United States Patent [191
[11]
[45]
Fukushima et a1.
[54] OPTICAL INTEGRATED CIRCUIT AND
0259832
61-85641
Yokohama; Kazumi Kawamoto, 92-6,
Torigaoka, Totsuka-ku, Yokohama;
Kenchi Ito, 850, Maiokacho,
Totsuka-ku, Yokohama; Masataka
Shiba, 2-1-5-602,
Royokuen-4-chome, Izumi-ku,
Yokohama; Akira Arimoto, 14-4,
Nakajimacho, Kodaira-shi, all of
Japan
[21] Appl. No.: 372,639
[30]
Jun. 28, 1989
Japan
Jun. 5, 1989 [JP]
Couplers” Appl. Physics 14/77, pp. 235-254.
G. Hatakoshi et al. “Waveguide Grating Lenses for
Optical Couplers” Applied Physics, 1984 vol. 23, No.
11, pp. 1749-1753.
'
M. L. Dakss “Grating Coupler for Efficient Excitation
of Optical Guided Waves in Thin Films" in Applied
Physics Letters 1970, vol. 16, No. 12.
Primary Examiner-Roy N. Envall, Jr.
Assistant Examiner-Hung H. Bui
Attorney, Agent, or Firm-Antonelli, Terry, Stout &
ABSTRACT
63-159105
An optical integrated circuit and an optical apparatus
Japan ................................ .. 1-141015
used in optical communication or an opto-electronics
apparatus such as an optical disk recording apparatus, in
which various kinds of aberration caused by wave
length variation of a light source when a semiconductor
[51]
Int. Cl.5 ....................... .. G11B 7/00; G11B 21/00
U.S. Cl.
. . . . . . . ..
360/44.12; 369/103;
369/109; 369/110; 385/14; 385/37; 359/571
[58]
OTHER PUBLICATIONS
J. H. Harris et a1. “Theory & Design of Couplers” Ap
plied Optics 11/10/72 pp. 2234-2240.
T. Tamir & S. T. Peng, “Analysis & Design of Grating
[57]
[52]
.......
3/1988
l/l986
Kraus
Foreign Application Priority Data
Jun. 29, 1988 [JP]
Dec. 3, 1991
European Pat. Off. ....... .. 369/4412
Japan .
61-29650 12/1986 Japan .
[76] Inventors: Atsuko Fukushima, 1545,
Yoshidacho, Totsuka-ku, Yokohama;
Yasuo Hira, 1534-36, Sugetacho,
Kanagawa-ku, Yokohama; Hidemi
Sato, 3404-35, Nakatacho, Izumi-ku,
5,070,488
FOREIGN PATENT DOCUMENTS
OPTICAL APPARATUS
[22] Filed:
Patent Number:
Date of Patent:
laser is used as the light source. An aberration correct
Field of Search ............. .. 369/4412, 44.37, 44.38,
ing grating having such grating pitches as to nearly
369/4411, 103, 109, 110; 350/96.ll—96.l5,
162.22, 162.17, 162.2, 162.24, 162.16; 372/102
satisfy the Bragg condition preferably with respect to a
certain wavelength of the semiconductor laser beam is
used. As a result, lowering of incidence (entrance) and
radiation (exit) coupling efficiencies of the grating cou
[56]
References Cited
U.S. PATENT DOCUMENTS
4,435,041
3/1984
Torok et a1. ..
4,718,052
4,737,946
1/1988
4/1988
Kondo et al. .
Yamashita et a .
4,779,259 10/1988
Kono et a1. ......... ..
4,797,867
1/1989 Sunagawa et al.
4,842,969
6/1989
.. 350/162.24 X
369/4412 X
350/9611 X
369/4412 X
pler and aberration such -as chromatic aberration can be
prevented. Even if a multi-mode semiconductor laser is
used as a light source, the characteristics of the optical
integrated circuit and optical apparatus do not vary
largely.
369/4412 X
Kawatsuki ............... .. 350/ 162.22 X
24 Claims, 11 Drawing Sheets
US. Patent
Dec. 3, 1991
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Dec. 3, 1991
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to nearly the diffration limit in order to read out high
density information recorded on an optical disk me
OPTICAL INTEGRATED CIRCUIT AND OPTICAL
dium. Therefore, the numerical aperture (NA) of a lens,
which is de?ned as the diameter of lens/focal length,
APPARATUS
5 must be not less than 0.45. In case of the above de
BACKGROUND OF THE INVENTION
scribed conventional focusing grating coupler, how
The present invention relates to an optical integrated
ever,
the wavelength deviation AA of the laser light
circuit and an optical apparatus used in an optoelectron
must satisfy a strict value represented as
ics device such as an optical communication apparatus
|A7t| =9.8 X 10-4 pm when NA is 0.45. Further, devia
or an optical disk recording apparatus, and in particular
to an optical integrated circuit and an optical apparatus 10 tion 6 of the laser light from the optical axis and devia
tion AN of the effective refractive index of the guided
in which various aberrations caused by variation in
optical beam must respectively satisfy the following
wavelength of a light source when a semiconductor
laser is used as the light source are corrected.
values.
Conventional'optical components used in ?elds such
as an optical communication system and optical infor
mation processing comprise bulk components such as
lenses, prisms and gratings mechanically assembled.
Since the above described optical components have
These values are extremely strict and are not at a practi
large outer dimensions, therefore, they cannot meet a
demand for size reduction. Further, they are expensive.
cal level. Especially, since 8 is small, the above de
scribed conventional focusing grating coupler cannot
be applied to an integrated optical pickup in which
high-speed access is implemented by propagating sur
Further, because of assembly using mechanical cou
pling, the above described optical components lack
safety for long time use and are inferior in reliability.
Because of these various problems, the concept of an
optical integrated circuit (optical IC) comprising a plu
face acoustic waves at right angles to the guided optical
25 beam and de?ecting the beam to the left and right with
respect to the optical axis by using the acoustic waves,
rality of devices integrated on a single substrate has
been introduced in recent years to study signi?cant
reduction in size and cost of optical components. That is
to say, an optical IC comprises optical components
obtained by integrating a photodetector device, a light
emitting device, a lens of waveguide type (thin film.
type) and a grating on a single substrate.
One component of the optical IC is a grating coupler,
which is a waveguide diffraction grating formed in an
resulting in a drawback.
A third problem will now be described. In the above
described conventional optical head fabricated as an
optical IC, a beam splitter for detecting a signal supplied
from the optical disk is constituted as a so-called copla
nar optical device. Therefore, the reception angle (i.e.,
magnitude of deviation of an incidence angle of light
with respect to a device which does not cause signi?
optical waveguide. The grating coupler functions to
35 cant deterioration in device characteristics and hence is
make an optical beam incident on the optical waveguide
and make the optical beam radiate from the optical
waveguide. The grating coupler is thus one of key com
ponents of the optical IC.
Concrete design methods of the above described
grating coupler are discussed in J. E. Harris, et al.
permitted) is small, and the detection range of a focal
point error signal is narrow. The range in which focus
ing servo functions is limited to a narrow range, and
characteristics deterioration caused by a change in
40
wavelength of laser light is signi?cant, i.e., so-called
chromatic aberration is signi?cant, resulting in a subject
“Theory and Design of Periodic Couplers”, Appl. Opt.,
to be solved.
A fourth problem will now be described. When the
optical disk is not in parallel to the above described
l1, 10 (1972), and T. Tamir and S. T. Peng, “Analysis
and Design of Grating Couplers", Appl. Phys, 14,
(1977), for example. As for optical components formed 45 conventional optical head, the optical axis obtained
when the light reflected by a surface of the optical disk
into an optical IC by using a grating coupler, examples
is introduced to the optical waveguide path again is
of their application to an optical head for optical disk
apparatus are described in J P-A-6l-85641 and J P-A-6l
deviated from the center axis of the beam splitter.
296540.
Therefore, an offset is caused in a tracking error detec
When a semiconductor laser is used as the light 50 tion signal, resulting in a subject.
source, the above described prior art has the following
SUMMARY OF THE INVENTION
problems. That is to say, the wavelength of light emit
ted by a semiconductor laser typically changes because
A first object of the present invention is to provide an
of nonuniformity in operation temperature and manu
optical integrated circuit in which the circuit character
facturing process for fabricating the semiconductor 55 istics do not largely charge and the beam utilization
laser. A ?rst problem will now be described. In case the
ef?ciency is high even when a multi-mode semiconduc
tor laser light is used.
A second object of the present invention is to provide
an optical pickup, i.e., an optical information readout
emitted wavelength is not single, the incidence angle
whereat the beam can enter the optical waveguide is
changed by the grating coupler, resulting in a problem
of lowering in incidence coupling efficiency. Further,
apparatus allowing de?ection of a guided optical beam
from the optical axis to the left and right at high speed
by using surface acoustic waves and allowing high
there is also a problem that the radiation angle changes
in the same way as the foregoing description when a
grating coupler is used at the exit side.
speed access.
A third object of the present invention is to provide
A second problem will now be described. When a
grating coupler having nonequal pitches and taking a
65
curvilinear shape is employed as a focusing grating
an improved optical head which effectively functions
even when the wavelength of laser light changes and in
coupler used as an objective lens of an optical head, the
which focusing servo effectively functions over a wide
spot size of a beam at a focal point must be reduced up
area.
3
5,070,488
A fourth object of the present invention is to provide
an optical head capable of reducing an offset of the
tracking error detection signal, which poses a problem
when an optical disk is so tilted as not to be in parallel
4
de?ne D so that the aberration correcting grating may
satisfy the Bragg condition with respect to at least one
wavelength among beams of M0) to Ml) or diffraction
may be caused in the vicinity of the Bragg condition.
to the optical head.
The above described ?rst object is achieved by dif
BRIEF DESCRIPTION OF THE DRAWINGS
fracting laser light by using a grating separately pro
FIG. 1 is a con?guration diagram showing an em
vided for the purpose of aberration correction. In ac
cordance with a feature of the present invention, the
grating period of the above described grating for aber
bodiment of an optical integrated circuit according to
the present invention.
FIG. 2 is a sectional view of an aberration correcting
ration correction is preferably so set as to meet nearly
the Bragg condition with respect to a certain wave
ent invention.
length of semiconductor laser light.
The above described object is achieved by integrat
recting grating used in an optical integrated circuit of
grating used in an optical integrated circuit of the pres
FIG. 3 is a sectional view of another aberration cor
ing, in a hybrid form, a grating having linear uniform 15 the present invention.
shapes, which radiates a guided optical beam from an
FIG. 4 is an oblique view showing an optical head for
optical waveguide to the inside of a substrate, and a
optical disk apparatus according to the present inven
focusing device functioning to converge the radiated
tion.
light to one point. Electrodes for exciting surface acous
FIGS. 5(a) to 5(3) schematically show fabrication
tic waves can be disposed on the above described sub 20 process steps of an optical integrated circuit according
strate, and especially on the optical waveguide.
to the present invention.
Thereby, the direction of radiation of the guided optical
FIGS. 6 and 8 show a second embodiment of the
beam can be changed, and an optical integrated circuit
present invention. FIG. 6 is an oblique view of an opti
can be used as a high-speed light de?ection apparatus.
cal integrated circuit with focusing devices integrated
The above described third and fourth objects are 25
in a hybrid form. FIG. 7 is an oblique view showing a
achieved by an optical head comprising a beam splitter
so formed on an optical waveguide between a laser
linear orthogonal coordinate system used in explanation
of embodiment. FIG. 8 is an oblique view showing a
beam incidence (entrace) grating coupler and a laser
light deflection effect implemented by an optical de?ec
beam radiation (exit) grating coupler as to detect a sig
tor.
nal fed from an optical disk, quarter a reflected beam 30
FIGS. 9a and 9b show a third embodiment of the
fed from the optical disk, and radiate the quartered
present
invention, which is an optical integrated circuit
beams into the substrate at predetermined radiation
comprising a combination of a grating coupler and a
Fresnel
lens. FIG. 9a is a sectional view of the third
two pairs of gratings respectively having nonequal
pitches and taking curvilinear shapes, so disposed on the 35 embodiment. FIG. 9b is a top view of the third embodi
ment.
left and right of the nearly central axis of luminous ?ux
F IGS. 10 to 11 show an optical head for optical disk
of the above described re?ected beam as to focus the
apparatus,
which is a fourth embodiment of the present
reflected beam on a photodetecting face of a photode
invention. FIG. 10 is a top view of its optical integrated
tector disposed on the outside of the end face of the
circuit. FIG. 11 is a side view of its optical integrated
substrate.
circuit.
Owing to characteristic con?guration according to
FIGS. 12 to 14 show the detection principle of focus
the present invention, the following operation effects
spot error signal of an optical disk. FIG. 12 is a top view
are achieved. That is to say, in case the wavelength A of
showing the case where the optical disk is located at the
the semiconductor laser beam changes from M0) to M1)
(where )t(0)>7t(l)), strong coupling of light to the opti 45 focal point. FIG. 13 is a top view showing the case
where the optical disk is located in front of the focal
cal waveguide is caused only when the incidence angle
point. FIG. 14 is a top view showing the case where the
a to the grating coupler is 11(0) and a.(l) respectively
optical disk is located behind the focal point.
corresponding to M0) and MI). In the absence of an
angles with respect to the optical waveguide plane, and
aberration correcting grating, the incidence coupling
FIGS. 15 to 18 show a sixth embodiment of the pres—
efficiency lowers because the angle of incidence onto 50 ent invention. FIG. 15 is a sectional view of a grating
coupler for incidence (i.e., an entrance grating coupler)
the grating coupler does not change even if the wave
having a rectangular grating shape. FIG. 16 is a sec
length changes. Therefore, a grating for correcting
tional view of an incidence grating coupler having a
aberration is installed to make grating period D and
blazed grating shape. FIG. 17 is a sectional view of a
inclination angle 8 proper. For beams respectively hav
ing wavelengths M0) and M1), therefore, diffraction 55 grating coupler for radiation (i.e., an exit grating cou
pler) having a rectangular grating shape. FIG. 18 is a
angles from the aberration correcting grating can be set
at certain speci?c angles 8(0) and 8(1), respectively.
sectional view of an exit grating coupler having a blazed.
Angles 5(0) and 8(1) refracts at the interface with re
grating shape.
spect to the substrate, resulting in angles 41(0) and (1(1)
of incidence to the grating coupler, respectively. Even
if the wavelength of semiconductor laser light changes,
therefore, angles of incidence to the grating coupler can
be made (1(0) and a.(l) respectively corresponding to
M0) and M1), and lowering of the incidence coupling
efficiency caused by variation of wavelength of the 65
FIGS. 19a and 19b show a seventh embodiment of
the present invention, which is an optical system of an
optical head comprising a combination of an optical
integrated circuit and an optical device of bulk type.
FIG. 19a is its side view. FIG. 1% is its top view.
FIG. 20 shows an eighth embodiment of the present
invention, which is an optical system of an optical head
laser beam can be prevented. Further, aberration such
having a separated objective lens portion of for detect
as chromatic aberration can also be prevented. For
ing information recorded on an optical disk. FIG. 20a is
its side view. FIG. 20b is its top view.
obtaining light having high strength, it is important to
5
5,070,488
FIG. 21 shows a ninth embodiment of the present
invention, which is an optical system of an optical head
6
-continued
(3)
3(0) : 6(0)
in which the re?ected light is returned from an optical,
disk to an optical integrated circuit again. FIG. 21a is its
side view. FIG. 21b is its top view.
FIG. 22 shows a tenth embodiment of the present
invention and is a side view showing an optical system
of an optical head used in an optical magnetic apparatus.
where
B(O)=incidence (entrance) angle with respect to the
diffraction grating at the wavelength M0)
'y(O)=radiation (exit) angle with respect to the dif
fraction grating at the wavelength M0)
In addition, d is conditioned to satisfy equation (4) so
that light having the wavelength 7t( 1) may be diffracted
in the vicinity of the Bragg condition.
DESCRIPTION OF THE PREFERRED
EMBODIMENTS
Preferred embodiments of the present invention will
now be described more concretely by referring to
drawings.
Ml) m
(4)
sin-7(1) — sinB(l) = "Pd
In FIG. 1, LiNbO3 crystals are used as a substrate 1.
In the vicinity of a surface of the substrate, an optical
waveguide 2 and a grating coupler 3 having a refractive .
index somewhat higher than that of the substrate are
where
B(l)=incidence angle with respect to the diffraction
grating at the wavelength Ml) (3(0): 3(1))
formed. Subsequently, glass blocks 5 and 5' for making
y(1)=radiation angle with respect to the diffraction
a beam emitted from a semiconductor laser 4 incident 20
grating at the wavelength Ml) Further, d, 8 and 0 are so
onto the substrate 1 in parallel are formed. A grating 6
defined that equations (5) to (12) below may be satisfied.
for correcting aberration is put between the glass blocks
5 and 5’.
The operation of the optical integrated circuit ac
cording to the present invention shown in FIG. 1 will
now be described. In FIG. 1, the grating coupler 3 is a
Cosmo) :
(5)
"3
diffraction grating having linear uniform shapes. An
some“) : N _ 2(1)//\
incidence angle a of a laser beam onto the grating cou
pler 3 is related to grating pitch A of the grating, effec
tive refractive index N of the optical waveguide 2, re
fractive index n; of the substrate 1, and wavelength 7t of
30
the semiconductor laser 4 as
N — MA
COSKZ =
(6)
S
<1i<0>= 010(0) + 0 - 121
<7)
(11(1) = (10(1) + e _ §
"3)
(l)
(9)
35
a2(O) = sin + [ "/1"S sina1(0)]
From this relation, the incidence angle a is uniquely
de?ned with respect to the wavelength A. In accor
_
'
(l0)
"s
dance with the present invention, therefore, the aberra
112(1) = sin +l: "/1 sina1(l):l
tion correcting grating 6 is provided in order to prevent 40
lowering of the incidence coupling efficiency even if
7(O)=az(0)—6—5+1r
(11)
the wavelength of the semiconductor laser 4 changes.
That is to say, the grating period d and the inclination
7(1) = ¢2(1)- 9 —- 5' + 1r
(12)
angle 6 of the aberration correcting grating 6 are made
proper when the wavelength of the semiconductor laser 45 where
a0(0)=angle of incidence to the grating coupler at
4 has changed from M0) to M1) (where )\(O)>)t(l). As
}\(0)
a result, the diffraction angles from the aberration cor
recting grating 6 with respect to beams respectively
a0(l)=angle of incidence to the grating coupler at
Ml)
having wavelengths M0) and M1) can be set at certain
specific angles 7(0) and 'y( 1), respectively. Even if the
a|(0)=angle of refraction from the glass block to the
wavelength of the semiconductor laser 4 changes,
therefore, the incidence angles to the grating coupler
substrate at M0)
can be made equal to (1(0) and 11(1) respectively corre~
sponding to M0) and M1). It is thus possible to prevent
substrate at Ml)
a2(0)=angle of incidence to the substrate at M0)
a2(1)=angle of incidence to the substrate at Ml)
a1(l)=angle of refraction from the glass block to the,:
lowering of the incidence coupling ef?cinecy and aber
ration such as chromatic aberration caused by variation
A=grating period of the grating coupler
of wavelength of laser light. Concrete examples of the
grating period d and inclination angle 6 of the aberra
d=grating period of the aberration correcting grat
mg
0=cutoff angle of end face of the substrate
tion correcting grating 6 will now be described.
First of all, d is conditioned to satisfy equations (2)
and (3) so that light having the wavelength MO) may be
diffracted by the aberration correcting grating 6 under
the Bragg condition or in the vicinity of the Bragg
60
5=inclination of the aberration correcting grating 6
m=order of diffraction by the aberration correcting
grating, which is set at —l
N=effective refractive index of the optical wave
guide 2
condition.
65
M0)
(2)
n$=refractive index of the substrate 1
np=refractive index of the glass block
FIG. 2 shows a preferred sectional shape of the aber
ration correcting grating 6 according to the present
7
5,070,488
8
A method for manufacturing the optical integrated
invention. For obtaining diffracted light having high
strength, it is desirable to use a diffraction grating hav
circuit shown in FIG. 4 and its operation will now be
ing a shape which satis?es the Bragg condition already
described.
A concrete example of a diffraction grating satisfying
described in detail. With regard to various optical de
vices of waveguide type such as the grating couplers 3
and 3' and the focusing beam splitter 16, the method for
manufacturing the optical integrated circuit as shown in
the condition heretofore described will now be de~
FIG. 4 will now be described by referring to FIGS. 5(a)
scribed. A semiconductor laser with M0) equivalent to
0.78 pm and MI) equivalent to 0.776 pm is used. On a
to 5(i g). In FIG. 5(a), LiNbO3 crystals optically pol
ished is used as the substrate 1, and Ti is so deposited as
to have a thickness of 24 nm by sputtering. The optical
Ti-diffusion optical waveguide comprising LiNbO3
crystals with n,=2.2 and N=2.209, a grating coupler
with A=4 pm is formed. A glass block made of BK-7
with np= 1.45 is stuck to an end face whereon a basic
laser beam is incident In this case, 0, 8 and d become
approximately 56°, approximately 100' and approxi
mately 1.6 pm, respectivley. Although in this case inci
dence angles of beams respectively having M0) and 7(1)
are equivalent each other with respect to the diffraction
grating, the difference in radiation (exit), angle becomes
15
waveguide 2 is then formed by thermal diffusion. The
above descirbed sputtering is performed under the con
dition that the high frequency power is 300 W, argon
gas pressure 0.5 Pa, and sputtering speed 4 nm/sec.
Thermal diffusion was performed by using an electric
furnace under the temperature of 1,000‘ C. in an atmo
sphere of argon gas for 2 hours and then in ?ow of
oxygen gas for 0.5 hour. The optical waveguide had a
surface refractive index n/=2.22 and was an optical
approximately 0.1‘. In this case, the ef?ciency of the
20 waveguide of single TE mode having an equivalent
diffracted beam (i.e., radiated beam of the aberration
refractive index N=2.209. The optical waveguide 2
correcting grating 6) becomes 90% or more when T is
may be fabricated by using the proton exchange tech
approximately 11 pm.
nique. In FIG. 5(b), a buffer layer 18 formed on the
An example of the above described aberration cor
optical waveguide 2 functions to prevent occurrence of
recting grating 6 will now be described. As a substrate, 25 peeling-off or cracking of a grating layer subsequently
BIC-7 glass 8 is used. On the glass, SiO; 9 is so formed as
formed. As the buffer layer 18, glass 7059 produced by
to have a thickness of approximately 11 pm by using a
Corning corporation was so formed as to have a thick
?lm forming method such as the CVD method. The
ness of 10 nm. Sputtering is performed under the condi
SiOz layer is ?nely processed so as to have a predeter
tion that the radio frequency power is 1(1) W, argon gas
mined shape by using photolithography. The glass
blocks 5 and 5: made of BK-7 and the above described
diffraction grating 6 are then stuck together with a
pressure 0.35 Pa and sputtering speed 0.2 nm/sec. In
FIG. 5(a), as a grating layer 3, TiOz was so formed on
the buffer layer by reactive sputtering as to have a
thickness of 100 nm. The sputtering condition is as fol
lent to that of BK-7. A periodic structure is formed by
lows. A TiOz target is used, and argon and oxygen are
the above described SiO; 9 and the binding agent, and 35 used as sputtering gas. Flow rate ratio between 01 and
the periodic structure functions as the aberration cor
Ar is 0.7, sputtering gas pressure 0.42 Pa, radio fre
binding agent having a refractive index nearly equiva
recting grating 6.
As for the form of the diffraction grating used in the_
present invention, a diffraction grating of re?ection
quency power 500 W, and sputtering speed 0.1 nm/sec.
In order to process ?nely the grating layer 3 and the
buffer layer 18 to provide them with shapes of optical
type comprising a re?ection ?lm 10 as shown in FIG. 3 40 devices of waveguide type. In FIG. 5(4), a resist 19 was
may be used instead of the diffraction grating of trans
formed on the grating layer 3 by using the rotating
mission type as shown in FIGS. 1 and 2.
application technique. Here, methyl chloride polysty
FIG. 4 shows an embodiment of the present inven
rene (CMS-EXR produced by Toyo Soda) which is an
tion, which is an optical integrated circuit effective as
electron beam resist was used as the resist 19 and pro
an optical head used in an optical disk apparatus. In 45 vided with a thickness of 0.5 pm. After the above de
FIG. 4, a beam radiated from the semiconductor laser 4
scribed resist was prebaked at 130' C. for 20 minutes, an
is incident on the grating coupler 3 from the substrate 1
electron beam 20 was applied to the resist in accordance
via a collimator lens 11, the glass block 5 and the aberra
with a predetermined grating shape. As for the radia
tion correcting grating 6, and is guided to the optical
tion condition, the electron beam diameter and the
waveguide 2. A guided beam 7 is deflected in an optical 50 amount of radiation were defined to be 0.1 pm and I6
de?ector 12 so formed on the optical waveguide 2 as to
uc/cmz, respectively. In FIG. $(e), after exposure with
use surface acoustic waves (SAW), and is radiated again
an electron beam, development was performed to form
into the substrate at a grating coupler 3' of- radiation
a mask made of resists. In FIG. 5(f), the grating layer 3
side. The beam thus radiated is diffracted by an aberra
and the buffer layer 18 were processed ?nely by ion
tion correcting grating 6’. After the optical path is so 55 etching. As for the condition of ion etching, CF4 was
changed as to be vertical by a glass block 13, the beam
used as etching gas and pressure of 3.8 Pa, radio fre
is focused to one point on an optical disk 15 by an objec
quency power of 200 W and etching time of 15 min.
tive lens 14. The beam re?ected by the optical disk 15
were used. In FIG. 5(g), after the etching, the mask
traces the identical path and is guided into the optical
made of the resist was removed. Various optical devices
waveguide 2 by a grating coupler 3’ located at the radia 60 of waveguide type such as grating couplers 3 and 3' and
tion side. The beam is then passed through the optical
the focusing beam splitter 16 could be thus formed. As
de?ector l2, bisected by a focusing beam splitter 16
for the SAW optical de?ector 12, an Al ?lm having a
formed on the optical waveguide 2, and radiated into
thickness of 100 nm was vapor-deposited and processed
?nely to form a predetermined comb electrode. The
the substrate. Each of the beams thus bisected and radi
ated is focused onto one point on the photodetecting 65 lift-off technique was used for the above described pro
cessing. As for speci?cations of the electrode. the cen
face of one of photodiodes 17 and 17' respectively dis
ter wavelength is 14 pm, the center frequency 250
posed on end faces of the glass block 5. The signal fed
MHz, and the number of pairs 8. With regard to the
from the optical disk 15 is detected.
9
5,070,488
aberration correcting 6, BK-7 glass was used as the
substrate 8. On the BK-7 glass, SiO; 9 was so formed as
10
the incidence coupling efficiency lowers. By using the
aberration correcting grating 6 having grating period d
to have a thickness of approximately 11 pm by the
CVD technique using SiCl4 and 02 as raw materials, the
vapor deposition technique or sputtering. In order to
process the SiO; 9 to attain a predetermined grating
equivalent to approximately l.6 um and thickness of
grating T equivalent to approximately 11 pm in accor
dance with the present invention, however, the beam is
shape
length, and incident on the grating coupler 3 at an opti
mum angle, resulting in a high incidence coupling ef?
by
using
photolithography,
photoresist
(OFPR800) was so formed by the rotating application
diffracted at a different angle depending upon the wave
technique as to have a thickness of 1 pm in the same
ciency.
way as the above described various optical devices of 10
Further, when the wavelength of the semiconductor
waveguide type. After the above described resist was
laser changes in the range of 0.776 to 0.78 pm under the
prebaked at 85° C. for 30 minutes, contact exposure was
condition that the aberration correcting grating 6’ is not
performed by using a photomask depicting a predeter
present, the incidence angle with respect to the lens 14
mined grating shape and a UV exposure apparatus.
After exposure, immersion processing in benzene chlo
ride was performed at 40° C. for 5 minutes, and then
changes by approximately 0.1", and the spot position is
displaced in the direction of jitter by approximately 8
pm. By using the aberration correcting grating 6’ hav
ing grating period d equivalent to approximately 2.5 pm
and thickness of grating T equivalent to approximately
development was performed. On the grating pattern
made of the resist, Cr was vapor-deposited Ultrasonic
cleaning in acetone was then performed to remove the
resist and form a mask made of Cr. Thereafter, SiOz was
?nely processed by ion etching using CF4 gas, and Cr
11 pm in accordance with the present invention, how
ever, the difference in incidence angle becomes not
larger than 0.01", and variation of spot position in the
jitter direction becomes not larger than 0.01 pm. Owing
formed. This photolithography technique can be ap
to the aberration correcting grating 6’ of the present
plied to ?ne processing of the above described grating
invention, dependence of variation of spot position on
layer and buffer layer as well. End faces of the substrate 25
the optical disk upon the wavelength is largely reduced,
1 having the above described devices formed thereon,
was removed. As a result, the grating pattern could be
the aberration correcting grating 6 and 6’, and the glass
blocks 5 and 5' made of BK-7 were cut at predetermined
and an optical head wherefrom more accurate informa—
tion can be read is obtained.
circuit as shown in FIG. 4. Operation of the optical
Constituent materials of the above described optical
integrated circuit include quartz, an SiOg glass sub
strate, a dielectric crystal substrate, an SiOg glass optical
waveguide and a metal diffusion optical waveguide.
Constituent materials of devices 3 and 3' include chalco
genide glass, Ti02, ZnO and ZnS. Constituent materials
scribed. The beam (having wavelength of 0.776 to 0.78
pm) radiated from the semiconductor laser 4 is colli
mated by the collimator lens 11 and diffracted by the
aberration correcting grating 6. The beam thus dif
the glass block 10 includes SiOz glass. In general, gen~
eral materials used to, form optical devices, optical
waveguides and thin ?lm optical devices can be used.
angles and polished. They were stuck together with a
binding agent having a refractive index nearly equiva
lent to that of BK-7. The semiconductor laser 4 and the
photodiodes 17 and 17' were coupled to the block stuck
together at their end faces to form the optical integrated
integrated circuit shown in FIG. 4 will now be de 35 of the aberration correcting grating 6 include SiOg glass
and a polymer compound. The constituent material of
fracted is refracted at the interface of substrate 1, and 40 The devices can be formed by these materials as well as
guided to the optical waveguide 2 in accordance with
the equations (5) and (6) according to the wavelength
and the periodicity length of the grating coupler. The
guided beam 7 is deflected by the SAW optical de?ec
the lithography technique and vacuum technique used
in fabricating semiconductors. Further, the above de
scribed optical head implemented as an optical inte
grated circuit can be applied to an optical head for laser
tor 12 and incident on the grating coupler 3'. Depending 45 beam printer, as well.
FIG. 6 shows a second embodiment of the present
upon the wavelength and the grating period of the grat
invention.
ing coupler, the incidence beam is radiated into the
In FIG. 6, numeral 1 denotes a substrate made of a
substrate in accordance with the equations (5) and (6).
dielectric or glass, 2 an optical waveguide so formed on
The beam thus radiated into the substrate is diffracted
a ?rst principal plane of the substrate 1 as to have a
by the aberration correcting grating 6' and reflected at
refractive index higher than that of the substrate 1. 3’ a
an end face of the glass block 13. The beam thus radi
grating coupler so formed on the optical waveguide 2 as
ated upward is focused by a lens 14 having mechanism
to have linear uniform shapes, 7 a guided beam propa
moved perpendicular to the optical disk 15 and focused
gated within the optical waveguide 2, 14 a lens for
onto a pit (information) of the optical disk 15. The beam
focusing the beam radiated from the substrate, and 21 a
reflected by the optical disk 15 is passed through the
focusing point (hereafter referred to as focal point, for
lens 14, the glass block 13 and the aberration correcting
convenience) of the radiated beam caused by the lens.
grating 6' and is incident on the optical waveguide 2
The guided beam 7 must comprise collimated parallel
again by the grating coupler 3’. The beam is then inci
beams.
dent on the focusing beam splitter 16 and bisected. The
It is now assumed that the wavelength of the laser
beams thus bisected are focused on the bisected photo 60
beam emitted from the light source is A, the effective
diodes 17 and 17’. The information of the pit is thus read
refractive index of the optical waveguide 2 with respect
to the guided beam 7 N, the refractive index of the
When the wavelength of the semiconductor laser
substrate 1 ns, and the period of linear uniform grating
changes in the range of 0.776 to 0.78 pm under the
condition that the aberration correcting grating 6 is 65 coupler 3')» In order to radiate an mth beam toward the
Substrate in a direction forming an angle 0 with respect
absent, the angle causing coupling to the optical wave
guide 2 changes by approximately 0.l°. Since the angle
to a substrate surface by using the grating coupler 3’, the
condition
to the grating coupler 3 is always constant, however,
out.
5,070,488
11
2):’ n,cos0 = 2T" N + 2%- m(m.' integer)
12
the radiation (exit) point of the beam as its origin and
has its Z’ axis in the direction wherein the beam is radi
ated when 8:0. Origins of these two coordinate sys
(13)
tems are made coincident each other. Further, the Z
must be satis?ed. As the diffracted beam, a (—l)-th
axis and Z’ axis form an angle 0. _
beam is generally used.
First of all, when parallel incident beams are incident
in a direction deviated from the Z axis by 8, the phase
matching condition in the xyz system is represented as
_
(14)
x
is satis?ed in addition to the condition expression (13), 10
the guided beam is radiated only in the substrate direc
tion and it is not radiated into the air, resulting in a high
KoNcos5 — 21r/A = Pz
(l6)
ef?ciency.
Assuming now that m: —l and 0:30’, the grating
period A is given by
where k,,=21r/80 , and Px, Py and P2 are components
of the radiated vector in the, xyz system. Since vectors
(15)
of the xyz system and x'y’z system are coupled each
20
other by the transformation
As an example, it is now assumed that LiNbO3 with
n,= 2.177 is used as the substrate 1 and the optical wave
guide 2 is so fabricated by thermal diffusion of Ti as to
satisfy the relation N=2.187. Further assuming that a
semiconductor laser with A=0.78 pm is used as the 25
light source, it follows that A=2.59 um and hence the
optical integrated circuit can be satisfactorily fabricated
(l7)
Px’
Py'
C059
=
0
—sin0
Px
0
1
0
Py
sinO
0
cost)
P:
Pz'
the radiated vector is represented in the x’y’z’ system as
by using the photolithography technique. Further, since
(18)
2
hr’ = kyqcosOJl —
S
sinli]
2
2
It;
"s
l
Z
:l _ [Ncosli - k/A ] + konacoso[
l
the condition (14) is also satis?ed at this time, the guided
beam can be radiated ef?ciently only in the substrate
Assuming now that the direction of the radiated beam
direction.
As another example, it is now assumed that Pyrex
and the Z’ axis form an angle ‘9 in case where the above
glass with n,= 1.472 is used as the substrate 1 and Cor 45 described LiNbO3 is used as the substrate, the approxi
ning 7059 glass having a refractive index of 1.544 so
mate relation ]6| :4) holds true in the range |8| 20.01
formed into a ?lm by sputtering as to satisfy the relation
rad. Further, as a result of refraction at a boundary face
N=l.520 is used as the optical waveguide 2. Further
between the substrate and the air, the relation ¢z2 | 6|
assuming that a semiconductor laser with A =0.78 pm is
holds true in the air. That is to say, the guided beam is
used as the light source in this as well, it follows that 50 radiated in a direction inclined from the Z’ axis nearly
A=3.l8 pm and hence the optical integrate circuit can
by 25 as parallel beams.
be satisfactorily fabricated by using the photolithogra
If a conventional objective lens for optical pickup is
phy technique in this case as well. Further, since the
used as the focusing lens 14, the radiated beam com
prises parallel beams. If 8 is sufficiently small as de
condition (14) is also satis?ed at this time, the guided
beam can be radiated efficiently only in the substrate 55 scribed above, therefore, aberration is 'so small that any
direction.
problem may be hardly caused.
As heretofore described, a large effect can be ob
Assuming that the focal length of a lens is 3 mm, for
tained in optical integrated circuits of the present inven
example, the focal point 21 moves by approximately 30
tion as compared with those of the prior art.
pm when 8:30.01. By positively de?ecting the guided
Aberration incurred when the above described wave
beam 7 by means of SAW (surface acoustic waves)
length deviation AA of laser beam, deviation AN of the
excited in the SAW optical deflector 12, therefore,
effective refractive index and deviation 8 of the laser
access and tracking correction of several tracks can be
beam from the optical axis will now be described.
performed as shown in FIG. 8.
It is now assumed that a linear orthogonal coordinate
By making the shape of the grating coupler linear as
system of left hand system has its Z axis in the propaga 65 heretofore described, it is possible to constitute a pickup
tion direction of the beam obtained when 8:0 as shown
objective lens system which is not signi?cantly affected
in FIG. 7. Further, it is assumed that another linear
by deflection from the optical axis Z of the guided beam
orthogonal coordinate system of left hand system has
7.
13
5,070,488
14
point f of the light. The guided beam 7 shown in FIG.
On the other hand, when deviation AN of the effec
tive refractive index and wavelength deviation of the
laser light source are caused, the direction of the radi
9b comprises collimated parallel beams.
The thickness of titanium oxide (Tl02) constituting
ated beam is deviated in the X2 (x’z’ ) plane by an angle
A6. However, the radiated beam comprises parallel
beams. Its magnitude is represented approximately as
the diffraction grating as well as the focal length f and
the aperture D of the Fresnel lens will now be described
in more detail. When a Fresnel lens is used as the objec
tive lens of the optical pickup head, the numerical aper
-AN
AA
A0: n,sin0 + AnSsinO
rad
When AN or AA appears singly in the above de
scribed case where LiNbO3 is used as the substrate, it
follows that
A0~ —0.92 AN rad
A0~0.3S Ax rad
ture NA must be made nearly equal to 0.5. The substan
tial numeral aperture of this lens depends on the nu
meral aperture of the diffraction grating. It is now as
(19)
(20)
sumed that the aperture length of the diffraction grating
in the x direction is Lx and the aperture length of the
diffraction grating in the y direction is Ly as shown in
FIG. 9b. When the beam radiated by the diffraction
15
grating is radiated in a direction forming an angle 6 with
respect to the surface of the substrate 1, the area of
bundle of rays incident on the Fresnel lens 22 is Lxcos0
XLy. Assuming that 0:30‘ and f=2 mm as in the
With due regard to the effect of refraction, the relation
|A0| < 0.01 rad is satis?ed in the range represented as
present embodiment, for example, relations Lx=2.30
mm, Ly: 1.15 mm must be satis?ed. In order to cover
all bundle of rays at this time, the aperture D of the
Fresnel lens must satisfy the relation
|a>.|§1.3><1o-2 um
(21)
These values are sufficiently large as compared with
those of the prior art.
In the optical integrated circuit of the present inven
tion heretofore described, a diffraction grating taking a
rectilinear shape is used in the grating coupler. Even if
AA, AN, AN and 8 have relatively large values, there
fore, the radiated beam comprises parallel beams. If a
high performance objective lens for pickup is used,
25
2
L15
2
L15
DéZX (2 )+(2)—l.63mm.
The beam is radiated from the grating coupler 3’ with
an amplitude in the x axis direction depending upon
exp(-a.x), where a is referred to as radiation loss coef
ficient. In order to assure the numerical aperture, the
condition aLxél must be satisfied. In case of the above
therefore, caused aberration can be reduced.
FIG. 9 shows an optical integrated circuit which is a 35 described example, it follows that
third embodiment of the present invention. FIG. 9a is its
l
sectional view, and FIG. 9b is its top view.
LX = 0.435 mm"‘.
In FIGS. 9a and 9b, numeral 1 denotes a substrate
comprising an optical material such as niobic acid lith
The value of (1 depends upon the refractive index and
ium (LiNbO3) having a refractive index 2.177, for exam
ple. Numeral 2 denotes an optical waveguide having a
refractive index higher than that of the substrate. In this
example, titanium was vapor-deposited on the LiNbO3
thickness of the substrate 1, the optical waveguide 2 and
the grating couple 3’. When LiNbO3 having a refractive
index n;=2.l77 and the optical waveguide 2 having a
substrate and diffused by heat to form an optical wave
refractive index N=2.187 are used and TiOg having a
guide having a thickness of approximately 1.5 pm. Nu 45 refractive index ns=2.4 is used as the loading layer
material of the grating coupler,'the relation (1:0.4 is
meral 3’ denotes a grating coupler so formed on the
satisfied by making the thickness of TiO; equivalent to
optical waveguide as to have linear uniform shapes. The
approximately 30 nm.
grating coupler 3’ is formed by an optical material hav
'Retuming to the description of the optical integrated
ing a refractive index larger than that of the optical
circuit having con?guration shown in FIGS. 9a and 9b.
waveguide. In this example, a titanium oxide (TiO2)
parameters of respective optical systems will now be
layer having a refractive index of 2.4 was formed on the
described.
It is now assumed that the wavelength of the laser
beam of the light source (not illustrated in FIGS. 90 and
in the direction of light advance by a pattern forming
technique using lithography. Numeral 22 denotes lens 55 9b) is )t, the effective refractive index of the optical '
optical waveguide. A rectilinear grating couple: having
a period of 2.59 pm was formed over a length of 4 mm
waveguide 2 N, the refractive index of the substrate 1
ns, and the period of the linear uniform grating coupler
means disposed on the optical waveguide 3 in front of
the diffraction grating 3’. In this example, a Fresnel lens
of transmission type having an aperture of approxi
mately 2 mm was formed on the optical waveguide 2 by
using the well-known technique. The aperture of the
lens is made equal to or larger than the width of the
the normal line of the polished face 23 of the substrate
guided beam. Although a bulk lens may be used as the
end face form an angle (b, total re?ection occurs when
lens, the Fresnel lens is preferred for the sake of inte
grating the lens on the substrate and achieving compact
con?guration. Numeral 23 denotes a substrate end face
comprising a re?ecting face cut and polished at an angle
of 30° with respect to the substrate surface. This inclina
tion angle is made equal Numeral 21 denotes a focal
3’A.
Assuming that the incidence direction of the beam
radiated by the grating coupler 3' and the direction of
n; sin¢> l
(22)
The present embodiment is included in the total re?ec
tion range represented by the expression (22), because
n;=2.l77 and (9:30".
15
5,070,488
It is now assumed that the focal distance of the Fres
nel lens 22 is f and a coordinate system has the center of
the Fresnel lens as its origin as shown in FIG. 9b. The
16
on the optical waveguide 2 by a diffraction grating
having nonequal pitches and taking curvilinear shapes
as to introduce the radiated beam of the semiconductor
shape expression of the Fresnel lens is given by
laser into the optical waveguide 2, a diffraction grating
6 for correcting aberration, the diffraction grating 6
constituting collimater means in conjunction with the
incidence grating coupler 43, a grating coupler 3’ so
(23)
formed on the optical waveguide 2 as to have equal
pitches and take rectilinear shapes, a Fresnel lens 22
comprising a diffraction grating formed as concentric
(where m’ is an integer) Since this Fresnel lens is axis
symmetrical with respect to the optical axis, the lowest
degree of aberration is 3. As described in Meier, J. Opt.
Soc. Amer., Vol. 55, No. 8 (1965), pp. 987 to 992, the
third wave front aberration function is represented as
circles, the Fresnel lens 22 cooperating with the grating
coupler 3' to radiate the laser beam propagated in the
15
optical waveguide 2 (in accordance with the advance
direction represented by arrows) into the substrate lo
cated outside the optical waveguide 2 at a radiation
angle 0 and focus beams totally re?ected by the end
face 23 of the substrate onto one point located on a disk
functioning as an information recording face (not illus
20 trated here but represented as 15 in FIG. 4), beam split
ters 24a and 24b for quartering the beam reflected by
the disk and led into the optical waveguide 2 by the
+ rlF + -l- KcosODx + sinODy)
Fresnel lens 22 so as to generate two pairs of, i.e., four
beams nearly symmetrically with respect to a plane
Using relations x=rcos0, y=rsin0, the function has
25
been represented by means of polar coordinates. Fur
ther, 7t’ is the wavelength of the actual light source.
passing through the central optical axis of the reflected
beam and perpendicular to the optical waveguide 2 and
for focusing the beams generated by quartering, photo
Regarding aberration of the ?fth or higher order as
detectors 25 and 25’ comprising photodiodes for respec
sufficiently small and neglecting it in the present em
tively detecting beams reflected by the information
bodiment, allowable upper limits of An, AA and 5 satis 30 recording face (disk) and quartered by the beam splitter
fying the Marechal condition are derived. That is to say,
and converting the detected beams in electric signals.
upper limit values of IAnI, [AI and [6| are represented
as
IIA
and a surface acoustic wave generating electrode 12 so
formed between the focus grating coupler and the beam
splitter as to de?ect the laser beam by surface acoustic
(25) 35 waves. The grating period of the beam splitters 24a and
24b become dense in accordance with the travelling
direction of light. Further, the substrate 1, the photode
tectors 25 and 25', and the semiconductor laser LD are
placed on a metal stage 36 comprising aluminum, for
|s| 2 2.0 x 10—3 rad
example.
1
Comparing the above described values of the prior
Operation of the optical head con?gured as described
art with the expression (25), it is found that the upper
above will now be described. First of all, the laser beam
emitted from the semiconductor laser LD is led to the
limits of the present embodiment represented by the
expression (25) are significantly relaxed as compared
with the grating coupler of the prior art having the
focusing function. As compared with the focusing grat
ing coupler of hybrid type using the conventional focus
ing bulk lens, the expression (25) is somewhat inferior in
|An|, but is improved in |8l and IAAI. If |AA| in the
expression (25) is allowed, the actual wavelength varia
tion up to :4 nm is allowed when a semiconductor
laser having a wavelength X=0.78 pm is used, resulting
45
optical waveguide 2 by collimater means comprising
the incidence grating coupler 43 and the diffraction
grating 6 for aberration correction and then convened
into parallel beams.
The design technique of the incidence grating cou
50
pler 43 and the aberration correcting grating 6 will now
be described. Regarding the above described two dif
fraction gratings as holograms of transmission type,
phase transfer functions for converting the beam emit
ted by the LD into parallel beams are calculated for
in a practicable level. With regard to |An| as well, the
actual variation of effective refractive index is 0.0035
respective diffraction gratings. Subsequently, depen
when LiNbO3 with refractive index n,=2. 177 is used as 55
dence of the above described phase transfer functions
the substrate and the wave 7 with effective refractive
upon wavelength is examined. The phase transfer func
index N=2.l87 is used. Since the variation value can be
tion of each diffraction grating is so defined again that a
suf?ciently'controlled in usual process such as thermal
diffusion and ?lm forming, no problem is posed.
change in diffraction angle caused by wavelength varia
the present invention. FIG. 11 is its side view. In FIGS.
10 and 11, an optical head according to the present
invention comprises an optical waveguide 2 formed on
Subsequently, the above described collimated laser
beams are propagated through the optical waveguide 2
and are incident on the substrate 1 located outside the
the surface of an optical substrate 1 such as a dielectric
optical waveguide 2 at a radiation angle 0 (30° in this
FIG. 10 shows in a top view a fourth embodiment of 60 tion may be canceled’ in each diffraction grating.
or glass, the optical waveguide 2 having a refractive 65 case) by the grating coupler 3' having equal pitches and
taking rectilinear shapes. The beams are then totally
index larger than that of the substrate 1, a semiconduc
tor laser LD for injecting a laser beam into the optical
waveguide 2, an incidence grating coupler 43 so formed
re?ected by the substrate end face 23 and are incident
on the optical waveguide 2 again. The beams passed