001 
shown in FIG. 19(c) is made on the surface of a substrate 300 by etching 
using a masking pattern 3051, and then, as shown in FIGS. 19(d) and 19(e), 
a masking pattern 3010 is formed again by a resist using a chrome mask 
4000 and a beam l.sub.2, an ion beam 302 is obliquely irradiated to etch 
the surface of the substrate 300 to thereby form a blazed hologram 2 
having a sawtooth-shaped cross section shown in FIG. 19(f). 
FIG. 19(g) shows a process by which the saw-shaped cross section is formed 
by the obliquely irradiated ion beam 302. FIG. 19(g) is an enlarged 
diagram of FIGS. 19(a) to 19(f). In FIG. 19(g), the portions 300a of the 
substrate 300 is etched by the obliquely irradiated ion beam 302. As the 
portion 3010a to be etched of the masking pattern 3010 is etched, the 
portion 3000a to be etched of the substrate is increased and finally the 
sawtooth-shaped cross section shown in FIG. 19(f) can be obtained. 
According to the present invention, since the oblique irradiation of the 
ion beam 302 shown in FIG. 19(e) causes an amount of etching to be reduced 
as compared with that of the conventional example shown in FIGS. 4(a) to 
4(d), an offset from an ideal sawtooth shape is reduced unlike the 
configurations A and B shown in FIG. 4(d) so that a cross section nearer 
to the sawtooth shape can be obtained. Further, the masking pattern 3010 
of FIG. 19(e) can be made a little wider as shown in FIG. 16. More 
specifically, as shown in FIG. 16, even if the masking pattern 301 
projects from the left edge of the convex portion 306 of the substrate 300 
to the left side by d.sub.1, the masking pattern 301 does not prevent the 
formation of the rectangular cross section because it is etched by the ion 
beam 302. The masking pattern 3010 may be rather positively widened to 
make the left side of the convex perpendicular. In addition, even if the 
masking pattern projects from the center line L.sub.1 of the convex 
portion 306 to the right side by d.sub.2, the masking pattern also does 
not hinder the formation of the sawtooth-shaped cross section because the 
substrate 300 begins to be etched after the projected portion has been 
etched. As described above, since the method shown in FIGS. 19(c) to 19(g) 
enables the masking pattern 3010 to be a little widened, the aligning 
accuracy of the chrome mask 4000 in the exposure in FIG. 19(d) can be 
eased. 
Next, it is considered that the blazed hologram 2 is used by being split 
into, for example, regions H1 to H4 shown in FIG. 17. In this case, the 
sawtooth can be made to a predetermined height [h in FIG. 19(f)] in any 
portion of the split regions H1 to H4 in such a manner that pitches P1, 
P2, P3, and P4 in a certain direction (X direction) are made substantially 
the same as shown in FIG. 17 and when the direction of the incident ion 
beam 302 is represented by a vector as shown in FIG. 19(e), the direction 
of the obliquely projected vector to the two surfaces of the blazed 
hologram 2 is caused to coincide with the X direction. The reason thereof 
will be described below. A cross-sectional view perpendicular to the 
surface of the blazed hologram 2 including the vector representing the 
incident direction of the ion beam 302 is considered as shown in FIGS. 
19(e) and 19(f). Since the incident angle of the ion beam 302 to the 
blazed hologram 2 is a constant, when the sawtooth is observed in this 
cross-sectional view, the angle of the inclined surface is a constant 
[.theta. in FIG. 19(f)]. When the pitch P to the X direction shown in 
FIGS. 19(f) and 12(a) is made to a constant, h=P tan .theta. also can be 
made to a constant. As described above, making h constant enables the 
phase modulation amplitude of all the split regions (H1 to H4) to be 
constant, whereby the diffraction efficiency of all the split regions (H1 
to H4) is made to be constant. Therefore, it is possible to make the 
diffraction efficiency of a +1 order diffracted beam substantially 100% in 
all the split regions (H1 to H4). Further, since the cross section of the 
hologram made by the manufacturing method of the present invention is very 
near to a sawtooth shape, it is very easy to increase zero and -1 order 
diffraction efficiencies and to reduce a -1 order diffraction efficiency. 
When the hologram is applied to an embodiment of a beam such as an optical 
pick-up apparatus shown in FIG. 6(b) and the like, the utilization 
efficiency of the beam is improved and a stray beam produced on a going 
path by the diffraction made from the blazed hologram 2 can be reduced. 
The provision of the blazed hologram having the cross section near to a 
sawtooth shape enables a difference between the diffraction efficiencies 
of conjugate diffracted beams to be made larger. Further, an optical head 
apparatus capable of detecting a signal having a good S/N ratio can be 
produced by using the blazed hologram. 
Note that the utilization efficiency E of a beam used at this time to 
obtain the signal is given by: 
EQU E=E.sub.z .times.E.sub.+1 (1) 
where, E.sub.z and E.sub.+1 represent the zero and +1 order diffraction 
efficiencies, respectively. Therefore, E.sub.z and E.sub.+1 need to be as 
large as possible to increase the utilization efficiency of a beam for the 
purpose of improving the S/N ratio of various signals, and thus it is 
found that the blazed hologram described above in the present invention 
must be used. Further, when the light beam 36 is incident on the hologram 
2, the -1 order diffracted beam 121 is also produced on the going path, 
and the beam of the -1 order beam on the return path which has been 
reflected by the optical memory medium 4 and transmitted on the hologram 2 
is incident on the photodetector 36 as a stray beam. Since the stray beam 
lowers the S/N ratio of a detected signal, the diffraction efficiency of 
the -1 order diffracted beam must be made as small as possible. For this 
purpose, it is found that the blazed hologram described above in the 
present invention is effective. 
FIG. 8 shows an example analyzing the ratio of the beam quantity of the 
above stray beam and the signal beam. In this embodiment, a design 
principle for approximately realizing a blazed hologram by four-level 
phase steps is given for simplification. More specifically, the 
diffraction efficiency of a hologram, a beam utilization efficiency on 
going and return paths .eta..sub.0 .times..eta..sub.+1, and the ratio of a 
noise component (.eta..sub.+2 .times..eta..sub.+1 =.eta..sub.0 
.times..eta..sub.-1) to a signal beam component .eta..sub.0 
.times..eta..sub.-1) are calculated and plotted, supposing that the depth 
of a first etching (the difference between steps) corresponds to 
.PHI.=.pi./2 in terms of a phase offset amount and using an amount of 
asecond etching (horizontal axis) as a parameter. Supposing that the depth 
of the second etching is .PHI..congruent..pi./3, it is found that the beam 
utilization efficiency of about 14% and the noise/power ratio of about 20% 
can be realized. 
FIG. 9 shows an example of the result of an experiment obtained by the 
error signal detection by a SSD system using FE=(S.sub.3 -S.sub.2 
-S.sub.4)-(S.sub.6 -S.sub.5 -S.sub.7) and the tracking error detection by 
a slit detection system and TE=S.sub.1 -S.sub.8 shown in FIG. 6(c) of the 
embodiment. When observed by a peak-to-peak value, a TE signal maintains 
about 90% of a maximum value with respect to an offset amount of 
.+-.+/-700 micrometers between an objective lens and the optical-axis of a 
hologram element (objective lens: NA=0.45, conjugate length; about 30 mm) 
and the offset of the TE signal is within .+-.10% of a peak value, so that 
two excellent effects of the present invention have been confirmed. 
Note that Table 1 shows the range of tolerance of a mechanical assembly 
error obtained by a computer simulation by comparing the hologram head 
(Type H-3) (SSD/slit detection method) shown in FIG. 1(b) with the system 
of the embodiment of the present invention shown in FIG. 6(c) (Type H-4). 
In the simulation, an assembly error as the tolerance was determined when 
a TE signal has a sensitivity lowered to 90% of an maximum value and an 
offset was 10% of a maximum value. It is found from Table 1 that the 
latter exhibits a tolerance which is greater than 2 times as large that of 
the former. Note that in the simulation, the same objective lens (NA=0.45, 
conjugate length L=30 mm) was used while having h=5.5 mm in Type H-3 and 
h=22 in Type H-4. 
FIG. 10 explains a method of determining the center of a fringe pattern 
when the holographic element by the SSD method according to the present 
invention is designed by a computer and shows the positional relationship 
of the points (x, y, z) of the surface 2 of the hologram element, a 
reference source R (O, O, O), and an object point O.sub.1 (x.sub.1, 
y.sub.1, z.sub.1), O.sub.2 (x.sub.2, y.sub.2, z.sub.2). Supposing that a 
wavelength is .lambda., the expression of a Fresnel zone plates with 
respect to O.sub.1, O.sub.2 is given by: 
EQU K=[(x-x.sub.i).sup.2 +(y-y.sub.i).sup.2 +(z-z.sub.i).sup.2 ].sup.1/2 
-(x.sub.2 +y.sub.2 +z.sub.2).sup.1/2 
where, i=1,2. 
FIG. 13 shows an example of a mask pattern drawn by a computer, wherein 
.lambda.=780 nm.times.2. (2010 and 2020 represent slit regions for 
detecting a tacking signal.) Further, the case in which the surface of a 
hologram is inclined with respect to the optical-axis of a lens can be 
easily determined by a simple consideration. As an example, a hologram 
pattern having a trapezoidal opening as shown in FIG. 18(b) can be 
synthesized by a computer to an optical head apparatus using a reflection 
type hologram element 2000 shown in FIG. 18(a) as another embodiment of 
the present invention. 
The embodiment of the present invention shown in FIG. 18(a) is an optical 
system simpler than an integrated optical system constituting the SSD 
system using a micro prism 999 shown in FIG. 3 (Japanese Patent Unexamined 
No. 1-118224 filed by Sony) and further has a higher freedom of design. 
The optical head apparatus of this embodiment is different from the 
previous embodiment in that not only a laser source 100 is integrated with 
a photodetector 36 but also the entire optical system is fixed on the 
optical-axis of a lens 333 for integration. With this arrangement, the 
conditions, under which the off-axis aberration of the asperical lens 333 
is caused, are restrained to thereby realize the micro objective lens 333 
having a short conjugate length, and thus the entire optical system can be 
easily accommodated in an actuator. As a design example, an asperical lens 
having a conjugate length of 10 mm could be realized with the weight of 
the entire optical system including a lens barrel of 1 g. 
A hologram element 2000 shown in FIG. 18(b) explains that the opening in 
the reflection hologram used in the arrangement in FIG. 18(a) is 
vertically asymmetric and optimized. Referring to FIG. 10, the method of 
designing the hologram element in this case is similar to that of a 
transmission type one, except that the hologram surface (x, y, z) thereof 
is inclined by about 45.degree. with respect to an optical-axis z. 
A great effect to the circumference-resistant performance such as dust 
resistance, moisture resistance and the like can be expected from the 
entire optical system accommodated in the sealed container. 
TABLE 1 
______________________________________ 
Calculated tolerances with tracking servo performance under 
displacements of respective elements to each direction 
Optical Type H-3 Head Type H-4 Head 
elements 
Direction 
Sensitivity 
Offset Sensitivity 
Offset 
______________________________________ 
Lens Radial .+-.350 .+-.350 
.+-.980 
&gt;1000 
HOE Tangential 
.+-.270 Don't care 
.+-.700 
Don't care 
Radial .+-.120 .+-.100 
.+-.370 
+370 
Lens .+-.260 Don't care 
.+-.900 
Don't care 
______________________________________